SECApocalypse - 1981 XJ750 SECA "Mad Max" bike

Had a thread for this already, but its OOOOOOOOOLD. Took me 4 years to build, and even folks who saw some of the early stages were surprised at what it now looks like. Front end is an adjustable Hossack style setup with 4.5" travel (forget the wheel rate, think its about 300 / in), back end uses the shock from a BMW r1200 Montauk with what amounts to a high leverage ratio, has 3.5 travel and wheel rate starts at 250 lbs / in going up to 45o lbs at end to travel (so a bit stiffer than stock, to MUCH stiffer)

Bike actually starts VERY nicely, just choke and throttle and BOOM, catches on the first turn. But once it warms up I have to turn the choke down and then its hard to start (needs ether) and won't idle unless I rev it a bit. Probably just needs normal tuning (timing / spark plugs / valve clearance), carbs should not be a problem though - they were well cleaned when the bike was torn down and then sealed up and stored inside. Despite the looks, the breathing from the carbs perspective should be near stock- the custom airbox has the same volume as stock (including the filter / snorkle protrusion) and the stock snorkle sits in a plate under that air filter, so all air that goes into the box has to pass through it.

Because of tuning, I haven't ridden it far yet, maybe a mile going around the neighborhood. Handling seems good, and the front brakes work GREAT (is the stock MC and rotors, with R6 calipers). Rear end seemed a bit freaky when braking, but I think that is because the tires is old and I was on a freshly surfaced road, plus there's now much less weight on the back end. Total weight is 500lbs dry, 265 front / 235 rear. I weigh 225, am 6'3" and have a long torso (hence the ergos) so a lot of my weight is also on the front.

 

do you put gas in that 2nd tank?
Oh, and my original thought.. ."What on earth?!"

 

The rear 'tank' is just a fender. Is the original, was very badly rusted. Bottom is cut out and inside sprayed with undercoat. The chain lock that Yamah aincluded with the bike (matching keys) runs through the fuel port and then around the frame, for use as a helmet lock.

Replacement tank is a from a Virago 750, has the battery mounted under it just forward of the seat. It's pretty much a straight swap, you just have to lift the rear end of the tank up maybe 4 inches (cut off orignal mount and welded it to a bar coming up) and plug the right side petcock port (no idea why it has two, I just welded a patch over it).

It looks pretty crazy, but should qualify as street legal. Turn signals are not required in my state, but it has em (though they are not currently wired up). The plate holder is LED and has brake, plate, and turn lights, and the headlights have small bulbs below the actual beam bulbs that can be set up as turn signals without much work (they are meant to be low-brightness aux lights). Even has a working horn (I beleive it might even be the stock unit).

The custom construction... would technically require a inspection and special registration or even a new VIN. I'm not gonna mention it at the DMV (the old VIN is still there, I didn't hack the neck any or weld to that part). Its been legitmately load tested (using a frame stand, turnbuckle, and crane scale) but I don't wanna argue with some half-informed DMV desk jockey about what a Hossack front end is.

Technically I didn't even alter the frame much... just welded & bolted some stuff to it. Like a bolting on whole new front steering & suspension setup. Which Is actually not THAT uncommon - trike conversions often work the same way.

 

I am left dumbfounded and speechless......but I think you do deserve some type of award!

 

How fast were you going when you hit that deer?

 

Whow....., I mean wow!

 

I like..

1) The mono-shock conversion
2) The airbox work around

 

That is a unique machine sebwiers where on earth are you, I'd need an engineers report for that to be legal in Aus'. I do kinda like it's one offness, it's your boat and I hope you float very happily on it.

 

I'm in Minnesota, USA.

I have no idea what the legal impact of the mods is, and may end up talking to a lawyer through a local bike group. There's a few ways to get custom build vehicles on the road here. The simplest is just to never mention the modifications when registering it, and leave the VIN stock. There's not any inspection when you just go to get a plate for an existing bike; its up to police on the road to cite you for issues. That was my plan, though its probably not strictly legal (the do ask if you altered the frame). If I get stopped by the wrong officer, it could be a big problem, but I figure if I ride cautiously and play it off as a serious and well researched engineering project (which it is, just with some art thrown in) when questioned, I should be OK. I've been told by others with similar projects that police are usually just curious, and maybe want a picture.

 

I was following your old build thread, and wondered what had happened to this project. I've been intrigued by the Hossack suspension setup for years but don't have the guts (or skills) to actually build one myself. Please get this thing tuned up and on the road so you can give us a real ride report on how it handles!

 

That's proving a bit challenging. Probably need to pull the carbs off and make sure they are 100%. Plus the $#@^& throttle cable issue pretty much stop any tuning before I can run it more than 2 seconds.

I did actually find a (very good) local shop that was willing (eager even) to take it on after seeing pictures and a video of it running. One other did not respond, and a second responded to say it would be impossible to tune because of the mods... without even knowing what the airbox setup was.

If I can find the money, I might just hand it off, because this is a very frustrating stage for me, and not my area of skills.

 

Where in MN? I'm down in Rochester.

 

Saint Paul.

I actually found a local shop to do my carbs. The advice on this site could probably get me through it, but I lack the time, patience, experience, and equipment to get the job done reliably before I need the bike running for an event in California.

 

Good luck with the shop... make sure to provide them with the address to "Church", along with contact info for Father Chacal. Lol

 

The virago 750 tanks had two petcocks because of the tunnel design of the tank. If you block off one side your only going to get fuel from the side with the petcock on it. I learned this the hard way. I have a virago 750 tank on my virago 920. The virago 920 only has one petcock.

 

Would you mind if I merged the two threads so people can see where the project started?

 

Well, the rear fender is just so obvious but brilliant! Never have seen it before despite it's obvious application...said tongue-in-cheek but good for you for seeing so far outside the box. I love the front end because mods that serve no function just aren't my thing but kudos on your build!

jeff

 

The antlers are to help him win the best mate during The Rut.

 

I'm very interested in your airbox mod here. There a build thread?

 

Nah, I guess not. I just got the bike back from the mechanics and have put about 70 miles on it, seems a good time to wrap up. I paid a whopping $650 to have the carbs rebuilt and the bike lambda balanced and dyno tuned; it makes 55 rear wheel horse on the rolling road. Might make more with a better pipe, apparently the cheap aftermarket MAC muffler that came with the 4-1 system the bike came with is a bit restrictive. I can't imagine what a less restrictive muffler would sound like; this one is pretty lout by my standards.
The runs GREAT, no stumbles, so I think money well spent, especially gioven I was in a rush to get it done and had no idea what was worng (it started but would run super hot and rev high or die). I had to contact 3 shops finding somebody willing to take the job on, most said it would never run. The old guy who did the work had me put the dyno video on his web page to prove to the other shops that yes, it WOULD in fact run.

Not really. Is pretty basic work (the first glass work I've ever done). I built a form out of styrofoam that would be the interior volume of the box (insuring this volume nearly matched the stock volume, I think its about 130 cubic inches), and covered that with packing tape. I covered it with layers of prickly fibers and nasty goop until I thought it was air tight. I took a bit of care at the front where the carbs would go; this was left with an open area big enough for all the boots, and the whole mess was set to dry each time with this area face down on a flat plastic surface, so that it had a flat, smooth front. To mount the carb boots, I made a plate from 1/16" aluminum that was the shape of this opening and had 2" diameter holes drilled in the pattern needed for the boots. It simply tapes to the fiberglass using high temp aluminum tape for furnace work. There is a 4" diamter PVC pipe coming out of the other end of the box. The mouth of the pipe is covered with a 1/8" aluminum circular plate that has an oblong hole cut in it for the stock air intake "snorkle"; combined with the stock volume, the airbox resonance (if not flow) should be the same as stock. The PVC pipe itself has a 4" cone style washable filter slipped over it.

The pictures are in this album, with some descriptions. There's actually a laser cut acrylic plastic plate in the airbox that I was gonna put the boots in, but I figured out pretty fast that was not gonna work. https://imgur.com/a/GzIm0

 

LOL, that explains why my range is only 70 miles! So I guess if I run out of gas I can stop and slosh all the gas over to the other side.

I went to a bike show yesterday, and won the biggest trophy they had. It was the prize for "Ugliest Bike". The other categories were Best Black / Red / Green / White Bike, Best Chrome, and Best Paintwork.
https://photos.google.com/share/AF1...?key=Z2w3WjR4LTdmOS02YzMwR1pNS01LQ1hEMDlmdDVB

 

BTW, I got a plate via the simple expedient of not mentioning anything they didn't ask about. Here's a crude video of it on the road; I mounted the camera on the bars pointing down at the front axle.

 

Added some farkle for my 700 mile trip to Detroit to join up with convoy to CA. Luggage rack, ammo can storage, 35 watt LED driving light / flood.

 

Well, the petcock may be on only one side or the other, but the fuel can still travel around the back end of the tank, from one side to the other.

 

Nope, not this tank. The tunnel runs from end to end, only bridged by the top of the tank. There's probably a solid gallon of gasoline that can not reach the left side petcock unless I tip the tank relative to level, either side to side or front to back. If I popped a big enough wheelie (or just accelerate hard) it might slosh some over, but it won't a happen when going around corners.

However, the setup on my bike WOULD allow me to add a tube that makes such a connection....

 

I feel confused, disoriented, and a little bit dirty when I look at your bike.

I wish you luck on your ride to California.

 

Luck was not with me. The bike handled great, but after about 200 miles the line for the oil cooler blew out. I was going up a hill at the time and did not notice the loss of power as being an engine problem, instead thought the hill was steep. The bike kept loosing speed and died near the crest of the hill. Later dis-assembly showed that the out-flowing oil line was routed very close to the header pipes, and turned brittle after prolonged exposure to heat.

Long story short, the engine is seized. However, it handled well enough to seem worth fixing, not to mention being quite a head turner at local meetups, etc. Simplest solution seems to be to buy another engine and swap it in. That might even be the CHEAPEST solution, given the cost of parts, special tools, my own time, let alone paying somebody else to do the job.

 

On the upside, once I got the damping & preload set right (both were at zero in the above video, because I'd been messing with range of motion tests and forgot to set them) it handled better than I had any right to expect. I can't say how a stock SECA handles, but I'd wager this one handles better... or at least, more like a sport bike, which is what I was shooting for. It LOOOVES to lean deep in corners, and while it takes some muscle change direction at low speeds, at higher speeds it feels very agile but also very planted. It comfortably eats speed bumps at 15-20 mph, with only a slight hop from the back wheel. The anti-dive geometry is very evident and effective when you go over bumps while using the front brake. I think I can honestly say its one of the best handling, best sprung (if stiff) bikes I've ever ridden (which isn't saying much, as that's essentially a handful of UJM's).

 

what's the weight vs conventional forks? looks real heavy

 

My total weight is 500lbs, afaik the same as the base bike. The weight bias is flipped - more on the front that rear. That's partly the shorter wheelbase and relocation of some components, partly the elimination of rear bodywork, and obviously partly down to new components up front. Rider positioning only emphasizes this bias - the bars are a full 8" forward of the stock position, and a few inches lower. I have a very long torso.

When I support the bike by the frame and let the wheel sag onto a scale with the shock removed, the scale shows ~45 lbs. That's wheel, tire, brakes, the whole unsprung weight. There's not much in the way of counterbalance to any pivots, so I think its safe to say that is representative of unsprung mass. Weight wasn't really a design goal, I selected materials on the basis of ease of fabrication & safety, so the steel is all quite stout to make for simple, strong welds. Most of the steel (for tubing etc) is 1/8" thick , which is plainly over-spec when you compare to the tubes used on the frame. I could probably shave down unsprung mass by at least a few pounds with a design that required more skillful fabrication and did not allow replacement of sub-components, but having the option to dis-assemble for re-work and replace sub components was a HUGE benefit in revising my design as I built.

I haven't tried the same unsprung mass measurement with a stock bike, but given I have one, maybe I should. I think I'd have to pull the fork spring and let out all the air to make such a measurement. To be fair, I'd also have to weight the fork springs and include half their mass, and be sure the fork has a full load of oil. I know I was surprised how heavy the triple tree / fork leg combo was, and can carry my whole system equally easily (accounting for bulk) so I suspect that in terms of total mass, they are in the same ballpark. I'd wager mine has more unsprung mass, but less steered mass. (Steered mass is another thing I have not tried to measure for either; its also tricky because rotational inertia is the figure you really want, so something heavy like the fork steerer being present in one case and not the other throws off the comparison. I'd be willing to put my fork legs against the stock ones; given they are shorter and not filled with oil and springs, they almost surely are lighter.)

Heavy or not, it seems to ride well. The spring is from the rear of a GSXR1000, and nicely damped, so its suited to controlling a heavier wheel system. The front wheel also has more 'sprung" weight pushing down on it, especially with a tall 225 lb rider on board, giving this spring more to work against. I took a few speed bumps at 15 mph and was very pleased with the behavior; the rear end hopped a bit, but the front felt quite stable and planted, and did not throw me around. I'm bummed I can't ride it more before the weather gets cold (and may buy a drop in replacement engine to fix that, rather than taking time to do a repair on either I have) because its a damn fun ride.

 

A quick and rough job of putting a stock bike up on its center stand, slipping a scale under the front wheel, and pushing down on the back end tells me the stock unsprung weight is ~40 lbs. That's the point at which pushing down slightly further seems not to change the weight at all, so I'm figuring its the end of spring extension and the start of negative travel. Obviously this is a VERY crude measure, and could easily be off by 5 lbs either way, but it fits with my impression that my system isn't grotesquely heavier than stock. If I have 15% more unsprung mass, that is probably offset by having at least 15% more load on the front wheel. The place where I noticed wheel hop is actually in the rear, which has near stock unsprung mass, but a lot less weight pushing down on it. The spring back there is very stiff (and highly progressive) and maybe should be softer but better damped.

 

well if you're ever curious my basically stock 750 seca has 191 lbs on the front tire and 270 on the rear according to my bathroom scale

 

191+ 270 = 461. How is your basically stock bike coming in 70 lbs lighter than Yamaha's claimed wet wait of 531 lbs? Only think I can think is maybe weighing one end at a time that way doesn't give results that reflect the total weight? Maybe you need to lift both ends equally for that to work?

f*** it. I'm heading out the the garage in the dark now.... (I keep a bathroom scale in there).

 

For my stock Seca, I got 225 lbs front, 265 rear... and then the scale was wrecked because one corner bent in and now it can't indicate over 100. No gas in the tank, which maybe accounts for some difference from the wet weight. I did the back last, so it was probably also a bit light. Like I said above, my bike was pretty much the flip of those - as I recall, about 230 rear and 270 front, with full (3 gallon) tank. (Yes, the back tire does lock up very easily; I pretty much just don't use the back brake unless I have extra load on the back.)

Oh, and I knocked my FJ over and the brake lever punctured the case and now oil is everywhere. f***, why do I get in this shit....

 

stock exhaust----mac 4/1 with supertrapp
center stand
atari----meanwel digital
steel bars----aluminum bars
stock mirrors----little bar end mirrors
tool kit
that chain lock thing
plastic part of the back fender
half of the wires
i don't have any gas in it and my scale reads me about 3lbs light by the doctors scale

 

Took the bike on a few rides, rode pretty well. Software and preload says my front shock is to soft, but its really not bad considering.

Won "ugliest bike" trophy at a small local show.



Went to take a longer trip, got about 250 miles and started loosing power. Didn't realize the problem because I was going uphill. Got to top of hill, engine died. Pulled over, bike was covered with oil. Poor oil line routing put it next to the header, it got hot and blew out, and I didn't have the oil light on my new gauges hooked up.



Got a Maxim with 15k miles and good compression numbers but bad carbs for $500 (minus wheels, seat, and tank). Swapped engine - got around the front rubber insert issue by machining the inserts down short enough to fit the SECA frame. Also got a Hyabusa gen 2 shock (50% stiffer spring than the gsxr1000 shock I was using) for the front end, allows proper ride height with no pre-load. Also new tires - Shinko 705 front and rear.

 

fugly is more of a correct term

you could add a rusty gas cap and carb hats to it to ensure winning two years in a row

that rear fender is a a nice touch if only it were functional

switch the mirrors to opposite sides and mount them in the forward handle bars as bar end mirrors.

 

How ugly is that going to get?lolz I like how proud you are of the ugly awards.

Whatever floats your boat and finds your lost remote...

 

I'm shooting for "post apocalyptic feat of mad engineering" ugly, so "fugly" fits. I'm not looking to add rust though. Any rust that happens, happens, but I'm not encouraging it, and if I have cleaner bits (with 3 bikes to choose parts from, I usually do) I use em. Shiney and Chrome, all the way to Valhala. (It also happens to be a bit of pre-apocalyptic engineering. I'm learning a lot about suspension setup, handling geometry, etc - part of the reason its so clunky is I wanted a lot of adjustability, which means a lot of parts and bolts. And obviously learning some fabrication skills as well; designing for adjustability, low cost, safety, and ease of construction didn't leave much room for beauty, so I went with brutality instead.)

More gas capacity would be very nice, I only got about 70 miles at a time from that tank, maybe could have gotten another 10 if I took it off and sloshed over the gas that is stuck on the side of the tunnel opposite the petcock. A tube joining the two halves would be a fairly easy fix, and that same tube could run to an aux tank if I also included a vent tube; that setup would also make installing a site gauge easy.
It would be nice to use the rear as a tank, but given all that is under there, probably not practical, even if I started over with a non-rusted tank (which I have now). Easier just to build a whole new rear section that is a combination rack / tank.

I don't want bar end mirrors because its gonna get sliders / crash cage / bash gaurd / other hardening against crashes. The extra set of bars will be extended to protect the controls in a crash, and work like a beefier version of the guards race bikes have to stop the brake lever from contacting another bike.

Should get fairly pretty, because the set of carbs I was running on the seized engine is still good. They were professionally rebuilt and dyno tuned to my intake / exhaust setup, and there's no reason they shouldn't work fine with this engine. The crap carbs, I get to pull apart and learn from without having to worry about them working or not.

At shows where it's a category "ugliest bike" is generally shorthand for "best home built" or "best rat bike" or "most interesting". The other awards at that same "show" were "best red / green / black / white bike" (all going to very nice imports) and "best chrome" and "best paint" (both going to traditional HD choppers). Of those prizes, "ugliest" was last awarded and had the largest trophey (the rest were plaques). So I think there's reason to be proud of it - the judges really liked the bike, as did a lot of people attending. And I even had some strong competition.


My boat floats pretty good. Considering the next show I'm going to is Moma Tried, where I'll be hooking up with a crew of folks from Milwaukee who were invited to make a special exhibition run in the flat track arena, I'd say I'm not the only one who likes the style.

When I broke down on the highway, I was actually headed out to CA to party with these folks. Their remote seems stuck on the same channel as mine. There was about 4,000 people at the (5 day) event this year.

 

Nice. Please don't interpret my comments as insults, no offense was intended. I've been following your build for a while, I was trying to be supportive. I am aware of the rat bike and rat rod phenomenon, it's just that all my vehicles, except my bike, are pretty ratty, not by choice, so it's not a direction I choose to go. Absolutely be proud, she's your creation, why wouldn't you be?

 

That's cool. Some people legitimately just don't know its a thing that people do on purpose (not that my skill level would allow significantly differently) so it seemed worth saying. Some folks, its just not for them. A few (many fewer than expected) actually get pissed off, but I'll admit, I went out of my way to annoy such folks by going against they styling conventions of multiple genres (anti-sloped steering vs chopper, downturned tail vs streetfighter, big custom airbox and broken bone lines vs cafe racer).

 

New engine is in, and everything is wired up like before (afaik) but I'm not getting any spark. Either my wiring is bad, or maybe the timing pickup in the engine is bad.

I can't use a normal wiring diagram because my wiring is literally a mash up of bits of the old harness that bypasses the entire dash and safety relay setup. I can literally run my bike without using the key (which just turns on the headlight / tail light / brake light circuits) - instead there's a big racing style "keyed" battery cutout that kills the entire battery connection for security and safety. Instead I needed to figure out how the TCI should be "minimally" wired.

There's two plugs on the TCI. One has 4 connections, and feeds the primary coils. It has 4 wires - black, grey , red/white, and orange. The black is a ground, and is connected as such. The red/white slits and goes to both coils - I assume its the coil ground. The orange and grey each go to one coil, I assume are the ones that have timed current. I'm assuming this one is correctly wired because the bike ran fine before, and neither coil is sparking.

The other plug in the TCI has 5 wires (and one empty spot). There's the grey, orange, and black wires running down to the engine / timing pickup. There's a black & white wire that I think goes to ground... but now that I consider it, maybe it is suppose to go to a positive that is switched by the engine run switch? I must have one of those, I wonder if I made a mistake and thought it should be connected to the fuses... which would do nothing except create a pos-to-pos loop when the switch is closed. So in this picture, the black and white wire needs to go that run-switch controlled hot wire (instead of ground), and then I should have spark?



There are two more wires coming up from the engine with the TCI timing pickup wires. One is blue, one is black & red. From what I can tell, these are the neutral and oil pressure indicator wires, and connect to ground when (respectively) the trans is in N / there is low oil pressure. My gauges have LED's for those (well, not oil, but I can use the turn signal LED, which is orange) but they were not hooked up... which is partly why I din't know I'd blown an oil cooler line. They most certainly WILL get hooked up now.

 

The R/W wires that split are coil power and connect to 12V - normally through the kill switch. The orange and grey connected to the TCI sink current through the coils as commanded by the TCI.

The B/W wire should be left open unless you have a side stand relay. When grounded it disables the TCI so no spark occurs.

Correct on the neutral switch. However, on bikes that contained the CMS the oil "level" switch is normally closed and opens if the oil level gets low. Since you replaced the engine with a Maxim you may or may not have the CMS version of the oil switch.

 

Cool, thanks! I think its safe to assume the R/W wire is set up OK, as I don't recall messing with it since the bike was running. Sounds like I just set up my TCI to think the sidestand was down. Ironic, given I don't have a sidestand switch (and am considering removing the sidestand - with the ride height lifted an inch, its not very stable and the centerstand is easy to use). I almost tried with that wire disconnected, but it was getting dark and cold. At 3:30 in the afternoon.

I think I actually probed the oil level wire with pos current and found it was an open ground (with the bike not running). Which is perfect, because it means I can simply wire up my orange LED through it, and the switch will be closed when starting the bike (when you light is normally on) and will turn off as the oil pressure comes up / come on again if it drops.

What is the CMS? That I'm aware, the engine is essentially identical - I don't need to switch to the TCI that came with the Maxim because of cam differences or some such, do I? I guess I should be sure to use Maxim spec valve clearance etc.

 

Just to clarify, the switch is simply oil level and has nothing to do with pressure. If it checks open in the motor with oil installed, then you should be good to go to wire it so the light illuminates when the oil level is low. To be sure it functions properly you would have to drain enough oil for the switch to close. After that, it should be checked whenever the oil is changed to be sure it is still functioning - simply turn the key on and the oil light should illuminate. On the Atari bikes, the CMS would have to be manually activated to ensure the oil level switch was still working.

One other note on the oil warning light; Yamaha actually went to the trouble to test the bulb for the oil light by incorporating wiring that would illuminate the light when the starter button is depressed. Something you could do, but maybe not a high priority.

CMS - Computer Monitoring System aka Atari. I believe the TCI for the 750 Seca and Maxim are identical, as are the valve clearances. Matching numbers for the TCI can be found under the pickup coil cover stamped on the timing plate.

 

Hmm, I see. Well, then I think the oil switch is closed when full. I suppose I can still find some way to use that (a light that is always on to show oil is in there is still maybe useful).

So, if I want actual pressure sensing, I need to install my own, eh? Might be easy enough, given I have oil lines.

 

Just to thank and confirm - this was indeed the problem. Unplugged the wire, and bingo, spark test was good. Looks like all I need to do is tie up the wires and mount the fuel system.

 

One of my headers was loose (missed tightening a nut) but otherwise runs well!

 

Bit of update.... lol, more than bit!

Got it running in 9/2017, blew the engine up because the oil line melted on the highway. Rode pretty good.


Replaced engine. Replaced front shock with Hybausa unit (850 lbs/in vs 650 lbs/in). Rides much better.


Built crash bars. Still plan a few things. A custom front shock linkage will improve the wheel rate curve to make it slightly progressive, and give me a bit of extra travel.