XJ750 Seca no power until around 3000 RPM

I have an '83 XJ 750 Seca that for the most part runs great but has always had a "flat spot" at low RPMs. From just off idle to about 3000RPM it has no power. Like hard to get going at a light kinda flat.

I live in Colorado at about 6000ft and most rides go up from here, so I’ve gone down to 116 mains. Plugs look perfect (literally exactly like the “best” picture in service manuals) and it runs great on the top end. It starts easy with the choke when cold and idles ok most of the time. Gets a little erratic when hot sometimes.

It has an aftermarket Vance & Hines 4 into 1 exhaust. Not my choice. I'd much rather have the stockers, but they're pretty much unobtainium these days unless I want to pay more than I paid for the bike.

If I had to make a guess, I would say it’s rich in the midrange. If it were a conventional carb, I’d raise the needle clip (drop the needle down) a notch or two and see how it does. But obviously that’s not an option on these bikes.

While I understand the basic principal behind a CV carb, I don’t really understand how to tune them in the midrange. I assume it has to do with the Pilot Air Jet and/or Main Air Jet. Logic would dictate that larger air jets would cause the piston to ride a little lower and therefore lean out the midrange. But which one? Would I go up a size on both?

Chacal has different sizes, but they run about $50 a set, so experimentation is an expensive proposition. Hate to guess wrong and end up dumping a lot of money down the drain.

Has anyone done much with changing out air jets? Any advice? Something else I should look at? Looks like there are different needles available too, but there seems to be even less info on how those affect things.

Oh, and before somebody asks, I have two full sets of carbs and have swapped back and forth with very little change. Both sets have been cleaned multiple times. Also checked valves and those were all in spec. Stock air filter. I am guessing the exhaust has messed with the midrange flow and thrown things off.

Thanks.

 

How does she run with stock Jetting? I ask because she should run way better at that altitude than you describe, even with stock jets.

The Vance and Hines kit was very good with stock jetting (from what I recall). I doubt that is the issue.

 

When I had the stock mains it was rich. Lots of soot from the exhaust and the plugs were a little black as I recall. It's quite a while since I made that swap so details are a bit fuzzy.

 

Taking off around 3000 rpm suggests the needle is about right, as well as the emulsion tube. What if you give it full throttle, does it still feel feeble?
Idle jets do a lot more than just idle, if you think its rich in the mid range try some smaller idle jets then open up the idle mixture to correct it?
The air jets do not control the height of the slides, they allow air in to mix (emulsify is the correct word) with fuel so you don’t get squirts out of the fuel jets. Bigger air jets will lean out the top end of the jet associated with it.
I would be checking you have the right springs fitted - did you swap the carbs complete? Is your air filter clogged?
My seca has a 4 into 1 V&H like yours, it rides fine (and sounds nice). I’ve no experience of the effects of altitude but @k-moe says it should be fine?

 

Speaking of air jets....are you sure that you put them back where they came from, instead of where the service manual says (incorrectly) that they go?

This is correct.

 

Yes, all jets correctly placed.

I did get some smaller pilots that I am going to try. I am in the process of cleaning the spare set again and put in the smaller jets. Waiting on new throttle shaft seals from Chacal too. Now just need to get the time to swap them and see what it does.

 

If you suspect throttle shaft seals, spray them with carb cleaner with teh engint running. If they are the problem the the idle will jump up even more.

 

Seals were definitely an issue. Not sure if they were the issue yet. Still in the process of splitting the rack and getting it all back together.

They were so dried out they literally fell out of the seats without even having to pull on them.

 

WOW!!! Those were the issue. Mine weren't nearly that bad and caused a hanging idle.

 

Well they were shot. But, given when you open the throttles the openings presented to the engine intakes within the carbs will be relatively huge compared to Hitachi's well engineered throttle bushings, I suspect the air will come in "the big hole" and not the throttle shafts. Thus negating any path via the throttle shafts.
I suspect you will either still have the issue, or will have found and rectified the real problem, thus blaming these seals.
We'll see, I guess.
At least you can cheaply buy the seals from @chacal
Actually, given you have two sets of carbs, with identical issues and as far as I can see no obvious cause, I might be inclined to look into borrowing a replacement TCI, just to eliminate this. Who knows, I may be wrong....

 

I may actually have another TCI in a box...somewhere. I have about 2/3 of another bike including a complete engine, frame, tank, seat, some plastics, a couple dashboards, and who knows what else up in the "loft". Haven't looked through that stuff since I bought it all in a package deal along with the one I ride.

My other thought was the intake manifolds. Most of them look ok but noticed one does have some cracks. Not sure if they are deep enough to leak. I tried shooting some silicone spray on them while idling and it didn't seem to change anything.

Mounted the carbs and got it somewhat idling. They seem to be reasonably balanced. Surprisingly they weren't too far off even after a complete disassembly and reassembly. I didn't mess with the YICS isolator when I checked them though, so that may not be accurate.

Still have to open the mixture screws way further than it seems like I should. I think I'm close to 4 turns before it will idle without surging. Haven't put it back together enough for a test ride yet but first impressions are that not much has changed.

 

Re the manifold cracks - see my comments above about small holes Vs big holes, and the fine cracks are usually just skin deep.
So the throttle spindle seals didn't help then? Makes sense - they should improve the idle consistency though.
Change the tci...
What about your float levels - are they set to get you -2 to 3 mm below gasket levels (from memory, please check I'm right with this setting).
Edit to add - if you're having to open the mixture screws that far (I don't understand the "surging" comment - surging is what you get with an over rich mixture - like a rhythmic rpm/sound - weak mix will give you a misfire - maybe its a US/UK terminollogy thing?)
So, if you have to screw your mix screws that far out, you have an idle jet issue (too small - which ties in with lack of power low down) or low float levels?

 

I "bench set" them according to the spec (17.5mm from gasket surface as I recall), but the one thing I couldn't get out when cleaning the carbs was the bowl drain screws. They are seized tight and I was afraid I was going to crack the bowl trying to break them loose. So I haven't been able to do the "tube" method.

Surging probably not the right term. Hanging idle maybe? Basically takes a while to settle back down after blipping the throttle.

 

Hanging idle tends to be caused by a vacuum leak.

 

For the bowl drain screws, I would recommend:
1. Heat
2. Easy out
3. Replace with socket head version from @chacal

 

Well you really need to get at least one working, and maybe swap between them to make certain your fuel levels are correct. Heat is your friend, a blob of mig weld on the end is your friend, then weld a bolt to it. If you succeed using an easy out go to the top of the class, you will be in single figures company...

 

Has this been fixed?

 

I put it back together enough to take a test ride. Maybe a slight improvement, but mostly the same.
Haven't had time to do a fuel level check or look for a TCI yet.
The "honey do" list has been getting the best of me. :-/

 

Ok, working on the fuel level. I managed to get 3 of the 4 drain screws out on this rack and 2 of the 4 on the other rack using a heat gun and a lot of cursing. So I have all 4 bowls with functional drains if I swap one from the other set. Interesting side note: I hadn't noticed before, but the bowls are slightly different between the two sets. I don't remember which were the originals anymore. I will probably swap back when I am done so they all match, but it shouldn't affect anything for these purposes. Looks like they just changed the casting a little to make them cheaper to manufacture.

I also decided to try and be clever and 3D printed a tool so I can check all 4 carbs at once. It actually works pretty good. It makes it easy to see the levels relative to each other. The top plate of the tool is exactly 3mm thick and aligns up against with the bottom of the carb body. A rubber band made from an old tube holds it against the side of the bowl so you can check hands-free. The idea is the levels should all line up with the black mark at the bottom of the top plate. The hardest part is that you have to get whole rack perfectly level side to side in order for all of them to be accurate. I did that using another tube with water in it, open at both ends and checked against the other end of the rack. (that's what the 5th slot is for)

I do seem to have some issues. #1 (left to right) is obviously way low. #2 and #4 look a bit high. #3 is close. Probably within spec, though perhaps a tad high as well.

I'm a bit perplexed why #1 is so low. I dry set them all to the same height. Makes me wonder if that float is bad. Might try swapping for another one.

Now the fun part - the tedious trial and error to try getting them all adjusted.

 

Close enough.

Solved the mystery of #1. Before I started the check, I decided to go ahead and replace the float needle valves with a new set I had from Jets-R-Us. Turns out despite them all coming as a set and having the same numbers stamped on them, one of them was slightly different. The spring hook was a different design and the machining was a little different on the seat. The main difference being that it sat up a little higher than the rest. I was able to get it in spec with some extra bending of the tab, but I'm trying to see if they can send me a replacement.

After all that I'd like to say problem solved, but it's still not quite right. It runs better. It's not quite as anemic on the bottom. But if I get the idle screw turned down enough so that it doesn't "hang" when you blip the throttle, then it idles too slow and will eventually stall unless I crack the choke ever so slightly.

I'm pretty much out of time. I am leaving on a 1600 mile trip with it next weekend. I've run with it this way for years, so I guess I can live with it a bit longer. I'll be going down to near sea level, so I went back to all the stock jetting. It will be interesting to see how it behaves at lower altitude.

 

Made it through my little excursion from Denver, CO to the National Motorcycle Museum in Anamosa, IA before they close down. The old girl ran like a champ. 1900 miles round trip running at 65-70mph most of the time except when we had to slow down for towns or stop for gas. Even had a couple passing runs in the 90's and one that got her all the way up to 101mph!

Temps were mostly in the upper 90's to low 100's including one of the days that was a stifling 107F. Miserable riding conditions, but the little Seca just purred along without a complaint. Burned a little over half a quart of oil for the entire trip. Doesn't seem bad for a 40-year-old bike with 45,000 miles on it running at those speeds and temps all day.

Of course, the idle is still a little off and it still has that flat spot, but I didn't spend much time idling or riding slow all week, so I barely noticed. The flat spot did seem a little less pronounced at lower altitudes, and certainly more power all around with all that extra air.

Glad we got a chance the see the museum before it's gone. Lusting after a nice Seca 900 they had on display that will be going up for auction next month. Too bad I can't afford it. Also stopped at the Speed Museum in Lincoln, NE and the St Francis Motorcycle Museum in St Francis, KS. A fun trip despite the heat.

Now back to the honey-do list for a while before I start fiddling with the carbs again.

Oh, and Jet-R-Us did send me a replacement for the mismatched needle valve. Didn't have time to install it before I left, but at first inspection it looks like the right one.