putting a 750 or 900 into a 650 seca?

hey guys does a 750 or 900 fit into the frame of a 650 seca?
cheers brenton

 

750 will fit, not sure about the 900.

 

Let me rephrase that, I know that a Seca 750 will fit onto a Maxim 650 frame (yes, bought the engine and tried to stuff it in, forgot the tach issues and yanked it, haven't gotten back to it yet). Good luck, sounds interesting, wish I had a 900.

 

what sort of tach issues ?
cheers brenton

 

I know that the 750 will fit into the 650.

 

650 Maxim has a mechanical tach, the 750 Seca has an electronic pick up on the coils. I expect that there are some models of 750's that have the mechanical hook up, but I failed to source the correct head for that application. I could have stuck with the 750, the mod is really no big deal except I haven't a spare eletronic tach for a Yamaha laying about. How's life down yonder? That storm cause you any grief?

 

nah theh storm was up qld , im down in sydney so i was allgood. but yeah we wont be havein bananna's for a while.
so you just need a electric tacho , that doesnt seem that hard.
has anyone got any ideas on the 900. i can get on cheap to rebuild?
cheers brenton

 

Engine Swapping

 

i couldnt seem to find out if a whole 900 motor and gearbox would bolt straight in ?, but it seems like the 750 will ?
cheers brenton

 

i'm pretty sure that the seca 750 and and the 900 share the same bottom end.The top end is where it takes up a little more room.Hence if you can shoe horn a 750 in a 650 chassis I would almost bet the 900 would go in. The biggest problem would be height.You'd have to modify those top frame tubes.Sounds like an in depth project to me.How deep do you want to dive in?

 

One guy stated that he had both the 750 and 900 engines in his bike at one time, so it should fit just fine. Consider that the turbo probably has less space to work with than the NA Seca, and you should be good to go. The 1984 XJ750R (not Seca) was virtually bolt for bolt a 900 Seca with a smaller bore and stroke. The engines are identical. The owners manual for the 84 750R was the 900 Seca manual with a small addendum included for the different 750 parts (carbs mostly).

 

cool well if i do it ill prob just go to a 750 then, the 900 that i found needs a rebuild anyway.

 

how much would the swap benefit you?

Is there that much difference in power between the 650 and 750 or is it just a better platform to build on?

 

Hi everybody... :wink:

I'm new here..

I had in a german forum xj650.de read that a 900 fits ok.

regards molgaard

 

hey thanks molgaard, ill keep that in mind.
the 750 is supposed to make 80hp over the 70hp from the 650, and the 900 is supposed to make 100hp :lol:
cheers brenton

 

The 900 also has a better flowing head and bigger cams. Would love to find a 900 around here, they're total beasts! But it's rare enough to find a complete XJ900, much less one that is being parted out. :cry:

What I'm doing is putting the 750 rotating assembly in a 650 motor. Why you ask? Well, mostly because all I have is 650's, and one of them needs to be rebuilt anyway. That with a set of 900 cams and some mild port work should make a pretty decent motor...

 

What is the advantage of the 900 cams? lift and duration? Won't you need to port to take advantage of that? Where is the restriction in the 650/750 engine? Are you trying to bolt the entire 900 head onto the 750 jugs?

 

Welcome Molgaard, hope you join us often. Thanks for the link, keep them comming!

 

The 900 cams have more lift and duration. Some port work is planned on the 750 motor that I am in the process of building. As far as where the restriction is, I'd guess the first restriction would be the small intake and exhaust ports, followed by the small cams. I'm not going wild with this setup, the 900 cams are only slightly larger than 650/750's, just trying to squeeze out a bit more performance without spending a bundle on custom cams/oversize valves/high comp. pistons, etc. If I were to find a 900, I'd use the complete motor.

 

Ok Now I got my head thinking. I'm tempted about the 900 engine in my 83 750 seca. Do I gotta change the carbs, coils, exaust? What other "accessories" besides the engine have to be swapped out as well?

 

I've got a 750 Seca engine in a 650 Seca frame. It fits w/o any problems, and is a nice combination.
There were a few minor things with the tach and oil level sender, but that was it. The 650 CDI box is physically the same, but may have an advance curve that's different. I don't know for sure, but they have different p#s, and I'm using the 750 box.

I was using the 650 airbox, but just got through reinstalling the 750's original airbox and filter. The exhaust is stock 650 Seca.

The 750 YICS engine weighs about 5# less than the non-YICS 650's.

An XJ 900 will also fit...I've measured :roll: You may need to pull the 900 engine to get the valve cover off, and I have a feeling that the petcock will be very close to the valve cover...but it should bolt right up. All the mounting points for the three XJ engine sizes are the same.

A 900 in a 650 frame is do-able, but you'd need to use pod air filters, and might have to tweak the exhaust pickup points a bit.

 

I've owned 2 Seca 650s & now a 750. I like the styling of the 650 better, but after owning a 750 it's look has grown on me. As far as the power, I was looking for more when shopping for a 750. For me it is just that bit more for acceleration on the freeways (passing), larger brakes, more comfortable seat, more stable (more weight). Don't care too much for the instrument panel, prefer old school. I've put on tires, steel brake lines, different handlebars, MAC 4-1, jetted the carbs and that's it, pretty happy with it. I too had considered putting a 750 into my last 650, but glad I went for the whole bike (plus just a motor wasn't around at the time, bike was).

 

The 900 engine "Goes-in" ... all the Motor Mounts are the same ... the 750 and 900 share the same Bottom End.

You do have to do some Mods to get a fuel tank on.
The 750 Maxim tank won't fit-in the horse-shoes and set down without the under-tank seam hitting the Cam Cover.

The angle was pretty severe. We didn't measure it. With the rubber piece that fits on the frame where the tank bolts-on too ... off ... we had a Stanley Rubber Sanding Block on it's side ... propping things up while tried to dream-up some possible way to keep it looking like a Stock-750.

Complicating matters for the Budget-Minded "Do-it-Yourselfer's"

The Exhaust system needed to "Do the trick" and keep the bike quiet enough to hear yourself think, retain the center stand and allow for easy filter access ... is MOTAD's 4 into 1 from Great Britain. HUGE money.

I haven't totally given-up on the idea; yet. I'm rebuilding the 900 mil real slow. Porting, polishing ... playing with it.

If the parts look pretty enough after I have it Media Blasted ... I'm doing it.
I'm going to go to Night School to learn how to TIG Weld.

 

Oh man, do you care to part with that 750? I could live without a Tach... 8O

 

There are stock Yamaha electric tachs that will work.

 

damn sounds like a lot of work.. i'll keep the 750 the way it is..

whats the quickest and cheapest way to get a lil more snot out of it?

 

If you shop at "Big & Tall" ... and, you ain't Tall ... you can bump-up to the next Main Jet.

It's not really a "Performance" Mod ... It just adds a little Power-on at the expense of economy ... to help keep you from needing higher rev's ... for today's Interstate Cruise-o-rama's ... when everybody is doing at least 80 and sometimes close to 90 ... happens a lot, out here, near Boston.

 

A bigger engine is the cheapest/quickest way to do it.

You can find a running 750 Seca for well under $500 if it's been crashed or neglected. Buy the entire bike, and you'll have all of the bits needed to make the conversion, except for a tach.

I paid $200 for a 750 a few years ago, and rode it home. I haven't had a tach since putting the bigger engine in, and really don't miss it.

It's not really all that much work: the engine R&R could all be done in a weekend, with the next weekend spent going through the carbs.

If you use the 650's original pipes and airbox with the 750's carbs, the jetting will be peachy if left stock.

 

From which bike do you get the tach?

 

If you have the round-body instruments, an electric XV920RH tach will drop in. It'll have a lower indicated redline, but otherwise it's the same externally. You'll have to make a couple of electrical adaptors to install it, but the tach itself drops right in, and the wiring is long enough to get into the headlight bucket.

An XS1100 tach will also work (I've got one to put in), and it's the same appearance as the XV tach, except the indicated redline is at 8500 rpm.

There are probably others that will work, and there's an article on the XJ CD that goes into this adaptation in more detail.

I haven't had a tach in mine since swapping engines last year, and may not get around to it for a while- the 750 is torquey enough that I haven't missed it yet.

 

The 750 will fit because thats what is in my Seca 650. You would have to get an appropriate electrical tach if you want a tach. I did'nt bother. I may at some point. Right now I'm looking to buy a Seca 900. Not to put in my 650 but to ride. If you're looking to get a 900 motor the local cycle salvage place near me has one but they want something a lot plus a core charge. I'd rather spend the money on a running bike.