1981 XJ750 Seca Drag Bike Looking For Information & Opinions

So I acquired this 81 Seca a coupple weeks ago as a back up for my Buell 1125r. Its quite modified compared to anything Ive read about on here so far. Its a wheelie bar bike. Its been claimed to be a 14:1 compression engine. Here is what I know so far

1981 750
~14:1 compression (Confirmed?)
Mikuni vm29 smoothbore carburators (confirmed)
Triple valve spring set up with titanium retainers (not yet confirmed)
Unkown cams (supposed to have a ton of lift)
4-1 Exhaust

Is there beefed up shafts out there? Same goes for the final drive?

I think the second gear shift fork might be bent from an air shifter. when you throttle into second it acts like id popping in and out of gear, can anyone confirm this is a probable conclusion?

Im probably going to buy a transmission and shift for asssembly off ebay and swap it if so

tested the compression going off of a compression test. I read max is 170 psi, The once cylinder i compression tested was 220psi, and this thing has a bunch of seasons on it. My question is with the compression, Could anyone point me into the direction of a high compression piston so i can know whats possibly inside of it?

The valve spring set up i was told it was out of a kawasaki

The carbs are obviously off of a kz1000/900

Ill identify the cams soon

Can i fit a bigger xj motor on the 750 frame?

Im sure more questions will come.

Thomas

 

Got some pics? I can’t help with questions but would love to see this thing.

 

I forgot to mention, I bought this motorcycle and trailer for 500$ it's not pretty, it looks better now, but of course like I said, back up bike


I don't keep stickers I don't earn. 20 years of history had to come off

 

Hmm first never believe the previous owner unless you know them. The compression readings are really high , you might want to rent another tester just to verify . While you stated you were going to check cams , perfect time to check valve clearance and adjust if needed . Sounds like you have a cool beast .

 

Did the previous owner provide you any paperwork regarding modifications? Any time slips? I wanna know what it does in the 1/4 mile!

 

The thing with cams and compression ratios (and cylinder pressure readings) is that one greatly affects another. With low lift and or duration cams you have optimum trapping at low rpms (hence why std cams give good low end torque).
As you add lift (and usually duration), you get more overlap, with valves open together, losing some charge down the exhaust, some coming back into the carb. This gives a lower reading on the compression tester.
Worst case for pump fuel is (real) high compression, with std cams = detonation, melted pistons etc.
So measuring compression on a modified engine tells you very little, unless you know how it was built - oh, and what fuel it ran on.
If you want to know what CR its got strip it...

 

I would contact the local racing orginizations and see if you can find the builder/racer.

I also would not try to run her until you know what class she raced in. Use the wrong fuel and there's potential for ruining the engine.

 

I know what she runs on, it currently has 119 octane in it. He was running 116 in it but 119 was available for me. She ran in the ET motorcycle class, which means nothing.

I do know the previous owner

I know static vs dynamic compression is different but it's extremely easy to slap on a compression tester to see if it's higher than stock at all.

My gauge isn't wrong, I checked with two already. I own an automotive shop up here in Washington, I have plenty of spares .

He ran on the bottom end of 12 seconds, he's a big dude though. I'm 5'8 maybe 220. He's probably close to 6 foot and on the better side of 260-300.

With the cams, he says it's an aftermarket cam with adjustable cam gears but he has them zeroed out.

I just didn't know if there was general knowledge on a high compression piston. I appreciate all of your feedback so far .

 

If you need a gearbox I just sold an 82 750 with a bad top end to one of our members from Seattle I’m sure he’d give you a deal on it

 

Any non-stock pistons will have to be custom made, or you might get lucky and find someone with a stash of racing parts that never got binned.
Even stock overbore pistons are hard to come by. There just never was much demand for either.

 

Interesting. Thanks for your input.

The guy raced it in heavy into the 2000s. Last sticker was a 2013. The gentleman couldn't hang on anymore because of hand issues. He's told me what he could about the build. Everything I've been told I've said here.

I plan on yanking the valve cover off later today to see what's inside. I want to degree the cams properly as well. I have a launch control sitting here that I'll probably throw on

 

Or you could pretend he couldn't hang on due to SPEED issues!

 

Haha maybe. Dropping the clutch with a wheelie bar is quite the ride

 

Can't seem to find a number on the cam. I measured .480 lift on the exhaust, having a hard time to get my magnet based dial indicator to stay still on the exhaust. Possibly a megacycle 250-40. Cams a bit beat up.

 

Alright, so here is an update. I decided to break her down a bit. Here was the problem with second gear.




I have a new transmission sitting here. Will put it in after ewverything else.

So here come some possible goodies. Let me know if something doesnt look normal or stock, i have no ides what im looking at when it comes to parts ID



These pistons look a bit more domed to me from photos of 750 pistons i have come accross, i could be wrong.



Intake port & Exhaust port. Any evidence of porting?



and here is the heasdgasket if it matters


Let me know what yall think

Thomas

 

Here is a close up of one of the pistons, definitely yamaha as it says it inside the piston

 

So im going to assume that this is a high compression engine due to the piston having a lot more dome than i have yet to see in a photo of another 750 piston. Also if you look at the photo here, the mill has taken material out of the bolt boss for the intake or exhaaust bolts. That being said, and with the valves being so close to the flat surface, i think i may be at the limit for taking off any more material.



Im going to see about getting the head CC'ed today, while im at my machinist, ill ask him to pop a valve so i can see the spring set up.

 

Hmm, rod bolts upside down - that's a 900 isn't it? Time to measure it up?

 

No sir, thats the way they go. They are shaped so they cant go the other way.

The engine is a 750

Thomas

 

Update

Combustion Chamber is 24cc, does anyone know what stock is? Ill be calculating the dome of the piston within the week if i have time and that will of course help me witht he comp ratio

No titanium at all. 2 spring valve springs exhaust and intake. Ill get the spring rate soon,

Thomas

 

well please forgive me, I am no expert on the innards of an xj engine, but the information I have is that the 900s had the rod bolts upside down, the 650 and 750 didn't. But if you're sure...
Bore area times stroke (in cms) times 4...

 

correct on the 900 bolts in "upside down " for the clearance

seca rods have 5g200?? numbers on rod. his are likely aftermarket

 

@Minimulty, let me apologise. I need to take any information I get from what's seen and verify it, because at this point nothing has been what it's supposed to be. Anyways I think you may be right. When I looked at my bore and stroke measurements, i assumed I measured off. I calculated just a second ago and they read ~67x60. That would be a 900 if my information serves me right. Plus the part numbers on the rods start with 31a which is the same the 900 rods start with (the ones I found on eBay).

Part number on the side of the road is 31a0001

 

The 2nd gear dogss should be undercut before you reassemble. That will prevent them from slipping out of gear under load. You can even reuse the original 2nd gear and save the other for a spare.
173-tranny-fix-1st-and-2nd-gears-dremmel-technique-a-pictorialwalkthru.html

 

I'll have to do that, seems like I'll need to get a new transmission anyways as the spare is for a 750 and I read that the 900 trans is stronger. I'll definitely do the undercut gears though. Thanks for the recommendation

 

you should run the short vin from motor through this link to positivly ID motor

http://www.dissingermoto.com

 

What ive found out so far

The engine code reads as a 81 750. First could digits is 5G2 which points to the 5G200 of the 750

Bore & Stroke of a 900 67x60

Head of what i believe is a 750 as it only has 4 cam towers per cam instead of 6 cam towers of the 900

Rods im 90 percent sure are from a XJ900 with 31A0001 stamped on the side

Still havent found a photo of any pistons that have the same size home as the ones i pulled out. They are yamaha brand though as per stamped underneath.

Head and Jugs are at the machine shop to find the cc of the piston dome and help me out with a compression ratio

Thanks for the help so far everyone

 

Forget everything i said in this. I must have read the gauge wrong. I miked the base circle and then measured the lift. Looks like the intake life is .380 exhaust is .360

The cam i mentioned wasnt even for the xj750. Must have been late when i measured and researched.

valves are standard size 33/28

 

What is the short vin on the motor

If you click on link I supplied above you can put it in that link and find out what your motor is. That would be a lot better can measuring up parts etc and guessing

 

If you read my message second to last, it says the code reads a 81 750 seca. I did use your link