Horsepower mods ? Cant find a tell all thread for the 750

OK does anyone know where I can find the specs and interchangeability of the crank rods and pistons for the 750 900 and 1100? From what I have seen there are many options for swapping things around.

Something like an 1100 crank and pistons with 650 rods???I have no idea if that is even a possible combo but you get the point.

 

I can hook you up with a 900 Plant.

I've got a 900 Motor you can acquire for $150.00

Base engine.
No rash.
Top and Bottom.
(Other XJ Parts might fit.)
Needs:
Cooler, Oil Filter Dome & Bolt, Starter, Manifolds and U-joint Boot.


PM me for Details.

 

Re: Horsepower mods ? Cant find a tell all thread for the 75

I can chime in adding that the Wiseco pistons and the larger "big bore kits" for these bikes are almost unheard of now. I purchased the 1196 Wiseco piston kit and shortly thereafter they were no longer available. No dump runs or real data numbers but this thing really came to life.

Jamie

 

OK ...I learned how to calculate CC's tonight. YAY!

So what I have found so far is that both the 650/750 have same size pistons and stroke which tells me rods are the difference. I cant seem to find an actual diagram or even just the listing of the parts with actual measurements. I have looked at the service repair manual for the 650 and 750 supplemental and all I can come up with are run out specs not journal diameters.
Please oh master gurus of the world of XJ, send me a schematic that I could use as if I were a machine shop...
I need to get together a comprehensive list of the 650 750 900 and 1100
I just need the crank the rods the pistons with pin positions and journal sizes.

can no one point me in the right direction. It seems so convoluted on google.

I have experience with this but of course it was honda so it was easy. I had a 550 bottom with a 650 head and 750 pistons. This may be more difficult but thats why I like it.

 

The dchc, two-valves-per-cylinder, plain-bearing engine is derived from the XJ650 Maxim powerplant introduced in 1980. The Seca 750 displaces 748cc, with a bore and stroke of 65 by 56.4mm. Besides having a larger bore and longer stroke than the 63 x 52.4mm, 653cc Maxim engine, the Seca engine isn't rubber-mounted. There are other changes as well. The 750's pistons are shorter overall, measuring 56.1mm from top of dome to bottom of skirt, and shorter from the center of the piston pin bore to the deck as well, measuring 25.7mm. The XJ650's pistons are 59.8mm tall overall and have a deck height of 26.2mm. But the 750's pistons are actually taller from pin center to skirt bottom, 27.8mm to the 650's 27.5mm.

Piston dimensions were changed to suit the longer stroke and hold down reciprocating weight in spite of the larger bore. The 750's pistons are just .17 oz. heavier, 5.64 oz. to a Maxim piston's 5.47 oz. The 750's connecting rods are longer, but otherwise dimensionally identical to the 650's rods.

The 750's crankshaft has the same size plain shell bearings as the 650, and the crank width is identical. Crank pins are the same diameter, but are repositioned due to the larger stroke. Like the 650, the 750 has a gear primary drive, the power flowing from crankshaft to clutch hub to mainshaft to layshaft to a middle gear shaft incorporating a spring-loaded ramp-type shock absorber (or cush drive) and from there through bevel gears to the driveshaft. The layshaft is positioned underneath the mainshaft in the lower half of the crankcases, to hold down engine length, with the middle gear shaft behind the mainshaft. The system is relatively simple for a shaft-drive, transverse four-cylinder engine, but still uses one more shaft than the simplest Fours. However, because the engine uses a gear primary directly to the clutch hub, it has the same number of shafts as the chain-drive Seca 550, which uses a link-plate primary chain to drive a jackshaft, which then drives the clutch hub through a gear.

The 750 engine uses a small link-plate chain to drive an auxiliary shaft, which carries the generator behind the cylinders, thus avoiding mounting the generator on the crankshaft and reducing overall engine width. And the 750 is narrow, 18 in. from crankshaft end cover to end cover.

 

so with that article you can put in 750 pistons into a 650 losing compression and 750 rods into a 650 gaining compression then a 750 head to lose compression(say for turbo...)or no head swap for a slight gain in compression.
what are the crank pins he is referring to? maybe even a crank swap for more compression(a great deal more IMO)
I digress though for that is what to possibly do with the 650. I need the same rough info to do something along those lines with a 750 using maybe a 900 or 1100.

I am really getting interested in all of this. enjoy.

 

The Seca's bore and stroke of 67.0 x 60.5mm give an actual displacement of 853cc. (The 750 measures 65.0 x 56.6mm for a displacement of 748cc, and the 650, 63.0 x 52.4mm for 653cc). The longer stroke puts crankcase space at a premium; the 900's rod bolts go in upside down to conserve crankshaft-to-case clearance.

In the cylinder head a single roller chain drives the double overhead camshafts. The combustion chambers have two valves each: 36mm intakes and 30mm exhausts. (The 650/750 inhales through 33mm poppets and exhales through 28mm pieces.) The 900's camshafts spin directly in the aluminum head and ride in four journals (not three as per 650/750 practice).

 

I think the only thing even remotely compatible to make the 750 stronger would be 650 2mm oversize pistons with a piston deck height gain of .5mm but from what I can gather good luck finding that. WOW. I almost hate to say it but seems to me that a good port polish and smooth the landings, with a feebay 818cc overbore kit maybe an 820 will pop up, and just ride it. there really is nothing out there for it. reasonably priced of course.

 

:idea: That's what we've been trying to explain.

Get it running PROPERLY and JUST RIDE IT. You may be surprised.

(The "get it running properly" will be enough of a challenge, trust me.)

 

hahahaha. well maybe if I run into a 650 I will have plenty of mods to do. or at least the 650 guys will have some food for thought.

 

Oh yeah. I got a smoking deal on multiple dyno runs. He has some experience with boring out these xj's and decking the head enough to get a 12.1 compression to still use pump gas. he can go wild and get even higher compression but I told him this was gonna be a daily driver lol. He put me on to two other piston companies that would make some pistons but I still have to check them out. He also is going to help me port and tune the carbs while its on the dyno.

So I will be getting a baseline dyno run, then a carb tuned run
and then a port polish run later and then
big bore time with a decked cylinder head(may as well do valves at the same time).

I will be posting DYNO #'s Yay!!!

 

That will be good. Let me know what you find about pistons.

 

Re: Horsepower mods ? Cant find a tell all thread for the 75

Going to EFI will give you LOTs more tuning options.

You can easily dial in your fuel mixture to as close to "burn it up" as you want.
You can easily advance the timing as close to the "blow it up" stage. Everything is electronically controlled according to your throttle position and load so you can tweak everything and with the proper sensors watch how well it behaves.
It's not dirt cheap, but spending $$ with a dyno and finite carb tuning will cost you some bucks regardless.

 

cheapest EFI I have heard of for these bikes was 900 bucks. I would love to go for that conversion but it seems too expensive. anyone know of a cheaper efi???? get me around 400 doll hairs and I will do it.

Plus the dyno guy is giving me the hookup and by dyno tuning the carb I just mean some quick jet changes and a few turns of the pilot screw.
plus It will be worth the money to be the famous xj dyno result provider to the stars for multiple mods .

mwuhahahhaha

 

Re: Horsepower mods ? Cant find a tell all thread for the 75

The EFI may cost you $900 (give or take $150)

BUT>> you can turn around and sell your TCI, carbs, pickup coils and some other misc stuff you won't need. Now you can easily be down close to $500 and the ability to plug it into your own PC and change every parameter as you wish with a couple clicks of a mouse seems very attractive to me.

 

oo la la. well since you put it that waaaaay.. :0

 

Took my head down to my machinist, he thought the intake runners looked like they couldn't get any better. I asked him about opening up the exhaust runner to the inner diameter of the exhaust gasket. He was gonna clean it up and see what happened. I guess I am going to be the guinea pig on the piston kit from ebay took the jugs off this morning, wrist pins were tight, probley no need whatsoever to bore it out but I am going to.

 

Re: Horsepower mods ? Cant find a tell all thread for the 75

heck yeah, lemme know what the kit looks like . I am in the middle of buying a house soooo I am not even touching my toys right now.

 

I like what you guys are doing with the 750, and will be great to see how it turns out. In my opinion if you want to go really fast buy a 550, there 36kgs less than the 750 and 6speed gearbox. There stock 50bhp with a top speed of 111mph, I have a friend that seems to think he saw 125mph back in the 80’s (highly unlikely). My old man got his up to 115mph redlining in the early 90’s, there just a much faster bike for there size chain drive means easy gearing changes for a tuned motor. Granted the 750 has more outright power, bettor top end, and faster standing quarter than the 550 but the 550 was a more popular bike at the time. Yoshimura made cams, exhaust and bore kits for the 550 plus a few other companies made tuning/race bits for them over the years. Basically what im trying to say is the 550 is a more tuneable bike, you can do more with it.

Keep up the good research
Al

 

Flow bench numbers are in. Headwork is complete.

1982 XJ750J Maxim

Stock intake CFM and Modified CFM​

41-41
81-83
108-112
112-118
112-122
112-124
112-125​

Stock exhaust CFM and Modified exhaust CFM
39-37
80-84
86-104
86-106
85-108
85-108
85-108​

All the numbers are by valve lift bold indicates .350 lift.

The average intake improvement 7%
And average exhaust improvement 17%

The stock exhaust to intake ratio was 86%, The modified exhaust to intake ratio average was 90%, and the average up to .350 lift is 93%

 

Any before and after pics of the port cleanups?

I will be cleaning up the ports on Pumbas head next weekend (the 820cc big bore) and I would be interested to see what you have come up with.