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'91 750 Motor Mod Options...

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crispone

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Discussion starter · #1 ·
So, planning to stuff a low-mile '91 motor in my '88~'91 variety-build project. New to the 750s, having been predominantly big-bore bikes for more than a couple decades, I could use some basic do's & don'ts and recommended performance combination options. Current thoughts:

- Either Yoshimura or V&H pipe
- Likely RS flatslides... but willing to listen to advice, including tuned CVs or other...
- No certain motor work YET...
- Rear will be a Fox
- Front currently USD setup, needing rebuilt (what is best? Or should I go to earlier, lighter right-side-up forks and tweak?)

What really lights these up? Will I need to get in the motor to sate my passion for power, or will it be "lively enough" with well-selected basic mods? How about a Yoshi ignition box? (I have a 1st gen Yoshi box...) Should I go a Dyna setup? What ignition advance? Etc...

Since the entire project is a "spontaneous motor purchase" that has begun to pick-up momentum, now I need to consider my build-plan more seriously to avoid picking up more miscellaneous parts I may or may not use, and get some better direction.

What do you guys who have "been there/done that" with the '91 motors and general 2nd gen builds suggest?

Thanks!

-crisp
 
#3 ·
How about the budget?
I'd go with rebuilding USD fork, an aftermarket rebuilt shock, 38 Mikuni or 39 FCR flastslides, ported head, stock CDI, full Yoshimura and probably this is enough for power.
The real challenge with 2nd gen build is about weight reduction...........trackdays or street project?
 
#4 ·
Dead fresh with a good pipe, you're looking at 105-110hp. Cams add a little but since your head is shim style, you can't afford to get too aggressive with the cams or the shims fly out when you miss a shift and then the motor destructs. Flatslides add response, not ultimate hp. If 105-110 isn't lively enough for you, you'll need a Bandit motor.
 
#5 ·
My advice is to first calibrate yourself. I also got spoiled by my big 1216 motors with 88-92 ftlbs of torque. So don't expect the rush. Members who have chimed in are correct in that 105-110 is fairly reasonable with 771 wiseco kit and Webb or Yosh intake cam. Others more knowledgeable than I can chime in on porting but I found that using the same theory on the 750 as an 1100 head is not the way to go. Big ports and big valves in my 750 head managed to drop the hp to 86. LOL Ended up using a 90 head and epoxy in the intakes to increase air intake speed. My engine with the above combinations, a gpwerks pipe and dyna made 112hp at 54ftlbs with Keihin 39 flatslides. The plus side is you really notice that the bike feels much lighter with the smaller 750 engine. For suspension I'd say this, I run USD forks on the big bikes with over 1500 bucks worth of mods provided by Traxxion Dynamics and they handle great. On the 750 AHRMA bike I run 86 1100 forks with gold valves and it also handles well. Simplified both can be made to work well just depends on what you want to spend.
 
#6 ·
I'd start out with a Yosh pipe and properly re-jetted 38mm CV carbs. I'd leave the ignition system stock. If you want to play with the advance, slot the plate so it's infinitely adjustable and only adjust it on a dyno, so you can see where you gain and where you lose. I'd put a normal front end on it, after having someone like LE rework it completely. I'd spend most of my time and energy shedding weight. Lots of things on a '91 that pack on the pounds.

It won't be an 1100, but that's a good thing, if you make it light enough.

JR
 
#7 ·
I'd start out with a Yosh pipe and properly re-jetted 38mm CV carbs. I'd leave the ignition system stock. If you want to play with the advance, slot the plate so it's infinitely adjustable and only adjust it on a dyno, so you can see where you gain and where you lose. I'd put a normal front end on it, after having someone like LE rework it completely. I'd spend most of my time and energy shedding weight. Lots of things on a '91 that pack on the pounds.

It won't be an 1100, but that's a good thing, if you make it light enough.

JR
 
Discussion starter · #8 ·
Wow, great feedback that is really helpful in formulating my action plan, guys! Here's my first impressions so far:

- Light-is-RIGHT!
- Let it BREATHE
- Bigger is not always "better"
- It's a 750... NOT a "Big-Bore"

I can always "add/spend" more, but maybe the "minimalist" approach makes more sense here. I can readily make it "look the part" and probably perform well with what I have laying around and a few trimmings to match. I am now thinking:

- 1100 front end (already sitting idle, off my original Frankengixer bike that is now a 2nd gen front) Maybe "cap-off" the NEAS?
- Either the CV38s (I have a pair, one with Factory Pro kit, supposedly) OR the RS38s I have laying around as spares...
- Shave as much weight as I can... and get it sorted and on the road with what I've got and "tweak" thereafter.

Since this motor is BONE DRY externally, and was apparently out of a 7,500 mile bike, I think I'll just start by cleaning it up and mounting it, get it running, then consider whether I want to do anything "extra". (Maybe freshen clutch, match intake runners to ports, mount whichever carbs I select and get them synced and tuned to the pipe I run...)

I'll have to do a sprocket/chain swap and get the Fox that I'm planning to use rebuilt... but overall, I've got most of what I need to do this thing readily in hand, I think.

So, next question is all about how best to shave weight! No secret there, I suppose... just look at each part one by one and figure out if it's necessary or not and if it is, how can I replace or modify it to bring the mass down.

Headed over to the cottage to stare at it shortly, maybe tinker later this evening after I sort my next move... really appreciate the input, gentlemen. :)


-crisp
 
#9 ·
I'd use a Showa front end off of a slingshot 750, not the first gen forks from your 1100.

Leave the motor internally stock, use a pipe and CV carbs to start with, then ride it. After you have 1,000 miles on it, decide if you want something more. The more power you make, the less rideable it will be out of the powerband, so be careful what you wish for.

I'd toss the stock bodywork, the insanely heavy headlight assembly and front fairing bracket and any provisions for a passenger. Buy some Airtech plastic, make a new front bracket from aluminum and mount a couple lightweight round headlights in it, use a single seat tail with a foam pad. I bet you could shed the better part of 100 pounds from what a bone-stock bike weighed.

JR
 
Discussion starter · #10 ·
Okay, Jeff. Will definitely consider all of that as well as what I have "in hand" as a stepping-stone to get there incrementally. Since I picked up the stock '90 body set in such clean condition in my favorite Blue/White for such a good deal, I'll start there and then re-evaluate. True on peakiness -vs- street driveability. I'll keep that in mind as this is NOT meant to do any serious riding beyond the casual squirt for my own weekend countryside riding... and of course, another local "conversation piece" wherever I make a stop! ;)

-crisp
 
#11 ·
Outsider, JR and RK3 pretty well covered it, my experience going from an 11 to a 750 was pretty entertaining for me personally. Became a fan after realizing that it wasn't really underpowered (duh...) as much as that it really just wanted to live at the upper end of the tach and demanded a lot more use of the gear box than on the 11 of course. The lighter weight, shorter wheelbase, higher corner speeds etc - all very amusing because it WASN'T and 1100. I kinda' fell in love. It was like dating a librarian who liked rough sex... Very proper out and about but liked to be ridden hard but I felt a little like I was abusing it until I got comfortable hearing it scream.
 
#12 ·
Lots of great ideas and feed back already Crisp - if you can, fit lighter wheels, an ally sprocket too (perhaps 520/525 kit), this will help with the bike's agility no end. I've Michelin Pilot Power tyres on my 1100, I like the feel of them, plus I saved 2 1/4 lbs !!
It's odd that the engine, although not much lighter than the 1100 unit does make it feel much lighter than the figures suggest - probably due to less weight transfer caused by the big motors immediate torque and resistance on the over-run. You sort of have to ride around this on the big engine, the smaller engine isn't as intrusive.
Nick
 
#14 ·
RE: Your Fox shock. Mine is at my suspension builder currently. I used up all the compression and rebound damping it had but needed more. He classified the piston and valving in the Fox as "quite antique." He's contacted Race Tech and thinks he's located a piston in their inventory which will allow much better performance. I race, so this is very important to me. I'll post up in a few months when the bike is back together and I've tested it.
 
#15 ·
750s are very nervous engines, you know..if you're used to 1100s you'll find a bit out of comfort with those hi-revving things. I always feel like off a spin cycle after an hard ride..damn..nassive use of gears and the whole noise using a racing exhaust make the thing scary..well, tearing off the bike you'll find a lot of unuseful brackets to tear off in order to reduce weight (just look at the front fender bracket...).Going under 200 kgs is not that easy without expensive light weight parts..long story short, it's not a 1st gen...that's it
 
Discussion starter · #17 ·
javadog said:
1100's are ponderous things that have a hard time getting out of the way of faster bikes.

Dangerous things, really.

JR
Click to expand...

Yup, yet when out on the open road one-up and lone-wolf on rural Midwest routes, all of those caveats sort of fade into obscurity with the big-dogs...

All you guys are making great points and share good observations, some very familiar and a few less so, not new, but "refreshers" for an old dude like myself who's riding style and machines have "morphed" over the years. Growing up in Japan, it was all about low-weight/small-displacement/high-revving machines with narrow power spreads and nimble, sometimes "twitchy", yet forgiving handling. The points on motor-mass behaving so differently between big-elevens and the three-quarter-horse are likely more a byproduct of the rotating mass within the motor than outright weight differential... combined with the less abrupt and violent delivery of torque, something I actually LOVE in my big machines. (Interestingly, my built '03 1143cc stroker seems to be BOTH a torque-monster AND top-end-rev-happy-screaming-E-ticket-ride... from hell, I might add!) Having let my very built-up RG 550 trophy slip away not too long ago, I now lack ANY really "peaky/nimble" ride for that "need-for-rush" without the "fear-of-God" that comes with getting too aggressive with the throttle on the stroker... which at upwards 180-190rwhp on tap, stretches the brain-stem and sends concussive strokes of jackhammer-pulses through my cardio while precipitating a Joker's grin that literally HURTS my facial muscles and wrinkles my eardrums from the tawt tension that is simultaneously being flogged with audio resonances that were never meant to ripple through nature's airspace in either God nor Darwin's grand scheme...

...at any rate, the 2nd gen Slingy may NOT be the ideal platform for a truly "NIMBLE" means to get there, but inasmuch as it's my FIRST MKII Gixer and I LIKE the understated and "pre-Mickey-Mousey-Clown-graphics" and "elephantine-mass" of later models who clearly "lost their way"... it's a great pleasure to both ponder these things and bask in the outpouring of both interest and clear affections those of you who have danced with the subject Beast have parlayed into a BOM of materials and recipe for this culinary feast of fast of which I aspire to dream...

Intrigued to reflect on all of this while the snow gently melts outside and my mind races with visions of velocity spinning in my pinwheel-irises like kaleidoscope eyes...

Saturday morning cup-O-joe two, lay down your elixir of life on my path to "number-next"... :D


-crisp
 
Discussion starter · #18 ·
Sacs said:
750s are very nervous engines, you know..if you're used to 1100s you'll find a bit out of comfort with those hi-revving things. I always feel like off a spin cycle after an hard ride..damn..nassive use of gears and the whole noise using a racing exhaust make the thing scary..well, tearing off the bike you'll find a lot of unuseful brackets to tear off in order to reduce weight (just look at the front fender bracket...).Going under 200 kgs is not that easy without expensive light weight parts..long story short, it's not a 1st gen...that's it
Click to expand...

Quite true, yet in light of my prior post and past propensity to tap-out the RG and keep it's aerospace-grade rubber-band-propulsion system burbling at a kettle-screaming-steam-streaming-full-boil and monster-modern-maximus-machina of overkill pinned through four gears of toe-tapping wheel-loft when the planets aligned and my biorhythms were in sync... in some respects, this new project will be somewhat of a "wind-up-toy"... just looking to make it "fun" without breaking the bank (oops, too late!?) and spending too much time with "false-starts" while I find my way to my destination...

Love the participation on this one, guys! Really appreciate the engagement from those who know the subtleties of which we speak and add value and depth to my experience as I test these waters anew. ;)


-crisp
 
#19 ·
The problem with the second gen bikes is Suzuki made everything weigh more. Even things like the mirrors could weigh a ton. You could kill a person with a mirror off of an RK, if you hit them in the head with it. A front fender from a '91 weighs as much as ten front fenders off of a 500 GP bike. Why? They do the same thing...

Bodywork can be four pieces, not the dozen that Suzuki used. The rear subframe can be simplified in a few hours. You don't need it strong enough to cantilever a 200 pound chick off the back. The brackets and misc. crap can be simplified and made from aluminum, not steel. Wheels can be lighter. Just look at every piece and toss what you can, simplify and lighten everything else. I took 150 pounds off of a 500 pound first gen 1100. That's probably going too far. I see no problem at all in getting to four hundred pounds, give or take, for a second gen 750, if you work at it a little.

JR
 
#20 ·
javadog said:
The problem with the second gen bikes is Suzuki made everything weigh more. Even things like the mirrors could weigh a ton. You could kill a person with a mirror off of an RK, if you hit them in the head with it. A front fender from a '91 weighs as much as ten front fenders off of a 500 GP bike. Why? They do the same thing...

Bodywork can be four pieces, not the dozen that Suzuki used. The rear subframe can be simplified in a few hours. You don't need it strong enough to cantilever a 200 pound chick off the back. The brackets and misc. crap can be simplified and made from aluminum, not steel. Wheels can be lighter. Just look at every piece and toss what you can, simplify and lighten everything else. I took 150 pounds off of a 500 pound first gen 1100. That's probably going too far. I see no problem at all in getting to four hundred pounds, give or take, for a second gen 750, if you work at it a little.

JR
Click to expand...
That's the nice thing about taking it down to the frame, picking and choosing what goes back on and how...

JR - I've looked at the sub frame and have considered what could be done, looking at the crossmembers, lets call them A-D, what could be done?

A - tank mount, probably best left alone
B - Tail mount, seat mount, again probably best as is
C - undertail fender mount, could go away or be drilled extensively?
D - Grab rail mount, drilled? Not sure I'd eliminate it as it's last in line for torsional forces..

As for the frame rails themselves, I've seen pic's of the lower ones drilled end to end like commonly done on brake torque arms.

And while we're at it -personally I like the look of the '91/'92 headlight but it is an anchor as weight goes...
 

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crispone
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