Suzuki GSX-R Motorcycle Forums Gixxer.com banner

1 - 20 of 24 Posts

·
Registered
Joined
·
39 Posts
Discussion Starter #1
Hey all!

I've been lurking for a while, but this is my first time posting (besides the intro post). My '86 750 is a spectacular bike, but is stuttering like mad at 3/4 - WOT. Here's what I know:

-Compression is solid on all cylinders
-142.5 jets (which I upgraded to 150)
-Yoshimura Exhaust
-K&N Pod Filters
-brought the carbs to a local shop to do a complete clean. God only knows if they actually got everything, but they're a reliable good shop.

Upping the jet size did seem to help a bit, but didn't eliminate the problem.

There's a set of BST36SS carbs on fleabay right now for $280ish shipped. Is that a good price? Should I stop throwing things at these carbs and just get the BST36SS and the Dynojet Stage 3, or can I get something solid out of these.

I have multiple other projects I'm knee deep in right now (Volvo 140 with Mikuni HSRs that I can't get to run lean enough, and a coolant leak just sprang up), so if you think I can solve this by going up another jet size or two, and then worry about the carb swap next winter, that's PROBABLY a better idea. Also if that carb isn't a smoking deal and better to wait, I'd love to know that.

Thanks all! Super excited to be part of the forum. I'd post a link to my CX500 build thread, but haven't reached my 10 yet!
 

·
Squid
Joined
·
3,564 Posts
If you can find a spare set of carbs for your bike in decent condition I'd jump on it. If I had more cash I'd be buying up heads, cases, everything before it becomes unobtanium.


Does the stuttering stop above 3/4 throttle?
 

·
Registered
Joined
·
39 Posts
Discussion Starter #3
If you can find a spare set of carbs for your bike in decent condition I'd jump on it. If I had more cash I'd be buying up heads, cases, everything before it becomes unobtanium.


Does the stuttering stop above 3/4 throttle?
I'm honestly not 100% sure. I'll give it a test out, but I think it just gets worse the more the throttle opens.
 

·
Registered
Joined
·
39 Posts
Discussion Starter #4
Sadly I can't link the fleabay ones, but if you look up bst36ss on there you'll see what I'm talking about.
 

·
Registered
Joined
·
39 Posts
Discussion Starter #6
Pretty common for the transmissions to skip on these bikes once they are clapped out.
Definitely not that - RPMs don't go any higher, engine just stops getting power. I'd say the only other option would be if somehow the ignition wasn't working at a certain RPM (CX500 Stator has a known issue where it won't go about 5k RPM if one of the coils is bad). Otherwise it's definitely carburetor.
 

·
Squid
Joined
·
3,564 Posts
Sounds like carbs yeah. I know that modern bikes and newer carbed bikes do something special to avoid what you're describing. The secondary throttle butterflies on my bike when removed will cause the same problem.


Srads use a solenoid to accomplish it, that's all I know though.
 

·
Registered
Joined
·
77 Posts
Is it carbs or fuel petcock that is causing you problems. If fuelling, could be either, perhaps test the flow from the tank to the carbies.
 

·
Registered
Joined
·
39 Posts
Discussion Starter #12
Is it carbs or fuel petcock that is causing you problems. If fuelling, could be either, perhaps test the flow from the tank to the carbies.
Ahhh good call. Any advice on how to test that? I have noticed that when the fuel is low at all the bike runs like utter junk. Could be the vacuum hose?

The BST36SS carbs I was looking at are ending today - is $250 a fair price for them or can I find them for less?
 

·
Registered
Joined
·
856 Posts
yeah, rule out everything else before you get to jetting. if it revs up all the way to redline and all cylinders are getting spark, chances are its not ignition related. are you absolutely sure the carbs and everything on them is factory for an '86 750? a bad fuel petcock not supplying enough fuel will only affect the bike after a few minutes of run time. for instance, my '86 im currently working on wouldnt want to rev up quick, revs were pretty slow and unresponsive. I found that the diaphragms werent rising up to throttle input as they should. I found the diaphragm slides were from a katana 600 and the cutaways on those are more closed up than the GSXR ones, not allowing enough air to pass over the holes to lift the slides
 

·
Registered
Joined
·
39 Posts
Discussion Starter #14
yeah, rule out everything else before you get to jetting. if it revs up all the way to redline and all cylinders are getting spark, chances are its not ignition related. are you absolutely sure the carbs and everything on them is factory for an '86 750? a bad fuel petcock not supplying enough fuel will only affect the bike after a few minutes of run time. for instance, my '86 im currently working on wouldnt want to rev up quick, revs were pretty slow and unresponsive. I found that the diaphragms werent rising up to throttle input as they should. I found the diaphragm slides were from a katana 600 and the cutaways on those are more closed up than the GSXR ones, not allowing enough air to pass over the holes to lift the slides
Oh god. Yeah I have no clue what is stock in there - I already know that they had ben rejetted before I picked up the bike, and obviously the filters and exhaust are different.

I'm pretty thoroughly leaning towards getting the Slingshot carbs and just doing it right.

I'll warm the bike up today, then shut it off (to let the float bowls fill), then start it and try to WOT. That should answer if it's petcock
 

·
Registered
Joined
·
856 Posts
keep in mind that if you do go with slingshot carbs, they will be much harder to shove into the manifold spigots. also you will need to thoroughly go through that set of carbs as well and replace all gaskets/o-rings to omit any possible vacuum leaks. Just stick with what you got and confirm that they are indeed 1st-gen 750 carbs, you can do this by looking at the main air jets and the pilot air jets. To see if its got an aftermarket jet kit in it, look at the needles and emulsion tubes. Refer to the sticky https://www.gixxer.com/forums/15-oil-cooled-gixxers/159718-understanding-carbs-rebuilding-forks-1100-shocks.html to see what the OEM jetting should be and confirm the set of carbs you have.
 

·
Registered
Joined
·
39 Posts
Discussion Starter #17
keep in mind that if you do go with slingshot carbs, they will be much harder to shove into the manifold spigots. also you will need to thoroughly go through that set of carbs as well and replace all gaskets/o-rings to omit any possible vacuum leaks. Just stick with what you got and confirm that they are indeed 1st-gen 750 carbs, you can do this by looking at the main air jets and the pilot air jets. To see if its got an aftermarket jet kit in it, look at the needles and emulsion tubes. Refer to the sticky https://www.gixxer.com/forums/15-oil-cooled-gixxers/159718-understanding-carbs-rebuilding-forks-1100-shocks.html to see what the OEM jetting should be and confirm the set of carbs you have.
Awesome, will do! Yeah I've been looking through that sticky like crazy
 

·
Premium Member
Joined
·
4,423 Posts
i agree; keep what you have, especially if you had them "cleaned" and seem to be in good shape. could also be fuel delivery, which could also be vacuum.

post photos of your bike and carbs. the 86 and 87 750 carbs are different, in that the 86 750's are 31mm and the 87 750's are 34mm, and require different jetting. i don't know the main jet sizes on the 86 750's, but i do know that my 87 750 34mm carbs, with k&n pods and yoshimura full race exhaust, is running well with 142.5 main jets.

also, make sure we are talking about mikuni or dynojet main jets. photos help here.
 
1 - 20 of 24 Posts
About this Discussion
23 Replies
6 Participants
cratcliff
Suzuki GSX-R Motorcycle Forums Gixxer.com
A forum community dedicated to the Suzuki GSX-R motorcycle. Discuss the GSX-R600, GSX-R750, GSX-R1000, and GSX-R1100, and more!
Full Forum Listing
Top