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1987 Gsxr 750 jet help

4K views 17 replies 6 participants last post by  rklontz 
#1 ·
Ok here's the deal. I picked up a 1987 gsxr 750 with after market slip on exhaust and k&n pod filters. The bike runs amazing with the filters off but bogs down WOT at 6-7k rpm when the filters are on. Due to this I determined the bike is running rich on the main jet. Pulled the carbs apart and floats were out of spec so put em all to 15mm, book says 14.6 +- 1mm. Also tried adjusting diaphragm needle clip to lean the bike out with only very minor improvements in bikes behavior. Upon closer inspection I found the main jets in the bike are 142.5 :shifty ; my book says stock for the bst34ss carbs is 112.5 so it looks like i'm about to begin the long process of jetting the bike properly. :sad

I am wondering if anyone has some advice on where I should start in jet sizes, I was thinking I would order 122.5 and 132.5 and go from there but seeing as I don't want to spend a ton of money and time ordering parts and taking the carbs apart again and again I figured I would ask here and see if anyone has any insight on what size I should get.
 
#2 ·
carbs suck, but understanding how they operate helps with diagnosis.

going from experience, first verify that you are indeed working with stock 87 750 34mm carbs. second, a 142.5 mj sounds about right if it is jetted to run pod filters.

i don't think it is the mj size. also, carbs with pods can be tuned to run properly without a flat spot.

read factorypro.com website for guidance.
 
#5 ·
Unable to find any good info on the factory pro site. Is the recommendation here to get new dual pods or should I try sizing down the main jet? It is 12 sizes higher than stock and it was my understanding that when you switch exhaust and switch to pods you should only go up about 4 sizes.
 
#7 ·
With pods on bike pulls much better over 8k than 6-7, however with the pods off power from 6k up is siginficantly improved all around including over 8k, The bike starts to wheelie in 1st gear before redline with pods off but does not with pods on. The bike used to perform better (never as good as with pods off) but recently avg temp where I live shot up about 40 degrees F and now the power glitch is much more noticeable.
 
#13 ·
going from memory, i think i am running either 135, 140 or 142.5 MJ on my "stage 3" 87 750 carbs with stock engine, and it runs excellent. that is what factory-pro recommends (i think).

are the needles tapered and adjustable?

again, i'm no expert on carb tuning, but 6-7K RPM probably isn't going to be the MJ. if the bike is new to you, then it will be a difficult diagnosis, because you don't know its history. could be a clogged fuel or air jets, needle height, mixture screw settings...check online for other resources for carb tuning.

112.5 MJ is for a stock airbox, which flows much less air than individual air filters.
 
#14 ·
Just put 132.5s in and took the bike for a test ride, it is performing much better, I am giggling like a school girl when I roll on the power. The flat spot from 6-7 is gone and it makes much more top end power. The needles are tapered and adjustable and I reset them to the middle clip because I figured it would be easier to tune either way from there. I am still running individual pod filters. My concern now is I leaned the mixture out too much. What should I be on the lookout for the next few times I ride it to make sure that is not the case. I know I should probably put new plugs in and check them over the next couple days but I am wondering if anyone has other tips. I am still wary that it's not solved yet because I don't believe I am lucky or clever enough to have the first jets I try work perfect.
 
#15 ·
And to clarify based on my understanding the jets used depend more on throttle position once you get over 5k or so rpm. The main jet circuit is activated when the vacuum pulls the diaphragm up. Due to the fact that the issue only occurs when WOT the main jet circuit is the primary suspect. The bike performs very well below 6k despite the pilot screws having been drilled and messed with by the PO.
 
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