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.
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.
They are the stock 34mm carbs. If not the main jet than what could it be? I have already tried adjusting the diaphragm needle and adjusting the float levels.
Sounds like the filters are the cause.
Perhaps clean the filters, or if they're the single K and N filters, change them, they're not that good - the dual pods are better.
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.
The problem you are having is related to the needle setting and not necessarily the main jet. They recommend starting with the main jet, so how does the bike run WOT past 8k rpm?
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.
Sounds like you have the right idea. I don't think you need to go as small as 122.5 as it seems it is pretty close already. If it were me I would just get a factory pro kit, and maybe some dual pods.
Okay I'll go for 132.5s and reset the diaphragm clip to the center, if I can find dual pods locally with a nice return policy I will pick them up and try that set up out. Ill post back with results as I go.
Could also be the emulsion tubes ovaling out and or worn internal o-rings.
I think I'm on my 4th set $100 for 4. Factory Pro sells replacements.
You didn't mention if had a jet kit or not.
Might just be the taper of the stock needle is not jiving with the pods and pipe.
The K&N RU-2922's will make it easier to tune vs individual pods.
If you are going to stick with the single pods I recommend getting a Factorypro config 30 kit and new emulsion tubes
Pulled the emulsion tubes to check for ovaling and they actually look really good so I am going to try to avoid replacing them. I've ordered jets from the local dealer and you guys are talking me into dual pods.
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.
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.
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.
I think the DJ stage 3 kit (which is commonly used for pods) uses a dj136 main jet which crosses over to a 127.5 mikuni jet so you very well might be to rich still.
Am I correct in thinking that it is better to run too rich than too lean? If I am happy with the power and I am likely running slightly too rich should I mess with it and drop jet size again or is the bike going to be fine?
If you are happy with it run it. Couldn't hurt to drop down a jet size, either it will run better or it wont.
Here is factorypro's tuning guide. CV Carb Tuning Procedures
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