StrayCats GSXR 750 Problem - Only VRF Forum Guys seem to be helpful

yea I fished mine with a guitar string, its clear and runs all the way thru to the pilot jet area.

does remind me to double check the idle port and transfer ports tho !

thanks sixdog.

 

Little late... just confirming.

Screw on engine side of carb: Fuel
Screw on airbox side of carb: Air

Never seen one differ.

 

Cheers, yea, thats what I figured. Im going to focus on the pilot circuit (fuel and air) tomorrow and see how I get on. I will say that id expect it to still "work" on the throttle though, but it doesn't .

one step at a time I guess....

 

You mentioned you ran a guitar string through the pilot jet area, but it does have me thinking (since you've essentially hit on trying everything)... When I was in the midst of my own Suzuki carburetor battle, I kept getting hung up on the progression holes. From your diagram, it appears it works the same way--and the same way a lot of carburetors work.

Can you feel air coming through them if you spray into the pilot from the other end? Better yet, can you spray some type of carb cleaner (*edit--maybe not carb cleaner if the o-rings are on the A/F screw. Drop of water and compressed air?) through and watch to see that fluid is coming out of both the hole with the A/F mixture screw and the progression holes?

It doesn't explain why you would randomly be dropping cylinders since it just wouldn't run at all if that circuit was clogged. But I think Squirrel mentioned a piece of o-ring or something breaking off and getting stuck in that circuit blocking fuel. If that were to happen, and depending on where that theoretical blockage was, it could cause either the progression holes or the progression holes + A/F idle hole to not receive fuel.

I really doubt this is the issue, but I was just thinking about it while catching up on your progress. I screen-capped a picture from a few years ago when I was troubleshooting. I added the red line just now to point out a potential blockage spot you could rule out by visually inspecting whether fluid sprayed through the pilot jet actually made it out through any of the holes on the pilot circuit.

Diagram isn't a motorcycle carburetor. I think it's still relevant based on the diagram you posted though regarding the latter half of the pilot circuit.

 

Thanks Matt, ill definitely be taking it apart again (Orings and all) an will again blow it all out. As I recall, I had air passing fine when I blew it out after the ultrasonic bath. but something has to be causing it.

The odd thing is, at one point #3 was the only one working, but most of the time #3 has been the most problematic,

UPDATE: I went thru carb #3 again , it was & is spotless. the guitar string /wire passes through the pilot circuit with no problems (air jet, to pilot jet , to mix screw etc), carb cleaner can be seen emerging from al the right places etc etc. blew it out with air again.

ill put them back on this afternoon. Im NOT hopeful.

Im going back to what NB and other have said next, valve clearances. Although the slides are moving fine, is it possible theres not sufficient suction on that #3 to pull fuel ? ie if the valves are too tight....id expect the #3 slide to be lazy in that case and its not, but I have too check.

 

Just "shooting from the hip" but could the problem be blockage of the fuel line feeding the #3 carb?
If so the bike would idle fine until the #3 carb fuel level in the float bowl fell below the main jet, thus starving it.
The blockage may not be complete, just enough to not keep up with the consumpion of fuel.

In the case of the #3 being the only one running, were the other three flooded?

 

No, thats all good. there are 2 fuel lines. one line feeds carbs 1 and 2 the other line feeds carbs 3 and 4.

Had all the carbs apart and off the rack, every last little piece and part off, and all the " T" fuel fittings are clean and new Orings. I also did a flow test with the bowl off and its getting loads of fuel. the bowls are full, its just not pulling fuel to the cylinder on #3.

Carbs are back on ...Again

but I'm yet to try it, may be tomorrow, ive been short on time due to other commitments

I dont think it will work, it maybe that valves are my problem, but it doesnt explain why the slide on #3 still operates the same as the others, we will see.

When # 3 was the only one running the others may gave been flooded and #3 was still on choke.

I think 100% of the time #3 has never run OFF Choke.

 

Thanks for the update. Keep at it. Will have a big smile when you finally knock it out.

 

would be nice. im getting to the end of my rope with this one.

 

May be time to take a short detour and work on the CBX750F?
\s

 

Any compression numbers?
Sent from my SM-N950U using Tapatalk

 

how could we be so far along without this question having been answered already ?

 

I thought he was waiting for the correct adapter. Since the fittings don't fit correctly, how do you get an accurate reading? Last I saw was numbers were "great" with a push in adapter

We've eliminated spark, so it must be fuel or compression related.

Carbs have been through multiple times, float level verified.. runs on choke so must be lean..

Are you sure the float is not distorted? The whole assembly seems to be made of plastic.. could very well be uneven across the float assembly.

Have you swapped floats between 3+4 to see if symptoms follow?

Sent from my SM-N950U using Tapatalk

 

Sent from my SM-N950U using Tapatalk

 

Compression on #3 is 180 psi

Not tried the float from 3 in 4 yet.

Just been doing things one step at a time, but now the fuel levels are even across all 4 cylinders . I drained the bowls and measured all 4

 

I have found the bowl volume test to be inconsistent

When you drain the bowl, you are also measuring the fuel in the line that is above the float valve waiting to enter the bowl.. throwing off your measurement. Might be different with the gsxr carbs and their inlet..

My opinion, and it can be wrong, but the clear tube method shows the fuel level in the carb with the float in operation. Granted you can't do the test with the bowls and their current drain fittings.

Dry plug and backfire through carb when off choke sounds like float level, blocked pilot circuit/lean , or valve lash/compression issue.

Would love to see valve clearance numbers and results of swapped floats

Hope you figure it out.

Sent from my SM-N950U using Tapatalk

 

i agree, but i think a suitably-sized clear tube to fit into the drain hole could be found and would be a useful test.

 

I believe it's time to move beyond the carbs. All of the hair splitting isn't justified, according to the information provided. I may eventually be proven wrong, but if I am, I would be cool with a positive outcome. I just have a gut feeling that the PO jumped through a lot of these hoops before and just wasn't prepared to spend any additional time or money on it, so they just sold the bike. It's just plain odd that it has 180 psi compression, but the plug shows no sign of fuel making it into the cylinder, the carb has been gone through multiple times and seems to be plenty close enough for it to at least run, but it still won't fire correctly. That is a lot of info to unpack. If it was suffering serious reversion, it would be pressurizing the float bowl on the compression stroke. Nothing adds up.

 

unfortunately these BST Mikuni's have no nipple on the bottom of the carb bowl and so without drilling and installing something (which I dont have on hand, it cannot be done

I bench tested the fuel levels in the bowls, and drained the fuel lines before the bowl contents were measured. any residual in the "T" connectors was minimal. I tested more than once and emptied #3 first, next time I emptied #4 first and there was no meaningful difference in fuel volume.

Pilot circuit is as clean as clean can be

We'll see what happens later today

 

One step at a time I guess, if it doesn't run properly today, ill be moving on to other things. there is a finite number of things this could be, I just have to keep plugging away at it I guess.

So, what am I testing today, what have I dont since the last failed test:

1) Reset fuel float levels, all 4 bowls have very very close volumes now - it should have fired the last time, there was plenty of fuel in #3, but for the sake of certainty I got them much closer. Ran a 2 day bench test to be sure nothing was leaking or over flowing. I tested fuel level of the bowls several times emptying the fuel lines before I emptied the bowls and emptying different bowls first....all good

2) I cleaned the carb #3 again, paying very close attention to the pilot circuit. I ran a guitar string & carb cleaner thru all the pilot passageways and there was No debris, and the wire passes easily through everything, I also blew it all out with compressed air. I re checked the pilot Jet and Main jet as well, all clean clean clean.

3) I re checked the pilot fuel screws, and seated them even more lightly before putting them at 2 turns out

4) did a fuel flow test on #3 carb with the bowl off, got a healthy steady flow of fuel out the bottom

Nothing above yielded anything that would make me say "a ha, I think that may have been my problem", so I dont expect any different results this time, and im prepared to move on to other areas to sleuth out the issue, ie valve lash & cam timing.

stay tuned for the update later today.