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Discussion Starter #1
Hi - I have an 86 GSXR11 that is farting on part throttle on number 4.
Start bike runs on 3 with 4 half running and farting.

I have tried the following.

Swap leads 1 and 4
Removed the bowl and the jets Main, pilot, air screw and used compressed air to make sure jets are clean and air ways best as possible.
Taken the carb top off and removed the slide and the rubber thing looks fine with no holes.

The header gets warm put not hot like the other 3.

I am out of ideas - new plugs all round, the plug gets a little black if run for to long but a 1min start up - fart, back fire remove plug and only a very light petrol smell.

Checked spark on 4.

Looked at removing carbs but what a nightmare with the airbox in place, I gave up it was so difficult and did the work on the carb best I could in place.

I think its petrol... needs vacuum to lift the slide... bowl is full of fuel.
drain 3 and 4 looks like the same amount.

Whats next?
thanks
 

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Discussion Starter #3
No have not checked compression.... will do.
I thought about it but that could be an nightmare...
 

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Discussion Starter #6
Assume guys .. new plugs installed.

I pulled the air box back as far as I could and looked at the carb slide.
Started the engine and pulled the air box boots back and the carb 1 slide flutters with throttle... but not carb 4.
Oh and compression was 150 psi..

That dirty rat carb 4 the slide did not move up or down.

So correct me if I am wrong but vacuum lifts the slides and no lifty on 4. Slide was not moving. = farty pants.
So lets assume the vacuum is not working, rubber thingy has no holes, Note only looked did not pull it a part, even used a torch. Could not see a hole in the diaphragm.

What next?
 

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Some times the holes are super small along the edge and can be very difficult to see. You have to pull the diaphragm out and stretch it to see them. Its also possible the inlet port for the diaphragm got some crap in it or the slide is dirty. I would clean and inspect and go from there.
 

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I would do a compression test after blocking the slide open a bit and making sure I held the throttle open. It might just be a weak cylinder.

When was the last time the valves were adjusted? Tight valves (typically the exhaust) will bleed off compression.
.
 
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