GPX750R - carb question?

[qzak]

Starts fine - idles fine

The problem
Open the throttle quickly and the rev's drop off and the engine falters- pops in exhaust.
Open the throttle slowly and the engine runs up to max rev's - no misses.
I removed the carbs and cleaned them, no obvious signs of dirt. Butterfly on carb 2 showed some signs of blackening, but this carb has also got a wrecker's mark on it so it may not have been in the same set as 1,3 & 4 ???
With the air box removed (for observation only)
1.Slow throttle opening gives smooth rise of all carb slides
2.Fast throttle opening all slides bounce up & down (engine misses) fuel splashy and visible in carbie throat.
I seem to have a problem with manifold vacuum? Any ideas???

[qzak]

Update

Well the bike went to the repair shop and the supposed cause is leaking rubbers on th air box - still can not see how this could cause the problem, but hey Iam not the expert here (question.: are CV carbs sensitive to airbox leaks?) As the bike is still at the repair shop (3 weeks later!!!) waiting for spare parts maybe air leaks are only part of the problem?Have noticed lately a lot of "mechanics" just keep replacing bits till it runs right - does that mean they take back off the bits that didn't fix the problem???

[78_KZ750_Cafe]

it means that training technicians in the art of carbeuration is a dying tradition. here in detroit michigan, a city of 3 million, the motor city, I had to hunt for a week to find a bike shop that would service my carbs and wasn't shady.

[elr658]

I just look in the mirror. Hey 750Cafe, try finding a CAR mechanic who knows anything about carbs. There is a niche open for anybody who can work on obsolete stuff.

[wiredgeorge]

Hehehe, I suspect you are right! My business is going great!

The intake rubbers that connect the carbs to the airbox won't affect the slides. This is ENGINE VACUUM coming from the combustion chamber opening those slides on your CV carbs. The diaprhagms are not seated properly under the lids or are ripped or torn. The airbox needing to be fitted might mean the bike won't start as most CV carbs are a bit fussy to being started when the bike is running lean. But if the bike is started, the intake rubbers don't affect the slides a whole lot. Air is drawn in via vacuum and that air is pulled into that longish slot on the intake side on top the intake venturi where the rubber connectors are located and this raises the slides.

Try mentioning your bike model/year if you need more specific help.

[qzak]

Thanks for that- the bike is a GPX750R 1986
I did finally fix the problem. serviced the carbs and softened up the rubbers - and behold it worked like new. I must admit it took me a while to work out how those worthy oriental gentlemen manage to "shoehorn" the rack of 4 carbs back in place without disturbing anything and then getting everything to seal again - but I did.
Best Regards

[nephilim]

How did you manage to get the airbox to seal to the carbs again? Damned if I can work it out...

[gpzrocker]

I've tried it all, finally got them on and ordered new boots: not a very cheap deal. But I can remove/install my carbs in a few minutes now, and consider it some of the best money that I have spent.

[gpzrider]

Too many mechanics these days are poorly trained in what an engine actually does and why. Computers have become such a large part of the industry that all they learn is how to hook up the diagnostic equipment and replace whatever it tells them is faulty. If that doesn't work, they just throw new parts at anything in the area until the problem goes away.

Just noticed how old this post is.

[qzak]

Hi
It may not be the way the experts do it but it works.
You need two strips of thin stainless steel [mine have a bent edge] about wood saw thick will do. [NOTE- all clamps seal springs must be fitted on rubbers but not in final position] Put one over cylinder rubbers - the other over airbox rubbers. slide carbies between the two strips and the rubbers are compressed back as the carbs slide into position. Slide strip 1 [Cylinder side] out slow letting carb mouths pop into place one by one. Slide strip 2 [Air box side] out slow alowing rubbers to pop on to carb one at a time.[check internal parts of the rubber fit into the carb properly] Position and tighten clamps - springs [airbox side] can be rolled into position [long fingers + screwdriver]
After you had to do it a few times you will be amazed how easy it is.
Design-wise the airbox looks like a cheap option to hard problem.
Regards

[nephilim]

That's brilliant!