Newbie with a 1980 GSX750

[DHW]

Gents,

Reading this topic with great interest.

 I have a GSX750 E that I’ve customised so can’t use the standard airbox.

The bike is complete but  I’m not good with carbs so entrusted a local tech to balance the carbs and set up ready for an MOT.
He couldn’t get the original carbs set up properly but had a set of Mikuni flat sides which I think are from a GSXR750 slabside. They are stamped 27A 10 and are 29mm.
He persuaded me these would be the better option. 
He serviced and balanced them allegedly!! and I eventually collected the bike believing it was ready to be MOTd.

Once home I noticed fuel coming from the float bowl overflows. Which would get worse. Also the engine rpm raced on and didn’t return to idle when the throttle was blipped.
I fitted new float bowl jets and a new float to replace the one with a pin hole in it. This fixed the overflow issue.

Also a broken accelerator pump lever was replaced.

Now it won’t idle below 2000rpm and if the throttle is blipped the rpm races away. It does this even with the throttle cables disconnected.

I now don’t believe anything the tech told me and don’t want to go anywhere near him again. 
 

Before I strip the flatsides again am i missing something obvious? am I flogging a dead horse and would I be better off refitting the original carbs and following Russ’s lead?

Advice would be welcome.

Edited October 8, 2020 by Blubber

[Gixer1460]

I'd say you've got an air leak on the inlet side - rubbers between carb and cyl. head is the favourite place, especially if carbs have been off and on a few times!

Learnt your lesson with the 'Tech' - i'm sure he relieved you of plenty of folding for his 'services'!

[Blubber]

Hi and welcome.

 

I just gave your issue it's your own topic to keep the other topic clean.

 

Now...  post an additional introduction in the General Chat section and you are all set up to go 

[DHW]

On 10/8/2020 at 4:53 AM, DHW said:

Gents,

Reading this topic with great interest.

 I have a GSX750 E that I’ve customised so can’t use the standard airbox.

The bike is complete but  I’m not good with carbs so entrusted a local tech to balance the carbs and set up ready for an MOT.
He couldn’t get the original carbs set up properly but had a set of Mikuni flat sides which I think are from a GSXR750 slabside. They are stamped 27A 10 and are 29mm.
He persuaded me these would be the better option. 
He serviced and balanced them allegedly!! and I eventually collected the bike believing it was ready to be MOTd.

Once home I noticed fuel coming from the float bowl overflows. Which would get worse. Also the engine rpm raced on and didn’t return to idle when the throttle was blipped.
I fitted new float bowl jets and a new float to replace the one with a pin hole in it. This fixed the overflow issue.

Also a broken accelerator pump lever was replaced.

Now it won’t idle below 2000rpm and if the throttle is blipped the rpm races away. It does this even with the throttle cables disconnected.

I now don’t believe anything the tech told me and don’t want to go anywhere near him again. 
 

Before I strip the flatsides again am i missing something obvious? am I flogging a dead horse and would I be better off refitting the original carbs and following Russ’s lead?

Advice would be welcome.

I checked over the rubbers again this morning. Carb three looked a bit suspect. Adjusted and tightened but not any better.

I noticed if I held the palm of my hand over the inlets of 1 or 2 the revs would drop and the engine would stall. The engine continues to run if I do the same on 3 & 4. ???

Carb 3 is different from the others as it has a different lever  on the throttle bar. It has no adjusting screw for setting the slide height. Is this correct??

Even more confused!!

 

[Gixer1460]

If you block the inlet of a carb - it gets no air so can't run therefore if it continues to run its got an air leak as I said before!

And yes #3 carb is different - it is the reference carb that you adjust the others to when syncing them up with vacuum gauges.

[Russ750ET]

Definitely sounds like a lean condition, which would suggest an air leak as Gix-er has stated.

A good starting point regardless is to record your current carb settings. GSXR carbs have been used on your model with great success by many people over the years, so I'm sure someone can confirm if your jets and settings are in the ball park.

The standard CV carbs can work well, I'm still working out some minor kinks with mine but I'm 90% there. Please note my engine is pretty highly modified and with a low restriction 4-1 pipe with very stubby silencer. So take this into consideration if you do decide to go the standard carb route with yours. You may not require such rich settings depending on your engine and exhaust set up. 

Edited October 12, 2020 by Russ750ET

[DHW]

Thanks. Confirmed what I was thinking.  These were new rubbers that I fitted. I’ve tried adjusting and tightening numerous times. Can it be anything other than the rubbers?

I remember seeing that these rubbers should be left and right I asked for this when I ordered them but they all came back with 454R marked on them. 
May get back on to the supplier. These things are not cheap!!
 

Thanks Russ,

I’ve downloaded a manual which gives all the jet sizes and settings so next step is to take them off and see what I’ve got.

Pretty sure it’s 2&3 that are causing the problem.
Going to do a compression test for a bit piece of mind.

Really can’t find any issues with the intake rubbers.

Cheers.

[Gixer1460]

The inlet rubbers are most certainly left and right handed to match the angle of the inlets cast into the cyl. head! Suzuki part #'s - 13121-45400 (LH) and 13110-45400 (RH) - clearly different!

Whilst a manual will spec the jets for the carbs - that will be for the CV types, you have GSXR slide types which are likely to require different jetting. You can try the same sizes but a dyno is likely the best way to get them correct. The other thing is the potential size difference of the inlets between the two carbs where they fit into the rubbers - likely to be different and so liable to leak air!

[DHW]

The manual I’ve downloaded is for the slide type and the correct model. 
I’ll source the correct rubbers and take from there.

Thanks again.

[Gixer1460]

47 minutes ago, DHW said:

The manual I’ve downloaded is for the slide type and the correct model. 

Correct model of what, carb or engine? Either way those carbs weren't fitted to a GSX750 engine so any jetting is guesswork!