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1988 GSXR Carb Sync Issues

7K views 43 replies 8 participants last post by  Rake 
#1 ·
Bit of advice if possible ?

Set my valve clearances on the 750J today and went about syncing the carbs afterwards.

Before sync and clearances I found the throttle response super crisp and it would come down the revs to idle super snappy but it wouldn’t idle.

If I adjusted the idle knob it would idle but it wouldn’t be anywhere near as crisp on the throttle and the revs would hang a bit on the way back down to idle.

So I though I’d sync and see if they were out which 1 and 4 were.

I proceeded to adjust 1 to 2, 3 to 4 and then the middle adjuster to sync 1 and 2 to 3 and 4 but they won’t sync and what more is that the revs hang coming down to idle again.

I haven’t stripped the carbs as I was hoping to avoid having to do that.

In a nutshell 2 issues

1) carbs 1 & 2 don’t sync to 3 & 4
2) rpm hangs and takes long to come back to idle if adjusted at the idle know under the carbs

Here is a video showing how I left them today.

Any help gratefully received

 
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#2 ·
At one point while I was syncing mine, they were like that. After a while I just stumbled on the sweet spot. Between the sync , and idle rpm. You aren't far off. Sure there are no leaks? Carb's , or vac sync tool?
 
#3 ·
Thanks for the reply.
I haven’t checked for vacuum leaks just yet but I’ll put that on my list for tomorrow.

Someone also suggested that the mixture or pilot screws might be lean so I’ll check those too as it could lead to the hanging revs issue
 
#5 ·
There are also small rubber caps around the choke sliders that also harden and start to leak vacuum.
Hanging revs indicates a lean situation. Could be adjustments that are way off, but usually is a vacuum leak.
Check those intake boots, the slider diaphrams, and the tiny o-rings where the synchronizer ports are.
 
#6 ·
Thanks All, went over today and tried to sync again at about 2K rpm - got them in sync but the overun is too long and revs hang when set to correct idle speed.

Sprayed carb cleaner over the rubber intakes and it did climb in rpm a little but not too much. I pulled the carbs anyway to insepct so will strip and have a check of the jets passages and tubes etc.

Spraying carb cleaner over the choke sliders did not cause the rpm to change sho hopefully not that.
 
#11 ·
Just to add that my son thinks the mains are staMode with 120 on them. He has Berger eyes than mine. Stock is 112.7

Float height was 13.55mm so not a million miles out.

Emulsion tunes look fine as do the pilot jets.

Can’t see anything part wrong with the carbs which are now all stripped for cleaning.
 
#12 ·
Well I finished setting up the carbs and fitted them only to find the same issue present.

So out came the easy start and I aimed at the intake rubber mouths, minor increas in rpm on pot 4, pot 2 and 3 caused more of an increase and pot 1 minor so it looks like the leak is present after all.

The cross head screws holding the boots to the head are solid and I can’t see myself being able to remove them without ruining them if at all.

Any tricks to get these off / fixed ?
 
#17 ·
I assume you are saturating the screws with penetrant. my favorite cocktail for this is 50/50 ATF and acetone. Works better than any off-the-self penetrant. I have used a impact driver wth success in similar applications, have you tired it to make sure there is no room? worst case scenario you can cut the bolt heads off with a dremel, slide the manifold off and them use visegrips/ heat on the remaining portion of the screw. Do you have a welder?
 
#18 ·
Are you using a JIS screwdriver or a ratchet? I usually have success with the ratchet, because I can give it a smack with a hammer, and then push inwards as I'm turning.
And then if that doesn't work...out comes the impact driver.
Again, as suggested above...soak them in penetrant overnight.
Be patient.
 
#19 ·
Thanks for the replies guys.

I am using a ratchet with the best fitting crosshead socket on and as soon as any turning force is applied it feels like the screw will head as the socket wants to tear the screw head.

Just doesn’t feel right.

I’ll get her stripped down again tomorrow and see if the impact driver will go in and allow me a hammer swing in terms of space.

No I haven’t tried any penetrating oil yet is I was under the impression that they were originally fitted with thread lock ?

I don’t have a welder or a dremel but if need be I’ll buy a dremel.

I was given the engine out advice on another group where the guy just took it out rather than attempting to remove them and having to take the engine out after anyway
 
#26 ·
Not trying to start a new direction here, but I had the same exact problem. Pulled the carbs, cleaned them, bench balanced, installed...crap idle, etc.

I had to just pull the jets and soak them all for a week, checked them, verified all clean and clear, bench balanced, installed, got the idle right, vacuum balanced - she runs great.

I had the same worries - cracked boots, vac leaks, etc. Clogged pilot jets on at least 2 of the carbs was the dealio... Not saying yours is a simple situation, but never underestimate the power of the dark side...or a clogged pilot jet.

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#28 ·
I have an 86 GSX-R1100 that I have owned since new. I have read all these posts and have had the same issues over the years. Yes, you need to replace those intake boots, and also check the rubber on the diaphragms. But what you are dealing with is mostly with the slow circuit of the carbs. Another thing to check and replace are the tiny seals on the shafts of the throttle plates. I discovered a vacuum leak there via carb cleaner when sprayed directly at each one. You have to remove the two tiny screws holding the plates to the shafts, and will need JIS phillips to do get them out, or grind them off. A real pain, but is the only way to remove those shafts to then replace the seal at the end. After 30+ years, they leak. I also have been fighting carb issues in the last couple of years due to only occasionally running the bike, and the gas in the float bowls will evaporate and clog the tiny passages. Had to remove and clean them a number of times, only to reoccur a year later. Now I drain them after a run, and run the starter to completely empty the passages while clamping off the vacuum hose to the fuel tap. The biggest improvement has come via recently begun using Sea Foam in the tank. It does not let that buildup occur.
 
#30 ·
Seams everyone likes spraying carb cleaner. Shits caustic. It's not good for anything it touches. Speeds degradation of soft parts, and strips internal coatings, which will eventually cause oxidation , and ruin your rack. Use Sparingly, and clean up quickly.
 
#32 ·
Latest update -

Air / Vacuum leak frustration.



Having identified the intake rubbers as leaking I bought new ones with new O Rings and fitted them today.



Hoping that the hanging revs would go away I then started her only to find the revs still hanging.



Out came the easy start and I found leaks where the carbs enter the boots and on the carb caps.



Stripped them off again and re did the caps bedding the diaphragms in with some grease to make the seal.



Put em back on and still leaking



Cap o rings are present.


So now the focus is on the caps and mouth of the intake boots.

Any advice ?

Link to video -

Thanks in advance
 
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