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Discussion Starter #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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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?
 

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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?
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
 

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Hanging idle is a common symptom of vacuum leak. If the intake manifolds are original it is very common for them to get brittle and crack.
 

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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.
 

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Discussion Starter #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.
 

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Discussion Starter #7
Hmm found a small piece of wood in place of spring 13268-17C10 at the cetral throttle adjustment mech - I guess that explains the difficulty in syncing carbs 1 and 2 to 3 and 4.....
 

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If yo
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.
I think you need to carefully inspect the boots. the fact the RPM did climb when you sprayed carb cleaner there is a strong indication of a vacuum leak.
 

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Discussion Starter #10
If yo

I think you need to carefully inspect the boots. the fact the RPM did climb when you sprayed carb cleaner there is a strong indication of a vacuum leak.
I agree, I have inspected them closely and no sign of any issue at all. All are flexible with no cracks or damage. One thing to note is that when I removed the carbs from the boots there was little to no clamping force in the boot, they came off too easily so the leak might be from the carb side of the boot.

The main jets have no markings so can’t tell the size, the jet needles also have no markings and 6 notches indicating after market kit but no more info.

I found a silicone repair on one of the carb tops which when removed revealed a large hole so I fixed it with epoxy and filler - if that was leaking it could explain it too.

All O rings seems good so I’m going to refit the carbs and check again
 

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Discussion Starter #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.
 

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Discussion Starter #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 ?
 

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Discussion Starter #14
An impact screwdriver should knock those screws loose.
Thank you, I dont see much space in there to swing a hammer but you sound like you have done it before so there must be a way...
I hope its just the O Rings that have failed rather than the rubbers themselves
 

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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?
 

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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.
 

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Discussion Starter #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
 

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Discussion Starter #20
Time waits for no man - I went over to the garage with a head torch on, carbs off and screws now soaking in plus gas. I have acetone and ATF ready too
 
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