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GS750 rebuild needs help


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unless the airleak was massive it wouldn't stop it firing on no 3,  just weak mixture, the bigger the leak the weaker it becomes (relatively) make sure the float isn't stuck open on no 3 if it is stuck open it will cause that cylinder to flood.

change all o rings, there should be 4, 1 on each of  the inlet rubbers replace and make sure they are well sealed to the cylinder head.

have you checked for a spark on no 3?

a good test for airleaks once you have put it back together and running is to spray wd40 or similar all over the inlet rubbers while running, if the revs raise/alter, then there,s a leak, no need to lag it, just a bit will do. if you are careful its possible to pinpoint the leak.

sounds like no 3 is flooding though.

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1 hour ago, mark35gun said:

unless the airleak was massive it wouldn't stop it firing on no 3,  just weak mixture, the bigger the leak the weaker it becomes (relatively) make sure the float isn't stuck open on no 3 if it is stuck open it will cause that cylinder to flood.

change all o rings, there should be 4, 1 on each of  the inlet rubbers replace and make sure they are well sealed to the cylinder head.

have you checked for a spark on no 3?

a good test for airleaks once you have put it back together and running is to spray wd40 or similar all over the inlet rubbers while running, if the revs raise/alter, then there,s a leak, no need to lag it, just a bit will do. if you are careful its possible to pinpoint the leak.

sounds like no 3 is flooding though.

Thank you.

I think your right, it can only be a small air leak, and so not this problem, I have ordered new o rings.

I checked the floats and could find nothing unusual.

I have checked for spark and have good strong regular spark.

I did the wd40 test, before with no result.

Again I agree I also think 3 is flooding. 

Although it does not leak when off or from the bottom of the carb.

 

Strange how it changed cylinders when I swapped the vacuum hose to a different carb  last time.

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7 hours ago, bob654 said:

Again I agree I also think 3 is flooding. 

Although it does not leak when off or from the bottom of the carb.

 

Strange how it changed cylinders when I swapped the vacuum hose to a different carb  last time.

If you take off the vacuum pipe from the intake rubber and suck on it, maybe some fuel comes out of it.  That would indicate a f*cked fuel tap. 

Edited by Captain Chaos
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10 hours ago, Captain Chaos said:

If you take off the vacuum pipe from the intake rubber and suck on it, maybe some fuel comes out of it.  That would indicate a f*cked fuel tap. 

 

I checked this and it worked as it should. Thanks for posting.

Edited by bob654
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What about this?

What if it is firing... Only very poorly? How much air leak from intake rubbers would it take to f up the mix enough to only ignite some fuel sometimes? IMG_20200510_170401794.thumb.jpg.2217a44ab06daf3d9f23229d45cd047e.jpg

The near one is from 2 the other poorer one is from 3 and was coated with old silicone.

 

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12 hours ago, Captain Chaos said:

If you take off the vacuum pipe from the intake rubber and suck on it, maybe some fuel comes out of it.  That would indicate a f*cked fuel tap. 

A better test would be to cap the vacuum port on carb #3 and run the bike with the petcock on Prime.

If the bike runs better, it's the fuel tap. Fuel taps generally only last about 30 years

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On 5/10/2020 at 6:16 PM, Blubber said:

I think nr 3 still has its oring only that much compressed it almost bonded in.

That seal is important . Hence the halfcocked attempt to seal . Replace the oring... fit a new one and forget.(y) 

 

All 4 would be my preverred action

NR3 ??

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I got some new o rings today so will fit tomorrow.

I dont think these have removable o rings. I have two like this and two metal and rubber with groove ( the real deal). Uh.

I thought about putting a ring on top and screwing it against the head.

Or else its new rubbers at 50 quid each.

IMG_20200510_170401794.jpg

IMG_20200511_194014557.jpg

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22 hours ago, Gixer1460 said:

The numbers on the rubbers should largely be the same with the last few increasing from left to right (usually) like CO10, CO20, CO30 and CO40.  Look on the Alphasports website and check what the part numbers should be - https://alpha-sports.com/suzuki_parts.htm

I did a search on the Alphasports site and i've got a problem! Early GS750's had slide carbs and a monoblock attachment to the head not individual rubbers! Later 750's had individual rubbers and CV carbs but the rubbers were slip joints held with hose clamps! Then I looked at the GS850 fiche - CV carbs and individual bolt on rubbers - two types a LH side pair and a RH side pair - part numbers 13110-45100 for RHS and 13120-45100 for LHS ! ! ! I wonder if someone has put an 850 head on the 750 bottom end? See here . . . .  parts #25 & #261283800692_AlphasportGS850CylinderHead.thumb.jpg.6ca7edd94db525b30723875056496be8.jpg

I think the inlets should all have o rings so you may potentially have the wrong rubbers and maybe the wrong carbs ? ? ? 

Edited by Gixer1460
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17 minutes ago, Gixer1460 said:

I did a search on the Alphasports site and i've got a problem! Early GS750's had slide carbs and a monoblock attachment to the head not individual rubbers! Later 750's had individual rubbers and CV carbs but the rubbers were slip joints held with hose clamps! 1283800692_AlphasportGS850CylinderHead.thumb.jpg.6ca7edd94db525b30723875056496be8.jpg

Errr, no. Not correct.

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I have been searching many sites and seem to find all 750  77/79 are part number 13110-45000 replaced with 13110-49000 unfortunately I cant find out if there is an o ring removable from here. The 850 and 1000 share this part. and  this does not  have sides, I think.

IMG_20200511_194014557.thumb.jpg.fd92ee294048d1173c24b1d3e2311216.jpg

 

https://www.Eblag.co.uk/itm/Suzuki-77-79-GS750-GS-750-GS850-GS-850-GS1000-Intake-Manifold-Boots-13110-49000/202730479839?hash=item2f33ada0df:g:oIYAAOSwqO5c1zgx

Cant seem to find good second hand ones and the new ones are 140 quid without importing from USA. Can I put an o ring on top and bolt to head?? A little basic only may work long enough  to test. Sorry if that sounds rough.

Edited by bob654
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4 hours ago, bob654 said:

I got some new o rings today so will fit tomorrow.

I dont think these have removable o rings. I have two like this and two metal and rubber with groove ( the real deal). Uh.

I thought about putting a ring on top and screwing it against the head.

Or else its new rubbers at 50 quid each.

IMG_20200510_170401794.jpg

IMG_20200511_194014557.jpg

They do have o-rings.

They're just deformed.

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23 hours ago, gs7_11 said:

They do have o-rings.

They're just deformed.

I can see the O ring in there. Just dig it out with that pick. If you need new boots, use CruzinImage.net

You probably need new boots. Air box tubes wouldn't , either

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Interesting, i have the same problem that cylinder 3 won't really work like the others. First i thought of the carbs and cleaned them again. Didn't really do the job. Then i sprayed some brake cleaner at the inlet rubbers while the bike was running and the damn 3 exhaust pipe instantly got as hot as the others and the rpm turned up. Checked the rubbers and they look the same like yours. Instead of just replacing the o rings, i decided to order new ones. Afters 40 years, probably not the worst idea. 

If this doesn't help, i will do that damn vacum test by capping it...God i truly hate vacum petcocks

Cheers and good luck with your problem

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