GS750 rebuild needs help

[bob654]

Hi. Thanks in advance. I have had a GS750 in the shed for some time. I'm getting close to riding, only today, when I fired her up, she wont fire on cylinder3...unless I take off the small vacuum hose ( I think its called) from the back of the tap for the  tank. When I do it wont fire on 4 and will fire on 3?

Ive got the hose ( A) running into the right side of carb 2. Is that right? I also have a loop of hose same dimension, going from right side of 1 to right side of 3 ( c). Is that right?

 

A = the hose from  ( vacuum ?? ) tank to carb 2
B= Main tank hose to rail
C= loop between 1 and 3
D= engine breather to airbox
E= clutch cable

Edited May 7, 2020 by bob654
add pics

[mark35gun]

do you mean vacuum pipe? or breather, some pics would help

welcome to oss by the way

[Blubber]

Hi and welcome to OSS.

 

please post an intro before you start asking questions.

This isn't fb.. 

[vizman]

Has gs resources started a vetting process or something?

[bob654]

24 minutes ago, mark35gun said:

do you mean vacuum pipe? or breather, some pics would help

welcome to oss by the way

How do you post pics please?

 

[mark35gun]

easy, you have done it in your first post

vacuum should be on the inlet side, dont matter which cylinder, i an not sure that c should be linked though

 

[bob654]

17 minutes ago, mark35gun said:

easy, you have done it in your first post

vacuum should be on the inlet side, dont matter which cylinder, i an not sure that c should be linked though

 

Inlet side?

Yeah sussed the pics upload-

 

This all looks good.Did the 1st pic help?

Should I try the small pipe from the tank on an other carb? Do I block off the others spouts on the carbs if there is no pipe on them?

Edited May 7, 2020 by bob654

[Gixer1460]

I think the hose 'C' in your picture is a Float bowl vent - if so they shouldn't be joined - doing so won't allow air in and fuel won't flow. Try disconnecting it and leaving both ends open for a start and see what happens?

[mark35gun]

whichever cylinder you use, make sure all the others are blocked off securely

@gixer1460 is right, they shouldn't be linked

[Gixer1460]

Just now, mark35gun said:

whichever cylinder you use, make sure all the others are blocked off securely

@gixer1460 is right, they shouldn't be linked

ie. only one vacuum source to the fuel tap.

[bob654]

Thank you I took your advice and removed the " loop tube"   'c'  in the pic.  

All 4 cylinders are now firing.

Any ideas how I plug the other 2?

Nearly time to go for a run. I have owned this bike for three years and never ridden it.

I installed Dynatek coils, and ignition update with new leads, new chain, and sprockets,  the exhausts were rotten, and I needed a new headlamp, ( USA import  ).The rear tyre was stamped, 1986, so a new set was added.

100 and one other things too with a new battery last week.

23,000 miles, 78, and original paint. 

 

 

Edited May 7, 2020 by bob654

[mark35gun]

if you are referring to the link pipe or "c" in your pic you don't block them,they are supposed to be open, they do want some pipe on them though long enough to route out of the way, just make sure they don't get kinked or blocked.

[bob654]

32 minutes ago, mark35gun said:

if you are referring to the link pipe or "c" in your pic you don't block them,they are supposed to be open, they do want some pipe on them though long enough to route out of the way, just make sure they don't get kinked or blocked.

Thank you. So what gets blocked off securly?

[mark35gun]

the vacuum

[Gixer1460]

13 hours ago, mark35gun said:

the vacuum

Why? If the vacuum pipe is blocked the fuel tap won't work ! ! ! There should be :

1. Main fuel pipe about 8mm ID from fuel tap to carb fuel rail. 

2. Vacuum pipe from carb 2 (in your case) to fuel tap about 4mm ID.

3. 2x Float bowl vents, each from between 1&2 and 3&4 carbs about 12mm ID - about 150 - 300mm long, left open and routed down towards back of engine cases usually.

 

[gs7_11]

The vent pipes should go over the top of the airbox to the little metal loop on the back, RHS, just big enough for 2 pipes to pass through.

6mm or 8mm pipe.

[mark35gun]

no no i meant block off any ports except the one that is being used for the tap, sorry i wasn't clear at all

[BigT]

None of the ports are blocked off

Fuel you have correctly

Vacuum to fuel tap, small port

The other 2 ports are open, as above

[mark35gun]

There is a misunderstanding here, all the vacuum ports, one per cylinder, either on the inlet tract or carburettor body, or both, should be blocked off securely, except the one that is being used for the vacuum tap.

if any/ all are left open or leaky then it will cause air to be drawn into the cylinders that is not metered by the carburettors, this means the bike will run very  weak, and will generally run roughly, it may not even run at all.

I have known the vacuum part of the fuel tap to be faulty allowing neat fuel to be drawn into whichever cylinder it is attached to ( usually 2nd from left), the pipe which is used for the vacuum  tap should be fitted securely and be airtight.

these should not be confused with the breather tubes which need to stay open as dealt with in the above posts

[bob654]

Thank you all, for all the shared knowledge. Im not able to do any more till Sunday.

I have also decided to  take off the inline fuel filter I installed, cos I believe it is too fine and starving the 4 cylinder a little.

I will post all findings and check for the metal loop on the back of the airbox, fit and route tubes, and block unused vacuum spouts. Thank you again.

Edited May 8, 2020 by bob654

[Gixer1460]

Re filter - I wouldn't unless you can be sure the tank is uncontaminated - shit in the fuel line / carbs is No.1 question / fault on OSS! And if the filter is new it will flow plenty unless its sized for a lawn mower - you want it running first, not breaking land speed records!

[bob654]

It's a universal Eblag filter. I'll get a proper one. Anyone know the flow rate for a 750?

[mark35gun]

Like gixer1460  says don't worry about it too much, new small inline filter will flow plenty, get it running properly first, however having the carbs off and give them a good clean out is always good advice, ultrasonic them would be preferable, 3 years stood at least, prob going to be  shit / varnish in the carbs, this will cause problems.

best advice i can give is don't skip  the carb cleaning step, it will save you hassle later on.                                                                                                                                                                                                                                                                                                                                                                                 

[bob654]

So started again today.

Bike starts right up, running on three.

Third cylinder no show again. I fear my oil level is increasing which would suggest its getting fuel and its not igniting and getting past the rings. I have good compression, over 120 on all cylinders.

I have good spark.

So it must be mixture, (guess).

I have the carbs off again and have rebuild and cleaned again carb 3 and 4, ( 4 for reference and they both seem good.

These are my inlet rubbers they all seem different and although very similar have different numbers stamped by the Suzuki Made in Japan logo.

 

The one with the rubber o ring is from 4 and the other from 3.

 

26, 19, 24, 39, they look identical except two have o rings and two don't ,the offending cylinder had no o ring and some clear silicone around it.

No cuts or tears

How poorly does it need to fit to loose air and upset the mix enough to stop it firing? 

My only other thought is the bottom adjustment screw on the bottom was further in than the others.

I cleaned and bench sync these carbs last week the all, have new Main and Primary jets, all other passage ways cleaned and the needle was raised 1 notch on all.

All input appreciated.

Edited May 10, 2020 by bob654
spelling

[Blubber]

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. 

 

All 4 would be my preverred action