GSX750es - unable to create vacuum in carbs. Won't start

[kimmenene]

Hey,
So a while ago I bought a GSX750ES from -82. It haven't been in use since -03 and a wreck so I bought I cheap.
As soon as I got it home I were able to fire her up, but only once or twice and only for a second or two.

Figured that she did not get fuel. Removed carbs, cleaned and put it on again. Still same.
Ordered a repair kit with diagrams and all jets except main and carbs look new and in perfect condition now. But she won't turn on.

Bought a vacuum pump to check if it's a leak somewhere. Petcock is alright, no leak.
But I am unable to get any kind of pressure via the vacuum hose to the carbs.
If I turn the engine I can see the needle moving very little, and obviously not enough to to suck any fuel.

I've replaced o-rings in the intake.
What else can I do?
I can't see how it's an issue with the carbs?

original air box. No other mods to the engine.

Any tips are greatly appreciated

[bikemtn]

The first thing I would do would adjust the valves. If that does not help check your compression.

[Rijko]

will the bike start with the petcock set to PRI ?
That way you circumvent the vacuum part.

If the engine starts, see which of the 3 hoses sucks air,
should be the 2nd from the right when sitting on the bike.
That's the vacuum hose.

Hope that's what is wrong, other issues may be expensive to solve.

[kimmenene]

No, won't start at all I'm afraid. A few jumps here and there due to the low pressure and small amount of fuel in the carbs.
Adjust/change valves is what I have to do, I guess.

One thing at a time. Good thing I'm not in a rush!

[Rijko]

No, won't start at all I'm afraid. A few jumps here and there due to the low pressure and small amount of fuel in the carbs.
Adjust/change valves is what I have to do, I guess.

One thing at a time. Good thing I'm not in a rush!

if the petcock is functioning ok and set to PRI,
the carbs should be filled to the brim ...

Adjusting the valves is good, hope that brings results.

[kimmenene]

No flow if set to pri... not even with all hoses detached from the tank and turned upside down. Faulty petcock?

[JTGS850GL]

If you have no flow in prime then the petcock is bad. Assuming you do have gas in the tank.

[Rijko]

No flow if set to pri... not even with all hoses detached from the tank and turned upside down. Faulty petcock?

yep ! That's the first thing to check now.
Hopefully it's just full of gunk, cleaning may suffice.

Don't worry about vacuum yet, testing the engine comes first.
If you get fuel to the carbs, next thing is to see if the engine runs.

My approach usually is to get things running with the least cost
and effort possible, if cost of getting the bike to where you want it to be is
too high it would be a shame to invest in it.

You could also test by hooking up a reservoir to the fuel line,
bypassing the petcock.

[kimmenene]

yep ! That's the first thing to check now.
Hopefully it's just full of gunk, cleaning may suffice.

Don't worry about vacuum yet, testing the engine comes first.
If you get fuel to the carbs, next thing is to see if the engine runs.

My approach usually is to get things running with the least cost
and effort possible, if cost of getting the bike to where you want it to be is
too high it would be a shame to invest in it.

You could also test by hooking up a reservoir to the fuel line,
bypassing the petcock.

Alright, I'll give it a look tomorrow! Fingers crossed.
Thanks for the tips guys.

[Rijko]

fuel from 03 is probably bad ... use fresh fuel !
good luck

[BigD_83]

fuel from 03 is probably bad ... use fresh fuel !
good luck

Absolutely...even ethanol-free gas that old is going to be full of water

[kimmenene]

fuel from 03 is probably bad ... use fresh fuel !
good luck

All fluids changed before first try! Don't worry
So I used a bottle to connect the fuel hose, and 2 turns she's on.
I removed the petcock but I can't see anything broken or missing.. Looks almost new.

Another strange issue I noticed once she was able to run was that if the exhaust pipes are on, she revs very high. Almost full throttle without me touching anything.
Without the exhaust she runs fine... Although very loud...
If they were clogged she would barely be able to fire up, right?
The exhaust is a bit rusty and a hole the size of a penny on it so she's due to a new set..

[Rijko]

good to hear she starts !

The high revving may not be related to the exhaust,
it may still have issues with carbs or airfilter, etc.
At least you now have something to work with

Petcock is next on the list, some repair kits are bad.
Especially compare the length of the axle of the diaphragm,
and the o-ring on it with the old parts.
I have had sets from K&L with a shorter shaft and thinner
o-ring, messing up the petcock functionality.
Are the old parts in working order ?
Maybe try to swap out some parts to see if you get it working.

So the exhausts are now mounted, is welding the hole an option ?
Just look for a cheap temporary fix to get the bike running nicely.
Btw even clogged exhausts should not be a big problem.

Airbox and airfilter are mounted, too ?

High revs usually means a lean mixture / gas shortage.

Try start the bike cold, and spray some brake cleaner
on the intake boots - if the revs change it is sucking air.
I read you replaced the o-rings but the boots may be bad.

Or a shortage of gas because valve needles are stuck in the
seats and do not let gas in, or a clogged jet ... unfortunately
you will have to recheck the carbs.

Do all 4 downpipes heat up the same ?
Oldskool method is some spit on your fingers and tap them
to find out, modern way is a nice infrared thermometer.
Sometimes you can narrow an issue down to 1 specific cylinder
that way.

[Rijko]

here's a very nice tutorial on how to clean and rebuild BS carbs.
If you have to take the carbs off again, this may come in handy.
Lots of little things in there that sometimes are overlooked.

http://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf