Rough running and no vacuum

[roosterrs]

Rebuilt tha carbs on my GPz750A3. Refitted with new rubbers and keyster carb kits. Float heights ok. Wound out the pilot screws 2 turns. Harris pipe fitted. Bike starts but runs a bit rough. Can't get the idle right. If I try to set the idle to 1000rpm and then blip the throttle it revs up and then takes a while to drop down again. Connectted a carbtune pro but can't seem to get any vacuum figures. If I turn the carbtune upside down it then seems to read near the 22mm hg figure. It sounds like it is pressurising the inlet rather than creating a vacuum. So I'm thinking maybe tappet shims?? Anyone have any ideas??

[ZedHead]

Might be worth a compression check, hopefully you will be able to eliminate this early in your diagnosis

[GPRD]

I've been having a similar problem, which I haven't sorted yet, but the next thing I've been told to try is to make sure the butterflies are closing properly, and relying on the pilot screws to provide tick over rather then being slightly open and the engine being able to suck more air in and reving higher, if that all makes sense!
Cheers

[Al]

Did you glue the rubbers onto the head?
ie. no air leaks.
Did you rebuild the carbs because it was running rough when it might have been valve clearances originally?
The carbtune pro can be turned upside down when the vacuum it sees is less than 8cmHg. It says in the manual that having turned it upside down the values on the scale are no longer relevant and it become a comparative measure rather than a numerical one.
How many turns out are the pilot screws supposed to be as standard? 2 seems a lot but i have no experience of the 750's

So it appears that your inlet vacuum is less than 8cmHg:
Air leaks at rubbers
Low compression values
Incorrect valve clearances
Incorrect valve timing


AL

[oldzed]

Would be worth checking your camchain timing is right if nothing else seems wrong.

1 tooth out will still run but the revs will hang

[roosterrs]

Bike has been in storage since 1999. Carbs were gummed up with old fuel so cleaned them all out. 2 turns on pilot screws is as per factory manual. I have changed the oil and filters, but havn't done anything else. Rubbers were not glued onto head but were new.
Might try another turn on the pilot screws to see if it makes a difference.

[Pigford]

Bike has been in storage since 1999. Carbs were gummed up with old fuel so cleaned them all out.

Will probably need a full ULTRASONIC clean if looking gummed up - very difficult to free all those little passageways otherwise

[GPRD]

I've been having a similar problem, which I haven't sorted yet, but the next thing I've been told to try is to make sure the butterflies are closing properly, and relying on the pilot screws to provide tick over rather then being slightly open and the engine being able to suck more air in and reving higher, if that all makes sense!
Cheers

This worked on mine last night.

[roosterrs]

Carbs have been ultrasonically cleaned and blown through with brake cleaner, carb cleaner and air line to ensure no blockages.
Will play with the pilot screws to see if it makes a difference, if not then will investigate further into the pilot circuit.

[roosterrs]

Pilot jets not making any difference. So carbs to bits again.

[roosterrs]

Took carbs off and applied pressure to the back of the carb in the pilot jet bore. Air was coming out of the pilot jet hole in the front of the butterfly, but i did notice that the air was coming out of the pilot jet screw thread, so it must be getting past the o ring on the pilot screw.

[Al]

How flat are the 'O' rings on the pilot fuel screws?
Depending how much air pressure you were using they may be fizzed or not. 100 PSI would bypass new ones!
Are these 34mm CV carbs?
Initially you said that you had no vacuum. If there is no vacuum then leaky pilot screw 'O' rings wont make any difference. Any advance on the valve clearance situation? How do they measure up? Whats the compression pressure figures?
If they are CV's are you using some sort of air filters. Either pods or airbox?
To generate vacuum internally they benefit from some form of restriction on the atmospheric side of the carbs.

AL

[roosterrs]

They were new o rings from the keyster kits. Clearances were ok with one cylinder at 0.025mm, two cylinders at 0.076mm and the rest in spec. In process of changing these now.
I havn't measured compression yet. Its a standard air box with standard new filter.
Yes 34mm CV carbs.

[Al]

Are the original 'O' rings any better than the new ones?
Would not be unheard of in my experience!
I have had reason to fill the pilot fuel screw wells with spray WD40 before suspecting something similar. Are these the American version CV's that had the pilot screws covered with plastic bungs?
If you take the plugs out and turn the engine over you should be able to see the sealing surfaces of the valves. If very rusty it may need some action there.
Did you also remove the small cylindrical alloy plugs in the float bowls that cover the pilot jets?
If so did you remove the pilot jets themselves?
Another thing to do may be to spray WD40 at the joint between the carb rubbers and the head to see if they are sealing correctly.

AL

[roosterrs]

These are uk spec carbs. From memory tge pilot jets were replaced but i will check when i remove the float bowls. Pressure in my air line is 5 bar or approx 75psi. Might remove the heads and relap the valves in if necessary.