Mastiff running problem

[ArtC]

I bought a 2002 Mastiff last August from a motorcycle dealer which ran for a while then started showing worrying signs.
I couldn't get it to increase revs above around 4,000 and then it was just dying, fortunately this happened close to home and I managed to nurse it back.
Thought it must be dirt in the carb so cleaned them out, no better and when I shut off the fuel the engine performance seems to improve. Since then I have found a loose connection on the coil which I have repaired, I also replaced the secondary diaphragm as I thought it might not have been lifting. The float height was a bit high and I have corrected that. I tried opening out the Pilot screw while the float was still high and it did improve things a bit but their is still no response from the throttle after it is opened just over half way. It still revs up better after the fuel valve is closed until faltering on being starved of fuel. I have replaced the fuel cock and filter. I'm sure the fact that it changes when the cock is shut off is symptomatic of something but what. Help would be very much appreciated.

[den]

did you find/clean the little filter gauze behind the float needle..

[ArtC]

Yes, cleaned that. Thinking about adjusting the Needle heights but am totally perplexed that it runs better when I shut off the fuel cock. Afraid I am very new to bike maintenance but if I can understand how and why things work I can normally muddle through I'm just a bit reluctant to do anything further until I can explain this phenomenon.

[DAVID THOMPSON]

try it with the fuel cap loose tank may not be venting and have positive pressure in the heat of summer
dave

do not splash any fuel out be careful

[ArtC]

Thanks, but I have tried that although I was sceptical because it wouldn't explain why it revs up when I cut off the fuel. I was thinking it might be electrical except that it would have to be intermittent because if it wasn't mechanical I wouldn't expect it to improve when starved by shutting off the cock.
I'm just getting a bit frustrated with removing and refitting the carbs while not being 100% sure of what I am doing. Backing off the Pilot screw improves the throttle response but only up to about 5,000 rpm when further throttle does nothing!!

[DAVID THOMPSON]

check the intake and exhaust valve settings
seem to remember some one had running problems till they set the valves

dave

also do these have valves that reduce the compression at starting rpm like or similar to a briggs mower motor

a unit like that that gets stuck could cause all kinds of problems ..

[den]

just study whats happening when you turn the peticock off..as the bowl slowly empties your leaning the mixture up..
have you checked for air leaks,,start the engine and spray around the carb area with easystart,( mist it ).if the rpms change then its surly drawing in the easy start from a gap/hole.
also.
the air pipe on carb no2 has a plastic filter on the end,,this sits close to the exhaust,,if it melts and closes you'll get the same problem.try it with that large 10mm pipe off.

ill ponder on for you ,

[ArtC]

Thanks for the latest input Dave and Den,

I was hoping that the problem lies with the carbs as the bike came from a dealer and they had to do a bit of unspecified work on it before swapping my previous bike for it.

The airflow through the secondary carb diaphragm breather tube is clear and I have checked for vacuum leaks at the inlet blocks/manifolds and just to be sure I have supplemented the captive O-rings with liquid gasket.

I have just opened up the carbs to check the e-clip positions and the Primary is 1 slot up from Rich and the Secondary is 1 1/2 down from Lean. However!! there is a spring missing from the Secondary which may explain the erratic mid range.

As Den says, the petcock is leaning things up when off so the mixture appears to be too rich up to 3/4 throttle so, sort out the missing bits, try leaning out the needles then look at enriching the main jets.

The bike doesn't have the original Exhaust, it has a BSM Future with a slight leak where it is bolted on, so I'm thinking the Main Jets might need increasing in size, from stock, anyway.

I wouldn't mind the trial and error approach if it wasn't so awkward to get at the adjustments.

Cheers,

Art

[den]

After some more pondering,
i remember when messing with the petcock and filter i had to suck on the tank breather tube to remove air bubbles trapped in the filter,,also, swapping out for a new air filter and air filter oil made a huge difference.

the first time i took my carbs off i replaced the bowl screws with hex head stainless steel ones,i can remove the bowl now without removing the carbs,

however , now you have found something missing , (spring) you got to replace that before you go any further..sounds like to found the problem anyways.