GB500 running problems

[danny baker]

I'm now becoming a Desperate Dan.
My GB misfires/jumpy in the mid range.
It starts and ticks over OK. It can get to 100mph no problem.
but it's the bit in between.  When standing still with the throttle open at 2500rpm the
revs will drop every few seconds to about 2000 and go back up again.
everytime the rev's drop I get a blowback through the airfilter.
I've cleaned the petrol tank out and replaced the petrol.
I've cleaned and checked the carb. checked the valve gaps, checked the timing with a strobe.
changed the plug, ignition coil & lead, electronic ignition, tried a spare carb.
Any ideas/advice would be welcomed

[002]

Sounds lean to me.

Check the airbox to carb rubber,for cracks or leaks around both joints.Air box side and carb side.
Then do the same for inlet manifold,check for cracks,a good fit to the carb and for leaks where it meeets the head.

When it is ticking over spray some WD40 around the joints in turn and listen for the revs increasing.
If so then that joint is your problem.

Jethro

[guest288]

I had a problem like this- it turned out to be the air mixture screw on the carb. Or is your carb jet worn since cleaning the carb???

[danny baker]

Tried spraying wd40 as above and did not make any difference.
My set up is....no air box. ramair filter, slow jet = 50. main jet = 165
2 into 1 pipe with a goldie silencer.
was going ok like this but it gradually got worst.

[danny baker]

looks like I've done it. I went over everything again and this time when putting the main needle into
the carb, I put some grease in the grove where the diaphragm sits. This made sure it stayed in place when putting the top of the crab on. Maybe it was not seated right and air was getting in.
I wonder if this is the same problem you would get if you had a small hole in the rubber.

[002]

looks like I've done it. I went over everything again and this time when putting the main needle into
the carb, I put some grease in the grove where the diaphragm sits. This made sure it stayed in place when putting the top of the crab on. Maybe it was not seated right and air was getting in.
I wonder if this is the same problem you would get if you had a small hole in the rubber.

"top of the crab"......"small hole in the rubber".....I say steady on !
You'll be saying"Suck,Squeeze,Bang,Blow"next !

Jethro