spark plug gap closing

[bansheemandan]

i just put a cool head on my buddies banshee, it runs pretty good, but after about 2 min. of riding it will backfire and the right side spark plug gap will close, it is not a bad rod bearing or piston hitting the plug, thanks in advance for any info

[nmdesertbansheeryder]

what size domes did you install?

[Fastassbanshee96]

I'm not sure but I've heard that deto can cause this to happen.

[bansheemandan]

they are 19cc domes, it has 164psi compression, i backed the timing back to 0, i was thinking detno to but it is only doing it in 1 cylinder and the plugs are tan color not white, should i run 50/50 pump gas and race fuel

[bansheemandan]

it also has 34 pjs, i bumper the mains from 160 to 175 and it had little affect, still has a slight top end hesitation

[BigRed350x]

Whats your squish clearance? Did you check the bore of the dome vs the bore of the piston? Does the dome squish angle match your crown angle of the dome?

[bansheemandan]

Whats your squish clearance? Did you check the bore of the dome vs the bore of the piston? Does the dome squish angle match your crown angle of the dome?

 

 

.40 squish and everything is good

[bansheemandan]

found the problem, it was detno, he toke it out for a ride without race fuel, blew the piston apart

[Jereme6655]

spark plug color and deto are two different things.

 

the plug color indicates whether your motor is receiving enough FUEL in order to ensure that the motor is not leaning out and causing a lack of lubrication.

 

detonation has to do with making too much compression and the volitility of fuel......aka.... how unstable the fuel gets when its squeezed. at 164 psi the fuel is being smashed so hard (remember that much compression causes heat) that the fuel becomes very unstable and then blam...it either self ignites from the compression or it finds that one small ding on the piston that sticks up a little....which gathers heat at a faster rate than the rest of the piston...so its a hot spot that can case ignition at wrong times.........all this makes deto.

 

so after all that........check compression when its rebuilt.....and either shoot for 150 psi on 93 pump fuel....or go with race fuel if its higher. And do a plug chop to ensure the propper jetting.