Dentonation

I've been working on engine detonation problem with my '72 combat engine. Here's what I've done so far. Retard full advance ingition to 26 degrees BTDC (Lucas RITA). Increased main jets from 230 to 240 - also tried 260, but ran poorly. I'm beginning to suspect current fuel is causing the problem, so will try a colder plug this weekend - NGK BP6ES or equivalent. I'm getting small spheres of what I believe is aluminum from the pistons on the plug ceramic. The engine is in top condition with roughly 185 psi compression each cylinder. Stainless sleeved original AMAL 932 carbs (932/19 and 932/20). I've tried Lucas Oils octane booster will only minor improvement.

illf8ed said:

I'm beginning to suspect current fuel is causing the problem, so will try a colder plug this weekend - NGK BP6ES or equivalent.

Click to expand...

I don't know what plugs you have been using, but NGK BP6ES is a hotter plug than a standard BP7ES,-for instance?

Remember NGK heatrange numbers go the opposite way to Champion.

Offhand, I say you may have too much compression for pump gas. IIRC, my 72 Combat had about ~150psi when fresh. Have you done some recent engine work? Head, pistons, connecting rods?

L.A.B. said:

illf8ed said:

I'm beginning to suspect current fuel is causing the problem, so will try a colder plug this weekend - NGK BP6ES or equivalent.

Click to expand...

I don't know what plugs you have been using, but NGK BP6ES is a hotter plug than a standard BP7ES,-for instance?

Remember NGK heatrange numbers go the opposite way to Champion.

Click to expand...

Yes,I was going backward on cold plug for NGK. Richard at Raber's said the same and sold me some BP8ES plugs. I've always used standard BP7ES or Champion N7YC.

Nothing all that new with the engine other than somewhat recent left exhaust valve and new rings. Shouldn't all combats have 180 psi compression unless a spacer has been used to lower compression? It ran fine from 1997 to 2007. The detonation has been plaguing me for the last couple of years - which is what makes be believe it's the gas. Gas also finally ate my fiberglass gas tank. I didn't have a problem until California got off MTBE and switched to ethanol additive.

If the BP8ES plugs don't cure this, I'm getting five gallons of race gas to see how that works.

David,

I believe the ~150 psi figure I gave was with a spacer. Been a long time ago. Sorry if I induced a FF.

Can you get avgas nearby? That works pretty good. If I had a Combat and wanted to do long runs I'd put in a spacer to knock compression down to where you can run it on currently available fuel. For some reason I'm always on trips on holidays and have to run some real trash gas. I'd build about 9 to 1 for a street bike these days. I'm running in that range on all seven of my bikes with no problems.