Popping on over-run.

The Original Tom · Post by **The Original Tom** » Mon Jul 16, 2007 5:42 pm

Not me, but a friend.
Is a lot of pop and bang on over-run a good sign of bore glazing? I think I heard that somewhere but I could be entirely wrong.
I only ask because mine's a very new engine and as hard as I tr I can't get it to do the same at all!
The timing and tuning has been done recently and it drives very nicely (if lacking that little 'spark' because of the age), but engine braking produces some pretty large noises from the exhaust!
It's a Rover stage 1 3.5 by the way.

Cheers
Tom.

sowen · Post by **sowen** » Mon Jul 16, 2007 5:51 pm

The popping and banging is unburnt fuel igniting in the exhaust when the throttle is closed but the engine is still running at higher revs, basically on overrun. Mine does it a little on overrun, and sometimes spits flames if the conditions are right (wrong if you're in the car behind

)

What carbs have you and your mate got? Some SU and Stromberg carbs are fitted with a little poppet valve in the butterfly to allow air through when the throttle is closed at higher rpm's.

Simon

Post by **Coops** » Mon Jul 16, 2007 5:53 pm

leaking exhaust manifolds?
mine does this, but its due to setting the ecu up for engine braking on deceleration,

The Original Tom · Post by **The Original Tom** » Mon Jul 16, 2007 6:34 pm

I've got HIF44/HS6 SU's (on military needles, code unknown), and I mainly run LPG (although it doesn't bang on either fuel), and he's got strombergs (standard) and no gas.

Also, in case it makes a difference, his engine is as it left the factory (and as a stage 1, it only had 91bhp from new), and mine is a complete bitsa. The main difference, other than age, is probably the fact that mine is running a more torquey cam than standard. If it was supposed to deliver 155bhp from standard, I'm probably pushing 170.

I thought it was unburned fuel but was told that it was exaserbated by glazed bores? Wonder where the guy got that info from.

ChrisJC · Post by **ChrisJC** » Tue Jul 17, 2007 6:56 am

SU carbs run lean on overrun, which gives this effect. As said, the poppet valves in the butterflies are supposed to cure it, but people (myself included!) tend to solder them up as the springs go weak and give a poor idle.

So there's probably nothing wrong with it.

Chris.

sowen · Post by **sowen** » Tue Jul 17, 2007 7:10 am

My first set of carbs were Strombergs, and I soldered up the poppets anyway. It didn't pop and bang, even when the throttles jammed open and I killed the ignition switch at 5000rpm.

My SU's do it a fair bit, and they have standard butterfly's fitted. I think the fuelling is all over the place as they are HS8's that came off a rover 2000tc without any change of needle. Runs fine and has loadsa power on an otherwise standard set-up. I guess its all down to the individual set-up in the end.

Excessive fuel can give bore wash, leading to bore glazing, if thats what the guy who brought up bore glazing thinks.

Simon

The Original Tom · Post by **The Original Tom** » Tue Jul 17, 2007 11:41 am

Ooooh ok. I get it now.

I assume by 'poppets' you mean the little sprung bits on the lower edge of the butterfly?
Some of my spare carbs have these removed and soldered up, but I left mine on the new SU's. I know they must impeed smooth air flow greatly, but as they were fitted any way I decided to leave them - they must have some purpose right?

Cheers
Tom.

TVRTASMIN · Post by **TVRTASMIN** » Thu Jul 19, 2007 7:06 am

The popping and banging can also be caused by a weak mixture at idle plus if there are any leaks in the exhaust system the problem will be exascerbated. (is that the correct spelling)