Backfire at it's worst

mdmax72 · Post by **mdmax72** » Fri Feb 17, 2012 7:23 pm

Anyone had a backfire like this??? Total destruction. Happened whilst having carbs set up on the rolling road. This was the second silencer in line not the first so a total head twister

l would have thought a backfire of this extent would have blew the first silencer not the second. Everyone was baffled by this at the rolling road place.

Gary71 · Post by **Gary71** » Fri Feb 17, 2012 7:26 pm

Looks like it's filled full of fuel vapour and then BOOM! It maybe that the gas speed in the second silencer allows the vapour to collect?

Is your car running a bit rich?

mdmax72 · Post by **mdmax72** » Fri Feb 17, 2012 8:46 pm

Nice punn Gary, not that l know off, last time the plugs were out they were nice and brown.

Car only went in to try and eliminate a flat spot through the rev range. I know this is a common thing with carbs but the tuner's fancied there chances.

Bruce M · Post by **Bruce M** » Fri Feb 17, 2012 9:34 pm

Probably not enough oxygen in the first silencer, plus the 2nd one would be colder so more likely to condense any fuel vapour into a puddle.

oliver365 · Post by **oliver365** » Sat Feb 18, 2012 7:08 am

Rich have you been on the VZI forum ?

you may get the answer there ! I bet you sh*t your pants

mdmax72 · Post by **mdmax72** » Sat Feb 18, 2012 11:07 am

I never as l wasn't present whilst it happened but the guys in the workshop it happened did from what the said.

Bruce and Gary you were going in the right direction. I'm Impressed. The guys at the workshop couldnt give me a feasible reason. Was you guy's refering from your own experiances??

Giving me a direction to do some digging and apparently any unburnt fuel lingering can be re-ignited if there is any leaks in the exhaust; or blowing on joints etc. On de-accelleration clean Oxygen can be sucked in and BOom. I do know there was a pin hole in the joint /clamp before the silencer box. So the puzzle may be solved??? Unless anyone else can comment?? Just had a new silencer configuration built from Rich at Turbo Thomas so don't want this happening again.... Learn something new everyday, l like that.

Gary71 · Post by **Gary71** » Sat Feb 18, 2012 1:32 pm

It may be that the way it was 'driven' on the rollers is somewhat different to the road, extended full throttle, lots of repeated acceleration.

Both conditions may be enriching more than even enthusiastic road driving. Don't know what carbs you have, but my accelerator pumps squirt a ridiculous amount of fuel on acceleration and use the same circuit under high load to add even more. Those following me have noted the smell of raw fuel occasionally

Bruce M · Post by **Bruce M** » Sat Feb 18, 2012 1:50 pm

What carbs are you running? Typically they have far too much accel pump enticement out the box.

I've had back fires when tuning carbs. When you mess with the mixture screws or balance (at idle) plenty of fuel get poured in so giving it a good rev every min or so is important to clear the pipes.

911hillclimber · Post by **911hillclimber** » Sat Feb 18, 2012 1:54 pm

I've been through a few back fires in the last few years on a 911 engine/PMO carbs.

To open the silencer in that fashion must mean the explosion/expansion of the fuel/air could not get out any other way quickly, so there must have been a lot of fuel to ignite, but you need the right fuel/air to get the explosion vapour ready to pop.

My thoughts would be the way the Rollers were driven up by the car.

At Bob Watson's, the run-up is very gradual, 1st to about 2000 rpm, 2nd the same , third is the power run from around 2000/2500 rpm to the red line, dip the clutch for the tranmissionlosses to show as the rollers slow down.

When in the run did the silencer pop?

mdmax72 · Post by **mdmax72** » Mon Feb 20, 2012 9:35 pm

Wasn't present when it happened so can't say at what point it happened.

The carbs are Webers 44idfs. New ones. They did change the pump jets to bigger ones as the the ones straight out the box seemed to be starving the engine if you put your foot down. Causing it to stumble before picking up.

Bruce M · Post by **Bruce M** » Mon Feb 20, 2012 10:46 pm

Hmm. Unless you have a big ass engine, I'd be surprised. If the pump jets are lean I'd expect misfires and spit back up the carbs. If the pump jets are rich the pick up will be slow and stodgy with a puff of black smoke from the tail pipe. Timing issues can also cause problems with pick up and mixture screws too.

I think the idf has a choice of two sizes 50 and 80 (ie big and bigger). However there is also a bleed jet (that no one ever looks at) that returns a % of the pump flow back to the float chamber. So using a larger bleed jet reduces the flow. New out the box, that bleed jet is a blank with no hole in it. So an '80' with a zero bleed jet is the biggest setting you get.

You also need to be careful with the fuel pressure. Too much causes the float chamber to overflow and petrol dribbles into the inlet. 2psi is a good place to be. Most pumps knock out more than that (stock t1 included).

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