popping back in carb
Turbo Mini Forums

Hi
I realise there are many causes of this problem, but what is the most common cause of the carb popping back? What are peoples experiences with this? Its freshly been built, not been set up on the rollers yet. Think I improved it by playing around with the mixture screw a bit. Does run a bit better with the choke out too which would suggest a lean mixture?
Its worse after changing up a gear, bogs down at the bottom and pops back. It does clear when the revs build up. Only reaching about 2 psi at the moment whilst running in.
Any ideas why its doing this please?
Thanks, James.

Sounds like the timings out if the carbs poping back. Sure its back fire that dose it

Sounds more like weak mixture to me.
Get it on the rollers and set up.

Yeah sounds like its running lean at bottom end, what needle, dashpot spring? Plenty of oil in there?

Audi s4 b5 - 470bhp & 486ft lbs

On 30th Apr, 2012 Brett said:
yeah stick the bit in and give it a wobble

Mega lean mixture

Lean.

In the 13's at last!.. Just

Brilliant thanks, that pretty much confirms then... Not sure what needle, the carb and plenum came from iain's turbo miglia he had so thought it should be around the right mixture already as he was running 150hp on low boost. I might try and richen it up so I can get to mitp and back which will be good running in miles then get it setup shortly after!
Thanks for all your replies!

May I be the first of many to suggest an afr gauge. Then you will be able to remove all guesswork

Audi s4 b5 - 470bhp & 486ft lbs

On 30th Apr, 2012 Brett said:
yeah stick the bit in and give it a wobble

Yeah I've been looking at them for a while, I'll definitely be investing in the near future!

Have you got oil in the dashpot? Part throttle lean off connected properly/blanked? Hole in the gasket etc etc

In the 13's at last!.. Just

Part throttle banked off, carb gaskets are new. Got 20 50 in the dashpot, a bit thick but should be ok for now!

There's no guess work here you need a rolling road set up and an aem lambda gauge......... or it will all end in tears I am afraid! Under boost

Sorry to be the bearer of. The news you don't want to here...... but all the advise above is spot on.

Pretty much sorted now! Took the dashpot off and found the needle wasn't in the damper properly. the spring bit was missing so the very badly filed needle was just dangling all over the shop. Swapped the whole top part of the carb over, set the fuel pressure at 6 psi at idle and its running lovely now! Even running cooler temp aswell! Still needs setting up but will be ok for now.

Don't even think about running boost until you have it properly set up or you'll kill it. More of the problems I read about on here are related to poor tuning and guesswork than anything else. Spend some money and get it set up properly on a rolling road, it will save money in the end.

It is going to be set up, just got to get it mot'd first.

6 psi fuel pressure!? Shouldnt it be between 3 and 4?