Backfire
Turbo Mini Forums

Hi all wonder if anyone could give me some idea's. I have an Aus SC12 supercharger kit which sucks through a NA Hif44.

I did the lake tour this year gave it a god hammering and all was well, had it sat a around a while and its developed a problem. on light throttle and low RPM, like when you approach a roundabout slowly and go to accelerate onto it, it bogs down like its cold and needs choke, then it backfires through the blow-off valve which is part of the kit. The car also suffers from a faint idle so when you let you foot off the car goes to stall then recovers.
Talking to the guy who builds the kit its sounds like its running lean, but I haven't changed any mixture settings from when its was setup on the RR? Once it gets going its OK.

The carb piston moves freely, I have removed the fuel pressure regulator as I'm only running a 2-3psi pump anyway and it was set to 2 psi, just in case it was faulty. I have fitted a new carb spring, cleaned everything up. tried a light weight oil,

I have changed the fuel tank as I wanted a larger one and I also wanted to check that there wasn't any crap in it..

I have cleaned out the carb fuel bowl, replaced the inlet and Sc gaskets.
We have checked the timing as I'm running a dizzy with no advance with it retarded. I'm no sure what it was setup as at the RR but its retarded from standard. (going to upgrade to Megajolt during the winter).

I have been looking at a rebuild kit for the carb? maybe the throttle spindle rubber seals?

anyone got any ideas, wanted to go to the Miniworld action day? might try to get it back to the RR, does anyone have any ideas?

Thanks for any help.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

try a thicker oil in the dashpot first to stop it rising so fast and leaning out, i get similar issues with mine unless i use 20/50.

Also check that the carb damper piston is OK.

What spark plugs are you running. Are they OK?

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Might sound naive,but have you checked the valve clearances recently, especially the inlets ?.

Have you got a modified float needle fitted. The modified ones have a little hoop of wire that loops over the brass tab of the float. I believe (may wrong) that this is to stop the float needle sticking shut. As the float needle get pulled down instead of relying on the spring. Mines was a little erratic until I changed to the modified type. But I had slightly different symptoms. I was geting unpredictable fuel starvation. I am running an eaton. My local rr upped my fuel pressure to 3.5 psi. Hope that helps

thanks for everyones input.

I have tried 20/50 first and same problem.

I have NGK 7's which I have swapped for new - same problem.

I've had the piston out and checked it modes freely + cleaned it up. I'm not sure what to look for in the damper the brass insert does look a little worn. Maybe its this?

Re-checked the valves ok.

The float needle is a standard one I haven't touched this?? mmm where did you get the modd'd type Burlen?

If the butterfly is opening but the piston isn't moving up this could cause it or am I wrong? I will check the damper and see if I can get another?

Edited by gr4h4m on 21st Sep, 2008.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

Jonspeed sold me the modified float. But it came in an su bag! I am sure you would get it cheaper from burlen. I will see if I have a part no.

If the butterfly is opening and the piston isnt moving,with the engine running,something is restricting its movement.Does it move when you rev the engine and lift off ?.You are not usually advised to swap damper assemblies from one carb to another,but it could be worth a try.There is nearly always some wear marks on the brass damper piston,which is not usually significant.

well swapped the dash pot damper piston and added a new spring, but still no joy.

The SC12 on my car can be switched off as it has an electric clutch, I have tried the car with it off and its better, I wonder if its a dizzy problem, too much retard, could also be why it runs ok on boost? I have another dizzy on a spare engine, The RR set the timing does anyone know where I should be to be safe? I'm running 7-9 psi at full wack.

Re-Checked the timing and its jumping around, so I think the dizzy is shot.

Edited by gr4h4m on 22nd Sep, 2008.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

well its getting worse so I have been lucky and got it in at our local rolling road, A&B. We think we have got it down to a duff dizzy, as when the timing light is on the car it seems to be jumping, so I can only presume that its the springs in the dizzy... I have a spare one which looks feels ok in the shaft, roll-on that mega jolt. will report back when I get he car back tomorrow.

just hope I can get it sorted for sat.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

well the jet in the carb was suck, the dizzy was knackers and the timing was moving all over the place. I think the spings in it are gone.
All sorted now by A&B our local Rolling road, they also modd'd my needle to make the most out of the high lift rockers.

roll on megajolt...

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

Hi mate.

Glad its sorted. Can be a pain tracking a backfire, just as it is a pain with a misfire.

Can you just confirm what you mean by jet ?

Main needle jet or float valve ?

Also, was this confirmed as a cause of fuel starvation as contributory to the problem or did A&B just put it down to sparks ?

Am interested as I know of a similar issue.

Ta.

Minivanless, but reluctantly happy living with the decision. There'll be another one day.
http://www.youtube.com/profile?user=minimadmotorman#p/u

main needle they said that the mixture screw wasn't moving the jet in the carb, I had had the bottom off about a week earlier and it was full of crap, I presume it was all over the place. I should have stripped it all down and serviced it. stupid I didn't now I think about it.
The problem was due both the timing and the fuel, the timing was jumping from about 0deg BTC to about 5!!! dizzy was well shot. plus I had added the high lift rockers causing it to run even more lean.

Can't wait to get my mega jolt installed.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

Cheers.

You'll love the features you get on the MJ. I've got Megasquirt and am gobsmacked with the feedback and flexibility. I'll load my map up on a separate thread.

Richard.

Minivanless, but reluctantly happy living with the decision. There'll be another one day.
http://www.youtube.com/profile?user=minimadmotorman#p/u