9 replies to this topic

[Sputnik]

Previously, my racer with a 296 kent cam (with the 220lbs double valve springs that comes as part fo the kit) backfired through the weber carb at high revs, but only one quick one at a time, I can feel a half a second of power loss, keep my foot down and the car just keeps on going after that as happy as can be..

I was told it is running to lean, but according to the mechanic it is fine.

Then, I was told it is either the points, condenser or leads, but now ....

In two of the corners, if I catch some traffic (yes, I am faster as some), my revs in 3rd drops below the cam and instead of the normal of cam 'splattering' it also backfires through the carb now suddenly, until the revs pick up again into the cam's rev range.

The question now is, what can it be. Is it timing of the cam, or ignition timing, or something else. I have very little knowledage when it comes to the internal of the mini engine, but learning fast as i go along.

Advice much appreciated.

[jakejakejake1]

I have had carb backfires twice now, one was caused by a vacuum leak causing a weak mixture, the other was caused when the valve gaps were set incorrectly. So it may be worth checking these things as they are fairly easy to sort.

[Sputnik]

Thanks Jakejakejake1.

I am running a 40 weber, and some how I do suspect a air leak somewhere, since the fisrt day I've fitted the manifold. Mechnaic recons the car, according to the plug colour, is running fine.

I will recheck the rocker clearance, might have moved a bit under the constant bashing

[jakejakejake1]

Just to add, both the problems I had left the plugs a 'good' colour, as the vacuum leak only affected the car at low revs, and for some reason the valve gaps didnt affect plug colour either.

[sonikk4]

Have you got Misab plates fitted between your carb and manifold or the old plastic type with two seal seals one either side?? Also have you over tightened the thackery washers as this will cause the seals to pop out. These are normally the main reasons for air leaks.

[Sputnik]

@jakejakejake1

I have about 6 weeks before the next race and I will spent some time to check for any air leaks. I just need to find out how ? I will in anycase replace the manifold gasket. I am not using a manifold gasket sealer, juts the gasket (new every time). Can that cause a problem ?

@ Sonnik4,

I have "metal" misab plates with two seals. Very interesting comment about the over tightening, as I did re-tighten it before the last race to prevent air leaks, and only after that it started to back fire at of cam revs. Might have over tighten them and going to check. Thanks, good point.

[jakejakejake1]

To find air leaks I normally use something like wd40, brake cleaners etc in an aerosol and spray around the gaskets with the engine running, any change in engine note/ speed will indicate that the fluid is preventing air from entering through the leak, thus preventing a weak mixture caused by the leak. Also check connections like vacuum advance and brake servo take off etc. And once you narrow down where the leak is occurring you can find a way to sort it out

And with regards to sealant, I have never using any and it doesn't seem to have caused any problems for me

Jake

[Sputnik]

Jake,

I assume that the car must be idling ? and not being lightly fueled by the accelerator ?

PS: Only seen your disclaimer now, but atleast what you suggests make sense Thanks.

[jakejakejake1]

Yeah the car must be idling, as if the mixture is weak enough to cause carb backfires then when you apply the fluid, the mixture will momentarily become richer due to less air getting in through the leak, and this will result in a slightly higher idle speed. That's assuming the problem is caused by a leak. You could also check the idle mixture using a gas analyser or colourtune etc.

Also it may be worth doing a compression test just to check the compression of the engine, as my car had similar problems when I burnt a valve seat due to the gaps closing up, this was causing poor compression in one cylinder, which at idle and below say 2-3k revs was causing bogging down, but once the car got up past this it was ok, probably due to the gas not having time to escape or something. This problem was also accompanied by terrible fuel consumption (20 mpg from a standard 998 drove carefully). Have you been experiencing unusually poor fuel economy?

Jake

[Sputnik]

Hi Jake,

I did not yet have time to do all tests as above as I have purchased a 1976 clubman and most of my spare time is to get the car back on the road and a full rebuild.

On the fuel consumption, I have no idea what it is as I am using the car for racing only, so actually not bothered.