Flat spot and spitting!

[minitravellerman]

My rally Mini has developed a bit of a flat spot recently. Basically under normal driving everything is fine, but put the throttle to the floor there is a monumental flat spot and "spitting", even dropping onto 3 cylinders. Very annoying when I'm in the middle of a lanes rally and autotest!!

Anyways, I fitted a Powerspark electronic module to replace the points and genuine Lucas sport coil and changed to NGK plugs as the Champions have tendencies to soot up under heavy load. Made no difference whatsoever - not an electrical issue as first through.

Ok - maybe it's a fuelling problem?

So bonnet off and out came the spanners - carb swapped over. No change!!! Richened the mixture and success - car driving a dream ... for 5 minutes then back to old tricks!!

What does anyone reckon - I've thought of everything now!!

Only other thing I suspect it could be is maybe burnt out valves, or air getting in through the manifold or something.
I have a rebuilt 12G295 head that I was thinking of fitting.

(by the way - it's a 998 A+ with Minispares stg 1 system and an 1.5" HIF44 carb)

[minitravellerman]

Should've said as well - I check compression and it's good across all cylinders, and there is a good spark. No spark jump either.

It's a real head scratcher!

[wantafaster1]

Try the points back in it just to see. I looked at everything including making sure no water in the fuel etc.
Did notice that since I took out the power spark the exhaust fumes smell better. Down with technology lol

[Smiffy]

Check and adjust the valve clearances.

[ianh1968]

Did notice that since I took out the power spark
the exhaust fumes smell better. Down with technology lol

Mr Vizard says that some electronic ignition systems will
RETARD the timing compared to points. In other words,
they will change the shape of the advance curve.

I am not in a position to discuss this as I am no expert.

... however, I did find that a BAD ROTOR ARM was the cause of
some major ills on a Morris Minor I was working on recently.

All the time it was driven slowly and nicely, it would be fine.
If driven "normally", it would start to splutter and then slowly
die.

Things that made no difference:
Cleaning 50 years worth of filth off the outside of the engine.
Giving the head a de-coke and re-seating all the valves.
Adjusting the tappets.
Changing the ignition leads.
Changing the cap.
Changing the coil.
Fitting an electronic ignition.
Buggering about with the mixture and timing.
Polishing the body-work.
Scratching my head.
Going for a "dump". (This normally works!)...

The rotor arm was the last thing that I changed and had been
totally overlooked. A new "Red" one was an instant cure!

Ian

[wantafaster1]

Yes it was strobed after the new electronic trigger was fitted and actually using the red rotor arm.

Hope some of these pointers can help you out Ian.

[wantafaster1]

Ian have you changed the fuel that you use, or additive? I bought some super unleaded and the car wouldn't have pulled your pants down, back on ordinary fuel and it was a different motor.

[minitravellerman]

Funny you should mention it - I got a full tank of BP Ultimate before the rally and she seemed to go well on it at the start. Back onto regular but will take a few more fills to get rid of the Super completely. Fingers crossed it sorts itself out with me having to pull the head off to see if there are any burnt valves.
I do feel she's taking in air at the manifold though.

In relation to the electronic gismo - it seems to be working well and I changed the rotor arm for a new red one. So I'm pretty sure now that the electrics are fine. I do want to get the timing checked properly. I don't have a timing light so is all done by eye so far.

Thanks guys for all the hints and comments. Hopefully I can get to the bottom of it this week. Got another autotest next Saturday.

[Lord Croker]

Are you using a water heated inlet manifold? If so, is it connected before or after the heater? I've had problems with carburettor icing which gives exactly the symptoms you describe, but only during the cold weather.
Regards, Rich.

[wantafaster1]

Hmm that's exactly the brand of petrol I tried and then abandoned.

[minitravellerman]

Are you using a water heated inlet manifold? If so, is it connected before or after the heater? I've had problems with carburettor icing which gives exactly the symptoms you describe, but only during the cold weather.

I'm using the Minispares water heated inlet manifold, but after my heater decided to take a leak over my left foot I took it out altogether.

Hmm that's exactly the brand of petrol I tried and then abandoned.

As will I now - I'll play a bit of fuel guage roulette to try to get rid of every last drop of it.



I was just thinking last night - the petrol pipe is very close to the manifold - I'll bet I'm getting fuel evaporation in the pipe. Wonder should I reroute it away from the manifold

[wantafaster1]

Have you got sorted out for tomorrow Ian?

[minitravellerman]

Not really!! She's going better but not cured. I haven't had time to reroute the fuel pipe away from the manifold to see if that's the issue. She goes well when she's cold - the misfire starts when all warms up - that's what's making me think it could be vapourising fuel.

I'll give it a shot tomorrow anyway for the craic. Are you coming to Nutt's Corner yourself?

[wantafaster1]

Unless I get massive handbrake improvement no. That's not really the problem, it's high speed reversing.
I have a bad back and with the bucket seats I can't look out the back window. Pity because I see they're doing a tutorial.