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MG MGB Technical - Odd ignition problem

Hi anyone out there any ideas.. 1979 mgb roadster..I had a dyno test last week which showed poor engine output topping at 62 flywheel hp at 3500revs and then flat until 5000 when it dropped. Mixture OK. Compression test gave all cylinders at 130-135 psi. Checked ignition and static was around 15degs with full advance at over 40 degs. When adjusted to correct static of about 7 degs the engine would not idle and performance was even worst. Distributor is quite new and vacuum system working fine (disconnected for timing test) . No pinking present. K&N filters . luminition plus s/S 3 branch single can Exhaust. Cylinder head has had replacement valve inserts and "gas flowed" some time ago according to receipts. Cant understand why engine will not run with correct ignition settings and why so down on power. Plugs slightly light on colour check but very clean . Could this be a worn cam ? Don't know the engine mileage.
a m morrell

First off
If there is a doubt about ign timing check that your harmonic balancer hasn't moved on it's rubber
Simple spark plug out and check for TDC alignment with timing pointer
willy
William Revit

It needn't be an ignition problem, it could be anything, especially if it has always been like it in your ownership and not suddenly or even gradually lost power over time. For example it could be anything to do with the cam, including its timing.

Could be fuel starvation, incorrect needles i.e. mixture across the range given the K&Ns and modified manifold.

In what way wouldn't it idle when the timing was backed off? It is normal to have to screw in the idle screws if the timing is backed off as you did, and unscrew them if it is advanced, in order to maintain a given idle speed.

If it wasn't pinking at 15 degrees static that says to me something is quite wrong, unless it is a low compression engine or has an extra thick head gasket or something. According to the book 130 is what you would get on an LC, HC should be 170 or so. This is measured hot, dry i.e. no oil in the cylinders, all plugs out, throttle wedged wide open. What is the engine number prefix?
Paul Hunt

Willy.. thanks for comment but this was one of the first things I checked as it had occured to me that this may have happened . Without a dial gauge but doing a "blind test" with probe on the piston TDC certainly appeared to correspond correctly to the crank and fixed timing marks. I think I will remove the crank pulley and have a better look at the rubber although would not the timing have appeared to be retarded rather than over advanced ?

Paul .. Thanks again ,,yes 130 is consistent with a L/C engine but it is marked with an H for high compression .

Needles were changed when K@Ns added and the tuning shop running the dyno indicated that the exhaust gas checks showed the fuelling was OK. Interestingly backing off the throttle does seem (!) to improve the power which would be consistent with a too lean mixture and the plugs 3@4 were rather on the light side

Engine would not idle at all with ignition set "correctly" although I did not try to pick up using the idle screw. Engine sounded horribly flat. I have yet to check the cam timing and that's tomorrows job I think as it is possible that the chain was changed incorrectly although cant see any evidence of this work in the huge history file. Everything in the engine is very clean indicating someone has worked on the engine in its recent past... although it has done very few miles in the last 10 years ..approx 10K .

I think performance was better ( a little perhaps)before last winters work on the engine bay which required removal of the carbs distributer and exhaust. More investigation by me required

mike


a m morrell

This thread was discussed on 22/12/2014

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