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MG MGB Technical - Ignition Puzzle


I have a puzzle, Just finished complete rebuild and overhall, Engine starts great, idles good, BUT, when reved up starts to miss badly. Connecting the timing light, and reading from under the car, ( who thought of that place to put the timing marks ???) I find that the car fires, at 10 degrees and then becomes irratic and fires at anywhere between 8-15 degrees. Egine is still idlying, vacuum advance is disconnected and plugged off. I do have a Rally Cam, installed and checking the vacuum, at idle it reads 18-19"hg. Which indicates to me that the cam and vales are timed just fine. Just thought I would toss this out to see, if someone had an idea, that I havn't thought of.

David 70B
David W

I would start on the advance weights in the dizzy , springs connectd , move freely ? Bearings in good shape ?
S Best

David. The change in ignition timing indicates a problem in the distributor system. This is the distributor itself, the distributor drive gear and its hold down piece, the cam, the cam sprocket, the timing chain, the timing chain tensioner and the crankshaft sprocket. You mention a "complete rebuild and overhaul". Did this include the distributor? Did you replace the timing chain, sprockets and tensioner? Did you release the tensioner so it is actually putting tension on the timing chain?

I think the first thing I would do is make sure the distributor is fully seated. If not, the drive dog could be making only light contact with the distributor drive gear. If the distributor has not been rebuilt as a part of your rebuild/overhaul process, it might be worth inspecting it. Best method would be to have it checked on a distributor test machine to see what it is actually doing on the test stand. If the distributor tests out properly, you need to look at the other parts of the system for the problem. Les
Les Bengtson

With the distributor cap off you should be able to turn the rotor anti-clockwise against spring pressure, when you release it slowly it should return all the way, and ideally there should be no slop. Slop can cause erratic timing and is caused by stretched, broken, or missing springs.
Paul Hunt

Have you made sure the spark plug leads are in the correct order....at the cap the #1 lead is at approximately 2:00...the firing order is 1-3-4-2 counter clockwise. I know this is basic but you be amazed how often this seems to happen after a rebuild.
Frank


Thanks everyone for your advise, I took it and started from scratch, setting the timing to 10 degrees before, pulled the plugs and checked for TDC, pulled the Distributor and stripped it down, did find that the weights were a little sluggish, also the rubbing block seamed soft? and worn, ( new from overhall) Installed a Petronix system, and flashed the engine up, ran like a sewing machine or I should say like a MGB should run. Drove around last night with no miss or hesitation. What did I fix, don't realy know, but what ever it's working.
David 70B
David W

David...was following this but not posting as everyone had it covered. Thanks for posting a follow-up. Good to hear it got solved. Sometimes it can be a couple contributory things so first solidifying the dizzy then going pertronix really makes the ignition side give dependable spark. Going Pertronix when my car gets back from the shop. Isn't it cool when you solve it. One more thing learned.

J.T. Bamford

This thread was discussed between 27/04/2004 and 29/04/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.