Triumph Repair: M.G. High Speed Miss, vacuum gauge, mg midget

Question
QUESTION: I have a 1969 MG Midget with a Crane electrical ignition taking the place of the original point and condenser. The engine is all original except for a new distributor and twin S.U. carbs. The engine accelerates fine up to 4500-5000 rpm's. At this point there appears to be a high speed miss which can be heard from the tailpipe. It sounds like a very rapid muffled popping sound. I've adjusted fuel mixture and timing and can't seem to eliminate the problem. Also, upon deceleration from high speed I get backfiring from the tailpipe. I attached a vacuum gauge which indicates I have worn valve guides. Could worn valve guides account for either of my problems. If not, do you have any other suggestions. Thanks so much for any help you may be able to provide.

Keith

ANSWER: Hi Keith,
I have not seen worn guides do that. First look closely at the exhaust system as a leak anywhere forward of the muffler and more likely up front will cause that.
If nothing is found there I would do a compression test and examine the lift of every valve for a worn cam lobe. Next examine the plugs for signs of a misfiring cylinder. Check the ignition timing and advance system and if nothing is found you should put it on a scope to examine the new ignition system.
Let me know,
Howard

---------- FOLLOW-UP ----------

QUESTION: Howard,
Thanks so much for your expeditious response. Compression should be 120 psi. I performed a compression test which had the following readings:120 115 120 125 which tells me, I believe, that the compression is fine. What info. can I expect to receive if I put the car on a scope? Can you also tell me the consequences of worn valve guides?

Thanks,
Keith

Answer
Keith, a scope can detail and pinpoint any and all problems in the primary and secondary of the ignition system.

Worn valve guides will show an excess of blue smoke on start up but diminish as you run. Also you will hear excess valve noise even if the valve clearance is set correctly. After time it will cause excess valve seat and face ware too.

Howard