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MG MGB GT V8 Factory Originals Technical - Engine miss at 2100 RPM

I was a beautiful day yesterday for our Spring drive. In total, we had approximately 50 British cars participating. My 3.9 fuel injected car ran perfectly, except for one issue. It developed a slight miss/hesitation at about 2100 RPM. This is an issue since it is the range where the RPMs drop when shifting and/or the range when crawling in traffic.

Some of the particulars on my car are 3.9L with ported heads and RV8 headers, standard cam, Mallory vacuum advance distributor and RPI Optimax chip. Everything in the motor is new or rebuilt.

At idle and all other RPM ranges it runs smooth as silk.

Any thoughts?
Scott Miller

Difficult to diagnoze - coil ? Fuelling related flat spot ?

Can you get it on the LandRover computer ?

RMW

Reversed coil polarity? Only mention it because I saw it happen once resulting in similar symptoms.
Dale

Is this a hickup at this one particular spot, or does it miss through 4-500 rpms around 2100? Is there moisture in the distributor cap or a crack in the rotor or cap? Loose plug wire or moisture around the cap terminals? Bad plug wire? Cracked boot? Plug wire touching a header tube? Broken wire or loose connection in the distributor that flexes with movement of the spark advance? Etc? Hope this helps.

Wayne
Wayne Pearson

Wayne- the hesitation seems to be in a very short range. Primarily, I notice it when shifting or when crawling in traffic.

When shifting, the RPMs drop to that level between shifts. As I accelerate into the next gear I have a hesitation, and then it is smooth throughout the RPM levels. If I keep the RPMs above 2100 during the shift, there are no problems.

I have wondered if it is a fuel map issue, a fuel pressure issue or a timing issue.
Scott Miller

Since you have a fairly mild engine, what happens if you accelerate from about 1500 rpms to 3500-4000 rpms in 3rd gear or say 1500 to 3000 in 4th gear? This should stress the ignition system pretty good and if all goes smoothly, I'd say the ignition is not at fault. Then it may be a lean throttle tip-in condition. Not being familiar with the hot-wire setup, I can't advise on how to trouble shoot, except to say look for a vacuum leak.

Wayne
Wayne Pearson

Check your vacuum advance....?
RMW

This thread was discussed between 23/04/2006 and 25/04/2006

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