Moss Motors, Ltd

I have a 1971 MGB that starts and runs fine, there is one problem i would like advice on, erratic running when trying to cruise at 40 or 50 MPH

with very little throttle, the engine has HS4 carbs, a peter burgess fast road head, a piper BBP270 cam, and a 1-2-3 ignition system,

any tips appreciated.

James denton.

Denton, You'll have to define "erratic" a bit more, but my gut is leaning toward a sticking valve.

It could also be an ignition problem such as worn plugs, wires, coil...etcetera. It could also be inadequate flow from the fuel pump at those revolutions.

How's the timing and dwell angle (if points instead of electronic ignition)?

 

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On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"

'68 Sprite 1275
'76 Midget 1500

Davey, what i mean by erratic is that when driving with very little throttle and trying to keep a constant speed of 40 or 50 the engine seems very lumpy, as if its misfiring, at 60 or 70 there is no problem at all, the electronic 1-2-3 ignition is programmed for the head and camshaft combination.

so the problem area is around the 2500 to 3000 RPM. plugs and coil are new.

 

First of all let me say I like your setup, those components should make for an excellent running engine. They appear to be well matched; I assume you are also using high compression pistons and a good free flowing exhaust. I am most envious.

 

The type of problem you described can be frustrating to diagnose and resolve. I suggest you reestablish or basically confirm baseline tuning. I would check; valve clearance, initial timing and points gap (not for you). Sounds like the plugs, wires etc are new; so I would not mess with these for now. And last I would confirm the carburetor tuning.

 

I know it’s a lot of work to do these things, but sometimes to find what is wrong; you have to find out is not wrong.

 

After doing these things I would connect a vacuum gage to the intake manifold and actually monitor it during driving conditions. There are some excellent web sites describing how to diagnose using a vacuum gage.

 

As a sidebar I had a problem with a recent rebuild where the engine would randomly misfire from 1500 to 2000RPMs under light load and ran out of power at 5000RPMs. After doing the above I finally turned to the exhaust, a new Monza system; it turned out that it was too restrictive for the longer duration cam causing vacuum fluctuations at low RPMs and excessive backpressure at high RPMs. Replaced the exhaust with no other changes, runs like a bat out of hell to 6,000+RPMs and misfire gone.

 

I hope this is helpful. Again, I do like your choices!

Thanks for your reply, especially where you mention the remedy involving the exhaust system, my car has an extractor exhaust manifold,but both dampers and the rest of the exhaust system are standard, so im going to do some homework on that one,

once again thanks for your reaction.

James Denton