MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - rough running under 3000 rpm

Last evening my 74 gt started to run really rough under 3000 rpm. Anything above 3000 it is just fine. I adjusted the points this morning and that dosn't seem to be the problem. Pulled the plugs and they look fine. It's almost as if I'm running on three cylinders until I get the rpms up there. Any thoughts? Tom
Thomas Strombeck

You could possibly have lost a plug wire. Remove each wire, one by one, and test continuity. If one is bad it will give a signal near infinity. Normal resistance should be a few thousand ohms. Also check the inside of the distributor cap for corrosion. If all that pans out, you might have a burned valve. You could check cylinder pressure in each cylinder. Or maybe you have a damaged vacuum hose or a bad vacuum advance canister, or a bad anti-runon valve, or your timing chain jumped a tooth, or...
you know where I'm going. Start with the simple tune-up stuff and work your way up the scale of disaster.

What area are you in anyway? Let me know if I can be of assistance!

Jeff Schlemmer

Thanks Jeff,
I'll check those things that you mentioned. I'm located in Mendota Heights. Just south of the river from the Twin Cities..
Thomas Strombeck

I'm over in Shakopee, but work in Eagan. If you can't figure it out, let me know and I'd be glad to bring over a few tools... If it was only firing on 3, you would have a wet or fouled plug. My bet is on an electrical short somewhere in the ignition though.

Did you try replacing the condenser? Do you have emissions controls still installed? Sorry - I didn't mean to add to the list...
Jeff Schlemmer

My car did the same thing, and it turned out that the carb needles were worn, as well as the jets...
Dave Pothecary

My 74.5 GT has a similar malady. It doesn't really miss but it gallups at idle - badly. I'm running it with Pertronics and at any rpm with my foot in the carbs it digs in and runs so well and so smoothly that I'm just putting up with it. This car only has 48000 miles on it (39000 when I bought it a year ago). Once ignition, valves and timing were up to snuff, I spent lots of time on the carbs - balance, mixture, idle speed, etc and could not tune it out. The other good news is that it runs without a hint of pinging on 87 octane. Maybe I'm running too retarded, but it's set at either 13 or 15 degrees BTDC - whatever the spec was - with the vacuum advance disconnected. Unless somebody knows something I haven't thought of, my next step will be to rebuild the carbs soon - unless I get the Olds 215 ready to go in first.

Mendota Heights - I used to work at Diamond Jims. It must be long gone now.

Thanks,
Allen
Allen Bachelder

Hey, maybe it's got something to do with Minnesota! I'm up in Wyoming, MN and seem to have the same problem! I've replaced my throttle shafts and tuned the engine numerous times to no avail. I'm putting out some dark exhaust so I must have it too rich. I'm not that patient with my tune up skills so maybe I'm skipping some steps or something. I'll probably try it again this weekend. If you guys come up with an answer, clue me in. I'm almost ready to cave and let Brian at BMC take a crack at it!
Tony Mason

Have you checked that the centifigal advance on the dizzy is free to return ?
S Best

For those of you wondering, I was just over to look at Tom's GT and it seems like the distributor is the culprit. Between a loose points plate, a loose ground screw, and possibly a mechanical advance problem, things could be straightened out with a rebuild. We got it running a little better, I think. What's strange is that it runs better when the engine is warm, and the carbs are in excellent shape, very well tuned.

Tony, are you sure you want Brian working on your car? You might get it back with a V6 in it! By the way, thanks for the doors. I'm going slow with the restoration, but they're finally mounted for the last time and fit perfect!



Jeff Schlemmer

I just want to take this moment to thank Jeff. What a heck of a guy. He drove an hour, looked at my engine, tagged along to drop my son off at a ball game, listened to my daughter complain, taught me a little bit about dwell meters, got my car running much better than it has been, and then refused to allow me to buy him a case of beer. The only thing more enjoyable than these cars are people like this. Thanks again for your help. Tom
Thomas Strombeck

Wow! It looks like you guys have made good progress since I last checked! Good going Jeff! Glad the doors are proving usefull. You're probably right about Brian! It's tempting to go the V6 route but I'm still a penny pincher. Maybe when I get to the MGB I'll consider it. In the meantime, I think I'll check my dizzy now. That sounds like a definite possibility.
Hey Tom, If you even have a need for another hatch, I have several spares.
Tony Mason

Interesting that this should come down to a distributor issue. I installed pertronix on mine about a year ago. I checked the vacuum and centrifugal advances at that time for vacuum leaks and freedome of motion and they seemed fine. But the idle did get worse after that. Running under load was good before and even better with Pertronix - so I've put up with it. I can't imagine the advances sticking on the advanced end of the curve because I can run the car on 87 octane and it NEVER pings!

So what could get messed up in a distributor while installing Pertronix that would cause rough idle?

Allen
Allen Bachelder

Allen, I don't think it's as much an issue with the pertronix as it is the state of repair of the points plate and its ability to rotate freely with the vacuum advance in operation. You also have considerably more dwell with the electronic ignition, so your advance curve is no longer accurate. You need to make certain you have the clearance under your (new) distributor cap and be absolutely certain that the advance weights are functioning properly. Removing the "slack" from the softer advance weight spring seems to help with throttle response as well.
Jeff Schlemmer

Jeff,

Thanks for the advice. While I checked freedom of movement on the plate, the vacuum advance and the centrifugal advance, I'll check again. I'll also check the cap for clearance, and snug up the softer spring. How do I adjust for the inaccurate advance curve? More advanced timing? It's at 13 degrees BTDC now.

Once again, thanks!
Allen Bachelder

Several have people have commented that they have to alter the timing quite a bit after replacing points with a Pertronix. This says to me that the phasing is quite different between the two, which shifts the firing point on the rotor. Because of centrifugal advance moving the points plate back and fore the firing point moves round the face of the rotor arm contact, which is why it has an arc and not a pointer, and is why it shows burning around a considerable portion of the arc and not mostly in one place. I have also noticed on mine that while there is a clean area on one end there is not on the other, which implies the phasing on mine are not ideal, which would leave a clean bit each end. However I never get misfiring at high or low revs so I assume it is within limits. However if the Pertronix moves this even further from the centre you may get weak sparking or even misfiring at either extremity of centrifugal advance. Clean the face of the rotor arm contact, run it for a while using the full range of centrifugal advance, and see where the burning reappears.
Paul Hunt

I had a similar difficulty until it was discovered that one of the SU pistons stuck part way open in the dashpot and would slowly return to position.
Dan Robinson

Allen, I wish there was a simple answer to your question. Unfortunately, there are so many factors that establish a "proper" advance curve, there is no set answer other than that rolling road tuning will determine your proper advance curve. I could tell you what my curve is, but unless you have the same modified head, modified carbs, cam, mixed ratio roller rockers, etc..., the curve I'm using won't suit your engine any better than what you have now.

I have noticed a very consistent power increase in every smog-free engine by reducing initial mechanical advance by removing the slop in the advance springs. This stabilizes the ignition advance at idle, so if necessary, you can advance your static timing to make up for it. You can reduce that slop by simply bending the current springs, or buy a spring kit for an american V8 such as an HEI advance curve kit. Usually they contain small bronze bushings you can use to shim the spring posts. As for timing, 18-20 degrees BTDC is not unheard of and advancing your timing is an option that may produce more off-idle and midrange power. I'd like to hear other peoples' results with changing these variables!

Jeff Schlemmer

Paul and Jeff,

You two have really improved my understanding of Pertronix and advance curves, but every single answered question opens the door for at least two more unanswered ones. Paul, maybe I'm confused or I misunderstood your comment, but I thought the centrifugal advance worked on the spindle rather than the points plate (which is moved by the vacuum advance, right?). Am I wrong? And, that question aside, does it make any difference? I would think advancing the plate advances the spark without changing the rotor, whereas the centrifugal advance moves the rotor without advancing the spark. Is this why the contact surface of the rotor is so wide? So it can deliver a spark within a 5 degree range or so? I'm suddenly aware of the two variables and I'm not sure what they mean.

Thanks!
Allen
Allen Bachelder

Allen; The complete cam spindle and automatic/centrifugal advance assembly is advanced, causing the rotor to advance, the cam on the spindle also advances timing. More RPM equals more advance dependent on the mechanical limits of the advance assembly. As you stated the vacuum advance controls the breaker plate, if there is sufficient vacuum the breaker plate advances the timing based on how much vacuum is applied and the built in limitations of the vacuum capsule. The the vacuum advance is added to the already present mechanical advance. The total distributor advance equals static advance plus mechanical advance plus vacuum advance. Clifton
Clifton Gordon

This thread was discussed between 17/04/2004 and 26/04/2004

MG MGB Technical index

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