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 - Ignition Timing-need help


The other day, I set the timing on my '80 LE. It has a 25 D distributor, pertronix electronic ignition, and the original Zenith carb on it. I set the timing according to the manual at 1500 rpm with the vac advance disconnected. I set the timing to 13 degrees. The car idles fine, but has no power when I rev it up to go. It doesn't miss and will rev up, but has nothing when I try to drive it. I also tried setting the crank pulley at zero degrees, with the #1 piston at the top of the cylinder. With the rocker arm cover removed, #1's valves are closed and in the same position ( so I guess cam timing is okay). Any ideas as to what the problem could be?

Thanks in advance.

Dave
David Plantz

David, I have a similar setup, and my timing is about the same as you describe.

Have you checked the diaphragm in your ZS carb? You won't get any power if it has a hole in it.

If its a timing issue, I'll let the experts take over.

John
John English


Hi John,

Yeah, the diaphragm is fine. I replaced it not that long ago when I rebuilt the carb.

Thanks,
Dave
Dave Plantz

Dave. Having a 25D4 distributor only tells you that the distributor has been replaced. (Your car may have been equipped with the Lucas 45DM4, or CEI, system--good system, or the Lucas 45DE4, Opus, system--bad system.)But, all you can say is, "The distributor has been replaced".

The Lucas 25D4 distributor was used in the MGB, and all other British cars, from about 1962 to about 1974. In the US, the basic specification number, stamped into the body of the distributor along with a four digit week/year code, was the same from 62 until about 68. After that, the specification, indicating the mechanical advance curve and the vacuum advance specification, changed on an annual basis as the Brits had to make the engines meet US emissions controls. Thus, you need to know what the specification number of the distributor is to have an idea of what the required ignition timing is. (My 68GT, which has the original Lucas 25D4 distributor to specification number 40897, requires a dynamic timing of 20 deg BTDC at 1,000 rpm as its basic timing. Other specifications vary from this figure.)

So, until you know what specification your distributor is, you do not have a clue as to what the timing should be. And, a clue is all you will have.

Over the years (something over 30)many of the distributors have been "rebuilt". Not always with the desired degree of competence. Paul Hunt noted that one rebuilt distributor he purchased only hand one mechancial advance spring. One I purchased had a mechanical advance cam which allowed the distributor to achieve 80% greater mechanical advance than was proper for my engine.

Thus, you need to determine what the specification number of your current distributor is. Then, you need to determine if that specification was ever used on the MGB. (Paul Hunt's website, "The Pages of Bee and Vee" give the MGB distributors and their advance curves.)Then, you need to determine if the advance curve of your distributor approximates the advance curve of its specification number.

With such information, we may be able to provide some focused assistance.

As to cam timing, that may, or may not, be an issue. What you need to look at is the amount the valves are opening. The cams for the rubber bumper cars are noted for being soft. They will wear down the lobes of the cam and the valves will not open to their proper depth. This can be checked by the use of dial calipers to determine the height of the valve spring cover above the height of the cylinder head or by using a dial caliper to determine how high the back end of the rocker arm comes up.

You could have several problems and there is not sufficient information in your post to allow us to focus on what may be causing your problem.

Les
Les Bengtson

David,
Sounds like the figure you are using is well out, so set the timing from the high end: plug the vacuum line and set the timing to about 30* @ 3000rpm. If you don't have an adjustable strobe, use the timing marks on the cover as your measure and put a dab of Tipex to indicate 30* on the crank pulley. Once you have set it at a steady 3000rpm, measure what the timing reads at 1000rpm to make sure your mechanical advance is working OK. It will usually have dropped to somewhere between 10-15*. If all is well, make a note of the figure and use that as your timing figure next time around to make the process quicker. As a final check, drive the car and listen for pinking. If necessary, back the timing off a degree or two until it just goes.
Steve Postins

These days, with so many departures from factory spec not least of which is fuel, the only practical way to set timing is as Steve describes which is to the maximum it will take without pinking at any combination of throttle, opening, revs and load. Even then if you live in a flat area and go touring in a hilly area you will probably find it will start pinking on hills. However with low compression North American engines you wil almost certainly have different pinking characteristics to high compression UK engines which I (and presumably Steve) have. A vacuum gauge can give a good basic setting taking into account all the variables on your engine (I got better performance *and* economy when using it on a different BL car, but ended up with the same setting on my MGB), but this will take no account of the vacuum and centrifugal curves at higher throttle openings and revs so you may still have to tweak it.
Paul Hunt 2

David, try setting the idle to the slowest speed you can get it to, then set the timing at about 10 degrees BTDC. This will get you close to where you need to be. By setting the timing at an advanced rpm, you never know exactly how far along the mechanical advance is in the distributor, especially in an old worn unit. Do this to set your baseline, then follow Steve's directions to check your total timing, and for final tuning. Feel free to go up to 4500rpms to verify the timing is "all in" as some distributors utilize a very heavy advance spring that requires a high rpm for full advance.

I've noticed that most worn distributors have around 24-30 degrees of mechanical advance measured at the crank, despite the stamping in the distributor that reads 10 or 12 (degrees.) Some provide as much as 35 degrees mechanical advance. In other words, don't trust the specs for your distributor.
Jeff
Jeff Schlemmer

The stamping in the distributor is in distributor degrees, whereas what you measure at the crank is crankshaft degrees. Crank degrees are double distributor degrees, so reading 24 when stamped 12 is correct. Only if the stop-plate inside the distributor has been ground down, or the weight holes and spindles are *very* worn, should you get more maximum centrifugal advance than the stamped value. However stretched, weak or missing springs will allow that maximum to be achieved at a much lower engine speed and this can cause mid-range pinking.
Paul Hunt 2

' Just read this thread, mostly because Dave is a friend and I was going to throw in my 2¢-worth about the Z-S diaphragm. But while the symptoms are identical, I see that diagnosis has been eliminated already.

But I have sure learned a LOT from reading these posts. So even though I wasn't the intended recipient, I have to say thanks to all! It's very likely that some of the explanations given here address some problems that have stumped me in the past.

Allen
Allen Bachelder

But Dave, have you checked to make sure the piston isn't sticking? If it's not lifting, it could cause the same symptom as a bad diaphragm.

FWIW,
Allen
Allen

Paul, my point was that a distributor marked "10" may give you 24 or 26 degrees advance at the crank because of wear. It should be corrected when rebuilt to have 20 degrees advance at the crank and most rebuilders don't make that repair. In fact, the original builders didn't appear make that check either. MOST distributors I check are wrong, with typically 2-4 degrees more advance than marked. And yes, I double check my measurements to verify the tester is accurate - unless my timing light and tester are BOTH wrong and coincidally match.
Jeff Schlemmer


Paul,

Thanks for the explanation of distributor timing and crank timing. I'll need to look at my distributor to see if it's 12. I think I remember it being 12 though.

Jeff,

I set the timing using your method and it helped. How do I check the timing up to 4500 rpm? How do I get a reading for this? More than likely I'll buy a eurospec distributor before much longer.

Allen,

Yeah, the air piston was sticking a while back. After cleaning it up, I think it's okay now. ' Will have to double-check. What can be done to get it moving more smoothly besides cleaning it with fine steel wool?

Thanks to everyone here for their help.

Dave
David Plantz

With so many variables, endless list of possible changes. It is hard to really know what the timing is or should be. I have found that at times on different cars even the timing marks themselves are off. (crankshaft marks) I have used the maximum vacuum settings to get things in the ballpark, then adjust for little to no pinking. I have found this to work almost all of the time. Timing at 4500RPM I believe would be not very usefull, as the maiximum advance in the distributer would have already been reached. IE same timing as at 3000RPM.

HTh

Ron
Ron Smith

Depends on the distributor, some max out at 2200rpm, some not until 6400rpm. Typically for a 45D distributor fitted to a North American spec it is 2600rpm, but David has a 25D of unspecified reference number. As Ron says the only real way with cars with worn parts, non-standard parts (like a 25D distributor in place of a 45D) and very different fuels to those in the 70s the simplest way is the maximum advance it will run without pinking. Other that that you are into the realsm of rolling-roads to determine the ideal curve for your engine, and finding and tweaking springs etc. to match. Bad enough with a 23D or 43D distributor without vacuum advance, much more difficult for one with.
Paul Hunt 2

David, 20 deg. Watch it go.
Victor Gardino (CO)

Ron, my whole point of checking the timing at as high as 4500 rpm is that maximum advance WOULD be reached. Most 25D distributors have one very stromg spring that doesn't fully advance until the 4000 rpm range. If you are trying to achieve a total timing amount of 30-34 degrees, you have to allow the mechanical advance to be "all in."

David, how is the car running now? Any progress? You would need an advance timing light to check the timing at full advance. They're cheap on Ebay or about $75 at Sears.
Jeff Schlemmer


Hi, Jeff. I set the timing at the lowest possible idle speed to about 10 BTDC like you suggested. That helped it quite a bit. My next move will be to check the cam for wear. Setting the timing helped the power a bit, but it's still not as strong as my '77 B.

Many thanks,
Dave
Dave Plantz

Dave,

You do not seem to be getting anywhere so let me see if I can help.

You state you have a 1980 MGB with a 25D distributor.

Here are my questions:

Is your car de-smogged. No air pump and the works?

You never mention what is the distributor number on this 25D. Is it a 40897?

Has your car been modified. ie; same pistons, same early cam, same single row sprocket?

Without this information it is hard to help soplease answer...

Ray

Ray 1977 MGB

Dave:

My 'BGT came with a new 45D distributor. If you want to see if a later distributor makes a difference, come on over.
Kemper

This thread was discussed between 22/11/2005 and 13/12/2005

MG MGB Technical index

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