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MG MGB Technical - Retarded Timing

Have a strange problem with a 79B. When the engine heats up to normal range, the twin stock fans kick on, and if at a idle, the rpm drops from 850 to less than 300, sometimes stalling if fans are on when coming to a complete stop. Engine has had a lot of work, new cam, rings, head rebuilt, crankshaft, bearings, thrust washers, and complete clutch, problem existed before all this work was done. When timing light is hooked up, strong steady idle with timing at 11 BTDC, with no vacuum hooked up (weber downdraft carb, new). Fans kick on, timing retards to 10 or 12 ATDC, engine stumbles, and low rough idle. Problem existed when the orginal Opus dizzy had the crane fireball electronic ignition installed, and has a different dizzy, lucus 45 points type now. Alternator was also changed to the Bosch unit, no difference. It seems that the coil, Lucas Sport coil, also changed from the Crane Fireball coil, no difference, fires a little later after the points open when the fans are on. New battery, no difference. Could it be a faulty ignition switch (white ignition wire to dizzy pos terminal losing power)? Seems the only thing left. Never heard of this problem before. Car starts great and runs very strong with or without fans on, until coming to a stop with fans on.
Thanks.
Bob Johnson

Bob. A most interesting problem. I also have a 79 for my personal use and a 77 that is my daughter's car. Both have the stock, two fan set up, points type distributors and the Weber DGV carbs. Neither demonstrates the type of problems you describe. Hence, I am at a loss.

As to the vacuum advance. You should have a port on the bottom section of the carb that will provide ported vacuum for the vacuum advance system. Perhaps this might make some form of difference.

It is my belief that your change in timing is due to the drop in engine rpms. The centrifugal advance is partly extended at that point and, when the rpms drop, the weights are retracted by the springs resulting in your change in ignition advance.

If you are new here, you do not realize that we have a secret weapon for electrical problems--Paul Hunt, a mechanical-electrical engineer who is very generous with his help and time. From what little I understand about electrical systems, mostly learned from Paul and a few others here, it would seem that the fans kicking on are robbing the rest of the electrical system of the power necessary to do the job. I also suspect that your dash tach is inaccurate. (Many are and there is an article on how to calibrate them on the web. However, I have never known a car that would idle much below 500 rpm and then only poorly. If your will idle below that, it may be the tach is inaccurate.)

It would seem that your problem is the fans are drawing more current than the alternator and battery are capable of supplying. A quick check of this would be to splice a toggle switch into the line to the fans, allowing you to turn off the fans when the problem seems to begin. If the problem goes away, it is definitely related to the current flow to the fans vs what is available for the remainder of the system. If the problem does not go away, it is related to that engine temperature and should be examined from that aspect.

If it is an excessive current draw from the fans, I would expect that the use of the headlights, turn signals, emergency flashers and windshield wipers would exacerbate the situation. From what I remember of Florida monsoon season, I would prefer to fix this problems sooner rather than later. Hopefully, Paul and some others will provide more definitive guidance in a few hours. Les
Les Bengtson

Hi..

A quick check with a voltmeter to measure the voltage fed to the coil would be very informative.

My UK spec 80 was fitted from new with a ballasted coil (ie a 6V coil with a ballast resistor).
The resistor is actually just a length of intentionally high resistance wire in the loom.
If you still have the ballast resistor in place, and a 12V coil, (or if the electronic ignition is fed from the ballasted feed) this could cause probs.

Don
Don

A little more info for the two replies. I posted a question some months ago about low idle with fans on, and was then convinced it was a mechanical problem, not being tuned good at idle. The engine really couldn't be tuned because there was a fast race cam installed by the PO, that fact I didn't know until the MG shop worked on the engine. I had also converted to the Hayden single puller fan, and installed relays in the light and fan circuit, that had no effect. Bad vacuum reading indicated other engine problems. I have a great MG repair shop nearby and the B has spent the last two months down there. Radiator was repaired, head was repulled and sent to their shop to redo the blotched job the machine shop (valves were sticking) did that I first used, pistons were pulled, and new rings installed, and a good honing job was done. Prior rebuild by PO had no cross hatch pattern in the cylinders, and the rings never set up correctly. Cam was replaced by the stock version Crane cam. I had replaced the dizzy (opus) with a new euro spec one, and their analyzing machine found every cylinder was firing at a different timing degree. This was caused by a defective bent dizzy shaft in the area where the points lobes are. They replaced dizzy. The engine is a 1000 % better performing since I got it back. They rewired and got rid of the relays, and reinstalled the stock fans, needed one new motor. Coil wire bypasses the high resistence wire, they cleaned up the wiring very nicely. As you can see, many problems existed that have now been dealt with. I will be bringing the car back to them, before the 500 to 750 mile return for head stud retorging, valve adjmt and oil change and a recheck. Just wish I could make the 500 miles event, but the stalling out in traffic is no fun.
Thanks.
Bob Johnson

Bob-
Check your battery for a defective cell. If the battery is producing less than 12 volts, then the system is underpowered and the additional current drawn by the fans will drag the ignition down. I know that in theory the alternator should be producing enough power to overcome this, but I had a similar ignition problem on my Honda Del Sol VTEC which defied diagnosis by every Honda-trained technician. I replaced the battery after the old one died, and the problem disappeared.
Steve S.

Bob, you stated

"Fans kick on, timing retards to 10 or 12 ATDC"

I don't think static timing should be less than
0 deg BTDC with most MGB distributors. The
engine won't idle well at timing retarded to
10-12 deg. ATDC.

There was a similar problem posted a while back.
The distributor springs were very weak, so
the static timing was actually set way too
retarded in order to obtain the proper advance
at 1500 RPM.

Les also mentioned the ported vacuum advance issue.
The stock 79 distributor used manifold vacuum,
which is high at idle. The 45D points-type
distributor, depending on number, uses ported
vacuum as Les points out, which is near zero
at low RPM.

If your distributor is erroneously set up with
manifold vacuum, then your timing will be excessively
advanced at low RPM, and you will also be
setting static timing too far retarded to
compensate.

I would carefully check your static timing,
source of vacuum advance, and distributor number
for proper vacuum connection. www.mgbmga.com
has a detailed list of distributor nos and specs.,
as does Paul Hunt's site.
Ronald

Also about dynamic timing -- it should be set with
vacuum disconnected on MGBs.

You can roughly set 0 deg static timing by observing
when piston 1 is at TDC. With the valve cover off,
watch when valves 1 and 2 are closed, 7 and 8 should
rock back and forth. The distributor rotor arm
should be at 1 o'clock (right under the connector
for plug #1 on the distributor cap.) I would use
this as a starting point and then recheck dynamic
timing. Make sure you don't have a broken or
missing spring in the distributor also.
Ronald

As someone else has said check the voltage, at the fusebox, 2nd fuse up (white/brown) both with the fans running and off. A 79 has an ignition relay operated by the ignition switch, the relay powers the coil and fan (from the factory, that is) via its own in-line fuse (white/brown to green). If there is a significant difference i.e. more than a volt it's possible the relay is going bad, or a bad connection to or from it on the brown or white/brown. It's also possible that the fan circuit is drawing excessive current and that is causing the volt-drop. I wouldn't normally expect a simple voltage reduction to cause such a major retardation in timing, but with electronic ignition who knows?
Paul Hunt

Over the weekend, I had time to look at this problem again. Set up the timing light, steady at 14 degrees BTDC at normal idle rpm. Fans kick on, engine stumbles while timing retards to 10 to 15 degrees ATDC. Turned the idle speed set screw on the weber a complete half turn to increase idle rpm well above 1000. Fans kick on, timing remains steady at 14 BTDC. Turn idle set screw slowly counter clockwise, about a quarter turn in, engine stumbles, and timing retards. The old chicken and egg question, is the timing retarding because of a electrical problem that involves the coil, and the retarded timing is causing the engine to go to low idle, or is it the pull on the engine that is decreasing the rpm and causing the dizzy centrifugal advance to diminish, thus the retard in timing. Guess the B goes back to the shop early and they can sort this all out. I did test all the battery cells in the new battery, and all had good equal readings.
Bob Johnson

Ah, sounds like Ronald had it. Are you using manifold vacuum? Are you measuring the timing with it connected? You shouldn't. You should set it to 14 BTDC with the vacuum disabled which will need the idle adjustment to be increased, then reconnect the vacuum, then reset the idle back down. If you measure the timing now you should see it is about 30 degrees BTDC. Seems to me the load of the fans is causing things to move past a critical point at which vacuum advance vanishes. With the timing set to 14/30 instead of -15/14 you should find the effect of the fans is less.
Paul Hunt

Re: voltage and ignition timing. A drop in voltage
would definitely cause a points or electronic
ignition to misfire. However, the electronic
ignition is triggered solely by the optical shutter
or magnetic sensor inside the distributor.
The electronic ignition can be thought of as
a relay controlled by the optical/magnetic switch.
Hence, low voltage may cause a failure to
fire, but cannot change the point (timing) at
which firing occurs.
Ronald

Bob. As I understand it, you have a "Euro-Spec", points type distributor. This particular dizzy is set up for vacuum advance off of the carb. In the case of the Weber DGV series this is a port near the base of the carb. For you to go from 11 deg BTDC to 10 or 12 deg ATDC with a drop of engine rpms indicates a distributor problem. As Ronald, and others, have noted, this is just not possible with a properly set up system. Paul Hunt once wrote me about a distributor problem he once had, due to bad springs, where, with the proper dynamic timing, the static timing was after top dead center. From reading your posts, you seem to indicate that when the engine is barely running the timing is about 22 degrees retarded from where it is at about 1,000 rpm. No properly functional distributor shows this much difference. Either you are taking the readings wrong or the distributor is totally worn out. Getting the shop which worked on the car involved is an excellent idea. Letting them know you are interested in learning about the car is another excellent idea. Much of what I have learned over the years has been due to the kindness of various mechanics who were willing to demonstate what they were doing and why. Les
Les Bengtson

Les, actually the motor is on it's 3rd dizzy, opus dizzy with crane cam electronic ignition first, replaced by a brand new faulty euro spec with the bent shaft that resulted in a different timing degree on every cylinder, (the shop found this out), and this was replaced by a spare matching dizzy they had, cleaned up and new points, etc....all of them showed the retarded timing after the fans kick on and engine goes to low idle. I believe opening the idle set screw is allowing the first progressive circuit in the weber to kick in, and help out the idle circuit, this engine just doesn't seem to like the webers at idle, this is a brand new weber from Brit Tek, replacing the old beat up weber that was on the car when I bought it. Both units had the same problem, so it isn't a particular unit. After the shop tuned it up, they capped off the vacuum plug on the base of the weber, and the advance relies on the centrifugal advance. With the new cam, it runs so strong and smooth throught the gears, I certainly agree with this. When I was tuning the engine, I was setting timing with vacuum plugged off, and then attaching the vacuum hose back up when I was done. I've been working on cars and outboard motors for over 35 years as a DIY, and have never seen a problem like this. Basically, the head is rebuilt, the engine is rebuilt, it's on its' third ignition system, and second carb, replacement alt, new battery, and the problem persists. It defies logic, maybe an exorcism is needed???
Thanks, Bob J.
Bob Johnson

Disconnect and plug the vacuum advance then measure the timing with the fans both on and off. If you still get the same difference then exorcism is needed. If not ...
Paul Hunt

Paul. I agree. There is something here which does not make sense. First, he is not running the vacuum capsule (personal choice, however, this dizzy was designed to be used with a functional vaccum advance connected to ported vacuum). This means that everything is being caused only by the centrifugal advance mechanism, if there really is a timing problem. We also see that there are about 22 degrees of centrifugal advance being applied at about 1,100 rpm. This from what is supposedly a new and correctly running distributor which has been checked out by a specialist who was competent to determine that the previous distributor, though new, was out of spec.

I think we need to step outside the box and look at such things as bad timing lights, marginal alternators or something beyond the normal, mechanical problems that are normally encountered. It would be interesting to see what the mechanic finds when he examines the vehicle. Les
Les Bengtson

Put the dwell meter on the coil last night and found a dwell angle degree of 84. The points slipped, and I set timing without checking the dwell first.. Guess I need to get used to having a dizzy with points again. Will be setting the points correctly tonight, and then the timing.....Bentley manual for mid to later years RB's lists the 75/76 dizzy as not having the vacuum advance connected, that dizzy is what the shop replaced the faulty eurospec with.
Bob Johnson

This thread was discussed between 27/04/2003 and 30/04/2003

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