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MG MGB Technical - Problems Timing '76 MGB

Hello. First off i'll describe the car: I have a 1976 MGB that has the emissions controls removed, carbs converted to twin SU HIF4s, camshaft replaced with new one for an early 70s B, distributor converted to early 25D with pertronix electronic ignition installed. Now, the problem is that when I try to dynamically time the car with a timing light, the car runs poorly and is somewhat difficult to start again when I have set the timing to around 13 degrees BTDC at 1500 RPM, vacuum line disconnected and plugged. However, the car runs fairly well when timed at approximately 35 degrees BTDC at 1500 RPM for some reason. I checked to see if the TDC mark on the timing cover lines up with the mark on the pulley when the piston is raised when firing on the number one cylinder by looking down the number one spark plug hole and the timing marks do line up in the correct spot, which would make it seem that there is nothing wrong with the pulley (as in it appears that it hasn't been changed for an earlier one by a previous owner and it appears that it is not delaminated). I have tried timing the car repeatedly and its always the best at approximately the same spot, 20 degrees off from where it should be. I really have no ideas left as to what's wrong, any help is much appreciated.
- Mike.
M. Carlile

Mike. You need to know the specfication of the distributor to have any idea of what the advance curve is. Then, you need to test it, either on a distributor test machine or on the car, to see if it is working properly. I have seen them with the mechanical advance weights rusted in place and no mechanical advance possible.

When the engine is running at a steady rpm, it will tolerate more advance than when accelerating under load. This is the principle behind the manifold vacuum take-off.

The standard advance for the early dizzy was 20 deg BTDC at 1500 rpm with the vacuum advance disconnected and the line plugged. If you can run 15 degrees more than this without pinging, I am surprised.

As to the accuracy of the timing marks--you need a dial indicator to indicate in the rise and fall of the piston to determine the true top dead center. Looking at the piston alone will not do it. It will be at the top for several degrees before and after the actual top dead center point.

You need to do some more checking to find out what your total mechanical advance is at about 3,500 rpm. You have a low compression engine, unless it has been rebuilt, which will tolerate more advance than an HC engine. The grade of gasoline you are using can also have an effect on how much advance the engine will tolerate--the low compression engines were designed to run on regular gas.

To provide any real help, we need to know the advance curve of the dizzy, the maximum advance you get to at what rpm and the grade of gas you are using. Les
Les Bengtson

Just for fun...

I'd also set the engine at TDC - then pull out the
distributor (2 bolts) and look to see if the distributor
drive gear cog (inside the engine) is rotated at it's
proper angle.

This isn't hard to do.

Most manuals that show how to strip & rebuild
a B engine will show this detail.
Daniel Wong

Les, i've only put premium fuel in the B (92 octane), the distributor number is 40768 with a 16 degree cam and free weights, not rusted in place (which i have found is not from an MGB, but the car's original 45D4 points distributor gave me pretty much the same problems, timing at around the same point). I have used a dial indicator in the number one spark plug hole and confirmed the TDC point at the marks do line up on the timing cover and pulley at the right point. I can advance the timing at maximum another ten degrees, to around 45 BTDC, but it starts running worse and worse and then dies at that point. I can also run the car at around 13-15 degrees BTDC but as i said, it runs much better and smoother at around 30-35 degrees BTDC and it starts up better there. I have not put the car under load because I am concerned about where it is timing and i don't want to push the ccar back home, so i have been warming the car up in the garage or driveway until i can take it off the choke then using the timing light. Daniel, i have also pulled the distributor a number of times now and checked the distributor drive and it appears to be in the right position according to the workshop manual. Is it possible that worn distributor springs could cause this problem? Thanks.
M. Carlile

Mike. Worn springs will cause the maximum dynamic timing to be limited (less than the full, specified advance) when you set the ignition timing dynamically. For instance, if your should have twenty degrees of mechanical advance at 1500 rpm and your dizzy, when new, gave ten degrees of mechanical advance, you were running ten degrees of static advance and ten degrees of mechanical. With this setting, your 16 degree cam (dizzy degrees equal one half the crankshaft degrees) would have given you a total of 42 degrees at max advance--the 32 from the mechanical advance plus the ten degrees static.

If the springs are worn, the weights are not restrained properly and, perhaps, you are getting 25 degrees of mechanical advance at 1500 rpm which means your static timing will be before top dead center and your maximum advance will be about 32 degrees overall.

It is common for an engine to tolerate more advance when at idle or operating at a steady speed. Hence, the real test of timing is taking the car out on the road and driving it to see what happens. If you hear pinging, you need to back off the timing some.

A couple of thoughts. First, you are running a higher grade of gasoline than your engine was designed for. This will allow more ignition advance than standard without pinging.

Second, check out the full advance of your distributor while on the car. Run the engine up to about 3,500 rpm and measure the advance (with the vacuum line to the vacuum advance disconnected and plugged). Then, compare the maximum advance you have obtained to the maximum listed for the distributor you are supposed to have installed (i.e. the factory dizzy--Paul Hunt's website has this info). Your engine, if still stock or rebuilt back to near stock specifications, should have a low compression engine. These will tolerate more advance than a high compression engine, especially when you are using the premium gasoline.

But, the real tests will have to be made on the road either under hard acceleration or when driving up hills under full throttle. Les
Les Bengtson

Mike, You mentioned changing the cam to an earlier version cam. Did you also switch to the double row sprockets and chain? Did you use factory timing marks to time the cam or did you degree it with a degree wheel? If your camm timing is off that could cause the engine to run best with a change in the ignition timing.

If you have not verified the cam timing you may want to try the quick check Chris mentioned in another thread. Turn your engine to TDC with number four cylinder on the compression stroke, both valves loose.
Adjust the valve clearance on valves 7 and 8 to .120".
Rotate the engine one turn to place number one cylinder at TDC compression stroke. Measure the clearance on valves 7 and 8. They should have the same clearance. If they do not gently rotate the engine so the clearances are equal. Measure the distance from the balancer timing mark to the TDC pointer on the timing cover to determine the approximate setting of your cam. The timing pointers are 5 degrees apart. If you are more than 3 or 4 degrees off in either direction your cam should be rephased.
Clifton
Clifton Gordon

Clifton, I did change to the double row sprockets and chain when changing to the earlier camshaft. I used the factory timing marks to time the cam. I am going to attempt to verify the cam timing tomorrow using the method you have suggested and i'll post the results. Thanks.
M. Carlile

This thread was discussed between 03/09/2004 and 05/09/2004

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