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MG MGB Technical - timing and back fire

I am puzzled. I am helping a friend get a car on the road and we cant seem to get it dialed in. When timing this car by the book, 10 degrees btdc it will spit and backfire and is weak. If you advance it well past where the book tells you to, it will run great. We then encounter the car wanting to turn over slow while starting as if the compression is too high. Once it starts though, it will run great and doesnt miss a beat or spark knock. Does anyone know why the slow start and why it is so far from where the book says? He is also running a weber dgv with a pertronix electronic ignition, would that change anything?
Bryan

If you have the timing too far advanced the engine will "buck" against the starter during cranking as the engine fires too soon and the starter can't overcome it.

As far as the poor running, has someone had the engine apart and is there a chance that they installed the timing chain off by one tooth? you may need to use a degree wheel and check the cam timing
John H

Check the mechanical advance in the dist. If it is stuck in the advanced position, it will do exactly this.

FRM
FR Millmore

Bryan,

there are different pulleys used on the B-Series engine that also used different timing marks. If you take out no. 1 spark and try to level out TDC, you can be sure about the position of the mark on the pulley.
BTW, sometimes the outer part of the pulley seems to 'wander' in its rubber mounting, leading to courious radings too.
One other thing to look for is the positioning of the HT cables at the dizzy. If they are set 90° off, it also can lent to the observations you are reporting upon. Just set no. 1 to TDC in firing position and check whether the rotor arm of the dizzy is pointing to the right lead.

Hope this helps

Ralph
Ralph

Check you have the direction of rotation right.
Stan Best

This is actually a pretty common event. I'm guessing its eaither an earlier car (pre-70) or has an early distributor in it. The primary spring in that distributor tends to fatigue dramatically, causing the advance to come in very early. You can get it to run better by advancing the timing at idle (to make up for lost advance that's already "used up" because of the weak springs). Yout total timing at 3000 rpms (probably 2000 in your case) will be the same as when the car was new and set by the book. You should consider having the distributor rebuilt including advance spring replacements.
Jeff Schlemmer

I did not meantion when this is set at 10 degrees btdc the distributor is retarded back as far as it can go. The vacuum advance unit is actually touching the oil pressure line. You rotate it about an inch forward and it will run great, which is around the location I have always seen distributors in B's sit at. If the timing chain was off a tooth, would the car still run? I am leaning towards the mark on the harmonic balancer being off.
Thanks for all your input.
Bryan

I agree with Jeff . I have an early B distributor (66) in my 73 B and had the same problem with a weak primary spring. But I believe your problem may be two fold, weak spring and or too high rpm when dynamic timing.
If you stactic time your engine and then check timing when running it will seem way advanced. This problem is exsasberated by the fact many cars will not idle down the 600 rpm needed for dynamic timing (early Dist) causing the centrifical advance to come on early. Ignition timing should be set with no advance coming into play. If you adjust your dynamic timing at this point you will be in effect retarding the timing.
A friend of mine who builds racing engines gave me a mess of dist springs for small block Chevy distributors, the length is very close. I found one close to the spring rate of the primary spring and with some tweeking it worked great, also cured slight pinging I had below 1900 rpm.
Charles O'Brien

I am experiencing the same thing. I too have a Weber downdraft 2-bbl carb and a Lumitronix ignition. My engine was completely rebuilt about 7k ago and an mild grind Isky cam was installed along with an alloy head.

I was experiencing a "lag" whenever I accelerated especially from zero up through the gears.

I noticed the timing mark on the crank really didn't seem to be "right" So, I moved the crank mark to the last timing mark and then notched a new crank mark at the first timing mark.. Then I proceeded to set the timing.

The "lag" is gone, more power, but now the engine bucks and backfires when decelerating and as well as diving at lower speeds around 2500 - 3000 rpm. This is after a fresh set of plugs were installed and the carb cleaned along with the K&N air filter.

Yes I do get a slight pinging upon occasion, especially under "load"

Now, if I retard the timing from my current setting, will the above condition correct itself??

Also the carb seems to running rich. The old plugs were balck with carbon as is the tailpipe.

How does one adjust the fuel mixture in these doawndraft Webers??

Any thoughts on this

Thanx in Advance

cheers

Gary :>{D
79 MGB
gnhansen

This thread was discussed between 17/09/2007 and 22/09/2007

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