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MG MGB Technical - NEW CAP & ROTOR NOW IT DOES NOT RUN

OK started up the 79 MGB drove it out of the garage, shut it off, replaced the cap and rotor and it has not run since.

Put old cap and rotor back on and it still does not run. The ignition was replaced with a Lumetronics unit 4 years ago. I thought perhaps I mis-wired the dist cap so I hooked up a timing light. Yes the timing light does light, so I'm getting spark.

I set the engine to TDC (removed # 1 plug and manually turned the engine using a screwdriver to "feel" the piston) and the wires to 1-3-4-2 turning anti-clockwise.

As it pouring rain I am not about to work on this problem until the weather clears.

I am open to ideas. What else can I check, or what have I done wrong??

The only elelctrical testing equipment I have is a timing light, and a circuit light.

cheers

Gary :{D
79 MGB
gnhansen

Maybe knocked the sensor (and/or chopper blade) out of line for the Lumetronics? I'm assuming it's optical. Are you getting a flashing timing light? If so, I'd try resetting the timing after checking the sensor is secure.
Steve Postins

Gary - "I set the engine to TDC (removed # 1 plug and manually turned the engine using a screwdriver to "feel" the piston)"

Both #1 and #4 are at TDC at the same time, so using a screwdriver to set #1 at the proper TDC doesn't work. Remove the plugs again and put your thumb over the #1 spark plug hole and turn the engine over. When the #1 piston is at TDC on the compression stroke (the proper TDC for the engine to run), it will push your thumb off the hole. Cheers - Dave
David DuBois

Check to be sure that #1 is at TDC on the compression stroke
(both In & Ex valves closed) - not on the exhaust stroke (Ex valve
open).

You'll need to remove the valve cover for this.
Daniel Wong

I'll second David's test for TDC on the compression stroke, much less bother than removing the rocker cover.

The other thing is to connect the timing light to No.1 and point it at the pulley while it is being cranked. This won't show the plug leads being two positions out, but it will show if the timing is about correct i.e. 8 to 10 degrees. Disconnect manifold vacuum while doing this in case vacuum under cranking is enough to cause confusion.

After cranking a bit check the plugs for a strong fuel smell, just in case a coincidental fuel pump or carb problem is the cause. If they smell strongly of fuel, and you have a regular and consistent flash from the timing light, then it almost certainly can only be lead order or timing.
Paul Hunt

OK.

David: I followed your direction and used the thumb over the spark plug hole. Perfect. Rotor now pointed at a "2 o'clock" position.

Pulled all the wires out and re-set them in order - 1-3-4-2 anti clockwise.

Almost started. Pulled all the plugs. All were wet, engine flooded. Cleaned and dried the plugs. Engine started held fast idle for a few minutes. Still stumbling. Replaced new rotor with old rotor and have new dist cap - stumbling gone. Runs great.

Next I will clean the carburetor then replace the plugs.

Paul: Yes I used the timing light but did not disconnect the vacuum advance. You are correct, I did not get a "true" reading on the timing marks. If I had followed your direction with vacuum advance I am sure my readings would have been more accurate and consistant.

Thanking all of you for your help.

Cheers

Gary
gnhansen

This thread was discussed between 15/11/2008 and 16/11/2008

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