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MGB MGB missing and perhaps knocking under load

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Jerry

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Just finished putting the head back (valve job) on a friends MGB and when we went to start the car there was no spark at the plugs. The pertronix test as dead, the coil kept changing the resistance across the terminals. Coil is probably original. Anyway, replaced both, which require new timing since the new pertronix put the unit in a different spot on the plate versus the old unit. Got it started and it revs fine sitting in the garage. But driving it, there is a miss about 3000 rpm and perhaps some knocking. So I assume the timing is still off. Would you guess that it is acting like :
too much advance?
too retarded?

Anything else to check?

Jerry
 
The obvious thing to check is to hook up a timing light and see what's happening. Have you done that? If not, how did you set up the timing initially?
 
mixture? lean spot on the needle compounded by a slightly lean mixture to start? (and always check oil in the dashpots)
 
I will check the oil in dashpots this am. I will also pull the spark plugs and look at color. Question on the Zenith carbs, how many flat of turn cause a big effect on changing the mixture?
Jerry
 
Try checking the resistance on the spark plugs with an ohm meter. I had petronix wires, and when tested one was at 400,000 ohms, the others in the 25,000 to 50,000 ohm range. Got some cheap NAPA low resistance wired, but I'm not running a petronix distributor. Easy enough to test, though.

What is your advance at 3,000 rpm? Is it within spec?

I'm no expert, but I don't think timing would generally cause a miss unless it was way, way off, and them more of loss of power, a backfire or a definite misfire. A knock, to me, is something else, sort of what you get when too low octane in the fuel. If you hold the engine at idle at 3,000 rpm, does the engine run smooth, or can you see it start to shake?

EDIT: I meant, "check resistance on spark plug wires!]
 
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Zenith carbs? Not SUs? Don't think you can adjust mixture on Zeniths?
 
I did put an ohm meter on the wires and they were close on the readings. I have not read the timing at 3000 rpms yet. I will check that tomorrow, what advance should I expect? It is Zenith carbs and they can be adjusted. I made it a little richer and will try it in the morning. More info, we had problems with the rings no setting so we replaced them. Compression prior was only 100 psi, and now is 125 with only 40 miles on new rings. So the compression went up 25%, so I am thinking that it needs to be richer fuel???
Found a new question for you. The vacuum advance has a tube that goes to a unit over on the servo. What does that unit do? and it has two wires to the unit, does it matter how they are connected. I know they were disconnected when the engine was taken out.

Jerry
 
I'm a MGA guy, so can't help too much. If you replaced the valve guides, a valve could be sticking as the guides heat up and expand. You might double check your valve lash. But a miss is probably ignition, unless can there be an air leak at the manifold. Listen with a tube. Finally, maybe it was always there and just wasn't noticed.
 
Update: put it on a CO2 and carbon machine. CO2 says it wants more gas. We adjusted as much as possible but the smog part of the carb will not allow much. Needle is old, so examination showed a flat spot, sanded it down. Also checked the timing again with another timing meter and that did some adjustment. So car drives better, more power, minimal pinging, but that miss at 2500-2800 when running steady is still there. Must be ignition but the Pertronix is brand new, rotor, wires and plugs have 700 miles on them. Coil is new. (3ohm).
Car is a 1978 .
I called Pertronix and they suggested a lower Ohm coil which I had. The instructions say a 3 ohm for a 4 cylinder, but I put the 1.5 ohm on as per their instructions. No change. Then they had me test the coil under load.
volt meter positive to the positive side of the coil, and ground the negative side of the coil. 10.3 volts. passed that test. so according to Pertronix, it is not their unit that is the problem.
Still stumped.
Jerry
 
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Another update with a solution:
We changed out the distributor (45D) for a 25D that came from a running car. No more MISS. So the first distributor has a problem but I don't know what it is . At least we know how to solve it.
We can buy new 25D and 45 D distributors. I think the 45D just has limited advance curve for emissions control. Any of you know the difference?
Jerry
 
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