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Missing Woes on my 72 B

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milomind

milomind

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72 MGB with Mallory Dual Point Distributor, and Weber DGV. I'm having some funny "missing" episodes with the B. It seems to want to miss, and sometimes badly so when it has warmed up. I changed the condensor on the dist, on the advice of a forum poster, and dynamically timed the engine, but the problem persists. I'm not very handy with points, but am very handy with a carb. Any advice on where I might look otherwise? It idles well, and doesn't diesel when I cut the ignition. I'm afraid I'm at a loss.
 
My '76 (single ZS Carb) exhibits the same "miss" at full operating temp too... errr more like an over-lean condition. However, I think my problem has to do with a fuel supply problem. I have a "see through" fuel filter under the bonnet, and when I experience this "miss" I've noticed the under bonnet filter is always empty. I'm not sure if it's at all related to you're problem, but I've been noticing with my car that the over-lean condition acts like a badly mis-firing engine....
 
I have a clear underhood filter also. I wondered about it possibly being the fuel pump, but I don't want to jump to conclusions. I have't noticed it unless I'm in gear however, so I can't see the filter when it happens.
 
I think transparent filters create much confusion. If you think about it, there is no reason that a horizontally mounted filter "should" be completely full. There is no way for trapped air in the upper part of the filter to get out.

If you suspect fuel delivery problems, measure how much is pumped out in 15 seconds. Disconnect at the carb & run the fuel into a container. If it is around 1/2 pint there is no problem. Most small engines could get by with 1/2 of this flow.
D
 
I have wondered ocassionally if those dual point systems' extra weight effect the action of the vacuum advance at all. Your symptoms (load only, warmer engine, etc) indicate to me that the timing may be slightly thrown off by the load. Have you checked the timing advance with a timing light while keeping the advance connected? Not so much the readings, but the smoothness of the advance as vacuum (engine speed) is increased and decreased? Check for vacuum leaks, spring tension and proper lubrication as well as the smoothness of the mounting plate on the distributor. Also make sure you have adequate wire clearance inside the distributor for full motion of the advance-and return? This is one place where cleaniness is next to godliness...and saves a whole lot of frustration under power.

Just a few possibilities to consider after you eliminate the possibilities of fuel starvation, which I identify with my "second gear highway test" Pick a nice open stretch of uphill rural road that can be driven at 30-35 MPH in second gear for a half mile or so...fuel delivery problems will become self evident under such conditions /ubbthreads/images/graemlins/rolleyes.gif

Good luck!

Bruce /ubbthreads/images/graemlins/cheers.gif
 
The distributor I have is a mechanical advance, but I have a feeling the problem is points related. I am just not very good with them, but at this time I don't have the funds to upgrade to electronic ignition. I am going to pull the distributor today, and regap the points, and lube the works. Hopefully that works.
 
I had a mallory dual point distributor back in the 60's and it is very important to get both the points adjusted the same. At that time mallory advised for the points to be set out of the car on a machine and block off one set with a piece of paper or thin cardboard and adjust the other to a recommended dwell value. Then do the same to the other one setting it exactly the same as the first. I used to do that in the car by turning the motor with the starter after disconnecting the coil hot lead so it wouldn't fire. It has been a long time ago but I believe the dwell on one set was half the total of the two together.
At one point I used a drill to turn the distributor with it out of the car and adjusted it by dwell with jumpers from the connections in the car.
Look at your instructions and see what it says. I believe both sets of points were set at .020" but that was in a V8
Bob
 
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