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Intermittent Stumble - Diagnosis?

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Shinsen774

Shinsen774

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I have a stock '64 MGB except for petronix electronic ignition and an aftermarket fuel pump. It has run perfectly for the 2 1/2 years I've had it. The last week or so, though, when cruising at about 2500 RPM more of less, from time to time the engine will stumble and lose power, like it's not getting enough petrol. I can mash the accelerator and coax it to come back strong. It runs fine most of the time, this only happens from time to time. At idle, it usually runs fine, but a couple of times it has started to stall, and did stall once. Started
right back up normally. I have the glass fuel filter with the replaceable element. Took it apart and it is clean. We have ethanol here, but that's not a recent development.

What suggestions do y'all have for diagnosis and repair? Thanks in advance.

Jim
 
First thing that comes to mind are the carb dashpots. Checked 'em lately? Or, with the change in temperature, maybe the oil isn't doing its thing...?
 
Jim, Have you checked your fuel pump pressure? The pump might be intermittently dropping off. Then again it could be in the ignition where it seems most of these problems originate and we think their fuel related. I had a similar problem where the engine would not idle properly and thought it was a fuel problem, found out it was the distributor which had a wobbly shaft. Had it rebuilt by Advance Distributors, and it runs perfect. Best move I ever made. PJ
 
Carbs seem fine. Fuel pump seems fine. My theories are bad gas, vacuum advance leak, or coil going bad.

When I suck hard on the end of vacuum line, there is no movement in the distributor at all. Air comes through the line constantly. I checked the line itself, no leaks. Would the syptoms I'm experiencing be consistent with a leaky vacuum diaphragm? If so, what's the fix and how difficult is it to repair? What does the timing vernier do and how should it be set?

I found a brand new new coil in my parts bin that came with the car when I bought it. It's the right Moss PN for the car and tested OK. Put it on the car just to see. The problem still occurred, not as bad; inconclusive I guess. The old Lucas coil looks like someone hit it with a hammer a few times and might have been on the car for decades anyway.

Thanks for your insight.

Jim
 
The vacuum diaphragm assemblies are replaceable as a unit, but as far as I know, not repairable. Moss might sell them, I'm not sure. I would think that if you need to replace the vac unit, the whole distributor should be rebuilt. If the shaft has any wobble in it, you'll never get the engine to run right. Advance Distributors does an excellant job of rebuilding them. Jeff will even set it up to your specific driving needs.
Here,s the link; https://www.advanceddistributors.com/
Advanced Distributors: Distributor Repair at its Finest!
 
I'd say he whips up a mean dizzy. He got a thumbs up from me.
IMG_6315.jpg
 
New fuel pump cured the problem.....but in the process I also discovered that the vacumm advance on the distributor isn't holding a vacumm. Next project is getting that off and sending to Jeff Schlemmer for a rebuild.
 
Distributor Doctor in England rebuilds vacuum units.
Had one done.
New innards, and once they cut it, looks like a new lip that covers both parts.
 
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