MGB & GT Forum

The 'clean plugs, sound like NO FUEL, to me.....................

******** UPDATE*********
So, I have checked the valve movement, pulled the plugs and rolled the engine over, watched and measured, everything is moving fine.

I turned to the carbs; See pictures for what I found ............................................

Checking that now. See update at bottom of thread.

Since the front carb feeds cylinders 1 & 2, and the rear carb feeds 3 & 4, I don't think your lack of firing on 1 & 4 are a carb issue.

I think I'd be looking closer at the distributor. Has something happened to the cap would be my first question. A coolant leak from the heater valve could cause shorting out inside the cap on 1 and 4.

If you have an old dist. cap put it on and see it solves the problem. And just as a double safety replace the rotor as well.

BH

The difference in the damper piston positions could possibly be a problem. When you balance the carbs at idle, the pistons are inactive. When you press the throttle, the piston on the left will encounter resistance later than the one on the right, so when you accelerate, the carbs are no longer balanced. I believe those pistons have to be identical to achieve smooth acceleration.

Recheck your firing order. " 1,3,4,2, CCW "Counterclockwise".

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Cylinder #1 and #4 have something in common. They fire 180 degrees from each other. Clean plugs suggest no combustion event, but do not show lack of spark necessarily. If the spark event is 180* out of synch, it would essentially be running on two cylinders, in your case, 3 and 4. #1 and #4 could be firing on the wrong stroke, and they would just be deadheading.

I would start at the beginning with the distributor. Set the engine to #1 TDC, such that the #1 valves are both closed, verified by actually looking at them rather than looking at timing marks. Then reset the distributor with the rotor pointing at #1 plug wire, which is commonly at about the one o'clock position. Then verify firing order counterclockwise from #1. These are the steps to take to set a new distributor. After that, you can set your timing.

I can't see a #1 and #4 failure attributable to the points or coil or condenser. Such failures would be more randomized, or more across the board, not isolated to two cylinders. Nor is it lack of fuel, since those two cylinders are served by different carbs. Whether you should have matching carb parts is a good question, but not likely to be your issue here.


GMc

What William was saying. check the float bowl. I was having a very similar problem. Did all that you did, timing, compression, etc. Not getting fuel...stuck something!!!

You're on the right track.

Stay Safe, David

Jeff Schlemmer's site Advanced Distributors has an excellent write-up on setting up a disturbed distibutor. I always follow his procedure, and use a compression gauge to find TDC on the compression stroke on #1, and an in-line spark sensor to determine when the spark occurs. You have to be certain you're on the compression stroke when you set the initial timing.

x2000

Pay attention to the evidence. 1 and 4 aren't firing. If they were the plugs wouldn't look like they were right out of the box. Why? Look for the reason and you'll find your problem. It is either no fuel which is unlikely given the nature of a front and rear carbs or no spark. I'd be checking out the spark coming to 1 and 4. Put a spark tester on those two plugs or plug wires. As others have said are you 100% certain you have the correct firing order on the plug wires? 1,3,4,2 CCW (probably not the problem as it was a sudden onset issue). Did you look at the distributor cap for flaws? Did you recently have a leaking heater valve?

BH

Unscrew the brass caps on the SU carbs and try the bent wire test to confirm if both pistons are lifting together.

And have you checked the mixture?

Backfiring through the carb is a dead giveaway for a round cam lobe. Check it out.

Jim

UPDATE ..............................
So I'm doing the "Twist" test and I feel that one of the pistons drops about halfway and stops. If I spin the piston in the dome I can feel it rubbing against the wall in one spot. (See picture) You can see in the pic, a dark spot on the dome wall. I took a piece of 1200 wet sandpaper and cleaned it up. Tested the piston again and still felt the rub. Checked the piston for damage or rubbing, none found. As for the test, I cannot get a close match on either dome.

No, they don't, I'm working on that now. I posted an update on the bottom of the thread.