I have a 351C-4v with fewer than 2000 miles on complete rebuild through short block and heads.

I have a backfire which I cannot tune out of the car. The backfire is coming from the passenger side tail pipe (cylinder 1-4). It occurs "sporadically" during idle all the way up through WOT. Although, the backfire frequency does reduce while idling when engine reaches 180 degress.

I checked the cylinder pressure and 7 cylinders register very close to 180lbs (+/- 3lbs), while the number #1 cylinder came in at a feable 135-140lbs. I changed the plugs....the carb is jetted just fine but after 100 miles, I inspected all plugs and found the #1 cylinder was fouled and all other plugs were clear.

So, it's seems evident the #1 cylinder is the source of the problem.

I pulled all plugs and went back to the cylinder pressure check. Using a squirt can which has a levered handle, I squirt one solid stream of oil into #1 cylinder and it still only registered 140lbs. With 2 additional squirts of oil (now a total of 3 shots), the pressure is averaging around 160 with a max of 170lbs.

I'm concerned that the 3 squirts are screwing up the accuracy of this test as it may be possible that the oil is now creating a seal at the valves.

Had I not shot in the additional 2 squirts, I was prepared to pull the passenger side head and inspect the valves. Reminder, these heads were fresh rebuilds (guides/3angle/seals/etc) only 2000 miles ago.

I don't have a leakdown tester.

GUYS: What do you suggest I do next ?

Thanks in advance

 

im not an expert on this kindof thing, but have you looked at the springs/rockers/pushrods etc for the suspect cylinder? might be worth a check but just guessing. Also, have you got new spark plug leads? maybe a faulty lead is causing the misfire and hence the fouling... or perhaps somewhere in the distributor?

 

This works for a 351w so it might work for a 351c too. If you are still using the original bolt down rocker arms, then after all the machine work, the valve adjustment might be out of range. Loosen the rocker arm bolt while holding the pushrod. Loosen it till the pushrod has play between the lifter and the rocker arm. Now slowly tighten the rocker till all the end play just goes away in the pushrod. Note the position of your ratchet and then tighten the rocker bolt down till it stops counting the number of turns. It should be at least 1 turn till it stops, but no more than 2 turns. If it's less than one, you need a shorter pushrod. If it's more than 2 you need a longer pushrod. By the way, I am getting this from "How to rebuild your small block Ford". If there is a different count for 351c than maybe somebody else know's what it is.

 

 

Sounds like a valve cracked open.Too much preload. Measure preload or just install a .060 pedestal shim and torque to spec. If you still backfire,,, Tear it back down