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EFV-8 Club Forum / General Ford Discussion / 21 stud backfires or pops @ 50 mph

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Discussion Topic: 21 stud backfires or pops @ 50 mph

deluxe40

08-04-2012 @ 10:41 AM

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I just solved a similar problem with my 21 stud. The hole in the new distributor gasket for the o-ring that seals the vacuum brake passage was too small. It interfered with the seal and caused a very small vacuum leak. I fixed it by making the hole in the gasket bigger.

supereal

08-04-2012 @ 6:18 AM

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Sounds like a vacuum leak. Even a small one causes the fuel mixture to lean out to the point it will preignite. Look for the obvious, such as a cracked or broken wiper hose or distributor vacuum tube that isn't seated properly. The intake manifold gasket may not have sealed, perhaps held up by a small piece of junk that wasn't removed from the gasket surface. You can check this by spraying some carb cleaner along the edge of the manifold while the engine is idling. It it speeds up just a bit, you will have found a leak. Another cause is crossfiring inside the spark plug metal conduits. If your plug wires are old, it is likely the insulation has some cracks. The old test is to spray some water along the conduits and listen for crackling and/or do it in a darkened place and watch fo jumping sparks. As speed increases, coil voltage goes up, producing the problem. The carb and distributor are seldom the cause of backfire, unless the throttle butterfly in the carb doesn't close completely, or there is a carbon track in a distributor cap.

This message was edited by supereal on 8-4-12 @ 6:26 AM

jtserenityln

08-04-2012 @ 5:59 AM

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The engine is a stock 1935 with cast iron heads and aluminum intake manifold. Oil canister filtering has been added.

40 Coupe

08-04-2012 @ 4:00 AM

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Backfire and pop are usually signs that the timing between the valves and the ignition of fuel in the cylinders is somewhat off. This could be caused by distributor timing problem since it fires the fuel or a valve problem. If the backfire occurs through the carburetor the problem lies with early ignition or an intake valve, if the backfire is through the exhaust an exhaust valve or late ignition fire. Since the engine is nearly new?? you may want to add some Marvel Mystery Oil to your fuel and oil, or remove plugs and squirt it directly down any open valves to lubricate the guides and valve stems, just in case they are sticking. Take a compression test of the cylinders. Remove the distributor vacuum brake piston from the top of the distributor and make sure it is free to move up and down and the leather brake pad has not fallen off the end. Put a vacuum gauge on the engine watch what happens to the gauge as the engine idles and the level of vacuum 15-20 steady? Nearly all of the carburetor adjustments are done by the re-builder before he returns the Carb to you. The adjustments for the idle mixture are at the base of the carb and only effect idle not 50 MPH driving. To adjust turn them in with your fingers until they stop (do not force) then screw them out about 3/4 of one turn. start the car and when hot then slowly turn one at a time in and out until vacuum meter reads highest and steady vacuum. Go back and forth between the two screws making small adjustments. When your finished they should not be out more than ABOUT 1 1/2 turn. After doing the above take the car for a ride and note any changes to performance.

08-03-2012 @ 10:53 PM

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I suggest you first check/trace the spark plug wires making sure the are connected correctly at the distributor. What year engine/car do you have?

jtserenityln

08-03-2012 @ 10:07 PM

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@ 50 mph 21 stud newly overhauled pops or backfires.. distrib. was professionally strobed and stromberg 97 professionally re-built. starts and idles great but seems to lack power upon acceleration then pops and/or backfires @ 50 mph or faster. Oil pressure & temp. are normal. I'm new to flatheads so can anyone give Me basic carb screw settings and any other tips to get this problem fixed... Thanks All***JT***

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