Backfire!!!!

gadgetgeeza said:
These ideas are brilliant - thank you. I share Steve's view that it will be found. The opinion seems to be split between fuel and electricals. What's confusing me is that after a short period of madness, it all dies down and off we go. So the problem is intermittent - don't you hate them. A loose connection, frayed wire (it has all been rewired). The fuel side is also interesting. Could it be some debris in the tank moving about and blocking the outlet from time to time? Has anybody experienced this?
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Debris in the system can be intermittent but usually requires manual intervention to clear it. Eventually debris makes it to the carb which doesn't clear itself. From your description I think that would be the least likely of everything suggested.

By contrast the fuel line issue I suggested is nearly always intermittent. This only occurs when the fuel consumption rate exceeds the fuel feed rate. If you sit for a minute the bowl has a chance to refill and the cycle begins again.

The line can be removed and visually inspected in about two minutes or less. If it proves to be proper then another minute to replace and it can be crossed off the list. It may not apply in your case but it's one of the simplest/quickest things to check.
 
One of my mates had this same problem,for FOUR years,tried replacing nearly all the electricals,,burned out the muffler,finally found a small stick stuck in the fuel tap.
Lawrie
 
logical assumption Marco. I had a boat that acted the same way, after a frustrating year, new fuel pump and numerous electrical parts, I found yellow plastic shavings from gas can spouts in the fuel line, not enough to plug it, but enough to restrict the flow under load.
 
My 28 Tudor has an exhaust whistle. My backfires sound like a handgernade thrown into a pipe organ.
 
gadgetgeeza said:
These ideas are brilliant - thank you. I share Steve's view that it will be found. The opinion seems to be split between fuel and electricals. What's confusing me is that after a short period of madness, it all dies down and off we go. So the problem is intermittent - don't you hate them. A loose connection, frayed wire (it has all been rewired). The fuel side is also interesting. Could it be some debris in the tank moving about and blocking the outlet from time to time? Has anybody experienced this?
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I have, that is why I mentioned it. It would happen (occasionally, mind you) after a fill up at the station. Maybe sloshing knocked some stuff loose or stirred it up. I bought one of those screens that mounts in the tank, on the shut-off and I have had no further issue.
 

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Just thought I'd give you guys an update. The fuel line is clear with no projecting pipe at the unions. There is no sediment in the sediment bowl. The carb float chamber is spotless and all the jets are clear. I have even checked that the gas/radiator caps are on the right way - they are, and the vents are clear. Oh - and the problem's still there!
I do like Smurkeys idea - also suggested by Lawrie. I have a small tank filter on order ready for when I get back from holiday. What a time for the wife to pick for a holiday! Jobs to do, cars to fix!
 
Check the literature on carb problems. There is a condition with the jet sizes that causes backfires. Jet sizing is rarely looked at because it is so much trouble. Bill G
 
Have you tried a different distributor body? Sometimes, the spark path to # 4 cyl will jump up through the body to the # 3 spark plug strap.
Bill W.
 
I like the thought of a dirty spark plug. Look to no 3.

You can do the "shorting out" trick (slow and retard the engine spark before hand) to see if it changes the engine operation any. If it doesn't, then there is a culprit.

Joe K
 
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Well I tried most of the suggestions and the problem is fixed. Thanks for all your ideas.
Embarrassingly the problem was driver error. I had it in my mind that the mixture screw was opened for cold running and then closed when warm - wrong. With half a turn open the car runs fine without pops and bangs. Just running too lean.
 
Hey, Thanks for posting the solution Gadgetgeeza, We all have done some things we didn't know the proper way to do it. Enjoy the ride, something else will probably happen soon enough!
 
I'd also be looking for the potential of a sticky exhaust valve. Doesn't have to burn't, just occasionally not fully closing. I have found this on a number of cranky old engines...although usually as the result of sitting too long. Plug color should show the cuprit...any one that has more black soot would be an immediate suspect.
 
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