Exhaust POPPING

[Bill]

(also posted in the Spider forum)

Have developed a "Popping" from the exhaust on acceleration until the revs get up to 4K or so. After that an occasional "Pop" while cruising. This started about 4 days ago. It does not sound like pre-ignition.

Here is what I have checked:
Timing is dead on (35 degrees advaced at 5K); new plugs gapped as before; cam timing right on; valves within spec; no fuel pressure light coming on except as it should at key turn; new fuel filters; fuel lines drained and flushed a couple of weeks ago; new points, condensors, and rotor, gapped and double checked; Long rod and pump gap set correctly and TA working properly; butterflys look to be set together and working correctly; new coil (Bosch Blue) and new plug wires (Magnacore 8.5's) checked for resistance and on spec. I think thats about it.

If I let off the accelerator a tad and sort of lightly pump it, "Popping" goes away until I shift gears again. It does sound more like a fuel shortage problem rather than an ignition problem, but after years of mis-diagnosing automotive conditions, I could be wrong.

Oh, Most of the new stuff was put on a month or so ago and has been working beautifully until now.

What have I forgotten to check? Any ideas?

As always thanks in advance,
Bill

[Roadtrip]

What the plug color look like? Normal mixture?

[Bill]

Thanks John for the quick reply,
The plugs are sort of a dark brown/grey shade, all four the same, and not wet or oily.

Sent you a PM

[Roadtrip]

Check PMs.

Check the electric vacuum switch. Under high loads, it's supposed to actuate and switch the ignition to the other set of points to slightly retard the spark. Maybe it's malfunctioning intermittently/momentarily.

Also, might check to make sure that you don't see any tiny fuel leaks from any of the fuel pipes from the injection pump to the injectors.

Is the pop accompanied by a momentary power loss?

I'm kinda miffed at this also, so am grabbing at straws.

Maybe Papajam would have an idea of what to check.

[WayneW]

A pop on acceleration could be caused by a valve that is occasionally sticking just a bit, allowing unburned fuel into the exhaust system. If it only sticks once in a while you will not see much of a difference in the spark plugs. The best thing to do to check this is to pull the plugs and turn the motor over by hand and look at the condition of the valves as they open and close. But even this is not foolproof. A treatment of Marvel Mystery Oil in the sump has been known to free up sticky valves.

-Wayne

[Bill]

Thanks folks, but we all missed it...
Bad condensors. Had changed out the points, plugs and condensors, then the problem started showing up about two weeks later. Been working on this for the last week or so, and decided to go back to square 1.
Pulled the new ones out, put the old ones in and we are back running like a scalded cat!
You just never know....

Thanks again,
Bill