padraigh

I have been having a problem once the engine is warmed up (180-210 deg) of surging where the car feels like it is lunging and the tach needle jumps to the right and bounces around for 10 sec or so. It might do this 2 or 3 times and then be fine.
I had this problem in July and it got progressively worse to the point of stalling.

I took it to the shop where they:
replaced the fuel filter and air filter
replaced the coil
replaced the plugs
tune up the carbs (pair of 3 barrel Weber IDA 3C)
valve adjustment
leakdown test #1 93 % #2 94% #3 85% #4 84% #5 92% #6 78%
replace ignition points

The mechanic considered the CD unit but discounted it as a cause.

The mechanic kept the car for 6 weeks, took it home at night, put a few hundred miles on it, and thought the problem was fixed. It was stalling regularly on him when he first got it so there was an improvement to where it does not stall out now but still surges.

It does it over the normal temp range 180-210.

Mostly observed at moderate speed ~ 35 mph and ~ 3500 rpm.

The tach jumped @ 4200 rpm but did not exhibit surge behavior.

At speeds > 40 mpr and engine speeds > 4000 it seems fine other than the one time that the tach jumped @ 4200.

A respondent to the rennlist site emailed me personally to say that he had a similar problem that was due to loose electrical connection on the CD unit. I checked all ignition system connections & everything seems fine as far as I can see.

Any idea what to look for?

Sound like CD unit?

Thanks

Rob Heath

So bringing an early 4 cylinder perspective (my '68 912 - also with webers) which may or may not apply...

don't see a mention of distributor or rotor being checked out. Hard to check out the distributor with out removing it and sticking it on a special machine (I think it is a Sun Machine) that is used to test, reset curves, etc. a distributor rebuild person would have one of these, not most mechanics. Also when enigne is off, Rotor should have spring tension, snap back to position, when turned slightly opposite rotation.

I ended up having my distributor re-built as I was not accelerating smootly at all, it made a really big difference. They just wear out like everything else on old cars.

Incorrect float level setting or clogged / partially jets in the webers can cause erratic behavior but have not heard about big surges / acceleration... mine usually let me know when they need to be thourghly cleaned when enigne starts to run / idle rough.

Just some thoughts.

Peter Zimmermann

Rob's post is excellent, often an ignition distributor needs more than new points. The first thing that should be done is to verify that the points were set with a dwell meter - not a feeler gauge. Then during timing adjustment, the advance mechanism should be analyzed with the strobe light. If anything suspicious is found, go to Rob's plan and have the distributor put on a machine.

Vacuum leaks can also cause a surge, but I'm a little puzzled by the narrow heat range in which the problem occurs. Did the mechanic use an aerosol spray and listen for idle speed changes while spraying the carb/manifold, and manifold/head areas?

Also, what spark plugs are you using, and what are they gapped at?