302 Ford with a 4100 4 barrel surges while at constant speed not under a load. I have reset float level, timing is good,spark plugs look good, new wires and points. I have checked for vacuum leaks. Idles fine and good on acceleration. I just purchased the car so I have no knowledge of the history but all parts look fresh and in good condition. Any ideas what might be the problem?
Just checked the vacuum advance. It is new and no leaks. I'v also sprayed either around the base and shaft pivots and they seem to be ok.
Had something similar happen on one of my cars and it ended up being fuel pressure related. The stock style replacement pump would increase pressure with RPM and start to flood at speed. Could also be the opposite and a starvation problem.
Could be a fuel supply issue. Lines still have pressure in them after shtting the motor off but do not have a pressure gauge on the motor.
All good suggestions, but look very closely at the breaker plate in the distributor. You did not state what year the engine is, but alot of Fords in the late sixties, early seventies would wear the breaker plate pivot bushings, which causes erratic dwell, and will give a real weird fluttery miss. Also check the shaft bushings in the distributor as this will also cause the same deal. Hook up a dwell meter and hold the RPMs where the miss is and see if the dwell isn't all over the place. Another symptom of this is, if you disconnnect the vacuum advance it will run better because the breaker plate isn't moving around.
What do you mean "timing is good"? If you have anything other than a stock type cam your timing curve needs to be mofified. Sounds like either a lean surge as above post mentioned or your timing is "hunting". If your cam has more duration than stock you need more intial spark advance and your advance curve needs to be quicker. Your vacuum advance will also need to be set so that its not oscillating while cruising (hunting). Since you don't know the history of the car you probably don't know the cam specs. I would start by disconnecting the vacuum advance and set your initial at 16 and see if it does the same thing so you can at least eliminate that possibility. Problems like this are almost always immediately blamed on the carb and usually are something else. That 4100 is a good carb, especially on engines not heavily modified.
Its probably in one of the responces above but another possibility.though slight, could be a worn timing chain and sprockets. Normal Norman
Thank you for all your responses! I believe that most suggestions point to the ignition. I will investigate all the areas noted above and appreciate any more suggestions given. I will post the solution once I have corrected it.
I'm pretty sure this is it. To check pull the cover of the diaphragm under the carb body in the front and look for fuel on the cover side of the diaphragm. A common problem with 4100s.
Had a similar experience with one of my cars, it was too much advance. Put some heavier advance springs in and it solved the problem. Pete
Weak coil can mimic rich condition. Modern fuel is lighter than the stuff it was designed for: might need to lower float level. Vacuum!! vacuum!! vacuum!! Hook up a gauge and read it at idle, while driving, coasting, etc. This can tell a lot. Cosmo
Had some time to work on all the suggestions. Only a couple things have made it run smoother. This is what I have found: 1. Distributor advance was running on ported vacuum. Switch it to manifold vacuum and it ran worse. Manifold vacuum is a steady 15-18 inches. Ported vacuum was not as steady and slightly lower, 10-12 inches. 2. Dwell stays steady at all rpm's at about 28 degrees. 3. Disconnected vacuum advance and set timing up to about 10 BTC (was set at 6 per stock engine specs) and it ran better but not great. 4. Sprayed starter fluid on all the seams and shaft pivots. No change in RPM. In fact spraying it down the carb throat made no rpm change. 5. Visual inspection of the distributor shows no excessive bushing wear and as I said dwell was very steady. What I have succeeded in doing is making it run even rougher at idle as well as the surge under no load exccept when set up like item 3 above. I focused on the ignition rather than the carb. I checked nothing on the carb that was recommended yet but it is my thoughts that it is more ignition than fuel. HELP!
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