259 hesitation at cruising

[dirtclod]

Our '64 cruiser has a '61 259 that has a slight surge or hesitation at steady highway speeds 65 mph plus or minus. It is stock except a 265 cfm Holley 2bl. She does fine on acceleration no hesitation there. I have been trying to figure this out and found two oil fouled plugs so I put in RH18y Champions in those two cylinders nothing changed and left the autolites in the other cylinders. I've cussed her and that didn't help one bit ( I did apologize to her) any suggestions on where to started looking next. I sure do appreciate everyones input cuz this ol' knucklehead sure needs ya.

[bige]

You could have a vacuum leak causing the car to run extra lean at cruise or your vacuum advance is over advancing the timing. Try a test drive with the vacuum advance disconnected and the hose plugged. If it gets better there could be a few reasons for too much advance but at least you know what the problem is and all that's left is to figure out what's causing it

[unimogjohn]

My newly running 63 Avanti R2 had a stumble/miss at about 2 to 2.5 rpm. I could not figure it out since my carb and distributer had just been rebuilt by Dave T. Well, turned out that I had not tightened down the distributer holding bolt tight enough as I was playing with adjusting the timing. Turned out that the distributor was walking back and forth, just enough to cause the problem. Worth a shot just to check yours.

[64V-K7]

Hesitation & stumbling are also semaphores of vapor lock...

[Ron Dame]

If the carb was jetted on the lean side for real gas, it will be too lean on E10, and that will cause a surge at cruise...as will a worn distributor

[dirtclod]

Welp I plugged the vacuum line and she ran a lot better but I can still feel a rythmic surge so I am leaning towards the idea of a rebuild on the Distributor and then we'll see after that thanks for everyones input.

[Milaca]

Out of curiousity, were the two fouled plugs the very front plugs on each side of the engine?

[bige]

Even though Prestolite distributors can have their problems you may just need a set of points and condenser and a careful check of timing especially where the timing lands at cruising RPM. Just check it at 2500-3000 RPM with the vacuum disconnected. I don't know what the factory recommends as total advance but somewhere around 32-34 degrees should be right. Adjust accordingly and hook up the vacuum advance.

It doesn't take but a few degrees extra advance to give you that surge. In fact if you have a good ear you can hear the engine hiccup a little just sitting in the driveway and bringing the rpm up to around 3000 and holding. If you detect a few muffled 'pops' you will still have the surge. When the pops go away the surge usually disappears with it.

ErnieR

[Bud]

It sounds to me like the fuel mixture is too lean. I've had to richen the mixture on my 62 Hawk with a late AFB and my 63 Avanti as I'm forced to use gas with 10% ethanol. If the gas that you use has ethanol in it, you may have to install at least one size or more larger main jets in your carburetor. It's possible that you have an ignition problem such as a worn distributor, but that will usually cause a problem on acceleration. An air leak somewhere in the intake system will cause a rough idle along with surging at part throttle. Bud

[dirtclod]

The fouled plugs were opposite corners left front right rear. We live in Utah at 4600 ft elevation and the carb I had in Calif. at sea level so that could be the problems. It seems to me I need the do a once over on the ignition and fuel system again. I put on the 4bl. intake last year and have tightened everything again trying to overcome any vacuum leaks. I'll get busy on all this cuz driving season is upon us. thanks to all again Doug