64con

I recently installed a 383 stroker in my 1964 roadster and have just over 300 miles on it. I have taken the engine up to 4000-4500 rpm a couple times and it seems to surge/oscillates. I thought it may have been ujoints because the rare time I took the previous engine to those rev's the car would do the same thing. I replaced the ujoints which were likey due for a change and it did smooth things out a bit on the highway untill the 4200 range. when I had the car home I decided to rev up the engine and it surges/oscillates at a stand still.
It has a 750 holley with vac secondaries, a winters intake. The advance timing is all in around 2800. 9 deg advance at idle. When I tuned the carb with a vacuum gauge it has almost 12 hg and showed no signs of vacuum leaks. I'm not sure where to start. It could be jetting or the intake is just not letting enough air in.
Another thing to note is that I had it at 4200 on the highway at a steady speed, it was smooth, but when I checked the same rpm in third, the oscillation was quit predominate. It's got me puzzled( which doesn't take much)

Anyone else get this resonent frequency oscillation at these rpm's?

GTOguy

I've had this happen with other makes, and it was the vacuum advance: too much advance causing break-up and surge at higher rpm cruise. You could disconnect it and plug it really quick and see if there is a change. It could also be lean or be another ignition issue. access to an old fashioned oscilloscope would be a big help here.

64con

I am using the same msd distributor. Hmm. That's a great theory.

skids

Is the msd hooked up correctly? Heavy red wire to battery terminal or positive lug on starter, and heavy black to block or chassis? Trigger wires should be routed separately,away from other wiring and magnetic pickup wires should be twisted together and routed separately to prevent extraneous interference.

65tripleblack

Was the same 750 Holley installed on both engines?
Can you further describe what you are feeling and hearing when the car "surges/oscillates"?
I assume that the fuel filter was replaced with the new engine, and I also assume that the fuel tank pickup screen and fuel supply line are the same.
Was the fuel pump swapped to the new engine?

Pop Chevy

The power valve in the carb is load sensitive (vacuum). Causing the car to be either too lean or too rich. You've got some tuning to do.

tbarb

Sounds like the power enrichment (power valve) is not enriching soon enough. If the engine runs OK at 4200rpm in high gear cruise the main circuit may be OK.

Do some research before you start with the carburetor and eliminate the other possible causes like have been posted above.

64con

Thanks for the replies. I had a oil leak that turned out to be the high priced braided dipstick that I snapped off where the tube meets the block. I have to extract it somehow. Then I'll get back to the surging issue. I just hope I don't have to pull the motor if I can't pull it out.

65tripleblack

An EZ Out usually works.
When you install the new tube, loosen the fit slightly by removing some material with a file and some emery cloth so that it is a sliding fit and not a press fit. Use Permatex between the tube and the block.

64con

Ezout is not working. The remaining tube is spinning around so I can't get enough bite with the ezout to pull it out. I am going to try again with a 3/8 course thread lag bolt and see if I can get that to bite. Thanks for reply!

64con

The 3/8 lag bolt worked. Whew!!!

64con

I plugged the vacuum line and the surge reduced to a minor oscillation. That's great news. Thanks for the help.

Frankie the Fink

Still might be worth looking at the power valve as PopChevy indicated. I had a bit of this problem in my '67 Chevelle and the PV was the problem...

64con

Different carb. Fuel tank pickup screen supply are the same. I did install a new fuel pump and libe up to thr carb with a bew inline filter.
Thanks for the reply

64con

Stock power valve. I will investigate. Thanks

64con

My plan is to take John Z's advice and get a b-28 can ( napa vc1810) and hook vacuum line to full manifold to get it idling nicely then get more into performance tuning it now that the surging at the 4500 rpm range is minimal.

Frankie the Fink

That usually means the 6.5 and that is not always the best depending on the engine/cam/etc setup

64con

GTOguy

I would have the distributor advance curve checked/dialed in. It sounds like it wants less total advance at high rpms if it's surging a lot less. I've had this issue with the limit bushings wearing out or falling out and allowing too much mechanical advance AND vacuum advance at cruise and higher. But fuel system issues, and bad ignition wires/points/condensor/coil can cause this, too. An oscilliscope is your friend here.....you can look at the pattern and see what's happening in real time.

65tripleblack

The PV only comes into play when engine vacuum drops under load (i.e: during moderate to heavy acceleration).
Remember that the OP stated that the engine "surges/oscillates" with no load, in neutral.