VCCA Home Forums GENERAL DISCUSSION New User Forum 67 250 I6 rough idle, weak spark?

Recently picked up a bone-stock 67 Bel Air 250/Powerglide with 65k miles. It idled a little rough (missing, I think) when I bought it, but no smoke. I went through timing, cap/rotor/points, plugs, fuel filter, vacuum advance, and everything else I could think of. All looked OK. Coil is new, plugs are clean. It does have new plug wires, ~5k ohm per foot. Seems high for a standard ignition set up. What do the experts recommend? My next step would be to put in a Pertronix system, but thought I'd check here first. Thanks for any ideas.

If it were my car I I would check the compression - but first take it out and drive it at a higher speed for about 10 mies to free up the valves and lifters.
If it is the 250 with the intake manifold built into the head the manifold could be cracked. Fairly common.

Also the distributor on that engine would wear out its bushings and the dwell would be all over the place.

Thanks for the feedback Gene. I need to pick up a few more diagnostic tools...

I have driven it 50-60 miles and had it up to 60. Drives fine on the road and even idles fine if I put the idle a little higher than I normally would. One thing that I forgot to mention in the original post is that the misfiring disappears if I disconnect the vacuum line from the vacuum advance and plug the line. With the in the vacuum advance connected it's at 15-20 degrees BTDC and the misfiring starts. I need to check the base timing again. The spec is 4 degrees and I know it's a bit higher than that. Mean anything?

Sounds like your timing is too far advanced. Check and reset the timing.

Yes, I would retard the timing a few degrees and see what happens.

It could also be the diaphragm in vacuum advance has a tiny hole.

I am also thinking a small vacuum leak. You can check the diaphragm if you have a hand vacuum pump. Or remove the distributor cap and rotate the points plate like it would move under vacuum. Put your finger over the connection hole and release the plate. It should not rotate back to the original position. If it does I expect that the diaphragm is leaking.

I cannot remember if you need to disconnect the vacuum advance and plug the line on that car when you set the initial timing. If that is correct you then will probably need to re-connect it when you set the idle speed.

Best bet is to check the shop manual.

Spent this weekend going through the suggested checks. Base timing is at 6, spec is 4. Doesn't seem like that would be it. Vacuum advance holds vacuum on its own and seems to be working fine. Checked compression. Spec is 130, cylinders measured between 155 and 140. I set the idle speed and mixture as per the shop manual instructions. There's a single vacuum line that runs between the distributor vacuum advance, the carb, and back to the transmission. I measured it at 25 PSI in park and 20 PSI in drive with the wheels chocked, the E brake on, and idle set to 550. Can't find a spec for what the vacuum reading should be, but I don't have a reason to believe that it's a problem.

I've driven it more than a hundred miles in the last few days. It runs and idles perfectly, as long as it's not in park or neutral. No smoke, and acceleration is better than I would have expected based on my experience with a 283/powerglide in a previous car.

Does anything else come to mind? I've got a carb rebuild kit and a Pertronix Igniter II on order. I'll rebuild the carb first and see how it goes...

I would suspect carbon buildup based on your compression readings. I would borrow a compression gage and check the readings again. If still high, I would warm up the engine to operating temperature, and at a fast idle, slowly pore automatic transmission fluid in the carburetor. Do not let the engine bog out, keep it running. When finished, pour enough to kill the engine and let it sit for a while.

Hi belair67
Just how much does the engine miss? Is it an irregular miss about every 1 to 3 seconds or so, and a few even closer together than that? Or worse than that?
The reason I ask is because Dad had a 1964 Pontiac Stratochief with a 230 cid six and a Powerglide, which up here in Canada was really a Chev Biscayne with a Pontiac body and wheels.
Right from new that engine had that irregular miss under pretty much the conditions you describe. Always did it idling, always disappeared as soon as you put it in drive.
Otherwise the thing ran great and got great mileage too. Dad was still doing the maintenance back then and I have little doubt he had a few extra degrees of advance dialed in.
Anyway. GM mechanics told us we would never get rid of that miss due to the amount of valve overlap those engines had. And we never did.
Don't know if your car has the same kind of miss as Dad's did, but Dad put 110,000 miles on that thing and it still idled the same way.

My specs from Chevrolet give 140 P> for the compression. I would also suspect carbon deposits.

Since the car is new to you:

If you have not already done so, drive the car enough to get the level of fuel in the tank down to almost empty. Then go to a name brand fuel station, and fill with the lowest octane (87 around here) fuel available, preferably non-ethanol. Do not try to burn higher octane fuel.

Many will argue that the independent stations sell the same fuel as the name brand stations, and in many cases this is true (but not always).

6 week old gas will be unpredictable. Higher octane gas will not perform correctly in the low compression engine.

Jon

Mr. Olson... That's it precisely. A little burp every second or two plus an occasional stumble of a few in quick sequence. Only in park and only with the vacuum advance connected. In drive it's quiet and runs like a watch. Maybe I just don't remember what cars without computer controls and fuel injection sound like.

I'll definitely check the carbon deposits angle.

Thanks everyone... Rob

Hey Jon

I'll definitely try using up all of the gas in the tank. I don't know where it came from or how old it is. It will be a bit of an adventure since the gas gauge doesn't work at the moment :-).

I figure that 300 miles should do it... with a gallon jug in the back just in case.

Rob