Manager Forum

This engine has a new carburetor and distibutor and it's been tuned up. It runs great going down the road but has a rough idle and the engine lopes at idle. We've checked timing and it's dead on. Not sure where else to look.

Any suggestions?

Sounds like a vacuum leak but i need more information. Is it a 6 or 8 cylinder, standard or automatic? A shot in the dark if it's an automatic check the steel line at the transmission modulator. I tried to think of a way to describe it but came up empty. If i remember correctly it's on the passenger side of the transmission about 3/4's of the way down or follow the steel tube that comes down along the top of the transmission. I've seen the vac hose pop off of the modulator and it will cause a roll at idle.

Was the carb and dist replaced to resolve this issue or was it brought in this way. I remember if the carb wasn't secured correctly it would break an ear and suck air. Another issue was on occasion there was a spacer below the carb and the gasket between it and the intake would leak. Another issue was the intake leaking on the internal side but that was usually the big blocks. A quick test was to take the breather cap off and see how much suction there was. The spacer plate may be used for the pcv valve and be aluminum, these spacers would rot out internally and cause a rolling idle along with plugging up. I know i'm jumping around here but things are coming back in dribs and drabs. With the breather off i would spray carb cleaner into the hole or use propane. If it effected the idle at all (pcv valve must be out of the valve cover and plugged) if the engine smoothed out the intake gasket would be a possible culprit. I think that there was an issue of the intakes twisting when they came off and becoming scrap metal. We would slap holly or edelbrock manifolds on them. I'll check back later in the day and if needed give you a call.

Is it possibly loping due to a bigger cam in it? Has this been a problem for a while or something that just started? Did they buy the car this way, that could be the bigger cam if they don't know the history on it.

If it's a bigger cam i hope the motorcraft carb is in the trash and a holly with a lifted manifold is on it otherwise the gas will liquefy in the intake and load up after idling for 5 or 10 minutes blowing that lovely exhaust that will burn your eyes out.

No new cam. All original type parts. Let me redescribe the problem, instead of loping it's more a rough idle and erratic idle only when in drive or reverse. It's fine in park or nuetral and runs great once you give it gas and are moving. When you come to a stop or are turning the roughness re-appears. Thanks

With car running warm with choke open place your hand over the air intake and see if it smooths out, if it does you either have a vac leak or lean carb. Another issue may be it's being timed wrong. Not sure if this is one but some of these had a dual diaphragm distributor. Unplug vac lines and see if it smooths out. I'm hoping a compression test was done to cover the basics, if not do one. With a timing light watch the timing mark, if it's jumping all over the place the chain may be weak. Under a load the chain will tighten. If you find it's jumping and you pull the cover because of the chain make sure the cover isn't damaged. This is only if it's a 8 banger.

I think steve's onto something. If when slowly covering the top of carb and idle smooths, could be vac leak or air/fuel mixture screw adjust. I've had lots of success in smoothing out an idle that way, can't be any vac leaks however. Keep us posted and good luck