Hey all,

84 T/A, CC Qjet. 355, 700R4, 3.73 gears.

I've been having some idle issues lately where the car runs 600-700rpm in park and 400-500rpm in gear, stumbling and barely running though it runs fine when driving around...only the idle seems to be an issue.

When I dropped my car off at the port in England, it was running fine, idle 800-900 in park, 600-ish in gear. When I picked it up at the port in California it was again running and idling fine. I finally drove it home to Salt Lake City which is at 4600 feet above sea level it started giving me problems with the low idle.

Would the altitude have anything to do with the change? Something else? What can I check? Any insight would be greatly appreciated. Thanks!

 

Yes the altitude does that.

For example I drove my 83 car all over the country; in Denver, the Sierra in CA, etc., it always did that.

Since the air is thinner, there is less of it (fewer molecules) per unit volume; but since the carb feeds fuel based on incoming volume, not mass, it will run richer as altitude increases. People who drive exclusively at higher altitudes often lean out their carbs; race cars often need to be jetted down at high-altitude tracks like Bandimere. Cars equipped with a baro sensor are able to compensate for this to some degree although far from completely.

 

Thanks for the reply Sofa,

I do have the baro sensor, not sure how well it works though as I've never changed it.

Is this something I can fix simply by bumping the curb idle up or do I need to adjust with the IAC and my dwell meter? Or, not wanting to do this as I don't have an A/F meter, but would I need to adjust my lean/rich stops?

 

Could try bumping timing 2-3 degs to help force higher idle, more complete burn but it does need leaned out some from the elevation change. Could try bumping idle screw to raise idle and try to reset iac. I am not familiar with qjets tho

 

My dad had to change the jets on his old lincoln when he brought it up to colorado springs. Not sure if different carbs work differently though, ive never worked with em much.

 

I'm going to start with bumping the idle and then checking the IAC I think, just to make sure it's still reading where it needs to. The altitude and O2 difference here could be significant enough to throw it off, but It h ought that's what the Baro sensor was for, still, won't hurt to check to make sure it's still in the "green" so to speak. If this moves my idle, I'll just drop it back down if I have to.

Without an A/F meter, I don't trust mysel fto touch the lean/rich stops to "lean it out", maybe try and find a tuner here who can borrow one to me or do it.

 

The carb's mixture control doesn't have enough "authority" to make up for THAT MUCH of a barometric pressure difference. It will take care of minor weather variations, and a little bit of altitude change; but not 5000' worth.

The timing suggestion is good... consider the thinner air as effectively lowering your engine's compression, which makes the mixture take longer to burn, which makes it want the spark earlier so it has longer to burn before it gets to TDC and is supposed to start making power.

 

That's where I'll start then, timing adjustment and IAC check. If that doesn't work then I'll manually adjust the curb idle.

 

I bumped the timing...it didn't seem to do anything with the idle. I pushed it all the way up to 12* BTDC, it's now set to 8, up from the factory 6.

I also checked the IAC with the dwell meter, and it was stuck on 50* without any deflection. So, I adjusted it until I got the meter to move, turning it out several turns until i got deflection. Now it's between 20 and 30* and deflects as it hunts for the proper reading.

To bring the idle up, I had to manually turn up the curb idle .

Without an AF meter, I can't adjust the lean/rich stops. Also, should I possibly look at re-jetting it?

In two years, if nothing else changes, I'm going to be moving back to Minnesota which at it's highest is only 1000 ft above sea level which will alleviate most of these problems.