911 3.0 Engine won't idle properly

[fst951]

Hi guys.

I did a tune up and replaced the cap rotors, plugs etc and the car still missed after extended periods of driving.

I then checked the fuel filter and man did it have all sorts of rust and shi- in it. I had the tank cleaned out and replaced the fuel filter to a late model unit.

I now have a surging idle that wants to die and runs just fine when it gets good throttle.

The car is a 1975 with a 3.0 liter engine.

I have played with the idle screw and the tried to trim the mixture out.

Any good protocols to do it properly? Also, where should the timing be set on this car? I am at 4000 feet above sea level.

Any help appreicated.

Thanks guy.

[bigchillcar]

how did the car run prior to the tuneup? in other words, why did you adjust the mixture? if you adjusted because of the idle, it could be true that the mixture is incorrect. one way to check how well the mixture has been recently is to examine the condition of your spark plugs. if you haven't driven too much to inflence a major change in their condition, examine them. i'm sure you know in general that if they appear oily, the mixture may be too rich, very white, the mixture may be too lean. if they look 'okay', i'd be tempted to try and remember how much you turned the mixture screw and return it to where you think it was. let the car warm to operating temp (180 degrees) and then set the idle speed at ~1000 rpms. next, check and see where your timing needs to be set either on a placard on the motor or on a 3.0 liter manual and set timing to that spec with a timing light. do you have ignition points on your 3.0? if so, you will need to reset the point gap after adjusting the timing. i can't recall what the 3.0 setting for gap is..on my 2.7 it is .012 inches i believe..your gap may be slightly larger. these adjustments should put you in the ballpark assuming you don't have major faults somewhere.

check for vacuum leaks with carb cleaner spray..an increase in idle speed would indicate a leak, which always results in a 'too lean' condition, but i think a hunting idle is more typically a rich mixture. anyway, once it's running, it wouldn;t be a bad idea to have a shop set the co on the car for you so you know your mixture is correct. good luck.
ryan

[fst951]

Hi and thanks for the response.

The car would run fine for about 30 miles and then the pluged fuel filter caused it to hesitate. I replaced the fule filter in order to eliminate the starvation issue. I am worried that when the engine was then tuned up, it needed to be readjusted for the increased fuel flow it was getting.

I must imagine that it is running rich, but I will pull a plug and see.

I will also use some carb cleaner and check for engine vacuum leaks. I think that then idle decreases with spraying carb cleaner into a leak, however.

Thanks and any further advice is certainly welcome.

[fst951]

Any more info?

[bigchillcar]

i see no reason for the mixture to have needed adjustment..sounds like you just had a dirty fuel filter, which caused hesitation, even perhaps intermittent stalling. that would be expected..the mixture itself was probably fine..with a new filter, it would simply return to getting the proper fuel that it should. unless the car had a long history of poor performance and fuel economy, my guess is that your mixture was okay..they don't tend to drift much once set. your problem was your filter, i think it has been exacerbated by fudging with the mixture without the proper tools to measure the resultant ratio.

i believe you will find that the volatility of carb cleaner will cause your idle to increase slightly, not decrease. mixture may be too lean now..any tendency to backfire now? if you can drive it until warmed to operating temp (right at 180 degrees), get out and set the idle to ~1000..then you'll know your basic idle speed is set for the correct temperature. then i would attempt to 'undo' whatever mixture adjustment you made..a click at a time. how many clicks did you turn?
ryan

[fst951]

Hi there.

There is no click, just a constant turn.

Thanks for the advice. Anymore would be appreciated.

[bigchillcar]

There is no click, just a constant turn.

actually, some report a click, others don't..clockwise will enrichen.
ryan

[fst951]

Hi there.

I will play with it this weekend and let you know.

Thanks!