Randomly going back into fast idle

[tonyfromjersey]

My car is running very well other than a minor quirk that just came up. It will go into fast idle as usual, and once the car warms up it will drop into normal idle (800 RPM or so). Every so often, and not on every drive, it will kick itself back into fast idle. There is no change in engine temperature, and no discernible other problems. After a little while it will go back to normal. This doesn't happen on every drive either.

It's a 77 with the stock carb.

Any ideas?

[dzyfngrs]

I had the same issue, only the fast idle was constant. Wachuko (Jaime) was able to isolate the problem with me by noticing that the butterfly valve would not snap closed. This led us to take apart the water activated choke mechanism and, sure enough, we found lots of crap preventing the choke to close. After a thorough cleaning my baby purrs. Hope this helps.
Cheers,
Frank

[tonyfromjersey]

Frank,

I'm debating a carb rebuild vs. buying a new carb from Mark Alison. Maybe start with your suggestion.

Tony

[So Cal Mark]

Tony, check the mechanical advance in your distributor. I recently found one that would hang up, especially after a freeway drive where the rpm was high for awhile. It would keep the timing advanced when you came back to idle, raising the idle speed substantially.
Of course check that the throttle is closing completely

[tonyfromjersey]

Mark,

Thanks, throttle cable is fairly new but may need some adjusting. I'm running a Crane electronic ignition system. I drove the car all day today back and forth to work and it didn't do it at all. I think it's time for a carb rebuild or swap. Problem is its finally driving season and I don't want to take it apart at all. Maybe when we get a rainy stretch.

Tony

[So Cal Mark]

isn't the Crane merely a swap for the points? If so you still have the mechanical advance in the dist.

[tonyfromjersey]

Mark,

I believe you're right. I'll look at it next week since it's an intermittent problem and it's Mother's Day weekend. If I have my head buried in the car I'll end up living in it.

Tony

[manoa matt]

Tony,

Before dropping some serious dough on a new carb, or even trying to open up the current one, check the accelerator linkage from the pedal to the carb. Its more likely the linkage is binding somewhere.

Take the white plastic gas pedal rod pivot brackets off, clean and lube, re-install.
There should be a return spring on the gas pedal way up by the firewall.
Make sure the accerator cable is lubed and not binding in the outer sheath.
Make sure there is a return sping on the cam linkage and that the balls are lubed where the socket fits over them.
Make sure there are no wires or hoses that will prevent the linkage from returning to the full rest positon.

My car does this sometimes when I come to idle at a stop. If I tap the pedal slightly it will return to the normal 800rpms. Despite me trying all the above it still happens, but very infrequently. I believe the throttle shafts on the carb are worn and that is what causes the sticking, as I see minor gas seapage around where the throttle shafts exit the carb body at the lower half of the carb base plate.

Alternatively there is a possiblity it's an electrical fault in the fast idle system, and the electrovalve is being triggered, opening up and increasing the idle.


When exactly does it happen? As you come to a stop the idle stays high, OR is it while you are sitting idleing, and then it fluctuates and goes up? The latter would indicate an electrical fault in the fast idle system.

[tonyfromjersey]

Matt,

It seems to happen when I'm driving along and come to a stop, and the car is back in fast idle. It will stay like this for a few minutes and then drop back down. Temperatures stay normal, no issue there. The throttle cable is new last year, but I will follow your advice on the linkages, etc.

Funny thing is I can go days without it happening, then it will do it again. I was also wondering if it was ambient temperature related, but its been all over the place this spring, 80 one day, 60 the next, and that doesn't seem to have any effect.

Thanks for the advice.

[So Cal Mark]

the situation I found with the mechanical advance sticking would occur after sustained hwy driving, usually at a fairly steady rpm. Sometimes you could blip the throttle and get it back to idle, other times nothing would help. It's pretty easy to check if you have a timing light, but of course you have to duplicate the problem

[tonyfromjersey]

Duplicating the problem is the hard part with this one. I'll start digging into it this week, I've been too busy to dig into it so far.

BTW, intermittent problems are one of the main reasons I no longer own my BMW. I was going to put my mechanic's kid through graduate school with that one.