Q-Jet Fast Idle Issues

[dz1087]

So back when the weather was Florida warm (80*+) in the morning, the truck had no issues at all and all worked as advertised -- press the gas pedal to set the choke, crank, fast idle, tap gas after a few seconds to take it out of fast idle, drive on your merry way.

Now with it getting in to the 60s and 50s in the mornings here, it is acting up a bit. It still sets fine and cranks fine. Still comes out after a tap on the pedal. However, now after coming out of fast idle, it places itself back in fast idle after I drive away. It will either stay in fast idle for about a mile, or if I stab the pedal come out of fast idle then go right back in, again for about a mile. Seems to not matter if I let it sit in fast idle for 10 seconds or 100 seconds after cranking either.

More annoying than anything, but it also doesn't seem to be the way it was designed either. Just wondering what I could do to fix it.

Truck is garage kept. Q-Jet had a complete rebuild accomplished by Q-Jet Power about 2200 miles ago. Using stock breather. Uses divorced choke.

[Troy Sr]

Was there grease put on the pivot point of the fast idle? If so maybe the grease has stiffened with the cooler weather?

[dz1087]

I'll have to check if any grease present has hardened, but how would that make it pop back in to fast idle? Seems like it would make it harder for it to come out of fast idle.

[68Gold/white]

A little dirt or crud can mess up the linkage there, also..

[RustyPile]

Sounds to me like it's working exactly as designed.. During colder weather the engine takes longer to warm up, so the choke stays engaged a bit longer, doesn't open as fast.. If you speed up the opening process, you might find the engine stumbles and stutters until it builds some heat.. You might try setting the fast idle speed a bit lower. Otherwise, I'd leave it alone..

[dz1087]

It is designed to go from slow/mid idle back to fast idle with light throttle input?

It pops out of fast idle, choke still mostly closed with a stab of the throttle, then as I'm easing down the street, it goes back in to fast idle. If I stab the throttle again, it again goes back down. Seems that it should stay down there, regardless of any grease or dirt. I'd imagine grease or dirt would prevent it from coming out of fast idle.

[geezer#99]

It’s working as designed.
When first started the choke coil heats up just enough in a short period of time to kick off the high idle. Heat comes from the exhaust crossover initially on the underside of the intake. The ambient air around the coil and cover also heats up. Once moving down the road that warm ambient air around the stove is cooled back down due to air movement in the engine bay. The coil cools just enough to kick the high idle back on. With prolonged driving the heat in the engine naturally rises and keeps the choke coil warm enough to keep the choke off and high idle dropped off.
Yours is likely set just right. If it kicks down and you can drive off without it quitting or stumbling then the cure would be turning down the high idle to an acceptable rpm.
The linkage rod from the coil to the carb controls the richness or leanness of your choke.
You can fine tune it by adjusting the length of the rod.
A strategic bend can change the length.
It can be tricky finding the length that acts just right.
I suspect yours is set right at it’s leanest length. Likely about perfect for your driving.
Be happy you’re living in a warm climate. In 40 below country setting chokes is way more fun.
I used to have a couple different linkage rods that I would change out depending on time of the year. Made them from wire coat hangers.

[Wrenchbender Ret]

The heat passages in the intake manifold may be plugged up or the heat riser valve on the exhaust pipe is stuck open. It may not be heating up the manifold to get heat on the choke coil.
George

[Orange Crate]

If you are feeling frisky you can check the set up of your carb to see where it is at. Check out this site:

http://vetteworks.tripod.com/qjet.htm

[dz1087]

Originally Posted by geezer#99

It’s working as designed.
When first started the choke coil heats up just enough in a short period of time to kick off the high idle. Heat comes from the exhaust crossover initially on the underside of the intake. The ambient air around the coil and cover also heats up. Once moving down the road that warm ambient air around the stove is cooled back down due to air movement in the engine bay. The coil cools just enough to kick the high idle back on. With prolonged driving the heat in the engine naturally rises and keeps the choke coil warm enough to keep the choke off and high idle dropped off.
Yours is likely set just right. If it kicks down and you can drive off without it quitting or stumbling then the cure would be turning down the high idle to an acceptable rpm.
The linkage rod from the coil to the carb controls the richness or leanness of your choke.
You can fine tune it by adjusting the length of the rod.
A strategic bend can change the length.
It can be tricky finding the length that acts just right.
I suspect yours is set right at it’s leanest length. Likely about perfect for your driving.
Be happy you’re living in a warm climate. In 40 below country setting chokes is way more fun.
I used to have a couple different linkage rods that I would change out depending on time of the year. Made them from wire coat hangers.

Sounds fun having to swap those out in the frozen tundra....

Thanks for the info. I was playing around with the carb last night and saw where the coil could cool and pull the fast idle circuit arm back up, so I suspect you're completely right - it's behaving this way based on the choke rod length.

Next step is to slow the fast idle circuit a little.