1969 CHEVROLET IDLING ISSUE

[STEVE POLLARD]

Ongoing issue with my 1969 C20...after driving the truck for 10  to 15 mins, the engine idle seems to drop to the point that it might stall out while at a stop light. I had the quadrajet rebuilt, I bumped up the timing from 4' BTDC to 10' BTDC....and I increase the rpm's. The truck starts up fine and runs great , but like I mention, once it's warmed up, that's when I start having issues. The engine set up is a 350 / 255HP with a turbo 400 transmission - all original, with the exception of a Delco-Remy hei that the previous owner had added.

Any thoughts ?

 

Thanks

[Ben Bruce aka First Born]

Been awhile since I messed with a Chevy. So may be way off.  Sounds like this may be happening when the choke opens all the way and the idle adjusting screw clears the fast idle cam. I would try adjusting the idle after it is warm. Just a wag.

 

  Ben

[joe_padavano]

6 minutes ago, Ben Bruce aka First Born said:

Been awhile since I messed with a Chevy. So may be way off.  Sounds like this may be happening when the choke opens all the way and the idle adjusting screw clears the fast idle cam. I would try adjusting the idle after it is warm. Just a wag.

 

  Ben

 

First, the curb idle and idle mixture screws should ONLY be adjusted with the engine fully warmed up and the choke fully off.  Has the OP verified that the choke does, in fact, open fully? Is the float set correctly? Has the float been replaced recently (the plastic Qjet float will absorb fuel over time and become less buoyant). You can check float level with the engine running using a gauge through the vent tube.

 

[STEVE POLLARD]

Joe, the float is open fully.... the quadrajet was completely rebuilt last week. I don't have a float gauge, so that is something I need to look into. I did increase the timing again ( between 10 &12 ) it sounded great, went for a test run and sure enough, on the ride back home, engine idle sounded different / decrease.. Just wondering... by increasing the timing, will that cause the engine to run hot ? 

 

Thanks for your input.

[joe_padavano]

Advancing the timing at idle will typically help the engine run cooler at low speeds. I notice you have a non-original HEI distributor. Why don't you start by telling us the exact configuration of the car and what has been done to it recently, so we aren't guessing and giving you bad info.

[STEVE POLLARD]

Sure.... 1969 Chevrolet C-20 pick up truck... engine is a 350 / 255 hp with a Turbo 400 automatic transmission... it has 88,000 original miles on it, purchased it last August....work done since I've purchase it:  all fluids and filters have been change, hose and belts, valve cover gaskets, new exhaust system, new brake flex lines.... and just recently the carb rebuild. The HEI was installed by previous owner. Just a nice old truck that was used for camping.

[joe_padavano]

Did you rebuild the carb or have it done? Did you have this problem before the carb rebuild? Did you replace the float during the rebuild (it is not normally included in a rebuild kit)? Have you adjusted the idle mixture screws using a vacuum gauge? Is the vacuum advance operating from ported or manifold vacuum?

[STEVE POLLARD]

No... the rebuild was done by a mechanic that I've known for over 30 years... The truck was running fine....... while driving it one day, I accelerated hard going up an incline, that's when the issue with the idle started.... I mention this to my mechanic and he believed that the accelerator pump on the carb failed...at this point we decided to do the rebuild.  He did adjust the idle mixture screws and check vacuum as well. Vacuum advance is operating from manifold.  But I would have to ask him about the float if that was included in the rebuild kit.

 

 

[joe_padavano]

9 hours ago, STEVE POLLARD said:

No... the rebuild was done by a mechanic that I've known for over 30 years... The truck was running fine....... while driving it one day, I accelerated hard going up an incline, that's when the issue with the idle started.... I mention this to my mechanic and he believed that the accelerator pump on the carb failed...at this point we decided to do the rebuild.  He did adjust the idle mixture screws and check vacuum as well. Vacuum advance is operating from manifold.  But I would have to ask him about the float if that was included in the rebuild kit.

 

 

 

The accelerator pump has nothing to do with idle - it doesn't even function until you press on the accelerator pedal. Your Qjet may or may not have a hot idle compensator valve.  If this is not working properly, that could cause a hot idle problem. I would also check ignition. Many "carb" problems end up being ignition problems. The HEI electronic module is notorious for having a failure mode under hot temps, but will work fine when cool. Of course, check carefully for vacuum leaks as well.

[Curti]

What would be the reason to replace a float if it doesn't leak?  

Personally I would ditch the aftermarket distributor and go back to stock.

[STEVE POLLARD]

Thanks for the info !

[Frank DuVal]

Quadrajets in the spread bore time had “foam plastic” floats. They become saturated with fuel and need to be replaced when that happens.  Back in the day I replaced many of them.....

[joe_padavano]

57 minutes ago, Frank DuVal said:

Quadrajets in the spread bore time had “foam plastic” floats. They become saturated with fuel and need to be replaced when that happens.  Back in the day I replaced many of them.....

 

^^^THIS.  The ethanol-laced cat urine that passes for gasoline today attacks the plastic float, causing it to absorb liquid and thus become less buoyant. A replacement float costs all of $6 at RockAuto. It's not even worth debating, just replace it as part of a rebuild.

 

By the way, there were never Quadrajets that were NOT in the "spread bore time"... ?

Edited August 25, 2018 by joe_padavano (see edit history)

[Bloo]

4 hours ago, Frank DuVal said:

Quadrajets in the spread bore time had “foam plastic” floats. They become saturated with fuel and need to be replaced when that happens.  Back in the day I replaced many of them.....

 

 I replaced scads of those back in the day. Those "nitrophyl" floats had a useful life of about 80K miles, and then they sink. The service manuals had a specification for weight, and you could theoretically weigh them and re-use them if they had not gained weight. If you did, the car would be back in a week or a month with a sunk float, and you would get to do the rebuild all over again.

 

NAPA had brass replacements, as well as the Nitrophyl ones. Maybe the brass was an upgrade, or maybe the earliest quadrajets had brass, I'm not sure. The brass ones make it much harder to get the float level correct, but last forever. Nitrophyl will fail, probably before anything else in the carb does, but sets up correctly on the first try. Pick your poison.

 

 

[carbking]

Early Q-Jets had brass floats.

 

There have been a number of DIFFERENT style Q-Jet floats. They look interchangeable, but they are not interchangeable.

 

Because of the failure of the Nitrophyl floats, some aftermarket brass floats were available 40 years ago to replace the Nitrophyl floats. These are not interchangeable with the early original brass floats.

 

Floats should ALWAYS be ordered using the carburetor identification number.

 

If you are buying a "new" brass float, check the box to see where it was made. Far eastern solder does NOT like ethanol. On the other hand, neither does Nitrophyl, but the Nitrophyl seems to be more reliable than far eastern solder. Avoid the "new" offshore brass floats.

 

As to float adjustment: I can never remember a Q-Jet with a float out of adjustment, if one used a new correct float, and a new correct fuel valve. There are maybe 40 or so different fuel valves for the Q-Jet. Don't automatically assume that your FLAPS kit has the correct one.

 

If float adjustment IS necessary: NEVER (one of the words I dislike using) bend the arm on the float. Instead, change the thickness of the gasket under the fuel valve.

 

If correct for the engine, and properly rebuilt, the Q-Jet is one of the finest carbs ever built, at any price, at any time, anywhere.

 

EDIT: and if you have to go into a Q-Jet, and it has the Nitrophyl float, don't bother trying to weigh it; "file 13" and replace it. Call it "lazy insurance" (so you don't have to replace it next week  )!

 

Jon.

Edited August 25, 2018 by carbking (see edit history)

[Frank DuVal]

11 hours ago, joe_padavano said:

By the way, there were never Quadrajets that were NOT in the "spread bore time

 

I got confused with the square 4-jets (4GC) of the late 50s early 60s, which were equal sized venturi Rochester's. Quad = 4. But the term Quadrajet was applied to the spread bore design..........?

[cahartley]

6 hours ago, carbking said:

Early Q-Jets had brass floats.

 

If you are buying a "new" brass float, check the box to see where it was made. Far eastern solder does NOT like ethanol. On the other hand, neither does Nitrophyl, but the Nitrophyl seems to be more reliable than far eastern solder. Avoid the "new" offshore brass floats.

Jon.

 

Neither do the crap floats coming from Mexico........ 

[joe_padavano]

8 hours ago, Frank DuVal said:

 

I got confused with the square 4-jets (4GC) of the late 50s early 60s, which were equal sized venturi Rochester's. Quad = 4. But the term Quadrajet was applied to the spread bore design..........?

 

Yeah, the 4GCs were sometimes called "Quad Jet" carbs, which has led to a lot of confusion. ?

[54vicky]

if single exhaust have the y pipe checked as the were known to delaminate causing engine overheat and sluggish getting worse until completely closing off.if your mechanic is old school he can check easily