Qjet performance and idle run on.

Need some help guys with a run on issue.

I have about 1600 miles on the rebuild. 454 +.030, 224/234, .569/.569. Timing is 34 all in at 2800. Initial is 10. There is no vac. advance at idle due to the TCS(cuts advance off in first and second MT). It has the original Q-jet rebuilt with the stock jets and rods. It runs great, but I have a couple of tuning issues to handle.

The AFR at idle is 14.5 to 15. Idle is 700.
AFR at cruise is 13.0 to 13.2.
AFR at WOT is initially 10-11 (pump shot effect?) and then climbs to about 12 to 12.5.
The spark plugs look good, perhaps a little dark/rich.

Here is what I am trying to remedy:

The engine has run on. Sometimes a couple of rotations, sometimes more. There isn't an idle soleniod. The good ole dashpot is still there and working. It idles around 700. The run on is probably due to running lean at idle. The choke blade is strait up and down when warm. It has about 1/8 inch to 1/4 inch of play. If I close it up with my finger by that play, the AFR goes down to about 13.5 to 13.7 and the idle picks up a little bit. This puts the idle AFR a little more in line with the cruise AFR. If I cup it slightly with my palm, I get the same effect which basically verifies a lean idle. Vacuum at idle climbs to 11-12 from 10 when I cup the carb.

Starting the car cold is a little bit of a problem. Needs to run about 15 to 30 seconds before it starts to climb to fast idle. Another minute or 2 before it hits fast idle. Perhaps the choke is set up wrong? I verifed that it has 1/4 inch play from full close before starting the car. Choke pulloff is functional.

I want to get above 13 AFR at cruise and increase the AFR slightly at WOT. I have figured that going up one size in rod will take care of the cruise. If I drop a jet size, I can accomplish increasing both the cruise and WOT AFR.

If I drop the cruise AFR, I'm afraid that my idle will lean even more causing more run on. I don't want to go into the idle circuits if possible. The idle mixture screws are out at 3 1/4 turns. The carb will pop out the carb on light throttle at about 2 1/4 turns. Richening it up more than 3 1/4 has no effect.

So, after a long winded explanation, to get back to the question, before I start tuning for better AFR, what can I do about the run on?

Thanks.

What kind of ign system do you have? No msd box or anything right?

Twin_Turbo said:

What kind of ign system do you have? No msd box or anything right?

Click to expand...

Stock coil (relatively new), stock dist(rebuilt) with a breakerless se vain type conversion instead of points.

Here is the Breakerless conversion. It worked fine b4 the rebuild. Plugs are AC R43t (stock)

http://www.breakerless.com/picturegallery.htm


Edit: Here's a link to the spark plug pics.

http://vettemod.com/forum/showthread.php?t=1341

Prolly not the ignition since there's no ext. amplifier that could cause the run on.

I would start by trying to fatten up the idle a little and see what it does.

It can be caused by too much air at shut down. The original idle solenoid was to reduce that air at shut down, and held it open key on. Lars had a cure for this, but I do not recall it.

big2bird said:

It can be caused by too much air at shut down. The original idle solenoid was to reduce that air at shut down, and held it open key on. Lars had a cure for this, but I do not recall it.

Click to expand...

The 70 BB with MT did not come with an idle solenoid. I was thinking that a lower flowing PCV valve might help, but then I think it would restrict air at all RPM's causing a richer mixture overall which I don't want.

I wonder if the carb can be modified for a solenoid.

I'll see if I can find Lars' post. Maybe even shoot him an email.

Here's a response that Lars gave on another forum:

Re: Deiseling at shut-off, lots of smoke at start-up...Need Help... (Double_0_7)

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Dieseling at shut-down is most commonly caused by idle speed too high. Drop your idle speed down just a tad, and it will solve the problem. If your car is equipped with the idle-up solenoid, be sure the idle speed is set with the solenoid, and that the idle speed screw on the carb is set to the lowest possible idle speed (about 400 rpm). This will solve the run-on problem.


Timing won't cause run-on, since the timing is off when the problem occurs. Just set your idle speed down to the lowest comfortable level, and make sure your idle mixture is not too rich. Run-on was a common problem in the mid-70's, and GM actually issued a Service Bulletin telling the dealers to lower the idle speed...

SmokinBBC said:

Here's a response that Lars gave on another forum:

Re: Deiseling at shut-off, lots of smoke at start-up...Need Help... (Double_0_7)

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Dieseling at shut-down is most commonly caused by idle speed too high. Drop your idle speed down just a tad, and it will solve the problem. If your car is equipped with the idle-up solenoid, be sure the idle speed is set with the solenoid, and that the idle speed screw on the carb is set to the lowest possible idle speed (about 400 rpm). This will solve the run-on problem.


Timing won't cause run-on, since the timing is off when the problem occurs. Just set your idle speed down to the lowest comfortable level, and make sure your idle mixture is not too rich. Run-on was a common problem in the mid-70's, and GM actually issued a Service Bulletin telling the dealers to lower the idle speed...

Click to expand...

Yep. That's the one. Can you lower the idle speed? The solenoid will bolt right on if not. Not a biggie.

big2bird said:

SmokinBBC said:

Here's a response that Lars gave on another forum:

Re: Deiseling at shut-off, lots of smoke at start-up...Need Help... (Double_0_7)

--------------------------------------------------------------------------------
Dieseling at shut-down is most commonly caused by idle speed too high. Drop your idle speed down just a tad, and it will solve the problem. If your car is equipped with the idle-up solenoid, be sure the idle speed is set with the solenoid, and that the idle speed screw on the carb is set to the lowest possible idle speed (about 400 rpm). This will solve the run-on problem.


Timing won't cause run-on, since the timing is off when the problem occurs. Just set your idle speed down to the lowest comfortable level, and make sure your idle mixture is not too rich. Run-on was a common problem in the mid-70's, and GM actually issued a Service Bulletin telling the dealers to lower the idle speed...

Click to expand...

Yep. That's the one. Can you lower the idle speed? The solenoid will bolt right on if not. Not a biggie.

Click to expand...

I'm going to try fatten up and lower the idle. I'll let you know how it turns out.

Well after many trial and errors, it looks like it needs a combination of lower idle and richer mixture.

I lowered the idle spead to 675 then 650. Car wanted to die a couple of times at 650. Still had run-on at 675.

I reset to 700 and fattened it up 1/4turn. That brought the A/F down to 13.8 to 14. Still ran on, but not as bad. Fattened it up another 1/4 and that brought the A/F down to about 13.6 to 13.8. A little deisel, so I knocked down the idle to approx 675-685 and bingo(I hope). No run-on after 2 trips and shutdowns....damn near took half a day. And I thought that I didn't have any more response past 3 1/4 turns before when I tried this.... BTW, I forgot to take the vacuum reading during the changes:crap:

I'll run it like this another couple of days and see if it is permanant. I tried just richening B4 and that only lasted a day or 2.

Thanks for the suggestions.

Anyone here real good at tuning Qjets? Hopefully next week I can get into tning to lower the A/F cruise/WOT.

Ah...bloody hell....

Today it deiseled again....about 2 to 3 revs. I can't go any lower on the idle and I don't want to richen it up any more than need be. Besides, when I get to tuning the carb, I want to run it leaner at idle.

Bird or anyone have pics of the 69, 70 or later qjet carb with the idle soenoid set-up? I need to dig through my books to see if I can find one. The parts are available, so I need to investigate this.

Have run into this many times and it can be a pain to remedy sometimes.

First guess would be that you have an intermittant problem with the carb.
By that I mean that all is good while you tine it, and then upon driving, it changes sometimes. The most common cause I have found was in a worn linkage system not alowing the throttle plates to close as per tuned EVERY time the throttle is released.
If when the carb was rebuilt, and throttle shaft bushings were not installed, then it is possible that the plates can bind and not return to the same possition every time. Eccessive air leakage itself around the shaft can be a problem, although it should be able to be adjusted out. I have seen some pretty bad ones. Just because GM allowed a certain amount of leakage new, doesn't mean it can't be improved upon to ease your fine tuning.
Also the secondary plates can hang up ever so slightly. I usually make sure there is a little clearance on the secondary linkage tang to eliminate that. It won't hurt operation at all.
All this is easy to see with the carb off, held up to the light and operating the linkage dozens of times.
Try adjusting the dashpot out of the picture completely.
You could also install a much heavier return spring just for testing.

The solenoid mounts to the 2 big holes near the accel pump on the linkage side. I can get a pic of it later today.

010752 said:

Have run into this many times and it can be a pain to remedy sometimes.

First guess would be that you have an intermittant problem with the carb.
By that I mean that all is good while you tine it, and then upon driving, it changes sometimes. The most common cause I have found was in a worn linkage system not alowing the throttle plates to close as per tuned EVERY time the throttle is released.
If when the carb was rebuilt, and throttle shaft bushings were not installed, then it is possible that the plates can bind and not return to the same possition every time. Eccessive air leakage itself around the shaft can be a problem, although it should be able to be adjusted out. I have seen some pretty bad ones. Just because GM allowed a certain amount of leakage new, doesn't mean it can't be improved upon to ease your fine tuning.
Also the secondary plates can hang up ever so slightly. I usually make sure there is a little clearance on the secondary linkage tang to eliminate that. It won't hurt operation at all.
All this is easy to see with the carb off, held up to the light and operating the linkage dozens of times.
Try adjusting the dashpot out of the picture completely.
You could also install a much heavier return spring just for testing.

The solenoid mounts to the 2 big holes near the accel pump on the linkage side. I can get a pic of it later today.

Click to expand...

I had the carb rebuilt by Lars G. He did not replace the shaft bushings..not needed. He set the carb up pretty well. It ran the first 1k miles issue free. The secondaries are closing all the way.

Will retarded timing cause run on? Reason I ask is because it has been running a little poorly at times lately. I checked the timing and it was at 8 initial. I thought it was at 10. I pulled the dist to look at the bronze gear and end play. Everything was good. Reinstalled, reset timing to 34 total which gave 12 initital. Of course...you guessed it, the idle improved significantly, so I reset the carb idle mixture screws and lowered the idle......2 rides and no more run on......for now:clobbered:

I would appreciate the pic when you get a chance.

Thanks.

The only times I have ever had runon associated with timing was with mileage engines, set a lot more retarded than 8, and with a lot of carbon buildup increasing compression and hot spots, and all together creating the dieseling effect. Even though yours was a little lean, I doubt with a new engine it would get hot enough in there for that, but jumping to 12 will definately cool things off.

Here are the pics of the solenoid.

Thanks for the pics.

Here is a pic of my Qjet before the rebuild. Doesn't look like I can add the solenoid.