I just replaced my Qjet with a new Holley 4160 (600) on my stock 350/300. It starts both cold and hot very nice but it diesels now at shutdown. Do I need to change the timing because of the new carb?

 

Check your idol RPM . It may be a little high.

Bill

 

It's set at 650RPM.

 

What I would try is to adjust idle mixture screws to obtain maximum manifold vacuum. This may increase the idle speed without increasing curb idle screw setting. If idle does increase then you can back off the curb idle screw to close the primary throttle plates thus decreasing dieseling. This worked on my Q-jet. What is your initial timing set at? How much advance do you have at idle with the vac. advance hooked up? Good luck with it. mike...

 

I don't have an advanced timing light. I set the timing to what the owner’s manual said, 4* BTDC with an old style light with the vacuum advance disconnected.

 

I am not advising you to do the same as me but here are my settings. 15 degrees initial, 11 deg. vac., 26 deg. at idle, 13 in. hg. manifold vac., at idle. This raised the idle rpm which allowed me to back off on the curb idle screw and close the primary blades enough to stop the dieseling. mike...

 

You guy's are confusing with timing advance, as there's no spark during shutdown when the run on(dieseling) is occuring. This condition is usually caused by hot combustion chamber carbon build up and low octane fuel. The new carb probably has a different idle fuel/air mixture, possibly adding to the problem. Maybe try some fuel cleaner additive's for awhile or bump up the octane you're using and see what happen's. Also, curious why the carb change, as a well set Q-Jet(while sometimes difficult) will easily show it's tail light's to most of these.

 

Can't imagine why you would waste time and money replacing your Qjet with the Holley on your base engine. You will gain nothing. Your Qjet will give you the best overall pereformance for your engine provided it is maintained.

 

it's a very poor replacement for the qjet

 

That is not what he asked and your irrelevant post adds nothing significant to this thread. I can tell you the exact reason he changed it. There are only 4 people in the world that can actually tune a quadrajet and maybe only 10 more than can actually read the book and make sense of it. I am sure that the qjet is an incredibly better carb but considering there are a million holley tuners for every 1 qjet I think he made the right choice. Wait one of the 4 just died.

 

i would give it more initial timing...like 10 -12 and that will increase your idle speed meaning you can wind down you idle screw on the carb meaning your butterflys will close more stopping the run-on like mike said.

 

Run-on (dieseling) is almost always caused by the primary throttle blades that are too open at idle. When the TB's are too open, fuel continues to be pulled into the engine where it's ignited by a hot combustion chamber.

In fact, many of our carburetor equipped Vettes came with an 'idle stop solenoid' that would allow the TB's to shut completely when the ignition was turned off. Here' an example:

https://www.paragoncorvette.com/p-34...-solenoid.aspx

But you can easily solve this without a solenoid.

Step One: You say your initial timing is set to 4 degrees advanced. Bump it up (advance) in 2-3 increments after test drives until you hear or feel pinging (knocking, pre-ignition, detonation, etc). When you've reached the the 'death zone', back your timing down 2-4 degrees. Most of us are running at least 10 degrees or more.

Increasing your initial advance will also increase your curb idle RPM. Lower the CI RPM to specs, which will close the TB's a bit.

Step Two: I bet you're running ported vacuum to your vacuum advance on your distributor. "Ported" meaning, there's no vacuum to the vacuum-advance at idle. With a vacuum gauge, find a source on the carb for manifold vacuum. "Manifold" vacuum meaning vacuum is present at idle. Then use that source for your VA.

Again, this will raise your curb idle RPM. Again, reduce it to specs, which will further close the primary throttle blades.

The two steps above will likely will solve your problem. If it doesn't, I have a couple of plan B's, such as cracking the secondaries open a bit at idle so that you can further close the primaries.

Also, running manifold vacuum to your distributor has several other advantages; a cooler running and more responsive engine that gets better mileage.

Good luck!

 

i could never get a good steady idle with my holley, i had to idle it at @1000 and it would drop to @600 in gear. and would load up the plugs and diesel when i killed it. idle air mixture screws had little to no effect.
i found the problem when i removed the carb. the primary throttle blades were open too far and the transfer slots (if thats what there called) were uncovered. i opened the secondarys a tiny bit. just a hair really and closed the primarys and now it idles at @850 and drops to @700 in gear and no more plug fouling and dieseling when i kill it.
it made a big diff.

 

OK, I drained all but about a gallon of gas from my tank (89 octane) and put 5 gallons of 93 ethanol free in. I also reconnected the Vac advance to what I think is an un-ported source on the carb. The run-on stopped, yippee!!! I advanced my timing to about 10* BTDC and the pinging increased during acceleration. I reset it to 4* and the pinging decreased but is still there while accelerating. What else can I do to resolve this issue?

 

buy a new timing light /confirm your timing mark ...... a motor that pings on 4 needs help before you even start messing with your carb. .

 

Did it ping at all with the q-jet? If not i would say your holley still needs tuned. I'm pretty sure detonation or pinging, or possibly even pre-ignition would be too lean of an air/fuel mixture, but i'm new at this and slowly learning as i go. I have the exact same carb and i am contemplating the same swap on my '81. good luck, i hope you get it sorted out!

 

Yes, it pinged with the Q-jet. It just didn’t “run-on” after shutdown. But now after changing to 93 octane the run on after shut down is gone. However, the pinging during acceleration was still there. So I retimed the initial timing to 10* and it seemed better but still pinged. My last hope was the mechanical advance. I replaced the springs with the stronger ones made for higher horse power engines and bingo, the pinging was gone. Wish I would have done this a long time ago.

As for the new Holley, I love it. It starts like a fuel injection car. I only have to tap the accelerator to set the choke and it fires right up. The only adjustment that I have made was the curb idle after the timing increase. I’M BACK ON THE ROAD!

 

+1....

I'll take a holley any day over a q junk....Parts are available at any auto parts store and they are simple enough you can rebuild one in the parking lot...

A q junk screws up out on the road, your SCREWED....

 

They're both great carbs (Holley and Q-Jet). They each have their advantages and disadvantages. To say a Quadrajet is junk shows a deep lack of understanding of the clever engineering behind the Q-Jet.

As far as ease of tuning, I can change the secondary rods of a Q-Jet in less than one minute, only having to remove the air cleaner.

In fact here's my comparison (I'm needing a good flaming):

"Groovy" Factor,..Holley

Fuel Mileage,.. Q-Jet

Tune-ability,..tie

Available parts,..tie
(Yes there are numerous Holley suppliers but there are probably 10,000,000 Q-Jet cores out there), plus plenty of sites such as this selling Q-Jet parts: http://www.carburetion.com/quadrajet.asp

Less time needed to rebuild,..Holley

Street Performance,..tie
(Comparing a 750 cfm Holley with a 750 Q-Jet and assuming both are tuned to the engine underneath)

Adaptability,..Q-Jet
(Q-Jets were intalled on anything from 390-HP L-36 Corvettes in 1969 to Pontiac V6's in the late 70's. It's air-valve design makes it hard to "over" or "under" carb an engine as often happens with a Holley)

Ability to meter fuel,..Q-Jet
(A Holley is more of a blunt instrument; A Q-jet is more refined. For instance the intricate power-piston set-up of the Q-Jet vs. the Power Valve of the Holley)

Still being made new today,..Holley

Control of fuel in the fuel bowl,..Q-Jet
(Ask drivers of off-road trucks about this,..or ask road racers about fuel slosh back through the Holley vent tubes when the brakes are aggressively applied,..or ask why Holley had to start making "marine" carbs)

Intake Manifold Options,..Holley
(Many more square bore options)

Adjusting the rate of opening of the Seconary side of the carb,..Q-Jet
(Instead of trying differnet springs for the vacuum (diaphragm) controlled secondaries of the Holley (a pain to R&R), the Q-Jet is controlled by adjusting the tension on the secondary air valve with a 3/32" allen wrench and small screw-driver. Again, less than a minute. Of course a Holley DP removes most all adjustability)

That's probably enough for a good flaming. I'll add more as we go.

 

Pull a few of your plugs and see if they might be running on the lean side. Post a pic if you can of the plugs.