Hi,

I know there have been numerous threads on this issue with some various opinions, however I will the question particular to my situation

1) I have had my car tuned and currently run a 1970 quadrajet that is off a truck when I run the numbers. Carburetor was rebuilt.

2) Car idles at 700 RPM.

I noticed the first time I drove the car home and parked it there was some dieseling and I have been on plenty of drives since and no dieseling until today, it was quite bad actually.

I did fill up earlier on in the day. First time it dieseled it was a different petrol station fuel to today, however both times I ran 98 RON. The tuning was done with 98 RON.

Should I take the car back to get the idle lowered or timing adjusted? The car does have a mild cam

Is there a reason why the dieseling would only occur occasionally?

Thanks,

NJ

 

Bad gas.

 

Bad gas, compression ratio on the high side, carbon buildup creating a hot spot. Shut engine off in drive or slip clutch a bit as you shut it off. Easier to deal with the symptom than pull heads to try to find the problem...

 

I would suspect that you're just on the ragged edge of any number of differing conditions that could permit dieseling (idle timing, octane level, coolant temperature, air temperature, humidity level, barometric pressure, etc).

My first questions are always, what is your idle timing set at, and is manifold vacuum used for a source for your vacuum advance canister.

 

When you say, take the car back to adjust timing I assume you don't know what degree timing you are at now? Did someone set the Initial Timing and not Total Timing? I do not know. Then, how much extra is your vacuum canister adding? Is it hooked up to a Timed vac port or Full manifold vac port?

For now, I would try to locate some Seafoam Gas Treatment on your side of the earth and slowly introduce that into the carb at a fairly high idle. That will remove any trace amounts of carbon deposits and remove any trace amounts of mosquitoes on your property.

Next, for any engine to diesel, it has to have a extra fuel supply and a remote spark. (remember, the sparkplugs are turned off) Its just that simple. No extra fuel, no diesel. So, I would check into the closing of the secondary throttle blades or lack there of. Its common for some carbs to build up some gum deposits around not only the throttle bore but the blades and throttle shaft itself. And in rare occurrences the secondary throttle blade spring is too weak to close the blades completely. Then you are introducing extra fuel.

I really doubt the octane level is the culprit here. I had the same issue for a week last yr. Never changed the purchase of octane levels but did find the secondary throttle blades open a pinch too far. They are adjustable on Holley.
And your 700 RPM idle is just fine.

 

Yes my mechanic cleaned out and refurbed the carburetor and set it up on the car. I am not aware of the timing details etc.

 

I finally got time to go back to the mechanics and he backed the timing off a touch.

He said the combination quadrajet from a 70's truck and the mild cam was probably the issue.

Does a cam require the engine to run a tad rich? And the hot car idling for a bit before shutting off is causing dieseling?

I do have a holley 3310 sitting around which needs a rebuild, is that a better option? I am potentially regretting rebuilding the quadrajet, I didn't initally mention to the mechanic I had a holley sitting arouund.

 

Qjets are great carbs, they are like edelbrocks, go through and blueprint all the components and fix the issues that have been caused by people not knowing what the are doing and you'll have a great carb with good millage

3310 is a great carb too, it's easier for better performance than the Qjet, but not as good for millage, so it's depending on what you want

 

Did your mechanic say why he backed the timing down?

 

I am just a weekender but I can tell you what I did to fix a nasty dieseling problem. I have an L46 350/350 that a PO put a Holley 4160 (3310-4) on. It had been sitting a long time. I followed the Holley docs to setup and connected the vacuum advance to the ported source. Car ran fine but would occasionally diesel. I found that if I kept the idle low at or just below 700 and waited just a second or two to shut it off it probably wouldn't - but that annoyed the crap out of me. In the course of screwing around with it I switched over to manifold vacuum. I know - I know - hot topic here - but it corrected the dieseling instantly. Is it possible the mechanic just plugged the hose back in the wrong spot and then changed it when you came back again? I am not familiar with q-jets but Google tells me they have both ported and manifold vacuum sources.

As for your Holley. Easy rebuild. Seems like the needle/seats are a bit of a p.i.t.a. and there was an ancient recall that I was unaware of and tripped over regarding the floats but it was a good project. Easy to adjust and work on if you're just leaning, which I am. Nothing like a gasoline fountain in the driveway to focus your attention.......

Good luck

 

Can you give me some info on that Holley recall issue? Thanks.

 

I probably phrased 'recall' incorrectly. I rebuilt it very carefully and had a bad flooding problem - that's what the gas fountain in the driveway was about. I spent some time on the phone with Holley tech support and there was a very helpful young man that was willing to look at a few pictures I sent him. The old yellowish looking floats were the problem and he was surprised because - according to him - they had been discontinued for years because of various problems. No spring on them and nowhere to attach one. Maybe I'm one of the last few who failed to get the message (there are still people out there unready for Y2K ....)

I replaced them with new Holley floats (black plastic) that do have that spring. So no, not a 'recall' because I had to buy the damned things. Maybe 'product alert' would have been better. To those who rebuild carbs all the time this is probably old news and simple stuff but to me it was well earned experience.

These are the old crappy floats. No, even with those notches you see nowhere to land that little spring.

 

Not surprised if you are still running ported vacuum advance. It was a late 60s / early 70s emsision control device that didn't work well even when new. It requires opening the throttle more at idle, hot exhaust manifold temps, dripping carbs, dieseling, and the required idle solenoids to prevent it were a common result. Just run manifold advance and rebuild the distributor advance and vacuum curves to match a pre-emissions mid sixties corvette, or the Chevy High Perfomance manual recomendations. It'll idle better, run better, run cooler, and not diesel. Yeah it won't pass 1970 emissions anymore.
Leigh

."If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more."

"For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts."
Tom

 

 

I am not sure to be honest...

 

NJey1985 - I fought dieseling issues with my current engine for quite a while - Without giving you a long and boring history of how I finally solved it, the culprit was a combination of 2 things: Idle speed was too high and initial timing was too low. You have to figure out what the engine wants, and getting there can be an exercise in fine tuning your ignition timing and idle setup.

For a long time, I ran on the "ragged edge" of both of these and still could not solve the dieseling without use of an idle solenoid to keep idle where I thought the engine liked it, and still close throttle blades enough to prevent dieseling on shut down.

This summer I revisited that, and bumped the timing a degree or two more, and finally got my carb's idle setting adjusted down enough via the secondaries to keep idle speed low (but not too low). Cam duration can effect how much timing your engine likes at idle, so the fact you have a mild cam suggests greater-than-stock initial timing may be required.

Every engine is different, but I personally find from experience, reading and research that these 2 factors are the most common contributors.

In my case, my cam is 230 degrees at .050 lift, and I finally ended up at 22 degrees of advance at idle, with an 800RPM idle speed. I adjusted the distributor so I am all-in at 32 degrees by 3500 RPM. I allow at most 10 degrees vacuum advance on top of that at idle, and the engine likes it. It starts easy, I have not experienced dieseling all summer at this setting, and find no evidence of any pinging.

I'm not saying this setting is right for your car, just relating my experience and results.

I would ask your mechanic why he backed the idle off, and if he adjusted idle speed to compensate when he did that. I find that higher initial timing allows you to close throttle blades more and keep the same idle; so if he backed off timing, he may have opened your throttle blades up - which can lead to this problem in some cases. I would bet you just need to experiment to find the right combined settings to eliminate the run-on.

 

 

Car has plenty of power when I put the foot down.

The car runs Comp Cams K11-242-3, initially when I got the car it was idling at 500 rpm and I didn't notice and significant dieseling. Now it's at 700 rpm. Car feels much better with the current setup.

I did get a second opinion and the mechanic who builds and tunes carburettors for race setups and he said that a standard rebuild kit gaskets and seals might not be enough for the Quadrajet. Rather than throw more money at it, I have decided I am just going to live with the problem short term and actually get a new Holley carburettor 750 cfm and get the car dyno tuned.

 

So I ended up getting a new holley carburetor 750 cfm and had the car dyno tuned and idle set up. First day of driving I was happy and then on the second day with a bit of spirited driving, I came home and then the dreaded dieseling happened again...are idle stop solenoids available for the new model holley's?

The car idles at 700 rpm and drives perfectly

https://www.holley.com/products/fuel_systems/carburetors/classic_holley/parts/0-80508S

 

You don't, nor does anyone else need a solenoid stop on the Holley. You still have other issues that are the cause of dieseling. You need to figure out what your Vac advance is doing at idle and where your timing mark is at idle. Are you using full vacuum port on the Holley not timed port? That Holley should have two small vac ports in the front. Put a Vac gauge on each one, one at a time. One should have no vac at idle (timed) and the other is full vac.