[Tonys72SS]

Working through a new Pro Systems carburetor on my Mark Jones built 467 and first things first, the carburetor is a beautiful piece! Went with a 1000 cfm Pro-Series 4150 from Patrick and the thing runs really nicely actually! Still working on the tuning of it, but the throttle response is nice and crisp, no bogs anywhere in the RPM range, and everything transitions really nicely. One thing I am working on is it likes to diesel on shutoff, sometimes barely does it other times, will do it for 3-5 seconds after shutoff. I have checked for vacuum leaks, and cant seem to find any. I pulled the carb off and tightened all the screws, so good there! What does not make sense is it still does it even when the throttle blades on the primary side are almost closed, as in the adjustment screw is barely on the tang for the throttle blades.

I was running a bit more timing initial, 22 degrees with a 36 total. It is running an MSD ready to run distributor so maybe springs are an issue?
I am at 2-1/8 turns out from seated on idle mixture screws with the secondary idle adjustment about 1 turn in and the primary side 1/8 - 1/4 turn in. Sitting at approximately 800 rpm in park and 700-750 in gear. Float setting is approximately 25% up on the sight glass according to Patrick's instructions. Idle air bleeds are 72. So at this point I am completely lost... it just does not make sense. I am waiting on a call from Patrick, but maybe someone else might have some ideas?

Other than that Patrick builds one heck of a carburetor!

 

[71malibuu]

Put a diode on your ignition lead to coil+

To test this theory prior to doing it, let the car idle and when you shut it off step on the brakes and turn on your headlights at the exact moment you turn the ignition off. No more dieseling.

 

[pockets]

Dieseling is caused by not enough ignition timing at idle, overexposed transfer slots, or rich idle. One of these things, or a combination of them.

What are your cam specs?

2 1/8 turns on the mixture screws are unusually rich.

Primary transfer slots need to be exposed as squares, secondary not exposed. If you cannot have that relationship to get it to ide, then you need more timing at idle.

 

[Tonys72SS]

Dieseling is caused by not enough ignition timing at idle, overexposed transfer slots, or rich idle. One of these things, or a combination of them.

What are your cam specs?

2 1/8 turns on the mixture screws are unusually rich.

Primary transfer slots need to be exposed as squares, secondary not exposed. If you cannot have that relationship to get it to ide, then you need more timing at idle.

The cam is a hydraulic roller 228/238 @ .050 on a 111 LSA. Lift is .570/.590.

I can go leaner on the mixture screws, but that will make it diesel worse than it is right now. I had the screws set at 1-3/4 turns out and the car ran great, but would diesel worse and last about 3-5 seconds.

I had timing set at 22 degrees initial and 36 degrees total and then came down to 18 degrees initial and 34 total. The car will idle just fine, and I pinched off every vacuum line to see it something was leaking and nothing on that front.

I was under the impression that a lean condition would make it diesel... but at this point what the heck do I know.

 

[oldguy67]

thinking you may have too much air thru the sec.side? set it to a 1/4 turn from seated. readjust carb mixtures.set screws 1 turn out and go from there. most of the time 4-corner idle carbs that i've had end up around 3/4 to 1 turn out. air bleeds are yours to play with;go up or down 3 steps at a time to get a reaction,going up leans out the idle which you may have to address? still try lowering the back blades first.

 

[pockets]

The cam is a hydraulic roller 228/238 @ .050 on a 111 LSA. Lift is .570/.590.

I can go leaner on the mixture screws, but that will make it diesel worse than it is right now. I had the screws set at 1-3/4 turns out and the car ran great, but would diesel worse and last about 3-5 seconds.

I had timing set at 22 degrees initial and 36 degrees total and then came down to 18 degrees initial and 34 total. The car will idle just fine, and I pinched off every vacuum line to see it something was leaking and nothing on that front.

I was under the impression that a lean condition would make it diesel... but at this point what the heck do I know.

If you cannot get it to idle without having the throttle blades open a lot, you need more timing at idle. Reset the primary blades for square transfer slot exposure, and the secondary 1/4 turn open from fully closed. Also check how many turns more you can open the secondary blades before the transfer slots expose. You do not want any secondary slot exposed at idle. This way you know what the max open is once the carb is on the engine.
Mixture screws to 1 1/4 turn.
When you start it and let it warm up, see what it idles at. If it is low, increase the idle timing. With that cam you should be in the 26-28 degrees at idle range, then centrifugal advance limited so you don't go past 36-38 at full advance.
Basically you want the amount of timing at idle that gives you the highest vacuum. The engine is telling you that is what timing at idle it wants.

Then, if you still need to raise the idle, open the secondary only, but do not open it to the point the transfer slots are exposed. Do not close the primary from where it was baselined at.

 

[Tonys72SS]

Sorry for the late response guys. Well after talking to Patrick about the issue I think I finally got it squared away! Patrick recommended stepping down a few steps on the spark plugs as far as heat range. So I did that and turned the secondary idle back and then got the timing up to 24 degrees initial and 36 degrees total. This allows the engine to idle around 850 in park when warm and approximately 700-750 in gear when warm. Also, got the mixture screw to approximately 1-1/8 turns out. It seems to like it there before I get a hesitation when I blip the throttle and it becomes a little sluggish off idle. So far has not been dieseling on shutoff after the changes but the best part has got to be the power out of this engine with the new carburetor! It definitely needed the upgrade from the 750 Holley it was running previously!

 

[pockets]

Did you have some cold plugs in it?

 

[Tonys72SS]

Did you have some cold plugs in it?

I didn't think they were that cold more in the middle of the heat range. They were 6's according to NGK but when I cross referenced them to anything lower, there was nothing available. I went to R42s from AC Delco and they seem to be working alright. Will have to continue to drive it and check the plug condition but so far so good.

 

[pockets]

I didn't think they were that cold more in the middle of the heat range. They were 6's according to NGK but when I cross referenced them to anything lower, there was nothing available. I went to R42s from AC Delco and they seem to be working alright. Will have to continue to drive it and check the plug condition but so far so good.

Iron heads, and 10.5:1 or lower I would be using an NGK 5 heat range, and projected tip. No need for colder.

 

[Tonys72SS]

Pulled the plugs after driving tonight and there was a slight hiccup on shutdown and they all look like this. The plugs have miles on them but overall not terrible in my opinion... what does anyone else think. These are the colder AC Delco plugs by the way

 

[oliver69]

I have the same 1000HP Prosytem carb and 468 vortecpro engine in my Chevelle , plugs are ngk 7052 range 5 I believe; I have slight dieseling only if I run low octane and in very hot summer days . If I run a premium fuel no problems , my timing is 19 at idle and 37 all in at 3000rpm . Have you tried a different fuel ?

 

[Tonys72SS]

I have the same 1000HP Prosytem carb and 468 vortecpro engine in my Chevelle , plugs are ngk 7052 range 5 I believe; I have slight dieseling only if I run low octane and in very hot summer days . If I run a premium fuel no problems , my timing is 19 at idle and 37 all in at 3000rpm . Have you tried a different fuel ?

I've been running the same premium octane fuel the whole time. Can't get anything higher than 91 octane here in California. There are some with 100 octane at something like $10 per gallon so haven't gone that far with it yet. But in other news, pulled the carb off last night, got the primary side adjusted to have the transfer slots square. The car will not idle below 1100 and timing set at 22 degrees with square transfer slots. Dieseling bad on shutdown, so how important is the square transfer slot? It will of course idle down if I adjust it or play with timing but now where do I go? I have just about had it!

 

[WHT/73]

Have you tried your old carb?
If so, does it do the same thing?
Have you spoke to Patrick yet?

 

[oldguy67]

lower the idle speed to it's best hot rpm,forget about the slots for now,anyway lowering the blades is not a problem as I see it.are the sec. blades a quarter turn in from seated? are you running 24* TOTAL idle timing or straight mechanical? see how it responses to 18* mech.at idle.let the engine tell you where it what's to idle at. at worst install an idle solenoid for shut down but you should be able to sort it out

 

[71malibuu]

Can you post a top view of that plug looking down the ground strap?

 

[rp930]

View attachment 724034

Pulled the plugs after driving tonight and there was a slight hiccup on shutdown and they all look like this. The plugs have miles on them but overall not terrible in my opinion... what does anyone else think. These are the colder AC Delco plugs by the way

Looks too cold.

 

[oldguy67]

try autolite 25s

 

[pockets]

You do not want the transfer slots exposed in the secondary side.
Only the primary. Therefore if you do have transfer slot in the secondary, close the secondary blades to get the idle back down. Leave the primary as small square slots.
Only if you close the secondary completely and the idle is still too high should you close the primary.

 

[Tonys72SS]

Here is what it looks like from the top.