Intermittant Dieseling - The 1947 - Present Chevrolet & GMC Truck Message Board Network

my new 72 · 09-22-2002, 01:24 PM

It only happens when the truck has been driven enough to get ggod and hot, and then driven relatively short distances afterward. It also tends to idle higher than it should when it gets warm? The timing is set to 12 btdc. The engine is a fresh built 383 bored .040 over Sportsman II heads w/2.02-1.60 valves. A crane energizer 274 cam w/ .454 lift. Topped with an RPM Airgap intake and Edelbrock 1411(750cfm) carb fed by a regulated Holley blue electric fuel pump. The ignition is an MSD 6al Super Hei unit. Overall compression is 10.2/1. What do you guys think could be the issue. Worn distributor springs and weights maybe? It also has a little hesitation on takeoff, but again this is only an intermittent problem. Thanks for the help guys.

I forgot to tell you that usually it starts right up after it has been started up and driven once, but after it deisels and I try to start it, I have to pump it a little to get it to fire. Thanks again.

my new 72 · 09-22-2002, 05:36 PM

Come on. Anyone have any idea.

loweredd · 09-22-2002, 05:58 PM

what kind of filter do you have on the line goin to the carb? I have there 60 dollar L shaped new one, and it lets all sorts of crap go through!! It floods all the time, which will let it diesel after you shut it off. This has happened to me a few times. Not even on cold days. Try that.

my new 72 · 09-22-2002, 06:03 PM

I have a slim glass filter with changeable elements. I have my fuel psi set at 5 1/2, that may be it too. I'll try a different filter and a lower psi setting and see if that helps. Anyone else??

*Zkast* · 09-22-2002, 06:08 PM

Right off hand it sounds like it has a small vacuum leak. When cold does it idle rougher than when warm? How high is "high" when its idling warm?
The reason it deisels is because of a few things. When the idle is high and the ignition is turned off, the engine is still spinning fast enough to draw in a fair amount of fuel. That coupled with a hot cylinder/head will create enough heat to ignite the fuel therefor causing it to "run on"

On a new engine there are serveral things you will need to look at:
Ignition timing - Check your timing, find out when the centrifugal advance comes in and make SURE its returning back to base timing at idle. Stuck weights could over advance the timing causing a high idle, excessive heat in the cylinder and run on.

Base idle and A/F mixture - make sure you have your carb dialed in right. Once you have your timing set and you know its right, start dialing in the carb mixture.

Until you get this problem nailed down there is a trick you can do to keep it from doing this. If you have a standard trans, just turn the key off and use the clutch to slow the engine down before it deisels. If you have an auto trans just leave it in gear and turn the key off. The lower RPM's will help stop it from running on.

Get your idle problem cured and I'm willing to bet your fix the run on problem too.

Good luck with it

my new 72 · 09-22-2002, 06:31 PM

When it is cold or just running around town(not on the freeway) it idles between 575 and 625 rpm. It is a little rough, but I figured that was just the cam, and when it is hot (after driving on the freeway) it idles at 850-900 solid. These are both in gear numbers, while it is in park it idles at 1050-1100in both situations. How do I check the centrifugal advance? With the timing light? What is the procedure? I think it is running a little rich to boot because of the hesitation on take off, but I knew I had to work on that. I have a vac gauge, could this be used to adjust the air/fuel mixture? Fine tuning has never been my strong suit. Thanks for the help Zkast, I really appreciate it.

CoryM · 09-22-2002, 06:38 PM

sounds like you are boiling the fuel in your carb (like me). Try lowering the float bowls slightly unless you cant without running it out of fuel at WOT. That should help a bit. Since this is a truck a insulating spacer may be a good idea, I dont have enough room to run one on my car. When you let the truck sit the heat travels up into the carb making the fuel very warm. Then you go drive it while its really warm and it over fuels, runs pretty lousy and diesels a bit when you shut it off depending how bad it is. Does the throttle response get sluggish when its hot?
CoryM

my new 72 · 09-22-2002, 06:57 PM

I only have 180 miles on this fresh build. What is WOT?? (just kidding I know what it is) I am running an Airgap intake, so the fuel should not be boiling like you say. I'll check it out though. Keep the ideas coming.

**Bowtie468** · 09-22-2002, 10:55 PM

Make sure your throttle plates close completley.

my new 72 · 09-22-2002, 10:59 PM

What would keep the throttle plates from closing completely? It is a brand new carb, so I never thoughtit could be anyhting other than the A/F mix adjustment.

*Zkast* · 09-22-2002, 11:04 PM

Before you get to involved in alot of different senarios it would be a good idea to run through your basics again. Timing, A/F, idle speed. Once that is all dialed in, then you can start to really get into it. Just MHO of course.

tom hand · 09-22-2002, 11:09 PM

What ever is causeing the idle speed to change is the problem. Maybe you have a vacuum leak that only occurs under certin conditions. Don't rule out the gas boiling because you have an air-gap, eldelbrock carbs are bad about that. If the motor is harder to start after you shut it off for 5 minutes you have the gas boiling problem. Find out what is making your idle speed change and it will disappear.

my new 72 · 09-22-2002, 11:23 PM

Thanks Zkast, I am going to adjust everything first, but How do I check the centrifugal advance, and what is the proper way of setting the A/F mixture? I have a vaccum gauge, but is that the proper way to set it, or is there something I am missing? Do I hook the vaccum gauge to the vaccum port that goes to the vac advance on the dist. and adjust untill I get certain number or is this totally incorrect and what is the correct way? Thank you again for your help.

tom hand · 09-22-2002, 11:28 PM

I reread all you posts and I believe I have a handle on your problem, tho you probably don't want to hear it. The problem is your air gap manifold. It does not warm up quick enough so you are setting your idle wrong. After your motor gets really warm, set it to the correct idle. [ 600 rpm] Now it won't diesel, but it will run like crap when it is cold. Maybe you can reset your choke ,[or run a manual one] to correct your problem. Remember performance parts for the most part are a compromise, for every thing you gain you loose some thing, but that makes it more fun when you find the right combo. If it was easy little girls would do it. I really hope you find the problem.

**Bowtie468** · 09-22-2002, 11:56 PM

Sorry Josh, i didnt explain myself. When i spoke of the throttle plates not closing fully, i was referring to your idle speed. When adjusting the idle screw you are in turn opening or closing the throttle plates. But you can go too far with it. Tom Hand seemed to explain it pretty clearly though. As far as other problems that would cause them not too fully close( or a least too close where they have been set to close at ) would include worn throttle plates ( which you dont have too worry about) and also a tight or sticking throttle cable. Good Luck !!

my new 72 · 09-23-2002, 04:30 PM

ttt

09-22-2002, 01:24 PM	#1
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	Intermittant Dieseling It only happens when the truck has been driven enough to get ggod and hot, and then driven relatively short distances afterward. It also tends to idle higher than it should when it gets warm? The timing is set to 12 btdc. The engine is a fresh built 383 bored .040 over Sportsman II heads w/2.02-1.60 valves. A crane energizer 274 cam w/ .454 lift. Topped with an RPM Airgap intake and Edelbrock 1411(750cfm) carb fed by a regulated Holley blue electric fuel pump. The ignition is an MSD 6al Super Hei unit. Overall compression is 10.2/1. What do you guys think could be the issue. Worn distributor springs and weights maybe? It also has a little hesitation on takeoff, but again this is only an intermittent problem. Thanks for the help guys. I forgot to tell you that usually it starts right up after it has been started up and driven once, but after it deisels and I try to start it, I have to pump it a little to get it to fire. Thanks again. __________________ I'm going to Law School, wanna debate? It's good practice. Last edited by my new 72; 09-22-2002 at 05:52 PM.

09-22-2002, 05:36 PM	#2
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	Come on. Anyone have any idea. __________________ I'm going to Law School, wanna debate? It's good practice.

09-22-2002, 05:58 PM	#3
loweredd www.loweredd.com Join Date: Aug 2002 Location: Struble IA Posts: 808	Floats what kind of filter do you have on the line goin to the carb? I have there 60 dollar L shaped new one, and it lets all sorts of crap go through!! It floods all the time, which will let it diesel after you shut it off. This has happened to me a few times. Not even on cold days. Try that.

09-22-2002, 06:03 PM	#4
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	I have a slim glass filter with changeable elements. I have my fuel psi set at 5 1/2, that may be it too. I'll try a different filter and a lower psi setting and see if that helps. Anyone else?? __________________ I'm going to Law School, wanna debate? It's good practice.

09-22-2002, 06:31 PM	#6
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	When it is cold or just running around town(not on the freeway) it idles between 575 and 625 rpm. It is a little rough, but I figured that was just the cam, and when it is hot (after driving on the freeway) it idles at 850-900 solid. These are both in gear numbers, while it is in park it idles at 1050-1100in both situations. How do I check the centrifugal advance? With the timing light? What is the procedure? I think it is running a little rich to boot because of the hesitation on take off, but I knew I had to work on that. I have a vac gauge, could this be used to adjust the air/fuel mixture? Fine tuning has never been my strong suit. Thanks for the help Zkast, I really appreciate it. __________________ I'm going to Law School, wanna debate? It's good practice.

09-22-2002, 06:08 PM	#5
Zkast Account Suspended Join Date: May 2002 Location: Olathe, Kansas Posts: 929	Right off hand it sounds like it has a small vacuum leak. When cold does it idle rougher than when warm? How high is "high" when its idling warm? The reason it deisels is because of a few things. When the idle is high and the ignition is turned off, the engine is still spinning fast enough to draw in a fair amount of fuel. That coupled with a hot cylinder/head will create enough heat to ignite the fuel therefor causing it to "run on" On a new engine there are serveral things you will need to look at: Ignition timing - Check your timing, find out when the centrifugal advance comes in and make SURE its returning back to base timing at idle. Stuck weights could over advance the timing causing a high idle, excessive heat in the cylinder and run on. Base idle and A/F mixture - make sure you have your carb dialed in right. Once you have your timing set and you know its right, start dialing in the carb mixture. Until you get this problem nailed down there is a trick you can do to keep it from doing this. If you have a standard trans, just turn the key off and use the clutch to slow the engine down before it deisels. If you have an auto trans just leave it in gear and turn the key off. The lower RPM's will help stop it from running on. Get your idle problem cured and I'm willing to bet your fix the run on problem too. Good luck with it

09-22-2002, 06:38 PM	#7
CoryM Registered User Join Date: Dec 1999 Location: Surrey, B.C. , Canada Posts: 874	sounds like you are boiling the fuel in your carb (like me). Try lowering the float bowls slightly unless you cant without running it out of fuel at WOT. That should help a bit. Since this is a truck a insulating spacer may be a good idea, I dont have enough room to run one on my car. When you let the truck sit the heat travels up into the carb making the fuel very warm. Then you go drive it while its really warm and it over fuels, runs pretty lousy and diesels a bit when you shut it off depending how bad it is. Does the throttle response get sluggish when its hot? CoryM __________________ 1970 heavy duty C-10 fleetside sport truck. Vancouver B.C. Canada http://www.geocities.com/chevroletc1070 "Take up our quarrel with the foe: To you from failing hands we throw The torch; be yours to hold it high." Lieutenant Colonel John McCrae, Canadian Army

09-22-2002, 06:57 PM	#8
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	I only have 180 miles on this fresh build. What is WOT?? (just kidding I know what it is) I am running an Airgap intake, so the fuel should not be boiling like you say. I'll check it out though. Keep the ideas coming. __________________ I'm going to Law School, wanna debate? It's good practice.

09-22-2002, 10:55 PM	#9
Bowtie468 Senior Member Join Date: Jun 2002 Location: Cecilia, Louisiana Posts: 1,638	one more possibility Make sure your throttle plates close completley. __________________ Chris click here to see my buildpage http://67-72chevytrucks.com/vboard/s...d.php?t=394656 ---------------------------------- 1979 C10 Silverado Big10 SWB Factory 454/400 Cardinal Red/Santa Fe Tan (original paint) 1969 Camaro X44 LS swap 1968 C10 SWB Fleetside (SOLD) 468BBC MultiPort TPI / Dynamic EFI P4 Black/Black 1963 Nova SS convertible (SOLD) 194 I6/ PowerGlide

09-22-2002, 10:59 PM	#10
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	What would keep the throttle plates from closing completely? It is a brand new carb, so I never thoughtit could be anyhting other than the A/F mix adjustment. __________________ I'm going to Law School, wanna debate? It's good practice.

09-22-2002, 11:04 PM	#11
Zkast Account Suspended Join Date: May 2002 Location: Olathe, Kansas Posts: 929	Before you get to involved in alot of different senarios it would be a good idea to run through your basics again. Timing, A/F, idle speed. Once that is all dialed in, then you can start to really get into it. Just MHO of course.

09-22-2002, 11:09 PM	#12
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	What ever is causeing the idle speed to change is the problem. Maybe you have a vacuum leak that only occurs under certin conditions. Don't rule out the gas boiling because you have an air-gap, eldelbrock carbs are bad about that. If the motor is harder to start after you shut it off for 5 minutes you have the gas boiling problem. Find out what is making your idle speed change and it will disappear. __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

09-22-2002, 11:23 PM	#13
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	Thanks Zkast, I am going to adjust everything first, but How do I check the centrifugal advance, and what is the proper way of setting the A/F mixture? I have a vaccum gauge, but is that the proper way to set it, or is there something I am missing? Do I hook the vaccum gauge to the vaccum port that goes to the vac advance on the dist. and adjust untill I get certain number or is this totally incorrect and what is the correct way? Thank you again for your help. __________________ I'm going to Law School, wanna debate? It's good practice.

09-22-2002, 11:28 PM	#14
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	I reread all you posts and I believe I have a handle on your problem, tho you probably don't want to hear it. The problem is your air gap manifold. It does not warm up quick enough so you are setting your idle wrong. After your motor gets really warm, set it to the correct idle. [ 600 rpm] Now it won't diesel, but it will run like crap when it is cold. Maybe you can reset your choke ,[or run a manual one] to correct your problem. Remember performance parts for the most part are a compromise, for every thing you gain you loose some thing, but that makes it more fun when you find the right combo. If it was easy little girls would do it. I really hope you find the problem. __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

09-22-2002, 11:56 PM	#15
Bowtie468 Senior Member Join Date: Jun 2002 Location: Cecilia, Louisiana Posts: 1,638	Sorry Josh, i didnt explain myself. When i spoke of the throttle plates not closing fully, i was referring to your idle speed. When adjusting the idle screw you are in turn opening or closing the throttle plates. But you can go too far with it. Tom Hand seemed to explain it pretty clearly though. As far as other problems that would cause them not too fully close( or a least too close where they have been set to close at ) would include worn throttle plates ( which you dont have too worry about) and also a tight or sticking throttle cable. Good Luck !! __________________ Chris click here to see my buildpage http://67-72chevytrucks.com/vboard/s...d.php?t=394656 ---------------------------------- 1979 C10 Silverado Big10 SWB Factory 454/400 Cardinal Red/Santa Fe Tan (original paint) 1969 Camaro X44 LS swap 1968 C10 SWB Fleetside (SOLD) 468BBC MultiPort TPI / Dynamic EFI P4 Black/Black 1963 Nova SS convertible (SOLD) 194 I6/ PowerGlide



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09-23-2002, 04:30 PM	#16
my new 72 faster, faster, faster Join Date: Dec 2001 Location: Houston ,Texas ,USA Posts: 1,354	ttt __________________ I'm going to Law School, wanna debate? It's good practice.

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