Diesaling Issue - Team Camaro Tech
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post #1 of 13 (permalink) Old Apr 14th, 08, 10:42 AM Thread Starter
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Diesaling Issue

I have a SBC in my 68 that has some aftermarket cam that I do not know of the specs because it was in the engine before I got the car. An edelbrock 1605 Carb with stock jet settings. The engine idles at 1000rpm in drive and 1200-1500 rpms in park, timing was set at 8* without the vac advance hooked up (vac adv hooked up the timing can not be determined by the timing tab) but believe to only be about 14*; the curve springs are not suppose to increase more till 2000 rpms.

I have found that the 3 reasons for Diesaling are: to much timing, to much idle speed, or leaking jets.
I think the timing is ok.
I can not get the engine to idle lower with setting it or it will shut off. I can not seem to get anything other then 500 rpms or 1000 rpms with trying to adjust the idle speed screw. Mixture screws does nothing for timing or Vacuum Hg.

Any thoughts?
Thanks
Ray
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post #2 of 13 (permalink) Old Apr 14th, 08, 11:23 AM
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Re: Diesaling Issue

I read a lot of issues here Ray... the run on (dieseling) seems obvious that it's caused by the high idle and the throttle butterflys being too far open. The fact that adjusting the mixture screws does nothing seems to support this. Make sure your secondary throttle butterflys are closed but not binding against the throttle bores. Back the adj screw off until the blades are closed and the screw is not seated then tighten it just enough to make contact and add an 1/8 turn max. You want the butterflys closed but not binding. Next adjust the Primary throttle butterflys down as far as you can without the engine stalling.

The only reason you shouldn't be able to get your engine to idle at about 800 rpm is if the cam is really big... You should be able to identify that by lope and performance as well it should have higher compression.

If you still can't get it to idle down start looking at timing, I don't know where you got the timing shouldn't climb until 2000 rpm, it really should be starting to move just a couple hundred rpm above idle and that is typically around 900 rpm unless it's got a custom curve. Do a basic check of your timing and write down the numbers. Read the Initial and the total both with the vacuum adv disconected and plugged at the source. Subtract the initial from the total to see what is built into the distributor. Once you know this you can do all your adjusting at idle and know what the total will be.

You might also want to verify your timing against #1 @ tdc to make sure your timing mark and dampmer are lined up properly. This is a common issue when folks can't dial in the idle because the timing isn't really where they think it is.

Now the trick to the deiseling is to close off the throttle and increase the idle timing to keep the engine idling. You can use the vacuum advance to help you with this by using a manifold source to operate it. This will add advance to the initial, I like to call the initial combined with vacuum advance "idle vacuum" it can really help with the dieseling and it can reduce the in-gear surging and the amount the rpm drops when placing an automatic in gear.

Oh, adjust idle mixture for highest idle or most vacuum, start them at about 1 turn out, maybe 1 1/4 and go from there. If you can't get this dialed in start looking for vacuum leaks, an internal leak in the manifold is the hardest to identify but really can make tuning impossible.

...Dennis

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post #3 of 13 (permalink) Old Apr 14th, 08, 12:16 PM
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Re: Diesaling Issue

Is it electric choke? I know i had the dieseling issue when i didn't have the electric choke hooked up, connected the 12v to the choke and everything was fine.

Rob

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post #4 of 13 (permalink) Old Apr 14th, 08, 01:30 PM Thread Starter
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Re: Diesaling Issue

yes I have an electric choke and it is hooked up.

I know I set the timing at 8* BTDC with the vacuum advance disconnected and the vacuum port on the edelbrock carb capped off. Then set timing as low as I could and keep the car idling while in park.

Then plugging the vacuum advance into the capped port shows the timing jump above the 12* mark on the timing tab.

Before doing all this I did find TDC by hand turning the motor and watching for the #6 Cylinders rockers to start moving.

I will need to check the butterflies and make sure they are closing.
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post #5 of 13 (permalink) Old Apr 14th, 08, 01:41 PM
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Re: Diesaling Issue

Quote:
The fact that adjusting the mixture screws does nothing seems to support this. Make sure your secondary throttle butterflys are closed but not binding against the throttle bores. Back the adj screw off until the blades are closed and the screw is not seated then tighten it just enough to make contact and add an 1/8 turn max.
That is the most common issue..and has to be sorted before moving or tuning anything else.

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post #6 of 13 (permalink) Old Apr 14th, 08, 01:48 PM
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Re: Diesaling Issue

Just for grins kick your initial timing up to 12 or 14 (no vacuum advance) and close down the throttle to bring the idle down. If you can get it below 1000 rpm adjust the idle mixture screws in until it stumbles and then back out just a bit until the idle steadys. Check the rpm and try adjust it down to 800 and see if it stays running. See what happens in gear next and let us know what's up.

Don't really drive it at this point because increasing the initial idle raises the total as well. It's important to know how much total you are running, you don't want wot pinging...

...Dennis

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post #7 of 13 (permalink) Old Apr 14th, 08, 04:43 PM
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Re: Diesaling Issue

Dennis:

Yep that's the way I do it. Ensure the idle stop (butterfly plate) screw is set just enough not to jam the plates in the venturi bore. NOTE: The Carb Mfgr will have the recommended clearances for adjusting idle stop screw to give a specific clearance between a throttle plate and venturi bore but I've found particularly with used carbs that these specs may not work because the carb has taken a ware set over time. Set spark advance (no vacuum) high - 13 degrees. With vacuum disconnected start engine and lower throttle adjustment while enriching idle air screws to a point were you can BARELY idle the engine (say 400 or 500 rpm - anyway to get below 800 rpm) with the throttle plates setting on the idle stop screws. From that point you can raise the Idle stop very slightly while turning out the idle air screw to increase idle to approx 600 rpm. You do NOT want to over do this and it may not be necessary as this will cause dieseling at shut down. Then you can use the idle air screws to hit 800 rpm. If you have it right, the idle air screw should be approx 3/4 to 1 1/4 and no more then 1 1/2 turns from its shut off to idle the engine between 600 to 1000 rpm. Under no circumstances, other then then say a choke kick-down or an A/C idle step-up solenoid, should the throttle adjustment prevent the butterfly's from resting on the idle stop screw while the engine is idling. Either a poorly adjusted choke kick-down or A/C solenoid can cause the motor to diesel on shut down when the Idle Stop Screw is properly adjusted.

Make sure there is no power going to the coil when you turn off the key. Sometimes depending on how a car is wired an alternator, which is spinning, can continue to supply power to the coil when the key is turned off and keep the coil energized and the engine will not shut down. Other then changing the cars circuits a Diode is placed on the alternators exciter wire to ground and prevent this.

NOTE: READ my follow up POST on "Carburetor Adjusting & Initial Distributer Advance" in this thread.

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Last edited by Z15CAM; Apr 15th, 08 at 02:24 AM.
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post #8 of 13 (permalink) Old Apr 14th, 08, 05:32 PM
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Re: Diesaling Issue

hehe, life is so hard in the stone age...

I just open my laptop, and tell it I want 700 rpm at 14.3:1 AFR... I also told it I want 26 degrees advance at idle (soon to be bumped up to 36...) and it just kinda happened. Can't really explain it, but the computer says it so it must be so.

700 rpm, 10" vacuum, 292 cam... rumpity umpity rumpity.

But yeah, the guys got it nailed.

Dieseling = too much air + not enough timing





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post #9 of 13 (permalink) Old Apr 14th, 08, 08:44 PM Thread Starter
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Re: Diesaling Issue

I tried a little test on the way home from work and was able to repeat it 2 more times each time I stopped on the way home. I left the car in Drive and the idle stabilize at 1000 rpms and turned the key to the off position and BINGO no dieseling. I then put the car into park.

I doubt this helps point anyone to a more specific thing to check but wanted to let you guys know.

I am really thankful for the information you people are providing and am going to attempt to look at all that you have mentioned this Saturday.

I have a couple of questions:
Do I need to start with the air/fuel mixtures screws all the way out and then turn a 1 1/2 in or all the way in and then back them out 1 1/2 turns?
If the butterflies are open (front or rear) how do I adjust them to close as long as they are not sticking? Remember this is an Edelbrock 1405.
Thanks
Ray
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post #10 of 13 (permalink) Old Apr 15th, 08, 01:58 AM
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Re: Diesaling Issue

Adjusting Carburetor and Initial Distributer Advance

Ok a little carb theory first. The engine is at idle when the throttle plates set between the Idle Transfer Outlet Slot located just above and the Idle Discharge Port located just below. The Transfer Slot discharges additional idle fuel when an opening throttle plate uncovers the slot. As the throttle opens additional fuel is introduced into the air stream of the venturi and prevents a lean condition before the main metering circuit engages. The angle with in throttle idle movement is very small so it is understandable that the position of the throttle plates is critical when the engine is at idle.

#1 - If the carb is new, the Throttle Plate Stop Screw should have been set by Mfgr and you may not have to adjust it; however, I would check the setting with the Mfgr's specs and ensure the throttle plates will rest against it without touching the venturi bores. You may have to loosen the Throttle Adjusting Screw and perhaps some choke links to ensure this. Some carburetors have to be removed in order to set the Throttle Plate Stop Screw.

#2 – If a vacuum advance distributer, disconnect the vacuum line and plug inlet to base of carb. If a mechanical distributer, do nothing. Turn in Idle Air Screws to seat then back out approx 1 1/2 or 1 3/4 turns for a starting point. Connect timing light - loosen distributer - start engine - set timing at around 14 for SBC or 18 degrees for BBC - lock distributer so you can move it by hand while engine is running - tighten the Throttle Adjusting Screw a few turns so engine will Idle High after start.

#3 - Start Engine - Adjust Throttle Screw to lower idle and let engine warm up and the choke kicks off - Wire the choke open if necessary. You should not have to adjust the Idle Air Mixture Screws.

#4 – Once engine is warm loosen the Throttle Adjusting Screw - you MUST MOVE THE DISTRIBUTER as you attempt to lower (close) the Throttle Plates onto the Throttle Plate Stop Screw or get it as close a possible, imagine loosing the Throttle Adjusting Screw 1 turn before resting the Throttle Plates on the Throttle Plate Stop Screw. Play with Idle Air Screws VERY LITTLE but ensure that you turn both screws equally. It is IMPORTANT to consider the Distributer and the Throttle Plates (controlled by the Throttle Adjusting Screw) as the VARIABLES and NOT the Idle Air Mixture Screws which must be considered FIXED.

#5 - You should get the engine to idle well below 800 rpm (400 to 500 rpm) with the Throttle Plates positioned very close to the Throttle Plate Stop Screw (This is the critical position between the Idle Transfer Outlet Slot and the Idle Discharge Port). I've no problems doing this with cams having 288 degrees or more of advertised duration and 850 Holley’s; such that the Idle Air Mixture Screws will set equally at ¾ turns before the engine leans and screw out to 1 1/2 turns before the engine over riches.

#6 – NOW last and Most Important, ADVANCE the DISTRIBUTER to increase the engine speed to your Desired Idle Speed, say 800 rpm, and LOCK IT. Record the Idle Speed and Distributer Degree Setting; such as, "800 rpm at 19 degrees" and keep the information in your Glove Box – whatever: This is the Initial Timing your Engine will Start and Idle at – every engine is different. At this point it is just a matter of minor adjustments to your Throttle Adjustment Screw and Idle Air Mixture Screws when you place the Air Breather on or set idle for A/C Loading. Adjust the Choke at startup when the engine is cold and do not mess with any other of your carb settings.

GEEZ! I've been running Mechanical Advance Distributes for so long I forgot to say that when reconnecting the Vacuum Advance the Timing will Advance and the engine rpm will Increase; so you will have to loosen the distributer and retard the timing back to the 19 degrees to obtain the 800 rpm Idle speed exampled here. These few degrees of retard, I believe, is designed for easy starting. Also when the throttle is opened while the engine is running the vacuum drops for the purpose of controlling ping and gives time for vacuum operated components like Carb Vacuum Secondary’s, Auto-Trans Vacuum Pods time to kick in... Etc. I don't believe that simply turning out the Throttle Adjusting Screw to reduce idle back to 800 rpm will drop the vacuum enough to retard the timing back to 19 degrees and besides you have just gone through a lot of trouble to establish the position of that Throttle Plate at idle.

Of course none of the above will be accurate if you have a vacuum leak or problems with the PCV system.

NOTE: I have, under certain circumstances, set the Throttle Plate Stop Screw at a position where the engine will Idle at the lowest possible rpm (say 400) and have the Throttle Adjusting Screw take it from that point; however, I would not recommend this for Vacuum Advance Distributors, Carbs with Vacuum or Electric Controlled Throttle close devices or if you have situations where you change the Initial Timing on regular bases, eg: Drag Racing, Degreeing Cam, Different Fuels ... etc.

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post #11 of 13 (permalink) Old Apr 21st, 08, 07:09 AM Thread Starter
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Re: Diesaling Issue

Saturday morning I got out in the garage and started making some of the above recommendations.

Check the butter-flies in the base of the carb to make sure the were fully closed and not catching on anything.

I first got the Air/Fuel mix screws set to 1 1/2 after finding out that the Pass side was 8 turns out and the Driver Side was 6 turns out.

I then backed the idle speed screw out to where it was not touching and then screwed it in just enough to touch. This was after messing with the airhorn tab thing that is on that side of the carb also. It was moving freely all over the place and the adjustment screw for it could not even touch the tab so I put a washer in to keep in line with the Screw. Set this screw to where it was between the 2 lines on the tab and then readjusted the idle speed screw.

I rotated the Chock cap to the suggested default point.

Got the car running but to keep it running I had to back the air/fuel out to a total of 2 1/2 turns on both sides and adjust the idle speed screw in 1/2 a turn.

Drove the car around and she idled at 600 rpm in gear after she warmed up. Plenty of stop lights and signs to make sure it was not just a one time occurance. Get back to the house and parked the car.

Sunday afternoon Temp about 10 dgrees cooler and cloundy day instead of Sunny. Humidity Level between the 2 days I do not know. Start the car up and cold she is idling at 900 in park and 400-500 in gear and she turns herself off.

I adjust the choke LEANER and no apparent help in getting her to idle on her own. When adjusting the choke cap I would stop as soon as I would see the air horn start to open up and then back it off just a little.

Now after warm up she is idling at 1200 in park and 900 in drive. Tried backing the choke cap back to default and no change in idle speed again, still at the 1200/900 rpm. I go for a drive and the first stop I come to the idle is 1000 rpms. Next light 500, Next back to 1000. Get on the highway for about 4 miles and we are staying at 1000 rpms everytime.

This morning on the way to work. She did not want to idle cold or stay running if I started to stop to change from Reverse to drive. Once I got moving for a little bit but still cold engine temp the next light was 400 rpm and attempting to shutdown. After she finally warmed up we are still at 1000 rpms in drive at a light.

First question has to do with the choke should I be adjusting this richer or leaner?
2nd question any more ideas of what is going on with this carb?

thanks
Ray
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post #12 of 13 (permalink) Old Apr 21st, 08, 08:43 AM
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Re: Diesaling Issue

The first half of your post seems to show that you have your HOT IDLE problem handled.
If you wire the choke open, can you still idle reliably at 600 rpm in gear HOT?

On the choke, there are basically 3 adjustments.
Rish/Lean: turning the black choke thermostat. This is an electric choke. It needs both power and ground, and assuming it has those, the choke butterfly should be fully open after the engine has been running for 3 minutes or so. Adjust the rich.lean to make it so.

Once that's done, when the motor is cold, the choke butterfly should snap shut when you jack the throttle. If it doesn't, something is wrong.

Fast idle: there are 2 adjustments here.

There is a link from the arm on the choke thermostat down to the fast idle cam on the passenger side of the carb. The link needs to be adjusted (bent) so the fast idle cam is completely below the screw, when the choke is open. (I think this is part of your problem) You can hold the choke open, jack the throttle, and check to make sure the fast idle cam dropped completely below the screw. If not, bend the link.


Fast idle speed: This is the screw on the pass side of the carb that touches the fast idle cam when the car is cold. Set it for the lowest possible stable idle speed under the following conditions:
Engine stone cold.
Jack the throttle once to set the choke (or a few times if your motor likes some "pumps" to start cold.)
Start the car without touching the gas pedal.
Immediately adjust the fast idle speed screw for lowest stable idle (check in gear too.)





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post #13 of 13 (permalink) Old Apr 21st, 08, 09:20 AM Thread Starter
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Re: Diesaling Issue

Quote:
Originally Posted by JimM View Post
The first half of your post seems to show that you have your HOT IDLE problem handled.
If you wire the choke open, can you still idle reliably at 600 rpm in gear HOT?
On saturday I thought I did but Sunday and today it is back to idleing at 100rpms and I did not adjust anything other then the choke cap. how do I wire the choke open? I know it has proper +/- hooked up I can feel it get warm/hot
Quote:
On the choke, there are basically 3 adjustments.
Rish/Lean: turning the black choke thermostat. This is an electric choke. It needs both power and ground, and assuming it has those, the choke butterfly should be fully open after the engine has been running for 3 minutes or so. Adjust the rich.lean to make it so.

Once that's done, when the motor is cold, the choke butterfly should snap shut when you jack the throttle. If it doesn't, something is wrong.

Fast idle: there are 2 adjustments here.

There is a link from the arm on the choke thermostat down to the fast idle cam on the passenger side of the carb. The link needs to be adjusted (bent) so the fast idle cam is completely below the screw, when the choke is open. (I think this is part of your problem) You can hold the choke open, jack the throttle, and check to make sure the fast idle cam dropped completely below the screw. If not, bend the link.


Fast idle speed: This is the screw on the pass side of the carb that touches the fast idle cam when the car is cold. Set it for the lowest possible stable idle speed under the following conditions:
Engine stone cold.
Jack the throttle once to set the choke (or a few times if your motor likes some "pumps" to start cold.)
Start the car without touching the gas pedal.
Immediately adjust the fast idle speed screw for lowest stable idle (check in gear too.)
I believe I understand what you are refering to but looking at the edelbrock doc for the choke kit the. Air horn tab and fast idle screw are on the driver side. which does correspond with the carb. I previous post I was refering to was the air horn tab with 2 marks and added a washer to but the washer may be preventing the tab from moving. Prior to the washer the tab that has the 2 lines and touches the fast idle screw would move upward and stay there with out the washer in place. Is this correct or should this return to where the fast idle screw should be touching it?
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