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#59959 - 08/22/10 06:19 PM Timing and Dieseling
snowman4839 Offline
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I'm trying to adjust the timing, idle mix, and stop my 250 from turning over backwards when I turn it off (diesel I think it's called). The factory says 33 degrees on the dwell and TDC for timing. I've tried setting it at 0 and 2 degrees advanced where it doesn't diesel much if at all but the mix screw has to be about 1/8 of a turn out of completely in for it to run well. Then when I set it to 4 degrees advanced, about a 1/4 of a turn is where it's best. But once I set it past 2 degrees advanced, it starts to diesel a lot. I've set it to somewhere around 12 degrees advanced (just a guess) and then I can turn it out about a full turn for it to run well. I've rebuilt the carb twice and double checked everything on the second rebuild. I've resealed the manifolds with a new gasket, replaced the internals of the dizzy and set the points to 30 degrees.

What should the timing be set at before it starts pinging? I use 87 octane.

Someone mentioned before that I needed to use an idle solenoid but I don't see how that helps. What's the point of the idle solenoid if I have an idle screw?
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69 Buick Special Deluxe. Intercooled Turbo Chevy 250 @ 15psi on a stock long block. It's kinda fast.

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#59960 - 08/22/10 06:58 PM Re: Timing and Dieseling [Re: snowman4839]
snowman4839 Offline
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Also before I rebuilt it, the best mix was about 2 and a half turns out when it was 2 degrees advanced.

I also have the stock jet I had in it before I rebuilt it.
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#59962 - 08/22/10 07:19 PM Re: Timing and Dieseling [Re: snowman4839]
Ken Offline
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Sounds like your idle speed is set too high - that is what an idle solenoid does, it drops the idle stop to shut off fuel when the ignition is shut off.

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#59963 - 08/22/10 07:22 PM Re: Timing and Dieseling [Re: snowman4839]
Ron Golden Offline
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Dieseling is caused by the engine idleing too fast (throttle open too far). Advance the timing, then adjust the idle mixture screw for the best (smoothest) idle, then set the idle rpm lower.

The distributor mechanical advance, vacuum advance and carb idle adjustments respond together and any one of them can effect the idle and throttle response. Sometimes you have to modify one, or all, of them to get it to run right.

Ron

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#59964 - 08/22/10 07:23 PM Re: Timing and Dieseling [Re: snowman4839]
snowman4839 Offline
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As expected, I just went out and moved the timing advance with a screw driver and it sped up.

It wasn't idling very fast, maybe 750

But wouldn't advancing it more than 4 degrees be dangerous to the engine and cause pinging?


Edited by snowman4839 (08/22/10 07:25 PM)
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#59965 - 08/22/10 07:34 PM Re: Timing and Dieseling [Re: snowman4839]
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I know you are new to alot of things mechanical. We never quit learning.
How exactly are you setting the timing?

It should go something like this.
1. Pull vacuum line from distributor and plug the carb end of hose.
2. Start engine and check timing with light. It should be idling and the timing # should be between 6-10 degrees BTDC.
3. When hooking vacuum line back up , the timing will go higher, maybe 20 ish.
4. Adjust idle back down to 750 RPM with idle screw and adjust mixture screw for highest idle and then bring idle back down with idle screw to 750.


Start with that. Tom
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#59966 - 08/22/10 08:05 PM Re: Timing and Dieseling [Re: tlowe #1716]
snowman4839 Offline
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 Originally Posted By: tlowe #1716
I know you are new to alot of things mechanical. We never quit learning.
How exactly are you setting the timing?

It should go something like this.
1. Pull vacuum line from distributor and plug the carb end of hose.
2. Start engine and check timing with light. It should be idling and the timing # should be between 6-10 degrees BTDC.
3. When hooking vacuum line back up , the timing will go higher, maybe 20 ish.
4. Adjust idle back down to 750 RPM with idle screw and adjust mixture screw for highest idle and then bring idle back down with idle screw to 750.


Start with that. Tom


Well i'm not knew to mechanics, just to actually working on a car

I pulled the vacuum line but forgot to plug the carb.

I can't really read the timing marker though. I see an 'A' at one end and a '0' at the other but I can't see marks in between. Is 'A' 8 degrees or 4 or what?

When I hooked the vacuum line back up when it was idling, It actually went a little retarded.(toward the 0 and away from the 'A'.

I tried turning the screw out 2 and a half turns and then I set the timing in increments from super-retarded to super-advanced and it never did start.
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#59968 - 08/22/10 08:12 PM Re: Timing and Dieseling [Re: snowman4839]
tlowe #1716 Offline
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There should be more #'s and dash marks on the timing indicator. Maybe take some time to clean it and look at it close with a light.
The 'A' stands for advanced, same as BTDC. That is the way the timing mark should go when hooking up the vacuum line.
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#59969 - 08/22/10 08:36 PM Re: Timing and Dieseling [Re: tlowe #1716]
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Alright, I wire brushed it a little more and found some marks. Assuming that each one is 2 degrees. I set it to 8 degrees which is just before the A.

I also just tried the vacuum advance again and I checked the idle without it, plugged it in, and then checked it again and it was either the same or retarded a degree.

I also looked at the vacuum advance and I'm not sure if it's working correctly. At idle it doesn't look like it's doing anything and as soon as I give it maybe 200rpm more than idle, it engages fully. When not idling, it then either stays at the fully advanced position when opening throttle or it sporadically moves back and forth between full open and off when changing throttle position. Does this mean that the mechanical springs are off or something? Isn't it supposed to engage gradually as you increase the throttle?


Edited by snowman4839 (08/22/10 08:49 PM)
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#59970 - 08/22/10 08:56 PM Re: Timing and Dieseling [Re: snowman4839]
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Vacuum advance comes in when the engine vacuum picks up - doesn't matter what the engine speed is - the induction system will be subject to vacuum whenever the motor is not loaded - so off idle vacuum going to full stroke of the vacuum actuator would not be unusual at all.

The mechanical advance that comes in does come in as a function of engine speed and is completely independent of the vacuum advance function.

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#59971 - 08/22/10 09:06 PM Re: Timing and Dieseling [Re: Ken]
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well I'm confused as to why even when I set it to 8 degrees advanced, it still only works when I have the idle mix is 1/8 out.

EDIT: I also just tried again and i can see how the timing gets way advanced when the vacuum kicks all the way in. I'm just confused because the mix is way off and it doesn't run very smoothly either.


Edited by snowman4839 (08/22/10 09:31 PM)
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#59973 - 08/22/10 09:59 PM Re: Timing and Dieseling [Re: snowman4839]
Mean buzzen half dozen A.K.A. Hank Offline
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Is it possible to find or borrow another carb to fix your problem?

MBHD
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#59974 - 08/22/10 10:02 PM Re: Timing and Dieseling [Re: Mean buzzen half dozen A.K.A. Hank]
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well the point is to get this one working. I'm going to get the 4 barrel setup I told you about in the turbo build thread soon but I want to figure out the problem with this one. What's the point of having a defective item if you don't learn from it?

I just took out the idle mix screw and blew out the passages with compressed air and I saw some gas some out the top and saw some mist come out of some other part. Put it back in and it didn't do much. Might've added a half a turn out for best mix
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#59975 - 08/22/10 10:35 PM Re: Timing and Dieseling [Re: snowman4839]
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Would something like metering rod adjustment or float height make a difference?
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#59980 - 08/23/10 06:51 AM Re: Timing and Dieseling [Re: snowman4839]
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What's the point of the idle solenoid if I have an idle screw?

What is your purpose: to get the correct answer, or to discuss the subject?

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#59984 - 08/23/10 10:05 AM Re: Timing and Dieseling [Re: panic]
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Snowman -

Tom's procedure is pretty good and should help you make progress.

Once you set the idle mixture screw to where the motor runs decently leave it alone and get your timing squared away - then adjust the idle speed screw for optimum idle speed and double check timing.

You should have no problem getting a 500 - 600 rpm idle speed unless you have a really lumpy cam and the motor will shut down nicely with the idle speed set this low.

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#59989 - 08/23/10 04:43 PM Re: Timing and Dieseling [Re: Ken]
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I think the symptom of no vacuum advance and then it all comes in with another 200 rpm sounds like a vacuum leak. That will throw idle mixtures way off and produce no vacuum advance at low idle.
When idling to set timing with vac line off, does the line have vacuum? It should. Tom
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#59991 - 08/23/10 06:24 PM Re: Timing and Dieseling [Re: tlowe #1716]
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Alright I made a video of what is happening to my advance when I rev the engine. http://www.youtube.com/watch?v=S--xJZM6tyc

Should I just re-re-rebuild the carb?

I tried Tom's method and the only problem with the procedure is I have to set the mix first and there's obviously something wrong with it because it runs best with an 1/8 turn out.

I set the timing pretty well because it barely pings on occasion

EDIT: Alright now this is just more F'd up than before. Now the idle mix screw doesn't really affect the idle at all and I just sprayed everything down with penetrating oil to find vacuum leaks with no success.

I also just looked at the advance and it doesn't move at idle whether it's plugged into the advance port on the carb or not

The last thing I was thinking of is the idle tube fits into the hole very loosly. If I remember correctly, it was pretty tight before. Would this let in extra gas?


Edited by snowman4839 (08/23/10 06:58 PM)
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#59992 - 08/23/10 07:00 PM Re: Timing and Dieseling [Re: snowman4839]
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Does the dizzy need a new vacuum advance?
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#59993 - 08/23/10 07:10 PM Re: Timing and Dieseling [Re: Sam Welch]
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I'm not sure. Does it? look at the video. Is that how it's supposed to work?
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#59995 - 08/23/10 09:05 PM Re: Timing and Dieseling [Re: snowman4839]
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the idle tube fits into the hole very loosly. If I remember correctly, it was pretty tight before. Would this let in extra gas?

What's an idle tube?

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#59998 - 08/23/10 10:52 PM Re: Timing and Dieseling [Re: panic]
snowman4839 Offline
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http://www.walkerproducts.com/_pdf/rochester1barrel.pdf
page 16. Part 25

It's a small constrictive tube that sits in the fuel bowl that is supposed to suck in the fuel for the idle circuit.
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#59999 - 08/24/10 12:17 AM Re: Timing and Dieseling [Re: snowman4839]
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Your vacuum advance is working properly.
You even have it connected to the proper port on the carb.
The correct port is called ported vacuum, which means the port is above the throttle blade. This port will have very low or zero vacuum when the throttle blade is closed. When you open the throttle (give it gas), this port will have vacuum and pull the vacuum advance, advancing the timing. Also, if you have everything out of adjustment and the idle speed screw is holding the throttle blade TOO far open, you will have vacuum on that port. This causes incorrect idle speed, incorrect idle timing and dieseling.
Many people incorrectly connect the vacuum advance to a manifold vacuum port that has vacuum at all times except of course WOT. Some people think they have to have vacuum advance at idle, BUT they have made other changes to their advance curve to compensate.

When you set or check the timing, pull the hose from the vacuum advance canister and plug the hose. Leave the other end on the carb and plug the hose at the canister.

As Ken said, idle speed should be 600 not 750. You can idle it down around 500, but make sure the oil pressure doesn't drop off when that low.

Set the idle mixture screw to X turns out. Yes, I said X turns. X turns out is the point where the rpm stays the same when you turn it out, but the rpm's drop off like rock when you turn it in. This is called lean roll.
This isn't a fixed adjustment
Use a tach for the mixture adjustment. When the mixture screw increases the rpm, use the speed screw to slow it back down. Finetune the mixture screw again.
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#60001 - 08/24/10 06:38 AM Re: Timing and Dieseling [Re: RichardJ]
Mean buzzen half dozen A.K.A. Hank Offline
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RichardJ,

Well spoken.

I was concerned he had the vacuum advance hose connected to full vacuum @ idle which someone stated that was the correct way to hook it up.


I think he needs to get that electric idle solinoid to work & that usually will kill the engine when the ignition key is turned off & really closes that throttle blade.
That should stop the dieseling.

He has a stick shift car/manual trans,500 RPM might be a bit low RPM for idle?

MBHD
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#60003 - 08/24/10 08:59 AM Re: Timing and Dieseling [Re: Mean buzzen half dozen A.K.A. Hank]
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Ouch, thanks - I had a similar problem with a tube coming loose and floating around in my Buick Quadrajet!

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#60004 - 08/24/10 09:10 AM Re: Timing and Dieseling [Re: panic]
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500 seems OK to me, if the cold idle speed is stable and the OP shows at least 10 psi.

In addition to the actual factory curb idle solenoid, which are pretty expensive new, you can bodge almost anything to work from another brand - all you need is to place the plunger (when extended) in-line with your existing idle stop screw, and slightly higher to raise the speed when the solenoid is hot.
You can use the really cheap (like $10.) 12 volt commercial surplus parts, but you want "push = hot" (not pull, unless you can make new linkage on the other side), and "continuous duty" since it will be hot whenever the key is on.
Here's a good pull type: https://www.surpluscenter.com/item.asp?item=11-3179&catname=

Now, the solenoid plunger is your new idle speed adjustment; if it's not threaded you can move the entire solenoid, etc.
Lower your original idle speed screw several turns, since it's now the "no air, anti-diesel" position with the ignition off.

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#60007 - 08/24/10 05:35 PM Re: Timing and Dieseling [Re: panic]
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alright well I'll look into getting a surplus idle solenoid

I just made a video of all the things I'm doing and what my engine sounds and runs like. It runs so choppily and I still don't have any idea what's going on. Just let me know what else you want me to film and I'll put it on. I just want it to run smoothly and have a smooth transition when I stop on it.

http://www.youtube.com/watch?v=FJOGxOfoiwA


Edited by snowman4839 (08/24/10 05:44 PM)
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#60008 - 08/24/10 05:56 PM Re: Timing and Dieseling [Re: snowman4839]
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Just looked at a cylinder firing by using a timing light and it seems to be constantly moving back and forth 50 rpms at idle.
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#60009 - 08/24/10 06:17 PM Re: Timing and Dieseling [Re: snowman4839]
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I watched your video. Is the concern currently only with what you perceive as rough running? It will likely never run as smoothly as a newer fuel injected vehicle, if that is a basis for comparison. My 85 Jetta with mechanical injection runs smoother than my 64 GMC with a carb. That aside, have you ruled out mechanical issues-has the engine been rebuilt ever? If not, could there be cylinder sealing issues, or a burnt valve, or even a wasted cam lobe? Have you gone back to basics? When I diagnose a vehicle at work for driveability issues, even with a scanner, the basics apply. Do the cylinders have sufficient compression? Correct fuel pressure and volume? How strong is the spark? Is the coil on its' last legs? Check with a spray bottle filled with water and spritz the plug wires-you may have voltage taking the path of least resistance, i.e. to the block, instead of the plugs. Hope this doesn't cornfuse you more.
P.S.-When checking for vacuum leaks, I like brake clean. While much more flammable, it does have less surface tension, doesn't leave residue, and therefore is more easily ingested by a vacuum leak.

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#60010 - 08/24/10 06:30 PM Re: Timing and Dieseling [Re: vanherk1]
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Well I'll try to go get some brake cleaner tomorrow then. It runs pretty rough as compared to idling at 800 or so. Is that just normal? Once it gets above 800 it just smooths out because you just can't feel the little bumps or is it because there aren't any bumps because the stronger suction makes the mix more constant and therefore stable?

No it's never been rebuilt but it only has 18,500 original miles on it. Some grandma used it to drive to church and the store iirc.

If you'll listen correctly, do you know what the whistling noise is in the video? It's very annoying and I can't hear it coming from any particular place

I also remember that it's run more smooth as silk before. I just don't remember what RPM it was idling at.


Edited by snowman4839 (08/24/10 06:37 PM)
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#60011 - 08/24/10 06:45 PM Re: Timing and Dieseling [Re: snowman4839]
Mean buzzen half dozen A.K.A. Hank Offline
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I like to use starter fluid to check for vacuum leaks,but is very flammable.

MBHD
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#60012 - 08/24/10 06:52 PM Re: Timing and Dieseling [Re: Mean buzzen half dozen A.K.A. Hank]
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well brake fluid... starter fluid... i'll just check what's at autozone tomorrow

Also even when the engine is warmed up, if I floor it, it dies and then stutters to life. Is that just the idle set too low?
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#60013 - 08/24/10 06:55 PM Re: Timing and Dieseling [Re: snowman4839]
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You might be just hearing air being pulled into the carb. 18, 500 by a grandma is almost as bad as 185,000 by a leadfoot. If that motor only saw city miles, there could be some carbon buildup internally. If the engine sat for a long time without being run regularly, there are other potential issues, like a valve sticking in its' guide, or stuck piston rings, or even corrosion on valve seats that can cause cylinder sealing issues. You're positive the odometer's correct? Also, one other note, don't always trust the rpm's shown on a (cheap?) tach-they can be inaccurate. 800 is a bit high, especially with an auto trans. Again, basics. Document changes as you go along, and go through things methodically. My last post is bare bones to getting something to run right. In response to your first question, the engine makes less suction, or vacuum, at higher rpm, and as rpm increases, the the mechanical defect usually is felt less exactly because the engine is turning faster. At least, that's what I think.

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#60015 - 08/24/10 07:02 PM Re: Timing and Dieseling [Re: vanherk1]
Mean buzzen half dozen A.K.A. Hank Offline
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I thought it is a manual trans car?
500 rpm seems a bit low for a manual 3 speed & poor rearend gearing.

MBHD
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#60017 - 08/24/10 07:51 PM Re: Timing and Dieseling [Re: Mean buzzen half dozen A.K.A. Hank]
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Don't have a 69 book, but do have a 71 book. Idle should be at 550 rpm. I would set it for 600.
The whistle could be a # of things.
#1 Vacuum leak (also causes weird fuel mixtures and missing)
#2 Exhaust leak. Had a guy bring a street rod to me. Said the new crate engine had a rod/ main knock and wanted it fixed. Turned out being a exh donut on sanderson headers. Sure sounded bad.
#3 put the air cleaner on. It is calculated into the mess by GM engineers.

By veiwing the vid, here are a few things. Slow down with the taping. I was trying to look at things in detail but you would not stay still long enough. Things like the firing order.
Your vac adv seems to work good. I like to see these inlines get 20-22 degrees adv idling. 8 degrees fixed plus whatever the vac can adds, which is around 15-20. My book says the 250 adds 23 degrees with 16" vac.
Is the timing vacuum port live at idle( sucking) or only when revved up alittle? I bet it needs it. Tom
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#60018 - 08/24/10 07:54 PM Re: Timing and Dieseling [Re: tlowe #1716]
Mean buzzen half dozen A.K.A. Hank Offline
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The timing port is working as it should,you do not hook it up below the throttle blade & produce full vacuum thus giving full advance @ idle.

MBHD
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#60019 - 08/24/10 08:20 PM Re: Timing and Dieseling [Re: Mean buzzen half dozen A.K.A. Hank]
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Why coundn't that be a option? My engine is very happy with 25 degrees at idle.

SBC and BBC use manifold vacuum for their dizzy's all the time.

I am going to have to check out one of those monojets to see the porting. Tom


Edited by tlowe #1716 (08/24/10 08:21 PM)
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#60020 - 08/24/10 08:30 PM Re: Timing and Dieseling [Re: tlowe #1716]
Mean buzzen half dozen A.K.A. Hank Offline
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It's most likely 118,00 miles ,dont think grandma burned up the clutch in 18,000 miles.


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#60022 - 08/24/10 09:25 PM Re: Timing and Dieseling [Re: tlowe #1716]
RichardJ Offline
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snowman, lets go back to the video and start all over. Just exactly where do you see 29 degrees dwell?? Between the focus and the jiggling, I couldn't read which scale was for 6 cyl dwell and I don't see a 29 anywhere. Try that again. The 33 deg and the video.
That meter has a low rpm scale that will be much more accurate than the incar unit.
Explain the deal with the duct tape on the manifold vacuum port that has nothing to do with the vacuum advance. Duct tape is not at all reliable for pluging vacuum ports when there might be suspicion of a vacuum leak. Get a rubber cap for it. Again, explain the deal with that port. Why isn't it capped? what was it used for if it was?

This engine should have "ported" vacuum for the distributor vacuum advance. It is sometimes called "timed" vacuum as "Mean buzzen" referred to it as.
We ask you pull the vacuum hose to the distributor and plug that hose so that there was not a vacuum leak while you were checking the timing.
Your video shows that you did not??????
_________________________
'67 GMC 3/4 292 4spd

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#60024 - 08/24/10 09:31 PM Re: Timing and Dieseling [Re: vanherk1]
snowman4839 Offline
Active BB Member
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Registered: 03/26/10
Posts: 579
Loc: Memphis, TN
Yes I'm positive about the odometer. The driver seat is in way to good of condition and the pedals aren't worn enough for 185,000.

How will I know if the problem is internal like valves sticking or stuck piston rings? Inconsistent compression test?

Should I take the head off and clean out the ports and clean out and reset the valvetrain?
_________________________
69 Buick Special Deluxe. Intercooled Turbo Chevy 250 @ 15psi on a stock long block. It's kinda fast.

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