1969 Dieseling 350, solved?
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Cruising
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1969 Dieseling 350, solved?
This 350 dieseling was driving me nuts. I finally completed an epic journey to fixing it. Thought I would post it and get some feedback. Bottom line...poorly set carburetor causes carbon build up and it's hard to get rid of.
Last edited by 68-WithSun; Jun 20, 2020 at 05:28 PM. Reason: Edited the document for clarity
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
I read your writeup. I'm one of those here who does understand what the big deal is with manifold vacuum source to the vacuum advance canister. If you don't understand, as you stated in your writeup, why don't you ask for an explanation, instead of just dismissing it?
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From: At my Bar drinking and wrenching in Lafayette Colorado
The OP states:
"Moving the vacuum advance source from ported [on the carb] to manifold just bumped the timing advance at idle from 12 to about 24. This kinda defeats the whole purpose of having it, so I don’t know what all those “manifold vacuum is the way to go people” were posting about."
That's the whole idea: You run manifold vacuum so you get the benefit of advanced timing (should be around 30 degrees) at idle. This advanced timing at idle allows you to reduce throttle blade opening, and it reduces combustion temperatures, both of which will reduce or eliminate run-on after shutdown while reducing carbon deposits. Automotive engineering 101.
Lars
Last edited by lars; Jun 20, 2020 at 12:15 PM.
Safety Car
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From: BC
Also, how is it possible that changing manifold to ported vacuum to reduce timing at idle would prevent run on when there is no ignition whatsoever when you turn the key off?? It actually runs hotter at idle with ported vacuum but when you turn the key off there is no ignition let alone ignition advance.
Last edited by CanadaGrant; Jun 20, 2020 at 12:28 PM.
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Cruising
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Thanks for the input. I was trying to get it to run cooler and maybe stop dieseling by changing the vacuum to the manifold and it just made it run so much worse I gave up and put it back onto the ported one where it ran sooo much better. That's why I dismissed it I didn't really understand why. With manifold vacuum it idles so bad and with ported it runs hotter but idles better. Still confused, but that's normal at my age.
Last edited by 68-WithSun; Jun 20, 2020 at 05:29 PM.
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What is your initial timing set for with vacuum disconnected?
When you tried manifold did you readjust the carburetor idle speed and mixture?
Last edited by MelWff; Jun 20, 2020 at 03:11 PM.
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Cruising
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When does ported "kick in" then? Initial is at 12 deg. putting the hose on ported does zip at idle. Putting it on manifold bumps idle to 26-28 deg like it should, but it runs so bad on manifold and so good on ported. ?? Lars is 100% correct. I was trying to get it to run cooler with more advance at idle, but couldn't get it to idle nice.
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Melting Slicks
Joined: Apr 2001
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From: Olmsted Falls, Ohio
OP:
It seems to me you don't have a complete understanding of how vacuum and timing relate.
Get a book and read.
Lots of threads here explain how to set timing. Yeah, you'll have to tweak timing again if you go from ported to full. So what?
This whole thread is that you don't understand how much vacuum is at idle on ported and when it comes in. Threads on that also.
BTW I did read your paper, but it is not correct and I hope others do not follow it.
It seems to me you don't have a complete understanding of how vacuum and timing relate.
Get a book and read.
Lots of threads here explain how to set timing. Yeah, you'll have to tweak timing again if you go from ported to full. So what?
This whole thread is that you don't understand how much vacuum is at idle on ported and when it comes in. Threads on that also.
BTW I did read your paper, but it is not correct and I hope others do not follow it.
Last edited by KenSny; Jun 20, 2020 at 03:35 PM.
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Cruising
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True. I don't quite have a grasp on it yet, but I have been reading about it.
Getting it off the page and into the real world is where I lose it. I edited the doc for clarity and to admit I am slightly confused.
Getting it off the page and into the real world is where I lose it. I edited the doc for clarity and to admit I am slightly confused.
Safety Car
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From: BC
What carb do you have? If it has the original 68 Quadrajet carb I don't believe it had a ported vacuum source as both the lower port on the front drivers side and the side port on the passenger side that comes out through the choke mechanism are both manifold vacuum. The front port was for vacuum advance and the side port was for the smog pump but both are manifold vacuum. I don't believe there was even a ported vacuum source on a Quadrajet until the early 70's. It should actually run smoother and cooler at idle on manifold vacuum as the idea at the time was that if you reduced the timing at idle throttle (ported vacuum) it would run hotter and burn off exhaust pollutants. It not only ran hotter but consumed more fuel doing it and ran rougher because of the low idle timing.
Last edited by CanadaGrant; Jun 20, 2020 at 06:57 PM.
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Cruising
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Thanks Grant, Its a one year old Holley 4150. I thought it should run cooler and smoother on manifold but it just won't idle properly. Maybe there's something else wrong with this motor. The vacuum needle bounces a little at idle. weak valve guides? Compression was okay at about 140 ish. I think I just stumbled upon the spot where it stopped dieseling by accident.
Melting Slicks
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From: Olmsted Falls, Ohio
Team Owner
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From: Crossville TN
The whole purpose of the timed/ported vacuum was to allow the engine to run HOTTER (combustion chamber temps) and burn off excess hydrocarbons. Strictly an EPA / environmental concept. Not what the engine wants, at all!
Timed/ported vacuum is NO VACUUM at idle, because the orifices that are to supply it are above the throttle plates at idle. No vacuum up there! When you begin opening the throttle, timed/ported vacuum quickly becomes close to manifold level vacuum.
Timed/ported vacuum is NO VACUUM at idle, because the orifices that are to supply it are above the throttle plates at idle. No vacuum up there! When you begin opening the throttle, timed/ported vacuum quickly becomes close to manifold level vacuum.
Melting Slicks
Joined: Apr 2001
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From: Olmsted Falls, Ohio
I think that's the point the OP is confused about.
Once past idle, ported vacuum is for all purpose at manifold vacuum.
So he's asking why the big deal. I see posts on this forum and the chevy forums asking the same thing. The answer is always the same.
Vehicles stuck on the 405 for hours were smogers, hence the the ported vacuum in the interest of reducing polution at idle.
I have triied both on my car after I installed the Holley. I did NOT see any diffference in engine temp between the 2, but then I do not sit for long periods at idle,
I did have to tweak idle and timing a little between the 2, but nothing major.
Maybe because my engine is basically all new except for the block and crank it makes a difference, I don't think so, but who knows?
Once past idle, ported vacuum is for all purpose at manifold vacuum.
So he's asking why the big deal. I see posts on this forum and the chevy forums asking the same thing. The answer is always the same.
Vehicles stuck on the 405 for hours were smogers, hence the the ported vacuum in the interest of reducing polution at idle.
I have triied both on my car after I installed the Holley. I did NOT see any diffference in engine temp between the 2, but then I do not sit for long periods at idle,
I did have to tweak idle and timing a little between the 2, but nothing major.
Maybe because my engine is basically all new except for the block and crank it makes a difference, I don't think so, but who knows?
Last edited by KenSny; Jun 21, 2020 at 01:37 PM.