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Timing Problem
https://slantsix.org/forum/viewtopic.php?t=33728		 Page 1 of 1
Author:  81dodged150 [ Wed Feb 11, 2009 12:28 pm ]
Post subject:  Timing Problem
I have an 81 D150 that i converted over to electronic factory ignition and ever since i changed the distributor its always had a miss. Ive tried timing it to the factory specs of 16 degrees and it will run fine until i hook up the vacuum advance then it starts to miss again. When its hooked up to the vacuum pod it advances the base timing of 16 degrees to about 20-30 degrees causing it to miss. Ive timed it witht he vacuum line hook on and off like your suppose to do and it will run fine both ways until you try to drive it. I dont know what is left to do because the valves are fine and the plugs and wires are right. Any help will be appreciated.
Author:  Reed [ Wed Feb 11, 2009 12:38 pm ]
Post subject: 
It sounds like the timing is being overadvanced or the vacuum pod is blown.

(1) Have you checked to make sure that the vacuum pod on the distributor holds a vacuum?
(2) Are you hooking the distributor up to a ported vacuum source? This means a vacuum source that does not provide vacuum at idle but does provide it at off idle.
(3) Is 16 BTDC the correct setting for the computer controlled timing or the correct tioming for the car where you got the non-computer distributor out of? The 16 degree setting for your truck is no longer correct. That setting worked with the computer because the computer (a) controlled the timing and (b) used a different pickup to retard timing during startup. You will need to set the base timing to the specifications of the vehicle the distributor came from.

It sounds to me like the distributor is attached to a vacuum port that provides a vacuum signal at all times and/or the distributor is overadvancing.

I can almost guarantee that you will need to recurve the distributor in your truck. Trucks and vans like slow mechanical advance curves and lots of vacuum advance. My advice is to carefully disassemble and lubricate the distributor you have now and make notes of the thickness of the springs on the weights, the number stamped on the vacuum advance arm, and the number stamped on the governor arm. Post the numbers here and we can help you further.
Author:  SlantSixDan [ Wed Feb 11, 2009 12:50 pm ]
Post subject: 
The vacuum advance pod is not blown. If that were the case, you wouldn't be seeing the timing jump when you connect the advance hose. There should be no vacuum applied to the vacuum advance when the engine is idling. If there is vacuum in the advance hose with the engine idling, then the hose is hooked up to the wrong port on the carburetor, or the carburetor is badly misadjusted or mechanically faulty.

An '81 would have had electronic ignition from the factory. When you say you "converted it to electronic factory ignition", do you mean you removed a Lean Burn setup (including a dual-pickup distributor without a vacuum advance) and installed a conventional electronic distributor with a vacuum advance? If so, that would help explain why you have vacuum advance being applied at idle: most of the Lean Burn carburetors do not have an appropriate ported vacuum takeoff to operate a vacuum advance correctly, so perhaps you just hooked it up to manifold vacuum; that won't work right.

What exact distributor did you install (where did you get it, and what part number is it)? 16° is probably too much initial advance, too, for an advancing (non-Lean-Burn) distributor; a better setting will be between 5° and 10° or so, depending on which (exact) distributor you have installed.
Author:  Reed [ Wed Feb 11, 2009 1:03 pm ]
Post subject: 
The vacuum advance might still hold a partial vacuum but have a slow leak. If he is hooking it up to a nonported vacuum source then he might be inducing both partial advance and a vacuum leak. Sort of like a choke pulloff diaphragm that still operates but also has a leak. It is posible, but not likely. I stand by my answer.
Author:  Ed Mullen [ Wed Feb 11, 2009 1:37 pm ]
Post subject: 
Quote:
most of the Lean Burn carburetors do not have an appropriate ported vacuum takeoff to operate a vacuum advance correctly
Is that true for my carb?

I am planning a lean burn conversion (HEI) and had hoped to use the ported vacuum intended for the EGR (see arrow) to run to the distributer (looking to control EGR by other means).

The pictured carb isn't what I'm running now, BTW. It is an old parts store reman carb. I had since had the original rebuilt by a local carb shop, but it's identical apart from some missing parts (seriously, everything under the purge valve cover plate was missing apart from the cover gasket.) Luckily I had hung on to the original because of procrastination, not any kind of foresight (put off taking it back so long I had lost the core receipt).

Image
Author:  SlantSixDan [ Wed Feb 11, 2009 1:43 pm ]
Post subject: 
Quote:
The vacuum advance might still hold a partial vacuum but have a slow leak
C'mon, Reed, the guy's truck has a sprained wrist and you're telling him to check and see if he's got a scraped knuckle. :roll:

His vacuum advance pod may have a leaky diaphragm — probably not — but it's irrelevant to his current troubles. Let's help him with the problem he's actually having, then we can worry about whether he's got additional problems like a leaky vacuum advance, dirty spark plugs, or worn-out wiper blades.
Author:  SlantSixDan [ Wed Feb 11, 2009 1:45 pm ]
Post subject: 
Quote:
I am planning a lean burn conversion (HEI) and had hoped to use the ported vacuum intended for the EGR (see arrow) to run to the distributer
Won't work at all. The EGR vacuum takeoff is venturi vacuum, not at all correct in strength or timing for distributor vacuum advance.

Your carb may or may not have a spark advance vacuum port. Match up the ports your carb has with the illustration in the Holley 1945 manual. Carburetor operation and repair manuals (and some training movies) are posted here for free download.
Author:  Ed Mullen [ Wed Feb 11, 2009 4:33 pm ]
Post subject: 
Quote:
Match up the ports your carb has with the illustration in the Holley 1945 manual

I have this port:

Image

It is hooked up to my ESA. I've been told (by someone on this forum--can't recall who) that the port to the ESA is not appropriate for a vacuum advance distributor, and that the ESA connects to unported manifold vacuum.

I guess I'll just take a vacuum gauge to it at some point before the ESA dies and forces my hand to do the conversion sooner v.s later.
Author:  rock [ Thu Feb 12, 2009 5:47 am ]
Post subject:  dodged, what state are you in?
dodged,

What state are you in? If you are in the land of the somewhat free down south, you can probably make some changes you can't make on the left coast.

For instance you can assure you are using a ported vacuum easily with any of several other carbs than what you may have, but probably not in CA and stay road legal. Then, just changing a carb is a waste of time in my opinion, unless you do so as part of a plan. For instance, Ted and I among others run 16 degrees initial advance...on highly built engines with recurved dizzies and ignition systems made to match. What Dan and the other guys are suggesting that if you changed what you already changed, you are going to have to get the vacuum back as it supposed to be and retard that timing. I first ran into the ported problem early on too, because an Edelbrock 500 has a ported inlet and I didn't know what it was, and the guys here straightened me right out. What you have is very fixable if you keep plugging away.

rock
'64d100
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