198 Oddfire Timing Marks

Can anyone confirm if these are the correct timing marks for a 198 oddfire? Sorry for the extreme close up, but for perspective these markings are on the passenger side of the timing cover.

my jeep seems to be running great, but I’m getting a timing measurement of approximately negative 5 degrees with vacuum advance hose disconnected and plugged, and approximately positive 15 degrees with vac advance connected.

FYI, I did confirm that the TDC marking on the pulley is correct.

 

To my knowledge there are two styles of timing marks for the Buick V6. Early marks are cast into the timing cover. Later marks were bolted to the cover.

The fact you are pulling 20 deg with the vacuum advanced tells me that you have it hooked to manifold vacuum instead of ported or the plates are too far open.

You need to set the timing with the vacuum advance disconnected. It should be approximately 5 BDC (the V6 will run more but that is the factory setting). I would run ported vacuum and it shouldn't change with the advance connected. If it still does, then you have another issue that is forcing the throttle plates above the port in the carb. This is usually caused by either retarded timing or issues with the idle circuit.

 

The marks are cast into the timing cover.

I am hooked up to a ported vacuum, not manifold.

I’m getting a reading of 5 degrees retarded with the vac disconnected and plugged.

Not sure I understand your last comment:

“ it shouldn't change with the advance connected. If it still does, then you have another issue that is forcing the throttle plates above the port in the carb. This is usually caused by either retarded timing or issues with the idle circuit.”

Based on that comment, it sounds like reading of 5 degrees retarded timing I am getting may be correct? And I therefore need to advance the timing a full 10 degrees to get to positive 5?

 

Retarded timing lowers the idle speed. This forces you to turn the curb idle screw in, opening the plates up inside the throttle bore. If the plates open enough, they will uncover the vacuum advance port, allowing vacuum to pull the advance at idle.

If you are 5 retarded, then yes, you need to bump it up 10 degrees to get to the proper timing. After you do that, you will want to readjust your curb idle, then redo your idle mixture. Once all that is done, your ported vacuum nipple shouldn't be pulling any vacuum.

 

Does a 198 have a ported vaccum fitting? That's more of an early 70's thing to reduce NOx at idle for emissions. I would think manifold vacuum is all that's available.

 

In fact, the 1962 Buick Special service manual says it's manifold vacuum: 1962 Buick Special Service Manual - Electrical Systems page 1 of 2

All the info you need to set the timing correctly is there.

 

By definition, ported vacuum goes to zero at idle. If the engine is idling and you measure vacuum, it's not ported vacuum.

 

As jeepdaddy2000 noted, there are cases where the throttle plate can be too far open at idle. Then the ported vacuum behaves like manifold vacuum because it's always below the throttle plate.

None of that should matter for a 198, there shouldn't be any ported vacuum ports on the carburetor to start with.

 

Maybe I am misunderstanding the ported vs manifold vacuum. My vac advance is hooked up to a vac port on my Rochester 2G that is above the throttle blades (at least it looks like it is). But it does pull vacuum at idle.

I found the attached image, and the port labeled “distributor vacuum tube” is where mine is hooked up to. So assume it’s hooked up correctly.

anyway, I adjusted the timing to 5 degrees advanced at idle, with the vac hose plugged and removed from the vac advance. When the hose is reinstalled, timing jumps to what looks like nearly 20.

I’m having a friend double check my work tomorrow, but does that sound right?

for what it’s worth, it was running pretty good before but seems to be running even better now.

 

Ported vacuum, sometimes called distributor vacuum, comes from a port in the venturi that is (supposed to be) physically blocked by the throttle plate at idle. Like a big gate valve, the port is closed at idle - no vacuum signal. On some carburetors, you can look at the underside and see the little hole, and follow it through the casting to the tube going outside. Usually the tube is located at the base of the carburetor, near the port, but not always.

Manifold vacuum comes from anywhere else. Above idle, ported and manifold vacuum are the same.

Fireball's point is that the 198 is too old to have this feature. Using ported vacuum for the distributor advance should turn vacuum advance off completely at idle, retarding the spark and lowering the combustion temperature of the burning fuel. Lower temperatures means fewer NOx emissions at idle (where the high temperatures burn the nitrogen in the air), though the HC goes up. The HC can be dealt with in the exhaust, initially by the air injection and later the catalytic convertor. The engine will idle great on manifold vacuum, but the emissions will be higher than with ported vacuum.

 

I can't speak for the Buick offerings, but every engine I've seen runs ported vacuum stock. My wife's old 65 Mustang ran ported vacuum. My 63 Rambler Classic runs ported vacuum. 1963 was long before anybody was worrying about timing related emissions.
Just saying (might very well be wrong) that ported vacuum has far outdated emission concerns.

 

I don't know - that was before my time!

 

That's different. Old Fords (and I guess Ramblers) use "Venturi Vacuum" which is more complicated. It had slightly reduced manifold vacuum at idle (but still had vacuum at idle), transitioned to venturi vacuum at high load, and had pure manifold vacuum at off-idle and overrun. As far as I know, GM never used this.

"Ported Vacuum" is strictly manifold vacuum that switches off *only* at idle and is pure manifold vacuum at all other times. This was introduced in the early 70s *only* to reduce NOx at idle.

Here's a description of the venturi vacuum system from a 1965 Ford service manual:

SPARK (DISTRIBUTOR VACUUM) CONTROL SYSTEM

The degree of spark advance in the distributor is determined by the strength of the vacuum acting on the distributor diaphragm. A high vacuum will increase spark advance. The carburetor is calibrated to provide the required vacuum to the distributor through an interaction of venturi vacuum and manifold vac-uum. Venturi vacuum is obtained through the pickup tube in the main venturi and is supplied to the distributor only when it exceeds manifold vacuum.

Manifold vacuum supplied to the distributor is obtained from a pick-up hole at the throttle plate edge (when the throttle is closed) and is metered to the distributor. An additional passage is always open to manifold vacuum. The purpose of the additional passage is to provide a higher spark advance at closed throttle during deceleration to promote complete burning of the fuel and greater efficiency.

Metering of the manifold vacuum to the distributor is accomplished through the use of the spark valve and restrictors in the vacuum channels. The spark valve is held open by the combination of vacuum and atmospheric pressure acting on the spark valve diaphragm to overcome the tension of a calibrated spring in the spark valve.

Calibrated restrictors in the manifold vacuum channels limit the flow of manifold vacuum to the distributor. Also, during low manifold vacuum periods, the bypass restrictor controls the reduction of venturi vacuum caused by bleed back.

At off-idle engine speeds, vacuum at the throttle edge is high due to a venturi effect created by the position of the throttle plate in the throttle bore. The high vacuum in this area is supplied to the distributor through the manifold vacuum channels for the required increase in distributor spark advance.

Upon acceleration and under wide-open throttle operation, manifold vacuum drops. When the manifold vacuum falls below a predetermined point, the spark valve closes, shutting off the manifold vacuum to the dis-tributor. The drop in distributor vacuum retards the spark advance. Venturi vacuum, now greater than manifold vacuum, supplies vacuum to the distributor, thus preventing a full spark retard.

As the engine load demands decrease, the increase in manifold vacuum will become greater than venturi vacuum. The increased manifold vacuum opens the spark valve and the higher vacuum now supplied to the distributor increases the spark advance for more efficient engine operation.​

 

If the 198 is running well, I would be inclinded to leave it be. There is also the chance your distributor is installed off by 1 tooth on the cam drive, causing your timing numbers to be off. If this is the case, the distributor housing may be twisted to an odd angle.
I often "power tune" the engine. I slowly advance the timing in increments, go out and drive it. At speed, I lug the engine a bit, listening for detonation. I keep advancing the spark until I get a touch of detonation/pinging, then I back off the timing to the point of no detonation, and lock down the distributor.
–Donny

 

Thanks everyone for their help. I had a friend double check my work and he said my timing is set right at 5 and looks good. Weather was beautiful today, so I drove around the neighborhood with the windshield down. Jeep is running great.

 

Interesting.
First, it is always neat to have someone give new information on a subject I feel I'm well versed in. That being said.........

The only mention to venturi vacuum that I can find is a Ford 94 application.There could and probably are more.
While I can't testify for all offerings prior to the 70's, I can say that my Rambler runs a stock Holley2300 and it is ported (I've rebuilt the carb). I worked for many years as a mechanic in a local 4X4 shop (specializing in Bronco's) in the 80's and 90's and rebuilt hundreds of carbs over the years. Most were Autolite and Motorcraft, there were also Carters, Rochesters and Holleys as well. Most were from the mid and late 60's through the existing time I worked on them and all had a vacuum port just above the plates. While the overall consensus is that ported vacuum is an emissions driven technique, I'm always trying to understand why these systems were in existence well prior to any tailpipe emissions concerns.

Thanks to you, I'm now tasked with further research on venturi vacuum advance.

 

And thank you! I need to research ported vacuum some more.

 

Aha, the text block I pasted above is for Ford "Loadomatic" distributors which is mostly a 1950's thing but live longer on 6cyls.

It does seem like 2100s and 2300s from the 60s had ported vacuum. Now I need to figure out why.