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Old 12-27-2010, 03:54 PM
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Modifying vacuum advance canister

I have reset my timing so the engine sounds better and it has better throttle response. It was at 14 initial and it was a bit sluggish. I jumped it to 18 initial which put me at 41 with mechanical all in at 2500. With vacuum I am at 58 total. The truck seems to run good at this point and does not kick against the starter. I am currently running on ported vacuum. I would like to keep the total down to 51. MSD says the vacuum can should only add 10 degrees but it appears to be adding 17. This is a 350 stock cam and I am using a dial back timing light.

Is it possible that the vibration damper has slipped since the 18 degrees is pretty high? Is it possible to restrict the vacuum hose to get the vacuum advance down to 10 degrees instead of 17. I'd rather not mess with changing canisters if I don't have to.

The other option would be to go back to manifold vacuum and try to get it to 36 and not worry about the total with vacuum.

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Old 12-27-2010, 04:07 PM
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You can put a limiter on the canister arm, Crane makes one that is adjustable in 2 increments. There are also some pictures floating around here at Hotrodders of home fabbed limiters that are simple to make, search in the timing or vacuum advance posts.

That 41 total mechanical is a detonation disaster waiting to happen, I would be restricting the mechanical advance to 36-38 first before I worried about the vacuum advance. When you are down hard on the throttle Vacuum advance drops off, but 41 mechanical total will be a rattle-fest with a heavy load behind the truck, and possibly even empty going up a grade.
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Old 12-27-2010, 05:18 PM
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I will back initial off a little but it was sluggish when I blip the throttle at 14. To stay at 38 I need to be at 15 initial which may not be enough. I have been using a dial back timing light which MSD says not to use so I might not be correct in my readings. The only other light I have is an old one where you attach a spring to the plug and then the light which might arch with my headers.

MSD forum had a post where they say to use a vacuum restricter in the line but noone seems to be able to find one. I assume it is just an insert with a small hole in it. The MSD is supposed to only add 10 degrees so my readings might be off.
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Old 12-27-2010, 06:13 PM
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Have you checked to see if you have the proper damper? Gm made more than one, that will put the timing mark in a different place. Also they have several timing tabs. You need to find TDC and see if yours in correct. 53 might be 40?
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Old 12-27-2010, 11:40 PM
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Quote:
Originally Posted by Mertz
I will back initial off a little but it was sluggish when I blip the throttle at 14. To stay at 38 I need to be at 15 initial which may not be enough. I have been using a dial back timing light which MSD says not to use so I might not be correct in my readings. The only other light I have is an old one where you attach a spring to the plug and then the light which might arch with my headers.

MSD forum had a post where they say to use a vacuum restricter in the line but noone seems to be able to find one. I assume it is just an insert with a small hole in it. The MSD is supposed to only add 10 degrees so my readings might be off.
The idea is to keep the initial where the engine 'likes' it, but to reduce the total timing back to 36 or so. This will require modifying the mechanical advance mechanism. The image below shows the slots and pins that limit the total advance available to the distributor. The slots can be brazed up to limit the travel, or if you have a selection of weights and cams for the HEI mechanical advance, you may be able to find a combination that gives you what you need for total advance.



An AR12 vacuum advance can or one of the adjustable cans from Crane or Accel would be a good unit if a new one was needed.

Using a restrictor in the vacuum line would delay- not reduce- the vacuum reaching the can. If you want to use the existing vacuum advance can, you will need to physically limit the vacuum advance can's travel w/a vacuum advance limiter plate like the Crane #99619-1, #99619 Instructions.
Or you can make one.

DETERMINING TDC will allow you to be sure the timing tab and damper are correct for TDC.

MAKE A TIMING TAPE to see/adjust the total timing, w/o needing to use a dial back timing light.

Good info on ignition timing (post 9)-Port or Full time vacuum
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Old 12-28-2010, 06:51 AM
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Put the initial back to 14 and try full manifold vacuum.
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Old 12-28-2010, 09:05 AM
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I'm with Cobalt. Mechanical should be limited to around 36, but depending on the combo the sky is the limit with vacuum. I crutched a low compression 454 by locking the mechanical, setting it to 35 initial, and the ported vacuum took me as high as 58. There is no set formula since there are so many possibilities with the engine.

As far as initial timing is concerned, don't worry about it. Set it to where the engine runs right and doesn't kick back, then tune the rest of the curve with mechanical and vacuum. Initial timing settings are very inconsequential as long as the timing is correct for the rest of the operation.
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Old 12-28-2010, 09:09 AM
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I read the articles in the past and believe that my timing mark is set to TDC. I had a number of opportunites to check it when I changed rockers and intake manifolds but I will check it again to see if anything slipped. The damper is the one that came with the truck and I believe it to be original so there is a chance it has slipped but it does not appear like it and there is no walking of the timing and vacuum is steady.

I believe I will also go back to manifold vacuum. I only have one full time vacuum source on the carb so I will add a tee for my vacuum guage. The only reason I went to ported was that it ran better on ported when I installed the carb. Now that I have fuel following properly it might run better on manifold. Right now the truck does not need a choke to start even in the cold of winter. I will see if it is the same with manifold vacuum. I don't drive it in the winter so it is not really a big deal.

The distributor is an MSD ready to run and I have changed springs and stops to get all the timing in at 2500 rpm.
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Old 12-28-2010, 09:40 AM
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That is the first time I have heard about a restricter in the vacuum line. This is not a valid solution to limiting vacuum advance and should be dismissed. As cobalt stated it will merely delay the action of the vacuum can. It would also slow down the return rate of the diaphragm causing the vac advance to be pulled in for a time period after the vacuum level drops off, not good.

Can you post the new values for the total mech advance limit and also the vacuum advance limit then.

The first post listed 23 mech and 17 vacuum advance, this is too much on both IMO. If you set the total with base and mech to 36 that would put your base at 13. What is the cam duration at? what type of heads.
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Old 12-28-2010, 12:25 PM
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This is a stock engine as far as the cam and heads go. It has an MSD Ready to Run distributor that I have changed the springs and the stop so that I get all mechanical in at 2500. It has been awhile so I don't remember the color of the springs. My numbers come from using my dialback timing light. I run the motor up to 2500 and check the light to see what it is reading. I connect the vacuum and do the same. That is were I get 23 and 17. Initial is at 18 with 41 (41-18=23) with mechanical and 58 (58-41=17) with vacuum. You are correct that initial will be at 13 if I got to 36 total with mechanical. I was at 14 and it was sluggish when I blip the throttle and really pickup up at 19 but I backed it off some to 18 because I thought 59-60 total was too much. I still think it is high but the throttle response if really good.
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Old 12-28-2010, 02:28 PM
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Throttle response is good because it likes that high initial, but you can't let the total mechanical get out of sight just to please the needs of inital timing. You should have a Black 18 bushing with the distributor parts kit, I'd get that installed so you can get your initial at 18-20 but not push the total over 38.

Adjust the vacuum advance degrees added(via travel limiter/stop) to maximize gas mileage, and to avoid pinging. This will take some road time to figure mileage gains, to find the sweet spot. This is where the Crane #99619-1 limiter plate is nice, as it is easily adjustable. It is really cheap too(like $4-5 bucks), you can't make an adjustable piece for that low cost yourself.
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Old 12-28-2010, 02:40 PM
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Thanks. I was thinking the same thing. I will check my kit and see what I have.
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Old 01-09-2011, 06:08 PM
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You could be off one tooth retarded on the dist gear,

Put a paint line across the balancer face you would see the line change.

How does it look even or wobble when its running ?
When mine spun it wobbled quite a bit.

I'd bet the timing is not correct.
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Old 01-09-2011, 09:00 PM
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I will try putting a line on the face but the line on the damper is very steady when I check the timing.

How would I know if I'm off a tooth? When I put the distributor in I had it set to tdc and pointed at #1. I never moved the crank after I took the distributor out so if it is off it had to be done before I got the truck and I don't think the original distributor has ever been out.
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Old 01-09-2011, 10:19 PM
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As long as the timing light indicates the timing to be"OK", the only thing the distributor being "off a tooth" would cause is for there to not be enough room to advance or retard the distributor w/o having the advance can hit the intake or firewall.

Last edited by cobalt327; 01-09-2011 at 10:27 PM.
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