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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine> Opinions, dumb idea: Using vacuum canister + vac advance to avoid hardstart 38'
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Topic Review (Newest First)
06-25-2010 09:15 AM
bubbahotep Barnym, that's an interesting possibility that makes sense. Up until I did this vac advance trick I think my phasing was off by the width of a full rotor tip at 38' timing too. My phasing was past the spark wire (my dizzy spins clockwise I thought) and my spark was always after the pole. So, that'd be even worse? My spark was technically already late but grabbing the correct terminal, add in 20' retard and I'm even later?

If I'm now phased perfect at 38' or 36' is 20' retard still an issue you think? I can, but never tried it, disable the retard feature but it's not adjustable. It's 20' or nothing.
06-25-2010 07:16 AM
barnym17 I think your problem is the 20 degree retard.10 would be better with 20 degrees the rotor is almost halfway between two terminals when it fires.It can fire the terminal ahead of it almost as easily as the one it is intended to fire so instead of 20 degrees timing retard occuring it can actually be 50 to 60 advanced if it fires to the wrng terminal.
06-23-2010 06:30 PM
bubbahotep Just an update: this was a failed idea with good intentions. Sure enough I was caught in traffic (ny) and on the brake at idle. After inching forward in traffic I lost all reserve vacuum from my canister which made the vac advance drop and then my "total" timing dropped so that dropped my idle timing and ka chugga ka chugga ka chugga. It was good throwing it into neutral but thats just not a fun driving experience. If I was just stopped at a light it was fine but the stop, inch forward, stop, killed it.

And yes it did wreck my phasing. I was past the firing by a good 1/2" I think. Way beyond the plug wire. Not good.

Now Im back to just 36' locked with the 20' start retard and using the msd 8981 computer to keep me at 28' idle timing. I'll do with the ignition "off" switch and be happy with that much.

Was worth a shot
06-04-2010 05:21 AM
Bases Covered

Hey Bubba, Well if you're already running a gear reduction starter it sounds like you've got all the bases covered. Starter and grounding issues already mentioned by others here was just starting to sound like where your real problem was. olnolan
06-03-2010 09:35 PM
bubbahotep Starter is MSD 5095, I thought thatd be enough gear reduction. Tiny starter too. Starter gear is as perfectly spaced as I could see. Goes in and out freely, no binding, no twisting, spaced according to MSDs little piece of paper that came with the starter.

My Ground cable goes from battery straight to an unused hole in the starter aluminum mounting block and then theres 2 or 3 more going to chasis. But its NOT many stranded. I thought the many strands were more for audio and easier bends. And the + goes to the starter direct. Both are also the length of the car (battery in trunk). Fairly new Optimum battery too.

The carb, thanks to info from this board, was properly adjusted last year on the plates where I had too much "square" exposed. Things right at factory setting now with a good 900 or so idle rpm.
06-03-2010 09:13 PM
Ground Cable

Hey Man, You may want to check your ground cable if it only hard cranks on occasion. Also starter shimming. Ground battery to block, block to chassis with the two cables under the same block bolt, block cleaned to bare metal. Remove solenoid and shim starter so it can be hand operated in and out of the flywheel with a smooth action at three separate points around the flywheel(sometimes the ring gears have a little run out). I went through this on my 406 boat engine and on the 454 motorhome. More food for thought. olnolan

Oh, I forgot, the type of cable you are using isn't the right stuff for 12vdc if its super stiff and doesn't have many strands, you want as many strands as possible to improve current flow to the starter. May be another thing to consider. olnolan
06-03-2010 08:56 PM
bubbahotep Starter is brand new MSD high torque with fat wires on both red and black. The wires are like 2 3/4" pipes. lol. They hardly bend at all, not too many strands inside and cost more than the starter if I remember

My reservoir wont get below the amount needed to maintain the vac advance. It is an adjustable vac advance can which supposedly can work as low as 5hg and seems to be somewhat true.

This 8981 MSD timing computer requires advance locked out at full in order to work. You give it your total timing and then I adjust my advance curve via a dial as well as my idle timing via another dial and the advance quickness via another dial. Its drawback is that its forcing me to start with 18' timing (after the 20' retard it gives).

Fbird is no doubt correct in everything and what he says about my hot hot start with gas mixture makes sense. I even installed a carb spacer thinking I had some percolation after the engine heated at shut off and caused the carb to dump gas. Didnt make a difference.

Engine never goes above 180-200' while running. Starter is 1" all around spaced from the headers, with the ignition switched "off" I can turn the motor freely so the starters not overheated. I can ALWAYS start the car. Just now and then I get that vrr vrr vrrrrrrrrr vrrrrrrrr.

OLDBOGIE - Ive had this thing at 50' idle and 50' driving around town at one time due to the dizzy slipping and never heard a ping. Must have been like that for months Id guess and I only noticed it when I felt I little stumble one day at idle. Possible there was just too much noise to hear the ping? That was years ago now though. Never sees more than 38 now and maybe 36 if I listen to Fbird
06-03-2010 08:46 PM
Vac Advance

Hey Bubba, While your solution to your problem at hand is pretty clever. I think that FBIRD&COBALT can help you set up for an engine that likes alot of timing. I have dealt with two big blocks that liked the timing but the starters didn't. If you can't crank your sbc without an interrupt and a timing box, you really should consider a gear reduction starter. I cured both the big blocks mentioned with gear reduction starters. Back in my Mopar days, cranking big cam, high compression, high timing engines was never an issue due to the factory g/r starters with no interrupt or box. Big cable, big battery, g/r starter= easy start. Worked on my 454 motorhome too. Used a later model 350 g/r on it. On the other two I used Powermaster. Food for thought. olnolan
06-03-2010 05:25 PM
Originally Posted by bubbahotep
my only loss the use of the vac advance for cruising light load since my vac can is just stuck at advanced fully?
That, and if the reservoir ever goes below the vacuum that is needed by the vac. adv. can to stay fully advanced, you will obviously lose timing advance until the vac. in the reservoir comes back up.

I had thought at first you were running a gnarly cam but a 236/242 @ 0.050" doesn't qualify for needing a locked-out advance dist., IMO.

If you set it up w/36-38 total timing all in by 2500-3000 RPM, w/as much initial timing as the engine "wants" (initial could be anywhere from 16 to 24, ballpark- the rest from mechanical), and use a vac. adv. as a vac. advance and have 10-12 in the can, the engine will run great for you. Use the timing retard to lower the initial for starting, like it was intended.

The ACCEL #31035 is an adjustable vacuum advance can for GM HEI that allows infinite adjustment to BOTH the amount and rate of advance. Comes w/instructions and tool.

To limit the amount of vacuum advance w/an OEM or some adjustable cans like the ADJUSTABLE VACUUM ADVANCE CAN KIT- Crane #99600-1: 99600-1 INSTRUCTIONS, you will need to physically limit the vac can's travel w/a VACUUM ADVANCE LIMITER PLATE- Crane #99619-1: #99619-1 INSTRUCTIONS.
06-03-2010 04:51 PM
lmsport What kind of starter do you have? With gear-reducted starters I dont even see this prob anymore, even on oval track cars with conventional headers after running at peak power for 30 minutes. I would stick with the locked mech adv if I had the timing setup you have and try to get some vac advance if you can, but if not its really not a big deal.. and get a better starter with big cables going to it.
06-03-2010 04:35 PM
bubbahotep FBIRD - Im totally agreed with everything youre saying and Ive read tons of your posts over the past year and learned a ton.

Aluminum heads, I was told 38' was acceptable, you still say 36? I'll take your word.

With the timing, in my case, Im not confused just in a bind with the MSD. I think this was mentioned in an older post of mine and is related this this new post this week: I use an MSD 8981 timing box for my advance curve because its so dead-on-balls precise. Its biggest downfall, as you're already pointing out, is that it requires the distributor to be LOCKED at full advance. It does the retard for hard starts but 20' from 38' on my engine is still sometimes a hard start.

So, thats what Im trying to use my vac advance for more initial start retard BUT keep the vac advance "advanced" even for WOT.

100% I see that I SHOULD go back to the weights and the vac can etc etc. But for now, isnt my only loss the use of the vac advance for cruising light load since my vac can is just stuck at advanced fully? (not going over 36 or 38 total advance ever)
06-01-2010 06:16 PM
bubbahotep F-bird, you think just dropping back 2' on my total will make much difference? The msd timing box can give me 26' idle and have an adjustable curve to eliminate weights. Thatd match your suggestion.

Id still be firing the engine at 16' with the 20' retard

But youre suggesting also using the vac advance as it was intended? Meaning, let the vac advance raise the timing to 48' (using 10' vac advance) at idle? Or are you suggesting i use the vac advance to get me to 26' for idle running only?

Also, you said about waiting to flip the ignition on to get better gas/air: isnt that what causes the starter to strain? I thought the starter strain was from the combustion pushing against the piston...cause im so far advanced.
06-01-2010 02:32 PM
Originally Posted by bubbahotep
This motors ideal timing 38' total, anything less is a loss of power on this motor. I idle at 30'

I know about vac advance and mechanical but had to remove the centrifugal weights to use the MSD timing box. Its their suggestion to lock the dizzy only because the msd computer does the timing.

All that aside I dont yet see why using the vac advance to give me my full timing is a bad idea. I know its not being used properly. Im using it more as a 'full advance' after startup with no regards for fluctuating engine vacuum
Mostly the problem is a lack of vacuum. In a"normal" motor manifold vacuum is quite high at idle and part throttle. Under that circumstance the vacuum can pulls in it's advance. As the throttle is opened the the vacuum falls and the advance it creates goes away while the centrifugal takes over with advancing RPM. At 10 inches of manifold vacuum at idle you're in a situation where a normal vacuum advance has retreated. The reduction in vacuum just falls further as the throttle is opened, so the resource you need for the vacuum advance keeps becoming less. The vacuum brake reservoir saves up a bunch of low pressure air, but it can't make the vacuum deeper than what's being produced in the manifold. It will, however, be able to supply some advance based on how deep the vacuum can be retained, this is a good idea and will work, but you'll have to trim an adjustable vacuum advance to work with the available low vacuums.

Way back you said this engine was cammed but not with all that much-do you know how much? Your description of this engine speaks to a lot of cam, or a miss timed cam if it isn't a big whomper stomper stick, or the compression is lower than you think it is. An engine that needs 38 degrees to run good also speaks to a slow burning chamber. So some or all of these could be players if this isn't a huge cam, what ever that really means. Please comment to these details if you know them. such as

-Cam duration and lift, timing events if you have a card or the cam maker and part number so they can be looked up if the cam is fairly recent and not an obscure maker.

- Cylinder head, whose and what chamber size.

- Pistons need crown configuration (flat, dome, dish & whether that dish is round or D, compression height.

- Rod length

- Cam coordination to the crank, straight up, advanced or retarded and by how much.

- Status of the distributor gear and if a roller cam status of the distributor drive gear on the cam,,i.e. wear.

- Carb mixture ratio, lean will give also you some of the headaches you have.

- Vehicle weight, gearing and rear tire size.

06-01-2010 01:56 PM
bubbahotep This motors ideal timing 38' total, anything less is a loss of power on this motor. I idle at 30'

I know about vac advance and mechanical but had to remove the centrifugal weights to use the MSD timing box. Its their suggestion to lock the dizzy only because the msd computer does the timing.

All that aside I dont yet see why using the vac advance to give me my full timing is a bad idea. I know its not being used properly. Im using it more as a 'full advance' after startup with no regards for fluctuating engine vacuum
06-01-2010 12:26 PM
Originally Posted by bubbahotep
Quickly: I have a 383 stroker weekend driver with 38' total distributor locked full advance and using an MSD timing computer for 20' retard on start and adjustable idle timing (I can idle anywhere under 38'). Been like this for at least 10 years, maybe 15?? The engine calls for 38' total timing.

No weights, no vac advance used for years.

Even with the MSD computer retarding my start timing 20' (goes down to 18' when the starter is cranking) I still get some hot days where the engine wants to spin in reverse and the starter is crying a little. I have a switch to kill the ignition which works. Usually its after driving, getting gas, and then going to restart the car (all warmed up and hot and gas flowing too well)

Today I tried this setup which appears to work well so I ask opinions:

I have a vacuum canister (for brakes) that holds somewhere around 18hg after I rev the engine. Need that for my brakes with the cam I have. My hg never goes below 10 when I press on the brakes and stays there when I hold the pedal down. At idle the engine wont get below 10hg but cant get higher because of the cam, thats just the average hg at idle give or take. But the brakes hold well and stops fine

I just hooked my vac advance on the dizzy, adjusted to kick in instantly, to the vacuum canister for the brakes. THEN, retarded my timing something like 20' (just a guess). The point is that when I first start the car I am NOT locked at 38', my distributor is probably locked at 18' now because I turned it back. After about 2 seconds as the vacuum canister fills (empties, you get the point), the vac advance kicks in at 5hg, raises my total timing back to 38' (which I checked to confirm). The timing holds because my vac advance never releases.

I ran the car around and never stumbled because the vacuum canister always had well over 10hg...enough to hold the vac advance canister in total advance. And, at WOT where I have no vacuum, it doesnt matter because the vac is coming from the canister which is still holding enough to pull the vacuum advance in.

Long winded but I wanted to cover the story. My idea is when I start the car I have maybe 18' on the distributor because its locked, the MSD timing computer throws in additional retarding for a better start, and then the vacuum advance pulls in once the car is started to get me back to 38' total locked timing. The MSD computer is still dropping my idle timing down and doing what it should.

YES, I know ideally I should have weights and vac advance canister hooked up. But aside from fuel economy, does anyone see this as a really dumb idea? Its nothing different than any other radical cam riding at total timing locked out except that my total locked out timing isnt in effect until the car runs.
If you're running locked in timing, having a vacuum advance is inconsequential. Vacuums intent is to pick up the low to mid RPM timing to overcome the loss of mixture density under partial throttle conditions. This does more than improve gas mileage in this condition, it greatly improves power in this stage. The problem with this is that long duration cams knock down induction vacuum regardless of throttle position. Lock in mechanical advance is used to overcome this deficiency in vacuum, but has a dark side which makes the engine difficult to start.

In your case the set up would probably be better with putting in 6-8 degrees mechanical to come up say starting from 2000 or 2500RPM and all in by 3000 RPM. Set the base at 10-12 degrees. With 30-32 locked in, the electronic retard dropping 20 will leave 10-12 degrees for starting. The final 6-8 degrees would come in on the top end of the mid RPM rev range where it can be used. Then run no vacuum advance.

Keep in mind that locked in timing is a feature of race engines, it's old school technology to overcome timing problems with big cams before the age of computer driven timing. It was also done to remove another failure prone part on race engines, especially those that see a lot of G loading in frequent and sharp corners, beyond that it serves no useful purpose.

In your case, running a total of 30-32 degrees up to 2000-2500 RPM is plenty. Frankly with 9.5 compression and 38 degrees of timing from the git-go, I'm surprised this will stay below the detonation threshold at moderate RPMs on 93. I'd guess that it does because the vehicle is light and perhaps "low geared" against it's weight and rolling/aero drag factors. Low geared in this case is a relative term variable with the work the engine has to perform against the load of the vehicle to be moved.

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