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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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Thread: Opinions, dumb idea: Using vacuum canister + vac advance to avoid hardstart 38' Reply to Thread
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Topic Review (Newest First)
06-25-2010 10:15 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 08:16 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 07:30 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 06:21 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 10:35 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 10:13 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 09:56 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 09:46 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 06:25 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 05:51 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 05:35 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 07:25 PM
F-BIRD'88 You're a very confused guy..... You make things a lot more difficult than they need to be.
reread my post and do exactly what I outlined. No more no less.

No ..the vacuum advance when adjusted properly will only add advance when you are cruising at high speed on the hiway. about 10deg.

It will modulate between 0 deg and 10deg of vacuum advance based on varfing engine vacuum, engine load and throttle position, just like it was intended.
When set up correctly with correct diaphram spring tension and limit it will add little or no additional advance at idle.
Reguardless of if you use ported or manifold vacuum.
Do not set the vac can to add all the vac advance at idle.
It would then just stay pegged at all times and not do what its supposed to do. You don't want that.
You don;t need 48deg at idle.
Your cam is not big enough to actually require locked advance.

26deg base static initial is just right. No box retard/advance required beyond the auto retard cranking function, little or no vacuum advance at idle needed 36 total mechanical is just right.
reinstall the advance weights and springs and create a 10deg advance curve in the distributor. 10deg only. no more no less. It should start just off idle speed and finish 2800-3000 rpm. No faster no slower.

adding 10 deg of additional vacuum advance at high speed cruise is just right.
The additional 10deg of vacuum advance must modulate based on the engine load/throttle input.
If the vacuum advance is too agressive, it will knock as you roll into the throttle from cruise speed.

Try my method of hot starting. When its real hot kick it over with no spark count 1-2-3-4, then throw the spark on. It will fire instantly as soon as you throw the switch.. The first few cranks will not fire cause the air fuel mix is just too friggin hot and way over rich. Crank it over to get the crap that wont burn anyways out of the cylinders and new fresh combustable stuff in that will burn. Then make up some theory....

Of the 5 years now on this car I've been running locked timing and a switch to aid real hot starting I have never had to attempt more than twice to start it. It starts instantly first time 90% of the time.
If the starter does buck back instead of fireing up instantly ,, on the first attempt using my method do not try to crank thru it. Stop and do it again.

38deg is more than nessessary 99.9999% of the time.
Usually indicates the jetting is not right and or the timing pointer is not accurate.

Use the auto matic spark retard function only to aid starting.
If this won't work your starter motor got pooched by the header heat.
Get a new one and a starter wrap and a starter shield.

When the engine pushes back on the starter its because that cylinder misfired, half hearted fired, (incomplete combustion) Cause the air fuel ratio is not right. (too hot too rich) When the air fuel mix is just right ( fresh air and fuel drawn thru the carb and into the engine) and not overheated, the engine fires instantly without push back.

try it.
06-01-2010 07:16 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 04:06 PM
F-BIRD'88 checked your earlier posts. With a 236-242 cam you are not quite up there to requireing locked out timing for best idleing etc.

But you do need a good bit of base initial idle timing.

I suggest you rework the distributor advance curve as follows.

26deg initial at idle,,,, 36deg at full mechanical advance. (2800-3000rpm)

That means you want a limited 10deg mechanical advance curve.
The mechanical advance can be limited to 10deg by various methods.
(search my old posts)

Then.... 10-- 12deg of vacuum advance max limited by a vac advance stop plate. The vac adv rate will need to be found by trial and error drive testing.
(use ported or full man vac as a source your choice , try both use what works best overall)

Then... when starting, employ 15deg-20deg of start retard for easy hot starting.

With this setup you probabily won't need the interupt switch to aid starting 99% of the time. But is real handy for the 1% when things get hot.
06-01-2010 03:32 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.

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