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Author
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Topic: Timing using vacuum
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Reed's '65 vert
Journeyman Posts: 40
From: Houston, Tx , US
Registered: Apr 2004
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posted 08-09-2005 03:36 PM
 
Has anyone had success with setting your timing by obtaining maximum manifold vaccuum at idle. Currently, based on the timing marks on the balancer, I've got it set @ 10 BTDC with centrifugal advance at 20 with very little vacuum advance. I've tried setting the initial timing based on vacuum and my vacuum and RPMs keep increasing until I get to 30 or so BTDC. My balancer is brand new and I've heard repeatedly that they can never be trusted, but I'd hate for it to be correct and cruise at 60 BTDC once my advance kicked in. On the other hand, it's a brand new rebuild and I'm getting 10-12 mpg from the following: stock 289 bored 30 over
C4 automatic
3.00 rear end
Autolite 4100 with 1.08 venturies (48 prim jets) Isn't this consistent with an engine that is not advanced enough? I spoke with a guy at Pony Carbs and he was adamant max vacuum @ idle is the only way to set static timing.
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indyphil
Gearhead Posts: 3394
From: Senoia, G.A. USA
Registered: Jul 2002
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posted 08-09-2005 03:50 PM
I guess its possible that your damper is "slipped" and what you think is 30 is really just 5 or 10 degrees.
Its possible to verify TDC - someone described this in another thread - by putting a bolt in the spark plug hole and seeing where the piston stops on its way up (mark the damper) then reverse the engine and see where it stops when rotated the other way (mark it again) then TDC is halfway between the marks. I guess I would be careful not to try to spin the motor too hard or you may put a little ding in the top of the piston and I think thats a bad thing to do  ------------------
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 08-09-2005 05:18 PM
Well it kinda depends on if you have a vacuum leak or if your carb is not properly tuned. I have a Pro Products damper and it is balls on with its marks. Not sure if i would use a bolt in the spark plug hole but i use my pinky finger instead. It will get you in the ballpark and let you know if it is just a few degrees or 30 degrees off. On my motor, the more i advance it the higher the idle gets. One more thing, are you running a stock points dizzy or a different non-stock dizzy? A duraspark dizzy that i had in my motor contributed to 44? mechanical by itself, so i was running over 50? of total. Way too much!! Check this site out for all your timing needs: http://www.foureyedpride.com/phpBB2/viewtopic.php?t=12752 ------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
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grego37
Gearhead Posts: 411
From: los angeles,CA,USA
Registered: May 2004
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posted 08-09-2005 05:29 PM
Is your pointer in the stock location?Ditto.
Id varify your TDC mark on the balancer with the pointer when the engine is at TDC.There are various methods of finding TDC.
If for some reason it doesnt check out (after checking it twice) You can install timing tape, or make a line with white-out on your balancer where true TDC is (at pointer) and use a 'zero' back timing light, or move your pointer to your balancer mark. Dont forget to re-adjust carb after changing timing. If you're running an MSD 6 series box and timing it with a 'zero'back timing light (depending on the brand)sometimes you'll get funky readings.
good luck
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Roadkill
Journeyman Posts: 80
From: Chesterfield, Mi, 48047
Registered: Jun 2005
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posted 08-09-2005 05:35 PM
I just put a Pony Carb in my 66 with a 302 V8. Timing instructions that came with it said dont use a timing light. I did exactly that, increased advance until a max vacuum 20. I then set idle at approx 650 in drive and adjusted idle mixture screws to specs. Not comfortable with that I checked it with the light and I was at 30 degrees, 21 centifugal total 51, kick in vacuum advance and it jumped way up. At low speeds it ran good, opening it up on the x-way it sputtered had some loss of some power just didnt feel right. What I ended up doing is using both ideas. I set the initial at the lowest degree (20) where the vacuum stayed at its highest (20) not highest RMP, I then changed the centifugal stop bushing from a 21 degree to a 18 degree for a total of 38 total advance. I put stiffer springs to slow the curve down and revving up the RMPs it seemed to peak at 42 degrees. Car runs good at this point no back firing and the power is good. I also put a post questioning if initial advance of 20 degrees is safe for the engine ( I did verify TDC, use a long pencil not screw) not much feedback at this point. And I did put a call to Pony Carbs that Im waiting for a call back. Sorry for the long post. Ed
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Reed's '65 vert
Journeyman Posts: 40
From: Houston, Tx , US
Registered: Apr 2004
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posted 08-09-2005 05:59 PM
Thanks for the replies. They seem to have highlighted my selective amnesia. Prior to bolting the heads back on, I triple checked the reading of the damper with the #1 piston at TDC and it was dead on. When I get a chance, I'll verify that it hasn't changed.If all checks out, does any one see a problem with running a higher static timing (and reducing centrifugal advance) in order to achieve max vacuum at idle.
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 08-09-2005 06:04 PM
quote:
Originally posted by Roadkill:
I set the initial at the lowest degree (20) where the vacuum stayed at its highest (20) not highest RMP, I then changed the centifugal stop bushing from a 21 degree to a 18 degree for a total of 38 total advance. I put stiffer springs to slow the curve down and revving up the RMPs it seemed to peak at 42 degrees.
You want total timing to be around 34-36?. You have that just from your mechanical weights. I would get a 10L or 12L to put in your dizzy and then set initial timing around 12-14?. Just subtract the 36? from your mechanical (a 12L will be 24?) and you have your initial that you want to set it at. Make sure that if the dizzy has a vacuum advance to pull the vacuum hose and plug the hose so you don't have a vac leak ------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
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grego37
Gearhead Posts: 411
From: los angeles,CA,USA
Registered: May 2004
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posted 08-09-2005 06:28 PM
I've allways been told to give an engine as much initial timing as it will take , where you can still start it back up when its hot.
If you reach a point where it doesnt want to start back up while its hot, back down little by little until it does.
Then work on your advance to achieve your total timing.Also,
Some people lock out their advance and just have fixed timing (where it doesnt change)
but use a Start/retard box , so it retards while starting.
Im not sure how practicle that is for the street though. Never tried it.
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Reed's '65 vert
Journeyman Posts: 40
From: Houston, Tx , US
Registered: Apr 2004
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posted 08-10-2005 12:37 PM
quote:
Originally posted by buening:
You want total timing to be around 34-36?. You have that just from your mechanical weights. I would get a 10L or 12L to put in your dizzy and then set initial timing around 12-14?. Just subtract the 36? from your mechanical (a 12L will be 24?) and you have your initial that you want to set it at.
Buening, that's kinda what I was thinking. Initially, my reluctor arm was on the 13L setting, but I recently switched it to the 10L to get a max of 20 deg. centrifugal advance. That plus my static 10 deg. yielded 30 right there. I was then wondering what benefit I was getting from my vacuum adv. I've set it so that it contributes around 5 deg to give my a total of around 34 to 35 BTDC. I've still got to play with the springs to give my the right timing curve, but from what I've read here and elsewhere, you want that to come in by 3000 RPM max. From what I understand, the vacuum advance is supposed to compensate for high vacuum, low load situations to improve fuel efficiency, such as highway driving. However, if the full mech. advance is set to come in by 2500 to 3000 RPMs, I don't know of too many conditions where vacuum advance would take effect without mech advance already being fully applied. The tach on my rally pac puts me at about 3500 rpms at 65 mph, but I don't know how accurate it is ( a restoration by "the TachMan" is in the future ). I just bought a cheap tach that I'm going to hook up temporarily along with a vacuum gauge so I can take it out and get some more accurate readings while driving. Hopefully, this will shed a little more light on the situation. Also, the shop manual states that mech advance should be no more than 14 deg and the vacuum advance no more than 11 deg. This plus a base of 10 would yield a total of 35 or so. Has anyone set their timing this way? If so, what were the results. This kinda confuses me because my distributor, assuming it originally came with the car, only has 10L and 13L settings on the reluctor arm. Sorry for being all over the place here, but I'm just trying to learn from you guys that have a lot more experience with this than I do.
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 08-10-2005 06:28 PM
Your vac advance kicks in when going up a hill at low speeds or just cruising around town and between stoplights. You're mechanical will probably start to kick in around 1300rpms or so, depending on your springs. And yeah most of the dizzys i've worked with all come in at or around 3000rpms. Any idea what kind of gears you are running out back? 3500 seems kinda high at 65mph.I too am running 10L weight, with 16? initial advance with the vac advance unplugged. The idea is to have the vac advance bring up the 16? initial to around what is needed (roughly 36?) so the motor will run efficiently at lower rpms, since the mechanicals haven't or just started to kick in. Most aftermarket dizzys don't have vac advance because they are made for racing and not too many race cars drive down the track at 1500rpms  I'm not sure how you have the vac advance set to help total timing. At low rpms, the engine has high vacuum and thus operates the vac advance. As the rpms increase, so does the vacuum levels thus reducing the effect of the vacuum advance. As the vacuum advance decreases, your mechanical advance is increasing. This makes for a happy engine at all rpms, if properly tuned. ------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
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Reed's '65 vert
Journeyman Posts: 40
From: Houston, Tx , US
Registered: Apr 2004
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posted 08-15-2005 11:08 AM
I did some playing around this weekend with my timing. I first set my timing by getting max vacuum (20 in Hg. steady ) and again, it was right around 30 BTDC. However, due to the increase in RPMs, some of this could from the my mech advance coming in. ( I haven't backed down the curb idle screw yet ). My reluctor arm in the dist is set for a max of 20 deg advance, giving me a total of 50 BTDC. I disconnected my vac advance for the time being and plugged the port on the carb.Due to the high initial advance, I re-verified TDC with my balancer and the #1 piston it was dead on. I got it out on the road and under light accleration, everything was great. I then got on it, and too my suprise, the secondaries opened and it ran as smooth as silk. Not a hint of pinging or detonation. I've heard that you want to run timing as far advanced as possible without any signs of detonation. However, has anyone ever heard of it being this far advanced? Would cam timing have any effect on my vacuum being maxed out at this high advance? I'm 99.9 % sure I indexed the crank and cam sprocket correctly when I re-installed them. I made sure the indexing marks were aligned before buttoning up the timing cover. Any thoughts/ideas?
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buening
Gearhead Posts: 317
From: Decatur, IL
Registered: Mar 2005
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posted 08-15-2005 03:17 PM
quote:
Originally posted by Reed's '65 vert:
I first set my timing by getting max vacuum (20 in Hg. steady ) and again, it was right around 30 BTDC. However, due to the increase in RPMs, some of this could from the my mech advance coming in. My reluctor arm in the dist is set for a max of 20 deg advance, giving me a total of 50 BTDC.
Was this the timing found with the vac advance removed and hose plugged? Do you have a tach to see where your rpms are? Also, is your cam stock or aftermarket. I cannot get anything over 15 in Hg but i have a little larger cam than stock. You kinda need to know what rpms you are at in order to determine if your mechanical timing has kicked in. In my opinion, 50 is too high and detonation is not always heard. Try bumping it down to around 16? initial to give you 36? total without vac advance (plug it back in after you set the timing) and take it out on the road. If it feels doggy then put it back on 30?. I just now got my carb dialed in with my timing. I could get my timing right but my carb was being a pain and it was doggy at low rpms. With a properly tuned carb and timing around 36? total, you should be surprised on how well that motor will run. I would venture to guess that if your motor likes that much timing then the idle mixture screws need adjusted. It's probably running too lean. ------------------
1970 Grabber Blue Mach 1 H-code
1970 Fastback
2003 Torch Red Mustang
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