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455rebel
Tribal Scout

USA
10 Posts

Posted - 28 Dec 2014 :  4:42:23 PM  Show Profile  Reply with Quote
455 with 400 heads, 150 psi cranking compression,
Stock ci intake and logs,lunati to monster cam lifters, comp springs,
Roller tip rockers,re worked q-jet,2.5 duals, rings and bearings
Rebuild,3 angle valve job,port matched home port job.
Hei distributor.
My question concerns hooking the vac up.
I have tried it on both port and manifold .
It idles different but that is the only difference.
It kinda sputters at slow cruise about 1200-1400 rpm when hooked to either.
Unhooked it runs fine. I want to hook it up for better mpg.
Initial timing is 15-16 btdc.
Vac can modified to start at 6 and limit to 12.
One stock spring and one lighter spring on advance.32-34 total.
All in by 3000rpm.
What to change to be able to use the vac adv?
Thanks

1970 amc rebel faded blue 4-door ,455 with 400heads lunati tq monster cam ci intake and logs q jet,turbo 400, posi 315 gears,

455rebel
Tribal Scout

USA
10 Posts

Posted - 12 Jan 2015 :  11:47:52 AM  Show Profile  Reply with Quote
No electricians on call???

1970 amc rebel faded blue 4-door ,455 with 400heads lunati tq monster cam ci intake and logs q jet,turbo 400, posi 315 gears,
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cortcomp
Coyote

USA
5338 Posts

Posted - 12 Jan 2015 :  12:04:08 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
Shooting from the hip, it sounds like too much advance too early if it's fine without it hooked up. At light cruising, you have a ton of vacuum and are adding your 12 right there (which, at light cruise, is usually fine but if it's bucking or sputtering, it must not like it.)

If it were me, as a test, i'd back initial timing back to like 12. Test it with vac can hooked up. If stumble is gone, turn initial back to your 15-16, and then limit vacuum can to like 8. I don't know if that's the right way, but it'd likely solve it if that's the case and i'd be happy.

Also, look at carb some too if you'd prefer, maybe you're on the edge of too lean and the extra vac advance is all it needed to push it over the edge. Maybe adjust your APT or richening the idle jets will be enough if that's the case. Without an AFR gauge, i really wouldn't know how to diagnose if it's a part throttle lean issue, but i'm sure others here would.
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Admin
Ye Olde Webmaster

848 Posts

Posted - 12 Jan 2015 :  12:40:50 PM  Show Profile  Reply with Quote
What do you mean by this statement?

Vac can modified to start at 6 and limit to 12.

Please explain.

What adjustable vacuum advance is installed? What is the manifold vacuum reading on your engine at idle? What is it at a steady 2000 rpm? What is it at a steady 3000 rpm.

"Dedicated to keeping the classic Pontiac engine alive."

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455rebel
Tribal Scout

USA
10 Posts

Posted - 15 Jan 2015 :  11:57:55 AM  Show Profile  Reply with Quote
I added a tab to the slot in the plate that the rod fits in with 6-7 vac applied to the can.
I then added a tab to the other end of the slot to limit adv to 12 degrees.
This is not a adj advance , this is a moded stock advance can.
Vac at idle 15 in park
Vac at idle in gear 12-13 @ 800-850 rpm
I will have to test vac at 2000 and 3000 rpm
Thanks

1970 amc rebel faded blue 4-door ,455 with 400heads lunati tq monster cam ci intake and logs q jet,turbo 400, posi 315 gears,
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Admin
Ye Olde Webmaster

848 Posts

Posted - 15 Jan 2015 :  2:48:43 PM  Show Profile  Reply with Quote
How does inches of vacuum (measured in terms of mercury (Hg)) relate to degrees of advance on a distributor?

When a combustion engine e.g. our Pontiac V8s are at idle rpm, the amount of vacuum shown on a typical vacuum gauge will be somewhere between 10" Hg to 22" Hg. The amount of vacuum will vary because of camshaft duration and overlap. What is important is consistency on our engines.

When the accelerator is depressed the throttle plates on the carburetor, for instance, change allowing more air and fuel to be drawn into the engine. Manifold vacuum decreases as rpm increase. At wide open throttle, manifold vacuum is low, close to nil. For purposes of discussion it is zero at wide open throttle.

Your comment, "Vac can modified to start at 6 and limit to 12" needs explanation, or clarification. The 6 refers to inches of vacuum. This is the measure of vacuum (sucking) in inches of mercury (Hg) when the advance begins to operate. The '12' refers to the amount of added advance the unit will provide. However, that number, 12 degrees advance, is not connected to how many inches of vacuum. Is it at 15" Hg or 20" Hg or something else? If your engine develops only 15" Hg and the 12 degrees of advance is achieved at 20" Hg that doesn't match the engine requirements to be effective.

This is the reason why I asked for an explanation on your comment, "Vac can modified to start at 6 and limit to 12."

So, if you believe that mechanically you have total timing of 34 degrees, all in by 3000 rpm, you need to know how much vacuum the engine is producing at that rpm. If it's 10" of vacuum at that rpm, what amount of vacuum advance are you getting? It could be only a few degrees. It may not be 12.

If you have 34 degrees mechanical and 12 you have a grand total of 46. This may not be enough for good gas mileage. It could be. You need to know at what point in inches of vacuum the vacuum unit gives you the 12 degrees advance.

I hope this helps.








"Dedicated to keeping the classic Pontiac engine alive."

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455rebel
Tribal Scout

USA
10 Posts

Posted - 16 Jan 2015 :  09:07:21 AM  Show Profile  Reply with Quote
I should have made it more clear, what I ment was with 6" vac on can I put a limit plate on the advance so advance would not start until 6" of vac.
I then put a limit plate on the other end of the slot to limit advance to aprox 10-12 degrees adv. I did this so when starting the engine the vac can would not start to pull untill the engine was running.I read about this mod somewhere on the web.
Really what I should have done was to buy a crane adj can, but I was trying to cut cost everywhere I could when I built the 455.
Now that Christmas is over I can start to spend a little each month on my car.
When can arrives and I have time I will play with it and see if I can work it out.
Thanks

1970 amc rebel faded blue 4-door ,455 with 400heads lunati tq monster cam ci intake and logs q jet,turbo 400, posi 315 gears,
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Phil
The Great White Buffalo

USA
7222 Posts

Posted - 16 Jan 2015 :  5:11:54 PM  Show Profile  Reply with Quote
Connecting the vac can to ported vac should take care of those concerns. Were you dealing with a hard start issue?

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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455rebel
Tribal Scout

USA
10 Posts

Posted - 30 Jan 2015 :  9:50:42 PM  Show Profile  Reply with Quote
No hard start issue, just looking for smoother crushing and a little better mpg.
Thanks

1970 amc rebel faded blue 4-door ,455 with 400heads lunati tq monster cam ci intake and logs q jet,turbo 400, posi 315 gears,
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