Can vacuum and mechanical advance curve tuning help my lopey cam bucking issue? - Page 4 - Hot Rod Forum : Hotrodders Bulletin Board -- Hot Rod Forum

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Old 10-27-2012, 04:09 PM
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Originally Posted by F-BIRD'88 View Post
Ya just yank the distributor and re stab it in a better position.

To see the effect of the increased idle timing and help you determine the exact amount of idle timng your motor
likes, now , before you get and install the 10deg bushing, you can remove the distributor and temp lock out the mechanical advance system so its fixed. (read the directions) Set the now fixed timing at 34-36deg.
and get the carb all reset. Now move the timing around and find what the motor likes at idle. (best idle manifold vacuum) best throttle response and idle quailty.
( between 24 and full 36deg timing at idle). If it likes the full 36deg locked timing, and does not protest at low rpm full throttle ( lugging the motor) then go with it. If it does ping a bit if when you lug the motor down
then go with the short timing curve.

You can check and determine what the motor likes by driving in first gear with no throttle input
idling along with clutch out in first gear. ( this is similar to what the motor sees on a automatic trans, while idling in gear.

Again as long as the motor does not ping when lugged, while using locked timing, you re good to go.

Locked timing ( no mechanical advance) + 10deg of vacuum advance.
Install a ignition power interupt switch if its hard to start up when hot with locked timng.

BE SURE the GM starter motor brace/bracket is installed.

try the locked timing. Those thumpr cams need lots of timing at idle. ( EGR effect at idle of the narrow LSA, extended ex duration and high intake/exhaust overlap)
This is a great idea, I'm gonna go try this.

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  #47 (permalink)  
Old 10-27-2012, 05:14 PM
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If the motor only needs 34deg timing, use 34deg. Crappy 87 gas won't work at all.
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Old 10-29-2012, 11:33 AM
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Originally Posted by Zerocyde View Post
@oldbogie, Yea, I've about had it with this thumpr. If I knew what it was I probably would have said no when my builder offered it to me.

I've been considering swapping cams this income tax, but I have no clue how to pic a cam that's gonna make it so I gain as much NORMAL drivability as possible without losing any power, because as annoying as that thumpr is, the car does get up and go.
Go here and read what I wrote on Friday the 24th. It shows what can be done with modern parts selction. Depends on whether you want to make noise or make power. See the attached file in that post.

comparing 1990 to 2001 build for unleaded premium fuel

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Old 11-02-2012, 10:49 PM
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Interesting read. thanks

Well I got the adjustable vac adv installed today. No fiasco like last time. I dropped the dist right in and hit the original timing right on the money. BTW I don't care what the initial is as long as it doesn't kick back against the starter. I just set the timing at about 2500 rpm and let initial fall where it may.

I preadjusted the vac adv to what I thought might be about 12 degrees. Again right guess.

So I have 34 deg at 1900 rpm centrifugal plus 12 deg vac at 10 inches vac. I measured the vac adv with the motor running and used a small hand vac pump. works nice.

The idle speed picked up 200 rpm so I need to slow this down tomorrow. The idle vac went from 9-10 in to 14-15 in and it is very smooth. In gear it went from 750-800 to almost 1000 so again I'll slow it down a little tomorrow after I take it for a nice cold weather cruise. The AFR did not change but I don't count this untill I get a good 1/2 hour drive. We'll see the final results tomorrow but it sure looks encouraging.

I have very lean jets and transition mix so we'll see how just changing the vac adv affects running and the tune up since I changed nothing else.

I also noted that the exhaust smell was much less. At least it didn't stink up the shop like it did before the change.
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Old 11-03-2012, 11:00 PM
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Well It was only 30 deg today when I left the shop. The car started right maybe 2-3 rev of the starter and it was running. Gotta love the mini starter.

Once the motor was warmed up I noted that the AFR was about .5 richer than it had been. I think what is happening is that the vac adv is setting the mixture off sooner and more optimum. This allows less throttle pedal (opening) and puts the throttle blades in a different location on the transfer slot...and a different AFR. What ever ...the car ran just great. I drove about 30 miles the long way to the breakfast and there was no lean surge like there was a couple weeks ago. After breakfast..couple hours it still was only 30 deg but the car started right up. I drove around a little then stopped and slowed the idle down a couple hundred rpm. Now the blades are tight in the bores so that's as far as I can go.

It's amazing that a simple change like this can smooth things out. I'm glad I elected to only change the vac adv then go after other items.

Next on the list is to richen up the idle feed restrictions one step (4 jets) to tweek the just off idle mixture. I think it needs to be a little closer to 14.7 rather than the 15.6-15.8 that it is right now. I'll try and do that tomorrow then later in the week I have a good 100 mile trip planned to check mpg for the last time this year.
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