No change when adjusting air/fuel screws?? - Page 2 - Hot Rod Forum : Hotrodders Bulletin Board
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  #16 (permalink)  
Old 02-17-2012, 06:22 PM
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I got the fuel pressure regulator installed and set at 5.5 psi, recommended by Edelbrock. My step up springs came in so I'll get some tuning done tonight.

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  #17 (permalink)  
Old 02-19-2012, 12:22 AM
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What cam is in this motorÉ Big duration need much more base timing at idle.
get this by shortning-limiting the mechanical advance curve limit stop.

Stock distributor is about 20deg,, you want around 10-12deg limited mech advance.
This will allow you 22 to 26deg inital base timing at idle. while avoiding excessive WOT advance.
Just about right for most street cams with 224 to 240".050.

if the cam is really racy (240+) you will need even more base timing at idle.
lock out the distributor mechanical advance.

Check the timing tab for correct true TDC location using a piston stop.
There are three different timing tab --balancer combos.

This will allow you to get the throttles closed down more at idle and active idle mix screws.

You cannot achieve the correct timing curve for a stable clean idle by advance spring swap and vacuum advance alone.
Give it the base timing required by shortening the mechanical advance curve limit.
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Old 02-19-2012, 07:51 AM
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The previous owner didn't know the cam specs and I haven't tore it down to get a casting number. I do know that's it a long duration cam by the lopey idle and low vac at idle.

When I first checked the timing, it seemed to be advanced a lot. I couldn't verify without the timing tape or advanced light. Now it seems like the timing was correct being advanced so far.

I did some seat testing yesterday. I tested at 12, 14, 16 18 & 20 degrees base timing. The power seemed to get better with more advance. 20 is the most I can get out of it right now, the vac advance is against the intake so I'll have to move it a few teeth.

Still no change from the idle mixture screws. It has a flat spot where it stumbles at part throttle, low rpm. It will continue to due this until I give it a little more gas. It now has a hesitation when you mash it. My plugs may be fouling.

Thanks for the help.
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Old 02-19-2012, 10:03 AM
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I remembered that I can just rotate the wires on the distributor. I thought my plugs were starting to foul. After pulling the #1 to get on tdc, the spark plug looks ok. Maybe a little lighter than I'd like. I thought I'd share.
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Old 02-19-2012, 11:43 AM
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Progress

PROGRESS! I moved the wires to get some more advance. Had to buy some more vacuum line. It's at 35 deg advanced and drivability is pretty good. I have no stumble when I punch it. The sputtering at low idle cruise mode is 95% better. It cruises pretty good. More importantly, I got some adjustment out of my idle mixture screws on the carb!

So I'm moving in the right direction. Does 35 deg advanced for idle timing sound right? I started by verifying #1 was at tdc and verified the mark on my balancer lined up with the 0 mark on my timing tab, which it did. To verify my idle timing I set my advance on my timing light to 35 and the balancer mark lines up with the 0 on the timing tab. 35 seems like a lot so I want to make sure. My vac advance was plugged as well. And could it take some more advance? I have no pinging or bogging or knocking at idle or under acceleration. It starts easy. Does it want more?

So with this much advance at idle, where do I need to start with tuning the vacuum and mechanical advance?
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Old 02-19-2012, 11:50 AM
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I adjusted the idle mixture screws 1 turn out and I'm pulling about 9.5in of vacuum. I got no change if screwed out any further.
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Old 02-19-2012, 12:36 PM
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I tried all of the step up springs. It's a coin toss between the blue 3" and yellow 4". I'm running the blue now.
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Old 02-19-2012, 02:44 PM
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There were three different damper lines and timing tab combos used on the Gen1 SBC.

DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC. If they're not, the timing can look whacked and you'll not know if the total timing is correct. And if the total timing is too high the engine can detonate, and that can kill it dead.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light. You can also buy a timing tape, get one that matches the diameter of your damper.

If the cam is big enough you'll need a lot of initial. Give it what it wants. Limit the mechanical so the total stays where it should be- about 36-38 degrees BTDC at 3000 RPM or less for most iron heads, less for fast burn and some aluminum heads.

If you're geared to turn less RPM than where the timing is all in by, and if this is often street driven, use a vacuum advance set up to give 10 degrees timing under light throttle cruise conditions. If the initial timing is over 18-20 degrees, try the vacuum advance hooked up to a ported vacuum source.
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Old 02-19-2012, 03:45 PM
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You need to work through the Edelbrock settings step by step, and it starts with getting it to idle right. You probably do need to change to a higher step-up spring (maybe as much as 6-7" if you have a very high lift cam), but that will not help your idle.

- 12-14 degrees initial timing is a good start - just verify that is what you have.
- Disconnect the vacuum lines on the front of the carb and put caps on the fittings
- Make sure the brake booster is connected to the rear vacuum port, or that you have plugged the port
- make sure the carb gasket is in place the the studs are torqued down evenly

- Now turn the idle screws all the way in, and then back them out about 1 and 1/2 turns.
- Start the vehicle and set the warm idle speed to about 800 rpm.
- Follow Edelbrock's direction on setting the idle mixture screws. If they don't work (nothing happens when you turn them), then you need to fix the problem first. It may be a vacuum leak, or the throttle plates may already be too far open at idle.

Once you have a good idle then run up the RPM and verify that the mechanical advance works, and that it comes in with about 18-20 degrees at the RPM you want. Then connect the vacuum advance up to the manifold vacuum port on the carb and verify that the timing advances. Reset the idle speed (not mixture) to compensate for the additional timing.

The next item is the step-up springs. Follow Edelbrock's directions and swap them out until you get a good transition from cruise to power mode.

From there you can use Edelbrock's directions, or you can install an AFR gauge and use that for selecting the right jets and rods.

Bruce
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Old 02-19-2012, 03:50 PM
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Now you need to recurve the distributor. yank it out and get busy.

Shorten-limit the mechanical curve to allow the increased initial at idle yet not over advance at high rpm.
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Old 02-19-2012, 03:56 PM
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If the engine is able to idle w/the way the carb is set now, get the timing/timing curve sorted out first, then do the carb.
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Old 02-19-2012, 05:30 PM
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I have no issues with idling. Today I was able to adjust the mixture screws properly after getting some more timing out of it.

After some adjusting today its driving pretty good. Cruise mode is ok, power mode is fun. I'll plan on dialing the timing in some more and then do some more reading in the edelbrock manual for tuning. Thanks for the help guys. I owe you a beer.
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