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Discussion Starter #1
I've been trying to get my enigine in tune, but i'm having trouble because the the timing and vacuum readings bounce all over the place at idle. Both smooth out around 1500 rpms. What could cause this? I'm frustrated to the point of buying a new carb.

My setup

327 ci sb
cam 292 dur 525 lift
holly 600 double pumper
victor jr intake
msd billet dist & 6al box
 

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Timing: could be your distributor, maybe the advance springs are advancing too early?? Timing effects vacuum too, check there first, then check the carb.
 

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That big cam in that small motor is not going to idle smoothly. However, the 600 DP should have a rich enough idle circuit to make it as good as it can be.

Start with ignition timing. That motor will want minimum 20-25° of idle timing. Usually you would achieve this with 12-14° initial plus 10° or so vacuum advance. However, your motor probably does not pull enough vacuum at idle to properly actuate the vacuum advance.

Thus, you'll need a distrubutor with a very adjustable mechanical advance curve. Set the mechanical curve to provide 16-18° timing. Set the initial at 20-22° for a decent idle. Your total will be in the 36-40° range, which should perform well.

Next, pull the carburetor, turn it upside-down, and look at the primary butterflies. Verify that they are not uncovering more than 0.020" of the idle transfer slot. If they are, increase the idle adjustment of the secondary butterflies until they just begin to uncover their idle slot. While the carb is off, install about a 3.5" Hg power valve. When running a large cam, the relationship between primary and secondary butterflies at idle becomes increasingly important. It requires trial and error to obtain the best results. Sometimes you'll need to drill small (1/8") holes in the primary butterflies to maintain the proper relationship between the primary butterfly and idle discharge port/transfer slot at idle.

What cylinder heads/compression are you running? If you're under 10:1 with that large cam, your engine might be a good candidate for a locked out mechanical advance. Be a little creative and lock the advance curve. Set the initial timing at 36-38° degrees and give it a try. Although a high torque starter may be necessary, this setting may give the best overall idle characteristics and performance.
 

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Discussion Starter #4 (Edited)
Thanks for the replies, your ideas give me plenty to try. I doubt the dist is adancing at idle, If I set the idle as low as 600 rpms, the vacuum and timing mark still bounce all over the place. By the way, the dist. is a mechanical advance, so the unstable vacuum isn't causing the timing to jump around. I think there is something wrong with the carb.

The cr is 10.5 : 1
The heads are 1.94 fulies
 

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Loose timing chain? Easy to check.

Pull all the plugs and distributor cap.
Put the balancer at TDC
Put a breaker bar on the 5/8" hex balancer bolt
Rotate the balancer back and forth at TDC.
See how much the balancer can rotate before the rotor begins to move.
 

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Discussion Starter #7
Actually, a loose timing chain makes a lot of sense to me. I'll check that this week and post my findings. Thanks for the help
 

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Discussion Starter #8
I checked the timing chain as paul suggested. I can move the balancer about 1.006 mm before the rotor starts to move. Is that excessive? A loose timing chain would reallyexplain the problems i'm having, but 1.006 mm doesent seem like too much play. any suggestions?
 

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This may seem over the top, but you could lock the mechanical advance, set the timing at 32/34 degrees and then try tuning the carb at about 700/800 rpm. This would tell you if the problem was the carb or if it was an erratic timing issue. Might also at that point try spraying some ether around the carb base and intake gaskets to see if there are any leaks.
 

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63SLEEPER said:
I checked the timing chain as paul suggested. I can move the balancer about 1.006 mm before the rotor starts to move. Is that excessive? A loose timing chain would reallyexplain the problems i'm having, but 1.006 mm doesent seem like too much play. any suggestions?
How did you measure that? What I was referring to was how many degrees on the balancer, but provided you were accurate then the timing chain is not loose.
 

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Discussion Starter #12
I measured how much I could turn the balancer without the rotor moving. I think 1.006 mm was about a mark and a half on my timing tab. I assume that's not excessive. I'm going to lock the advance on the distributer and see if it cures my problem.

Thanks,

-Christian
 

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I would check for vacuum leaks at the base of your carb and all you vac. hoses. If you have a msd box check your ground make sure that it is good and tight. If that all checks out try to borrow a carb that you know is good to try.
 

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Discussion Starter #14
Thanks for the advice, I'll try all of the suggestions his weekend and let everyone know how I make out.

-Christian
 

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Discussion Starter #16
I still haven't figured it out! I tried to lockout my MSD mech adv distributer, but you have to remove the gear. I couldn't figure out how to get the roll pin out to remove the gear. So, I changed from the stock springs and cam to the lighest springs and smallest cam. This brings the advance in early and limits the total advance. I forget what the number are, but I have a ton of initial timming now. The throttle response is much better! However, I still have the bouncing vacuum and timing problem.
 

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The roll pin is removed with a roll pin punch (has a little knob on the end)

What is your calculated (actual) compression ratio? What is your cranking compression (4 puffs)
 

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remove all the spark plugs too.
 

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Discussion Starter #20
Looks like I have some tools to buy! My wife will be so happy!

Can anyone reccomend a roll pin punch that I can buy online?

How about a compression tester?

With help from this board, I am determined to figure out my tuning problems.

Thanks again!
 
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