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Discussion Starter #1
I am having a great deal of trouble getting my timing and carb tweaked. The 275hp 327 runs best at 8 degrees static, 18 with vac (vac on maniforld) and 24 degrees of mechanical advance. The engine hesitates badly as I roll into the throttle. If I advance the timing, the hesitation is worse. When I retard the timing the car does not hesitate, but the enging dies if I take my foot off the gas quickly. Each time I adjust the timing I use a vac guage to readjust the idle jets, and adjust the idle speed. The spark plugs look like the mixture may be rich, they are black and sooty, with a tan spot on the end on the electrode. Floats in the carb are set properly. No vac leaks. Any ideas?

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68 convertible
 

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Did you do anything to the engine before this happened? Is it mostly stock?

I had that problem and I did fix it but I did a lot of things at the same time so I can only guess which one was the true culprit. Start by verifying the mixture screw adjustment. Back the mixture screws out a 1/4 turn more and see if the stumble improves any. The accelerator pump needs to be working perfectly as well.

The biggest problem I was having was the ignition advance backing off as the throttle was cracked. The manifold vacuum drops instantly and the canister will back off causing the RPMs to drop. If the distributor is "idling on the curve", the centrifugal timing will back off as well. These 2 things were causing the engine to have a bad hesitation at tip in.

You can check to see if you are "idling on the curve" by hooking up a timing light and watching the timing as you back off the idle speed screw. If the timing drops below the static setting as the RPM drops below your idle speed, you are idling on the curve and you need to limit the centrifugal advance in the distributor. You can do this by reducing the length of the slot in the autocam plate either by brazing it closed a bit or welding and dressing it up to give you the advance you need.

Give that a check and see what you have.

-Mark.
 

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Discussion Starter #4
Thanks for the tip Stingr69. Tried your suggestion and am not "Idling on the curve". The advance holds tight as rpm's are dropped below idle, and stays put until the mechanical advance kicks in at, I would guess, is about 1500rpm. The carb is a Holley 4150 series, 600cfm with an electric choke. The carb is only 6 months old and was running well before I limited the total mechanical advance in my distributor to 24 degrees. Prior to the distributor adjustment the car ran well, but was hard to start when warm because the static timing was set at 18 degrees, the vacumn was hooked up to the ported source and added to the mechanical advance, not the idle. Any help is greatly appreciated!

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CCPEAT,
I am going to walk you through the setting procedure as I think that is the problem. Starting with the centrifugal advance, you have 24 degrees in the centrifugal advance mechanisim so you need 12 degrees static initial timing with the vacuum advance plugged. Set the static advance at 12 degrees at the idle RPM spec you want when you are finished. Lock down the distributor and now hook up the vacuum hose to the manifold vacuum source(not ported). The idle speed will now jump up quite a bit. You will then need to lower the idle speed screw to get the idle back down to the original RPM spec. Now adjust one idle mixture screw and reset the idle speed RPM. Adjust the other mixture screw and reset the idle speed again. Repeat this mixture screw adjustment process a few times until you feel it is perfect. Now test drive and see how that feels. If you have a stumble just off idle you can try backing out the mixture screws an additional 1/4 turn or so. Let us know how that worked out.

-Mark.

P.S. - if she won't idle down you may need to adjust the mixture screws to the baseline one and a half turns out from lightly seated to start with.
 

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Discussion Starter #6
Thanks for the suggestions. I have tried many different timings, adjusting the idle mixture each time. If I move the vac line to the ported source, and set the timing to 10 degrees, which in this case is both static and idle, the car does not hesitate as bad when rolling into the throttle. When it does stumble, it fires back throught the carb. Next I will try using a more agressive acceleration pump cam to try to get the mixture a little richer under acceleration. Most of the posts I read on this site say the vac should be hooked up to the manifold source, but that provides 20 degrees at idle, and the hesitation is horrible. I am curious what changes any of you have made when installing a Holley 4150 onto a 275HP 327 to the stock Holley setup. Did you change the primary jets, accelerator pump cam or orifice from the ones that came stock with the carb from Holley?

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You sound like you are on the right track. I think the backfire at tip in is a lean condition and adjusting the squirt will hopefuly cover that up for you. The idle and transition circuits are not able to do it all by themselves. If all else fails and you can't tune it out, you can always go back to ported advance and just be done with it. Manifold advance is realy nice for taming a wild cam or high heat loads at idle.

-Mark.
 

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look to see if when your choke is full open if the squirts are hitting it. I had this problem and found the squirt was hitting the choke plate and dripping down. I put a small tierap on the rod to keep it from opening so far. I was a 600 holley

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69 convertible
LT1 4L60E
 

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Discussion Starter #9
I finally got the engine running very well and felt I should post what I did to solve the problem so others might benifit from my frustration! The set-up is: Static timing at 12 degrees; vacuum to a ported source; using a Crane vacuum advance canister set to start pulling a 9 Hg; total mechanical advance of 25 degrees all in at 2800 rpm; Primary accelerator pump has a .0135 shooter and uses the blue cam in the number one position; secondary accelerator pump using .0135 shooter and an orange cam in the number one position; Power valve is 9.5. Enging now idles with 16 Hg vacuum, idle mixture screws out 2 turns.
Runs like a scalded rabbit! No hesitation at anytime. I was unable to use manifold vacuum because with the crane vacuum canister adjusted to the heavyest position, it still advanced the timing under hard acceleration. My enging is creating a lot of vacuum with the Torquer S/R heads that have 2.02/1.60 valves with 1.6 rockers and the stock 275hp cam. Produces 16Hg at idle, 20 Hg crusing at 55 and 22Hg crusing at 70mph. Vacuum never drops lower than 8 Hg under hard acceleration.

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