cam advance and vacuum

[lewtot184]

the stock avs is an emissions carb. the idle circut is too lean, especially automatic trans carbs. fix this first. you'll be surprised at the difference.

 

[texas69bee]

Thanks again Lewtot. It is a 4 speed. I will give that a try first. Seems to be the easiest possible fix.

 

[Meep-Meep]

I calculated compression to be at 9.5. Motor was decked, .004 below deck and heads are at 75cc

That's perfect! Now do a blow check. Hopefully you get in the 170 PSI range. With 12-15 deg this thing should be snappy and love a dual plane intake, even if stock.

 

[texas69bee]

Thanks MeepMeep. I will try to get at it this weekend if possible. I have another 7 day work week. To many people buying these Suburbans and Tahoes. Thanks everyone for the help. I will keep you posted of the results.

 

[texas69bee]

I've played with the idle mix, vacuum advance and initial timing. What I ended up with at 750rpms is 14hg of vacuum but with an initial timing of 21 degrees. Idle mix screws are as far out as they are going to get. I checked for leaks and all seems tight. I have not done a compression test yet. I know I'm reaching for excuses why the vacuum isn't higher but what affect does plug gap have and not running with mufflers. I am running with a gap of .045 and just the head pipe. Stock gap calls for .035 but I went with the MSD Blaster 2 coil thinking I would get a hotter spark and then need a wider gap.

 

[cdr]

set the idle at 800 to 850

 

[texas69bee]

I would imagine the vacuum will increase as I increase the idle. So I guess that brings up another question. What is typical for idle rpms? Is 800-850 typical for non stock cams?

 

[cdr]

not all engines are the same, i would say about 800 rpm idle, like i said on timing on my earlier post & see how it runs.

 

[62 Dart Convertible]

I would imagine the vacuum will increase as I increase the idle. So I guess that brings up another question. What is typical for idle rpms? Is 800-850 typical for non stock cams?

Yep vacuum will increase as you increase idle speed. Nobody could tell you the correct idle speed for your combination with any accuracy as it's dependent on so many factors. Luckily for you with a 4 speed, at least you don't need to worry about converter load-up etc. I'd say 800-850 with your cam should be about right, but you will know if it's too low if it stalls too easily.

Let us know how you go!

 

[62 Dart Convertible]

Idle mix screws are as far out as they are going to get.

evidence that what was said earlier is the case here -

the stock avs is an emissions carb. the idle circut is too lean, especially automatic trans carbs. fix this first. you'll be surprised at the difference.

I would say your carb should/could be jetted/adjusted using an AFR as it's likely holding back your motor. Or get a performance aftermarket carb that's sized right that will likely be real close to ideal ootb.

 

[Cranky]

Have you done a compression check yet? Just another tool to see if the engine is doing what it's supposed to be doing. Low idle roughness can be other things besides the cam acting on it. The lower the idle, the less signal the carb sees. Also, if you have any cylinders that are not producing PSI that's close to the others can also play part. As for timing....play with it. I can tell you about at least two combos that liked 50 total and we didn't even care what the initial was at that point lol. Both engines were very low compression tho. Just saying, all engines are different and not all fit into the same box.

 

[texas69bee]

UPDATE
Here is what I have done and the issues seem to be getting better. Originally had the gap at .045 due to the extra voltage in the coil. I re-gapped all the plugs back to .035. This took care of the erratic idle(sort of). It now holds within a 30 rpm range at 750. I redid the preload on every lifter. I went from an 1/8 of a turn to 1/16. The pre-load called for .003 and the 1/16 got me there. The vacuum at idle is now at 12hg and shoots up right away as I slowly increased the throttle. I have the initial set at 12deg. I've been making adjustments with the allen wrench inside the vacuum canister and this seems to be helping a ton. By doing all this I have been able to take the air fuel screws back in at least 43 full turns. Almost there but still more work to do.

 

[bigmanjbmopar]

43 turns wtf? lol

 

[texas69bee]

WTF is right, I just reread that. 3 full turns not 43. Big finger hit both keys. lol

 

[bbcharger71]

Man I'm glad I found this thread. I've been searching for actual vacuum number for this cam. Please update us when you get it dialed in.