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Old 06-16-2010, 01:12 PM
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383 stroker timing and carb ???'s

Hello everyone,
I am a newbie to the auto performance world. I am a boat and motorcycle mechanic. So please don't flame me to hard. I recently built a 383 stroker for my Tahoe. After I got it installed I am having some mild running issues that I need some assistance with. Hopefully some of he gurus here can help out. Here is a list of what I have in the engine:

Eagle 383 Stroker kit
Speed Pro hypereutectic -5.0cc flat top - 9.7:1 compression bored 30 over
Lunati cam - 515 lift, 246 @ .050 tappit lift
Holley 750 DP ProForm
RPM intake
Summit Racing 72cc heads
full Mallory Hyfire ignition system with Distributor
Timing 32* @ 2500 rpm
I hope that is enough info on the motor

My problem is that the motor seems to load up and it has a rich smell at idle. Once I am going it runs ok, but not great. It is a little bit sluggish to take throttle off idle. Once I get it taking throttle though it revs great. I do not know much about how to adjust/tune the carb so I am not sure where to go. I have readjusted the valves after it has been warmed up and that did help a good bit. I am also not to sure if I have my timing set correctly. So at this point all I can do is research and hope I can find answer to make her run better. That is how I found this forum.

Well that is it in a nutshell. If there is any information I left out that might help with a diagnosis please just ask and I can give you any info you guys might need.
Thanks in advance,
-B

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Old 06-16-2010, 01:28 PM
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What's the initial timing set at? I have a similar cam in my build and it required 24-26 initial and 34-36 total.
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Old 06-16-2010, 01:48 PM
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I am not sure about the initial timing. Are there 2 different ways to set the timing. I was told to set it @ 2500 rpm. My ignition system does not have a vacuum advance. Please elaborate.
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Old 06-16-2010, 02:36 PM
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Quote:
Originally Posted by Siddhi
I am not sure about the initial timing. Are there 2 different ways to set the timing. I was told to set it @ 2500 rpm. My ignition system does not have a vacuum advance. Please elaborate.
Initial timing is set at idle and total timing is set at whenever the distributor stops advancing which is normally around 2,500 - 3,500 rpm.

Hopefully someone who's more knowledgeable than I am will drop in and give you a list of solutions to fix the stumble problem. Could be you might have too much cam for those heads. I'm not sure how well they flow. Could just be a carb tuning problem (adjustment / jetting, etc.).
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Old 06-16-2010, 07:07 PM
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Search this site for 383 timing, there are tons of amazing facts about your exact situation

I have a similar setup and you need a high idle timing to burn the fuel. When the car is just idling in park, check the timing with a light. You need something like 26' of timing just at idle and then advance fully from there to 32 or 36 or whatever is ideal for that build.
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Old 06-17-2010, 07:34 AM
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Thanks for the replies! Question though! If I set the initial timing doesn't it change when I set the WOT timing?
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Old 06-17-2010, 07:42 AM
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Quote:
Originally Posted by Siddhi
Thanks for the replies! Question though! If I set the initial timing doesn't it change when I set the WOT timing?
You'll have to physically limit your distributors timing advance so you'll get the initial timing you need and the total timing. Take a look at this thread for the how to.

http://www.chevelles.com/forums/showthread.php?t=253368
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Old 06-17-2010, 08:13 AM
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Thanks V8!
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Old 06-17-2010, 08:25 AM
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A couple things to consider:

A 246 @ 0.050" cam will want a lot of initial timing- something around 20 BTDC. That only leaves 12 to 16 of timing to the mechanical advance.

I'm a firm believer in using a vacuum advance. Using a vac. adv. hooked to manifold vacuum allows you to add timing at idle w/o affecting the total timing. It also allows the carb primaries to be closed more than w/o the vac. advance. This 'cures' problems w/having too much transfer slot exposed. Too much transfer slot exposure causes a bad idle and poor off-idle transition as well. An indication of too much slot is if the idle mixture screws do not affect the idle (or not much effect) when turned in or out.

Then, there's the power valve. It could be wrong for your engine. You need to see what the vacuum gage reads at idle w/the timing sorted out, and go from there. I wouldn't be surprised to see it needing a 3.5-4.5 PV- but you need to check the vacuum to know for sure.

If you set your initial to 20 BTDC, there's no doubt in my mind that you'll be over-advanced when the centrifugal is fully in. So, you'll need to employ some method of limiting the total advance.

Depending on what distributor you're using, this can be easy- or not, depending.

So, what distributor do you have now?
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Old 06-17-2010, 08:33 AM
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I have a Mallory Unilite. How do I get a vacuum advance? There is no place to hook one up?

Last edited by Siddhi; 06-17-2010 at 08:40 AM.
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Old 06-17-2010, 08:40 AM
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Also on the Holley Proform the idle mixture screws; are they Air or Fuel? I have 4 of them. I bought the carb from a friend that had it on a BBC. I am wondering if the jets are wrong myself. I believe it has #60's in it. I did rebuild the carb when I got it. So whatever power valves came in the kit is what I used. I have a feeling the way it is running that the carb could be most of my problems. However everyone I talk to tends to lean towards timing as my issue.

Building the motor was the easy part. Now getting everything tuned right is becoming a booger. Also I did not mention I have hooker headers on it. I do not know how much that will make a difference.
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Old 06-17-2010, 08:51 AM
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Quote:
Originally Posted by Siddhi
Also on the Holley Proform the idle mixture screws; are they Air or Fuel?
Both. The screws allow a pre-mixed emulsion of fuel/air into the airstream. To change either the ratio of fuel or air of this emulsion requires mods to the carb, the A/F ratio of the idle mixture is not changed by how far in or out the idle mixture screws are.

Quote:
I have a feeling the way it is running that the carb could be most of my problems. However everyone I talk to tends to lean towards timing as my issue.
I'm betting it's BOTH.

Quote:
Building the motor was the easy part. Now getting everything tuned right is becoming a booger. Also I did not mention I have hooker headers on it. I do not know how much that will make a difference.
If you can work w/the replies from the members here, you will prevail.

You need an accurate timing light and a vacuum gage for starters.

You might want to verify that your timing marks are accurate by using: DETERMINE TDC
AND
MAKE A TIMING TAPE. The temporary timing tape will allow you to read the timing at full advance if you don't have a degreed damper or a dial back timing light.
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Old 06-17-2010, 11:06 AM
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Cobalt thanks for all the info. It sounds like you are pretty knowledgeable.

I do have a dial back timing light and I have verified TDC. I have never made a timing tape but that sounds like great advice. Thanks for the link! I also have a vacuum gauge. Would you mind elaborating on the correct way to use the vacuum gauge for timing and Carb adjustment?

So the 4 screws really do not do me any good for a/f mixture. Motorcycles are so much easier! lol!

I have been thinking about just buying a new carb as the one I have came from someone else used. I have been looking at the Demon brand. However I see there are Speed Demons, Road Demons, and Mighty Demons. If I was to just bite the bullet and purchase one do you guys have any recommendations as to which one might be better?

So with the Mallory Unilite is there a way to disable the mechanical advance to set the base timing?
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Old 06-17-2010, 11:26 AM
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Quote:
Originally Posted by Siddhi
Would you mind elaborating on the correct way to use the vacuum gauge for timing and Carb adjustment?
Someone posted a good site that showed various vacuum gage readings and the interpretation for them, hopefully someone will re post a good site.

The short answer as to using the vacuum gage to set the idle mixture is to aim for the highest reading. In your case, you really won't be using the gage for setting timing, although you can keep it hooked up and see what the timing changes do to the vacuum.

But the gist of the initial timing is that you need more than what it now has.

Quote:
So the 4 screws really do not do me any good for a/f mixture.
Well, the 4-corner idle system does adjust the idle, as far as how much fuel is entering the engine, and from where (front or back- usually this will be equal, or nearly so for best distribution).

Quote:
I have been thinking about just buying a new carb as the one I have came from someone else used. I have been looking at the Demon brand. However I see there are Speed Demons, Road Demons, and Mighty Demons. If I was to just bite the bullet and purchase one do you guys have any recommendations as to which one might be better?
I see no reason the carb you now have can't be worked with. Given the correct tune, it should be fine, IMHO.

I would see what the best idle vacuum was that I can get, divide this in half to get the PV rating.
EXAMPLE: 13 Hg vacuum reading divided by 2 = 6.5 power valve. If your reading divided by 2 lands on an even number you should select the next lower power valve.
EXAMPLE: 8 Hg vacuum reading divided by 2 = 4 power valve. Since there is no #4 power valve you should use a 3.5.

Quote:
So with the Mallory Unilite is there a way to disable the mechanical advance to set the base timing?
You shouldn't need to disable it- it shouldn't begin to add advance until significantly above idle. If it DOES start to add advance at idle, the springs are too light.

As long as you don't see the advance rising or moving all around at idle, it should be fine to set the initial timing the way it now is.
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Old 06-17-2010, 11:51 AM
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So when setting the idle with the vacuum gauge, I adjust to get the highest vacuum reading. Does this result in having different adjustments (# of turns in or out) for each screw (i.e. one screw is 1.5 turns out and one is 1.75 turns out, etc.)? Or do you set one screw for highest vacuum and adjust the others to be the same # of turns out?

Well Cobalt you have given me some good info and a better understanding as to where to start with. I plan to tinker with it when I get home. I have to change my header gaskets 1st. I have a mild exhaust leak that I need to fix. I did not know that there were specific gaskets for headers and I put the stock manifold gaskets that came with my head kit on. Since I am on the subject are there any recommendations as to which gaskets to use. I am planning on using copper gaskets with a light coat of copper permetex.
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