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Old 10-03-2005, 06:07 PM
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Carb question??

I have posted a similar question before on this subject but am still confused about something.

I have a 283 with a tunnel ram and two holley 650's. Being as this is too much carb for the motor, I need someone to explain something to me.

If the carbs produce too much air for the size of the motor, why does the motor run rich and not lean. If you turn down the fuel it won't run at all.

I don't understand the theory about the air and fuel mixture. Hopefully someone can explain that to me.

I know the carbs are too much for the motor so I'm not looking for advice on carb size, just the theory about adjusting the air and fuel.

Thanks.

Bob
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Old 10-03-2005, 06:20 PM
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Quote:
Originally Posted by Bob23T
If the carbs produce too much air for the size of the motor, why does the motor run rich and not lean. If you turn down the fuel it won't run at all.
They don't produce air per say, they're just bigger holes to breath through. That means that the velocity of the air as it enters the venturis is a great deal slower than with one carb. Therefore the fuel doesn't get atomized as well. Combine that with the extra length of the runners and you have a recipe for chitty combustion. Meaning you're probably dumping a lot of raw fuel into the exhaust simply because it didn't mix with the air before it got the chambers. A few big drops don't burn as well as a bunch of little ones.

That said, it probably runs rich because the carbs are.....

a. Not tuned as well as they could be.

b. Not designed for use in a 2x4 situation.

Larry
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Old 10-03-2005, 06:28 PM
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Not very often...but sometimes you will hear me say, "I agree 100%" and this is one of those times!
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Old 10-03-2005, 06:29 PM
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You also have double the idle restriction size as compared to what the engine wants. The best thing to do would be to do the V shaped wire trick. Stick the same sized wire into the idle feed restrictions on both carbs. You will know when you are in the ballpark when you have all the mixture screws at around 1 to 1 1/4 truns out.

You can also lean the idle mixture by drilling the airbleeds, however this will mess with your circuit timing and you don't want that.
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Old 10-03-2005, 06:44 PM
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Quote:
Originally Posted by Bob23T
I know the carbs are too much for the motor so I'm not looking for advice on carb size, just the theory about adjusting the air and fuel.
Because the volume of the intake itself is so large in relation the ci of the engine {and each cylinder in particular} air velocity suffers and allows the fuel particles to "drop" out of the mixture.
Is what needs to be done is to try to increase velocity or vacuum in the intake. Smaller carbs will increase the vacuum a slight amount and will dramatically increase the velocity ofd the air thru the venturi....but since you want {or need} to keep these carbs we will attempt to work around it.
You can add material to the inside of the intake itself, and even add a plenum divider if possible. A plenum divider will allow each cylinder to pull thru half as many barrels thereby increasing velocity thru the carb.
Keep in mind that you manifold vacuum is low and the powervalve is what modulates the fuel curve. If you run a stock PV on a low vacuum engine it has the possibility of being wide open even at idle! The goal in this application should be to use a very low numbered PV. Say a 4.5. This will keep the engine on the idle circuit longer..thereby helping it run leaner. If the PV is too low of a number it will hiccup and pop back thru the carbs. To combat this possible side effect you may need very large pump shot with a long duration, like a 50 cc on each primary side with a 26 or 27 squirter.
Another place to look is at the dizzy. In this particular situation a spark curve that has no vacuum advance {you have little vac to work with anyways!} and only a few degrees of mechanical advance. Something like 22-26 initial and 36 total, you may need more as this is an inefficient setup you have but it really depends on the rest of you combo {I'm not sure what you have by way of gears, cam, compression, converter}..
Tiny breathers will also make one run rich as it induces vacuum {but not good vacuum, it's upstream of the venturi} and pulls extra fuel thru the carb. I know that's a lot but it's where I'd start{low numbered PV & tight timing} and go from there.
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Old 10-04-2005, 04:24 AM
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Thanks for the great information. This at least gives me a starting point.

I do have one other question. What would happen if there was a restrictor plate that completely blocked out the secondaries??

Thanks again.

Bob
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Old 10-04-2005, 05:09 PM
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Why not disconnect the secondaries. Just keep their idle circuit active.
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Old 10-04-2005, 05:55 PM
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Quote:
Originally Posted by Bob23T
What would happen if there was a restrictor plate that completely blocked out the secondaries?
It would run the same up to the point where the secondaries normally opened. After that it would run lean, if it would even rev any higher.

The divided plenum is a better idea.


Larry
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Old 10-04-2005, 06:36 PM
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Quote:
Originally Posted by lluciano77
Why not disconnect the secondaries. Just keep their idle circuit active.
Yep, tried that, did'nt help.
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Old 10-04-2005, 06:38 PM
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Quote:
Originally Posted by coldknock
The divided plenum is a better idea.


Larry
How would you go about that??

Bob
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Old 10-04-2005, 06:58 PM
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I suppose the plenum doesn't have a removable top?

Make a cardboard template that fits the plenum between the carbs and cut a piece of aluminum to match it. You might have to then split that into two pieces just to get it in there if the top of the plenum isn't removable. You can use Moroso A-B epoxy or get someone with reaaaally nimble fingers to weld it in place.

Larry
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Old 10-05-2005, 08:32 PM
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Quote:
Originally Posted by Bob23T
Thanks for the great information. This at least gives me a starting point.

I do have one other question. What would happen if there was a restrictor plate that completely blocked out the secondaries??

Thanks again.

Bob
Sure, why NOT ?

And why not use progressive linkage on the carbs so you are essentially just running the primary 2 bbl on the rear one, then the primary 2 bbl on the front one ???
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