Jetting question

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Biteme
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Jetting question

Post by Biteme »

Hi all.
I’m putting a 650DP I’ve had on a 400HP engine onto a 500HP engine.
Do I increase the jets a few sizes?
Cheers
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Re: Jetting question

Post by mag2555 »

Since your expecting a 20% increase in HP I would start off on the safe side and bring up the primary jets by 2 sizes and the secondary s by 3 sizes and also maybe start off with a spark plug one heat range cooler then what you where running.

If you have gone up in the motors cid and or cam size to reach this 500 hp level then you may also need to play around with the idle air bleeds on the primary side, and I do not think that Carb stock has screw in ones do it may need to get modified.

All of this being said I think that yanking 500 hp out of a 650 Carb is starting to push it into the start of its limits!
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Re: Jetting question

Post by Tuner »

630 HP with 50 year old 4777 650. Engine is for a Dirt Late Model

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Re: Jetting question

Post by rebelrouser »

I always understood that the carb added fuel to the air being pulled through the venture of the carb, the venture does not know how much HP or engine size that is pulling the air. sometimes where the rpm of the engine wants fuel may change, and you have to compensate for that with a new combination of jets or air bleeds. Most of the time idle and part throttle is what needs attention when swapping carbs.
If it was jetted right before it should still be jetted right. By adding a richer jet you give yourself a safety margin on a new combination, if you were right on the edge before.
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Re: Jetting question

Post by MadBill »

In my experience, most basic unmolested Holleys trend rich at high RPM, in which case you might have to jet down for the new combo. I'd start with what worked best with the current engine.
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Re: Jetting question

Post by rustbucket79 »

I would start with it jetted as is and see how the car cruises first.

With all the different carbs I’ve run on the dyno, with normal fuel, it’s pretty rare to have to deviate more than a couple of numbers from stock, regardless of the engine it’s on.
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mt-engines
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Re: Jetting question

Post by mt-engines »

Biteme wrote: Mon Sep 07, 2020 3:01 am Hi all.
I’m putting a 650DP I’ve had on a 400HP engine onto a 500HP engine.
Do I increase the jets a few sizes?
Cheers
No, unless its not calibrated right to begin with. and also why a 650?
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Re: Jetting question

Post by Biteme »

Thanks for all the replies.
I have a 500hp 372 sbc which has a few niggly issues I’m ironing out.
Usually has a demon 850ad on it but I want to put the 650 on for a bit to try and isolate a problem.
650 was previously on a 400hp 360 sbc.
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