Many of you probably already know this, but I just learned it and want to share it with anyone who has not heard this before. If I am wrong please correct me. . .

Concerning tuning of an Edelbrock carb vs. tuning of a stock CV carb, this is how it was explained to me:

"Bigger needle"=smaller diameter=more fuel flow, similar to bigger jet on stock carb

With the stock carb, a constant velocity of air flowing through the carb is controlled by the vacuum slide. As more air is drawn through the carb the slide opens more, keeping the velocity of air passing over the fuel pickup relatively constant. That is why as your engine is modded and gets more powerful you have to get bigger jets to supply more fuel.

With the Edelbrock carb, the air velocity through the carb changes with different modifications. Because of this, depending on setup, a stock 35hp 660 or a modified 60hp 740 could both use the same needle. You get a lower velocity with the lower power output so it pulls in less fuel. You get a higher velocity with the modded engine so it pulls in more fuel.

Now a 40hp engine with a long, slow intake tract will need a bigger needle than a 40hp engine with a filter clamped directly to the carb. The slower intake charge of the stock setup is not as efficient at pulling gas from the jet, so you need a bigger needle to get the same amount of gas into the engine.

So what this all means is that when I go from my stock 660 to a 10.75:1 686, and a direct carb mount filter I should be dropping down a needle size from a 21E to a 19E because the intake speed will be higher and more efficient. With the stock carbs I would have to go UP a few jet sizes with the change.

The main point is that with the Edelbrock carbs you don't necessarily need to go with a bigger needle when you increase power. The carb sort of automatically adjusts the fuel flow.

I hope someone finds this info helpful, or at least interesting. Let me know if I am way off track, and add anything I forgot. Thanks!

RR1

 

This is a very interesting, thanks. Yesterday I installed carb kit on my raptor. So far I like this set up very much. Come up with 19E needle and 12 clicks adjustment. What is your needle size and how many clicks you run ?
rnuts