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post #1 of (permalink) Old 12-06-2001, 05:57 PM
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How to \"jet\" an engine

How do I decide to use bigger or smaller jets?thanx

jsjps
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post #2 of (permalink) Old 12-06-2001, 06:10 PM
 
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Re: How to \"jet\" an engine

If you go up more than 3,000 feet you will need to use a jumper wire. There are jets in the carb. If you go up in altitude you will need to jet down, to let more air in the carb, (lean it out). If you go down much in elevation you will need to intall a larger jet, therefore adding more gas, runs richer. I have never re-jetted a car carb before but i have rejetted many 4-wheelers and from what i gather they are about the same.

Headed for Gunnison, soon....

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post #3 of (permalink) Old 12-06-2001, 06:27 PM
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Re: How to \"jet\" an engine

Whats a "jumper wire and where does it go?thanx

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post #4 of (permalink) Old 12-06-2001, 07:55 PM
 
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Re: How to \"jet\" an engine

NEVER UNDER ANY CIRCUMSTANCES "DRILL" A CARB JET!!
An immense amount of time, millions of dollars and countless thousands of hours of effort go into getting the passages correct, and it only takes one chimp with a drill bit to screw that up...
If you need bigger jets, buy correctly made larger flowing jets, don't just take a drill to them...

When you 'Drill' (mangle) that jet, you can actually increase the size of the passage in the jet, but DECREASE the amount of fuel that will pass through it. I've seen it over and over again.
(Let's see if the 'Drill' guy can figure that one out... Anyone that 'Drills' jets won't be able to...)
------------------------

First, we need some info on your carb and engine.
Operating vacuum, idle vacuum, what RPM you normally run, what your top RPM is, How many miles the engine has on it, ect.

Then, you will have to buy a vacuum gauge, and some new spark plugs.
Get back here when you have those items and we'll take you through it.
Ask for 'Carb Rejetting Help' and I'll keep an eye out for the post.

This is actually much easier to do correctly than it is to screw up, but most of them do it the hard way anyway...

So many cats, so few recipes...
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post #5 of (permalink) Old 12-07-2001, 01:07 PM
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Re: How to \"jet\" an engine

I personnaly would use a J-79. It's a gas hog but no ones going to beat you on the 1/4 mile.

Actually, I want to read Aaron's "How To.."

Karl
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post #6 of (permalink) Old 12-07-2001, 01:22 PM
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Re: How to \"jet\" an engine

In answer to TR's "Drill guy" question above... if you increase the size of the hole in the jet by drilling it the venturi effect is dimenished and fuel deliver acutally decreases. So Aaron, do I win a prize?

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post #7 of (permalink) Old 12-07-2001, 02:21 PM
 
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Re: How to \"jet\" an engine

Nope LEVE, you have the correct idea, but you are trying to apply aerodynamics (Venturi flow) to liquid movement (hydraulics)...
Check your private for the correct answer... (you almost had it!)

Anyone else care to try...?...

So many cats, so few recipes...
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post #8 of (permalink) Old 12-08-2001, 03:18 AM
 
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Re: How to \"jet\" an engine

The drill does not make as smooth of a hole.

85 CJ7 4inches 33ssr's infinite projects
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post #9 of (permalink) Old 12-08-2001, 05:37 AM
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Re: How to \"jet\" an engine

I'm looking at some jets now with a magnifying glass and it looks like the lead in side is tapered down to the final jet size.
My guess would be that by just drilling and not changing the lead in taper, well that's not a good thing.

CTjeepnut


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I'm sure glad I own a CJ!
post #10 of (permalink) Old 12-08-2001, 11:18 AM
 
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Re: How to \"jet\" an engine

DING! DING! DING! DING!
We have a winner!

That entry and exit taper and radius on the entry and exit of the 'Jet' make a ton of difference in how the liquid fuel behaves...
If you just drill the passage, you ruin the approach, and a square corner (or worse yet, a square corner with scratches, gouges or burrs) makes for a real hard time for the fuel flow. Lots of turbulence, and tumbling of fuel, and all that activity slows the fuel flow down.

We have several sets of close tolerance jets here that have the same hole size in them.
The length of the passage, and the contour of the passage entry and exit are the only difference.

(You have no idea how much hair I pulled out trying to figure that one out!!)

'Drilling' the jets will affect not only the amount of fuel delivered, but the timing of the fuel delivery, and the characteristics of that timing...
Coming off the idle circuit and into the primary circuit in particular, and at the very top end of fuel delivery capabilities (Top RPM).

I'm sure 'Drill Boy' will chime in now what I've given the answer....

So many cats, so few recipes...
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