jetting help (progress update ---need more help!)

[Zach310]

Race ported 61X cylider
Ported/epoxied cases
210 lbs compression
vidler total loss
just using stock 38s w/ stock reeds

need jetting, at least a starting point

 

[Zach310]

anyone have any ideas?

 

[Matt_E]

Stock jetting is 1.5 N&S, 115G springs, 70 pilots, 130 mains.

I think it's a given you will have to jet up.
What pipe are you using?
What flame arrestors are you using?
Chokes removed?
Stock reeds?

 

[Zach310]

Factory B bipe w/ limited chamber
stock reeds
chokes removed
prok arrestors

i was assuming up.. hope some one has some ideas!.. finally got the blaster togeather.. just dont' want to blow er up right away

 

[Matt_E]

I would lower that popoff....retain the stock N&S (1.5), put 65gm springs in.
Use 80 pilots, 150 mains, and work your way down.

Consult the Mikuni tuning manual for the proper sequence.

 

[Zach310]

i know how to do all of that except change the pop off

 

[Matt_E]

Change out the spring under the arm. You'll see it when you change jets.

 

[SUPERTUNE]

Don't use that big of main jet...start out with 135-140 at 1 turn out, use 80 gr. or 95 gr. springs. I feel 65's are too low.

 

[Matt_E]

Listen to Chuckie...he knows his stuff.

 

[Zach310]

that low of a main jet with the ported motor?

 

[robertg]

I am running a 760 cylinder with 35cc domes, mild porting, B pipe. I have a 135 main, 75 pilot, 1.5 needle and gold spring. The same jetting worked with a ported 701 cylinder. This is in a Superjet.

 

[SUPERTUNE]

YES! Look at it this way...if you turn up the fuel pressure...does the main jet run richer or leaner? Think about it....

 

[waxhead]

Think about it this way
When you drive along the road n a car with the window open
hold a piece of paper in your hand
The high pressure in the car tries to force it outinto the low pressure inside

ok now double your speed
what happens
the differenc ein pressure between the car and the outside are greater
the paper tries harder to escape

when you put a bigger engine in or a more efficent engine then the air speed in the carb is higher thus a lower pressure in venturi

this means that the fuel is being forced into a lower pressure area than it was on the stock / smaller engine

so therefore the jet flows more fuel it has a greater pressure behind it

 

[ANT]

Your all ported and everything, why don't you run a Mod chamber on your pipe?

 

[Zach310]

ok here is where i am at :
started with 2.3 needle/seat 95 gram spring
80 pilot 145 main

super rich
went to: 135 mains -- still rich on the bottom.. would hold at 4200 rpm then snap would run all the way up to 7360 rpm.

lowered spring to 80 gram spring.. still ruffly the same.. little bit better

then went to 70 pilots 2 turns out and worked my way down to 1/2 turn out.
it was little improved again.. some groggy ness and "like some one hit a switch' after it ran at 3900 rpm (300 rpm improvement)

Last change was to put a 65 gram spring in and that got me to the same result but the snap is commin @ 3700 now..

Were or were to go from here

 

[Matt_E]

Why are you lowering the main jet to clean up the bottom end? That doesn't make sense.

Also, why do you have a 2.3 N&S in there? That popoff is likely WAY too low, flooding out your bottom end.
Go with 1.5 N&S, and the jetting recommendations given. (i.e., 38psi instead of 15psi!)