Cant find answer on carb tuning

Mike Grafas

Mike Grafas

Location
Point Pleasant, NJ
Ok maybe this will be a quick answer or maybe it wont be
97 rn that I just bored to 82mm and freestyle ported, ada head 190psi, b pipe, enhancer etc
70 pilot
135 main
Dual 38s
prior to doing the bore it ran great, piston wash was good and very smooth power

Now that I have bored and ported it I cant find out how to jet the top end, I did all the tuning myself previously but I'm at the point where I don't want to lean seize so here is my question

All the carb tuning sites and forums I have read over and over they all say tune for max rpm, BUT what they don't say is where is the INITIAL starting point on any given motor unless you it is a stock boat and you use stock setting

If I start my initial starting point at say at 1 1/4 out on the high speed with my same jetting, I assume I will be lean so when I go to do a full throttle run ill be way too lean

ok so I will bring my high speed out to 2 turns, except how do I know if I will be lean still

I want to start rich on the top end but I don't know how to find out if I'm overly rich before I run it

What if I bring my high speed adjuster out to 2.5 turns, what if that is still too lean and I wont know it until I'm going wot and it shuts down

I'm already certain I need to go up a jet size on my main to 137.5 but again, where do I set my adjuster to be on the rich side without doing a wide open pass

I hope this makes sense of what my question is, but if its not I can probably word it better
 
Mike Grafas

Mike Grafas

Location
Point Pleasant, NJ
Mike W said:
Follow the FP instructions but go to a 72.5 or 75 pilot.

H/S 1 1/2 turn out from closed
L/S 1 1/4 turn out from closed

http://www.factorypipe.com/media/instructions/yamaha/701SJ03.pdf
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well I did my break in so far up to 50% throttle, still have yet to do up to 90%
but the low end felt great, it was the top end I am worried about, being its ported I don't know how rich I need to go as a starting point to be over rich

the last thing I want to do is go wot for a few seconds and lean seize from being too lean
 
Mike Grafas

Mike Grafas

Location
Point Pleasant, NJ
I'll try those BUT my question is how does one figure out the correct jetting for a given motor

Is there some sort of formula

What if there was no starting baseline like forums or other motors to compare to and one must find the correct jetting

You can't really say start anywhere because they're would have to be a guideline to getting somewhat close
 
kevbo

kevbo

Location
dfw
Open the top adjusters until you lose power under full throttle acceleration. Then slowly close them until it pulls hardest. At this point it will be a little rich of peak rpm at top speed, which is safe.
 
Mike Grafas

Mike Grafas

Location
Point Pleasant, NJ
kevbo said:
Open the top adjusters until you lose power under full throttle acceleration. Then slowly close them until it pulls hardest. At this point it will be a little rich of peak rpm at top speed, which is safe.
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ok I understand that but for example, lets say someone takes a 701 and bores it to 84mm, now they are at a 754cc, lets also say the port it, and add a pipe.

how would that person know what a good starting point would be for their motor, even like these engine builders building 1100cc engines, how do they know where to start with their jets, because one could say ok I'm going to jet it rich to start and be safe, but how would they know what is rich. is there a baseline to go by with cc/exhaust port opening/flow rate?

I wouldn't put dual 38s with stock jetting on a 84mm bore, I would obviously jet up because of the larger displacement, but how much to jet up. Cant say ok ill go up 2 jet sizes and try it, because if you are lean and don't know it and do a wot test and stick a piston or lean seize then there goes the top end
 
motobehrens

motobehrens

X
Location
Gilbert, AZ
You have a lot of the same concerns I used to have. When it is overly rich, you will know it. Be it two turns out, or two jet sizes up. It will be lethargic at full throttle, stumble, maybe excess exhaust smoke, low rpm, low mph, etc. To be on the safe side, if you have no idea where your jetting will end up, throw in some overly big high jets at two turns out and start working your way down. You will feel when it starts to run cleaner as you turn in the screws, and your rpm will increase. Pay attention to your tach. As I'm tuning, I log my rpm and mph for each jet change or partial turn of the adjustment screw.
Historical data provides your baseline, but you still want to start rich anyway.
 
Big Kahuna

Big Kahuna

Administrator
Location
Tuscaloosa, AL
Mike Grafas said:
Is there a reason to lower pop off? Just looking for an explanation because right now low end is spot on, not hesitation or bog
And just curious about lowering it if I'm using stock flame arrestors, is it because of the larger bore and greater intake vacuum?
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if your low end is spot on then you are over thinking this. Go out and ride the ski. do a high speed plug chop. (Wide Open, Hit the stop button without letting off the gas - Pull plugs and inspect). See what your high speed setting have the plugs looking like. If white, Turn your screws out 1/4 turn and repeat. If Black, turn in 1/8 and repeat. if you cannot get it to richen up any, then go up 2 jet sizes, same of you cannot get it lean, go down 1 size.
 
Mike Grafas

Mike Grafas

Location
Point Pleasant, NJ
Big Kahuna said:
if your low end is spot on then you are over thinking this. Go out and ride the ski. do a high speed plug chop. (Wide Open, Hit the stop button without letting off the gas - Pull plugs and inspect). See what your high speed setting have the plugs looking like. If white, Turn your screws out 1/4 turn and repeat. If Black, turn in 1/8 and repeat. if you cannot get it to richen up any, then go up 2 jet sizes, same of you cannot get it lean, go down 1 size.
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I do get what you are saying about over thinking this but I am just replying to other replies, someone said to lower pop off, I was asking why would I lower pop off if I still have stock FA with all three screens, unless the manifold pressure is decreased then I would lower pop off
Also my original question was how does one know where to start on jetting and I believe it was answered, basically historical data. Others who have tried different set ups and were grateful enough to put their data records on the internet
 
Big Kahuna

Big Kahuna

Administrator
Location
Tuscaloosa, AL
Its alot of trial and error. You know where stock jetting is on oem 38's. You add some mods, fuel requirements go up. raise jetting. best answer is to talk to your builder. Go with what he has suggested.
 
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