90 SJ 701 carb setup/tuning

W

WillieD

Location
Tennessee
I've got a 90 sj with a 701 61x/61x, single 46 mm carb. Trying to figure out the best setup for it. I found a chart on mikuni website that says 150 main, 120 pilot, 2.5 N&S, and 80 g spring; which puts the pop off at 14 psi according to their chart. Also says 1 turn on the low speed and 1.5 turns on the high. I'm in TN and thats the only place ill be riding it. Is there a better or more reliable jetting/N&S/spring setup? Thanks
 
maxboost

maxboost

Super tuned
Location
Ia
I tried those exact settings last year since that’s what Mikuni calls for, I had same setup 61x/61x with sbn 46, it was rich because I always battled fuel dribbling through the high speed circuit, I’d set your pop off around 25-28 and play with the jetting and screws. If you look in the jetting database in the driveline section you’ll see my whole setup, I think there is also some other similar ones.
 
wmazz

wmazz

Location
Riverside, CA
2.0 w/80gr spring.

A rich low speed (pilot) circuits kill top end, read my blog on pwctoday and learn
how to adjust and possibly rejet the low (pilot)circuit for best performance, and
reliability (starts easy, doesn't load up while idling or 5mph zone, the engine doesn't
quit while idling and hard to restart).

If the carb was used, rebuild with genuine Mikuni parts, not the cheap needle and
seats, or carb kits you find online. In the long run you will save money.


Bill M.

Purchase a few extra thick carb base gaskets.

Stick with seat & spring combo's like 2.0/80gr, not pop-off psi goals.

Low speed jet, seat, spring, low speed (pilot) screw combinations are
way more important than using a pop-off psi suggestion.
 
Last edited:
maxboost

maxboost

Super tuned
Location
Ia
@wmazz could you elaborate a bit more on what you mean by choosing the right seat and not necessarily choosing the right pop off psi? Just curious how you tune for optimal seat diameter or maybe you could post the link to the pwc blog? Thanks
 
maxboost

maxboost

Super tuned
Location
Ia
wmazz said:

(They only allow 1000 words per blog)​

Jetting Basics, Part 1​

Jetting Basics, Part 2​

Jetting Basics, Part 3​


@maxboost I will answer on Thursday.


Bill M.
Click to expand...

I found the blog last night, it was some good reading. One question tho,

in the idle drop test you say “on your initial low speed setting, if your low speed screw ends up all the way in and it still runs then increase pop off by reducing seat diameter and if your screw is farther than 3/4 turn then lower pop off.

in the jettings basics part 2 you say “if your screw is less than 1/4 turn or more than 3/4 then to adjust low speed jet?

I hope I’m not causing confusion, just picking your brain :)
 
wmazz

wmazz

Location
Riverside, CA
maxboost said:
@wmazz could you elaborate a bit more on what you mean by choosing the right seat and not necessarily choosing the right pop off psi? Just curious how you tune for optimal seat diameter or maybe you could post the link to the pwc blog? Thanks
Click to expand...
First off, why did people choose low pop-off with large needle and seats?

It all started with go-karts, and snowmobiles. Their carbs were similar to the Mikuni BN,
and they were manufactured by Walbro, Tilitson, Mikuni, Keihin, and possibly others. A big
difference with some of these carbs was the absence of a main jet ck valve that prevented
a bogg after the throttle was chopped and re-accelerated, some carbs didn't have o-rings
on the high and low speed screws to prevent vacuum leaks.

The big difference between the old style carbs and the newer CDK2, SBN, SBN-i carbs is
the only other way (besides the low & hi speed screwes) to tune out problems on (old style
carbs) was to change the needle and seat, cut or stretch the spring, or change the lever height.
But that is different on the new CDK2, SBN, SBN-i carbs, because you have more options to
assist the low to high transition. I prefer to use low speed jets, not springs.

Back in the 1980's it was generally believed that engine failures on JS440/550's were lean
failures, and that increasing the seat diameter allowed more fuel into the float chamber.
But that was not a fix for the thousands of piss poor pipes, performance accessories, and
engine mods to engines that had leaking main seals in as little as 2hrs of break-in.

In 1986 the first 650 X2 was released as Kawasaki's first 2 seater jet ski o_O. There was a lot
of new performance accessories available for the 650, and many of those parts contributed
to countless 650 engine failures that appeared to be detonation by a lean running engine.

~1990 - 1991, when the SBN was released, most mechanics and engine builders did not
equate the engine failures to impellers, and a pump set-up that matched an engines peak
HP rpm. Using progressively larger needles and seats (up to 2.8) and cutting springs was
a normal to hopefully reduce the possibility of a lean engine failure.

There was many other contributing factors to engine failures, such as the Ocean Pro cylinder
head that increased compression and increased squish clearance ~.120" There were bad jet
specs. Many BN carbs had small main jets. Too much compression was another cause. Most
people were using compression gauges that were reading 30psi low. So when they milled
a head for 185psi, they really had 215psi.

All this was stacked on top of an engine that was over-propped. Companies like Skat Trak
sold their impellers on the idea that, as you increase the performance of your engine, you
need a larger impeller. People like Klemm perpetuated the idea that impellers were like
sprockets, and if you wanted more speed, just add a larger impeller.

At least PJS noticed the problem of 650's with straight pitch impellers, so they made an
aluminum exit nozzle that was at least ~10mm's larger than the stock plastic exit nozzle.

During the 90's while at PJS, I answered the tech line calls, and "what is pop-off," was a popular
question.

Pop-off is just a number, a tool used to diagnose and rebuild a carb. Pop-off, or the needle,
seat, spring, low jet, and low speed (pilot screw) affect the starting and idling, the low speed,
and the transition from the low speed to the high speed circuit, in carburetors like the CDK2,
SBN, SBN-i.

Pop-off was not the cause, or a fix for engines that failed. Lowering pop-off didn't make them
any more reliable than they were before the mod, and if pop-off psi was close to the fuel pump
pressure, that caused problems. A low pop-off caused many engines to fill up with gasoline
(hydroloc) while sitting on the beach.


Bill M.

Most people will never see a transition problem caused directly by the spring. That is the
sort of things that happen on Walbro weed whacker carbs, not CDK2, SBN, SBN-i.

There is no reason to lower pop-off when replacing the stock flame arrestor with an aftermarket
flame arrestor. If the carb(s) can not be adjusted out with the low & hi speed screw. That is a sign
that a larger problem is the cause.

When reassembling carbs, you should test the pop-off while disassembled, and again, after
it has been reassembled. The pop-off should be the same.

According to Mikuni: The 1.5 needle and seat is capable of supplying an engine the contents
of the fuel pump without restriction. No need for larger needle and seats than a 1.5, or 2.0.

On most dual OEM carbs I prefer 1.5 & 80gr spring.
 
Last edited:
You must log in or register to reply here.
Top Bottom