Super Jet carb jetting - urgent

[hbski]

Hi folks,

I have a SN with a groupK 718 sleeper kit (milled head, massaged carb and intake, ported) I'm addinga B-pipe and called Harry for jetting specs. he gave me:

140 pilot, 160 main, 1.5 seat, 24 pop off

I opened her up to put in the new jets and found that it already had a 2.5 seat and it seems odd to me to reduce the seat for adding the pipe. The carb was originally from a 650, GroupK already set it up for the 718 sleeper. Can someone chime in on whether to leave the 2.5 or swap to the 1.5 or something else entirely?

One other slightly odd thing, the metal disc on the diaphragm was bent, could that have been done on purpose for some reason, or should i just straighten it (seems like i should)?

Thanks,
hbski

 

[Tyrant1919]

Jetting seems close. Diaphragm disc should be straight. Not positive on what your seat and popoff should be.

 

[Vumad]

You are confused about how the needle and seat work. The n&s is not a jet. Smaller doesn't mean leaner and bigger does not mean richer. It is an assembly. The size of the needle and the weight of the spring (and to some degree the bend of the rocker arm, but do not bend the arm) work together as an assembly to set the popoff. A higher popoff is leaner and a lower popoff is richer. The parts you use don't matter as much, just the out one of the complete assembly matters. A 1.5 with a weak spring can be richer than a 1.8 with a heavy spring. Refer the the mikuni manual.

The mikuni manual says that a 1.5 needle and seat will flow all the fuel that a sbn carb can flow. Bigger needle and seat isn't magically better. Actually, the manual says to run the smallest needle and seat possible because smaller seals better. Group I is telling you to run the most efficient needle and seat to get the right popoff. Bigger is not better.

See mikuni manual, page 16, center of the page under "note"

 

[Vumad]

As a side bar, you are making a mistake. You are not trusting your engine builder and are coming to a crackpot community to ask inexperienced guys like me for advice.

If you have enough faith in the engine builder to pay that kind of money for an engine, you should probably trust him on how to run it. I'm sure he would have answered the question if you called him.

If you feel that this community is a better source of information than your builder, get a new builder. However, I disagree. I can't tell you if those jetting specs are right, but I can tell you trusting the community of over your builder is a mistake.

 

[Tyrant1919]

Well said!

 

[Vumad]

A 1.5 N&S with a 65gm spring only gives 32psi popoff (mikuni chart). You should probably get clarification from group K, because based on the mikuni manual, you can't get a popoff of 24 with a 1.5 N&S.

A 2.5 with a 115gm spring gives 19 popoff, so based on the mikuni charts, you can't get the popoff you want with any of your current options regardless of spring choice.

 

[hbski]

Thanks for the reply, I do trust Harry Klemm, but he is also an extremely busy man and unavailable at 10pm on a Friday night, or on the weekend for that matter. So hoping it was an easy answer I posted here so that I may finish the SJ and head off to the lake for a few days.


Also keep in mind that this was on the tail end of a couple of rounds of e-mails with GroupK about the jetting and getting several different answers because Harry was out for a while due to emergency eye surgery.

Thanks again for the reply
Cheers,
Don

As a side bar, you are making a mistake. You are not trusting your engine builder and are coming to a crackpot community to ask inexperienced guys like me for advice.

If you have enough faith in the engine builder to pay that kind of money for an engine, you should probably trust him on how to run it. I'm sure he would have answered the question if you called him.

If you feel that this community is a better source of information than your builder, get a new builder. However, I disagree. I can't tell you if those jetting specs are right, but I can tell you trusting the community of over your builder is a mistake.