How do I find out what size jets I have?

[IDoSeaDoo]

I have a 1997 SPX with a 787 engine. It has the twin carbs. Last year, I blew up the engine, and have put in a new one. Last year though, I rebuilt the carbs and re-tuned the pop-off pressure. Im not sure what I set them to, but will find out. It has just a mesh screen for a flame arrestor, so I'm not sure what the guy before did to the carbs. How do I find out what jets I have, what should they be? What should my pop-off pressure be?

I also have a 92 gts with a 580, what should its pop-off pressure be?

Thanks!

 

[Dr Honda]

If you think someone was messing with the carbs... then the only way to know is to take them apart, and read the little numbers on them.

Assuming that you are running the stock air box, and flame arrestor... then your pop-off should be:

97 SPX: 23-43 psi
92 GTS: 22-29 psi

 

[IDoSeaDoo]

on my 787, the flame arrestor is just an aluminum mesh screen. I can drop corn kernels through it... should I have higher or lower pop-off? what about the high-speed needle, would I get anything out of turning it out a little more w/ less restrictive flame arrestor?

 

[Dr Honda]

Can you post a picture of it?

 

[IDoSeaDoo]

Don't have a pic yet, but upon close inspection last night, it seems that the previous owner simply took out the fine steel mesh from between the coarse mesh. The coarse mesh looks just like the kind on the stock flame arrestor off my 580, except the 580 has other, finer stuff in the middle. I can maybe get a pic later today.

Went out yesterday to tune the 787. It has a serious mid-range stumble. Starts right up, and idles great, but open her up and it chokes, then reves up to full throttle. I tried turning out the low speed idle screws from 1 turn to 1.5, I did not perceive a difference. Am I going to have to lower my pop-off? If I do that though, I'm afraid I'll make it harder to start The high speed is set to factory (0 turns MAG, 1/4 PTO).

Can anyone tell me why PTO and MAG high speed adjusters are different?

Thanks!!

 

[Dr Honda]

A lower pop-off won't cause hard starting. A leaky needle and seat will. I personally would double check the high speed check valve (behind the crescent shaped part in the carb) and check that all the transition ports are clear. If they are... then drop the pop-off to the bottom of the specs above.

 

[Dr Honda]

Can anyone tell me why PTO and MAG high speed adjusters are different?

Thanks!!

Under hard acceleration, the crank can twist, and it causes one jug to have more timing than the other. The extra fuel helps keep the piston cool. If you set both to "0"... you would probably stick the PTO piston. (or melt it completely)

 

[IDoSeaDoo]

How would I know if the Needle/Seat or HS Check Valve are leaking? Would the check valve be obvious when I take it apart? I rebuilt everything in the carbs, it should all be new/clean, but I will rip it all apart after work today.

 

[Dr Honda]

I like to refer people to this thread....

http://www.seadooforum.com/showthread.php?33643-Even-the-Pro-s-screw-up-once-in-a-while.


But, you need to use a small pump to check for needle leaking. The check valve will be obvious. if it's not clean and flat... it will leak.

 

[IDoSeaDoo]

Checked the pop-off pressures on the carbs, they were actually LOW!! Mag was 25, PTO was 27... it didn't start hard or anything... there seemed to be some sort of residue on the high-speed check valve (and surrounding area). Today I'm going to spray all that down with carb spray. Jets are stock (142.5 and 70). What color is the 65gram spring (I had a Mikuni kit).