Help with 787 high speed jetting

[BoLongo]

Seadoo specs large high speed jets in the 787; the setting for the high-speed adjustment screw is set to 0. You can only make it richer you can’t make it leaner

I want to be able to have the same stock fuel mixture when I turn the high-speed screw; 1½ turns out. I understand that I will have to lower the size of the high-speed jet, so the high-speed screw becomes active. Turning it in will make it leaner and turning it out will make it richer.

I will have EGT gauges so I can monitor internal temperatures to prevent piston damage. I have been all over Group-K and other racing sites, but I am not finding the information I'm looking for. I want to make my two 787s runs as efficiently as possible. I know the engineers at Seadoo are coming from the point of view of making the engine run as rich as possible to prevent engine damage. I want to get a little closer to the line between running too lean and running too rich. Why? Because I am OCD like that.

Before I make any changes to the jetting, I will establish a baseline exhaust temperature using the stock setting other than the high volume fuel pump which I have already converted. From there I will slowly lean out the carbs noting each change in exhaust temperature. 80% of the time I will be in the 5000 to 7000 RPM range; so I’m focused on the high speed jetting.

I do not want to buy ten different sets of jets, so I am trying to estimate the best set of high-speed jets to start with. Currently, my high-speed stock jets are 140.

Any input would be appreciated.

 

[Kingsspeedshop]

What's done to the 787..is it all stock or what?..that will make a big difference in jetting

 

[BoLongo]

1999 Challenger with dual 787s both engines are stock. I cruise at 5000 RPMs and WOT 7500 RPMs

 

[Matt Braley]

GroupK does have a write up on putting 717 carbs (38mm) on a 787 though (40mm original carbs). They got better efficiency without loosing a lot of performance.

Since your tuning I think you need an accurate tach. 7500rpm is Superstock fully ported race gas 787 stuff. these are cheap and all you have to do is set them to 2.0 and wrap the wire around a plug wire. They display engine hrs when the engine is not running. http://www.ebay.com/itm/TRAIL-TECH-...ash=item2ec52293f5:g:-IcAAOSwEeFVDIKd&vxp=mtr

As far as your original question goes.... I would not run less than 140 highs on a 787. BRP did want to be in the safe zone but they still had to answer to the Government emissions folks.

 

[BoLongo]

Matt,

My stock tachs were showing about 7200.
I just bought the TRAIL TECH on Ebay, an extremely inexpensive tool to be able to compare to my stock tachs and an hour meter as well! Very nice. Thank You

If I stay with the 140s, I will have no control over the high-speed fuel circuit. I guess I need to get my base temperature reading from the EGT gauges before I start thinking about leaning out the engine. I was trying to get a jump on purchasing the smaller jets online.

It might be that Seadoo hit the sweet spot with the 140 jet, but I have to believe that they are going to be at least 10 to 25% on the rich side. The rear cylinder has a 142.5, so that has got to be 15 to 30% rich. I would not be thinking about any of this without EGT gauges that would be foolish.

 

[BoLongo]

Is this the article you are referring to?

http://www.groupk.com/restogsx785.htm

 

[Matt Braley]

When I think of 787 jetting I'm thinking about 96 specs. 70 lows and 142.5 highs with 30ish psi pop off. In later years they had accel pumps and those where able to lean the jetting.

With an accurate and fast updating tach like the TTO you can use it to tell where your sweet spot is. If attempting this I'd go down to 135's on the high and start my needle 2 turns out from closed. Note the WOT rpm and turn them in at 1/4 turn intervals until the you stop gaining rpm. Then open them about an 1/8 to give back 50ish rpm. Confirm the "sweet spot" by checking spark plugs (search chop test) and add a little fuel when needed to a cylinder with the slight opening of the high speed needle.

 

[mikidymac]

I can understand the need to tinker but honestly on a stock ski you are wasting your time and money. These Seadoo's are tuned almost perfect from the factory for performance and reliability. Going a tiny bit leaner probably won't even be noticeable. It is not like they are jetted super rich from the factory and blubbering and fouling plugs.

 

[BoLongo]

When I think of 787 jetting I'm thinking about 96 specs. 70 lows and 142.5 highs with 30ish psi pop off. In later years they had accel pumps and those where able to lean the jetting.

With an accurate and fast updating tach like the TTO you can use it to tell where your sweet spot is. If attempting this I'd go down to 135's on the high and start my needle 2 turns out from closed. Note the WOT rpm and turn them in at 1/4 turn intervals until the you stop gaining rpm. Then open them about an 1/8 to give back 50ish rpm. Confirm the "sweet spot" by checking spark plugs (search chop test) and add a little fuel when needed to a cylinder with the slight opening of the high speed needle.

Matt,
Exactly the information I was looking for. Thank You

Mikidymac you are right, but this happens to be one of my many OCDs

 

[mikidymac]

If you are going to all the trouble you might as well get a pipe and more compression and have some fun. :thumbsup:

 

[BoLongo]

This a Challenger 1800 boat with two 787s. The boat weighs about 2600 lbs with a full tank of gas and five people on board. I can go through 40 gallons of gas WOT in under an hour. Even a 10% improvement in gallons per hour makes me happy.

 

[racerxxx]

The wind and current will affect your final outcome.I'll let you down easy. Just get used to the gas bill and enjoy the boat. It's smiles per gallon , not miles.

 

[BoLongo]

I'm still on my first tank of gas. I have spent more time standing on my head in the engine compartment than I have on the water. You know when I'm upside down in the engine compartment my frown is a smile