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WMAZZ - GREAT technical support

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Quote Originally Posted by wmazz View Post
Quote Originally Posted by billman100 View Post
Have a 2.5n/s with a 115g spring so I should be at 19psi for pop-off. Looking for the jet sizes I should get.
You still haven't said anything about the return line restrictor size? Did Riva have any recommendations
for the high speed screw and low speed screw?

I have never jetted based on pop-off. 19 psi is normal for your set-up, but over the years I have seen
differences in gauges.

First I recommend you try to insure the method you use to test the pop-off is as consistent as possible.
I normally use a gallon sized zip-loc bag and wd-40 to completely submerge your needle and seat with. I
prefer to use an air compressor to test the needle and seat for leaks and pop-off pressure. I can post my
tester if you need to see it.

When testing, the first time the needle pops off is out of range, and the result should be discarded.
The second and third tests are used as valid tests. Be sure to keep the needle and seat submerged.
I use the plastic bag as a splash guard. What ever the pop off is with the lever level with the casting.
Small changes in lever height will effect your low jet size.

The Jet setup Riva recommended should work, but I think the mains are too small for your application.
You have big bores with a relatively short stroke. IMO it is much safer to increase the carb bore
diameter with longer strokes. The big bore is the reason for the main jet difference in my specs based
on Riva's recommendations. There is a good components in your set-up that will help with reliability,
and I have recommended specs that can be easily fitted to your set-up with the proper testing first.

Ideally with larger diameters you need to maintain similar pressure differentials (negative pressure inside
the venturi, and and atmospheric pressure) compared to a smaller diameter carb. When you change to
a larger carburetor, and the venturi is a larger inner diameter and the secondary venturi is missing, or
larger than original we need a few workarounds to keep the right amount of fuel supplied to the engine.

Reverse jetting is one way to achieve this, and the Riva specs are basically reverse jet specs. The 2.5
seat may have the same effect as richer jetting with a 2.0 seat.

I have two different approaches: (1) is similar to Riva's recommendations, 140 low, 155 high, 2.5 seat,
115g spring, 1 turn out on the pilot screw, 1 1/2 turns out on the high, and a 30 to 35 return line jet.

(2) is to use a 130gr Sea Doo spring (270-500-374), 150 low 150 high, 2.5 diameter seat, 1 1/4 turn
out on the pilot screw, 1 1/2 turns out on the high, and a 30 to 35 return line jet.

Before you ride your Yamaha, evaluate your initial low jet, seat diameter, spring, and pilot screw settings
on the trailer first using my Idle drop test. Afterwards we can talk about the mains, when the low speed

My Idle drop test
Perform the idle drop test on a trailer. If you are simply power tuning, testing stock carburetion for best
performance with the intent to rejet, you should set the pilot (or low speed) screw to stock settings to
save time.

If you are developing your own jet specs or testing jet specs you found on line, you should set the pilot
screw set to a rich setting. For example: 1 turns out, and warm up the engine.

(2) Set the idle to a low rpm. A good rule of thumb: You should be able to hold your hand behind the pump.
If the pump pushes your hand away, the idle is too fast.

(3) Slowly turn the pilot screw inward (clockwise) until the engine wants to die, or quits. If you are
working with duals or triples, turn each pilot screw inwards with 1/8 turn increments until the engine wants
to die, or quits.

(4) The engine will run smooth out as it approaches a good pilot screw setting, but as it leans out, sometimes
the engine will struggle to keep running. If it does, shut the engine.

(5) Make a note of how many turns it takes to bottom the screw (initial setting). If you are working
with duals or triples, use the same process Average how many turns it takes to bottom each screw. The
average is your (initial setting).

(6) Reset the pilot screw(s) to the (initial setting).

(7) Turn the pilot screw(s) outwards (counter clockwise) turn beyond step 6. This is the
(final setting).

(8) Restart the engine, and check the idle. If the idle speed has increased, and repeat steps 2 through 7.

(9) Evaluate the test results.

(10) Did the engine continue to run with the pilot screw closed?
If so the low jet is to rich. Reduce the low jet 2 sizes, and repeat steps 2 through 6.

(11) Did the engine die, or want to die while you were turning the pilot screw in?
If so, how many turns out? If the (initial setting) turns out is greater than 3/4, the low jet is to lean.
Increase the low jet 2 sizes, and repeat steps 2 through 6.

(12) Continue jetting until (initial setting) is 1/4 to 1/2 turn out.

(13) (Final Setting) should end up at 3/4 to 1 turn out.

Let me know if you have any trouble with the Idle Drop Test, or if you are ready to deal
with the mains.

Bill M.
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