bhambrook

Hello Bill:

I am having an issue with my sport spec SeaDoo HX. Whenever I run the engine on the trailer, it idles just fine. But, whenever I rev the engine up, it will not return to idle, despite the throttle being released. Instead, it maintains the rpm that I revved the engine to last. And, I have to turn off the engine and restart it in order for it to return to normal idling rpm.

Please note:

My carburetors are running slightly rich, but I don’t think this would cause this problem?

I rebuild my carbs with an aftermarket part (i.e. not genuine mikuni parts)

I only experience this problem with the ski out of the water, that is, this problem is not apparent when I am riding the ski.

Thank you very much for all your help!

Brent

Water Worx

Turn the low speed screws out 1/4 of a turn & back off on the idle screw....

bhambrook

It appears that adjustment solved my issue.

Thank u,

Brent

Water Worx

Cool

bhambrook

The gremlin has come back. After adjusting the low speed screws, the problem went away. But, I ran the engine yesterday and I am having the same problem again. Any recommendations? How much pressure loss over time is acceptable when holding slightly less than pop off pressure?

Mr. Bill

Loosen up the throttle cable.

bhambrook

It is not the throttle cable. Note: I pressure tested the carbs (including the fuel pump) and there are no leaks. I can't figure this one out? Unless the problem stems from using an aftermarket carb rebuild kit as opposed to a genuine mikuni kit?

Water Worx

That would be a good start.....

bhambrook

I further adjusted the mixture setting and idle setting and this appears to have corrected the issue I was having. I ended up having to set the mixture less rich than I had it set with stock air filters. Because aftermarket air filters create less suction, does this make it necessary to lean out the low speed mixture setting, while at the same time having to enrich the high speed mixture setting?

Thanks for your help!

Brent

Mr. Bill

No, what that effects is manifold pressure ( intake vacume drops with less restrictive filters ) This is usually compensated by installing the next size needle valves and using the oem springs, or by installing lighter springs. The idea is to lower the pop off pressures.
Never should there be a case of needing to turn the oem carb adjuster in further taking away some fuel, unless you have also changed to larger low speed jets that have added fuel to the mixture.

Normally when we add filters, we make jetting and pop off changes to offset the extra air entering the motor and the loss of some of the motors vacume from having less restrictive filters.

Before you go futher, do a very close inspection of your intake manifold looking for any cracks that may have formed ( 720 manifolds are very easily cracked even on stock motors ). Then if no cracks, remove the linkage from one of the two carbs that ties the two carbs together. Now, back the idle scew off so that it does not touch anything. Try to snap the linkage back onto the carb without it moving the linkage arm at all ( you may have to adjust it in or out to fit perfectly ). Look into each carb to verify both throttle plates are fully shut and you have some slack in your throttle cable. Once all of that is done, you can readjust the idle speed screw to make it idle at exactky 3000 rpm out of the water so that once put into the water, it will idle at 1500 rpm.

If you cannot make it work right after all of that, you have either screwed up the rebuilding, or have it jetted way off, or there is some sort of air leak into your motor from a crack or a bad crankcase seal.

It is mandatory that you pressure test the carbs after fully assembling them if you used aftermaket kits as the diaphrams may be resting on the needle valve levers and allowing the needles to leak gas into the engine. If so, bend the levers inward only slightly, then reassemble and retest.