Exhaust Valves and surging

[prairieboy]

Recently did a top end rebuild on my 787 Speedster motor. They bored the cylinders 0.5mm over.

Took her out on the water and was doing proper break in procedures but I noticed the engine would surge around the 5 to 5500 rpm mark. I'd back off and it would be "ok". But just doing some quick testing I took it up past 5500 rpm and the surging seemed to stop. I didn't want to push too hard.

I talked to a couple of places and one theory is that my raves are not functioning properly. The idea that the over bore and the springs getting weak and so on sounds plausible especially at the 5000 rpm mark as if I recall correctly that's when the raves start to operate.

So a couple of questions. Does this seem to be a logical place to start? I mean, if I were to buy a rebuild kit, would I also need the guilotine. The old ones were in good shape and shaved down for the over bore. One place I checked said that new guilotines would need to be shaved down, but a seadoo mechanic said that not on a 0.5 mm bore that there is still plenty of clearance with new ones.

Another possible cause could be an air leak and the mechanic said to check the water strainer, but I can't seem to find reference to one in the shop manual. Said the o-rings on the strainer get old and start to suck air.

So many questions and chasing a few ghosts, but I thought I'd ask the experienced folks on here!

 

[1983]

Why did the engine require a new top end I the first place ?

If you didn't repair something then it will smoke the top end again quick, especially if fuel supply related.

 

[prairieboy]

It was fuel related. One cylinder had good compression but one was dead so rebuilt carbs, fuel lines and new gaskets on fuel pump.

 

[prairieboy]

Anyone have advice?

 

[1983]

I would lean towards fuel system related over raves.

The boat guys should chime in soon on locations of items to check.


Rob

 

[racerxxx]

Original standard bore pistons are 82.00mm

First over pistons from seadoo are 82.25mm

The above can be ran WITHOUT cutting rave valves

82.50mm and up NEED the raves cut. At this dimension you have made the vale and piston line to line zero clearance.

On the boats you can feel the rave RPM at about 4500rpms when they open.......provided where you have them set. It can feel like a surge when they bounce in and out, make sure the raves (red caps) are set the same as the other engine. If the raves aren't leaking I'd say run them for now, or if you want to prove them out, swap the entire assembly from the other engine. It's a little work, but free trouble shooting. Have you re-checked the compression?

I would have no problem on a full throttle run for a few seconds to see if it does it at higher rpms. If it's gonna let loose it's now or never, LOL.

 

[Seadoodoc]

^^^^ rave valves opening. Validate by tightening caps all the way down and see if it changes rpm. Or remove cap and spring and test drive. You will loose bottem end power but can vaidate. That surging was reason SD made valves adjustable.

 

[jhjesse]

Yep all good advice. Maybe back the red caps out until flush with the top of the rave cap then screw in 1 turn or the thickness of a nickel below the top of the rave cap. That is usually a good place to start rave valve adjustment.

Any hesitation or bogging when accelerating? Maybe after a long idle in the water (no wake zone) hammer the throttle and see if there is any hesitation. If so then the carbs/fuel system needs to be examined.