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1999 speedster, full throttle hesitation

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robomatic12

robomatic12

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I rebuilt and cleaned the carbs of my speedster which has twin 787's last week. On my first test ride, if you went from idle to full throttle suddenly it would go up to about 4,000rpm and stutter for a bit then continue to 7,000rpms. If you go slowly with the throttle sticks there is no problem. From what I have researched it seems like it's lean when going full throttle suddenly and I'm suffering from hesitation.

I rebuild the carbs using the silver springs which gave me a pop-off of 45psi and had the low speed at 1.5 turns and high speed at 0 turns.

I changed the pop-off to black spring and got 38psi pop-off. I just took it out again tonight and it was running better but still hesitated a bit. I richened up the low speed to about 1.3/4 turns and it was better but would still hesitate perhaps every 1 out of 4 times.

Any tips? Should I go lower pop-off or tune the needles better? The carbs originally had 30psi pop off and functioned flawlessly with 1.5 turns out on the low speed.
 
I was thinking of doing, just time consuming to keep pulling both carbs off both motors. Unless anyone else has a better idea?

The only thing I can think of, I did not do an accelerator pump adjustment after the rebuild. I'm going to check tomorrow if it's out of adjustment.
 
a "Stuttering" mid is either:

1) Too much fuel
2) The RAVE valves are not opening.


If it's was lean... the engine would try to die. Also... the Low-mix screw has NOTHING to do with fueling once you crack the throttles. In the Mikuni carbs... the pilot jet is before the low mix screw... so, you will only get as much fuel as the JET will allow. The low speed mix screw is to get a stable idle.

Second... as a tuning tool... the pop-off has little influence past 1/4 throttle. (Unless it's very high, or very low)

If you think that you need more "mid" fuel... then you need to put in a larger pilot jet. BUT... knowing that the Seadoo jetting is very good... I would set the carbs back to the factory specs, with a pop-off that is on the low side of spec.


THEN.... what I would do, is make sure the RAVE's are clean, and moving free. If they are... open the adjuster 1 turn. It will probably remove the dead spot.


This is all assuming that there isn't a mechanical problem. (like a water leak in the regulator, leaky exhaust, or low compression)
 
I agree with Doc, I think it is a RAVE valve sticking. Mine had a slight hesitation like that when I first got it and cleaning the RAVEs fixed.
 
Thanks for the help guys, I still haven't gotten around to pulling the carbs back off. But both motors exhibited the same hesitation after the carb rebuild, I'm guessing for some reason the lower 30psi pop off that was in it before works better with this boat. I'll try it out.

My only thoughts were that it would be so coincidental for both the rave valves to stick at the same time as carb rebuild.

Sidenote: Can I get away with reusing the RAVE valve gaskets, I'm redoing top end at the end of season anyways.
 
robomatic12 said:
Sidenote: Can I get away with reusing the RAVE valve gaskets, I'm redoing top end at the end of season anyways.
Click to expand...

Normally...no.

BUT.... if you are doing a top end... is your compression low? If it is... don't worry about a hesitation because you probably won't be able to get rid of it until the engine is mechanically right. Low compression cases a whole list of issues... one is low exhaust pressure. (as in... wont' push the RAVEs open at the right time)
 
The only reason I was thinking of doing a top end, still need to check compression but I am only getting up to about 42mph with just myself in the boat (I'm only 150lbs). Both impellers need replacing or refurb but would that alone account for the 10mph I'm missing?
 
When I bought my boat, the impellers were in rough shape along with wear rings I was maxing out at 42mph (dash so who really knows) and it took forever to get there. Anyways I replaced wear rings and shipped my impellers to SBT. Just that service got me to 52mph. Listen to doc and foster, clean up the rave valves there is an excellent article, just search for Rave Valves and I believe it was rampage who originally posted it. It's a great write up, getting a baseline on your compression is never a bad thing either.
 
robomatic12 said:
The only reason I was thinking of doing a top end, still need to check compression but I am only getting up to about 42mph with just myself in the boat (I'm only 150lbs). Both impellers need replacing or refurb but would that alone account for the 10mph I'm missing?
Click to expand...

At 42 MPH, what RPM's are you getting? Should be 7200.
 
Dr Honda said:
Normally...no.

BUT.... if you are doing a top end... is your compression low? If it is... don't worry about a hesitation because you probably won't be able to get rid of it until the engine is mechanically right. Low compression cases a whole list of issues... one is low exhaust pressure. (as in... wont' push the RAVEs open at the right time)
Click to expand...

Finally got a compression gauge, all four cylinders were at 150psi. So I guess I do not need to worry about doing a top end anymore. Gotta give the RAVE valves a good cleaning now!

Side question: One of my engines crank seals are slowly leaking, when stored over winter it fills up the crank. Can I drop in a new crank without re-doing the top end? Or is that silly
 
If it is a crank seal, you would see it fill much sooner than that. It would fill in a couple days. It is probably just the rotary valve seals, most of them leak over the winter. When you are actually using it through the summer, do you have problems with oil in the cylinders?
 
Nope, it never fills up enough to cause any troubles. Just smokes a little bit more than the other motor at first start-up.
 
Sounds like a leaky rotary valve. Totally normal. Clean those rave valves and you should be good for now.
 
Took the boat on the water today, it was hard to start. Probably took 15-20 seconds with the choke on for the engines to fire up. Then they require a few more starts to fully catch and stay running. Anything I should look at first?

I'm going to replace the little plastic inline fuel filters tomorrow just incase.
 
Once you had them running, did they start back up easy if you shut them off? These are old 2 stroke motors. If they have sat for a while, mine take 15 to 20 seconds to start. That is normal. Thats why i always start them at home in the driveway first before i head out for the day. Make sure you are leaving the choke pulled all the way out initially till they fire.
 
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Yeah they start back up easy with 1/4 throttle applied, I read that in the manual. Seems to be working better than with no throttle.

How long do you run them at home? Just until they get running?
 
yeah i just start them till they actually run. then shut them off. you dont need to hook the hose up. just long enough to warm them u. then when you get to the lake they fire right up.
 
Did a bit of work over the winter, I removed and cleaned all the carbon deposits off the RAVE valves and ensured they are moving up and down smoothly with new gaskets. Checked engine compression, both at 150psi. Put on some new SBT reman props and wear rings.

Took the boat out for a quick cruise today and still have the 1/4 throttle to full throttle problem. It falls flat on its face and feels like its loading up. Sometimes one of the engines will die. If I back off of the throttle and try to go back to full throttle it seems to clear out.

Should I still try to back off the RAVE valves one turn? Is there anything else I can check? I still have the black springs in the carbs giving me a pop-off of 38psi. This problem occured on both engines as soon as I rebuilt them last year, should I go back down to the shiny silver 65 gram springs that were in there before that gave me a 30psi pop off?
 
I turned the rave valves so they are flush with the plastic cap, no change in the way the boat runs.

Any ideas? Is this all due to the pop off? I can't find anything definitive as to what the pop off effects.
 
The pop-off really has no effect after about 1/4 throttle. Having said that, why did you increase the pop-off in the first place? I usually set them to factory spec.

Also, did you rebuild the accelerator pump when you did the rebuild? The accelerator pump squirts gas into the carb when you push the throttle forward. You may have a leak in the pump diaphragm. Also check the hose that goes from the engine to pump. If this is leaking it may also cause the hesitation.
 
No I didn't rebuild the accelerator pump. I would just have to grab a new diaphgram for it? I will check those things.

I just put what springs I thought were oem in when I did rebuild. Didn't think too much about it since the factory has like a +/- 12psi which I don't understand.
 
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