Twin Rotax engine RPM difference

[HighTrim]

I have a '97 speedster with twin rotax engines. At about mid throttle, they are about 1500 rpm different. How do I adjust this short of adjusting the throttles to different positions.

 

[Dr Honda]

Do they idle the same? Do the peak the same?

If they do... and it's just a mid issue, then it could be light cavitation on one of the engines.

 

[HighTrim]

You have it right on, they idle the same, and seem to peak the same at WOT, it is just the mid range throttle that is off. How would I go about solving the cavitation?

 

[HighTrim]

After googling this, I read from some other members with the same problem. They mentioned a bad hold shot, which I am also having a problem with. It really revs up but takes a while to get on plane. If I ease into it it will slide up smoother, but dropping the hammer just gets rpms and bubbles, lol

 

[Dr Honda]

Then it's time to check the wear ring, and carbon seal on the driveshaft.

The carbon seal is under the gray plastic cover behind the engine, and I just made a post on the wear ring. (It's a sticky on top)

 

[HighTrim]

Ty for your advice.

So is the carbon seal on the exterior of the boat? Hard thing to change without a manual, I ordered one but not here yet.

 

[HighTrim]

....and could I ask why the carbon seal? The dealer told me that you only replace that if the boat is taking on water through the hull?? I am new to jet boating and trying to learn all I can.

regards, chris

 

[dsw222]

i had the exact same issue two weeks ago. last weekend i installed new impellers and wear rings and that took care of it, just like honda said it would

 

[HighTrim]

What was the cost total dsw? Did you do it yourself? Any tips?

 

[dsw222]

200 per impeller, 35 per wear ring, 10ish for the impeller tool, and a few bucks for blue locktite and a 23mm socket (apparently not a common size cause i went to 3 stores to find one)

a tip... dont listen to the people who say "it will take 20 mins per impeller". if its your first time, it will take a lot longer because you dont know the tricks about how to get the driveshaft aligned with the impeller splined while slipping on the rubber gasket and lining up the 4 bolts on the pump, etc, etc, etc. Even if you did it before, i still dont see it taking 20 mins

it took my friend an i about 6-8 hours from start to finish, including several trips to hardware stores to swap two impellers and two wear rings

its definitely not hard... it just takes time and its a bit frustrating at times.

 

[HighTrim]

Tks, I figured it would take more than that! I know the marina charges 3 hours labour per ring! They also told me like 80 bucks just for the part? Did you buy yours at a dealer or online?

 

[dsw222]

I got everything online... the wear rings from SBT for 35 each. They also have an impeller exchange program for 25 (you send them your old impellers) which is a great deal, but I got solas concord impellers from overton's for 200 each

 

[ProbeGT]

I've got the exact same problem on my '98 speedster

Throttles are equal up to 3000RPM, then left engine revs a lot more (1000-1500 like you) and WOT they rev about the same.

The high revving engine (left) also jumps to 7000RPM when I floor it, while the other gets to 5000-5400 then after a second, gets to 7000. Would my left engine cavitates? All this time i tought i had a carb problem on my right engine...

Is there a way to check the carbon seal? I've checked my impeller/wear ring clearance and it's under 0.0020" for both pumps.

 

[ProbeGT]

I got everything online... the wear rings from SBT for 35 each. They also have an impeller exchange program for 25 (you send them your old impellers) which is a great deal, but I got solas concord impellers from overton's for 200 each

Could you please give me your feedback about those concord impellers once you'll try them? What did you gain, what did you loose?

 

[dsw222]

well, i am having carb issues so i cant speak for the hole shot... but my top speed before installing the concords was about 61mph and now it reached 64mph

keep in mind that my one wear ring was all torn up causing cavitation... and it was like that since i bought the boat.... so i dont actually know how it performed with stock impellers and good wear rings

but i'm very happy with 65ish mph, so i cant complain about the concords. i hope to have a much better hole shot after the carb rebuilding that i am doing next week. i will let you know how she runs after that

btw... i also noticed that the engines tach out at 7000 RPM instead of 7200 RPM with the concords. i dont think this is a big deal since i'm getting mid 60's top end though

 

[ProbeGT]

No offense, but your speedo must be wayyyy off.

A speedster 16' with the same engines as you (lighter and shorter boat) do about 55-58MPH in perfect running condition (on GPS) mine does from 50 to 58 depending of water conditions (speedo) wich means probably 48-55 in real life (GPS)

If your challenger really does 64MPH, that would be insane.

I had installed a concord impeller on my old sea-rayder boat. I had gained a little hole shot but i had changed my wear ring so i'd have never known if the impeller really helped...

Well i might give a try on that carbon seal, it's not too costly and if it works, I'd be very glad

 

[dsw222]

No offense, but your speedo must be wayyyy off.

A speedster 16' with the same engines as you (lighter and shorter boat) do about 55-58MPH in perfect running condition (on GPS) mine does from 50 to 58 depending of water conditions (speedo) wich means probably 48-55 in real life (GPS)

If your challenger really does 64MPH, that would be insane.

I had installed a concord impeller on my old sea-rayder boat. I had gained a little hole shot but i had changed my wear ring so i'd have never known if the impeller really helped...

Well i might give a try on that carbon seal, it's not too costly and if it works, I'd be very glad

I figured the speed was off because i haven't seen many 60+mph boats on here... but you asked how the concords ran compared to before... I used the same measurement for both (even though it may be skewed), so the difference should still be accurate... i.e. the concords gave me 3 ish mph over stock

I just got the boat a month or two ago, so i'm still getting a feel for what works and what doesn't work on it... i'll have to take GPS out with me after i rebuild my carbs and see how far off my speedo is

Good luck with the carbon seal

 

[HighTrim]

Dealer told me that carbon seals only need to be replaced when there is water leaking into the bildge?? Im new to jet boating though and am only going on what I am being told