Matching stock impellers = less speed??

[MNdrifter]

I've always had poor top end speed with my boat. Best ever was 40mph solo with 1/2tank and just a ripple on the water.
I pulled my pumps yesterday and noticed that both impellers have the same number stamped on them.
6S810. I'm inferring from the shop manual that these are both Stbd impellers.

Would that contribute to reduced top speed?

Stbd impeller

• MNdrifter
• Oct 5, 2015

 

[Scottintexas]

I would think it would have some effect. Did you buy your boat new ? If not, it's "possible" the previous owner had them reworked and changed the pitch on the port one.

 

[MNdrifter]

I bought it new.
I remember seeing another post about both impellers being the same from the factory.

 

[Scottintexas]

if your manual says they should be different (my '07 manual clearly states it) I would think there would be some type of "issues" if they were the same.

I would show it to the dealer you bought it from and see if they will step up for you to make it right with Yamaha

 

[Ronnie]

If they are the same model and have not been repitched and/ or the throttles have not been recalibrated you should see a difference in rpms between the engines throughout the throttles ranges but especially at cruising speed and above. There are a few theories on why the stock impellers are pitched differently, I not an engineer but the ones that make the most sense to me are:

1. One side is more efficient at taking water into the pump than the other since both impellers spin in the same direction but the angles of the pump intakes are different on each side of the boat. This is supposed to be a large part of the reason the boat turns better to one side than the other.
2. The pitch of one of the impellers is varied a little (according to my shop manual for the 242 Ls one of the impellers has a .5 to 1.5 degree difference in pitch from the other) to compensate for the difference in performance from one pump to the other.
3. By varying this pitch of each impeller the rpms will better match throughout the throttles range and the boat will track straight(re) than it would have if it had matching impellers.

Is your boat's top speed impacted by your current impeller set up? It depends on which stock pitch they have (assuming neither has been repitched), since one is already optimally pitched per Yamaha's specs. A general way to tell is by looking at the rpms at WOT (wide open throttle), a properly pitched and un-damaged impeller should allow you to get to the stated maximum rpm smoothly (with little or no cavitation).

Altitude is another thing which impacts performance as does the condition of the implellers and engines driving them. Since I don't know how to measure impeller pitch if I was in your shoes I'd remove them and send them to Impros in California for an evaluation, recommendations, refurbishing and possible repitching. They can be tough to get s hold of on the phone but I've found its worth the effort and have done business with them several times over the past decade. Incidentally, I bought two Solas impellers from them for my 242 Ls. They are the same model Solas but one or both have been repitched (to what I don't know since this information is proprietary to impos) and they perform as I wanted (I.e., better acceleration / hole shot with little or no loss in top speed and the Rpms match throughout the throttles ranges). New implellers are around $400 each parts and labor but refurbing / repitching is only about $100 each. Plan on at least a month of down time if you don't have a spare set so the offseason is the best time to send them in. If you can't get them off the shaft by yourself consider bring the assemblies into a local pwc shop.

 

[swatski]

Would that contribute to reduced top speed?

I think it is likely. As @Ronnie said the pitch is different, the port will be higher. If you are maxing out your RPMs by rev limiter, you could use higher pitch to still get to max RPM and gain speed.

 

[MNdrifter]

I forgot to mention that my boat always pulled to port last year if the engines were at the same RPM.
I lengthened the starboard steering cable to compensate.

If I read the shop manual correctly, the port impeller is supposed to have a more aggressive pitch.
For the same RPM that should push more water out the port pump and correct the pull to port.
It should also increase overall thrust and make more speed.
Now I'm excited to get the correct impeller.

I have 230hrs on the boat. There is some evidence of wear on the wear rings.
If I replace the impeller, how important is it to replace the wear ring?

I'm expecting the impeller to be replaced under warranty. Do I have a case for having the wear ring replaced too?

 

[Bruce]

If the pump housing (wear ring) is worn or expanded and causes cavitation that will damage the new impeller.

I would not install new impellers into old pump housings.

 

[MNdrifter]

Yamaha asked for pictures of each impeller showing the model number stamped into it. Sounds like I'm going to get a new port impeller.
I will replace both wear rings too.
Since the boat is going into storage next week, I'll just pull the impeller shaft and have the new impeller installed before next year.

 

[MNdrifter]

Follow up.
New impeller was installed at the beginning of the season. Speed topped out at 38 and the port engine will not make full RPM.
I gapped and replaced all spark plugs, that improved 100 RPM.
I adjusted the throttle linkage to make sure that both sides were getting full rotation.
The hull is clean.
I removed the air filters to see if it wasn't getting enough air. That gave me 100 RPM.
I'm still only getting to 7600 Port and 8000 Starboard. I should be getting 8200.
Is there a calibration that needs to be done in the ECU to allow the port engine to make full power?

 

[Gym]

Sounds like you covered most common issues @MNdrifter. I'm just wondering...did you check the silicone on the pump intakes? Missing silicone WILL cause cavitation resulting in performance loss. Use a feeler guage to check the clearance between the impeller & wear rings all the way around. Again, out of spec equals performance loss. Are you getting any rattling noise which could be cone bearings? Has anybody done a compression check on each engine?

 

[Bill D]

If you haven't, check the height of the reverse buckets to make sure they are going up all the way at full throttle. My engines top out around 7,800 rpm at WOT.

 

[MNdrifter]

@Gym, Wouldn't cavitation result in slower speed but higher RPM?
With the throttles forward there is about 1/8" between the top of the jet outlet and the bottom of the reverse gate. When the reverse gates were too low in the past, I could feel vibration in the throttles. I don't feel any vibration now.
I don't hear any rattles or vibration. The clearance seams to be good. I find it difficult to measure clearance between 2 curved surfaces.
I haven't done a compression test. I'll add that to the debug list.
First I think I will pull the plugs to check the gap and coloring.

 

[Gym]

@MNdrifter. Not sure about the higher rpm. I guess that would depend on the severity of the cavitation. Certainly if you're pushing more air than water higher rpm would be the case. Cavitation can be heard and felt.

With regard to my suggestion about checking the clearance between the impeller and wear ring, I was suggesting you pull the pumps and measure the clearance & check the cone bearings while you're at it. This does not involve the reverse gate although that was a good call on your part. I'm not sure of your boat's history but if you are not the first owner there may have been impeller/wear ring issues that may not have be addressed properly. Measuring the clearance would be a good indicator.

One more item to check. While running at wot with your air cleaners off check to make sure the carburator (tfi) butterfly valves are fully open so your engine is breathing properly.

 

[Gym]

By the way @MNdrifter what spark plug are you using and what gap?

 

[Scottintexas]

did you visually verify the new impeller is the correct part and they didn't mess up and give you a new "wrong" impeller again,

 

[MNdrifter]

did you visually verify the new impeller is the correct part and they didn't mess up and give you a new "wrong" impeller again,

Good point. I will check the PN to be sure. I suspect it is the correct part because with the old impeller I always got full RPM. The new impeller should have a more aggressive pitch so it makes sense that it would load the engine more.

The plugs I'm using are CR9EB with a .75mm gap. That is what the shop manual calls for. I pulled them yesterday and rechecked the PN and gap. All looked like new and had the correct gap.