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Ask RSRBOB Your Own Question
RSRBOB, Technician
Category: Boat
Satisfied Customers: 1078
Experience:  Former Yamaha Factory Service Rep, Dlr Line Tech, Serv Mgr, G/M
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Im back. I have a problem with a yamaha ls2000 jet boat that

Customer Question

Im back. I have a problem with a yamaha ls2000 jet boat that I could use help with
Submitted: 2 years ago.
Category: Boat
Customer: replied 2 years ago.
(Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  RSRBOB replied 2 years ago.
HI, I am RSRBOB and I would be happy to discuss your LS2000 with you. I have a lot of experience with that particular model.
Customer: replied 2 years ago.
Let's start with doing this via chat/text since I think it will take some trial and error.Here is a description of my issue:The issue is that my starborad engine is only hitting 6200 RPM's at WOT while it used to go to ~7000 and the port engine continues to hit 7000.Now the details:
- I rebuilt the starboard carbs last season. I also redid the jet pumps including the impellers on both sides.
- For the carb rebuild, i didn't change out the springs per SBT's recommendations since I was not prepared to play around with pop-off pressure. I also have to admit that the kit came with many gaskets/diaphrams and I am wondering if I did something wrong there.... the kits were generic for the 2 different models of mikuni carbs.
- boat is an LS2000 with twin 135's. I have the fuel/air separator pump from GroupK and this was already installed last year when the engine was working.
- I have Sierra fuel/water filters vs. standard OEM filters. These have always been in place as well
- compression is the same across all cylinders and both engines (99 according to my cheap harbor freight gauge). This is the same as it has always been for meThe boat runs great but I know that the starboard engine should hit 7000 RPM and I want to address the problem.1) I changed the impellers/pump drives to see if it was an impeller issue and the problem continues. Not the pumps/impeller.
2) I checked compression and it is consistent
3) I put in new plugs and did a plug test. All cylinders appear to be running based on the color of the spark plugs. I am not sure if there could be a misfiring problem though, i.e. plug wire, CDI?
4) When the engine hits 6200 RPM, I pulled the choke and the engine bogs down. I think that means that the problem isn't because the engine is running lean.
5) I did adjust the idle so that the engine would be at ~1200 at idle. Previously it was a little low.
6) High speed jets? I can't reach the adjusters with the arrestor in place and I am hesitant to remove it since it has a gasket that I don't want to have to replace again. Do you think the high speed adjusters could be the problem? I will say that rebuilding the carbs has you putting these at a certain # ***** turns for each carb and I had instead tried to put them back to how they were (i.e. I counted the # ***** turns when I removed the high speed jets)What would you do to troubleshoot this? Ideas?
Expert:  RSRBOB replied 2 years ago.
HI and thank you for the additional information.Many points to discuss here.First, although both engines will turn about the same RPM's, they will never turn exactly the same RPM's, even with everything being identical between both engines. The reason for this is called pump loading. Both prop shafts rotate in the same direction, however, because it is a V shaped hull and the intakes for the jet pumps are at different opposite angles, one pump is being loaded essentially up hill, and the other is being loaded down hill. The down hill engine is the port or left engine. Therefore it will always turn more RPM's than the starboard or right engine.Some more facts to remember when trying to sort this out. First, Yamaha states that they consider any RPM within 10% of the other engine to be within factory specs. Therefore, if you are turning 7000 on the port engine, the starboard should be turning a minimum of 6300 RPM at wide open throttle.The next fact that Yamaha mentions when addressing this issue of different peak RPM's from one engine to the other, which by the way is not unique to the LS2000 but to most of the twin engine Yamaha jet boats, is the tachs are not guaranteed 100% accurate. My experience with those tachs is they are close, but certainly not perfect. If you want to check the variation on yours, connect your port tach to starboard engine and see how close it reads to what the starboard tach read before. Whatever that difference is, you have to factor in mentally when reading your stock tachs. I use an inductive digital tach when addressing RPM related issues, but that is a $400 tool that most owners aren't interested in acquiring at that price.The next issue I have is taking advice from SBT. Everyone has their own opinion but the people I associate with are not big fans of SBT. I do not know if you got OEM Mikuni carb kits or if SBT is marketing their own. If they are after market kits I personally would not use them, opting for genuine Mikuni. The carb rebuild kits you got fit 2 different mikuni Super BN carbs which is very . Yamaha uses a different Super BN carb body than most other manufacturers. It should have been pretty obvious which bits fit your carb and which didn't. As long as they looked like they were correct, they were.OK, JA is acting up so I am going to post this and continue on in another answer. BRB.RSRBOB
Expert:  RSRBOB replied 2 years ago.
Back!Continuing on. I had tried to include this 3x previously and it kept erasing it, so I am trying here.My next big question mark is your compression test results. You stated you were using a Harbor Freight gauge, and you said that like you know it is not a professional instrument. It most certainly is not. I have tried the HF compression gauges and found them to be very inaccurate. The fact that you have EXACTLY 99 PSI across 6 cylinders on 2 engines just reinforces my suspicions that the gauge is not giving you accurate information. For them all to be the same like that, it sounds like the needle is stopping at 99. I urge you to borrow, beg or steal one (don't really steal one, everyone knows its wrong to steal), but get a high quality gauge that will give accurate results. Also, when you are checking compression on a multi cylinder 2 stroke engine, leave the spark plugs in on the other 2 cylinders that you are not checking at that time. The engine actually cranks over faster with the plugs in than it does with the plugs out. It is also the way Yamaha tested the engine for compression when they set their factory specs.If all the spark plugs are coming out about the same color, that is a good sign, It means the adjustments can't be off too far. I am curious if you have reduced MPH with the one engine not showing the same RPM as the other too. Are you?As high speed screw adjustments, Yamaha specs read 3/4 turns out from lightly seated (three quarters of one turn, not 3~4 turns) on cylinders 1 and 3, and 1 full turn out from lightly seated on cylinder #2. Usually when Yamaha does this, they are providing an extra measure of cooling on cylinder #2 by running it slightly richer.When you are doing your plug readings, you should be warming the engines up on one set of plugs, then shutting it down, swapping over to brand new spark plugs and immediately going to the test RPM you want to get plug readings for. You should hold it at that RPM for at least 1 minute, and the longer the better. What you don't want to do is put the new plugs in at home, go down to the dock, launch the boat, idle through a no wake zone, then run at other throttle settings until you get to the RPM you want to test. That method contaminates the spark plugs with readings from different circuits. For example, lets say you are lean on the main jet, If you were idling it for awhile to get to your riding zone, the carbs typically are really rich down low, and you may not have run it long enough at the RPM you needed to so you could get accurate plug readings, In your case, you need WOT plug readings. If you didn't follow that procedure before, you should consider re-doing the spark plug reading test too.If you could answer those questions for me, we can go from there.ThanksRSRBOB
Customer: replied 2 years ago.
I can tell that you really know what you are talking about. Unfortunately, this is a hard one to diagnose. Let me answer the questions from above and then you may have some next steps.1) Both engines use to run at the same RPM's (~7000) prior to me redoing the impellers, impeller housing ducts, and carb rebuild (problem engine side only for the carb rebuild).
2) I used SBT Mikuni carb rebuild kits which are made by SBT as far as I know. They are substantially cheaper than the Mikuni OEM kits. It would be significant work and $$ to simply redo as a way to troubleshoot this. I may have to get there
3) Compression - I am not concerned and I chalk this up to the harbor freight gauge. I did try an auto zone but not with much better results. Than I bought another Harbor freight and I am getting the same readings. I borrowed my neighbors older tool which didn't actually screw onto the plug hole and I had better readings at around 108. I do not think compression has anythign to do with the problem given the consistency across both engines.
4) I followed the plug test procedures similar to what you described above. I didn't idle on the new plugs so I think they are fine- I am wondering if this could be a spark plug wire issue. Just a question
- I guess as far as the carb goes, I could swap the carbs and see if the problem follows the engine. I really don't want to do this but it might have to be a next step. I would need to use new gaskets and that would be more $$ for troubleshooting unless you think I can get away with using the same gasket just for temporary troubleshooting. Thoughts?What would you do next?
Expert:  RSRBOB replied 2 years ago.
Thank you for the additional information.
I am glad to hear you were able to get higher compression readings with a different gauge. I still find it unusual that it isn't varying by a pound or two. Beyond that, I don't suspect compression is an issue either.
I was also not suggesting to replace the carb kits you bought with OEM, but for future reference, I would avoid aftermarket parts because you truly get what you pay for.
I am not 100% clear on one thing you said. The impellers. You re-did your pumps. Did you get new impellers? Did they come from SBT? Or did you have yours repaired/rebent? You asked what I would do next, and if you have changed impellers or had them worked on in some way and the problem showed up right after that, I would be swapping the pumps left and right and taking it out to see if that affects anything. It sounds like the one pump has a bigger bite than the other.
You did not respond to the top speed in MPH now compared to when the engines were turning about the same RPM. If you are down almost 1000 RPM I would expect a noticeable drop off in top speed.
Perhaps before you swap pumps, try this. Get out on the water and be running about trolling speed or slightly higher. Say no faster than 10 MPH or so. Now, hit the starboard engine wide open and see if it pulls 7000 RPM when you are not forcing water into it @ 55 MPH. Then do the same test for the other engine. My thinking here is that with the lower MPH there will be less resistance to the pump and perhaps allow it to turn more RPM's.
As far as swapping the carb banks, that is actually a good idea, after you exhaust the possibilities of the pump. The gasket below the carbs is actually a foam covered aluminum plate that lends itself well to being recycled a time or two. I don't forsee a problem with you using those gaskets over. I would try to leave the bottom half of the flame arrestor assembly bolted to the carbs however. That is a paper gasket and is about guaranteed to tear upon separation. If you disconnect the oil lines you should be able to leave it on no problem. It might complicate the extraction of the carbs, but it still can be done. If not, and you want to save the bucks and invest some of your time, you can cut yourself new gaskets for that out of a manilla folder. If you wanted to get real fancy you could go to the auto parts store and get some real gasket material and make them out of that. That gasket is about as close to not needed as any component on your boat, so don't sweat that too much.
Let me know.
Customer: replied 2 years ago.
1) compression - the gauge is very hard to read to the 1 degree. Compression would not seem to be the issue
2) OEM parts - got it and I will follow your advice from this point forward.
3) Top speed - I believe I have gone from a top speed of about 49 mph to 45 mph. I will do another test and report back
4) Impellers were reconditioned by SBT. You send in your old ones and they send you back a reconditioned impeller. Since I also suspected the impellers, switching the pumps/impellers was the first thing that I did. No luck
5) Switching the carb racks - I definitely do not want to remove any oil lines but I understand what you are saying. These are installed with after market oetiker clamps so that they stay on given the problems with oil lines falling off. The gaskets are not that much of an issue if I need to buy them and I learned something new about the manilla folder. I may just have to do this step towards the end of the season.Next steps:
1) what do you think about my question on the spark plug wires? Are they easy enough to swap? They go into the CDI box (?) and I am not sure if they are just something that plugs in
2) Could cavitation be the issue here? Would that limit the RPM's and any idea on how to work through diagnosing that as a cause?
3) I will try your test of the starboard engine WOT with the port engine at low RPM.
4) do you agree that it doesn't sound like it is a Lean problem since when it hits 6250 RPM, it doesn't fluctuate and that adding choke causes it to bog versus increase the RPM's?Thank you for continuing to help.
Customer: replied 2 years ago.
Please see above as well but I also wanted to add more (questions above).I swapped the tachs and I ran the boat out of the water. Both engines hit ~7500 out of water. Does that tell you anything more? Does this mean it is more of a drive issue (i.e. cavitation, impeller although probably ruled out, other??)?I also replaced the fuel/water separator filters although that is just routine and I change them yearly.I am headed out on the water now and I expect to be able to answer:
1) top speed versus previous top speed of 49/50 MPH
2) results of running starboard WOT while keeping the port engine at 2500-3KAlso, is this Q&A billed at the total of $26.00 upon completion? I want to make sure I don't go over that scope of consultation.
Expert:  RSRBOB replied 2 years ago.
Thank you for the additional information. I look forward to the latest test results.
As far as the charges, as you can well imagine they do not let us as their experts have any influence over the charges. I only get a small cut of that anyhow.
Customer: replied 2 years ago.
1) I confirmed that the boat only goes 45.5 MPH now. Prior to this problem it ran 49/50 MPH both readings using GPS
2) I tried running just the problem engine at WOT while only running the other at 2-3K. The engine still only hit 6200 RPM'sI hope to figure this out
1) When I ran the boat out of the water yesterday, both engines ran WOT at 7500 RPM. Does this tell us anything? In other words, if it was a carb problem, high speed jet adjustment, a throttle/valve adjustment, etc, would this be the case?
2) spark plug wires a possible problem?
3) next steps?
Expert:  RSRBOB replied 2 years ago.
Thank you for the additional information.
I too hope to figure this out very soon too!
Although we have discovered some new helpful information, I seem to be missing a couple of critical points.
First, I am not clear exactly what was replaced in the pumps, especially the impellers. If you have a different impeller in the problem side, it is possible the bend is at the high end of acceptable tolerances. I am wondering if it has more bite than the other prop and therefore limiting peak RPM. I have had a similar experience where I got a custom bent prop that the prop shop told me would pick up RPM and it actually dropped peak RPM. If the props are not the original, I revert to my previous suggestion of swapping the pump/prop assemblies left to right and re-testing.
The other piece of information I am missing is what did the good engine do when you were running 2-3k and it at WOT.
There is a remote possibility the carbs are the problem but again, as best I understood what you were telling me, you didn't go into the carbs until after the problem cropped up. Meaning, you went into the carbs to address the problem, going into the carbs didn't precede the problem.
Also, although you said something to the effect of your plug color test was about what I described, I am not sure what you did do that was different.
I am not trying to be difficult here, but as you can see, there are certain pieces of information that I need to have, and can't have them skewed in any way for it to be valid information for me to help you with. If the plugs are always coming out the same color after strictly WOT operation for extended periods of time, then we should be safe ruling out carburetion as a potential problem. This is where I rely on you and your assessment to form my opinion.
If you want to check ignition at WOT, use an automotive type inductive timing light and connect it to a plug wire and go run the boat at WOT and see if you have continuous spark. You will have to test one cylinder at a time but at least you can eliminate that variable.
The fact that it does rev 7500+ RPMs on the hose is encouraging. At least we don't have a twisted crankshaft. Obviously there are other things going on under operation such as the resistance the pump offers to the engine and the back pressure of the water on the exhaust when the boat is on the water.
If everything we have done is accurate so far, my gut instinct is you have a bad prop on the problem side. I don't believe it is any other component in the pump assembly because every other problem would produce cavatation and excessive RPM's with reduced thrust. That is the opposite of what you are experiencing.
Customer: replied 2 years ago.
Let me try to answer the questions that you have
1) i changed the impellers and the impeller ducts. They are reconditioned impellers. While this could be the problem, I already tried swapping them across left/right and the problem still remains (I noted this in the initial explanation above)
2) When I ran the good engine at 2-3K and the problem engine at WOT, the good engine ran consistently and didn't do anything that I would note.
3) No - the problem cropped up AFTER (not shouting, just emphacizing) the carbs were redone but the problem is that I did a lot of things all at once which was the result of what ended up being a cooling system problem (clogged exhaust assembly).
4) I don't have a timing gun but maybe after you read this you will have other ideas.Thank you
Customer: replied 2 years ago.
I wanted to provide an update but please see my prior post since that is new information that had requested.I am in the middle of swapping the carb racks with the goal of seeing whether the problem follows the carb rack. In other words if the good engine now has the same problem, RPm symptoms that tells me that the problem is the carb rack. Remember, this is the rack of carbs that I rebuilt.The problem is that the air cover gaskets (2/engine side) are torn as you said they would be. I may try to reuse what is there knowing that it is only temporary. New gaskets are on order but they will not be here until Friday. Should I go through the effort of making my own gaskets or will a few tears in the older gaskets introduce new problems (i.e. excess air leaking?)Thank you
Customer: replied 2 years ago.
I have been asked to rate the answer yet I don't have an answer yet. I sent some updates but have not heard back. Will you be responding?
Customer: replied 2 years ago.
FYI - I swapped the car racks and the problem engine continues to peak out at 6k. That tells me it is not the carbs.
Expert:  RSRBOB replied 2 years ago.
Thank you for the additional information. As I stated in a previous post, I had little hope of finding the problem in the carbs. Because you said the problem existed before you rebuilt the carbs, or more specifically that was WHY you rebuilt the carbs and the problem persisted, I was fairly certain the carbs were not your issue. As far as I can tell, what you have not done, nor answered, was EXACTLY what parts were changed and replaced when you worked on the jet pumps. My real suspicion is that the impeller for the right engine has a bit more bite in it than the left. I suggested that if you were going to swap anything left to right, it should be the pump assemblies. I also never got an answer as to why you rebuilt the pumps in the first place.I am happy to help you but by ignoring my suggestions and inquiries, there isn't much I can do for you. I understand it is easier to swap carbs than jet pumps, but if there is not any evidence to suggest it is going to help, it becomes a waste of time.If you prefer I opt out and let another expert assist you, I will gladly do that for you. But understand, he too will need you to cooperate and follow his suggestions. Asking him what to do then doing what you want probably isn't getting you any closer to resolving your issue, and in reality negates any need for the input of an expert/professional.RSRBOB
Customer: replied 2 years ago.
Sorry for any of the miscommunication but I am wondering if you actually see each of my posts or maybe you are just reading the last post since you log on.Let's see if you can look at my numbered points below and respond to each of them letting me know if this answers your questions. I have been trying to do this above but i am sure I have confused things for you.1) The timing of this RPM problem - the problem occurred at the beginning of this season and is not the direct result of work that I did last year on the 1 carb rack or the replacement of the jet pump assemblies. I did the carb work to address what turned out to be a clogged exhaust chamber causing the boat to limit RPM's to 3500. This new problem of the 1 engine began this season and I really can't associate any new work with the timing of this problem. I am sure that is the smoking gun but I just can't piece together any direct cause/effect.
2) You asked about the impellers. let me break this down and you can respond to each point.
A. The impellers were not replaced due to this engine RPM issue. They were replaced last year after I ingested a piece of wood. I figured the impellers and the impeller ducts were in bad shape so I replaced them as more of a maintenance/upgrade step.
B. Last year when I replaced the impellers, I replaced the actual impeller (refurbished from SBT), replaced the impeller ducts (pwc performance parts), the impeller housing (new from SBT and this is the type with the removable, plastic liner) and in one case, I needed a new drive shaft because they couldn't get the old impeller off without destroying the original drive shaft.
3) Carbs - I swapped these not because it was easier but because I had already swapped the pumps. While this was about 6 hours of work for me and some cuts, I thought it would be a great variable to eliminate to address some of the unknowns around fuel.
4) impeller swap - I like your idea of it being the impeller and a different bite. Don't forget, these were remanufactured by SBT so they are suspect. I was hearing from people (i.e. GroupK) that the impellers for each side are different but I called SBT and they said that for this year of boat (2001), each side is the same. In any event I had swapped them and didn't see a difference. One thing to note is that when I swap the jet pump assembly, I did not swap the impeller housing itself. The new impeller housings (last season) have a plastic ring insert instead of being made of stainless steel and I was wondering if that may be spinning inside the housing and causing issues? Again, these inserts were put in last year and were there before this new problem.
I would like to continue working with you and sorry for any problems with doing this over email. Let's use a number system like I did above so we can make sure nothing is missed. Fingers crossed that we get this resolved. Thank you
Expert:  RSRBOB replied 2 years ago.
Thank you for the additional information and clarification. Allow me to digest this and get back to you this afternoon.
Customer: replied 2 years ago.
RSRBOB, were you going to reply or did you want me to solicit another expert?