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very low RPMs 8HP yamaha

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  • very low RPMs 8HP yamaha

    Hi. I have an 8hp 2-stroke yamaha with 8.5 x 7.5 prop on an inflatable dinghy. I don't have a tachometer handy but there is very little difference in RPMs between half throttle and WOT when the boat is fully loaded. When it is just me with little gear, it moves fine and I can hear the engine achieving high RPMs, but add another person and the usual gear and the engine can't rev up much at all and we snail away.

    If I put an 8.5 x 7" prop, will it make a difference? Online estimates say 200rpm per inch of pitch, so that would be 100rpm, which is negligible. I didn't see any other prop options for this motor. Thanks for your help.

  • #2
    start with a compression test

    make sure your motor is running on both cylinders
    Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - http://forums.iboats.com/forum/owner...988-rogue-2420

    Past Boats
    1970 Wooster Hellion - Merc 9.8
    2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
    1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

    What’s behind you doesn’t matter.Enzo Ferrari


    • #3
      Thanks Scott. I have only used the engine a few times since I got it, we did a compression test back then with the seller but I'll do it myself tonight to make sure.

      When you say to make sure it is running on both cylinders are you still referring to the compression or something else like spark plugs?


      • #4
        Also worth mentioning. The engine idles great after starting up, but after running if I let off the throttle too fast it shuts off (just about anything is too fast). I have to VERY gradually bring the throttle down for it to not stall out.


        • #5
          Originally posted by cyborgkiter View Post
          Also worth mentioning. The engine idles great after starting up, but after running if I let off the throttle too fast it shuts off (just about anything is too fast). I have to VERY gradually bring the throttle down for it to not stall out.
          That sounds like you need to clean the carb. You probably have two other issues. One is your hull is a power hog and you can't get the speed high enough to let the motor sing. The other is the Yamaha. The small ones are finicky and underpowered. I had a plastic boat with a Yamaha 4 and even with prop work and tuning the motor to perfection it would not move that hull. What is your boat specifically and what is the max rated HP? The other stalling issue and poor speed is that it is only runnin on one cylinder. They start run and rev fine on one but with half the power. Pull one plug wire at a time and make sure it's firing on both cylinders. And yes that 7" prop will only help but not much. I did that on my 4 and it was a waste because it was so underpowered the few extra RPM were not enough.
          Last edited by Stumpalump; December 6th, 2017, 11:10 AM.


          • #6
            Its the boat at the beginning of the video below. 16ft inflatable flat bottom kaboat. 16ft sounds big but its very narrow. Weighs less than a tandem hobbie kayak. Kiting gear isn't that heavy either, and I weigh around 170 and my friend less. Small cooler and the boating/safety gear. Will check on max HP on the boat but I remember it being 8 or 10HP on the placard, although the new one (same exact boat) says 15HP 2-stroke max on the website.



            I'll check the plugs tonight too.


            • #7
              Compression on lower cylinder is ~92psi and upper cylinder is ~95psi. The spark plugs had some black gunk on them as shown in the picture and I can see more crud down the spark plug holes. Can't test if it is running on one cylinder since I don't have a place to start it up tonight. Will try tomorrow. The gunk on the spark plugs is sludgy. Any thoughts?
              Last edited by cyborgkiter; December 6th, 2017, 07:06 PM.


              • #8
                Cranking it with spark plugs in hand produces spark on both plugs and coils.

                Also, just had the engine serviced before using it for the first time since it was sitting for a good 6 months after I bought it, mechanic said he cleaned the carb and changed the sparkplugs. Sparkplugs look new. That gunk is from about 4 hours of run time.
                Last edited by cyborgkiter; December 6th, 2017, 07:35 PM.


                • #9
                  If you are new to inflatable boating, there are some issues you should know about or be aware of :

                  An inflatable needs to work inflated to it's max working pressure range which is 3.0 PSI min and 3.5 psi max, only readable with a pressure gauge, so buy one as a starter.

                  If an 8 HP is not max rated for a large inflatable, you need to go boating with way less passengers/extra load with a healthy engine and,correctly propped through a prop maximization towards a less pitch prop.

                  To get the max out of that portable engine need to buy a good quality induction tach, would recommend a Hardline hour/tach, test current prop as usually loaded and check max wot revs achieved on flat calm water with engine trimmed to 90 deg, Ideal is engine revving towards middle to max wot range, although max is preffered. That engine should rev around 4500 to 5500 rpm.

                  Don't know about Yams, on other engine brands going one less pitch will pull wot revs 400-600 or way more revs on an inflatable or light boat. If happens that you're running an 16 footer inflatable probably a 8 HP won't make it to plane, but will achieve a faster fast displacement speed which is good compared to be lugging the engine and don't know anything about it.

                  Report your max wot rpm range, will work from there..

                  Happy Boating
                  Last edited by Sea Rider; December 7th, 2017, 07:48 AM.

                  Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers


                  • #10
                    Thanks! Tach is on its way. Hope I get it for this weekend. I did use about 3 gallons of old gas in it so that may have clogged up something or caused the gunky deposits. I am planning on pouring in 1/4 - 1/2 a can of sea foam in the full 3gal tank that has fresh gas from last weekend and running it as usual. Then maybe out on the water put some in the intake. The pitch would only go down 1/2 an inch if I downgrade from a 7.5 to a 7. Havent seen anything with a lower pitch for this engine.


                    • #11
                      Compression on lower cylinder is ~92psi and upper cylinder is ~95psi.
                      Ayuh,.... Welcome Aboard,..... Yer motor is worn out,....

                      Normal compression for as gas motor is in the 150 psi to 170 psi,.....
                      A worn motor from above 100 psi to 150 psi,....
                      Under 100 psi, it's a rebuildable core motor, not a runner,....


                      • #12
                        Looking at the pics, plugs are completely fouled, internal cylinder looks carboned up, pouring Sea Foam into tank will take to long to see near inmediate results if that's what you're aiming for. Drill a 2-3 mm diam hole right through carb plastic silencer. Buy a can of Power Tuner, Carbon Tune Up, on muffs and with engine ON sufficiently warme up and geared forward spray small shots of product, engine will tend to shut off, once revs recuperates repeat the procedure several times. Engine must be revving evenly between 2 to 2.5 K revs between shots. Give a last overdose product blast till engine dies. Watch spinning prop while product is being applied.

                        Let product sit and do its homework right for some hours, install new well gapped spark plugs, go for a solo wot spin as if stealing the combo for some minutes to burn and clear product out. Afterwords can pour sea foam on tank and use it regularly if plan doing so, This product is good for new engines or the ones that usues it on constant basis, takes to long to remove carbon buil ups on ones that never have used it before and are already carboned up.

                        Check Yam compression tech specs for that particular engine to know if can still work for some more time or need a new set of piston rings, new pistons and rings, whichever. Make a compression test after product application, compression should have rised a bit and engine to run much smoother.

                        As don't know which is the max wot rpm range achieved as loaded, with a tach and healthy running engine condition will certainly know, only then can go for a prop maximization, not before. Will check inflatable specs to have an idea and get the max out of that combo.

                        Happy Boating
                        Last edited by Sea Rider; December 7th, 2017, 08:39 AM.

                        Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers


                        • #13
                          Does the power tuner smoke up a lot? I don't wanna be mr. fog machine at the marina like the seafoam intake cleaner videos show.

                          Can I pour seafoam into the cylinder through the spark plug hole and let it sit? Turn over a few times to get it covering everything in the cylinders.

                          next steps I guess: Install tach on saturday. Take boat out with 1/2 can of sea foam in the 3gal gas. once out, do the process of spraying power tuner in the carb intake and letting it sit while I go kite . Check compression back at shore for any improvements.

                          No luck finding compression specs for the 8HP 2-stroke yamaha.


                          • #14
                            Yep, those carbon removers will smoke worse than an asian poker gang smoking in a close room, but do their homework right. If smoking is an issue at the marina, perform that described process at home, not on a barrel, must use muffs for engine to rev geared. Spraying product through spak plugs holes won't to a thing, waste of time, nice carbon build up removal its done in small steps as described if wanting the job done fast.

                            When installing tach you must adjust spark firing order for tach to read correct wot rpm, just follow written instruction and will be fine. Researching that 16 footer inflatable, it's for a 15 HP-OB, one down issue, has an inflatable air deck which needs to be inflated rock solid for inflatable to glide properly over water level, if not expect a marshmellow ride with poor water performance along excessive hull drag.

                            Happy Boating
                            Last edited by Sea Rider; December 7th, 2017, 03:15 PM.

                            Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers


                            • #15
                              Sign up today
                              Looks like I might need a bigger engine.

                              Weekend test:

                              Prop: 8.5 x 7.5
                              Test Load: 400lb (300 2 people plus ~100 motor and safety gear)

                              Max speed: ~11-12.5mph

                              1 can of seafoam in 3gal of gas cured issue of letting off the gas and the engine dying. Idles nicely now. I think I was going significantly faster but we went out with minimal gear (just boating safety stuff and some water). I didn't spray the intake with cleaner yet.

                              WOT RPMs (manufacturer range: 4500-5500):
                              4700-4800 with trim pin at the lowest position
                              4400-4500 with trim pin at one up
                              Need to test without the pin maybe (is that ok?)

                              Idle: 1450 rpms (manufacturer specs 900 +/- 50)
                              need to turn this down.

                              This was a test run, I usually never go out without gear. Next test with usual load:
                              Expected load: 500lbs (2 people plus usual gear)
                              Expected RPMs: under 4K
                              Last edited by cyborgkiter; December 18th, 2017, 11:39 AM.