boat wont plane, low rpms

[justinbmurphy]

Hi, I'm new here and have a problem with a 1996 Silverton 31' flybridge. It is powered with twin 350's, I have completely serviced them and they run great, spin up to 4k in neutral, no missing or hesitation, new plugs, oil, filters etc. My problem is that the boat wont plane, when you come out of the hole the motors only spin up to 3k and 3,100 rpm. I have scrubbed the bottom, pulled and balanced and polished the props, drives are all the way down, tabs are up. Motors run great, no missing or shuttering, however they just wont spin up over 3k under a load and therefore the boat wont plane. Any help would be great, thanks in advance, Justin.

 

[Silvertip]

Re: boat wont plane, low rpms

Reving those engines to 4K in neutral is a good way to destroy them. Besides, an engine running on half its cylinders will rev to the moon with no load. And don't you want the tabs down for hole shot? Do you have a bilge full of water?

 

[justinbmurphy]

Re: boat wont plane, low rpms

no bilge in the water, i tried it with the tabs down, i always ran them up.

 

[studlymandingo]

Re: boat wont plane, low rpms

Is this a problem that has just begun to occur? Did the boat plane properly last season? Have you checked your ignition timing? Are the motors efi or carbureted? What is the pitch of the props? Are the engines lugging hard trying to get the 3k?<br /><br />I'm with upinsmoke...don't rev your engines unless they are under a load. When loaded, do they seem to run smoothly? Give us as much information as possible.

 

[justinbmurphy]

Re: boat wont plane, low rpms

My point with the motors is that they do spin up to their max rpm in neutral in that I don't think it is a control cable issue. Yes boat planed properly, however it hasnt been ran in over a year. I havent timed the motors, however, they seem to be in time, no missing, idle properly. carbed motors, factory props, I didnt notice the pitch. The engines dont lug at all under a load, they dont miss and they dont strain to reach 3k under power, that is why I am stumped, I ran each one out on their own (other motor in neutral) under power to see if one was lugging and causing the other to strain, they both reach about 2700 rpm when ran independantly under a load. Thanks.

 

[hustler70]

Re: boat wont plane, low rpms

your props have too much pitch you will get 100 more rpm for evry pitch you go down i recomend going to a marina and asking about the best prop

 

[studlymandingo]

Re: boat wont plane, low rpms

If it did well last year with the same props, something else is going on. Is the same gasoline in the tank from last season? You could have some varnish in the jets. Get some Sea Foam and pour in the tank per the directions.

 

[solidwaste]

Re: boat wont plane, low rpms

rilo <br />Listen to these guys I had the same problem couldnt get more then 2k with load on motor. But when it was in neutral I could rev engines to 5k no problem..Being new at this I was told never to rev engine in neutral so I dont anymore..I was then told engine would rev high rpms in nuetral someone told me to check see if I had lost a cylinder or 2..Geuss what I walked out to engine took off cowl and low and behold a plug from the right 2 cylinders was disco so I was in fact runing on 2 cylders not 4 and in neutral it ran 5k on 2 cylders..First you need to see if you have fire to all cylders than check the rest..Go to autozone pick up a spark tester it runs $4.00 than check each plug wire for fire or no fire..Bet youll find no fire on at least 2 of them if fire on all of them then you know its not a ing problem..

 

[Silvertip]

Re: boat wont plane, low rpms

Have you added some weight to the boat since it last ran correctly? Did you move weight to the stern? Think about changes you may have made other than engine related. I'm beginning to suspect old fuel as the culprit and I'll say it again -- STOP REVING THE ENGINES IN NEUTRAL. Did that get your attention? You'll blow them up at the dock. To check if throttles are opening remove the flame arrestors and with the ENGINE OFF, move the throttles wide open. The throttle plates should be vertical. The throttle plates are below the choke plates so you may have to open them manually.

 

[budsbud66]

Re: boat wont plane, low rpms

I have some expierence with SBC 350's and i agree 100% with SOLID. I think you are probebly short a cylinder. when a small block runs on 7 it is very hard to notice, but will greatly affect performance. ( I dont bieleve in bad gas causing this big of a problem unless your carbs are gunked ) Since you say its not missing, and running smooth, i doubt the carbs are screwed up, but i wouldnt rule it out.)<br /><br />I'm not fimliar with Marine 350's but why would it be detrimental to rev them a little without load?<br />I can go outside and rev my 358 to 6k RPM with no worries. ( the cam is a bit bigger but thats not the point )<br /><br />Just my 2 cents. i dont mean to rattle anyones cage..

 

[justinbmurphy]

Re: boat wont plane, low rpms

Thanks all for the help, I will check to see if all the cylinders are firing. Upinsmoke - I UNDERSTAND ABOUT REVING A MOTOR! I think everyone here would spin the motor up in neutral for an instant to see how it sounds and runs. I appreciate your help but there is no need to be condescending in your posts.

 

[Texasmark]

Re: boat wont plane, low rpms

team: I'm going to say whiplash is why you don't.....like slack in gears and things that mesh.....timing chain for one. Other thing is that engines tend to miss when not loaded and this missing can cause the whiplash that could lead to damage. Used to be real obvious on carburated 4 cycles. Run them in N and they cough and sputter. Put em in gear and they settle down real smooth.<br /><br />---------------<br />rio: What you said about only dropping 300 rpm's with only one engine running vs when both are running is interesting. You are doubling the load on the engine and only dropping the rpms by 10%. Doesn't sound like a problem with the engine to me......but obviously you have one.<br /><br />You have to back up and think thru this. You had this boat and it ran fine. You put it up for a year and now you have problems. So what could have changed in that year? With solid state ignition I am going to say that it's not the problem. The fuels of today don't (seem to) varnish up like they used to; but the violiatle part, the part that gets the octane up does evaporate......something to consider.<br /><br />Other thing is that these current fuels attack rubber; so things like vacuum hoses and all may have deterioriated.<br /><br />Other thing is critters. Did you get visited by mice or something that may have built a nest in the wrong place.<br /><br />When you mentioned running each engine separately you said they went from 3k to 2700. That says that both engines perform the same under load; heavy load. <br /><br />Both aren't going to drop cylinders simultaneously.....and you aren't going to pull that hard, evenly between the engines, if one has dropped one or more cylinders. <br /><br />Your throttles are separate, so if one was malfunctioning, like wouldn't go to full throttle, the other surely wouldn't be doing it too.<br /><br />You have to look for a common thread....what's common to both engines.....FUEL. <br /><br />Or the boat. What in the boat has changed? Cleaning the hull and all is very important but what else could be different from last year?<br /><br />HTH<br /><br />Mark

 

[budsbud66]

Re: boat wont plane, low rpms

Im not familiar with marine small blocks, but i have great interest in learning them.. <br />Does the tach read the rpms of both engines? or are there two tachs? <br /><br />Do both motors share the same coil? I cant imagine they would. <br /><br />I still am kinda locked on a sparking problem. <br /><br />like mark said thoe.. we need to know the whole truth, was it really put away running 100%, untouched. and now your having problems??????<br /><br />and, somethings fishing judging from this..<br /><br />

riloedebusk i tried it with the tabs down, i always ran them up.

when your taking of you want the tabs down to help you plane.. make sure your trim is all the way down... maybe there is nothing wrong with the motors at all, it could be a case of forgetfullness over the winter..

 

[QC]

Re: boat wont plane, low rpms

Texasmark is on this. If the data is correct, I think these are each running on all 8 holes and this is either a linkage issue, fuel issue or a boat issue.<br /><br />Are you sure you got your original props back? Are you sure they didn't add some pitch? Positive? Beyond any doubt?<br /><br />No additional weight in the boat at all compared to last year? Water, Fuel, Stuff? Contraband? Mother-in-law? <br /><br />I am not sure that you do understand about revving (I am not talking about the RPM damage debate). There is no way you are proving that the linkages allow full throttle by revving in neutral to 4000, pretty meaningless data although I don't think you are hurting them. IMHO, they would go to 6000+ and then boom if you held the throttles wide open I am not 100% positive about the 6000+, but I do know that 4000 RPM in neutral proves nothing. You can check your linkage for full throttle without them running . . .<br /><br />BTW, tabs down to plane and then adjust for max speed at cruise RPM (when you get this performance issue straight).<br /><br />team_starcraft, Independent tachs and coils, all independent except fuel . . .<br /><br />Edit: BTW, Welcome to iboats!

 

[Texasmark]

Re: boat wont plane, low rpms

Hey QC thanks.<br /><br />Mark

 

[Texasmark]

Re: boat wont plane, low rpms

Back in the game. Thought about what I said last night and I'm not sure that fuel is the issue.<br /><br />We are looking for a common thread. Rilo mentioned he cleaned the hull. Well the hull is definitely a super common thread and given the size of it, could be the determining factor.<br /><br />So my question is, how can you, Rilo, be assured that you cleaned the hull adequately to not be causing this problem?<br /><br />What did the hull look like, and feel like last year when it ran out the way you wanted it to?<br /><br />Mark