Boat doesn't plane

[SilverSS07]

Hey guys. I have a 1975 Bonito 15' tri hull with a 1973 50hp Mercury Thunderbolt 500. I'm new to working on outboards but not new to working on engines . I took her out for the first time a few weeks ago and it would bog and die at anything over idle so I rebuilt the carbs and ran all new fuel line. Also put in a new set of plugs. Took her out again yesterday to adjust idle mix and she'll plow now but when I give it more throttle to attempt to get up on plane it revs up some for a few seconds and raises the front end some then starts to sound like it's bogging down. Sounds like it's flooding to me. If I don't pull back on the throttle it will die after maybe 5 seconds. Sorry I don't have a tach yet but I would guess it's revving up to around 2500-3000 rpm when attempting to accelerate; no where near peak RPM. I replaced the prop on the boat with a new one that had the same pitch- 13. Checked spark today with a timing light and it has spark on all cylinders. Ran a compression test also and it came back 110, 110, 105, 100. The next thing I plan on doing is rebuilding the fuel pump. It has 2 6 gal plastic day tanks that have fresh gas in them mixed 50:1 and the vent is open. Also she revs up fine in neutral and starts instantly cold or hot and runs fine until you try to get up on plane. Thanks for the help!

 

[SilverSS07]

Re: Boat doesn't plane

Got done with rebuilding the fuel pump tonight. Hopefully will have time to take her out this Wednesday and see if that helped.

 

[Texasmark]

Re: Boat doesn't plane

Just because it dies when coming up on plane and it recovers when you cut the throttle, you are just taking the load off the engine. I think you are starving for fuel. Assume you got all the jets cleaned out when you did your carb kits. After you fix the pump and go back out, you might squeeze the bulb when it loads up and see if it recovers, if it loads up again.

Then moving to another arena, if you got the same pitch/style prop then that shouldn't be the problem.

Moving on to the boat, does it feel like it's heavy; when moving it around by hand, maneuvering around at slow speeds? Any clues that it may have sit out in the weather, become filled with rain and possibly water logged the flotation?

What do you have in the boat when you go out? Just you and basic necessities?

Mark

 

[SilverSS07]

Re: Boat doesn't plane

Thanks for the reply. I soaked the carbs in carb cleaner for a few days while waiting to receive the rebuild kits. The "rebuild kits" only included gaskets though lol. Jets and needles appeared to be in good shape. Floats were good also. Used a digital caliper to check float levels, etc. Blew them out with compressed air also. The boat doesn't seem to be heavy at all. No indication that it has waterlogged foam. According to the previous owner it was redecked a few years ago and everything underneath was in great shape. I can tell the floor and transom are solid but havne't seen it. When I went out there were 3 of us- maybe 400 lbs. I tried dropping them off at the dock and going by myself but the boat was the same.

 

[SilverSS07]

Re: Boat doesn't plane

I read somewhere on here that I should be able to pump up the primer bulb and it should stay hard for several hours without starting the engine. I pumped mine up and checked it an hour later and it was soft again. Got a new bulb so going to replace that as soon as it stops raining. I also read that it was better to have the bulb closer to the motor so going to try moving it also.

 

[SilverSS07]

Re: Boat doesn't plane

Finally got to take here out today but No luck. Got a new mercury bulb also. Squeezing the bulb didn't help when trying to accelerate. Also Tried hitting the choke but that didn't help either. Engine seemed to be surging at idle and didn't run very smoothly. After running a while the idle tomorrow would decrease to where it barely will stay running. Whatever it is it seems to be getting worse. Going to start checking the electric side of things I guess.

 

[mr 88]

Re: Boat doesn't plane

I would buy a manual. Does your distributor advance as well as your carbs opening all the way when moving the throttle? There is a link n sync procedure and you have had the carbs off. So my guess would be the linkage is out of sync between carb and distributor or something like a throttle cam was not installed correctly where it meets the distributor.As far as the idle goes where are the mixture screws set ? You may be flooding it at idle and fouling the plugs in the process.

 

[SilverSS07]

Re: Boat doesn't plane

I have a seloc manual for it. The distributor does advance and the throttle blades are both completely open. I've been playing with the mixture screws to see if it makes a difference. Been running them between 1-1.5 turns out. I'll check the linkage. I know I put it back in like I took it out bit it may not have been put in correctly in the first place lol. I'm also going check the timing and ohm the coil and sync the carbs and see if it helps.

 

[SilverSS07]

Re: Boat doesn't plane

Well checked coil and linkage and sync'd carbs and no change. A friend of mine told me about a retired marine mechanic he knew that worked from his house so I took the boat over there a few days ago. Picked it up today. He told me that he checked everything out on the engine (stator, timing, spark, fuel, etc.) and that it was bogging because cyl 3 had low compression of 90 psi. Told me I need to rebuild the powerhead. I asked him why my #'s were fine and he said it was because he did a compression test "under a load" which he stated was with the LU submerged in a tank. When I did mine I had it on the ears. Does this sound right to you guys? I got 105 on my gauge on cyl 3. I might check the compression on it again tomorrow with it submerged.

 

[webrx]

Re: Boat doesn't plane

I usually do my compression check in Neutral, so not sure how being submerged is gonna change that unless he is doing it in gear. 15 lbs out between cylinders could mean you have an issue, and it sure sounds like you have a cylinder not firing. Have you tried pulling wires one at a time while the enginer is runniing to see if you have a cylinder that is not responsive? it could be a bad plug, bad wire, bad coil, etc - not sure if yours has the 4 coils like my 76 does or not. The one motor I had the hardest time with was one with a bad coil that fired fine at idle, but under load it failed. the only way I found it was to replace it and it solved the problem. Similar scenario to you, idle ok, but not great, and bogged/died under load.

Dave

 

[SilverSS07]

Re: Boat doesn't plane

Thanks for the suggestions. Mine only has 1 coil. Unfortunately all I have is a multimeter and a spark tester but it isn't adjustable. I don't have a DVA yet either. Anyway- I decided to seafoam the old girl and run another compression test today. I also borrowed a friend of mine's gauge to compare against mine; although his gauge is about 20 years old. On my gauge I got 110#'s on all 4 cylinders. On my friend's gauge I got 125#'s on the top 3 cylinders; I couldn't fit it on cyl 4 with the adapter on. Just for the heck of it I put it in gear and spun her over also as this "mechanic" claimed to have done and still got 110#'s on all cylinders. Pretty sure the guy was full of it. Probably just let my boat sit there a few days then told me I needed a powerhead. Anyway- since I had her sitting in a tub of water and all my tools readily available I turned the idle mix screw down to 3/4 turn out from seated. RPM increased and the engine idled much smoother. Would go into gear and not die. Also I could hammer down on the throttle with it in the tub and it would rev with no problem. Could only do this for a second though because it would blow the water out of the tub. Would it be ok to leave the idle mix that lean? What could cause it to run better leaned out?

 

[mr 88]

Re: Boat doesn't plane

Compression is compression and readings should be the same with or without load. Not a great idea to rev it to the moon without a load on the prop. You can take the prop off and the water will stay in the tub,load free of course which is not the same as being out on the water.The fuel air mixture is different and most two strokes will run better leaned out,to a point,then it's a fried piston.Try dunking it and see how it accelerates,if it stumbles you may have to richen up the idle jet.

 

[CharlieB]

Re: Boat doesn't plane

There can be a wide variance in quality of compression gauges and the readings each can have when used on the same motor.

Generally, if the motor will fire and run, compression is not the major issue. even when down to 95 psi, a motor can still make reasonable power, not as much as originally, but the boat would still get on plane, just not quite the top speed as original.

Idle mixture can contribute to difficulty getting up to speed. Two strokes do not have an accelerator pump, the idle mixture must be sufficiently over rich the provide the additional fuel for acceleration. Once up to speed, the motor is only drawing fuel thru the main jet so the idle no longer passes fuel into the motor.

Idle mixture must be perfected IN the water idling IN gear. This provide the correct exhaust back-pressure and load on the motor.

Adjust each idle mixture screw so as to achieve the 'Best Idle Speed'. This is the starting point, from here you will need anywhere from 1/8 to 1/4 turn MORE fuel. Start at 1/8 more, then test acceleration IN gear by very rapidly opening the throttle to WOT.

Any lean cough, miss, bog, or hesitation indicates a need for MORE fuel. Open each idle mixture screw just the width of the screwdriver blade slot then retest acceleration. Repeat to perfection.

You do not want to run an excessively over rich idle as it can load up the spark plugs and cause fouling and misfiring on acceleration. If the the motor seems to accelerate fine with the idle mix set at just 1/8 turn more than Best Idle speed, then turn each screw IN just a blade width and retest acceleration until acceleration 'Just' begins to be less than stellar, then reopen each that last bladewidth, that near perfect idle setting.

Once done with idle mix, and you have already synch'd the carbs, that leaves ignition quality.

Verifiy the intensity of spark using an adjustable gap spark tester set to 7/16 inch. If you cannot get a hot blue arc then your coil is getting weak or the plug wires are leaking energy and not delivering enough to adequately fire all plugs under full load.

A weak ignition can start and idle fine, rev to the moon in neutral, yet fail to make much power under load. As load increases, intake vacuum drops and cylinder pressure rises. This requires the ignition to draw maximum voltage in order to arc across the increased resistance of more air squeezed into the spark plug gap. It is CRITICAL that the ignition can arc a hot blue ZAP over a 7/16 inch gap in free air. Anything less and the motor will only pull up to the limit of the ignition voltage.

 

[SilverSS07]

Re: Boat doesn't plane

Took her out again yesterday. She would idle great, start right up, and plow water fine. When given throttle tomorrow would go up to Maybe around 2500 and No higher. Then after Maybe 5 seconds it would start to bog and die if I didn't pull back on the throttle. When pulling the throttle
back once it as Maybe 1/3 from idle speed engine would rev up a little and smooth right back out. I've got a spark tester and tach
ordered.

 

[SilverSS07]

Re: Boat doesn't plane

They're taking forever to get here! Guess I shouldn't have gotten super saver shipping lol. Just a quick update- I borrowed a friend of mine's Snap On Compression gauge and came back 125#'s on cyl's 1,2,4 and 123#'s on cyl 3. She's a pretty healthy old girl! Now I know my great Harbor Freight comp gauge reads about 15#'s low.

 

[Maxz695]

Re: Boat doesn't plane

Bump- Did it plane for you recently? Have you had this boat for some time. A tri hull sounds heavy and that may be overproped. I was thinking cylinders and rings wear woren out but it could be as simple as using a smaller prop say and 11 Pitch but no more than a 12 pitch.

 

[SilverSS07]

Re: Boat doesn't plane

No it has never planed. I bought it a few months ago as somewhat of a fixer upper. Got everything straightened out on it except for this. When I got the prop I told the guy I wanted the same thing that it had and took the old one with me. The prop that was on there was frozen on and then some. It was the older style of prop that didn't have the removable hub. If it was overpropped when I tried to get up on plane would it still start to bog down after a few seconds?

 

[mr 88]

Re: Boat doesn't plane

Okay A 1973 hull that has a subfloor with foam in the middle is probably waterlogged at this point. A trip to the scales may be in order,you may be carrying more of a load than you think you have. I would think a 500/15'/glass hull and trailer would weigh in around 900-1000 lbs.If your over propped the motor will not rev up to its' rpm range,just gonna lug along and make that engine work.Under propped it will jump up on a plane very quickly then start to over rev at wot .I would guess that if the motor was on there so long that the prop was frozen on ,that at some point it was the right pitch for the boat.Hull and or engine,start with the hull.

 

[Maxz695]

Re: Boat doesn't plane

I have a V hull on my bass boat and with the 12 Pitch it got up on plae fairly well but not imediately. With the 13 pitch it took alot longer. Because you have the Tri hull you have alot more boat in the water that needs power to get it up there. True the hull may be wet inside causing even more of a problem. I would suggest trying an 11 Pitch Prop. Also if you have more than one person on the boat and alot of gear cooler etc. It compounds the stress on the engine. and it will never get up on plane. Also make sure all culinders are firing. If you are getting water in the engine then the bottom cylinder is not firing all the time if at all. This could happen with a bad lower seal or through the exhaust divider plate and gaskets

 

[SilverSS07]

Re: Boat doesn't plane

Well I got my new tools but haven't had a chance to take her out or test the spark. I know all cylinders have spark going to them. I'm going to see if the place I bought the prop will swap it out for an 11p. Usually when I go I have woman and 2 kids so I would rather give up some top end because I know normally it will have more weight in it than just myself. If it is due to the prop though would the engine start to cut out and die like it does? Or would it just rev up some and stay there but just never plane? It starts to bog down after about 5 seconds when trying to accelerate then will die unless you pull back on the throttle. I think there is something else going on than it just being overpropped although that may contribute to the problem as well.