25hp two stroke performance issues.

[FISHFINK]

guys
last weekend i bought a used 1988 yamaha 25hp two stroke short shaft outboard. its mounted on a very light 14ft aluminum jon boat.
it started up on the first pull at the guys house, and every time i tried to start it. runs good while idling and revving it up on the rabbit ears.
today i took it out for its maiden voyage. it started up on the first pull, then i got underway. taking it easy easing into the throttle. once i got going, i then went to full throttle.
the front of the boat came up, then went on plane pretty quickly. although it was going much faster than my old 15hp 4 stroke motor, to me it seemed like it wasnt performing to its full potential.
i was at full throttle for about 1 minute, then had to back off the throttle because of an under pass and bridge pilings coming up.
once i backed off to about half throttle, went under the bridge, then went to full throttle, nothing happenned. it was kind of stuck at half throttle. the tiller grip was turning, but the engine was not responding. i kept backing off the throttle, and it was slowing down, then i tried to let it idle, and the motor shut off. i pulled it about 50 times and could not get it to start.
thankfully i had my trollig motor to get me back to the ramp.

once i got home i wanted to flog the carbs with power tune spray, (as it had worked for me on numerous occasions,
i checked the air mixture screws (?), and the top carb was 2.5 turns out and the bottom carb was 1.75 turns out. i took them out, along with the bowl drain screws, flogged then thought both ports, put it back together with both at 2 turns out, and got it to start.... obviously it was smoking from the power tune spray, but once i got that cleared it would start on the first pull... i then flogged the carbs again with power tune and am letting it set for a few days.
i hope to test it again next weekend when i have time.

do any of you have any ideas as to what might be my problem ? is there different passage ways in the carbs for idle and full throttle, like an automotive carb ?
and what is the correct adjustment on the mixture screws ? i have them now at 2 turns out.
hopefully its something simple, and if i have to have the carbs rebuilt, hopefully that would be all that is wrong with it.
the previous owner said it had been setting in his garage for a while, and he always drained the carbs before setting for storage.

thanks for nay help you can offer.

 

[orbanp]

Re: 25hp two stroke performance issues.

Any chance that the motor overheated?
Check the compression.

Peter

 

[FISHFINK]

Re: 25hp two stroke performance issues.

thanks peter...
no, i doubt it overheated today, since it pumps water pretty darn good. i didnt think to check compression... i will have to dig up my old tester and will report numbers tomorrow.
thanks again.

 

[FISHFINK]

Re: 25hp two stroke performance issues.

ok guys,
i finally did the compression test. once cylinder, the top cylinder, was 150psi after 5 pulls of the cord, and the bottom cylinder was 135psi, with the same 5 pulls of the cord. i did the test with the other plug installed. hopefully thats the way i was suppose to do it ?
i think i read somewhere on here that the cylinders should be within 10% of each other ? if so i should be fine there, correct ?

hopefully tomorrow after work i will start it up and try pulling one plug wire at a time, to make sure each cylinder is firing.

anything else i should try while its running here at the house ? if everything seems fine, i am hoping to test it in the water again this saturday after work. im hoping that it setting for a few days with the power tune spray in the carbs will clean them up, if thats what the problem was....

oh, how many turns out from bottom should i have the air mixture screws set it ?

thanks much guys.

 

[FISHFINK]

Re: 25hp two stroke performance issues.

oh, since the compression seems to be good, does this eliminate bad reeds ?
if reeds were bad, would that make a bad compression reading ?
thanks much... i will report how it runs tomorrow after pulling one wire at a time while running...

 

[orbanp]

Re: 25hp two stroke performance issues.

Hi Fishfink,

The compression values look excellent, no worry there.
BTW, you check compression with the other plug taken out, ignitions grounded, throttle at WOT... Still your numbers are good.

You could have problems with reed valves, irrespective of compression, but then you would have the engine running poorly all the time and not intermittent.

You should check out your fuel system. When were the rubber parts replaced last? Gas with ethanol is not easy on rubber parts, especially on old rubber that was not meant to tolerate ethanol.
You could also fully take apart both carbs and clean/rebuild them, it is not clear how much dis-assembly you did during cleaning. Check the diaphragm and check valves of the fuel pump on the lower carb. Might as well get carb rebuild kits.

For setting the carbs a factory manual would help. If you do not have one, proceed as follows:
Check if the throttle plates are in sync, if they do open together, close together. Adjust the linkages to sync them.

From the carb parts diagram the needle is to adjust air. It should be the idle air adjustment. http://www.**********/parts/search/Yamaha/Outboard/1988/25SG/CARBURETOR/parts.html

Start your engine and warm it up, on your boat. At idle (in gear) adjust first one needle. Find the setting where the engine run smooths out. Close a bit the needle (this regulates air, this will make the mixture a bit richer). Do the same with the other carb. Check the idle speed, adjust that if necessary. Go through the cycle again.

This is in principle how you should adjust your carb.

Any chance of loosing ignition as the engine and ignition components warm up?

Good luck, Peter

 

[99yam40]

Re: 25hp two stroke performance issues.

Need to pick up a service manual and set to factory settings

Not real sure on that old of motor but the pilot fuel jet and pilot air jet are fixed and sized for motor, the pilot screw adjustment is what allows more or less of that mix into motor,
so out lets more in and in lets less in.
Factory spec work well when carb is cleaned properly

 

[FISHFINK]

Re: 25hp two stroke performance issues.

thanks so much for the info and help, peter, and 99yam40.....
glad to hear my compression values are good... even though i didnt do it with the other plug out.
also glad to hear that my reeds are probably good, since it runs so good at idle.... good news.

i also am suspecting the fuel system. the guy i bought it from two weeks ago said it had been setting in his garage for some time.
i have not taken the carbs apart, but have sprayed a bunch of the "mariner power tune" spray in the carbs through the air mixture port and through the bowl drain.... that spray has worked wonders for me in the past, on my 15hp 4 stoke engine and my lawn mower and generator.

i fired it up tonight after work and its running perfectly... half a pull with my bad arm and it starts. seems to be running smoother since the power tune spray, but the real test will be on saturday when i take it out again for a test run. although i am very good mechanically, if it still gives me problems under full power, i may take it to my mechanic. i have never taken a boat carb apart before...

and while it was running at idle, i pulled each plug wire off one at a time and it did effect the running, so both cylinders are firing.

not really sure if the ignition system is failing while running, but i do know it was only under full throttle for less than one minute... i dint think it had enough time to get hot.

i do have a repair manual on order, but it hasnt arrived yet....

my brother told me to get some "startron" fuel additive, he swears by it. says it treats fuel and cleans carbs also ?
i will try it also...

hoe difficult is it to rebuild the carbs on my own ? again, i havent done this type of repair before, but have taken automotive carbs, holley and edelbrock apart and rebuilt them years back...

again, thanks so much for the input guys... and i will let you knwo how it runs when i take it out on saturday after work.

 

[orbanp]

Re: 25hp two stroke performance issues.

Hi Fishfink,

You should take apart your carbs and clean them, no amount of additive and other miracle cleaner will get rid of the dirt in them! I would do it before I took out the boat again. Probably chunks of dirt is bouncing around and is blocking intermittently your carb.
If you have worked on cars and carbs before no need to take the engine to a mechanic, this is no different, it is well within the capabilities of a "home mechanic"!

Good luck, Peter

 

[FISHFINK]

Re: 25hp two stroke performance issues.

ok guys,
took it out for a test run yesterday after work, and does the same thing. starts right up on the first pull. give it some throttle and it runs good for about 15 seconds, then starts slowly slowing down, no throttle response. once i got it home i noticed when i pump the primer ball, there is gas coming out of the bottom carburetor. has to be a stuck float, or needle pin, something... i assume. i aint messing with it anymore, dont have the time, so its off to the mechanic hopefully by weeks end..... hoping it only a carb rebuild..
thanks

 

[FISHFINK]

Re: 25hp two stroke performance issues.

ok guys....
took it to the mechanic, he rebuilt the carbs, and WOW.... she runs perfect now.... clocked it at 31mph on my smart phone GPS.
thanks for all the help with this matter...