Yamaha 40hp Lacking Power

[swall]

I have a 1990 yamaha 40hp that lacks power and dies when cruising. boat is struggling to plain. I have had it to the shop and they have replaced. CDI control unit, fuel pump, cleaned all carbies checked compressions all good swapped fuel tank replaced fuel line and bubble. The boat shop have told me that the boat has done a seal in crankcase(?) and it is not creating enough vacuum to pull fuel into the top 2 carbs. is this possible. they have quoted $4000.00 for the repair. spray fuel into the top 2 carbs and the boat performs as it it should, also hit the choke while motoring along and boat comes good as well. Any help greatly appreciated, I have read lots of forumns and can't find any that have done this seal.

 

[JustJason]

Re: Yamaha 40hp Lacking Power

it's possible you could have blown the top seal under the flywheel, but it's certainly no 4000.00 fix.

Start with the basics....
What are your compression numbers?
Do you have good spark on a Steven's (or eqiv) spark board?
Have you verified, with a light, your base and advance timing?
Have you changed the fuel filters?

Best bet is to find a new shop.

 

[swall]

Re: Yamaha 40hp Lacking Power

Thanks for the reply JustJason.

the compression numbers unsure but have been told on 3 seperate occasions that they are excellent.

timing i have checked and advance timing all checks out good.

where would i obtain a steven's spark board or are you talking a tune scope?

have looked but doesn't seem to have any fuel filter fitted?

Where do I go next?

 

[turn2]

Re: Yamaha 40hp Lacking Power

With the motor running ,put your hand over the carbs to stop all air intake. One at a time if both are sucking hard to draw air you have no vacuum problem. If the problem [carb or cylinder] has no draw at all ck the reed valves behind the carb. If it is sucking ck for trash in the problem carb. will not always be visible. I am assuming this is 2 stroke. A new power head installed would not cost 4000.00. I agree with finding a new shop.

 

[swall]

Re: Yamaha 40hp Lacking Power

Thanks Turn2. I will give it a go and post back.

 

[b2theill]

Re: Yamaha 40hp Lacking Power

usually when i see some type of case leak on a 2-stroke i see a significant running issue at low speeds. from what i understand you are having off idle running problem, notable at 4000 rpm. you have not had a recent prop change have you? and second, if you are having a power loss off idle into mid to high range rpm drop your gear case and confirm you are not getting sometype of exhaust restriction i.e. carbon, melted tunner. being a older two stroke i would not be suprised to see the motor having high carbon deposits in exhaust chambers and tuner.
just an idea for you to try. good luck

 

[swall]

Re: Yamaha 40hp Lacking Power

Thanks b2theill.

is definetly higher rpms idles and runs fine at lower rpms. have not changed the prop at all still original. I will drop the gear case and have a look at the exhaust.

Thanks every one for their help.

BTW definately 2 stroke, 3cyl 40hp, 1990 model done very low hours, just to confirm

 

[99yam40]

Re: Yamaha 40hp Lacking Power

Sounds like a carb problem to me or a fuel delivery problem.
If you have good compression,good spark, and at the right time(timing checks OK), then it has to be fuel/air ratio.
You can check fuel delivery with a pressure/vacuum gauge.
Timing with a timing light at different RPM.
Spark with a spark board.
To clean carbs properly they need to be completely disassembled and soaked or use a ultrasonic bath and then brow out all passages with compressed air. Reassemble and adjust all linkages and adjust carbs as the manual calls for.

 

[swall]

Re: Yamaha 40hp Lacking Power

Thanks 99yam40,

Am working on the boat today so will keep every one posted on what i find. hoping just something simple but we will see.

 

[swall]

Re: Yamaha 40hp Lacking Power

Just am update.

first disconected carbies so as could operate them individually 1 and 3 operate and rev engine 2 flat spots and does nothing.
i then proceeded to strip the carbs and found quite alot of water and rubish in them especially 2. reassembled will carry out decarb on engine and water test tomorrow. Hopefully thats all it is..

am now going to reclean and recheck all the fuel system after the shop had had it for 8 weeks and suposingly did all this???????

Whats the best way to clean out a fuel tank?

hopefully the boat will be all operational tomorrow....

 

[turn2]

Re: Yamaha 40hp Lacking Power

drain well rinse with alcohol then gas. Leave the lid off and turn tank upside down somewhere dry and let it evaporate dry or use a dryer. Use a glass jar or something clear to catch some of the new gas , collect it from the main hose that hooks directly in to the carb . The jar will show any trash or water. Dont hook the hose until you get clean gas at the carb.

 

[99yam40]

Re: Yamaha 40hp Lacking Power

Don't forget to clean out the fuel pump and lines. Trash and water trapped in them will go directly into the carbs and cause you to pull them again and clean.
Fuel/water separator type filters are worth every penny you spend on them by cleaning up the fuel to carbs.
What kind of tank do you have? Built in or portable? Built in tanks are a pain to clean.
Don't forget to do the sync and link procedure for adjusting everything once you reinstall the carbs

 

[divinginn]

Re: Yamaha 40hp Lacking Power

There is a jet underneath a rubber plug in the carb,pull the plug out and unscrew the jet and blast with carb cleaner and poke with a small wire to clean it. I would do all 3 carbs,I went through the same thing with my 88 40 Yamaha last summer.

 

[swall]

Re: Yamaha 40hp Lacking Power

once again thankyou to all. finished off the carb re bild today after let soak over night. seems to run ok and all carbs seem to be working now...
just wandering where i would find the sync and link instructions for these carbies and also the adjustment specs. (i have a manual but seems to be useless when it comes to adjustments etc) the link rod between the carbs broke on reassembly so have to get one of them tomorrow. have cleaned the fuel tank and got clean gas coming out of the line now. removed fuel pump and drained and cleaned the supply filter as per manual hoping that's all it is after 8 weeks.

clened the jets nder the black caps as well.

won't get a chance to rn it now ntil next week end so hoping all is well.

i want to catch some fish!!!!!!!

 

[99yam40]

Re: Yamaha 40hp Lacking Power

read this thread to see if it will help. with your motor being 1990 I am not sure how the link and sync would go, but all the carbs need to be at the same position and opening the same amount to make sure all cylinders are running the same. My C40TLRX 1999 motor is 1 1/2 +/- 1/4 turns out from lightly seated

http://forums.iboats.com/showthread.php?t=366637

 

[swall]

Re: Yamaha 40hp Lacking Power

Hi All,

Just updating this thread. re assembled carbs cleaned fuel pump and fuel line and tank. now seems to rev on all cylinders on loaded so will have to put it in the bay and give it a run loaded.

Another question on no. 2 cylinder at idle seems to blow a puff of smoke hesitate then start runing again. any ideas.

wold a decarb help this engine. it is almost 20 years old and wold be surprised if it has ever been done..

it seems to rev up to abot 1300rpm fine don't want to go any higher with no load.

thank you again for all the help.. will post results after water test on weekend.

 

[99yam40]

Re: Yamaha 40hp Lacking Power

how can you tell that cylinder #2 is giving the problem you talked about and not the other ones?

"Another question on no. 2 cylinder at idle seems to blow a puff of smoke hesitate then start runing again. any ideas."


A decarb is a good idea in my opinion.

Did you pick up a manual that gives you the info you needed for adjusting everything?

 

[swall]

Re: Yamaha 40hp Lacking Power

how can you tell that cylinder #2 is giving the problem you talked about and not the other ones?

"Another question on no. 2 cylinder at idle seems to blow a puff of smoke hesitate then start runing again. any ideas."


A decarb is a good idea in my opinion.

Did you pick up a manual that gives you the info you needed for adjusting everything?

Hi 99Yam40, thinking about it there is no way of telling that it is definetly no.2 just seems when you pick up the revs just off idle blows a puff of smoke and then runs like normal again. (Carb 2)

i have set the mixtures and synced the carbies as per the manual relating to my engine. but original settings on the pilot screws seemed to be wound out alot more than the specification.

i will carry out the water test and decarb on the week end and go from there. seems to pick up revs alot easier than what it was before the carb overhaul.

Again thanks, Am fairly new to outboard motors but am a motor mechanic by trade.

 

[99yam40]

Re: Yamaha 40hp Lacking Power

"i have set the mixtures and synced the carbies as per the manual relating to my engine. but original settings on the pilot screws seemed to be wound out alot more than the specification."

If the carbs were full of trash and water I would think that they had to crank them out just to get it to run. I would be raising H**L with the shop you took it to. Did you read the thread and see that Rodbolt said the the spec settings are usually the best as long as thay are clean

 

[rodbolt]

Re: Yamaha 40hp Lacking Power

if it coughs and blows smoke you still have some trash in a carb.
what your hearing and seeing is a lean backfire in the crankcase assy.