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Have a 90 HP Yamaha...used with jet drive... unsure of

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year...one plate unreadable... the other...
Have a 90 HP Yamaha...used with jet drive... unsure of year...one plate unreadable... the other partically.. 6H L 355427.....CAOT?RX... best I can tell... believe to be between 98 and 2004.... has carburators (3)
Compression, 131. 133, 135... new plugs, coils, carbs cleaned, 18 gallon tank... put 2 cans of SeaFoam... ran better...then returned to running bad even with Seafoam....starts fine...fuel pumps not replaced but have good pressure.... fuel lines replaced...
Does not have tach...so rpms are estimated and to use for reference... will run at 3000 rpms... barely on plane...14- 16 mpg.... gets on plane... will jump to 4500 rpm... 22-25 mph...still on plane running easy... drops to 3000... then to 4500... then to 3000 then 2000 rpms... more fuel does no matter..... never misses... just instantly loses or gains power.
Submitted: 1 year ago.Category: Boat
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Answered in 10 hours by:
10/18/2016
Marine Mechanic: Jason, Marine Mechanic replied 1 year ago
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16,297
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Good morning, my name is ***** ***** am I speaking with?

I read what you wrote. Did you have any specific questions for me, or did you want me to explain how we would troubleshoot this type of symptom in the shop?

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Customer reply replied 1 year ago
Jason,This is Doug. I want to know how to troubleshoot & make engine go to WOT and stay there.
Marine Mechanic: Jason, Marine Mechanic replied 1 year ago

Hi Doug,

The engine needs 3 things to both run and make full power. What it needs is compression, timed spark ignition (spark happening at the right time) and the proper ratio of fuel to air. When I get engines in the shop that do not make full power, this is what I do to troubleshoot them, I also do it in this specific order in order to both be thorough and keep from making mistakes.

What I start with is engine compression and spark tests.

1. Compression test. You already did this and your numbers are fine.

2. Spark test. The easiest thing to do here is to hook up a timing light to each plug wire one at a time and see if the light flashes or not. Flash means spark, no flash means no spark. No spark can be something as simple as a bad spark plug, that was dropped on the floor before you got it, or a bad plug wire, bad coil or bad CDI. This needs to be checked with either the engine in a test tank and hooked up to a dyno, or with the boat in the water and under a load. If you get no flash/no spark on a cylinder what you would do is swap the plugs first. After that swap the ignition coils and see if the problem follows the coil or not. If it does, there is a bad coil, if it does not, there is a bad CDI box.

3. If compression and spark are good, make sure the fuel filters are all changed.

4. Next is to get a small outboard tank (like a 3 gallon) and fill it up with known new fresh gas and try it. If the engine runs better on the portable tank there is an issue with either the fuel in the boats tank or there is a blockage or air leak in the fuel pickup and related hardware on the boats tank and related lines.

5. If no change, next is to hook up a fuel pressure gauge between the pump and carb. Fuel pressure needs to be in the 3 to 7 psi ballpark at max speed. If it's not, the fuel pump is weak and needs to be changed out.

6. Lastly, if you got this far and the engine STILL is not running correctly, everything else has been ruled out at this point, and the problem is inside of the carb. They are either dirty inside or one of them has a sticking float now allowing fuel into it.

If the troubleshooting is done in that order, and no steps are skipped, the root cause will ultimately be found. Also keep in mind there is always a chance there is more than 1 thing wrong with the engine. So if for example if you pull apart the fuel pump and change the filter inside of it, and it looks nasty but the boat still does not run correctly, try not to get discouraged. The older the boat is the odds of it having more than one problem goes up. I couldn't tell you how many engines I have worked on that have come in and the boat has bad gas in it, all the filters are trashed, the ignition system needs a tune up, and the carb is clogged as well. It definitely does happen.

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Marine Mechanic: Jason, Marine Mechanic replied 1 year ago

I don't know how you want to leave this but that is where you stand on your end and what needs to be checked. I think that should do it but I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.

Thank you
Jason

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Marine Mechanic: Jason, Marine Mechanic replied 1 year ago

Was there anything else Doug?

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Marine Mechanic: Jason, Marine Mechanic replied 1 year ago

It looks like you read my answer without replying back or putting through a rating for me. Please keep in mind the website did charge you to speak with me, but they will hold your money and not credit me for helping you out if no rating is put through. If you have a further question on the issue by all means please feel free to ask. If not, kindly select a rating for my service. If you have a problem with the ratings box or can not see it please let me know.

Thanks

Jason

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