Hey everyone,
My 1999 Mariner carburated 150hp 2.0L engine is not hitting the top range of the rpms. I'm used to hitting about 57-58mph with my procraft at 5500 rpms, but recently I'm only hitting about 5000-5100 and barely hitting 50. I do premix my oil at 50:1 and have replaced the plugs within the last 6 months. I had ethanol issues a few months back and replaced all of the hose lines and clean out a bunch of tube chunks from the fuel pump screen. However, I did check these to see if there were any remnants of hose tubing preventing fuel flow, but all was clear. The engine runs great and doesn't miss a beat, just no top end.

I suspect that perhaps the carburetor jets may be clogged (especially since I premix). Does this sound like the potential issue?

I picked up some SeaFoam to try and clear the jets (without getting too invasive and rebuilding the carburetor). However, I'm afraid to use the additive since my oil is also pre-mixed. Is this not advised for pre-mixed fuels?

Any suggestions would be great! Thanks!

You can run Seafoam in premix gas without a problem. As to the rpm's when did it start to slow down? Since the weather has gotten hot? All boats slow down some when the weather gets as hot and humid as its been lately. Steve at Steve's custom props recommends you drop down 1 pitch in the summer due to the heat and humidity. Does it run ok all the other times besides wide open?

Sounds kinda like its runnin' on five but think ya might lose a few more rpm than that. I'd be lookin' at tha fuel delivery stuff from tha tank on. Might be sumpin' as simple as cable adjustment. Ya might wanna track dowm an oem manual cause ya probably gonna need ta dig a little deeper.....

Not a big fan of additives 'cept for a shot a kwik-clean ever once in a while....if its a motor I'm takin' apart just ta look at ever so often...jmo

If you had hoses brakedown from ethanol the carbs need to be gone through. Probably has small pieces clogging the jets. This will also cause you to run lean if that is the case. I would have the carbs looked at if you don't feel up to doing them yourself.

One of the main things missed when you experience ethanol damage to the fuel lines is, the manual fuel pump. The diaphragm gets brittle and those hose fragments get in their too.

Thanks for the info guys. I decided to give a less invasive tactic a whorl first and Seafoamed it. However, after running the tank, this didn't fix the problem. She is still running steady at ~5000 rpms. I think you guys are right and maybe the carb jets are gummed up. I've already tore apart the fuel pump a few times when I was cleaning hose fragments out of the line. However, I didn't change out the seals and filters. I just cleaned it out good and put it back together.

I'm decent with mechanics, but I've never torn apart a carb before. I see several adjustment screws (6) across the carbs. Are these for idle adjustments? I'm ordering a shop manual before I take anything apart. I figure if I'm going to tear into the carb, I should get a kit and replace all the seals while I'm at it.

Those are just idle air adjustment screws. Count the number of turns from all in and put back to the same when re-assembling. Check the little things first, are the carbs going full throttle, check all cylinders for spark. If all else fails you may need to have a leak-down test done on the motor as you are talking about a 15 year old motor.

Really impressed with this part. Something ANYONE that's gonna take it on themselves ta do tha work needs ta do very first thing..........good for you.

http://publications.brunswick.com/ProductDetails.asp?ProductCode=OB%2DSM%2DGT135

Best $100 I ever spent.

Copy and past link above in your browser. This is the factory service manual Mariner was a division of Brunswick along with Mercury you will need your serial number to get the correct manual for your model.

Personally I would prefer to get a factory service manual for your specific model of motor. Other manuals cover several models. Carb kits come with instructions. It is so much easier now days with cell phones. Take several pictures before removing the carbs.

If you had ethanol induced fuel line decay then check your jets. I had fuel line problems on mine and some of the jets had pieces of rubber in them. Might also try hitting the enricher while running but I would not run it much. If you not getting feed to a cylinder you could end up with worse problems.

Thanks for the input everyone. You guys have been very helpful in my research... even before tearing anything down.

I'm hunting down the carb seal kits now. You're right Gary, there are a couple things I need to check. I haven't watched to make sure the carbs open full, and (even thought the plugs are new) I need to do a spark test (because the plug wires are not). I did check to make sure all of the throttle cable adjustments were still tight and hadn't moved. Despite this engine being a '99, it had very few hours (probably less than 50) on it. I knew the guy, he had it professionally winterized each year even if he only took it out once all season.... but that still didn't stop ethanol issues either.

I'll probably end up ordering the Mercury Service manual recommended by 97fordnut. It's definitely pricey, but I figure it beats paying $30 bucks for a piece of garbage. Do they provide an online version and a hardcopy?


Also, I found these videos to be helpful since these are the same carbs I have.

https://www.youtube.com/watch?v=sjr27WOvvPU
https://www.youtube.com/watch?v=okWLa4h8ZNY
https://www.youtube.com/watch?v=59wKtTN0t1U

These show removal of all the jets too, which you guys seem to agree can get clogged due to ethanol issues.

See if this will download. Be very patience. it take several minutes...

http://boatinfo.no/lib/mercury/manuals/135-150-175-200.html#/20

Perfect. This is pretty slick... Thanks.

Yeah it is. Just wish I could remember how/where I found it!!!