'99 Merc stalling out on fast takeoff, smooth takeoff ok

[wn6ngp]

I've got a 99 Merc 90 hp. If I am sitting idle and slam the throttle forward the motor stalls out. If I put it in gear and slowly increase the throttle to 2K rpm I'm able to go thru that point and run full throttle 5K-6K rpm no problem. I ran some Sea Foam thru the fuel line and it fixed the problem. Slam full throttle , off she goes. Then after about an hour of running the problem came back. I am suspicious that some of the small fuel line hoses are deteriorating and clogging jets in the carbs so I am planning to replace those original small fuel lines.

My question is, are there other O rings or plastic things on the down side of the fuel filter that also need to be replaced? I am hoping to avoid removing the carbs. It had older fuel in the 35 gallon tank so I topped it off with 30 gallons of fresh fuel thinking/hoping that was the problem but that did not help.

tks
Don

 

[racerone]

A " no $$ spent " test is to open the low speed mixture needles a 1/4 turn at a time.

 

[Texasmark]

Agree on the low speed tweaks are too tight and bad fuel lines. The fact that it returned smells of such. I had an '02 version of that engine and at 10 years old the gray OEM fuel line started flaking internally....brown semicircles of crud. Found pieces of that and internal black fuel line specs in my carbs which gave me similar hole shot hesitation and lugging. The main culprit was in the bottom of #2 bowl where upon spraying with compressed air dislodged a piece of black fuel line that I couldn't see prior to the air blast that would intermittently get in the way of the jet fuel flow and bog the engine down.

Once you finish checking all the lines you may need to clean out your carbs and if your carbs are like mine (Merc used numerous carbs on the mid range engines over the years), not only take the bowl off but up on top of the carb is a metal plate with a couple of screws and I found black crud in there too on #1.

 

[wn6ngp]

thanks for feedback racerone and Texmark.
out of curiousity, Texasmark where are you? I'm in Georgetown just north of Austin although I experienced this down at Matagorda bay. Don't worry I won't look you up and come pester you. This is happening after a couple of 30 mile runs down at Baffin Bay so it could be those extended runs dislodged some crud. Regardles, its time for those hoses to go.

Anyway, sounds like good news in that besides the fuel lines there are no other plastic or rubber parts that I have to worry about. So if I'm lucky and replace the fuel lines and then clean out with Sea Foam AND the problem doesn't come back then I'll be ok.

A couple of years ago I had a similar problem due to not running it for about a year. In that case it would only hit on 2/3 reliably at high speed. So it was the high speed jet that got clogged. Direct Sea Foam into the fuel line fixed that problem and it never came back.

I am hoping that I don't have to remove the carbs as it seems like once they are removed, jet needles are adjusted etc, then you area always twidling around with them in the future. At least that's my experience with chain saws and weed eaters.

This has been a great motor for me, I just need to give it some TLC. Its been so good that I don't even know where the carb adjust points are located.

 

[Texasmark]

N.E of Dallas. When you pull your hoses, take a knife and make a slit down one side. Then force them open and bend the hose sharply. If you see cracks then you could have pieces of hose in your carbs. On got it all with......don't forget the fuel pump...diaphragm is rubber as are the metering valve seals internally.

Removing the carbs is not that big of a deal. Just pay attention to what you are removing and put it back like it was. Uncouple the linkage right at the carbs and don't diddle with the rest of the linkage unless you want to reset your link and sync. You can turn your low speed jets in to a soft finger-thumb snug, counting the revs/partial revs as you go so you can clean the carb and get the needle back where it was. You hear a lot about having to do a kit but I didn't a couple of years ago when I cleaned mine out. Engine was a 2002 and in 2012 or 13 I did the first removal and checkout. Put everything back like it was with no kit and no new gaskets. Just ensured I put the parts into the indentation in the existing gaskets and snugged up the screws good. I guarantee you if you weren't a believer in Sea Foam, if you saw how clean my carbs were (other than the pieces of hose) after 10 years of service you would/should be after that. absolutely no...nada.....nil...crud of any kind other than the hose and Sea Foam can't correct for that problem. Also the crowns on my pistons are spotless.

On long runs on the coast. I sold a boat to a guy that burned out a piston making a full hour run at WOT with an overloaded boat. Had he not been overloaded or cut her back to cruise periodically, or maybe checked his impeller before he went maybe he wouldn't have had the problem. I had the rig new to 7 years and never did a thing to it....Johnson 125 V4. Yeah it was time for a new impeller.

Good luck

 

[wn6ngp]

Racerone, kudos for you. I took things apart, cleaned them, everything amazingly clean even in carb bowls. Hoses looked good. Still poor idle and takeoff. Turned carbs 1/4 turn richer and voila, problem solved and now I also know that I have clean carbs and good fuel lines.
tks