Mercury 115hp problem help please

Ross Honey · Nov 20, 2010

The engine starts fine and in neutral ticks over and revs up the problem is that there is no real power in forward gear with the throttle in the full on position. It also clunks into and out of reverse. I have just got the boat which a Nab 21 and with the set up of motor should cruise at 30+ knots ......it currently does 5 flat out. The boat had not been used in over 2 years prior to me getting it. Any help guidance greatly appreciated

Chris1956 · Nov 20, 2010

Re: Mercury 115hp problem help please

OK, Lets start at the beginning. What year Merc? What kind of boat? Take a compression check and see that you have spark to all cylinders, Post results.

Ross Honey · Nov 20, 2010

Re: Mercury 115hp problem help please

Thanks for the prompt reply it's driving me nuts at the moment. The boat is a Nab 21. A dory twin hull work boat. Have a look at this link to show you what it looks like http://www.boatshed.com/nab_dory_21-boat-24337.html I am told that the engine is a 2000 model . All cylinders have a good spark and compression. That's all the info that I have at the mom apart form all the carbs have been striped and jets cleaned and I can't wit to get everything functioning properly. I really appreciate your assistance

Ross Honey · Nov 20, 2010

Re: Mercury 115hp problem help please

Also one other points is that it clunks into reverse and then back into forward gears

Wingedwheel · Nov 20, 2010

Re: Mercury 115hp problem help please

Could be a number of things on the shift problem, clutch-dog, cam or follower, yada, yada. I guess the first question to ask is did you check the LU lube and if so, what did it look like?

daveswaves · Nov 20, 2010

Re: Mercury 115hp problem help please

The prop hub could also be spun, what rpm is it turning when you are "flat out"??

Wingedwheel · Nov 21, 2010

Re: Mercury 115hp problem help please

Yea Daveswaves, I thought about the prop hub also, but that wouldn't explain it popping in and out of gear would it?

CharlieB · Nov 21, 2010

Re: Mercury 115hp problem help please

Sounds like you have the in-line 4 cyl of the 2 + 2 design. These motors idle on the top two cyl's only, the lower carbs do not have an idle circuit and do not receive enough fuel to fire until the throttle is opened.

As the throttle opens an accelerator pump squeezes a measured amount of fuel to both lower cyls to cause them to fire, air flow thru the carbs then carries fuel for continued firing.

Carb link & synch is critical as well as accelerator pump setting and cleanliness of the accell pump injectors in the side of the lower cyls.

Check this link and download the manual for this motor, read and re-read the section on fuel, then start checking/cleaning the accel pump and injectors.

http://forums.iboats.com/showpost.php?p=2072211&postcount=3

daveswaves · Nov 21, 2010

Re: Mercury 115hp problem help please

Wingedwheel said:
Yea Daveswaves, I thought about the prop hub also, but that wouldn't explain it popping in and out of gear would it?

No, but he likely has more than one issue, the only way to tell from a far is to eliminate possibilities. With more input from Ross things should start to fall into place.

Ross Honey · Nov 22, 2010

Re: Mercury 115hp problem help please

Hi guys a big thank you for your assistance so far. Have checked the prop and thats fine. So will check the accelerator pump next have you any tips on doing this.??

Ross Honey · Nov 22, 2010

Re: Mercury 115hp problem help please

daveswaves said:
The prop hub could also be spun, what rpm is it turning when you are "flat out"??

flat out is about 5 knots when the revs pick up the boat feels like it going to go onto the plane but the motor will not rev high enough under load

Ross Honey · Nov 22, 2010

Re: Mercury 115hp problem help please

You can really feel the acceleration when the second set of carbs try to kick in but even at what it currently full throttle the engine will not rev enough meaning that flat out is just above tick over

daveswaves · Nov 22, 2010

Re: Mercury 115hp problem help please

Ross Honey said:
You can really feel the acceleration when the second set of carbs try to kick in but even at what it currently full throttle the engine will not rev enough meaning that flat out is just above tick over

Ok Ross, lets start from the beginning, with the engine off, advance the throttle with the engine in gear and make sure the carb butterflies open all the way.

Ross Honey · Nov 24, 2010

Re: Mercury 115hp problem help please

Have checked the butterflies and all open full so no problem on that issue

daveswaves · Nov 24, 2010

Re: Mercury 115hp problem help please

Ok Ross, follow Charlie B suggestion, download the manual and follow the link and sync procedure and fuel system troubleshooting guide. you mentioned that the carbs were removed and cleaned so likely your link and sync is off.
You have fuel, spark compression but it needs to happen at the correct times. Thats what the link and sync does.s

Mercury 115hp problem help please

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