Carb Question on 115 Merc Inline 6

[fishaholic911]

Motor: 1987 Merc inline 6 115 hp.
Carbs: WMK-41
Fuel: 89 octane unleaded

My carbs have .076 main jets and .094 vents in them and the Mercury parts diagram and clymer manual says that the mains should be .074.

The merc parts list says .074 for the wmk-41-A and .076 for the wmk-41B. I assume that mine are the wmk 41-A since starting with the top carb following the wmk-41 it says 1A, the middle carb 2A, and the bottom carb 3A.

My Questions:
1. Are my carbs the "A" type carbs?
2. What is the difference in the A and the B type?
3. Should I change the main jets to .074

Note: The elevation where I live is considerable below 2500 ft.

I do experience some rpm variations at rpms bellow 5000. These variations are very suttle. you can not even feel it in the boat, I just can hear it and see the rpm guage move up and down ever so slightly.

I have also experienced a miss when going from idle to wot very quickly. When I throttle it up slowly it doesn't seem to do this. The jet size may not have anything to do with this.

I have done a timing, lync, and sync on this motor and I check it periodically to make sure everything is right where it needs to be.

Thanks in advance for any info on this issue.

 

[emckelvy]

Re: Carb Question on 115 Merc Inline 6

You should take those vent jets out altogether, and plug the crossover hoses with a small B-B. You may find the vent jets have been leaning things out enough to make it surge a bit. Another Merc 'better idea' that didn't pan out.

On the slight miss/rpm variation, it could be as extreme as the stator going out-of-spec slightly, to cracked coil, or bad spark plug wires. Run the motor at night and if you get a 'light show' it's time for new plug wires.

ADI coils have been known to crack, then they'll arc to ground. You'd have to pull each coil out and inspect it front & rear for cracks or burn marks.

And I wouldn't trust Clymer on jet sizes; I'd go directly to the Merc site, www.mercurypartsexpress.com , and check the parts diagrams for your motor. Usually there's a lot of footnotes on the diagrams and you might find out something about your carbs.

HTH.........ed

P.S. just thought of something else, if you have an "idle stabilizer" connected to your ign system, get rid of it! Those things have been the cause of a lot of motors getting 'nuked' and may be what's wrong with yours.

 

[fishaholic911]

Re: Carb Question on 115 Merc Inline 6

I did disconnect the idle stabilizer a few months ago. To be honest, I don't know why they even put it on there because I couldn't tell any difference in the way it idled with or without it.

I went to the web site that you mentioned and according to their parts diagram I need the .074 main jet. That is if mine are wmk 41-A. I am assuming that they are since the top carb says 1A, Middle 2A, and bottom 3A which follows the wmk 41 stamped on the front. If someone could tell me the difference between the a and b carb that would help too. I talked to the local merc dealer and they couldn't even tell me the difference.

what is the purpose for those vent jets and the crossover hoses anyway? If I take those out and plug the crossover hoses am I going to need to reset my idle screws and will it make it run richer or leaner?

 

[fishaholic911]

Re: Carb Question on 115 Merc Inline 6

also would I leave the vent hole open after I remove the vent jet or would it need to be plugged?

 

[fishaholic911]

Re: Carb Question on 115 Merc Inline 6

^

 

[emckelvy]

Re: Carb Question on 115 Merc Inline 6

You don't need to plug the vent hole once the vent jet is removed. In fact, this hole must be left opened.

The jet was designed, in concert with the "back-drag" hose connected to the float chamber and the carb's throat, to pull a partial vaccum on the float chamber at higher rpm, in order to lean the mixture out for better economy.

'Course we know now that Inlines don't like to be leaned out at all !!!

So, once you pull the vent jet out of the top, the float bowl vent is completely at atmospheric pressure and even if you didn't disconnect and plug the back-drag hoses it wouldn't make a whole lot of diff.

But, no sense in having a hose (albeit very tiny hose) hooked to the carb sucking air, thus the reason to plug it. Some of the other Inlines Guru's have found a BB to be of perfect size for doing this but you could use something else of suitable size to plug the hose.

Hope that clears things up a bit.............ed

 

[Plainsman]

Re: Carb Question on 115 Merc Inline 6

Just to clairify for myself, as my carbs have this hose as well, if the vent jet is removed from the float chamber, do not plug it. Leave the hose off the float chamber, but do plug the other end, at the throat? Is that correct?

 

[fishaholic911]

Re: Carb Question on 115 Merc Inline 6

Yesterday I took the carbs off and gave them a good cleaning. I put the .074 main jets in to replace the .076 that I had been running. Removed the vent jets and instead of a BB in the crossover tube I just removed the tube and used rubber vacuum plugs to cap off the nipples on the float cover and at the throat. When I got everything back together I set the Idle mixture screws exactly where they were before. Double checked sync and timing and all was good.

When I tested the boat at the lake I noticed at wot that it ran smoother than ever before. The only thing is that the idle was alot faster than before. Nearly 1200 out of gear instead of the 900 it was before. I double checked the idle mixture screws and they were set exactly in the same position as before. I did not have my tools with me at the lake so I am going to go back sometime this week and reset the idle and the idle mixture as to clamscanino's method described in his timing, lync, and sync post.

emckelvy, do you think the rubber vacuum plugs that I used on the carbs will be ok?

Thanks

 

[emckelvy]

Re: Carb Question on 115 Merc Inline 6

Most folks leave the back-draft hoses on the carb and disconnect the end at the float bowl, then plugging with a BB.

But I don't see any reason why capping both sides off would hurt anything. As long as the vacuum plugs are fuel-resistant.

Your idle speed probably increased due to the main jet change; smaller main jets would allow just a wee bit less fuel up into the idle passages and maybe you were too rich on the idle mix.

That's the only thing I can think of that would have caused a pronounced change in idle speed, besides a vacuum leak.

The Clams Link 'n Synch should fix it right up, I bet......ed

 

[timmathis]

Re: Carb Question on 115 Merc Inline 6

Yep. It sounds like you are on the right track.
Only thing with the rubber plugs is they are known to crack after a few years. Just keep a eye on them and replace as needed.
Good Job , And good Luck, Tim

 

[jimmbo]

Re: Carb Question on 115 Merc Inline 6

'Course we know now that Inlines don't like to be leaned out at all !!!

ed

Keep in mind when the 'back drag' feature was added to the carb in 1975 or 76, gasoline was a much different bunch of chemicals than it is today. Todays gas has additives(oxidizers) that reduce emissions but unfortunately for 2 strokes it chemicaly leans out the mixture. Ethanol blends add to the problem only making it worse

 

[Plainsman]

Re: Carb Question on 115 Merc Inline 6

So should I remove mine?

 

[jimmbo]

Re: Carb Question on 115 Merc Inline 6

It probably is a good idea. the engine will burn more gas but it will run cooler and get more oil.

 

[adtodd]

Re: Carb Question on 115 Merc Inline 6 - idle problem

Re: Carb Question on 115 Merc Inline 6 - idle problem

1988 Mercury 115 Inline 6 with "enricher" choke system

I'm new to this forum and have read with great interest concerning these inline 6 motors. I can appreciate the knowledge and experience shared here. Hopefully, some of you can offer information to help me.

I'm having idling issues with my motor, and this is a bit concerning when fishing in the Gulf Coast bays with tides and currents. The motor runs pretty well at faster speeds, but won't idle well. It begins to stall, and can be hard to restart and keep running. It seems like the motor begins starving for fuel at idle. The motor did set up through the winter without draining the carbs or any other storage prep.

I've changed the fuel pump and fuel hose/primer bulb coming from the tank, but that hasn't resolved the issue. After reading some of the posts here, I thought I would clean the carbs. When I removed them, I noticed that the top and bottom carbs were WMK-41's (41-1B1408 and 41-3B0918 stamped on the sides), but the middle carb is a WMK-39. My first question is what is the functional difference between the 41 and the 39? I would imagine these all need to be the same. The floats are not filled with fuel, but I haven't checked anything else on the carbs yet.

After I pulled the carbs off, I did see that the hose from the enricher valve was split where it attaches to the barbed fitting on the motor just left of the middle carb. This could certainly be a fuel or air leak point, but would that affect the low-speed operation?

Any thoughts or suggestions would be most appreciated to help me step through this effort in the right order.

Thanks,
Andy

 

[GA_Boater]

Re: Carb Question on 115 Merc Inline 6

AD - this thread is almost 7 years old. You should start your own thread instead of piggy backing on an old thread. You will get more answers to your questions.

Welcome to iboats, just follow the rules. It keeps people happy and you will get better/more responses. Thanks for your understanding.

 

[adtodd]

Re: Carb Question on 115 Merc Inline 6

Thanks for the help! I'll start a new thread as recommended.