merc115 bogs - carb qestions

[nmast98]

This is a 1987 115hp merc tower of power, model 1115727, ser# OA227416. Is there any reason that the vent "jet" on the top carb is the same orifice size(.074") as the main jet while the other two carbs' vent jets are .096" as the service manual specifies? The top carb(all three recently rebuilt) shows up as the culprit in the "spray bottle of gas sprayed into the throat while the engine is bogging on take-off" test(Thanks to all of you for the many great troubleshooting suggestions on this forum!!!). I can provide more details, but you 115 experts probably know what I'm talking about. My local merc guy says don't touch it - they probably cut the orifice size to cool down the upper two cylinders by reducing the atmospheric pressure in the bowl at 2500-4500rpm more than the lower four. I (totally ignorant but I did sleep in a HI once and have a MSEE) say someone screwed up when the carb was assembled and put the wrong jet in. Help! This intermittant problem has been driving me nuts for about 3 years now.

 

[joe Hetz]

Re: merc115 bogs - carb qestions

Neal!!<br /><br />Don|t know if this will help or not but I have a 94 75HP Merc. and it is basically the same engine and I was having trouble with the top cyl.also. I could not get the top cyl. to adjust with the idle mixture screw and took the carbs. to clean and rebuild. I noticed one thing and that was the ible mixture passage onthe top carb. had one small hole and the other carbs. had two larger holes. I ended up breaking the fuel connector off the top carb. as I thought it screwed out but it was pressed in and had to get a new carb, and lo and behold the new one had two holes in idle mixture passage and it now runs like it should but no one knew why the old carb. only had one hole so you might be right after-all. Just something to dwell on. Good Luck

 

[nmast98]

Re: merc115 bogs - carb qestions

Joe:<br />If by the idle mixture passage you mean that small chamber with the lid that comes off only by destroying it, then mine has 4 holes in it.<br />I don't know about your carbs, but mine have a black rubber tube that goes to the bowl cover and is used to lower the pressure in the bowl at 2500-4500rpm, leaning the mixture. The vent jet size adjusts the pressure. <br /> Since I posted this, I've had two people tell me to disable this feature on these carbs by either taking the vent jets out and tossing them or slipping a needle bearing into the black rubber tube that goes from one side of the carb to the bowl cover to effectively block it. Seems this "bogging on take-off" is a fairly common problem with this engine as it ages, in fact one guy thought he remembered a service bulletin from mercury on this about 15-20 years ago( If anybody out there has it, I'd sure like a copy). The manual is pretty clear about the vent jet size and says nothing about the top carb having a different size. I'm going to try these solutions tuesday and I'll report results.

 

[Hooty]

Re: merc115 bogs - carb qestions

Neal, If you don't get any satisfactory answers here, go to:<br /><br />www.screamandfly.com<br /><br />They're some Merc wizards that hang there.<br /><br />g'luck & c/6<br /><br />Hooty

 

[nmast98]

Re: merc115 bogs - carb qestions

SUCCESS<br />Today I pulled out and discarded all 3 vent jets, and turned all 3 idle mixture screws out 2 full turns, put the boat in the water and the engine performed beautifully! How sweet it is! Until today, I've never been able to push the throttle lever all the way forward in one swift motion and have the engine accelerate smoothly - I've always had to move it slowly, even when it wasn't bogging.

 

[joe Hetz]

Re: merc115 bogs - carb qestions

Neal!!!!!!!!!!!!!!!<br /><br /> Its great to hear you got the old Merc. running again. where did you hear about taking out the vent jets?

 

[nmast98]

Re: merc115 bogs - carb qestions

Joe:<br />I ran the same question on ScreamandFly and got <br /> http://forums.screamandfly.com/showthread.php?s=&threadid=18391 <br /><br />Hooty was right - see above.<br /><br />"boneman"