Merc 150 V-6 idle problems

[dicktaber]

Just bought an 83 Nitro 17'er with an 85 XR-2 150 HP Merc. The boat sat in the owner's garage for three years without running (fuel drained and winterised). Compression is good (125-135 on all six) and on the muffs, it starts right up and idles smoothly. On the water, it starts but misfires at idle (pop/chuff out the exhaust) and will die frequently, especially after pulling back to idle after a run across the lake. If I use 1400 rpm as a no-wake speed it runs great; pull it back to 1200 and it will die. Pull it back into neutral and it will sometimes run fine at 1000 rpm and sometimes die. It will always restart, and the bulb stays firm at all times. Checked all six plugs and they are firing, none looks fouled, and as I say, it runs great above 1400 rpm. I also pulled all six slow speed jets and checked to see that they were unobstructed. I do have both a Clymers and a Seloc manual. I'm thinking maybe carb synchronisation or fuel pump diaphragm (too rich at idle?) Big question is, why does it run perfectly on the hose? Less back pressure? Really hard to fix when it seems fine in the driveway and only misbehaves on the lake (one hour away). Any help, suggestions, etc, are greatly appreciated.

 

[jim dozier]

Re: Merc 150 V-6 idle problems

My guess is that you will find that one or more of your cylinders is not firing at idle. It may have spark but isn't firing. First start pulling plug wires (best to ground the loose wire) 1 at a time and see if you can find one cylinder that isn't pulling its share. Then concentrate on the mixture for that cylinder, too much or too little. If it happens to be the cylinder that your fuel pump is attached to, check the fuel pump diaphragms before you go any further.

 

[dicktaber]

Re: Merc 150 V-6 idle problems

Thank you for the advice. I pulled the boat out on the street, hooked up the muffs, and fired it up. While running at idle (1100 rpm) I pulled each plug wire in turn. Very little drop in actual rpm (less than 100) but I did notice the exhaust note change as each plug wire went off then back on. All cylinders are firing. Now, when it's in the water there is a bit more back pressure, right? As I said before, I have already verified that all six low speed jets are clear--is there an associated air passage I should be checking? How about the small vacuum hoses--I can see the ones on the port side but on the starboard side they're hidden behind the ignition components. Might I have a vacuum leak that under load is causing an idle misfire? Here's the part that puzzles me--I've alway thought that if an engine spits back through the carbs, it's lean, and if is pops or fires in the exhaust, it's rich--at least that theory seems to work with four strokes...am I overlooking something obvious here? I thought I read in another post where someone said that if your fuel pump was leaking internally, you'd be rich at idle, maybe enough to cause symptoms, yet at larger throttle openings the excess fuel is negligable. Once again, your suggestions are welcome and appreciated!<br />Thanks, Rich

 

[jim dozier]

Re: Merc 150 V-6 idle problems

Leaking fuel pump diaphragms can cause the engine to run rich or lean depending on where the leak is. I have a Suzuki DT85 that was having a rough idle and ran rough until about 3500 rpm where it would run fine up to WOT. I initially thought it was the carbs, but ultimately the problem turned out to be a hole in the inner fuel pump diaphragm that was squirting fuel directly into the intake port of the bottom cylinder (the one the fuel pump draws vacuum and pressure off of) effectively killing that cylinder (too rich) until the demand at higher rpm could keep up with it.<br /><br />Sometimes a leaking fuel pump just sucks air and can cause a lean condition due to inadequate fuel supply. Similarly bad fuel line connections can suck air.<br /><br />Your carb float valves may be stuck open or improperly set causing a rich idle.<br /><br />You can't evaluate and set the idle properly on muffs without the backpressure and drag of the prop in gear. Maybe if the lake is that far away you can set up a tank to submerge it in that is big enough to get it in gear.

 

[greasemonkeyozi]

Re: Merc 150 V-6 idle problems

Im not exactly familiar with your engine type, but let me tell you about something that could possibly fix your problem. Have you ever heared of something called sea foam? <br /> I have a 75 70hp motor that I had recently rebuilt. Rebuilt carbs, electrical system, fuel system, and all that good stuff. She ran pretty good, but not 100 % until the other day. I had purchased this stuff at napa, about $6.50 an arisol can. Took my plugs out, sprayed it in each cylinder, let set for an hour, and started it right up. sprayed it in each carborator and watched all kinds of stuff blow out of my exaust. I took her out on the lake that evening and she came out of that water like there was no tommorrow. I guess this stuff cleans your engine out.<br /> I hear people talk of this stuff on this site frequently, and how good it is. If it doesnt fix your problem, it would be good to have used it in your motor anyway. I hear that you can use 1/2 can per 3 tanks of fuel. Just spray it in your cylinders, let set over night, and spray in your carbs while running.

 

[The Marine Doctor]

Re: Merc 150 V-6 idle problems

A POP at idle will be a lean condition. As the fuel charge is ignited it Flashes Or POPS rather than burns. <br /><br />IF the problem goes away at higher speeds..the idle circuit is to blame in the carbs. Remove and rebuild the carbs...this should take care of the problem.<br /><br />To explain the POP better...imagine a balloon..if poked with a pin it POPS...the energy is now expended. The balloon falls to the floor.<br /><br />Now take a different balloon...blow it up but do not tie it. Release the balloon. The energy is expelled slower than the popped balloon. Which moves the balloon around the room.<br /><br />This is what is needed to push the piston down the cyl.<br /><br />I hope this helps you out..its kinda rough. <br /><br />TMD

 

[greasemonkeyozi]

Re: Merc 150 V-6 idle problems

Hey marine doctor, I learn something new from you guys every day.

 

[dicktaber]

Re: Merc 150 V-6 idle problems

Thanks Doc--<br />When (if!) the weekend comes I'm going to look more closely at fuel pump and vacuum lines. Regarding the "pop", it was more like a "chuff" which lead me to the rich/lean, intake/exhaust theory I discussed earlier. Care to comment on that? If I can figure out whether it's rich or lean it will help narrow it down--oh, BTW, when idling, if it's "chuffing", engaging the choke (momentarily) has no apparent effect; that's another reason I'm thinking rich...<br /><br />As far as the Seafoam goes, I bought a can a while back, and I'll give that a try, but it will probably wait until the motor is running correctly. I can't think of any reason that an internal cleansing will help a jetting problem, an ignition problem, or a vacuum leak...

 

[KCLOST]

Re: Merc 150 V-6 idle problems

Knot2much,<br /><br />I don't want to cause you to worry about anything more. But you mentioned that at Idle the rpm's are at 1100... This sounds a little high to me... I think it should be more around 700 rpms..<br />I could be wrong, but based on my experience with my 86 merc 175 I think 700 is where you want to be... You might want to check on that.. If I'm wrong, sorry!

 

[dicktaber]

Re: Merc 150 V-6 idle problems

...and the latest update. My local Merc shop says that if the exhaust relief ports are partly or fully under water, as is sometimes the case, the engine will load up, die, etc, so trim it up. I'm going to try that tomorrow. Additionally, I bought a fuel pump rebuild kit and completed that task today. I'll get back to you, and thanks for all your help. <br /><br />Thanks, KCLOST, I'll have to check the manual but I seem to recall that the idle stabiliser kicks in at 900 rpm? Also, as I recall, factory tachs aren't known for accuracy at the low end; that's why most manuals tell you to use an inductive tach and timing light to set idle mixture/speed/timing etc.<br /><br />Thanks to one and all and I'll report back, but I just know this thing is gonna be tons o'fun when I get it working right!

 

[dicktaber]

Re: Merc 150 V-6 idle problems

Back to the lake today, same story...runs great at 1400 rpm and above, but below that will sputter and die. Fuel pump made no difference, trimming it up made no difference, spark on all six is good at idle, all cylinders are firing at idle on the muffs but in the water it seems like I'm running on four or so. All low speed jets are clean, fuel is fresh, and boy does this thing run great at 3500-4000 rpm--ran for about 1.5 hours total at cruise power. But any no-wake zone is an exercise in frustration. Any suggestions before I pay big $$$ for guesswork at the local Merc emporium? <br />Thanks again, Rich

 

[dicktaber]

Re: Merc 150 V-6 idle problems

bump

 

[dingbat]

Re: Merc 150 V-6 idle problems

Have your stator looked at. That motor has both a high and low speed stator and when one of them goes out, you'll loose either the high end RPM or your low end RPM

 

[The Marine Doctor]

Re: Merc 150 V-6 idle problems

The engine will not start if the low side does not operate.<br /><br />Does the engine have a Small black box mounted near the front? This is to advance the timing at idle. The name of it slips my mind atm. It is possible that it is not working.<br /><br />TMD

 

[dicktaber]

Re: Merc 150 V-6 idle problems

That be the idle stabiliser? I was told that the motor would run fine without it so I will disconnect it and see if that helps. A local Merc guy told me that the stator would cause this problem but I don't want to spend a bunch of money on something that "might" fix it.

 

[dicktaber]

Re: Merc 150 V-6 idle problems

Okay, it's winter and the boat is sitting tight. I have a Mercury manual as well as the Seloc and the Clymer and have reviewed all. Stator has been tested IAW Merc manual specs. Carbs will be coming off this winter for rebuild. Just bought a set on eBay that I can play with and get familiar. One last question--the carbs I bought are for a 200 HP V-6 (mine is a 150HP V-6). Can I install the new ones as-is? I am guessing jetting is different for the higher-HP motor--what else is different?<br />Thanks again to all...

 

[dicktaber]

Re: Merc 150 V-6 idle problems

Okay, complete Seafoam job (DeepCreep)--no improvement. Complete removal, disassembly and soaking of carbs, blew out all passages with compressed air, new gaskets, (carbs looked spotless inside). Back together, synchronized in accordance with Merc manual--no difference at all. Still hard to start (enrichening circuit is working), "chuffs" when cold, dies occasionally at or just off idle, and runs great the rest of the time. (Runs great on the muffs, only in the water does it act up.) Got all the ohm readings for the stator from the Merc manual, and no significant variation--I recall that one measurement was off by like two ohms. As resistance varies with temperature, I attribute this discrepancy to that. I'm looking for answers. Do I buy a new stator "just 'cause"? I can't find anything in the book about the small (vacuum?) lines running around the motor but someone told me that they are check valves/bleed valves...any ideas?

 

[outboard tech]

Re: Merc 150 V-6 idle problems

The low speed noise you describe is a lean spit; if the carbs are indeed thoroughly clean (nothing you spray into them will achieve that--they need to be disassembled); you most likely have one of the throttle shutters open. Place the control to neutral, remove attenuator (airbox) and confirm the shutters are all the way closed--if not, loosen the screws on link arm and reset them--test to water--should run at 750 rpm in gear, no throttle--none of this can be checked on the hose.

 

[dicktaber]

Re: Merc 150 V-6 idle problems

Ben--<br />Thank you, but as I said, I fully disassembled and rebuilt all three carbs, even though they looked spotless before cleaning. I had the airbox off and synched the carbs in accordance with the manual. The throttle plates are all closed at idle. What is disturbing is the fact that zip/zero/nothing has changed before/after the cleaning and resynching. So, is this really a carburetion problem?

 

[outboard tech]

Re: Merc 150 V-6 idle problems

If you're satisfied the carbs/timing (and this is critical) are propperly sync-ed---and you still have the lean spit at low throttle ( by the way, they were right about exhaust backpressure and trim, but you need to trim down at idle to achieve a richer mix)......and you're sure ignition is OK--you have a fuel problem. The cam on the throttle MUST not touch the follower at idle; worn out cables or misadjustment can cause your problem. Also one of your bleed hoses could be off (possibly behind the switchbox/solenoid assembly)and that can cause idle problems. A motor as old as yours should have the hoses replaced by now anyway, as they get brittle at the fittings, and tear off. You probably didn't need to build your carbs, but that's what you run into when you're guessing. Everything you describe could be diagnosed in minutes by a good tech testing on the lake.