Merc 3.0, odd stumbling issues.

[SimpleM]

1987 3.0 Alpha 1, purchased mid April with 172 hours on it. I have always had to give it a little throttle to get it to start whether hot or cold, and it will only idle out of gear for about 60-90 seconds before it seems to load up, run rough and die. However it will putt along in gear at idle without dying most of the time. (could slight mis-alignment of the shift interrupter and engine vibration cause this problem in out of gear situations?)

My initial problem (about 3 hours after test drive and purchase) was with the engine not reaching WOT, which then progressed to stumbling and not running with any power above low rpm at all. Trim pump operation pulled down rpms badly as well. I discovered I had a loose distributor cap and when I tightened it the boat ran fine. (however needing throttle to crank and the out of gear idle issues were still present)

Fast forward 2 weeks and 12 additional hours on the meter; the boat is beginning to stumble at higher rpm and under acceleration, as well as a couple of backfires. Also, when I operate the trim motor it is dropping the rpms again. Assuming it is a loose distributor cap again, I tighten it, but actually it was already very tight. So, I take the distributor cap off and inspect; everything looks reasonable. Points plate screws will tighten about 1/2 a turn. I then spray some WD-40 in the cap for possible moisture and re-attach. Crank the engine and it runs like crap, worse than before... I figure it is the WD -40 just burning off and give it a minute @ 1500 rpm. Still runs like crap. So I check firing order; correct. I re-remove the distributor, and discover the metal arm on the rotor is broken off and laying on top of the points plate. A-ha... Likely the culprit. I checked the bottom of the rotor, and the key notch was chewed up completely. This could explain the gradual worsening of performance at WOT and under acceleration. So... I go out and buy a new rotor. (On Memorial Day Monday... thank heavens it is a well populated area) I figure that I might as well do the dist. cap and plugs at the same time. Armed with a cap, rotor and plugs I return to work on the boat. I only do the cap and rotor to start with. (Old car habit, change one thing at a time to isolate cause... guess it applies in boats too.) When I test drive it stumbles a bit at first, but as I run it about 1/2 a mile it smooths out quite a bit. I am disappointed the cap and rotor did not fix it completely, so I decide to change the plugs. Old plug inspection reveals 2 perfect plugs out of #1 and #2, but #3 and #4 are significantly dry black. Assuming the plugs were affected because of the slowly disintegrating rotor, I put the new set in. (however why just 3&4 if it was rotor?) Crank, (still having to give some throttle, still wanting to die our of gear) idle out of slip, hit it, and WOW. Smooth as silk, great power, smooth acceleration, trim pump not affecting the rpms at all.

I wish this was where the story ended with the standard happily ever after, but alas, after about 3 hours of playing with the kids tubing and kneeboarding the stumbles and trim pump rpm pull-down returned. I ran it up to WOT just before I parked it and it did smooth out quite a bit, but not to the point it was right after the rotor, cap and plug change. It was twilight when I parked it so I did not pull the plugs to inspect, but I bet I will find #3&4 will be black. Where do I start? Coil? (tach has jumped a bit and never been consistently accurate at higher rpms) Timing? Points? Condenser? Carb? Fuel/water sep.? Carb filter? Last compression check was fairly solid and all cylinders within a few psi of each other so I feel this is ignition or carb related. I would like to get this problem fixed so I can tackle the idle/cranking issue. I guess they could be related if it is a carb issue.

Help. Please...

 

[Don S]

Re: Merc 3.0, odd stumbling issues.

Sounds to me like a full tuneup (points, condenser , cap, rotor, and spark plugs) along with setting the dwell, then the timing, then adjusting the idle speed and mixture controls on the carb, you know the regular maintenance items, may cure your problem.

 

[jerryjerry05]

Re: Merc 3.0, odd stumbling issues.

Check for water in fuel. Then do a good tune up.Make sure timing is in spec.
Dwell is VERY important in that motor.Make sure it's to spec.Jerry

 

[SimpleM]

Re: Merc 3.0, odd stumbling issues.

Would you go ahead and replace the coil? It's a fairly cheap item and the one on it looks like the original from 1987. As for dwell, I have a service manual, can it reliably lead me through the process, or should I take it to a tech? Last question... the fuel water seperator... is that the same thing as the upside down bell shaped filter attached to the mechanical pump, or is it something else all together? I'm not near my boat so it will be this weekend before I can attmept to svc any of these items. Just trying to make a list and notes to take up there this weekend.

Thanks

M...

 

[stacyt]

Re: Merc 3.0, odd stumbling issues.

Sounds to me like a full tuneup (points, condenser , cap, rotor, and spark plugs) along with setting the dwell, then the timing, then adjusting the idle speed and mixture controls on the carb, you know the regular maintenance items, may cure your problem.

May I ask what dwell is?

Thanks

Stacy

 

[SimpleM]

Re: Merc 3.0, odd stumbling issues.

May I ask what dwell is?

How long the points stay closed to send energy to the coil in a points/distributor ignition system. It is expressed in degrees of rotation of the distributor shaft and must be less than 1/4 of 360 degrees in a four cylinder. (otherwise the points would never close) Usually it is somewhere around 60 degrees. The rotor will make contact with a spark plug lead while the points are closed, discharge the energy, and the process starts for the next cylinder. As I have understood it, it is a slightly different process to adjust with a marine motor as compared to an automobile, hence the question I asked about self adjustment versus a technician.

M...

 

[Coors]

Re: Merc 3.0, odd stumbling issues.

http://forums.iboats.com/showthread.php?t=208547
Don's start list.

 

[fixb52s]

Re: Merc 3.0, odd stumbling issues.

The dwell is set by either opening or closing up the point gap. Of course you will need a dwell meter to check it. It is different than older cars (GM) because the autos had a little door that opened up, allowing you to insert an allen wrench to move the point gap on the fly. With the boats, you cannot have this door because of the sealed design of the distributor. So you would have to check it, pull the cap, adjust the points, reinstall the cap and check the dwell. Repeat as required. Overall, it is not that bad to do, so you should have no problems doing this yourself.

From the sound of it, you might be running a little rich. It could be from weak ignition or an overly rich idle mixture. #2 and 3 will load up sooner than 1 and 4 simply because they are closer to the carb. The path to the outer cylinders gives the air/fuel charge a little more time to mix than the center 2. A weak (out of adjustment) ignition system will not properly fire the charge, hence the rich cylinders.

Like suggested, get the ignition sorted out (sounds like the points/dwell is off to me), then the timing, then the carb adjusted.​

 

[87Searayseville]

Re: Merc 3.0, odd stumbling issues.

Hey Guys,

Good reading here. I am having trouble w/ setting the dwell on the merc 3.0 and wondering of the manual is a misprint because it lists the 4,6,8cycl engines with the same dwell specs. Is that possible?

Can anyone confirm the dwell settings for the 3.0 and the points gap for me because it seems if one is one the other is off.

Thanks

 

[Maclin]

Re: Merc 3.0, odd stumbling issues.

The points gap is only a starting point. Set the gap then check dwell, adjust the points as needed to get the dwell in range.

 

[87Searayseville]

Re: Merc 3.0, odd stumbling issues.

Great! Thanks.

I feel better now as I could not get both of them exactly on at once but is still ran fine.

 

[Maclin]

Re: Merc 3.0, odd stumbling issues.

Dwell can be measured in percentage or degree of angle.

Dwell angle in degrees will be the same no matter the cylinder count. Most engine specs show dwell angle in degrees.

Most modern dwell meters measure true dwell angle, they need to know the number of cylinders and whether it is 2 cycle or 4 cycle.

Some more "economical" dwell meters measure percentage. They just need to know if it is 4 cycle or 2 cycle. They also have a conversion table to follow to translate the readings into dwell angle in degrees.

Dwell percentage will be different based on number of cylinders, it gives the duty cycle percentage that the points are closed during a rotation of the distributor. When measuring percentage there will be a table for your meter that shows what the percentage relates to in dwell angle for the number of cylinders.

For example a 28-32 degree dwell for an 8 cylinder would show on a dwell percentage meter as a percentage between 62 and 72 degrees. A 6 cylinder's percentage would be between 47 and 54. A 4 cylinder would show percentage between 31 and 36. All approximate.

SO My recommendation if you are looking for a dwell meter is to be sure to get one that can show degrees without having to look it up on the back of the meter or from a table in the manual for the meter. Like I just did.

 

[doublehelix]

Re: Merc 3.0, odd stumbling issues.

WOW! I am having the EXACT same issues. I have a 2000 Bayliner with a 3.0 but mine is electronic ignition. And I find it INCREDIBLE that I broke my rotor button just like the original poster did. Did you buy your parts from NAPA or are they Sierra parts? Quality control issues? It seems that mine broke where the tab was attached to the rotor button. The plastic weld seems to have failed. Check this out: http://davesnsanity.blogspot.com/2008/07/more-boat-problems.html
My engine is stumbling going off idle to part throttle. At part throttle the engine is loading up and stubbling too. I thought that it was accellerator pump related (not getting a big enought squirt of gas as the throttle increased) but it seems to be squirting quite well. At 3000 RPM and above the engine runs fine so it would seem that the high speed jets are working as designed. It seems that there could be junk/dirt jamming up the low speed jets. My dist. cap and rotor only had about 10 hours on it. I replaced it last fall before winterizing the boat. I too took the cap off thinking moisture and I too was shocked to find the rotor button in two pieces. At this point I have the carb off and will do a rebuild and a replacement of the in-carb fuel filter (thats the only one I have on my boat). Since it runs fine at higher RPM that would eliminate fuel delivery (pressure) problems, right? Also, that would eliminate moisture in the fuel too, right? Anyhow, does anyone have a part number for a MerCarb model # 807504, built in 1999? Thanks.