3.0 mercruiser runs rough when warm

[forplayboat]

I have a 1987 Sylvan 18'8" with a 3.0 Liter mercruiser and an alpha one outdrive. When I start the boat when it is cold (on the muffs or on the water) it starts right up and idles ok until it is warm then the idle settles down and it purrs along nicely. After being on the water for a while it becomes hard to start after it is turned off. The temp runs at about 160 degrees and does not overheat. It idles erratically after that and seems like it wants to die. It even coughs a little (like a small backfire or something). It is sometimes difficult to get it back to the dock and this whole thing makes me a little nervous. The erratic idle seems to be worse in the dreaded "Minimum wake zone", at full or increased throttle is seems a lot better. The carb was rebuilt about 4 months ago and I have done a lot of work on the engine(new starter, fuel pump, manifold and riser, alternator, cap and rotor, plugs, wires, etc. etc.)I just put in a new fuel/water seperator also. Any info on what could be the problem would be appreciated as we still have a couple of months of boating weather left down here.

 

[Blk-n-Blu]

Re: 3.0 mercruiser runs rough when warm

For Play, You may be experiencing a "Vapor Lock" situation the problem is described in a Service Bulletin, Have a look here.<br /> http://www.boatfix.com/merc/Bullet/87/87_12.pdf <br />Hope it helps.

 

[forplayboat]

Re: 3.0 mercruiser runs rough when warm

Water cooled fuel line? Huh. I,m not sure I have the required high volume fuel pump either. Hmmm.<br />Thanks for the input, I'll do a little research.

 

[Boatist]

Re: 3.0 mercruiser runs rough when warm

For Play<br />I would bet on a carburetor problem. Vapor lock is usualy not a rough idle problem, yes it can cause hard restarts.<br />You say the Carburetor was rebuilt about 4 months ago. Was it rebuilt to try and fix this problem or for some other reason?<br /><br />What is the RPM SPEED at idel after the engine is warmed up?<br /><br />I would start by warming up the engine until the problem starts. Then I would pull the flame arrester and check the choke plate to make sure it is fully open. I do not think it is a choke problem but must make sure first. Next I would make sure your idle speed is at least 800 rpms. If not adjust idle speed until it is. Note weather the engine is now running smooth. If it is the idle may have just been set to low. If it is still running ruff then I would turn the idle mixture screws 1/4 turn and note the direction. Then see if it gets better or worse. If it gets worse then turn back to where it was and turn 1/4 turn in the other dirrection. Still make no difference put back to where it was. <br /><br />Other problems could be float set to high in the carburetor so fuel over flows into the engine. This would help when the engine is cold but flood the engine when hot. Also check the clear line going from the fuel pump to the Carburetor, there should be no fuel in this line. If it has fuel the fuel pump is leaking fuel into the carburetor.<br /><br />Make sure the carburetor is tight on the manifold, all 4 bolts snug.<br /><br />If you find no problem with the carburetor then I would check the points with DWEll meter.<br />Good luck.

 

[forplayboat]

Re: 3.0 mercruiser runs rough when warm

Thanks Boatist. I will concentrate on the carb first and make sure all the adjustments are what they should be according to the manual(Mercruiser). I am not familiar with the points system so I really dont know what I am doing there but will give it a shot. Thanks for the input, I really appreciate it.

 

[Don S]

Re: 3.0 mercruiser runs rough when warm

I've seen many bad coils that break down when hot that cause the same problem. Check around the coil tower for cracks also. If you see sparks jumping intermittently that could cause the rough running.

 

[forplayboat]

Re: 3.0 mercruiser runs rough when warm

Sorry in advance for the rambling post. Today, took the boat out to do adjustments on the water(and enjoy the day too). I started with cleaning the distributor area first; weights, springs etc., I then gapped the points to .016 as stated in the manual. Started the boat and it seemed to start better. I then hooked up a dwell meter to see the dwell at 30, the manual says adjust between 39 and 45, so I did this adjustment and it ran worse than the manual gap method. Put it back to .016. Better again. Cheked the timing while at idle(about 800) seemed to be pretty close (8BTDC), so i left it alone. I then adjusted the carb per the manual and the boat runs fine at idle and at low to moderate speed. As soon as I go to WOT the valves clack like there is no oil in the engine... really pretty bad. No valve noise at cruising speed up to about 3500 RPM's. Any faster than that and clatter, at WOT trying to get on plane - lots of clatter. What the hell??? Any one have any ideas. I am very mechanical and understand the principles of how this motor works. I have a Merc manual also. Please help. Thanks,Jim

 

[ron7000]

Re: 3.0 mercruiser runs rough when warm

could the "clatter" be backfiring?<br />sorry to ask, but when the clatter happens at wot, what's your oil pressure?<br />my best guess would be since it's a 1987 (and how many hours?) with points and mechanical distributor is that the distributor might have slop in it causing erratic timing resulting in poor idle and backfiring at full throttle. Normally, since you have points I would say start by replacing the points with electronic ignition (pertronix). But depending on the tools available, if you got an adjustable timing light you could try to observe timing while you're running; at a constant rpm or above 3000 or so you should get a consistent reading on the crank marking. If not, then you would need a new distributor, adding an electronic module to it wouldn't fix the problems.<br />As always, it's a good idea to hook up a manifold gauge to diagnose. Poor valve seating or stem seals will cause your problems also. On the 3.0L, it should beasy to do, along with a compression check. Verify the engine is sound internally before tuning the carb and ignition.

 

[jackrg]

Re: 3.0 mercruiser runs rough when warm

This clatter could possibly be the exhaust shutter going bad. Mine makes a similiar<br />noise.

 

[Boatist]

Re: 3.0 mercruiser runs rough when warm

For Play<br />It seem like you are making progress. I do not have a manual for your motor but a few thing seem odd. My 1980 3.0L wants a dwell of 31 to 34 degrees and 4 degrees BTDC timing. Now that you have your boat idleing better you can probably slow the idle to about 600 RPMS. On my boat I make the final adjustment on the water by shifting in foward and reverse. I adjust as low as 500 rpms and shift if it stumbles and tries to die then I increase idle until it steady then just a little more.<br /><br />Your WOT noise I am guessing is preingition. It could be caused by a couple of things. First are you sure it should be 8 degrees BTDC? My 3.0L is 4 drgrees. Second are you sure you read the timing marks correctly? What RPMS are you turning at WOT? If I am right and it is spark knock/preingition it can also be caused low grade fuel (87 octain). My engine when first switch to unleaded needed 92 octain or I would get spark knock when hot. Since I pulled the head and used a thicker gasket will run on 87 with no problem.<br /><br />Last thing it could be is valve float. I do not think this is it because in your post you said at WOT would clatter before got on plane. Valve float is normaly only at very top RPMS.

 

[waterone1@aol.com]

Re: 3.0 mercruiser runs rough when warm

My 2 cents worth. You said that you adjusted the point dwell and it ran worse, so you re-adjusted it back. Remember that timing and dwell interact. If you changed the dwell without correcting the timing, it might have "run worse" untill the timing was corrected. I would suggest setting the idle to spec, setting the dwell with a dwell meter then adjusting the timing to spec. Timing should be as exact as possble, if it is a little "off" I would prefer it to be toward the retarded and not toward advanced. When that is done, re-check the idle rpm's. And by the way, if you have not checked or replaced your fuel filter lately....not a bad time to do it.

 

[Don S]

Re: 3.0 mercruiser runs rough when warm

I guess the first thing that needs to happen is find out which engine you really have.<br />What is the serial number for your engine, is it an 86 or 87 engine, is it a 140 or a 120. Any markings on the valve cover perhaps?<br />One thing I do rememeber about the 140's is the Delco distributor bushings going bad. Have you checked for excess side play in the distributor shaft. You have to check all directions as it will only wear in one direction.<br />Below are the specs I find for an 87 140/3.0L<br /><br />

<br /><br />

 

[forplayboat]

Re: 3.0 mercruiser runs rough when warm

Thanks for the help everybody!!<br /><br /> I am using Merc manual # 13 and got my tune up info from page # 4B-2. After looking at this again it gives info for a "Prestolite" distributor, not a "Delco". It gives the tune-up specs as Dwell; 39-45, point gap;.016 and timing as 8BTDC. This is the only breaker point type of distributor listed in this manual.<br /> <br />DonS; I have a seloc manual for this motor also and it gives the same specs that you posted. I havent used the Seloc since aquiring the Merc manual but I see now that I should have compared the info given in both. What gives with the Merc manual?? I will re-do all the point and timing work and see how it goes. Thanks again everybody, I'm trying to squeak out the last bit of boating weather that I can.

 

[forplayboat]

Re: 3.0 mercruiser runs rough when warm

Engine is serial # oc303193, 3.0 litre.

 

[Don S]

Re: 3.0 mercruiser runs rough when warm

What gives with the Merc manual??

You're using the wrong manual. You need manual #10 for that serial number. Look at page iii of your manual (it's only a few pages in at the beginning) The serial numbers for book 13 start at 0C856451. Your's is well below it.

 

[forplayboat]

Re: 3.0 mercruiser runs rough when warm

OOPS. I guess that should tell me something about the local Merc dealer. Gave him the S/N and he sold me this manual. Hope he'll take it back with a little grease on it. Thanks, Jim

 

[Don S]

Re: 3.0 mercruiser runs rough when warm

And I kind of blew it too. I just looked at the front page of the service manual you gave and seen the manuals started at OC856451, now that I have taken your serial number and plugged it into the parts program, it comes up as an outdrive. So basically you gave us the outdrive serial number, not the engine.<br />Look at the tag on the engine.<br />It should be: 0B832941 <br />It's still service manual #10, but you would have real problems getting engine parts for the number you gave.