Merc 3.0 bogging when run in the driveway

[craze1cars]

Well, remember I'm a new guy so please help me out...my first 4 stroke I/O and I'm not sure what to expect here.<br /><br />Maiden voyage on this 1992 3.0 Mercruiser beast will be this weekend. It's finally fully assembled, adjusted and seems ready to go. Today I put on the muffs and fired it up in the driveway for the first time since I purchased it in the fall and pulled the motor out over winter to do structural repairs, followed up by new bellows, shift cable, trim senders, impeller...all your old boat maintenence stuff.<br /><br />It started quickly and easily, idles quite smooth and steady at about 950/1000 rpms. Shifts correctly, temp and oil pressure guages looked healthily in the mid range. However it seemed to occasionally stumble (bog) and die when I give it gas from idle, and sometimes also when I bring the rpms back down to idle from a short 2,500 rpm rev. If it dies, it would restart easily by a quick hit of the key. I had it running at least 15 minutes, so I know it warmed up completely. Choke was wide open and worked properly. Plugs look good, dark dusty brown and clean, gapped to .035 spec, compression is 165 to 170 on all 4. I shot some carb cleaner in it while running just to make myself feel better.<br /><br />I guess I'm not so sure a driveway test/tune is going to answer many questions for me. Can I expect the exhaust backpressure created by being properly in the water to help it to run better? Or if it's occasionally bogging/dying in the driveway, will it probably do the same when I drop it into the water this weekend?<br /><br />I'm also hoping/praying that it might just be an "old gas" situation. There's probably 5 gallons of heavily stabilized last fall's gas in the tank. Before I drop it in the lake Saturday I'll top it off with fresh stuff to dilute the old.<br /><br />Any thoughts or words of wisdom? I do have a paddle! Thanks in advance...

 

[Don S]

Re: Merc 3.0 bogging when run in the driveway

Did you set your timing and check to be sure the advance is working. Do you have the EST distributor, and if you did, did you put it in base timing before adjusting the timing?<br />After the timing is set, you need to adjust the idle mixture.

 

[craze1cars]

Re: Merc 3.0 bogging when run in the driveway

I always get these EST/DIS things confused and my book is out in the shop...I do have "Digital electronic ignition" (no points, funny shaped GM style coil with the 2 prong plug...not the old round cylinder style) but it does have a conventional distributor cap and rotor. Which one is that?<br /><br />But to answer your question, no, I have not touched the timing at all. Nor the carb. Just pulled the whole motor as a unit and dropped it back in as a unit and hooked everything back up the way it was. Bought this beater from a neighbor who did run it quite a lot last year and it was running good at the time. Nonetheless, I fully acknowlege that it's due for a complete tune-up and I will probably do it in the very near future, maybe next week.<br />Let's see....cap/rotor/wires/plugs/set timing, probably carb clean/rebuild...have I missed anything? So thanks for the tips on that end.<br /><br />I guess I'm mostly curious if anyone here has found that the backpressure on the exhaust of being in the water has a significant effect on the way these motors run. I'm planning a short 1 to 2 hour test-drive this weekend, and I suppose I'll find out the answer for myself if I give my starter a workout.

 

[Don S]

Re: Merc 3.0 bogging when run in the driveway

You have the EST, the digital doesn't even have a distibutor cap.<br /><br />What did you do to the engine while it was out.<br /><br />No, the back pressure when in the water isn't going to make any difference.<br />Get the tuneup done before you go to the water, maybe not the carb rebuild, no reason for that yet, but getting the timing set and the carb adjusted will tell you if the carb needs a rebuild.<br /><br />BTW, you don't want that engine idling over 750rpm when you put it in gear if you want your drive to last. 1000 rpm is way too much.

 

[craze1cars]

Re: Merc 3.0 bogging when run in the driveway

I did nothing at all to the engine while it was out, I was doing structural repairs to the wood/fiberglass mounts and had to pull it to get it out of the way.<br /><br />Thanks for the tips...I questioned my RPMS being that high...book said 650 to 750 if I remember right. I put an inductive tach on it while I had it idling just to make sure my dash tach was accurate...it was.<br /><br />I'll go find myself some plugs/wires/cap/rotor for this thing tomorrow...I might as well get it over with and it's quick and easy. What are your (or anyone else's) thoughts on these tuneup products? OEM Mercruiser only? Or are there acceptable substitutes that may offer better performance and/or cheaper price maybe from NAPA or somewhere else? Thanks again for any tips of the trade.

 

[craze1cars]

Re: Merc 3.0 bogging when run in the driveway

This is just a thank you to Don S. for giving me the little push I needed to do the basic tuneup...here's what I found when I dug into it:<br /><br />1. Plug wires date code 1991<br />2. Had Champion spark plugs...actually had a different thread depth when I compared them to the correct AC plugs I replaced them with, likely were incorrect.<br />3. Aftermarket distributor cap, severely corroded and worn contacts.<br />4. Rotor absolutely fused onto the shaft, I had to break it off into 3 pieces to remove it. Burnt contacts and corroded.<br />5. Base timing was retarded by about 2 to 3 degrees from proper setting. After fixing that, I checked the electronic advance, it appears to be working like it should, advancing smoothly as RPMS go up.<br />6. Idle speed set too high (you already knew this), most likely set this way by someone to compensate for the other problems.<br />7. Carb air mixture screw...set just about right.<br /><br />Anywho...a $60 part donation to the marina for a bag full of new OEM stuff, and an hour or two of wrenching, tweaking, and tuning, and I now have a boat that is idling beautifully in my driveway at about 725 rpms, seems to have a responsive throttle with no bog, and not dying anymore. Amazing what a basic tuneup will do for a motor...I guess I've been wrenching on maintenance free computer systems in newer cars for so long I forgot to address the basics of these old fashioned systems!<br /><br />I'm sure these improvements will help make my maiden voyage on Saturday considerably more enjoyable.<br /><br />So thanks Don for all your help with this and all my other questions I've posted over the past couple months. Consider me a satisfied customer!

 

[Don S]

Re: Merc 3.0 bogging when run in the driveway

Glad to hear it worked for you. Now you know why I always start off with "Did you do a tuneup" <br /><br />It's amazing that people think a tuneup is such a minor thing, but on a boat, it's a major factor in how it all works together. <br /><br />Hope your test run/maiden voyage Saturday turns into a weekend fun trip.

 

[craze1cars]

Re: Merc 3.0 bogging when run in the driveway

Maiden voyage has come and gone. <br />everything went great! Thought I'd add a note though, regarding my question about effect of exhaust backpressure while being in the water...<br /><br />As I already stated I had it running beautifully in the driveway after I did the full tune-up. After putting it in the water, I couldn't keep it going at first...idling too slow, 450 to 500. So I turned the idle back up to about 700 to 750ish and re-adjusted the mixture screw again, and problem solved, ran like a champ with proper idle all day and didn't die once. After towing it home I put the muffs on to run it for a couple minutes to flush the lake water out of the cooling system, and it was idling up again about 1,000 to 1,100. Discussed this with a neighbor who went through a similar experience...dialed in a beautiful driveway tune, but had to do a few carb tweaks after it was floating on his first run.<br /><br />So I really do believe the exhaust backpressure has an effect on tuning, especially idle speed. I'm wondering if I should have adjusted the timing while it was in the lake, but didn't bring my timing light with me. Might actually do that next time I go out just to satisfy my curiosity (hope it's a cloudy day!).<br /><br />Nonetheless, it obviously needed the tune-up. But I thought I'd share my experience for the benefit of others who may do their tuning with muffs in the driveway.

 

[Don S]

Re: Merc 3.0 bogging when run in the driveway

So I really do believe the exhaust backpressure has an effect on tuning

It should ONLY affect the speed, nothing else. The idle mixture is set at a certain rpm and shouldn't change.<br />Something is still wrong. One would normally see around 50 maybe 100 rpm drop when in the water, but between 300 and 400 is way too much.<br />Rechcheck your timing and idle mixture and make sure your settings are at the specified rpm.

 

[waterone1@aol.com]

Re: Merc 3.0 bogging when run in the driveway

This boat is raw water cooled, right ? In my opinion, a 4 cyl. raw water cooled engine can show a 200-250 rpm difference between muffs and in the water. An eight cylinder or a fresh water cooled, not that much, but I have seen several 4 cylinders do that. If you are really getting 300 -400 rpm difference, that is a bit much. I would be wondering about the intake/exhaust manifold.

 

[craze1cars]

Re: Merc 3.0 bogging when run in the driveway

Yeah, it's raw water. I might play with it a little more and actually re-do the idle speed/mixture/timing sequence while idling in the middle of a lake. And I may have exaggerated the RPM difference a little bit...I based it on the boat's tach which does have a tendency to drift at times. But there is no question I have at least a 200 to 300 rpm diff between in and out of the water right now.<br /><br />Then again, it ran beautifully in the water, so I guess I'm inclined to just leave well enough alone. Anyhow, thanks again for both of your input.

 

[csanna2]

Re: Merc 3.0 bogging when run in the driveway

I had the same problem with a motor that sat for awhile. It ended up being the carborator.<br />I put a new one on and problem was solved.<br />One of the mechanics that work for me sugested this. The advice paid off.

 

[trog100]

Re: Merc 3.0 bogging when run in the driveway

the smaller the engine the greater the drop in engine RPM caused by back pressure caused by the boat being in the water will be..<br /><br />it makes sense to set idle speed in the water not guess at it in the driveway.. it will also drop when it goes in gear.. with a dog clutch it wants to be as slow as it can be consistant with keeping running reliably when its dropped into gear..<br /><br />it all needs doing with the boat in the water to get it right..<br /><br />trog100

 

[waterone1@aol.com]

Re: Merc 3.0 bogging when run in the driveway

Most people do their own work at home in the driveway....so it just makes sense to set the idle a little high on the muffs, knowing it will drop when the boat is put in the water. If you have to "touch up" the idle once the boat is in the water, that is fine. I would not go thru the mixture, timing, etc. That should be fine as it is.