Merc 800 stumbles when going into gear

[Moody Blue]

1971 Merc 800 new to me last year. Had it on the water for the first time today after following the sync and link procedure. Got the motor to start easily and idle well in neutral at 700-800 rpm. Whenever I shift into gear it immediately dies. I backed out the idle stop screw to allow me to experiment and I was able to manually (throttle cable disconnected) rev the motor just a little to get past this hesitation then it would idle at 800rpm in gear very well. When going back into neutral, rpms then would jump to 1200.

I did the idle mixture adjustment thing over and over and I'm certain I've got the optimum setting.

So here is the situation in a nutshell. I can get it to idle at 800 in neutral but dies when put into gear.

I can bump the throttle to keep it running when going into gear then adjust the idle speed (per sync and link) down to 800. Problem is when I go back into neutral, the rpm's jump to 1200. There seems to be a dead band of about 400 rpm that I can't get around.

So I set it up to idle at 800 in neutral and took it out onto the lake for a shakedown cruise. Motor ran strong and flawlessly for 2 hours. I noticed during this time that the tendency to stall when going into gear was not nearly as bad as originally, and I could shift into and out of gear with some reliability. There was still the hestiation like it was going to stall but it seemed to recover and keep running on its own more often than not.

Hole shot response was fantastic. No hesitation at all.

Any advise on what to look at to fix this stumble problem?

 

[Karl_Childers]

Re: Merc 800 stumbles when going into gear

Change the sparkplugs

 

[Moody Blue]

Re: Merc 800 stumbles when going into gear

Change the plugs?

 

[emckelvy]

Re: Merc 800 stumbles when going into gear

Are you adjusting the idle mixture in gear, after you warm it up, then adjusting for stable idle speed? You have to set things up in the water with a load. Adjusting in Neutral will only get you close.

800 rpm idle speed is just a ball park setting. Set it up for the slowest, stablest idling speed in gear, adjusting both idle mixture screws and the idle stop.

Once you have that, check your hole shot. If you don't have a good hole shot, go 1/8 turn CCW (rich) on both idle mixture needles. Then check hole shot, readjust if you're still stalling out.

If you find you need to increase idle speed to get it to idle reliably in gear, that's what you'll need to do. I've had 'em such that they wouldn't idle much below 1000 rpm and then jump up to around 1200 out of gear. Hopefully when you get yours 'tweaked in' it'll be better than that.

Sometimes, having a large differential between stable idle speed and idle in Neutral is indicative of a worn-out motor, or worn reed blocks. Not much you can do about that except tear it apart, unfortunately.

If your motor has good compression and you can get it to idle acceptably, maybe you can live with a tad high idle. If not, it might be time for a teardown/inspection. It doesn't take much wear on the reed blocks to cause a cranky idle.

HTH & let us know..........ed

 

[Moody Blue]

Re: Merc 800 stumbles when going into gear

Yes, adjusted in gear in the water. After some more reading, I'm thinking maybe the 3-4* BTDC at initial throttle pickup is not enough. My manual (Seloc ) states 5-7* whereas the post by ClamsCasino suggests 3-4*. Will readjust and try again on the weekend.

The curious thing is that I could slightly goose the throttle as it went into gear and it would get past the stall tendency then idle smoothly. Not too concerned with a slightly high idle (800 -1000) but I want a reliable shift and idle.

Pretty sure I have the carbs adjusted to the sweet spot. Like I say, it idles well and the hole shot is flawless.

Compression is excellent on all 4 cyls. I did check spark on all cyls but not spark strength. Could a weak spark be a cause of this stumble?

Am I on the right track?

 

[rtpassini]

Re: Merc 800 stumbles when going into gear

Moody!!!! I thought you were making a post mocking me because of my problems!haha.

Anyways. Im not sure if your motor has this sticker, but on the front of mine, it has a sticker that says .172 BTDC and 27 degrees.

not sure if that means anything to you or helps you.

 

[Moody Blue]

Re: Merc 800 stumbles when going into gear

Rt, thanks for that info.

I do have the same sticker but it refers to the max advance timing and no longer applies because of the different fuel quality today vs 40 years ago. The accepted number today is about 21* for max advance timing.

Maybe between the two of us we will get this stalling thing resolved. I'm back at it this weekend and am determined to get it fixed. I'll keep you posted.

 

[rtpassini]

Re: Merc 800 stumbles when going into gear

ill be getting the new waterpump housing by then too. so ill be working on mine this weekend.

as far as motors go, i know you are supposed to use the lowest octane possible without knocking. and also, the best spark you can get (obviously).

is it not normal for it to idle higher when not in load? every car ive driven bumps up in neutral. even the forklift at work that runs on LP does.

i thought 800-1000 for shifting into a gear was bad for it? no?

 

[Karl_Childers]

Re: Merc 800 stumbles when going into gear

Does it shift ok on the hose in the driveway? I am guessing that when you put it in the water, you are increasing the backpressure and thus messing with the compression. Compression affects spark. If your spark plugs are bad, this could cause your problem. I have had a perfectly good looking, fairly new plug cause me to tear down my carb on my motorcycle 4 times. Spark plugs are cheap, give it a whirl, you might get lucky, and I might be right for once this century.

 

[Moody Blue]

Re: Merc 800 stumbles when going into gear

Karl you are right, plugs are cheap and the existing plugs came with the motor so I have no history on them. Will take your advice and get a set of new plugs.

 

[Karl_Childers]

Re: Merc 800 stumbles when going into gear

Hey Moody, Just curious what the outcome was with this? Knowing the cure makes the symptoms not as scarry in the future.

Karl