'76 Merc 850 won't throttle up in fwd

donspaulding · Jun 24, 2005

Hello all! This is my first post and believe me, I wouldn't be posting if I hadn't spent three hours searching google and iboats for an answer. A little history: I have a 1979 16' Mark Twain V-hull with a 1976 Merc 850 Outboard. This boat was run hard for 3 years and then sat for 2 years before I got to it this spring. I put new gas in it with a fuel additive. I put new plugs in and tested compression on the top three cylinders (all showed about 112) since my tester wouldn't fit in the lower one because of the lower cowling. I also drained the gearcase oil out of the lower unit (the oil was black but didn't show any signs of water or metal shavings). My problems: The motor starts easily. It will slow/regular/fast idle all day long both on the muffs and in the water. It used to not drop into gear in reverse, but after adjusting the shift cable at the engine, I got it to drop into reverse without losing forward or neutral. All shifting/throttling works on the muffs, but it won't go in reverse in the lake, and won't throttle up past a fast idle in forward. Any ideas why it would behave so differently under a load? Any help is appreciated. I can provide pictures if they would help.

Bob H. · Jun 25, 2005

Re: '76 Merc 850 won't throttle up in fwd

Sounds like you need to start over. The problem is probably in the shifter, springs, balls, etc. Now that you've tried to make corrections at the engine you'll need to start over after taking the shifter apart.

donspaulding · Jun 25, 2005

Re: '76 Merc 850 won't throttle up in fwd

I assume you mean taking apart the shift controls at the helm. I had done that last year before I had a manual, so it might be in there. I'll rip it open again and see if I put it back together right the first time. Again, does anyone know why would this make it behave differently in and out of the water?

donspaulding · Jun 30, 2005

Re: '76 Merc 850 won't throttle up in fwd

OK, I tore apart the shift controls. Everything looked to be in order. It just doesn't seem like the shift lever is "catching" the throttle until it's almost all the way forward. It won't catch at all unless the neutral warm up lever is all the way back, but then the engine is running at too low an RPM to drop into gear without killing it. Here's my new question: Is there any reason why I couldn't increase the idle speed so I could drop it into gear with the neutral warm-up lever all the way back?

Chris1956 · Jul 1, 2005

Re: '76 Merc 850 won't throttle up in fwd

Don, The warm up lever needs to be all the way down, or the control will not allow you to throttle up. You can raise the idle speed by turning the idle stop screw inward. Try to keep the idle speed below 1000 RPM. if it won't rev up while in the water (underload) it is possibly a too lean carb idle mixture(need to reset it), dirty carbs or some link and synch issue.

donspaulding · Jul 11, 2005

Re: '76 Merc 850 won't throttle up in fwd

Thanks Chris. After talking with someone at a Marina, I was informed about the warm-up lever needing to be down. It makes sense now why it wasn't throttling up. I tried to increase the idle speed but when I went to start it up after adjusting the screw it wouldn't start. The starter motor spins full speed but it isn't being thown up into the flywheel. My manual says this is because of a "gummy" Bendix drive. Anybody know a good/easy way of lubing it up?

Chris1956 · Jul 12, 2005

Re: '76 Merc 850 won't throttle up in fwd

Don, Sometimes if the bendix drive has too much grease on it, it sticks in the down position. If this is the case try to wipe off the grease and maybe use some WD-40 to try to clean it up. If there is some rust on the bendix, they can stick as well. Is so, lube it with light grease or oil, and work it until it operates smoothly.

donspaulding · Jul 12, 2005

Re: '76 Merc 850 won't throttle up in fwd

Can this be done while it is still on the engine?

donspaulding · Jul 13, 2005

Re: '76 Merc 850 won't throttle up in fwd

Nevermind. I took another look at it and it didn't appear to be sticking at all. I could spin it up by hand. I took a look at the rotation and noticed it was moving the wrong way. Looks like I just had the cables reversed on the battery, DOH! Now I can test out the raised idle speed to see if that does it. I'll let you know.

Chris1956 · Jul 13, 2005

Re: '76 Merc 850 won't throttle up in fwd

Don, You might want to check to see if your rectifier is still good. A lot of times, when you reverse the battery cables, it blows the rectifier.

donspaulding · Jul 14, 2005

Re: '76 Merc 850 won't throttle up in fwd

The rectifier for the charging system? Do you know any good way to check it? I have a digital multimeter, but my manual's not the clearest on how to check it out. You can view my boat here. Or for a close-up of the area near the rectifier, try this

donspaulding · Jul 21, 2005

Re: '76 Merc 850 won't throttle up in fwd

OK, I took it out a couple more times. I'm back to my original problem. After the engine warms up, I can drop the motor into forward and it will run at low (no wake) speed. If I try to move the throttle forward, it acts like it starts to speed up, but then immediately bogs down and dies. It does this no matter how fast or slow I increase it. I think I can rule out fuel/air starvation since the engine will run to WOT while in neutral. Is this correct? My newest idea is the prop. It fell off when I first took the boat out last year. We found it in shallow water and I bought a new nut and washer and put it pack on this spring. But I keep reading about a "thrust washer". Is this something that could have come off with the prop? Would that cause the engine to bog down as it tries to accelerate in forward, and reverse gear to almost not work at all?

donspaulding · Jul 21, 2005

Re: '76 Merc 850 won't throttle up in fwd

Also, what does it do? Will it hurt the engine if I run without it?

Bob H. · Jul 22, 2005

Re: '76 Merc 850 won't throttle up in fwd

Running WOT on muffs can cost you an engine - it needs a load. Also, with no load the engine may rev but not rev under load. Is the fuel bulb firm when it bogs down? The easiest fuel items to check first is to replace the fuel pump diaphram (dirt cheap) and check the fuel tank vent. From there you'll need to clean the carbs if the problem continues. The carbs are probably running lean. Do not run without a thrust washer if your prop calls for one. Running without the thrust washer will cause expensive damage to the lower unit. The thrust washer will not cause the engine to bog down. It sounds like you still haven't adjusted the shift/throttle controls correctly. While you're checking things it wouldn't hurt to change the lower unit oil and check for water intrusion(muddy looking oil).

donspaulding · Jul 22, 2005

Re: '76 Merc 850 won't throttle up in fwd

Thanks for the reply Baitis.

The easiest fuel items to check first is to replace the fuel pump diaphram (dirt cheap) and check the fuel tank vent. From there you'll need to clean the carbs if the problem continues. The carbs are probably running lean.

What I need to know is what happens when I throttle up in forward that's different than speeding up the engine in neutral. All of the above items should affect the engine regardless of whether it's in gear or not, correct? If it's fuel or air starvation, it should happen anytime the RPM's increase, right? Does everyone agree that there's no way the abscence of a thrust washer (which my prop calls for) will cause the engine to bog down? And I just changed the lower unit oil.

Joe Luedee · May 24, 2008

Re: '76 Merc 850 won't throttle up in fwd

Hey Don, i realize its been awhile since you posted info re your 850 merc, however, i just got a 76 850 free... and its doing the same thing your motor did... Did you ever resolve your problem? If you did what was it... ive tried everything and cant resolve it... motor has 120lbs compression all cyl., bright blue spark all cyl, i rebuilt the fuel pump and carbs... i cant believe the timing would be off so far as to cause this problem ... ? any help appreciated tnx

'76 Merc 850 won't throttle up in fwd

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