85 HP Merc

[nmhellraiser]

Alright, I've got an 85hp motor that is giving me problems. Unsure of the exact year b/c NM doesn't require a title for motor and boat, just for the boat. Anyways, I think it's a late 70's. Here the question, I took it out today and it ran great for about 5 minutes after that it would start and idle fine but when you put it in drive and throttle it up, it dies. I went and replaced all the fuel lines, bulb, and connectors as well got a new 6 gallon tank. I can get it to full throttle but I have to squeeze the bulb as I'm advancing the throttle. Once full throttle it runs fine as long as I don't go back down to idle. Any ideas? I know it's a fuel issue but I'm stuck as where to start. Filter maybe?

 

[dwco5051]

Re: 85 HP Merc

There is a filter screen behind the fuel pump cover if this is a mid 70's 850. One screw removes the cover. Be sure to have a new "O" ring for it as they should be replace when cleaning the screen. Trying to stop a leak by overtighting the cover only results in a cracked cover. I have replace several of these covers over the years because of cracking. Pump the primer bulb hard and check for leaks around the cover. Best to keep a couple of these rings on hand so if you get a bad load of gas and have to clean the screen you will be prepared.

I would recomend rebuilding the fuel pump. On an engine that old it may solve the problem. If not it will prevent any future problems with the pump.

 

[nmhellraiser]

Re: 85 HP Merc

yeah it has the screen behind the screw. Is it possible to do away with the screen and put an inline filter in? If the fuel pump is the problem, it wouldn't effect the way the motor runs once throttled up? From what I can tell, I'm not leaking gas anywhere. The bulb gets pretty firm and even when I have to pump it before throttling up, it's fairly firm.

 

[crem1]

Re: 85 HP Merc

Once you check out the fuel pump, start checking out the carburators. Remove and soak in solvent and install new carb kit

 

[nmhellraiser]

Re: 85 HP Merc

Well I think I may have found the problem. There was a split in the gas line at the very end that ties to the fuel pump. It was enough to see the brass fitting on the fuel pump. Replaced that line and looked at the pump and filter. The diaphram on the pump looks good and is pretty tight. The filter is clean not a trace of trash. I'm thinking that split might have been enough to cause the system to lose pressure when trying to throttle up. It wasn't leaking gas though. We'll see this afternoon when I put it back in the water.

 

[nmhellraiser]

Re: 85 HP Merc

Alright, I'm back to this damn boat again. Since I have installed new cables and controls, I now have reverse. I also put a new fuel pump kit on today. Now here's my new issue. It ran fine last week other than me having to pump the bulb to get WOT but I didn't have reverse. Now I have reverse and it runs like crap. It's surging dropping 400-500 RPMs while at full throttle. It didn't do this last week. I'm guessing it's a fuel issue but maybe a bad fuel pump diaphram?

 

[Chris1956]

Re: 85 HP Merc

When you rebuild the fuel pump, you installed the new diaghram?

If it runs better when you pump the primer bulb, it is likely a bad fuel pump. However, I am betting that you have other issues, like dirty carbs.

 

[nmhellraiser]

Re: 85 HP Merc

I'm not real sure what to think of this boat. I did put a new diaphram, gaskets, and lines on when I did the change. My bulb is loosing pressure and has to be pumped to run properly. I do have another bulb to test. I've run probably 40 gallons through this with in the last 2 weeks, it likes the gas. I can run it up to 6k rpms without problems now but I usually hang around 5k. Today it completely burnt the end off the spark plug on number 1 cylinder and wouldn't throttle up once I replaced the spark plug. I had to pump the bulb, attempt to throttle up, and choke it all at the same time to get it over 2k rpms. After 2k, it runs fine. It seems to be lower rpm's that I'm having problems with.

 

[backyard mechanic]

Re: 85 HP Merc

The fuel pump diaphram operates off engine compression. What about the compression? No mention of it being checked.
STEP 1 check the compression! Especially after burning the end off a plug! Is the timing and sync off? Burning lean also means burning too lean on your oil for lubrication.

Go back to your 2 stroke basics here and start looking at fuel delivery at WOT. How does it run at idle or mid range? By the way... Be sure the vent on your tank is open.

Any fuel or air leaks? Are the reeds in good shape?

 

[nmhellraiser]

Re: 85 HP Merc

I have not checked compression yet. It runs pretty well in all rpm's. I think I have a fuel issue. It idles goods. I always mix a little more oil than what is required for a 50:1 ratio. I'm putting the old fuel pump back on today and replacing the bulb. The boat planes out fine around 3K rpms. I will check the compression on 3 of the 4 cylinders today. The 4th is too far down there to get my compression gauge in there. If the carbs were dirty, I'd figure it would run rough at idle and full throttle. I feel like this one is starving for fuel in the upper rpms and burning the plugs up.

 

[dwco5051]

Re: 85 HP Merc

What plugs are you using? To the best of my knowledge the motor used surface discharge plugs. My factory manual for this model for 1976 is 23 miles away so I can't check the plug specification right now. Be sure you are not replacing the plug with one with too far of a reach and you are knocking the end off. Aluminum melts much lower than steel so I doubt they are burning off and not doing any other damage to the piston.

 

[Chris1956]

Re: 85 HP Merc

Those motors use the surface gap plugs like Champion L-76V or L-78V. Other plugs might have bad effect.

 

[nmhellraiser]

Re: 85 HP Merc

yeah well that was my problem. I did some research this morning b/c my other boat has surface mount plugs. This one had regular plugs in it and though nothing of it since this is what was on it when I bough it. Compression checked out 115-120 on the cylinders. Changed bulb, diaphram, gas lines. Ran good today. The only issue I have is it was set to idle at 1k but it would almost die when shifting into gear. I just set it to idle 1500 and it seems to be fine.

 

[CVX20SPRINT]

Re: 85 HP Merc

Sounds like the idle circuits are plugged(or at least partially plugged) in your carbs as that idle speed is too high.I'd still clean the carbs and do a lync and sync.

 

[Chris1956]

Re: 85 HP Merc

You need to check the link and synch, and if the carbs are clean, adjust the idle mixture so the motor will idle at 90RPM or so, and accelerate cleanly.

 

[krawczmwk]

Re: 85 HP Merc

I have 50 hp Merc and was wondering if you have any advice on how to perform a link and sync. thanks mark

 

[Chris1956]

Re: 85 HP Merc

There is a post on Merc Inlines Link and Synch. Try the search function.

 

[Moody Blue]

Re: 85 HP Merc

adjust the idle mixture so the motor will idle at 90RPM or so, and accelerate cleanly.

Whooooooooa, that is a well tuned machine . A little rough I imagine though

 

[nmhellraiser]

Re: 85 HP Merc

That's the thing, it will idle perfectly at 900 but it seems like it drops rpms when you shift into gear. I'll do a link and sync as well as adjust the mixture and see what happens. So far it fish's really good so I'm happy.

 

[krawczmwk]

Re: 85 HP Merc

This maybe a stupid question. Can I use a car timing light to set the timing on an outboard motor.

Thanks