1975 Mercury 850...

[Texas3141]

I have a 1975 Mercury 850....do they ever really idle right? I have taken my boat out several times this year and have had issues at times with the motor dying off as it idles for a couple of minutes at 900 RPM. The bulb is good and my lines are clean. It usually starts with ease but with the engine trimmed up with the lower unit still in the water of course. I was told that it should start with the engine trimmed all the way down...it doesn't as easily.

Once started and warmed up it will sit in neutral at approx 1100 rpm. When it's put in gear at idle speed and 900 rpm there have been several occasions when it dies off. I normally have to go slightly past the idle notch to make sure it doesn't die. It sucks because no one else can operate the boat in order for me to have any fun. I replaced my powerhead about a year ago since my other one had low compression on one cylinder and was having issues with idle. It was doing about the same thing this one is doing now. I have completely broken down both carbs and cleaned them. I found that the idle/low speed jet was clogged in one of the carbs. I cleaned both well and no more blockage. I thought this would fix the problem but it acts exactly the same as before. I decided to check compression on this powerhead. At operating temp is (1) 124psi (2) 121psi (3) 124psi (4) 120psi. I don't think its a compression prob. Another thing is that after about 4400 rpm it sometimes surges down and back up. Other times it will run good at those levels. My main complaint is the idle issue. Will a goofy trigger or rectifier cause these issues? Thanks in advance for any advice.

 

[BossMaverick1]

Re: 1975 Mercury 850...

When you rebuilt the carbs, did you happen to replace the float needs and seats?

My thoughts are:
A: Leaky float needles
B: Bad carb floats
C: Incorrect float height
D: Incorrect jet settings
E: ????

 

[Chris1956]

Re: 1975 Mercury 850...

The idle mixture on that (and most of the IL4/6 motors) needs to be set just right to get a good idle. Likely why Merc added the idle stabilizer to newer models. The idle mixture likes to be set lean for good idle quality and must be set rich enough to allow smooth acceleration. So, mix up 6 gal of fresh 50::1 fuel. Exactly 50::1. Now take some time on the water to set the idle mixture just right, lean as possible with good acceleration. Now if you keep the fuel fresh, it will idle reasonable well.....Best you can do, assuming link and synch is correct.

 

[Texas3141]

Re: 1975 Mercury 850...

When you rebuilt the carbs, did you happen to replace the float needs and seats?

My thoughts are:
A: Leaky float needles
B: Bad carb floats
C: Incorrect float height
D: Incorrect jet settings
E: ????

Thank you for your reply. The jets are a fixed size with no adjustment allowed. I will try a different setting near the float top. Both floats are new.

 

[Texas3141]

Re: 1975 Mercury 850...

Thank you Chris1956. I have battled with this type of engine for about 5 years and know how difficult it is to get the "sweet spot". I just wish someone would tell me that its something different I guess. Haha. I spent part of my Monday with my neighbor at the helm as close to idle as possible while I sat in the back messing with the idle screw trying to get it just right. I never seem to get the same result twice though. Temperamental I guess.

 

[emckelvy]

Re: 1975 Mercury 850...

Another issue that can cause poor idling are missing bleed restrictors. They are plastic and over time will get sucked out. There are 3 of them on a 4-cyl and you'd need to pull both intake covers to verify they're missing. Part #11 in this diagram:

Mercury Marine Model 850 85 HP(4 CYL.) CYLINDER BLOCK AND CRANKCASE ASSEMBLY Parts

Another posssiblity is worn reed block labyrinths. Unfortunately the only way to check for this is to pull the powerhead & split the crankcase to gain access to the crankshaft. The edges of the grooves should be sharp and well defined, if they are pounded over they should be replaced.

A guy on the "b.mercury-inlines" newsgroup (parent group "news.bigweek.org") has devised a way to recondition the reed blocks, by machining out the worn center, and installing a newly machined split-center which is pinned to each reed block half. I was over at my buddy Doc Frankenmerc's shop a while back and he showed me a set he got from the guy, they looked real nice.

I definitely would check for missing bleed restrictors first, though, a lot easier than pulling the powerhead.

On the surging, if its not a carb or fuel pump issue, you might want to pull the distributor and check for "Trash-in-the-Cap". I've had quite a few that had a misfire from trash/debris causing arcing inside the cap.

Check the spring-loaded carbon contact in the cap, it should be free to move in-and-out. If it's all gunked-up, pull it out (twist as you pull) and clean.

An effective way to flush all that gunk out of the cap is to flood it with carb cleaner, it'll blast that nasty stuff right out. You can clean up the contacts in the cap & on the rotor with some medium sandpaper. Use caution inside the dist. trigger, the rotor Does Not come off the shaft, and the trigger pickup can be quite fragile if banged upon.

On the trigger thing, they can start running a bit screwy when the trigger is in its first stages of failure, but typically it's not long before it stops sparking altogether. You might try running the motor in the dark with the cowlings off, and check for arcing/leaking spark plug wires. Even a leaky spark plug boot can cause problems. If you can get it in the water and idle around, you can use a spark tester ("firing indicator") on each spark plug wire or snap an inductive timing light around 'em, one-at-a-time. You'll be able to see right away if there are any inconsistent sparking issues.

HTH.............ed

 

[merc850]

Re: 1975 Mercury 850...

I have an 850 too and I found crap in the lower carb filter.I took out the filters because there's one after the fuel pump and in the tank to filter the fuel.

 

[emckelvy]

Re: 1975 Mercury 850...

I have an 850 too and I found crap in the lower carb filterView attachment 200341.I took out the filters because there's one after the fuel pump and in the tank to filter the fuel.

Yeah, Merc actually issued a Service Bulletin to take out those little screens, 'cause they get plugged and cause fueling problems.

 

[Texas3141]

Re: 1975 Mercury 850...

Sorry I've been at work today and was unable to check my thread. 10 hr shifts are a pain in this heat. I will check for missing bleed restrictors like you suggest. Never heard of them before. I appreciate all the responses I've gotten today. Wow.
Also, a local merc repair place mentioned bad reeds before on my other power head. He said the way to look for them is if I saw some fuel spray going out from inside the carb throat. I completely broke down my other power head and saw that those reeds were good. Ill check into those bleed restrictors. Thanks again

 

[Texas3141]

Re: 1975 Mercury 850...

Oh and I haven't mentioned that I have replaced all spark plug wiring including the carbon piece with the spring below it before. My main coil is aftermarket from the aftermarket CDI switch box. I left the original trigger on this power head because I didn't want to change it out. I do have a brand new cdi aftermarket trigger sitting already installed on my other power head's distributor. Thanks for all the replies.