1975 Mercury 850 ?

[tompowers85]

I have a 1975 Mercury 850 That i have rebuilt new boysen reeds, piston rings, rebuilt carbs, new gaskets, water pump, everything. I have been trying to tune the carbs using clams great link and sync process and i cannot get the engine rpm's below 1100 rpms without stalling when it goes into gear. When the engine is in gear it runs at 800 rpm's. The engine will run at 600 rpm's in gear but when i shift back into neutral and back to gear it stalls. I just cant figure it out. The carbs are so clean i can eat out of them. I have all new fuel lines and a brand new gas tank. When i prime the bulb fully a little bit of gas will trickle out of the jets. I have also found that when i run the engine and shut it off i have to prime the bulb again for it to be able to shift into gear, if i do not prime the bulb it will stall out even at 1100 rpm's. If anyone has suggestions please help. I have put so much time and money rebuilding my entire boat and engine (4 year project) i just want to get this working before i fix it with a match.

 

[whiplash921]

Re: 1975 Mercury 850 ?

How's your fuel pump?

 

[tompowers85]

Re: 1975 Mercury 850 ?

just put the pump in last week

 

[ricksrster]

Re: 1975 Mercury 850 ?

just put the pump in last week

Did the fuel pump include the check valves?

 

[Laddies]

Re: 1975 Mercury 850 ?

Is the engine on a small boat with a higher pitched prop or is the prop to much pitch for the boat? Another thing that will cause that type of problem is a engine that has been overhauled and the bores were out of round and instead of boring just rerung, if thats the case some times they will finally set the rings and some times not, depends on how much out of round or taper the cyls are and how lucky the owner is.

 

[tompowers85]

Re: 1975 Mercury 850 ?

The check valves were ok. I checked them with the test in my manual. I do think the pitch on my prop is much. I have a 21 pitch prop i would like to get a 17 pitch prop. I had the boat out today to run it around a bit. I had fully opened up for a few minutes and i was only at 3200 rpm's wich is about 2300 under max rpm's. The boat was still going very fast. I have checked the compression on each cylinder it is 150 on all.

 

[ricksrster]

Re: 1975 Mercury 850 ?

I have a 1977 850. It is on a 16'4" Trihull. Total weight about 1500lbs. I use a 13" pitch prop to get 5500 RPM. Unless you have a really small boat a 21" prop will probably be too much for the 850.

 

[adrian62]

Re: 1975 Mercury 850 ?

Have you checked the float levels?, cause mine was doing the same thing and finally I raised the floats and low and behold I got more R.P.M's, or another thing to check is the needle and seat valves they may be worn out but can't be seen with the naked eye, I know it's very frustrating, cause I've been dealing with this all summer, and finally figured it out yesterday.