1973/74 850 thunderbolt

[Strahm]

First let me tell you a little bit about my engine. it is a 73 or 74 mercury 850 85 hp i have had and used it steadily for the past 3-4 years, before that it was sitting in a barn for 10+ years.
when i got it, if i remember correctly i changed the spark plugs, and thats about it engine wise. it ran pretty decent for sitting in a barn the past ten years, so instead of put money into it i drove it for a few years. it runs good at full throttle, aside from having a little bit of a stutter when going from slow to fast (and this is only sometimes)
the worst part is, idling most of the time i have to tap the choke button to keep it going. also starting takes raising the idle lever all the way up and holding the choke button.
I am the only one who can drive the boat, as i know when to hit the choke button and when not to to keep it running.

when i pump the fuel bulb, if i pump a lot gas seems to come out of the carb.

my plan is to rebuild both carbs
my question is, should i rebuild anything else while im at it? i dont want to spend a lot of money as it is an old boat and there is no telling when it may have a serious problem, but if it should be done and is fairly simple/cheap i will do it now.

what do you guys recomend doing? nothing has been done to it in 10+ years.
water comes out of the "pee hole" steadily so im not sure if the water pump/impeller should be changed?

note i am pretty handy, and have access to every mechanic (car) tool you can think of, but i am no boat expert so if it is too hard to do i dont want to do it unless completely necesary. i would rather have a decent running boat than one in pieces or running worse because im not capable of doing the work.

 

[jwbandy]

Re: 1973/74 850 thunderbolt

I recently bought what i believe to be a 1975 850 thunderbolt. I have all most all the same problems. The motor does not want to idle and also needs to be choked at full throttle. It also throws fuel out of the carbs after pumping the fuel bulb multiple times. After the motor is running when i turn the idle throttle down and try to put it in gear it almost always dies so i have to pull the idle throttle down and put it gear at the same time.

Recently checked the compression. 1,2, and 3 were all around 120 lbs but cylinder 4 was at 80. I'm sure it has a lot to do with my idling problem but isn't my only one.

I also started to check my wiring and found that someone has tried replacing some of the wires. I replaced burnt wires and found someone tried replacing the wires where the control wires come into the motor. Haven't had a chance to look into the wiring coming from the control box but whoever tried working on them spliced the wires very low with no room to work.

Not sure what steps I need to take to make the motor idle but not looking to spend more money then i need to. I'm sure i need to put new rings in to solve my compression problem and rebuild the carbs.

I would recommend checking your compression. After reading other post checking the compression is almost always mentioned when troubleshooting.

 

[Chris1956]

Re: 1973/74 850 thunderbolt

Strahm, Clean and rebuild the carbs. Install new gaskets, floats and inlet needle and seats. Carb idle mixture screws are initially set to 1-1/2 turns open, and adjusted to be a bit richer from there.

Rebuild the water pump with a new impeller, wearplate and all gaskets. Check the idle pickup and max timing. Specs are 4-6* BTDC for idle pickup timing and 21* Max advance. Max advance was lowered from original recommendation, when we figured out how lousy the gasoline is today. Clean the dist cap. The rotor is non removeable, and $$ so treat it like a piece of glass as it is fragile.

JW, you seem to have more issues than Strahm. If your motor has a distibutor, my advice is the same. If it has 4 coils, the timing specs are different. Post back after carb cleaning/adjustment.

 

[full_restoration]

Re: 1973/74 850 thunderbolt

Hi, new to the forum, and have a 1973-76??? 850 as well. Similar problem with the lower carb spitting out fuel into the engine compartment when running and when pumping primer bulb. Chris1956, is it required that i rebuild the carbs? or would it be possible to just take it apart clean it? Reason i ask is that i am trying to get this thing splashed asap, and at a low cost. Thanks

 

[1960 Starflite]

Re: 1973/74 850 thunderbolt

I had the 1500rpm idle problem also. However my engine is older than yours, 650 with magnito. I went thru the carbs, fuel pumps, points, timing, ect. Still wouldn't idle below 1500. I pulled the magnito again and while checking the plug wires with ohm meter, I bumped the leads across the magnito cap posts and my meter showed a reading. I found very small cracks in the cap which let stray high voltage to fire plugs at random. Replace cap and all was fine. May not be your problem, just to offer another possibility.