850 Thunderbolt won't start, need help!

[truman3]

I recently purchased a boat that has a 1976 mercury 850 thunderbolt on it. The motor will not start. The previous owner said that he did a compression check, which turned out good. He said that it wasn't getting fire. I checked it out myself and it is getting fire. The fuel line needs replaced, so the carbs aren't getting enough fuel, so I squirted some gas in the carbs but still would not hit. I was wondering if anybody had any suggestions on what to check.

Thanks

 

[bryanwess2000]

Re: 850 Thunderbolt won't start, need help!

1st don't trust anything someone says about a nonrunning motor. Its been
my experience that they flat out lie or don't know and are just repeating
something they were told. After you confirm you have good even compression
and good spark 3/8" or better on a tester, then check your fuel pump, carbs
and do a link and sync. The old 1970's inlines had bad wiring, I had to redo
alot of wiring on mine and make another wiring harness. Read the post
awakening a sleeping outboard, it's a sticky at the top. good luck.

 

[rcam97]

Re: 850 Thunderbolt won't start, need help!

Bryan is right. I just reworked the ignition system and wiring harness on my 77 850. I also rebuilt the carbs. If you rebuild the carbs go ahead and get new floats, needles and seats. If the original floats are still in there they will probably deteriorate after you reassemble the carbs. They did on mine so I had to take mine apart a second time and replace the floats. Also go ahead and put a new complete water pump kit in while your working on it. Of course I checked to make sure I had good even compression across all cyliders before I put all that money in it. Hope this helps. You should be happy with this engine if it you get it running good, mine is pretty quick for a 85 hp engine. Oh, almost forgot, be careful removing bolts on this engine because they are corroded alot on these old engines. Also check your wiring for bare wires and shorts.

 

[truman3]

Re: 850 Thunderbolt won't start, need help!

I did a compression test and it had good compression on all 4 cylinders. I also went through the carbs. The motor now will hit every once in awhile now but will still not start. Any ideas?

 

[rcam97]

Re: 850 Thunderbolt won't start, need help!

The first thing is that you need a repair manual it will help you alot with this because it contains all of the specs for the timing, carbs, and everything else. The idle mixture screws on the carbs need to be 1 1/2 turns out from lightly seated to start with then when its running you will fine tune this. Also make sure your fuel pump is pumping fuel to both carbs. Pump the fuel bulb up on the fuel hose and start the motor over, if no start then loosen the brass bolt on the front of the carbs at the bottom on the float bowl. If fuel leaks out then you are recieving fuel. There is the possibility that there is not good fuel pressure even still though. Also the float height for the carbs needs to be 11/16" from the base of carb to bottom of float while holding the carb upside down. Another big thing I can see that might be wrong could be the timing. Remove all plugs except #1. Hook timing light to #1 wire and have someone turn the engine over with the key while you check the timing. You need a manual to do this though because you have to make sure the timing pointer is lined up correctly to get a correct reading on the timing.

 

[JaseBosto]

Re: 850 Thunderbolt won't start, need help!

If you can't get it to run by spraying gas in the carb, try to crank it over at night with the cowling off. If you see fireworks in the wireing, that may be your reason sonny.....

 

[truman3]

Re: 850 Thunderbolt won't start, need help!

Ok thanks for the info. I am going to order a manual today. Probably won't have time to work on it until this weekend but I will post what I figure out.
Thanks again.

 

[Bob_VT]

Re: 850 Thunderbolt won't start, need help!

The older Mercs were notorious for having the wires break down and short out.

A manual is a great suggestion and so is cranking it in the dark.

 

[truman3]

Re: 850 Thunderbolt won't start, need help!

I adjusted on the floats in the carbs and also checked out some of the wiring. The wiring is in pretty bad shape so I replaced some of it and plan on rewiring most of it. I finally got it to start and it will run for awhile then it will die. I had to give it quit a bit of throttle to keep it running. I am going to try to adjust on the carbs. Any other suggestions?

 

[rcam97]

Re: 850 Thunderbolt won't start, need help!

Glad to hear you got it running! After you get the wiring straightened out, get your timing set properly and adjust throttle and idle stops. Then fine tune your carbs. Then drive it a good bit, letting it sit is one of the worst things you can do.

 

[truman3]

Re: 850 Thunderbolt won't start, need help!

I am trying to get the wiring all figured out. I have found the internal wiring harness which is part number 414-3369 or 73369, but I am having problems with the external wiring harness (from the controls to the engine). The one that is on it now is part #45876, but I am having problems finding that part. I have found a couple but according to the pictures that is a 7 pin harness and when I look up the internal harness it shows that it is a 6 pin harness. Does anybody have any insight on that or know the part # to the harness I need? Thanks.

 

[truman3]

Re: 850 Thunderbolt won't start, need help!

I have got the motor to where it will run with the fast idol on, but as soon as you put the fast idle all the way down it dies. It doesn't try to run at all, it is like you just turn off the key. Any ideas?

 

[Moody Blue]

Re: 850 Thunderbolt won't start, need help!

Have you gone through the link & sync procedure yet? Could be that you motor is just not properly set up.

The procedure is here

http://forums.iboats.com/showthread.php?t=168855