850 thunderbolt

[sweet duck]

I have an older 850 thunderbolt that is not working to well. Last week I took it out and found that it would not come up to full rpm. When I did try to bring it up it would die. I took it home and replaced the fuel lines along with a new bulb and hose from the tank. The next day I took it back out again and it ran better but not right. in order to get it up to 4500 rpm I had to choke it as I pushed the throttle up. Also, when running at a slow troll it would die. Like it was running out of fuel. It runs great in my driveway but terrible under a load. Any ideas would be appreciated.

 

[Jake_MN]

Re: 850 thunderbolt

Sounds like a fuel issue. Clean the carburetors. Link and sync.

 

[bktheking]

Re: 850 thunderbolt

Stator stator stator, look at my other posts about 850's, the stators go bad and they won't run worth a damn.

http://forums.iboats.com/showthread.php?t=491516

The other issue could be water intrusion into the cylinders but test your stator first.

 

[sweet duck]

Re: 850 thunderbolt

Link and sync? I'm new to this. Are there directions somewhere?

 

[sweet duck]

Re: 850 thunderbolt

Where and what is the stator. I looked at your other posts and that sounds like my problem also.

 

[bktheking]

Re: 850 thunderbolt

Stator does 2 things, provides power to the switchbox for spark and provides charging of the battery. It is located under the flywheel. I made a youtube vid of how to change one but I haven't tested the motor yet so I haven't had a chance to finish the video.

All I did was test it, verify it was bad, remove the upper cowl from the motor, set the flywheel TDC so I could replace it and know where the timing marks go, remove the 8 bolts from the flywheel, lifted it off and remove the 4 allen screws holding the stator down, disconnected the wires and installed the new stator in reverse.


Battery disconnected of course.

 

[Jake_MN]

Re: 850 thunderbolt

Out of curiosity, how would engaging the choke counteract a failing stator? Not saying you're wrong, just genuinely curious.

 

[bktheking]

Re: 850 thunderbolt

I see the point however it could be one of two things, carbs or stator/switchbox, the stator test is much quicker than ripping off carbs and cleaning them.

 

[Chris1956]

Re: 850 thunderbolt

Does that motor have a distributor? If so, the stator plays no part in the ignition. If choke tends to fix the problem, it does point to carbs being dirty.

 

[bktheking]

Re: 850 thunderbolt

A year or serial number would help, should have asked long ago.

 

[sweet duck]

Re: 850 thunderbolt

The serial number is 4725445. that is the number on the front of the motor. I have the carbs off already and cleaned. I will be putting them back on sometime this weekend. Hope to have it in the water monday. hopefully I get the carbs syncronized properly. I will ohm the stator first though. thanks for the help.

 

[Jake_MN]

Re: 850 thunderbolt

Good luck. If you can please report back with the results. I'm interesting in seeing what you find.

 

[bktheking]

Re: 850 thunderbolt

I ran that SN, couldn't find squat.

 

[sweet duck]

Re: 850 thunderbolt

Ok, just pulled the boat out of the water. After cleaning the carbs it ran better, but not great. When I give it full throttle it comes right up to where it should for about 15-20 seconds then dies. I can start it up again and do the same thing. runs good at idle but still acts like it is lacking fuel at high speed. Also, it seems to be using a large amount of fuel. I was on the lake for approxamately an hour and went through 3 gallons of fuel. This thing will cost a mint to run.

 

[gbrowne9]

Re: 850 thunderbolt

take it out one more time and squeeze the primer bulb when it starts to die. if it runs better replace the feul pump diaphram.

 

[PHE]

Re: 850 thunderbolt

Agree with gbrowne9 on diagnosing the fuel pump. It might not be keeping up. Usually if the diaphram goes bad it runs rich because of leaking fuel into the crankcase. I've heard that the check valves in th fuel pump can get degraded by the alcohol in current fuel. Especially if they are original. If the check valves leak, it will not be able to keep up the fuel flow at full throttle. Instead of buying just the diaphram, get the check valve kit. It contains the check valves and diaphram for around $20.

 

[sweet duck]

Re: 850 thunderbolt

I took the fuel pump apart and found that the diaphram was cracked and leaking. Rebuilt the pump today and hope to put the boat in the water the first of the week. Thanks for the help I will let everyone know what happens.

 

[79Merc80]

Re: 850 thunderbolt

As for fuel consumption, your engine should burn about 8.5 gallons per hour at WOT (~10% of HP rating