1976 Mercury 85 hp Thunderbolt.

[Bob1944]

How is fuel drawn from the fuel tank into the combustion chamber on the above engine.

 

[chum1]

Re: 1976 Mercury 85 hp Thunderbolt.

Just like a snowmobile, vacuum pulses act on a diaphragm in the fuel pump through check valves sucking the fuel out of the tank

 

[Bob1944]

Re: 1976 Mercury 85 hp Thunderbolt.

Would you think the replacement of the diaphram would pump enough fuel in the engine to keep it from stalling as it throttles up?

 

[SeaKaye12]

Re: 1976 Mercury 85 hp Thunderbolt.

That's a tough question to answer. What else have you checked?

The pick-up screen in the tank could be clogged.

The hose from the tank to the engine could be collapsed internally; even though it looks OK from outside.

The vent on the gas tank could be closed.

You might be sucking air someplace in the gas line due to a loose or faulty connection.

A question:

When the engine stalls; is the primer bulb hard, soft, or collapsed?

Another question:

When you advance the throttle....does the engine just die immediately (no hole shot) or does it advance to WOT and then stall after it's been up there for a few seconds?

Post some more details and I can tell you more about what to look for.

Chuck

 

[Bob1944]

Re: 1976 Mercury 85 hp Thunderbolt.

We have three gas tanks, one new, and it happens on all of them.

It is an immediate stall on throttling up in the lake with a load on the engine.

I never noticed if the bulb is hard or soft.

points and plugs are good, good spark, timing is good. Once it is up and running it runs great, like it is a strong powerful engine.

I can't think of anything else off the top of my head.

 

[chum1]

Re: 1976 Mercury 85 hp Thunderbolt.

This may (also)be a spark issue under load, if there is a way you can just trailor the thing into a lake and run it under a load ( it must run under a load) hook up a timing light to each individual spark lead and see it it fails.

 

[jerryjerry05]

Re: 1976 Mercury 85 hp Thunderbolt.

Ethanol,not a good thing.Needle/seat,pump diaphram.Crap in the carb.Hoses delaminating.bad check valve in the squeezie.
I'd go with the diaphram first.J

 

[Chris1956]

Re: 1976 Mercury 85 hp Thunderbolt.

Bob, That motor does not have points. Immediate stall upon throttle up is indicative of dirty carbs, incorrect idle mixture or both.

Did this trouble just start? Was the motor layed up a long time? is it new to you?

You might take a compression test and a spark test and post the results.