1977 Mercury 850 Thunderbolt test drive results

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MichiBoater

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My first trip to the lake with my 1975 Sea Ray SRV 180 - new to me last fall and all winter making it look pretty. Here's a summary of the trip:
1. Starting motor - pump bulb hard, full forward timing advance and choke - starts and idles high then idles slower when timing advance pulled back . So far so good
2. Put in forward gear. Only runs at idle/trolling speed for a minute or two. Throttle unresponsive all the way forward. Then throttle starts being responsive. Eventually goes to full speed - boat planes and runs great.
3. Slow back down, idle near dock. Put into reverse and stall. More bulb pumping, timing advance, and choke. Seems to work fine.
4. Shifting gears. Most of the time causes stall. Bulb pumping seems to help.

It has new plugs, new gas (50:1), and lower unit new lube (old looked good). Making good "pee stream" at all speeds.

Next step: rebuild fuel pump?

Any other ideas?

Boat lists pretty hard to left most of the time too... maybe I'll take the shark fins off and see if that goes away.
 
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emckelvy

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Jan 16, 2004
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Re: 1977 Mercury 850 Thunderbolt test drive results

Well, the bulb pumping is a good clue and points to a possible fuel pump problem. These are pretty simple to rebuild and the fuel pump diaphragm kits aren't too expensive, so maybe it's a worthwhile maintenance item to do, regardless.

Other things such as tank venting issues, bad primer bulb, plugged carb filters can cause fuel starvation so maybe check into those too. Merc has put out an upgraded bulb with a better design of check valve, your local Merc dlr should have those in stock.

When you rebuild the fuel pump, pay special attention to the check valves. If there's a bit of 'trash' or any kind of debris caught in a check valve, it'll cause leak-by and there goes most of the pumping power.

Most of the time there's nothing wrong with the check valves, but you can get them separately as replacement parts, too.

Check here at iboats or at places like www.boats.net www.crowelymarine.com or www.boatfix.com for parts.

Boats.net and Crowley Marine have parts diagrams so you can look up the part numbers specifically for your serial # range. Once you have the Merc numbers you can also plug them into the Boatfix search engine to get the equivalent aftermarket Sierra part numbers.

NAPA carries their own rebranded version of Sierra parts, so that's a good resource for parts. Best to know the numbers as most places aren't too well-versed in looking up marine parts. But if they have the part in the local warehouse, they can usually get them to you pretty fast.

Note if none of the above helps, it may be time to tear into the carbs for an inspect/clean. Especially if the motor sat for a long time before you bought it.

One last thought, check your fuel for water contamination, ethanol-blended fuels suck up water like crazy and 2-strokes really don't like that much! I had a guy come to me last year with a carb full of water, his tank was the same way. They don't run so well with 99.9 percent water-mix!!!!

HTH & G'luck.........ed
 
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MichiBoater

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Re: 1977 Mercury 850 Thunderbolt test drive results

Thank you! Great background and resources.
Another clue I forgot to mention is that the throttle lever from neutral (straight up) to full on (90 degree forward) moves easily but only seems to increase engine speed in the last inch or two of travel. In other words, push forward into gear, keep rotating, keep rotating, and only near the end of the travel does it seem to make the motor speed up. Does that ring any bells?
 
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MichiBoater

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Re: 1977 Mercury 850 Thunderbolt test drive results

Thanks for the suggestion. Sounds like I need a tachometer to get that done right.
 
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