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Stalls under load

R

RGP

Regular Contributor
650 merc, 4cylinder, ser.# 2606971, thunderbolt ignition, 2 carb.

Starts easy. Revs eager in neutral. Idles down. Put it in gear at idle and OK. Ask for any throttle up from idle and it stalls. Throttle up slow or fast, no difference, it stalls.

Carbs are clean. Ignition is 7/16" spark on all four. Comp is 120 on all. Gas is clean and present at carbs.

Could this be a timing issue? Can anyone lead me through a timing procedure?

Thanks.
 
I assume you set the idle mixture screws at the sweet spot...did you add 1/4 turn CCW to richen the mixture after finding the sweet spot? Is the throttle opening up consistent w/the amount of throttle you give it?
 
Yes the idle mix was set up to the sweet spot, as you described. Throttle opening looks to be consistent.

Two years, two mechanics. Same problem, never any improvement or change.
 
Either the idle mixture is still too lean, or the carbs are opening too soon--the spark should be advanced virtually all the way BEFORE the carbs open.

Are these outboard mechanics, or auto mechanics?

Jeff
 
Thanks Jeff.

Will try all of your suggestions.

First mechanic worked on pedal cars. Second one works mostly on tricycles. But the price was right. Over $400 between the two.

Thanks again.
 
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You need to find a good / experienced outboard mechanic in southern Ontario.--------------These things are easy to work on.
 
..."First mechanic worked on pedal cars. Second one works mostly on tricycles."

Hummm.... Think I may have found your problem!

Jeff
 
fastjeff said:
..."First mechanic worked on pedal cars. Second one works mostly on tricycles."

Hummm.... Think I may have found your problem!

Jeff
Click to expand...

I aint no outboard mechanic by any means, but I had a friend with a similar motor with the similar problem. He did just what you did (carb clean, mixture, etc). What ended up fixing the problem was the carb rebuild kit because the that rubbery pointed tipped thing that allowed gas into the bowl was worn. In turn, she was flooding out.

Who knows, maybe yours is a different case, maybe its the same. But, a carb rebuild cannot hurt because you'll always gain fuel economy.

I hope I helped you with my limited knowledge.

Robby
 
Found that the carb throttle cluster was not set up right. High speed set screw was replaced with a really long bolt. It was over riding the throttle cam which controls the initial carb opening. Fixed all that. Cam now runs its full travel, gradually opening carb. Then the set screw picks up on the second finger of the throttle cluster and continues opening to wide open.

That should all sound correct. Right? I was certain that this was the fix. It was way, way off.

Water test: NO DANG DIFFERENCE. Tried adjusting slow speed idle screws on carbs for sweet spots, no change.

Will try putting in new needle valves. Carbs are so clean it's ridiculous.

Any thoughts out there?

Thanks.
 
WHEN are the carbs opening? The spark should be nearly fully advanced BEFORE the carbs start to open.

If that's already the case, try drilling the holes in the butterflies out one drill size (1/32 " larger). That's what I've done many times to eliminate bogging. Merc did this themselves on later motors.

Jeff

PS: The attached photo is not your carb, but typical.
 

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Thanks Jeff,

Carbs open up initially from idle, about 1/4, by action of throttle cam on distributor. Remaining 3/4 of opening is controlled by set screw actuating against 2nd finger of throttle cluster.

You indicate that advance must lead throttle, by a lot. I will see if I can adjust any more linkages to delay initial throttle opening.

Thanks
 
..."Carbs open up initially from idle, about 1/4, by action of throttle cam on distributor"

That's the problem! On my little boat (with its custom Merc power head) I run 100 % full spark advance BEFORE the carbs begin to open.

Jeff
 

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Jeff,

How did you achieve this, mechanically?

I now have the linkage restored to very close to what was originally manufactured. The initial 1/4 opening, by throttle cam, seems to have been the intent of the design. Throttle cam is built onto side of distributor. Full advance and then throttle opening would require some involved fabrication, or am I missing some major adjustment?

Thanks.
 
Update:

Timing was way off. Motor was running with too much advance.

Re positioned timing belt. Arrow on dist. pulley lines up with two dots on flywheel, which lines up with sticker on flywheel shroud marked #1 TDC. Checked that #1 really is at TDC. All good.

Motor will start only with full neutral throttle and run about max 1000 rpm. Back off neutral throttle and it stalls. Can't get a decent idle. Only runs with throttle plates opened about 1/4 and dist. near full advance.

New carb needles and seats. Sync and link as manual describes. Srtong 7/16" spark. 120 psi all around.

Thanks
 
Update:

Carbs were wrong. From a 50 hp. Found the right carbs and installed. Test on muffs good. Water test, same problem. Stalls under any load.

Removed power head. Split block open. Bottom crank bearing missing 10 ball bearings. Not a trace of them. Bottom crank seals shot from bearing damage. Bottom piston banged up, cylinder bore scored but not ruined. Bottom reed valve is very loose on crank, worn from excessive crank play due to bad bearing (theory).

What a mess. Where did those bearings go? Anyway, it explains the stall under load.

Thanks to all who tried to help.
 
I have most of the parts you'd need to repair it: piston and rod, reed cage assemblies, etc. Free. (Just pay postage.)

Let me know.

Jeff
 
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