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what am I missing?.. it STILL wont stay running shifting into gear

C

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Regular Contributor
I have this 850 that I disassembled and freshened with new rings, restrictors, upper and lower main bearings, new gaskets, new spark plugs, carb and fuel pump rebuilds, with all new fuel lines...

it is a 1975 merc 850, the specs call for 2-5 initial advance, and 27 max advance.. I have it within the specs

after learning how to set up the carb/advance sync, I have gone over it several times to set and reset and to insure I got it right and havent missed anything.

it starts a bit hard, but otherwise runs great and idles absolutely smooth, but for the life of me, I can not get it to stay running when shifted into gear in the test tank... on the muffs the problem does not show itself..

the flywheel and distributor gear are in their respective positions....the pickup advance is set to 3btdc... the max advance is set to 26btdc.... the idle in neutral is 950 in the test tank.... the prop is a thin bladed 17pitch... the gas is NON-ethanol, mixed as close to 50:1 as possible.

I have adjusted the carbs to max rpm using a TA100 smart tach to get the highest rpm from the motor, and found that to do this, the screws arent adjusted the same amount of turns out, but withing 1/2 turn from each other... the engine idles smooth without a hickup..... as i turn the screws out another quarter to richen it, the engine idles rougher and there is a bit more smoke from it.....

I have tied shifting it into gear, at all possible carb settings, and it tries to run, but it immediately begins to run rough or misfire, and then dies... what else can I do?

would turning the distributor gear a notch clockwise help?... or counter clockwise?... maybe new plug wires?.... a higher or lower initial advance?

where do I go from here? Thanks Lou
 
Lou:

"I feel your pain." (Bill Clinton)

It sounds like you have both done everything you can, and you've also done it right.

I too have had this same problem, although with three cylinder Mercs in my case (same basic motor less one hole). In my case--and you aren't gonna like this--the problem was excessive blow by. To wit:

When the bores are badly worn exhaust gases push pass the rings to contaminate the fresh air/ fuel charge in the crankcase. That murders idle and low speed power. My engine (actually two different triples) would start up fairly well, and idle down to a shakey 1,200 rpm (but no lower), then die soon as they went into gear. Sound familiar? If I was lucky enough to get the motor to stay running in gear, it would plane the boat right off and go like stink at WOT. But Lord help me if I stupidly let it idle done again before I was back at my trailer.

Your particular motor is famous for running too cold at low speed and wearing their bores out prematurely. They had a HUGE impeller and needed a thermostat desperately.

What is the compression readings for the cylinders?

Jeff

PS: One of my expert Merc guys explained to me that when a motor like yours arrived at his dealership, the only fix was another power head. Sorry.
 
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Worn cylinders and worn reed blocks is the problem. Cylinder wear on these are do to side thrust of piston not temp....always around the ports.
 
the compression readings are all within 5psi of one another at 140psi... when I had it apart to do the rings, I used a hone lightly on the cylinders, and they did have some wear at the ports, but using a bore gauge the measurement was hardly peceptible and was only noticed after I used the hone... a very small amount.

I took the reed blocks to the local merc shop, and the mechanic said they show no signs of wear... it starts a bit hard, but idles smooth (after warming a minute) in the test tank with the advance at idle, set to 3btdc... when it gets shifted into gear it is still at 3btdc.... when the advance starts to increase, the throttle plates begin to open... I was hoping to hear that a different advance setting would make a difference as it was shifted into gear... frustrating!!!
 
My triples also had 140 psi compression, yet...

How low can you get it to idle, in neutral, in a barrel, and stay running?

Jeff
 
Compression is only a test to see if cylinder will support combustion process it will tell you nothing about cylinder condition. I have seen scored cylinders with good compression. Always hone with a rigid hone not a spring or ball to see if cylinders are worn, the worn areas will show up as dark untouched area by the stones. I have seen very few inlines that the cylinders wasn't worn.
 
-I used an engine hone (3"stones) to break the glaze on the cylinders and they did have some wear at the ports, but using a bore gauge the measurement was hardly peceptible and was only noticed after I used the hone... a very small amount.

-an open air spark test? is that where you pull the plug, insert it back int to the wire and crank the engine over to see if it fires?... yes, all 4 were firing when I had the plugs removed and grounded when setting the sync/advance....

also, with the motor running I can remove the plug wires one at a time and the cylinder will drop out, so I know its running on all 4... BUT... I have not tried to get the motor at a very low rpm, and then removing the plug wires to see what cylinder may not be contributing. I suppose it could be possible that at a 500 rpm idle, one of the spark plugs or wires isnt delivering its load into the combustion chamber, but that is usually a high rpm problem..

-the idle setting..... as it is set up with the carb butterflies fully closed and the minimum advance screw set to 3btdc, it idles very smooth at 950... for test purposes to see if the carbs were still being held open a bit, I have tried pushing the butterflies closed, and they are fully closed....

the only way I can seem to lower the idle is by backing off the advance or adjusting the mixture screws one way or the other.... richening does not help it stay running when shifting into gear in the tank, neither does leaning it out.... neither does squeezing the primer bulb while shifting it.

-what I have not tried is backing the advance off to near 0 and seeing if it still idles smooth or if it remains running when shifting into gear.. neither have i tried setting it up with more advance at idle/shifting rpm.... and that leads back to my question, does anyone have any thoughts on whether it would help to change the idle speed advance so it has a better chance of staying running when shifting into gear, and if so, which direction should I go with it?
 
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Idle speed is controlled by backing off the timing.-------The further you can move it back the better.--------------------You must realize that idle speed is controlled by retarding the timing.-----------The throttle plates are closed we say.------------------They are not realy closed as they let a controlled / calibrated amount of air in.-------------The reason is obvious to me.-------------A 2 stroke engine must inhale fuel and thus oil to have lubrication at idle!
 
Pulling plugs and inserting them back in their respective wires,cranking engine over,looking for spark is not the proper method for checking spark. Spark needs to jump a preset gap of 7/16ths of an inch on a home made or store bought tester. Joe Reeves has a simple way of building one with a piece of 1x4 and finishing nails,some alligator clips,solder and wire
 
I will try this on all the wires... while it was running and I removed the #4 wire, the spark was strong enough to jump out of the end of the boot to ground, so I know that one has the power...
 
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