'76 650 Merc carb adjustment and spark problem

[hankll]

Just got this motor, was running fine except when I go to full throttle then its acting like its too lean, and sputters and almost stalls. Yesterday, I adjusted carbs, just slightly richer, about 1/8 turn. It started fine but acted like it was way too rich. so I adjusted carbs back again. It started ok, so I thought I was ready to take it to the lake. When I got to the ramp, it wouldn't fire a lick.

When I got home, I pulled plugs, I had spark, and the plugs weren't dark oily, but smelled of gas. It seemed like gas and spark were getting to cyl's where it should be. Then I noticed #1 cyl wasn't sparking anymore. Pulled coils off, cleaned metal plate and back of coils, which had a small ground? wire which I carefully cleaned. Re-assembled, I then had nice spark on #1 but none on #3. I'm scratching my head for several reasons. Why would it be running one day and not the next. Are the carb adjustments so sensitive that it won't start. I sprayed gas in each cyl and still did not get a sputter.
So the adjustment screws are beside each carb throat, correct? What is the amount of turns to get close to proper mix adjustment?
Previous owner said if it ever flooded it had to have the plugs cleaned, but they are a closed end type of plug. Not sure about them, never seen this type of plug before.

 

[Moody Blue]

Re: '76 650 Merc carb adjustment and spark problem

There is an excellent post under FAQ section that describes how to sync and link the carbs with the ignition on these motors. Included is the procedure for setting up the carbs. Read it a couple times then have at it. It really works.

 

[hankll]

Re: '76 650 Merc carb adjustment and spark problem

ok, thanks, that's what I need - instructions.

 

[jbrown270]

Re: '76 650 Merc carb adjustment and spark problem

the mixture screw on the carb only affects the mixture at idle and slightly above. the full throttle mixture is determined by the jet size. If it is running lean, then the carbs need to be cleaned.

 

[hankll]

Re: '76 650 Merc carb adjustment and spark problem

The idle mix screws are beside the carb throats behind the access plugs on the front carb cover? thats what I adjusted. I also adjusted the idle speed adjust screw on what I believe was the carb sync connector. That may have been my problem according to the Freq Asked Question section about setting carbs On Merc earlier years. Seems to have something to do with Spark advance timing. If so I really messed up.

 

[jbrown270]

Re: '76 650 Merc carb adjustment and spark problem

yes, that is the idle mixture. an 1/8 off a turn would not make much of a difference. 1-3/4 turns out from lightly seated is a good setting to get the motor started but it is a little on the rich side which is ok. the idle speed screw would make it idle faster (if you turned it clockwise) but it wouldn't affect your timing at full throttle so you probably didn't mess anything up. The intermittent spark issue is your main concern right now. Could the motor sustain full throttle?

 

[hankll]

Re: '76 650 Merc carb adjustment and spark problem

Last it ran, once I got it past all the sputtering, and it was on plane, I was satisfied with its power output. In fact I cut it back a little to keep a smoother plane. Its got a lot of slack in the control. It goes from shift into gear and slight RPM increase to a little past 10 oclock position is dead with know response till almost 8 oclock position, which is where it begins all the sputtering (or did). Push it on down to stop, and it finally catches and smooths out and picks up throttle

 

[jbrown270]

Re: '76 650 Merc carb adjustment and spark problem

yeah, the throttle is the same on a friend of mine's merc 650, little affect until it gets to the 10 o'clock position. the first thing i would check is that the butterfly valve in the carb are opening fully. the distributor should swing out as the throttle is advanced. then when the throttle is around the 10 o'clock position, the throttle will continue swing out and the valves in the carb will start to open in unison. it could be timing but that will usually just make the motor not reach full throttle. it could be dirty carbs but if it smoothed out at full throttle then that may not be your problem. for some reason, it could have been loosing spark at or close to full throttle making it sputter. you may just have to listen to the motor and pick a starting point. here is the link for the link and sync procedure.
http://forums.iboats.com/showthread.php?t=168855

 

[hankll]

Re: '76 650 Merc carb adjustment and spark problem

Well, I've spent last 2 days trying to get this Merc 650 running. So far new spark plugs, nothing changed - then checked and cleaned fuel filter - nothing, bypassed the fuel connector at the motor with straight connector - nothing.
I am getting spark at spark plugs when I ground the ends to the head bolts. I have gas in the cyl, and I have gas coming out of the carbs. I adjusted the carbs to 1 3/4 turns out on the idle mix screws. So I'm about done with my testing. I don't have a timing light. Where is best place to get a reasonable priced one at? I could try to home in on the sync an link procedure with one. A friend who "knows" all about Mercury motors says its a "magneto" that needs to be replaced, which I believe he must mean Stator or maybe the Trigger. My question is if I have spark, why would I need a stator or trigger? I'm about at my ability to go further. Does any body have a suggestion.

 

[jbrown270]

Re: '76 650 Merc carb adjustment and spark problem

is the gas puddling or dripping out of the carbs? you might want to check the float level. here is a diagram that shows how.
http://forums.iboats.com/showthread.php?t=318980

 

[hankll]

Re: '76 650 Merc carb adjustment and spark problem

Jbrown270 - yea, seems to be a little. Not a huge amount. But I've tried to clear the gas out of the cyl by warm up lever all the way open and no sneeze or cough or anything. I don't understand why if I put the idle mix settings back to where originally set, why wouldn't it start. The link and sync seemed to be something that would help if I could start the engine. Or maybe I didn't fully understand use of the timing light. Was it just cranking over the motor while timing light trained on the tang. And where is the degrees showing for BTDC markings. Ok, I'll see if I can get to the carbs and remove and check float levels. But again, it doesn't make since that I can't get a sneeze since it was running the night before. Is the trigger the timing mechanism for the throttle when its advancing throttle up? Could the trigger be bad suddenly?

 

[jbrown270]

Re: '76 650 Merc carb adjustment and spark problem

just a little gas in the bottom of those carbs is plenty to keep it from starting, especially if the boat is in the water. if you still have spark, your trigger should be good. to test the timing, you turn the motor over with the starter.

 

[hankll]

Re: '76 650 Merc carb adjustment and spark problem

I'm totally stumped. I've done everything except rebuild carbs, and replace electrical parts. I have great spark, I have gas in the cyl. I used starter fluid with no result. This was after I checked for water in fuel, which came up based on the gas I bought on the way to the lake the morning after it had last been running. I checked compression (bought a new compression gauge outfit), and had great compression - #1/ 136 #2/ 139 #3/ 132 with no immediate leak down. I've taken off the flywheel, inspected the stator and trigger. Thought of doing that as just covering my bases, cause spark is great. It just acts like its off time enough to not be firing at proper compression point. I guess I have to take it in to a mechanic. The carbs look extremely difficult to get to. So although they might need t be rebuilt and reset the floats right, I'm gonna pass on that.

 

[hankll]

Re: '76 650 Merc spark problem -FIXED

Re: '76 650 Merc spark problem -FIXED

I was looking the problem in the face and just didn't have knowledge enough to figure out what was wrong. There were 2 things that messed with my head. I had spark, and when running the carbs were adjusted too lean. When I adjusted them to a richer setting, it started the whole cascade of problems. The stator was bad on the secondary connection, it turned out. So apparently when the plug was in the cyl under compression, there wasn't enough spark to fire it off. But yet when I pulled plugs and checked them for spark, I saw "a nice fat juicy spark", produced by the Primary winding of the Stator, but had I seen the spark with a good stator, I would have known I did not in fact have a nice fat juicy spark. Once the stator was replaced the carb adjustment went back to a normal 1 1/2 turn approx instead of closer to 3/4 turn like I found them before the motor stopped running. Just leaving to take it to the lake and see it do what its supposed to do.