Sync carbs on 1975 75 Johnson?

[Thirsty Turtle]

Well here is my story I have not been able to get this motor to come to WOT since I bought this boat a month ago. So far I have rebuilt the carbs, replaced fuel pump, fuel lines, fuel tank, primer bulb, spark plugs, and powerpack. Compression has been checked three times and is 150 lbs on all three cylinders. Spark has been checked and is good. The motor starts and idles fine. When in the water I can run the high speed idle up and it will open up and run fine. As soon as I put it in gear and try to accelerate it will start bogging down, it will not quit but if you back off the throttle it will smooth out. It is on a 14ft flats boat that may weigh 1500 lbs and has a 17 pitch 3 blade alum. prop. I am not sure if the prop is my problem or if the timing or link and sync. I am not sure how to check this and does this sound like this might be the problem or something else. Pleas help as I am ready to light a match to this thing and I don't feel like waiting two weeks to have it looked at by a shop.

 

[jtexas]

Re: Sync carbs on 1975 75 Johnson?

c'mon, thirsty, you're on the two yard line, just one more little push.............

Prop is not too big for that motor on that boat. 150 PSI is outstanding for that vintage.

Are the throttle butterflies closed at idle, horizontal (and not beyond) at WOT, and parallel throughout their range of motion?

Does the roller hit the cam right on the mark?

Does the timer base move all the way to its stop without sticking when you advance the throttle?

Does the timer base start its motion *before* the throttles start to open?

Are the choke plates all completely open (except when the choke is engaged)?

 

[Thirsty Turtle]

Re: Sync carbs on 1975 75 Johnson?

c'mon, thirsty, you're on the two yard line, just one more little push.............

Prop is not too big for that motor on that boat. 150 PSI is outstanding for that vintage.

Are the throttle butterflies closed at idle, horizontal (and not beyond) at WOT, and parallel throughout their range of motion?

Does the roller hit the cam right on the mark?

Does the timer base move all the way to its stop without sticking when you advance the throttle?

Does the timer base start its motion *before* the throttles start to open?

Are the choke plates all completely open (except when the choke is engaged)?

Throttle plates are closed at idle, horizontal at WOT, and parallel throughout.
Yes timer base moves all the way to stop smoothly and starts to move before the throttle starts to open.
And choke plates are open unless choke is pressed in.
I just checked the adjustment and the follower on the carb plate was not between the two marks on the cam when the throttle just started to move. I adjusted the screw where it falls between the marks but I am not sure if this is enough to make this big of an impact on my engine.

 

[jtexas]

Re: Sync carbs on 1975 75 Johnson?

if the roller hits the cam too soon, even a tiny bit, throttles open before timing advances, engine wants to run but the spark is late -- typically shell just die off at the get go, unless you really feather the throttle, nurse it up, then she can run.

If it's idling at less than 900-950 in neutral, lower unit submerged, turn it up a tad.

Hold an index card in front of the carb throats, one at a time, bout an inch away, while the engine's idling. a light mist of fuel is normal, but if that card is getting wet, you need new reeds.