stutter when going to full throttle

[freddyray21]

1972 Mercruiser 120 hp compression 135-135-128-135 new tune up with plugs, cap, coil wires and points. Timing is right on. Engine idles great never gets warm. Oil change about 25 hours ago good and clean still no sign of water in oil. Checked carb and get good squirt of gas on both sides when opening throttle so accelerator pump is okay. Fuel pump has good pressure right at 4lbs and steady. wot runs about 4100 if tach is right. Think the tach is a little off. My dwell tach meter shows it to be about 300 off. Cruises all day at 3400 real smooth. Checked the springs in the dist. They all seem to be working fine. When I open it to full throttle it stutters almost coughs then takes off. It's kind of a bugger when you have a skier out there. It's not a bad stutter, but it's still there. Any ideas?

 

[Don S]

Re: stutter when going to full throttle

Check your carb adjustment, too lean or too rich on the idle adjustment will cause it.
You also didn't mention you set the dwell and did you set it before setting the timing ? ? ? ? ? ?

 

[fishnfiend]

Re: stutter when going to full throttle

Common. Do a search for "bog" and you'll get more than you want. If the stutter is after you're already in gear (as in taking up slack in ski rope, idling in gear), then it's probably inadequate fuel through the carb. Ron7000 has a bunch of posts after researching float level - he found that a higher-than-factory float setting is necessary to supply fuel to off-idle transition circuit in Merccarbs. Rasing the float improved my off idle "bog."

Carb has to be extremely clean - the transition openings are very small and easily clogged.

Until you git her figgered out, you just have to feather the throttle a little more on take-off. You'll find how hard you can push and and keep rpm's up. Sounds like you know your engine well otherwise - you'll get this one (hopefully before this ski season is over!)

Have fun.

 

[freddyray21]

Re: stutter when going to full throttle

yes dwell is right on 30 degrees and steady done before the timing.

 

[freddyray21]

Re: stutter when going to full throttle

stutter is after skier is in position when I hit the throttle to open pull them up. They get up anyway I'd just like to get rid of the stutter.

 

[freddyray21]

Re: stutter when going to full throttle

I do know the boat well and I thought I had put possibly all the information needed to keep Don from asking me a question about it but I was wrong.

 

[freddyray21]

Re: stutter when going to full throttle

Don, will idle adjustment affect the timing? The one thing I did after setting the timing was adjust the carb to get it idling smooth. If I changed the idle mixture do I need to re time it?

 

[Don S]

Re: stutter when going to full throttle

I do know the boat well and I thought I had put possibly all the information needed to keep Don from asking me a question about it but I was wrong.

That's why I like to actually talk to our customers and ask the questions that are needed to help me troubleshoot the problem. If you knew the information I needed, then you knew what needed to be checked and you wouldn't need to ask here.

Don, will idle adjustment affect the timing? The one thing I did after setting the timing was adjust the carb to get it idling smooth. If I changed the idle mixture do I need to re time it?

It won't affect timing, but it will affect how much fuel is already in the engine when you hit the throttle.
If it's already too lean, then the accelerator pump just isn't giving it enough fuel to compensate for the too lean adjustment. IF it's adjusted to rich, the when you hit the trottle there is now to much fuel and it takes the hesitation to burn the excess fuel.
With carbs and points, it's a fine balance of adjustments to get it to all work together correctly.
EFI makes the computer (ECM) figure it all out.