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140 Mercruiser bStumbleb MidThrorrle

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"Hi. I'm new to the board

"Hi. I'm new to the board. I work in the parts dept. of a boatyard. Anyways, we are working on an older (1985) Mercruiser 140, and there is a stumble when we throttle it up. Our technicians have tried just about everything, and as it sits it should work, everything is in good order, clean, etc. I tried to locate a technical bulletin on the subject, but nothing seemed relevant. I was wondering if anybody has encountered this same problem. It basically just 'stumbles' for a second or two when we try and throttle it up from low to mid RPMs, and this is unacceptable since the customer uses the boat for water skiing. Any help would be appreciated. Thanks"
 
If the accellerator pump is ad

If the accellerator pump is adjustable set it to give more fuel or a longer stroke. Are the advance weights in the dist. rusted and hanging up?

Take each plug wire off and check it w/an ohmmeter while shaking and bending it.
 
"Guy, thanks for the response.

"Guy, thanks for the response. I will forward the info to our technicians, and let you know how it goes. We appreciate it."
 
"Well, I guess they checked th

"Well, I guess they checked the advance weights yesterday, and they are fine. I am not sure if they checked the distributor wires. Are there any other known issues endemic to this drivetrain which might cause stumble? The accelerator pump is not adjustable, but they are considering drilling additional holes to make it adjustable. Any thoughts on this? Is this something anybody has tried?"
 
If you have an osciliscope hoo

If you have an osciliscope hook it up to the ignition and have someone watch the spark pattern when it stumbles to rule out ignition problems. Worn distributer shaft MAY throw timing off for a moment.

If the carb is original (23 yrs.) then there are issues of worn bushings and other parts that when combined could create the problem.

Check the accellerator cable and linkages for slack when accellerating.

Look for wasp nest in fuel vent and check the vent line for sagging that could hold fuel. Loosen the fuel cap to see if the hesitation goes away.

Did they pull the top off the carb and look for old gas varnish granuals (looks like sand). They can get caught in the fuel metering.

If they find varnish granuals you can be sure they are in the fuel pump and will keep returning. The fuel pump diaphragm leak nipple that connects to the carb via plastic tubing will be plugged w/old gummed varnish meaning the pump has leaked at one time.
 
"I hate to ask an obvious ques

"I hate to ask an obvious question, but what about the quality of the fuel? Water in the fuel? Old fuel?"
 
"Carb has been rebuilt and tes

"Carb has been rebuilt and tested twice. Mercruiser tech support pointed out that there are actually two accelerator pump rebuild kits available, so we re-ordered the correct kit (initially, we used whichever one we had in stock) and are going to replace it. After talking to the techs I myself am unsure whether this will have much effect because upon inspection it seems the wrong kit will not fit, but we have our fingers crossed that correcting the accelerator pump problem, if it does exist, will fix the stumble problem. I will pass on the electrical information to my boss, and thank you for your help. We really appreciate it. As far as fuel goes, there are several technical bulletins from the manufacturer about using anything but pure 87 octane or better without alcohol or other additives. The first thing we did was refual the vessel, but thanks for the advice anyway."
 
One pump is straight and the o

One pump is straight and the other has a hooked top. Glad to help. Tell us if it is cured.
 
"I'm experiencing the same

"I'm experiencing the same problem on my 470, One thing I did that helped was to put a light duty spring on the acc. pump linkage. I have read thru some past posts and one fix reported was going to an electronic ignition which Guy also recomended but I haven't had the chance to try yet."
 
"If the carb is a Mercarb 2 bb

"If the carb is a Mercarb 2 bbl, I beleive they are noted for hesitation or bog on hard acceleration. Someone correct me if this is wrong, as I have never owned one.

Your customer may have to re-educate himself to gradually giving it WOT by use of a smooth stroke over 1 second or so, as opposed to just hammering it. Most carbs don't like overly rapid throttle up. The gas flow typically lags the airflow; thus the mixture goes lean, and you get bog. The accelerator pump attempts to compensate for this, but its not perfect.

Just my thoughts.
Rod"
 
I am thinking of adding anothe

I am thinking of adding another hole in the accellerator pump lever for adjusting to get a longer pump stroke from idle to try to cure it. I'll post it if it works.
 
Guy;
That's a good idea a


Guy;
That's a good idea and I can't see how you could go wrong.
Worst case scenario: Put the rod back in the original hole if it does not work or does not help.

Rod
 
"Just my opinion but an accele

"Just my opinion but an accelerator pump can easily be checked by giving the throttle one pump and watching to see if a solid prolonged squirt is injected into both holes, No?
Bert"
 
That's how I check one. J

That's how I check one. Just thinking of a little longer pump stroke will provide a few dribbles more fuel to decrease the stumble. What is your opinion Bert?
 
"Guy
Depends on when the stum


"Guy
Depends on when the stumble happens. Kind of tough to say without experiencing it. My experience has been that if the engine shows any type of life on the first instant then the accelerator pump is probably getting the fuel in there as it should. There are so many other things that can have an effect on the engine both in the carb and out of it. In this case the stumble appears at mid throttle which could mean a power valve, metering rod, ignition advance even a vacuum leak. I'd go through it all, as it sounds like they have. I don't think any modern day engine has a chronic stumble problem, there has to be something causing it.
Bert"
 
"Well, we fixed it! The origi

"Well, we fixed it! The original gasket has been superceded by a thicker one, by the manufacturer, so we changed that, and replaced the accelerator pump, and now it works fine. Thanks for all of your help. We definitely appreciate it."
 
"Bert:

My carb is the Merc.


"Bert:

My carb is the Merc. 2BBL. I can put another accel. pump in her if need be; will check the fuel spray today. When I got it the base gasket was damaged so I made one from some gasket material I had. It has worked fine until this last month. When I give it 2/3 to 3/4 throttle after a brief or complete warmup it will sometimes stumble until I pull back on the throttle some and give it another shove and she takes off. It even does it if I troll awhile @ 600 RPM and hit the throttle. I'll start w/new plugs and see what it needs. Thanks.

I haven't done much boating due to the cost of fuel per trip to the lake. I'm by myself which makes it tough on me at the end of a day when I am exhausted from exposure to the summer heat which affects my muscles. I have to do all the trailering and putting the boat under cover, etc. which makes me not go to the lake knowing what I have to do at the end.
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"
 
Guy;
Sounds like you need som


Guy;
Sounds like you need some younger "shipmates" to help with the work load.
How about some like MistaHays has on his boat?

Rod
 
Rod:

I would have to throw


Rod:

I would have to throw away all my guns since I would be found dead w/the barrel of one stuck in my derriere. I have two sons that are gone now but come home a few times a year and we hunt and fish together.
 
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