40 HP hesitation/bog

[LarryMc]

Yamaha 1999 40HP, 2 stroke, non oil injected (model C40TLRX), prop lower end replaced with an aftermarket OB jet.
The carbs were rebuilt last fall to correct a hot starting problem.
A "shock treatment" was performed at the same time.
Plugs are new
Timing is set to specs (7* ATDC & 25* BTDC
The compression is 120, 120 & 115 from top to bottom cylinders.
Motor starts easily under all conditions and runs great after warm up.

The problem I am having is that the motor bogs down or hesitates during the "holeshot" when first run during the day or after a long interval when fishing (several hours). The symptoms are this, the motor accelerates to about 4600 RPM, (RPM numbers are approximate) hesitates and drops down to 4400. It will stay at 4400 for a second or two, recover and then accelerate to 4900 and then drop down to 4700. It will then recover again and accelerate back to 4900. It will run at 4900 for a few seconds and then climb to 5000. It will run at a steady 5000 until the motor is shut down again. The problem only occurs when the motor is cold. If the motor is run again within three or four hours the bogging does not occur.

Fuel line and bulb were replaced with a new one.
I have adjusted the pilot screw, but it doesn't seem to make any difference.
I always run fuel stabilizer and the motor never sits for more than a week or two.

I am convinced it is a fuel issue or carb mis-adjustment but don't know where to go from here. Any suggestions?

Thanks

[LarryMc]

Need to add that I bought the motor used about 18 months ago and it has had a bogging/hesitation problem since I acquired it. But the problem seems to be getting worse lately. In the beginning it would only bog down once before recovering and running normally. Could be due to cold weather???

[jb123]

sounds like fuel delivery to me..i would clean the carbs.try an aux gas tank, pull each plug at a time while running, listen for the rpm change, or, take off airbox , choke each throat at time with your finger and again, listen for the rpm change, to isolate the problem.

[Bay Catt]

Don't think cold weather has much to do with it however cold air in the intake does have some effect on how well you burn fuel. I had some of the same symptoms several months ago and they realy had me running in circles for awhile. I changed both the male and female ends of the fuel line connections at the motor(between the primer and the carbs) some times the o-ring goes bad and you could be intermediatly drawling in air. Then changed the water/fuel seperater. And finally ran some fresh gas(no stabilizer) out off an auxilairy tank. Also changed primer and fuel lines from tank to seperator and from seperater to motor. Rpm's leveled out with no hesitation. I think mostly it was air getting into the fuel line. All these fixes are inexpensive and a good way to prevent potential problems later. Just out of curiousity let me know how you make out.

[Bay Catt]

Just read your second post. Being you bought used motor 18 monthes ago and problem increased since then try changing the fuel connection end at your motor. It,s called a pipe joint. Part #6G1-24304-02-00. About $25.00. Double check that number. Not sure of your exact model. You may also what to take a look at all fuel lines from that point all the way to the carbs. Sounds to me like air getting into the fuel lines.

[slim]

have you checked the timing to see if it is fluctuating relative to rpm change? this might be a good idea before you go any further.

try pumping the fuel bulb, if that helps u got fueling issue

[LarryMc]

Bay Catt,
I replaced the pipe joint and fuel line to the fuel filter/separator when I replaced the hose and bulb. Didn't replace the carb fuel lines because they looked good.

I was looking at the motor yesterday and noticed that there was a large spot where fuel had sprayed on the inside top of the motor cowl. Checked the intake silencer and noticed that the top bolt on one side had backed out a little and there was a gap of about 1/8 inch between the right side of the top carb and the intake silencer seal. That could be the reason that the problem has gotten worse lately.

I plan on taking the boat out tomorrow and I'll find out if that is part of the problem.

I don't think that it is a timing problem since the motor will run well at any RPM once the initial hesitation and bogging stops.

I had thought that the problem might be that the fuel was draining down from the carbs when the motor sits for a while without being started, but the bulb stays hard after it is first pumped up no matter how long it sits during the day.

[LarryMc]

Update.
Pretty sure I have found part of the problem. I'm hoping, anyway.

After I found that one of the top silencer bolts had backed out, leaving a gap between the top carb and seal, I checked the other bolts in the silencer. It appears that over time the star washers on the bolts had sunk into the plastic material of the silencer, which reduced the clearance between the bolt ends and the carbs when the bolts were tightened. The bolt ends were hitting the carbs before they tightened up enough to seal the silencer to the carbs. I put a 1/4" lock washer under each bolt head, this allowed the bolts to tighten up enough to seal the silencer without hitting the carbs.

I took the boat out today and after letting it run at idle rpm for a few minutes, I gradually opened the throttle up and it accelerated to 4900 RPM with only a very slight hesitation at 4500. There was no drop off of RPM as there was previously, just a second or so of holding at 4500. After about a minute or so of running at 4900, it climbed on up to 5000. I then closed the throttle and then opened it fully a few times, the motor accelerated to 4900 with no hesitation at all, but never reached 5000 for some reason. I fished for about 2 hours and when I quit I cranked the motor up, let up run for a few minutes at idle and then opened the throttle wide open. No hesitation at all to 4900, but still never revved over 5000.

Now I need to figure out what is causing the slight hesitation at 4500 and why the motor didn't rev to 5000.