Bayliner Mercruiser 4.3L Throttle Dead Spot

[nazmen1]

My 1997 Bayliner Capri 1850 with a 4.3L Mercruiser runs fine at idle and at WOT. Accelerating from 600 RPM to around 1,200 RPM the engine bobbles and gasps. At 1200 rpm there is a considerable improvement in the smoothness of the engine from which I can then accelerate to full throttle where the engine runs perfectly.

Last season the carb was rebuilt, new fuel filters everywhere, fresh gas and it ran fine until this week. I use ethanol free gasoline.

My suspicion is the accelerator pump but question that since the engine bobbles in the mid range (600 to 1200) rpm. If the accelerator pump then it would run smoothly at the mid range and just hesitate on accelerating the throttle. Also since there was a rebuild last season I don't understand how the pump could have broken down and the engine has run fine since the beginning of this season.

After reading some posts it could possibly be that something has gotten into the low idle circuit of the carb.

 

[Bondo]

My 1997 Bayliner Capri 1850 with a 4.3L Mercruiser runs fine at idle and at WOT. Accelerating from 600 RPM to around 1,200 RPM the engine bobbles and gasps. At 1200 rpm there is a considerable improvement in the smoothness of the engine from which I can then accelerate to full throttle where the engine runs perfectly.

Last season the carb was rebuilt, new fuel filters everywhere, fresh gas and it ran fine until this week. I use ethanol free gasoline.

My suspicion is the accelerator pump but question that since the engine bobbles in the mid range (600 to 1200) rpm. If the accelerator pump then it would run smoothly at the mid range and just hesitate on accelerating the throttle. Also since there was a rebuild last season I don't understand how the pump could have broken down and the engine has run fine since the beginning of this season.

After reading some posts it could possibly be that something has gotten into the low idle circuit of the carb.

Ayuh,..... A carb rebuild only lasts til the next time crud or water gets to the carb, then it's square 1 again,....
I think that's where yer at,....

I install Fresh fuel filters every spring, regardless their age,....

 

[HT32BSX115]

My suspicion is the accelerator pump but question that since the engine bobbles in the mid range (600 to 1200) rpm. If the accelerator pump then it would run smoothly at the mid range and just hesitate on accelerating the throttle.

Howdy,

Yes it could very well be the Accelerator Pump. 600-122 RPM is pretty much where it needs to work well. higher than you have enough air flow through the carb to get good fuel flow via the venturi.

Just because it was rebuilt (by whom?) doesn't really mean it was a "good" rebuild. Other things to check can be timing and timing advance. The advance is "fixed" meaning it's either via the flyweights in the dist or electronic. if the timing is not changing in relation to the RPM, it *could* cause a hesitation.

I think your ignition is a T-bolt (IV or V) system and it's likely NOT the problem unless someone change the dist to a mechanical one.....so unless the timing is simply "off", I would look to the carb.


Regards,


Rick

 

[nazmen1]

Thanks Rick

I did the rebuild last year using Mikes Carberator's rebuild kit and his videos as a guide which could not have been more helpful. I had no problems and it went smoothly. The parts were all high quality including the accelerator pump. I have since ordered another accelerator pump, but I was wondering if it could be something else.

As I said the boat was running just great until this week, which probably rules out the the timing issues, but not if the accelerator pump failed or something got into the low idle circuit.

In any event I am looking at taking the carb off the engine and installing another pump and will clean out the rest of the carb hoping to find some dirt in there.

Your thoughts would be helpful.

Mike

 

[Volphin]

Mike, did you use carb acid to soak it overnight when you rebuilt it? Carb cleaner spray won't remove deposits from internal passages. You didn't miss a check ball did you?

 

[nazmen1]

Yes I soaked in the gallon of cleaner from Advanced Auto. I am planning to soak the venturi cluster, clean the interior passages and replace the accelerator pump when I get home from vacation. Regarding the check balls, I was careful to remove them and place them back, as per the rebuild video from Mike's Carburetor.

Between 675 and 1200 rpm the engine makes an uncharacteristic gasping sound is the best way to describe it. At idle it is fine, above 1200 it sounds normal, but the sound in the midrange is atypical which leads me to the venturi cluster or dirt in the internal passages of the carb. That too baffles me because the fuel has to pass through 2 filters to get to the carb.

I am planning to shoot a video this afternoon if the weather clears here to record it.

Appreciate you response, comments and thoughts. Please let me know what you think.

 

[nazmen1]

Second vacation week on Squam Lake in New Hampshire and needed to fill up my gas tank, 15 gallons to fill my 30 gallon tank.

I live in New York where I have been using ethanol free gas since last summer. In New Hampshire the state apparently doesn't allow the marinas to sell ethanol free gas. The local marina sells its gas with 10% ethanol but treats the entire gas supply with Star Tron. When asked the attendant told me that the owner decided to deal with the ethanol issue by adding the Star Tron to avoid customer dissatisfaction. I was hesitant about adding this gas to my ethanol free supply.

Last night I started my engine which had been sitting with the new mixture of gas since the morning. The dead spot was gone. The engine has run several times since, from idle, through the mid range to WOT and is running perfectly. The gas is the only thing that has changed since last week.

I checked the Star Tron website and Amazon to discover that many people have experienced improvement in the way their engines ran. Star Tron advertises that it cleans out gum, varnish and carbon deposits. My suspicion was that mid range circuit in my Merc carb was clogged and was planning to take the carb apart and clean when I got home next week.

If there is anyone else who has had a similar experience with Star Tron such as this I would like to hear about it, just curious. Thank you.

Mike

 

[Bondo]

Second vacation week on Squam Lake in New Hampshire and needed to fill up my gas tank, 15 gallons to fill my 30 gallon tank.

I live in New York where I have been using ethanol free gas since last summer. In New Hampshire the state apparently doesn't allow the marinas to sell ethanol free gas. The local marina sells its gas with 10% ethanol but treats the entire gas supply with Star Tron. When asked the attendant told me that the owner decided to deal with the ethanol issue by adding the Star Tron to avoid customer dissatisfaction. I was hesitant about adding this gas to my ethanol free supply.

Last night I started my engine which had been sitting with the new mixture of gas since the morning. The dead spot was gone. The engine has run several times since, from idle, through the mid range to WOT and is running perfectly. The gas is the only thing that has changed since last week.

I checked the Star Tron website and Amazon to discover that many people have experienced improvement in the way their engines ran. Star Tron advertises that it cleans out gum, varnish and carbon deposits. My suspicion was that mid range circuit in my Merc carb was clogged and was planning to take the carb apart and clean when I got home next week.

If there is anyone else who has had a similar experience with Star Tron such as this I would like to hear about it, just curious. Thank you.

Mike

Ayuh,..... I donno's I'd be givin' All the credit to Star Tron,....

Ethanol is a mighty powerful solvent on old gas tank residue,...