New carb, fuel pump, low WOT issue now.

[Pilotn8]

I have a new to me circa 1968 mercruiser 160 with the Chevy I-6 250. Previous owner babied her and had just replaced carb, starter, battery, and fuel pump. He says out of routine maintenance and I do believe him. The carb is a Rochester 2bbl GC with auto choke. Took it out for the first time and at wot I can’t get past 3,000 and then after a bit slows down to about 2700 rpm. I ran one tank empty and right before it starved out it jumped to over 4,000 rpm. I also sucked 10 gallons in an hour and a half at 2700 rpm and to me it smells rich. Am I just chasing a carb adjustment issue and does anyone know this carb? Other factors are lake was at 7700 and carb was installed at 4400. I also have this giant metal plate above the prop on the drive that I’m not sure what for. I’m putting that in a separate post for help on purpose. Thanks much for your time reading this.

 

[Scott06]

I have a new to me circa 1968 mercruiser 160 with the Chevy I-6 250. Previous owner babied her and had just replaced carb, starter, battery, and fuel pump. He says out of routine maintenance and I do believe him. The carb is a Rochester 2bbl GC with auto choke. Took it out for the first time and at wot I can’t get past 3,000 and then after a bit slows down to about 2700 rpm. I ran one tank empty and right before it starved out it jumped to over 4,000 rpm. I also sucked 10 gallons in an hour and a half at 2700 rpm and to me it smells rich. Am I just chasing a carb adjustment issue and does anyone know this carb? Other factors are lake was at 7700 and carb was installed at 4400. I also have this giant metal plate above the prop on the drive that I’m not sure what for. I’m putting that in a separate post for help on purpose. Thanks much for your time reading this.

Normally at altitude you need to rejet leaner and go down in prop pitch. In the thinner air the engine wont produce as much power due to thinner air. Check the plugs likely based on what you outline and its running rich and I suspect they are black.

If it starved out just before it died there probably was less gas in the float bowl leaning it out momentairly. You probably need two steps leaner in jets. Unfortunately you have to take the float bowl off to change the jets.

I suspect your engine which is similar to what my grandfather had in his 1966 /150 hp I-6 and redlines at 4200 rpm or so. If you lean it out and go down 2" in prop pitch it probably will be set up ok for 7700 ft

Heres a thread discussing high altitude jetting on a 3.o but same idea and similar carb. Also talks about advancing timing 2 degrees
https://forums.iboats.com/forum/eng...outdrives/413899-merc-3-0l-high-altitude-jets

 

[Pilotn8]

Thanks for the help. I’ll pull the jets and see what size they are and go down a few. It came with two other props, I’ll check those out for pitch.

 

[Pilotn8]

Thanks for the help. I’ll pull the jets and see what size they are and go down a few. It came with two other props, I’ll check those out for pitch.

 

[Pilotn8]

Okay, got the jets pulled out and they look like 55’s. Going to order some 52,53,54’s and put the 53’s in to start following the two sizes smaller advice. The prop is a -17 it says. By 2” do you mean two inches smaller diameter or two degrees lower pitch? I would think that means pitch but is the inches symbol also pitch? I have a 13 pitch one in the front storage and am acquiring a 15.

 

[Scott06]

Pitch is also measured in inches not degrees. The prop will have two numbers on the hub usually next to the part number. Like 15 x 17 , 15” is the diameter, 17” is the pitch. You want to go down at least 2” . If you have a 13” pitch I’d be worth trying considering how high altitude you are. Generally 2” up or down in pitch will lower or raise WOT rpm by about 400 rpm.

you should read you Spark plugs before You pop the new jets in and keep an eye on them as you lean things out. You are better off being a tad rich than lean to prevent detonation

 

[Pilotn8]

I’ll pull the spark plugs and check for carbon buildup. I ran it a bit at home and when I revved it up it belched some carbon so I think I may have inadvertently cleaned some carbon off. Is that a thing?

 

[Pilotn8]

Been at work a lot, but got around to pulling a few plugs. This is 3,5, and 6. Pretty significant carbon fouling. Probably should replace them all at this point.

 

[Pilotn8]

Took it out with new plugs at 700 feet above home altitude. Ran decent for the first few minutes and then started bogging again. Still smells like it’s running way too rich. Spark plugs confirm a rich condition. Should I adjust the float level or just re-jet it? Might also take it to the people that installed the new carb and say fix it but I don’t trust shops a ton.

 

[Scott06]

I’d check the float and needle and seat for proper adjustment and drop. Yes it’s running really rich assuming float level and needle and seat are functioning correctly yes go down in jet size. Last time I saw plugs like that I had a choke stuck closed fouled it to the point where it wouldn’t run.