67 merc thunderbolt 650 Boggs under load

[glastron_cruzer]

I have a 1967 merc thunderbolt 650 on a 73 Glastron v-140 trihull rated for up to a 70hp. It has a 15 pitch prop, New spark plugs, 119psi across all 4 cylinders and carbs cleaned last year. When I'm in the boat alone it gets up on plane like it should and reaches speeds of 28mph (gps). As soon as I get another person in the boat out takes twice as long to plane and doesn't plane if 3 persons. I was told that my file line going to the motor may be to large of diameter 3/8 and that I should try 5/16 instead? Also on tuning the carbs I read that the lower carb should be adjusted at idle in gear and the upper carb adjusted at wot, is this true?

 

[jbjennings]

Re: 67 merc thunderbolt 650 Boggs under load

I have a 1967 merc thunderbolt 650 on a 73 Glastron v-140 trihull rated for up to a 70hp. It has a 15 pitch prop, New spark plugs, 119psi across all 4 cylinders and carbs cleaned last year. When I'm in the boat alone it gets up on plane like it should and reaches speeds of 28mph (gps). As soon as I get another person in the boat out takes twice as long to plane and doesn't plane if 3 persons. I was told that my file line going to the motor may be to large of diameter 3/8 and that I should try 5/16 instead? Also on tuning the carbs I read that the lower carb should be adjusted at idle in gear and the upper carb adjusted at wot, is this true?

Did the same guy that made the 5/16 fuel line comment make the carb adjustment comment?
I would get that boat across the scales and see if it's got water in the hull. You might also consider checking spark on all cylinders. What do the spark plugs look like? Any difference in appearance of one plug vs. the others?
Just some ideas,
JBJ

 

[racerone]

Re: 67 merc thunderbolt 650 Boggs under load

Run and find another mechenic !!!-------3/8" hose is just fine.-------------Perhaps they meant to say that you adjust the low speed at idle and the high speed at full throttle.--Trouble is it has fixed high speed jets , not adjustable.-------So you can open up the low speed mixture jets an 1/8 turn at a time.--See if that helps.--Verufy that you have proper spark at each cylinder.----Check out the weight of the boat too.

 

[glastron_cruzer]

Re: 67 merc thunderbolt 650 Boggs under load

The guy that told me about the fuel line races old mercs professionally talked to him at the boat show. I just read in a forum about the idle screws and was just curious cause that didn't sound right. The boat has been always covered ever since the floor was redone and really hasn't seen to much rain. Boat weighs 600lbs un manned. Would it make a difference if I went back to say a 21p 2 blade prop that these engines originally came with or just stick with the 3 blade and maybe go down in pitch? That 650 should push a 600lb boat load with 3 people up to around +-27 mph with the 3 blade 15p prop In my opinion or am I wrong? Would the stator have anything to do with lack of power under a load only?

 

[racerone]

Re: 67 merc thunderbolt 650 Boggs under load

You have not answered the question about strong spark on each cylinder.----There may be a stator under the flywheel for charging the battery.--------There may be issues with the stator in the distributor as well.--------Have you cleaned the distributor cap ?-----------Do the spark check too.----Spark must jump a gap of 3/8" or more.-----------The 2 idle jets are adjusted at idle.--------High speed is not adjustable !

 

[glastron_cruzer]

Re: 67 merc thunderbolt 650 Boggs under load

I have no idea about the spark when I got it tuned up last summer the mechanic that tuned it didn't mention anything, I would assume he would check all of that...

 

[racerone]

Re: 67 merc thunderbolt 650 Boggs under load

Well, that boat with a 65 HP motor in good condition appears to be quite " sporty " as per U-tube video.------I would say 35 MPH with 2 folks.----Something is wrong with your boat or motor.----Trouble shooting is the path forward.

 

[glastron_cruzer]

Re: 67 merc thunderbolt 650 Boggs under load

Yeah I'm rebuilding the fuel pumps to see if that will help! You wouldn't happen to know why some of these engines have 1 fuel pump and others have 2 do you? And if they are the same kit for both?

 

[BatDaddy1887]

Re: 67 merc thunderbolt 650 Boggs under load

Another thing to check is perhaps your props hub is starting to give out. Try your spare prop and see if that has any effect.