mac 21 ob

[bob miller]

my mac 21 ob is blowing plugs ...i'm running a vision tunnel...438 prop...silver bullit pipe...24 thousanths head clearance...60% s&w fuel...os needle...os water pick up....lawless lower...a lot of tiny metal specs attached to head button...carb set slightly rich...any sugestions ?....thanks...bob

 

[Rokieboater]

my mac 21 ob is blowing plugs ...i'm running a vision tunnel...438 prop...silver bullit pipe...24 thousanths head clearance...60% s&w fuel...os needle...os water pick up....lawless lower...a lot of tiny metal specs attached to head button...carb set slightly rich...any sugestions ?....thanks...bob

Maybe bearings going out on motor.

Robert

 

[Mark Poole ModVP]

Is that a typo or do you really mean .024" head clearance?

 

[bob miller]

Is that a typo or do you really mean .024" head clearance?

i really mean .024 bob

 

[bob miller]

my mac 21 ob is blowing plugs ...i'm running a vision tunnel...438 prop...silver bullit pipe...24 thousanths head clearance...60% s&w fuel...os needle...os water pick up....lawless lower...a lot of tiny metal specs attached to head button...carb set slightly rich...any sugestions ?....thanks...bob

Maybe bearings going out on motor.

Robert

bearings feel ok....smooth

 

[Bill Britton]

Somethings going south in it. Tear it down and check it thoroughly. When they eat plugs they are telling you that somethings not right. Keep running it and you will find out the hard way to.

 

[FloridaScaleBoater]

"a lot of tiny metal specs attached to head button..."

This should be telling you something, IE: check your bearings!!!

 

[tnrcboatracer]

0.024" is way wide for that motor. I run my MAC21's down at 0.008" HC. Does your 0.024" number include any piston dish?

 

[Buckshot]

Is that a typo or do you really mean .024" head clearance?

i really mean .024 bob

Is .024" normal for O/B's?

I/B's run around .005" to .008".

Like the others stated, check/change the bearings and look everything over.

 

[tom galdys]

Bob, That boat was SCREAMING yesterday. UNTOUCHABLE. They always run best before they pop. The metal on you piston could be the 1/2 dozen plugs that pooped out. But they are right, the bearings are going. Tom

 

[bob miller]

Bob, That boat was SCREAMING yesterday. UNTOUCHABLE. They always run best before they pop. The metal on you piston could be the 1/2 dozen plugs that pooped out. But they are right, the bearings are going. Tom

i took the mac apart today ...this motor really has me stumped...i flushed out the bearings with wd40 and they feel very smooth ...the piston has a nasty groove on the side...the sleeve has no scratches or grooves on it ,feels very smooth....i will need a new piston and sleeve...bob

 

[Marty Davis]

People have been all over your problem.

If your head clearance is .024" then the engine is probably making 1.7hp.

If you move the head button to .006, it will make 2.3hp and not work very hard.

Your problem is that the engine is working its butt off trying to move the boat using only 1.7hp or less.

If you lower the button, you will not burn plugs and the engine will not have to work as hard to push your boat.

You might also have to put in new bearings, but maybe not. The specs are probably detonation because you have the needle squeezed down so lean that it is detonating. You will want to be just at the point where the engine goes from a 4 cycle (rich) to a rich 2 cycle. NOT LEAN....

 

[tnrcboatracer]

The grooves on the piston is from a plug element wire getting hung on the edge of a port and scoring the piston. You might be able to clean it up.

 

[bob miller]

The grooves on the piston is from a plug element wire getting hung on the edge of a port and scoring the piston. You might be able to clean it up.

i think the groove on the piston is too deep to reuse...i.m can't understand why the sleeve was not damaged...bob

 

[bob miller]

People have been all over your problem.

If your head clearance is .024" then the engine is probably making 1.7hp.

If you move the head button to .006, it will make 2.3hp and not work very hard.

Your problem is that the engine is working its butt off trying to move the boat using only 1.7hp or less.

If you lower the button, you will not burn plugs and the engine will not have to work as hard to push your boat.

You might also have to put in new bearings, but maybe not. The specs are probably detonation because you have the needle squeezed down so lean that it is detonating. You will want to be just at the point where the engine goes from a 4 cycle (rich) to a rich 2 cycle. NOT LEAN....

i bought the motor used i think the button was modified...maybe the head shim was accidently was not used...bob

 

[bob miller]

Is that a typo or do you really mean .024" head clearance?

i really mean .024 bob

Is .024" normal for O/B's?

I/B's run around .005" to .008".

Like the others stated, check/change the bearings and look everything over.

i checked all the bearings and they feel smooth....the only damage i can spot is the grooved wall on the piston....i removed the plug elements from the piston top with fine emery cloth...i also removed plug elements from the head button with fine emery cloth...i appreciat every one trying to help....bob

 

[julianconde]

i forgot to mention that i'm using mc9 and odonell blues plugs....usually the element is sucked out of the plug...the pipe is set short...bob

thats the same plug i use on my Mac 21 OB and they run really well hardly ever eat a plug on my boat only 3 for a hole season but the element stayed with the plug it never sucked it up. You can ask bill britton and a few boaters here on IW my Mac HTB 290 runs very good with those plugs and my tuned pipe is flush with the header really short and it hauls the mail no issues. I have never used the odonell blue plugs cant comment on that.

 

[Jerry Wyss]

Bob, That boat was SCREAMING yesterday. UNTOUCHABLE. They always run best before they pop. The metal on you piston could be the 1/2 dozen plugs that pooped out. But they are right, the bearings are going. Tom

i took the mac apart today ...this motor really has me stumped...i flushed out the bearings with wd40 and they feel very smooth ...the piston has a nasty groove on the side...the sleeve has no scratches or grooves on it ,feels very smooth....i will need a new piston and sleeve...bob

Those motors are tougher than a pine knot,, the piston & sleeve is very that way too. Its a difficult engine to get a head

clearance on due to it being very tight in the "pinch". Are you sure it has .024"? cause I don't think they come with enough

head shims to get there.

I must say here they are not the most user friendly engines around for the average boater. Case volume and crank induction timing is way too much IMO. If you get it running right you'll know it, it will be the loudest engine on the pond by far. The term

"its was screaming" can have its own level of correctness from person to person as well.

This "short pipe" buisiness has me a little concerned too,, with a 183* exhaust port timing I would expect the pipe length in

the 8 3/4" area, no?

 

[julianconde]

from my experience with the engines are 100% positive results its a screamer i dont know the pipe lenth never got a chance to measure it but i have no spacing from header to tuned pipe.

im just using a stock X438 with no mods and it runs really fast and very consistent actually one of the most consistent that i have ran all year. i only had 4 glow plugs and 2 shafts go bad on it all year and i raced it in district 12 and in Chesapeake VA all year and charleston SC.

great engines,good performance and great speed thats why i have a docen of them on hand lol. there that good to me dont know about everyone else. Never had any problems actuall i wanna go faster whats the next size prop i should use ? i have a Octura X438 right now stock just sharpened and Balance

 

[Jerry Wyss]

There's a guy (Eric Bourlet) up in namba dist 8 that wins with one all the time. He is one super boat driver and could

probably win with near anything.

Props can be all over the map , but I know the 438 is one that some ran. I set-up one on a XM lower unit for my best

boating bud,, without a doubt the most cantankerous engine I've ever fooled with. When you hit the very narrow spot

they can light the pond on fire, but its razor edge spot for max performance from my experience.

I tend toward a broader margine needle for a heat race engine,, off one click either way is just not my thing.