Mac .67 lack of low end

[Bill Gibson]

I raced my 82 Executone for the first time a coupla weeks ago and was really happy with how it ran except for one thing....unless its my lack of experiance driving scale hydros, this thing seems to have no low end or mid range throttleability......its either full on or it falls off the pipe. I was told that Andy at CMDI can sleeve the carb and it helps, but lord knows it sounds like he has a full plate right now, so id rather not bother him with this at the moment....im using a black AB pipe at 11 3/4" and im using an X-455 2 blade prop. Perhaps an experianced scale driver would be able to drive the boat like this, but for myself with at best on a good day average driving ablilty, its a HANDFULL! Oh, its a Phil Thomas 82u55 hull, and i LOVE how it handles! Many Thanks, Bill

 

[SayMikey]

I raced my 82 Executone for the first time a coupla weeks ago and was really happy with how it ran except for one thing....unless its my lack of experiance driving scale hydros, this thing seems to have no low end or mid range throttleability......its either full on or it falls off the pipe. I was told that Andy at CMDI can sleeve the carb and it helps, but lord knows it sounds like he has a full plate right now, so id rather not bother him with this at the moment....im using a black AB pipe at 11 3/4" and im using an X-455 2 blade prop. Perhaps an experianced scale driver would be able to drive the boat like this, but for myself with at best on a good day average driving ablilty, its a HANDFULL! Oh, its a Phil Thomas 82u55 hull, and i LOVE how it handles! Many Thanks, Bill

I ended up using a Macs 13 on my 82u55 it would not come back up with out one. Fast when it was up but if it went down i could not get it back up. Id also consider Barr cutting the prop to unload it. I like a x455x3 on that hull You are using large fuel lines and in the tank also?

 

[tnrcboatracer]

Another option is to sleeve the transfer ports to increase the torque output of the MAC. I have a custon R.G. prepped motor for sale if interested.

 

[SayMikey]

Another option is to sleeve the transfer ports to increase the torque output of the MAC. I have a custon R.G. prepped motor for sale if interested.

Phil Thomas runs a Mac and dont have any low end concerns,id say its drag from the hull being an 82u55 where is the drive dog? How deep is the strut?

 

[Terry Flynn]

If you know for sure it the carb. You could try the OS 9B carb. A guy in our club had better luck with this carb in his scale vs the MAC.

 

[P THOMAS]

The Mac carb should work ok, the 9bOS is good too. How worn is this Mac? maybe it has lost some fit. They can be squeezed to renew the performance. Steve Wood i think sell the tool.

Probaly combination of the pipe and fuel draw, The AB pipe has smaller stinger fittings you could try too. The 13cc mac muffled about 12 inches from the glow plug should work, the tanks MUST have larger brass tubes and lines. I assume you have the front radio box, that makes the tank fuel lines longer which causes problems.

You can add some ride pads to the sponson bottoms about 3/4 wide 1/16 ply and let it overhang the back of the sponson about 1/8th", this will help it get up easier too. The 455/3 is about as small as you need. The Mac needs a bit of load to run right. With the 455/3 it will probally top out before 60mph. Losen up the ride till blowoffs are a problem. Adjust the strut lower till it stays on the water and just add 4 oz nose weight on windy days.

 

[SayMikey]

Probaly combination of the pipe and fuel draw, The AB pipe has smaller stinger fittings you could try too. The 13cc mac muffled about 12 inches from the glow plug should work, the tanks MUST have larger brass tubes and lines. I assume you have the front radio box, that makes the tank fuel lines longer which causes problems. You can add some ride pads to the sponson bottoms about 3/4 wide 1/16 ply and let it overhang the back of the sponson about 1/8th", this will help it get up easier too. The 455/3 is about as small as you need. The Mac needs a bit of load to run right. With the 455/3 it will probally top out before 60mph. Losen up the ride till blowoffs are a problem. Adjust the strut lower till it stays on the water and just add 4 oz nose weight on windy days.

Ah Phil is da MAN Happy B-day by the way........

 

[Bill Gibson]

Okay, first Thanks for all the responses! I hope that all of you that responded will read this so that you can add some more if you want.....Im running medium fuel line at the moment...when i bought the motor from Andy he said not to use large fuel line for this motor.....evidently you guys have found something out as my friend Joe Wiebelhaus also told me to do this the day we tested the boat, so i will go to the larger fuel line....sorry Joe..I shoulda listened to ya! The boat is set up pretty much to the last letter of Phil's instructions...strut is 1" deep, no bend in the shaft, so the prop has lots of angle. My fuel tanks are in front of the engine, so im pretty sure fuel draw is no problem...Phil, i Made a radio box to fit just behind and on the left side of the engine as i did on my sport 40 hull...dont know if you remember me or not...i do have an X-455 3 blade prop that i havent tried on the boat yet......what should it do for the boat that the X-455 2 blade that im presently running not do? I have a 13cc Macs nitro pipe but i dont know if i can squeeze it into the boat when i bolt a muffler on it. I have also heard about the stingers that are used on the AB pipes but have no idea which one to use and know nothing about them! Im sure that more stick time with the boat will help alot...the first time the boat was tested, Joe drove it and had no problems, but he is also a VERY good driver! Thanks so much, all of ya! p.s. the motor is very low run time.

 

[P THOMAS]

The smaller pipe stingers would give a bit more pressure, might run a smaller one when cool out and larger when hot.. depends on your boat and setup. I have one of those pipes but not much experaince with it. mac 13cc pipe is what I have been running.

The 455/3 would hook up better bit more speed. You can pinch the tips of the x props too to get a few more mph.

 

[Tom Foley]

Bill , lets try sleeving the carb first , very easy to do . Second , I have a muffled Macs 13 cc pipe you can try . Third hold that mother on the pipe , get the prop unloaded and then the throttle will work both ways ...you have to get the motor cleared out and free and then you can work the trigger both ways !! I also have a three blade you can try . Lets go racing Bro ! :lol:

 

[Bill Gibson]

Bill , lets try sleeving the carb first , very easy to do . Second , I have a muffled Macs 13 cc pipe you can try . Third hold that mother on the pipe , get the prop unloaded and then the throttle will work both ways ...you have to get the motor cleared out and free and then you can work the trigger both ways !! I also have a three blade you can try . Lets go racing Bro ! :lol:

Thanks Mang....I know that you ALSO told me about the fuel line deal! See you EARLY tomorrow A.M........Its gonna be FUN!! Ive got the rx battery chargin for your 20 hydro so you'll be good to go!

 

[SayMikey]

Bill , lets try sleeving the carb first , very easy to do . Second , I have a muffled Macs 13 cc pipe you can try . Third hold that mother on the pipe , get the prop unloaded and then the throttle will work both ways ...you have to get the motor cleared out and free and then you can work the trigger both ways !! I also have a three blade you can try . Lets go racing Bro ! :lol:

Thanks Mang....I know that you ALSO told me about the fuel line deal! See you EARLY tomorrow A.M........Its gonna be FUN!! Ive got the rx battery chargin for your 20 hydro so you'll be good to go!

No need to sleeve the carb ive sen guys run the .550 with little to no hesitation. Pipe will fix it

 

[FloridaScaleBoater]

Bill , lets try sleeving the carb first , very easy to do . Second , I have a muffled Macs 13 cc pipe you can try . Third hold that mother on the pipe , get the prop unloaded and then the throttle will work both ways ...you have to get the motor cleared out and free and then you can work the trigger both ways !! I also have a three blade you can try . Lets go racing Bro ! :lol:

Thanks Mang....I know that you ALSO told me about the fuel line deal! See you EARLY tomorrow A.M........Its gonna be FUN!! Ive got the rx battery chargin for your 20 hydro so you'll be good to go!

No need to sleeve the carb ive sen guys run the .550 with little to no hesitation. Pipe will fix it

Mike, remember we don't have "good air" down here.

Alway's humid, and most of the time HOT

 

[Ken Retallick]

Im running a blue head Mac .67 in my scale boat, same prob as you, no low speed torque at all.

Best fix i found is An Exhaust throttle. I can get on and off the throttle any time and know it will get up and go.

 

[John Knight]

Bill,

I have found over the past few years that the MAC 67 NEEDS the large fuel tubing along the east coast. Medium is fine for pressure but not for fuel. I found that out myself. The extra large is too large and overloads the engine.

My 67, in my former Roadrunner and now in my Crapshooter, has excellent low end torque, top speed and can throttle up and down at will, with the .515 MAC carb. It runs better for me with the .515 than the .550, more consistant. Also using an AB parabolic. However, you are pushing a lot more boat than a rigger.

John

 

[SayMikey]

Bill , lets try sleeving the carb first , very easy to do . Second , I have a muffled Macs 13 cc pipe you can try . Third hold that mother on the pipe , get the prop unloaded and then the throttle will work both ways ...you have to get the motor cleared out and free and then you can work the trigger both ways !! I also have a three blade you can try . Lets go racing Bro ! :lol:

Thanks Mang....I know that you ALSO told me about the fuel line deal! See you EARLY tomorrow A.M........Its gonna be FUN!! Ive got the rx battery chargin for your 20 hydro so you'll be good to go!

No need to sleeve the carb ive sen guys run the .550 with little to no hesitation. Pipe will fix it

Mike, remember we don't have "good air" down here.

Alway's humid, and most of the time HOT

We have crummy air also, June July and August 90% humidity and 90plus temps. I know from my experience the Macs pipe will help a bunch. I ran the Macs muffled in mine. Seems to be just with that hull, My t-5 and RedMan like the ab67 or Cooper......

 

[Bill Gibson]

Thanks all you great scale guys.....im gonna stick a Macs pipe on it and try an X-455 3 blade prop on it and see what happens....seems to be the easiest thing to try, and ive got the stuff to do it with...for once! Ill let ya know how it goes...hopefully it'll be a HAPPY post!! Thanks again, Bill