Mac 45 with a differant carb installed

[Scott Schneider]

:unsure: Any of you MAC gurus chime in please. :unsure:

Does the MAC 45 SF need to have that dump carb on it to run right?

I know when on the bench it's sloppy rich and a real pain to keep it lit, but once on water it idles down fair.

I have a few older "RPM" .470 throat carbs that would match the bore of stocker and with low mix adjustment would make starting and overall handling before the launch so much easier.

But with the MACs high compression and over all fuel thirsty attitude, will having a carb thats much leaner down low in the rpm range effect it's ablity to run right once under some load and milling? Or is this a GREAT idea being I have some good O'l carbs to use? Scott

PS. I have a .454 bore and a .425 bore RPM carbs as well, smaller any better?

 

[Don Ferrette]

DON'T DO IT!! Raise your base idle speed to keep it running. Trust me on this one. :unsure:

 

[Lay26]

i agree with Don don't change the carb on the Mac45. alot of engine heads think it's the best thing to do, but if you ever spoke to andy brown he'll tell you that the carb is the best for the engine. on your radio hit the idle up button and let it idle high.

 

[Scott Schneider]

DON'T DO IT!!    Raise your base idle speed to keep it running. Trust me on this one.  :unsure:

Thanks for the firm statement of not to do it :huh: But being I am a motorhead and am no stranger to all the varrables that make engines run differantly.

Can you guys please be more specific as to why it is not a good idea?

I have been racing the .21 MACs and a carb swap IS the thing to do and have great success in running other carbs on them.

So what makes the .45 any differant?

:unsure: Scott

My argument is this: The stock carb is never shut down to 1/6 or 1/8 open, On the bench or on the water. So if we have a carb that can be closed this far and hold rpm down and running stability nice and steady on the bench, yet at 1/4 throttle where you are typically at while launching and milling be at the same degree of richness as the stock dump carb is.

Why is this not the better way to set up?

I am not questioning Andy and his prowess in figuring out what works well, Only to resolve that the carb issue has more to it possably than I am understanding? or it's not as you beleave either? :blink:

 

[AndyBrown]

Scott,

You are running O.K. with the O.S. carb on the MAC 21 because it is a smaller bore version.

Some who use other brand of carbs on the MAC 21 that are the same bore as stock (.355"), experience detonation and plug problems.

They are then requierd to make engine mods. to get things working right.

After all of this extra work they don't have an engine that mills or does anything better than the stock engine would have done.

As for the MAC 45 and those RPM carbs you have, the same will be true.

If you use the .470" RPM carb you will have worse throttle response and more problems with Plugs and detonation.

If you go all the way down to .390" bore throttle response and detonation will not improve until you prop down so much that speed is no longer competitive.

If you are experiencing Throttling and detonation problems with the MAC 45 it is more than likely caused by a an incorrect set up or some internal engine problems that we had with the early MACs if you have an early version.

P.S. I finally got the aluminum drum in the boat, but the weather has been rotten for the past week so I have not had it to the pond yet. Will let you know about it soon.

 

[Scott Schneider]

Thank you Andy for the reply, I would agree with the .21 engines. I use either O.S. 2D carbs at .340 or 20G at .380 with no change other than minor head mod to add a @ 1 1/2 degree angle to 1/2 of the squish band. I still have good torque ( for a .21) but have GREAT bench idle and super on water throttle controll and RPM well into the low 30K range.

What I really need is an indepth WHY? As I have stated once above 1/4 open on carb no metering is happening other than the high needle for max flow.

So WHY? does a carb of the same size, same tuned lenth, simular intake bell and fuel passage flow within carb make the engine run differantly? Is it a mixture curve issue?

And WHY? would throttle responce be differant if carb is the same size?

I am willing to learn and listen but I am not seeing the whole story? Please inlighten me. :blink: Scott