More Mac 45 Carb options?

[Scott Schneider]

:unsure: Another possable cure for the slobber rich bottom end on the stock Dump carb could be to add a hole in the side of body that would be uncovered by carbs barrel at partial throttle.

<_< K&B has done this for years as have other Mfg's with dump style carbs. Some have a screw that covers hole and is adjustable for quanity of air that is allowed in.

The hole in the side of carbs body is uncovered when the barrel is rotated to the closed side and allows air in through a calibrated or adjustable size hole, this hole is designed to bleed off some of the vacume signal from carbs closed barrel and theirfor leans down the delivery of fuel out main discharge nosile. The hole allows air into the barrel area leaning down the mixtrure while it's uncovered, as you open throttle the hole gets covered up by the rotated barrel and carb meters normally.

<_< Has any one played with this mod on any of the Mac stock carbs? Scott :unsure:

 

[TimD]

Scott,

Sounds like a feasible option to me!

Word of warning though - air bleeds can be a real pain to get right!

People who struggle with idle mixture adjustments have no chance of getting an air bleed working properly - that is why a lot of manufacturers have deleted them from carbs in the last few years.

Tim

 

[AndyBrown]

Scott,

There is more to all of this idle issue than just the carb. The internal workings of the engine and the pipe can have as much effect as a low side as an idle needle.

Yes we have tried the an air bleed hole, but as Tim said, it can be a pain because a different pipe, fuel ect., can have an effect on it's preset mixture, possibly making it too lean for a reliable launch.

The MAC 45 is the most difficult of all the MACs to achieve a good idle. This is particularly true for the earier versions.

The newest version idles well and this is not do to the carb alone.

Again, tell me about your MAC 45 set up, and have you run it?

 

[TimD]

Andy,

Has anyone tried the MAC45 with an exhaust throttle? I'd bet that a few different venturi designs would be needed to find the optimum arrangement considering it's behavior with a carb, but the end result could be very interesting

Tim.

 

[Scott Schneider]

:huh: No Andy I have not run this new engine yet. Being that it's winter and building time, figured I would do some R&D on a problem that has plagued every Mac 45 user I have ever been witness too, myself included.

My consern is a fuel capisity/use issue and being the.45 engines are running so rich on the bench that some fuel could be saved before the real racing has started.

Like you I too have Allen terry building me tanks and in most .45 riggers it's damm tough to get much more than 12 to 13 oz of fuel on board. Tring to save fuel when i can and need / want the best idle for minimum hassles on the bench. Scott

PS. I have opted to use the AB black pipe for top rpm and not the parabolic. Set at @ 10 1/4" from engine center to weld. :blink:

 

[Scott Schneider]

:huh: Keeping post on page 1 so Andy will respond. I hope. <_<

 

[Anders_Martinelle]

Scott,if you can make the tank a little bigger............

I have a STD Eagle SG tank that holds 12 oz and that is to little for the MAC 45 when it runs hard.

It have happen that on the last lap i have gone out of fuel .

So now i have built two new tubs so i can have a fuel tank that can hold 15oz.

I run my engine in the 31k range.....

Anders

 

[Don Ferrette]

"So now i have built two new tubs so i can have a fuel tank that can hold 15oz.

I run my engine in the 31k range....."

15oz?? How soon do you launch in mill time?? I don't go in earlier than 1 minute 30 seconds to the start. B)

 

[Don Ferrette]

"PS. I have opted to use the AB black pipe for top rpm and not the parabolic. Set at @ 10 1/4" from engine center to weld."

Scott-

For what it's worth I ran my new MAC45 today to break it in & stock carb worked great. I do however run the parabolic pipe & the combo works great on my sport 40. John Finch ran his stock MAC45 in his SG45 with the parabolic & it flat screams RPM wise. Launches great, mills super & instant power ........... at 9 1/4" pipe length!!!

 

[Scott Schneider]

:huh: I don't go in earlier than 1 minute 30 seconds to the start. <_< <_< <_<

If it starts and stays running thats great, if not?

Don, how much fuel you keeping onboard? Scott

Andy, please don't think I am slamming Mac 45s or the carbs. I am just trying to get some indepth info on the subject and to date not been very sucessfull.

 

[AndyBrown]

There are many MAC45/SG combos out there running the stock carb and finishing and winning races. I ran my SG 45 with a MAC 67 (.550" carb) at the 2000 IMPBA nats. Finished every heat and tied for 1st place. Had to run really hard to stay in front of Stu Barr and Mark Grannis that year. The stock 45SG tank held enough 60% nitro to do the job.

 

[Anders_Martinelle]

Don,i used to launch between 1.45-1.30 before but that was to early so now i have to put it into the wather at around 45sec mark. :blink:

Then i have fuel so i can come back to the ponton.

12oz is not enough....or i have a MAC that is exceptionaly thirsty <_<

By the way,i use the std carb and it work`s just perfect.

Just bored the inlet alittle bigger for fuel flow.

It has the same fues consumtion as my 90 boat.............

Anders

 

[Don Ferrette]

Scott-

John (Finch) runs a standard SG45 which has a smaller tank than the SGX. My MAC45 is in a Phil Thomas sport 40 and has 14 oz tank which seems to be plenty so far.

 

[Ian Inverarity]

Andy,

I didn't see this topic until recently. You say engine internals and pipe can have as much effect on the low side of the carb as second needles etc.

I suspect I have a problem in this area with my K-90. I have tried the original carb (no low end needle), an OS9B (low end needle) and an A-90 carb (tapering slot type metering) and none worked well. The engine runs very poorly on the stand and is difficult to keep running and near impossible for me to tune. This is by far the most significant reason in why I no longer run this motor.

I use either a Macs 15cc muffled pipe or a Bolly P92 pipe.

What should I be looking at?

Ian.

 

[Anders_Martinelle]

Ian,that sounds odd with the K90 problems you have?????

I have been running the K90 with the std carb w/o low end needle and have never seen any problems with it <_< .

I have tested alto of pipes with the K90 and the MAC 15cc pipe is one of the most "safe" combos to get to work together...

When you say "The engine runs very poorly on the stand and is difficult to keep running and near impossible for me to tune." you have some setup problems.....

I know that it can be rich on the pontoon but then it runs very well under load on the lake.

How is the setup with your K90, what mods have you done??

The K90 is a rocket.

Anders

 

[Ian Inverarity]

Anders,

On the water throttling has not been too bad, particularly with the OS9B. With the stock carb the motor would tend to fuel up too quick and put the plug out. Thus I stopped using the stock carb.

The OS9B metering is not that fantastic either, if you ever put one on a flow meter and open and close it, they go full rich as soon as the low end needle pulls out of the spray bar at about 1/3 throttle. On the stand the motor would start fine on 1/3 throttle or less, but as soon as I tried to rev it a bit it would fuel up so quick I would have to go to 3/4 or full just to keep it running, then come back to about 1/2 throttle to keep the revs reasonable. As I said tho, it actually worked okay on the water.

But this very poor transition meant I cant tune the motor. I tune by ear using throttle transition on the stand. With such poor running I cant tune my K-90 and always seem to be too rich or too lean, and if the conditions change day to day I am out again. It has cost me way too many races for me to persist with the motor.

The motor has a 40 thou shim under the liner and I have increased the drum timing to 215 total, closing at 65 degrees ATDC. I have made my own cyl heads to use 25% nitro that are the same geaometry as I have been using for years in may A-90HP etc. Perhaps the only other mod of any significance I can think of is I did a bit of work to the drum housing in the area below the carb just to smooth the flow path out.

Setting aside the issue of top end performance, the poor running on the bank leading to my inability to set the mixture right is why I dont use this engine any more.

Ian.

 

[Don Ferrette]

"But this very poor transition meant I cant tune the motor. I tune by ear using throttle transition on the stand. With such poor running I cant tune my K-90 and always seem to be too rich or too lean, and if the conditions change day to day I am out again. It has cost me way too many races for me to persist with the motor."

There is a fix for this, I experienced similar traits with the OS 9B on my CMB 90. First take the low end needle out, count number of turns to remove it. The taper of the needle is too abrupt & you will need to alter it. Dave Marles shows what to make it look like at this link-

http://www.prestwich.ndirect.co.uk/techcarb.htm

It's easy to do, I just chucked mine up in the lathe & used a small flat micro file & very softly held it on the needle with lathe running & filed the taper to the new shape. Then put the needle back in but go about 1/2 to 3/4 turn less in. The combination of the original taper & probably having the low end set too lean to begin with was possibly the source of your woes. Being too lean on the low end setting would make it act like you describe since you'd be fattening up the main needle to make is sound right on the stand.

 

[Ian Inverarity]

Don,

You could be right, and I was aware of the mods suggested by Dave Marles on his site, however I spent a fortune and bought the carb brand new, so wasn't about to mess with it! Talking to some mates at the racing they basically suggested re-profiling the needle as well.

But there seems to be other problems as well as the K-90 ran poorly with an A-90 carb, which work great on the A-90HP's and A100's I have used.

Ian.

 

[Anders_Martinelle]

Ian,sounds like two things for me that is not correct.

#1. If you run std Kalistratov linear in the engine i suggest not to lift it.

The fit in a K90 is quite loose when it`s new so rise the ports the "hard way".

The loose fit can generate this symptoms.

If you run the Rossi linear it`s different.

#2. Increse compression. As you describe "always seem to be too rich or too lean" is a good sign that the compression is to low.(0,7-0,8cc bowl with 0,3mm clearance)

Also if you increse the compression you will find that the engine can "burn" the fuel more easily in lower range.

Mac 15cc pipe set at 320mm and go down,i run 295mm with the 13cc pipe and 305mm with the 15cc pipe.

Anders

 

[Ian Inverarity]

Anders,

The bowl volume is 0.76cc with 0.28mm clearance. If I go too lean the engine will detonate, which tells me I am close to the compression limit for that nitro content.

The shallow taper of the K-90 liner should mean it is less sensitive to raising? I personally dont feel this is the issue, but I could be wrong.

For those pipe measurements, are you measuring to the end of the diverging cone or the start of the converging cone?

Anyway the raising of liners, my cyl head designs, carbs and pipes all work on the A-90's, thus I get back to my original question which is what other engine factors affect how it runs at lower speeds? There is something about my K-90 that makes it run like a dog, and I want to know what it might be! The motor has had a couple of P&L's and several sets of bearings too, all run the same.

Ian.