More Mac 45 Carb options?

[TimD]

Ian,

Now that I have a little spare time I will have a good look at that engine for you. I'm sure we will find something that is causing it to run this way as it is not like any other K90 I've seen run.... I don't think itis a carb issue but it does sound induction or compression related to me.

Anders,

What % nitro are you using with the K90 setup you listed?

Tim.

 

[Anders_Martinelle]

Hmmm,sounds strange Ian............

Shoud be OK with that setup, just #1 left.

Check it inside and see if you can see anything strange in it.

My MAC 84:s i have felt OK when you turn them around but when i ran them the conrod rubbed against the case.....

What timing did you get with that ring??

I have just tested 0,75mm ring in mine, 0,04inch is 1,016mm.

Timingwice i have 187-190 exhaust and 130 on transfer and boost on my engines.

Tim,my std fuel is 40/40/20, 20% castor.

Anders

 

[Ian Inverarity]

Anders,

The liner shim is 0.82mm

It gives exhaust 189 degrees, transfers 131 and boost 129. Pretty much in the ball park of what you are runniung by the sounds of it.

Tim,

I dont think it is the carb either, but I have been trying to use the carb to solve the problem, which hasn't worked. The compression I am using is in the ballpark of Anders figures too. :huh:

Nitrocrazed racing: Stumped!

 

[Anders_Martinelle]

It should be ok then but not ..............

Do you have the full cirkle chrank or the old one??

Old one is better.

I have 5 engines that i use this setup on and all of them run close to the same...

At the moment i have no more idèas <_< .

Check so the head botton is tight when you run so you don`t get wather in the engine when it`s warm or so..

Or you don`t have a small pore in the chrankcase or......

Talk about "off topic" but what`s a......

Anders

 

[Scott Schneider]

:angry: Talk about "off topic" :angry:

I would say so guys. Half this thread has nothing to do with original post !!

Its great, but start you own thread so others may follow it if interested. Scott

 

[Ian Inverarity]

Anders,

Full circle crank.

I am pretty sure that the water jacket seals okay too!

Nitrocrazed racing: Totally off topic. :angry:

 

[AndyBrown]

The .040" sleeve shim gives too much Transfer/Boost timing for 25% nitro.

Remove the Sleeve shim and you will be amazed at how well any of those carbs work.

 

[TimD]

Andy,

I have his motor at the moment and plan on doing exactly what you suggested!

I was also thinking the case or drum housing might be sucking air from somewhere.

Scott, Sorry for getting side tracked! I'm still curious to know if anyone has tried Exhaust throttle with a MAC.

 

[AndyBrown]

I have a friend who runs an exhaust throttle on a MAC 45 in an SG and it works great.

 

[TimD]

Kewl,

Did he have to play around with venturi's to get it right or did an "off the shelf" venturi work OK?

Gotta love the throttle response with E/T's - shame they are soooooo messy

Tim.

 

[AndyBrown]

He built his own Throttle and Venturi. Worked on the venturi for awhile til he got it right.

 

[TimD]

He built his own Throttle and Venturi. Worked on the venturi for awhile til he got it right.

Thanks Andy,

I thought that may have been the case as the motor does behave differently to most when using a carb (thus this whole thread ). I would suspect the distance from spraybar to drum face would be fairly critical on this motor. Any chance of us seeing yourself going to this setup or is carb still the preffered method? I guess that is as good a way to judge the performance than anything else

Tim.

 

[Ian Inverarity]

Andy,

Thanks for the tip on the liner shim. What would cause the timing to be a problem as it relates to nitro-content? :huh: I must admit I have never considered timing as it relates to nitro %.

Ian.

 

[Ian Inverarity]

Tim,

Maybe a better thing to try would be leaving the liner shim in, go for 50%, and get the original head button adjusted for proper clearance.

Ian.

 

[Anders_Martinelle]

Remove the shim and you will have the fit also....

Anders

 

[TimD]

Ian,

I'll put it into the 80SG for the 26th and try it on 50%. Then I can swap the head button and take out the shim and see what happens.

No 25% for testing tho'. :lol:

Tim.

 

[AndyBrown]

Yes, 50% would help and 60% will be better.

For me the K-90 worked best with .030" sleeve shim and 60%.

If you go back down to stock liner timing it will work good with the 25% and not so good with 60%.

Kalistrotov does all his testing with 20%.

Ian,

To answer your question about the Nitro/ timing relationship; Nitro makes heat.

When you run low nitro the combustion temp is low. If you make Big ports with Big (High) timing you are dumping a lot of fuel into the cylinder. All of that fuel wants too put the fire out.

So, for low nitro you need low timing (less mixture entering cylinder) to maintain cylinder temp. As you increase nitro you can begin to increase the mixture mass entering the cylinder.

 

[TimD]

Thanks Andy for the info.

Any suggestions on a good starting prop with the SG / K90 combo? H667? Running at sea level.

Tim.

 

[AndyBrown]

H667 should be good.

Also, see my edit above.

 

[Ian Inverarity]

Andy,

Thanks for the tips!

But still I have more questions on this subject. :blink: I run A-90's as well and do not have the carburation problems that I get with the K-90. I run the same fuel (25%), same plugs (Mc8's or 59's), same pipes, carbs with same bore (12mm), same head bowl volume, shape and clearance and actually I use higher transfer timings on the A-90's. The A-90's have a larger bore, shorter stroke, same rod length, but have disc induction. Is there something unique or different in the porting or induction system of the K-90 that means it doesnt like low nitro and high timings? :huh:

Ian.