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Discussion Starter #141 (Edited)
Before you guys waste time turning the Reed cage around I would strongly advise reading this thread...



also remember that these CAG engines are actually a copy of the Robin Subaru NB411 brush cutter engine my guess is they put the read cage back there number one to save space and allow room for the gas tank and reason number two plenty of lubricant for the crank bearings and connecting rod they weren't worried about cylinder charge efficiency and on a weed eater you don't want an engine that will rev unlimited RPMs and then blow up.
I've read most of the thread and can't find what you meaning.
Get this crankcase and your problem is solved.
View attachment 159320
They are out of stock unfortunately
 

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In Kimjons thread we learn that if you turn around the Reed cage the wind from the connecting rod catches the edges of the reeds and induces reed flutter making the engine actually lose top end power...
 

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Discussion Starter #143
I have ordered a cylinder head for the 40mm cag engine.
A minarelli vertical ac head. Uses the same bolt pattern.
159367

I think it will work good. Might need some work in lathe to make the squish right.

I couldn't wait for trying the 44mm cag engine.
But since I haven't machined the cylinder or the cylinderhead i had to use a 44mm 12mm piston instead of the flat piston i will use later.
I think the exhaust timing is 155°. So very low.
But it had much low end and was a pain in the *** to start. Squish was to tight. 0.3 i think. So very tight. The low duration and crappy exhaust and jetted a bit rich so it didn't rev so high. Like stock i think. But with very much torque.

So now I want my parts to be machined so i can make some power and revs.
I hate to wait for machining the parts.
 

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When you set the cylinder higher, you can rais exhaustport and squish. Take a 0,4mm gasket to get 0,7 squish and mor exhaust timing.
 

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Discussion Starter #145
Yes, That's correct.
I wanted to try it with as high compression as possible. And try the head. Never had a 2 piece cylinder head before.


I have an spacer that is 2.85mm thick. So that's raising the duration pretty much. The flat piston are a bit higher too.
Top deck of the cylinder needs to be machined 2.6mm.
With gaskets the squish will be 0.5.
With the 2.85 spacer and gaskets the duration is exhaust 175° transfer 125°
Blowdown 25.

I think it will be pretty good when it's finished.
The enlarged transfers seems to work good.



The 40mm engine specs are
Exhaust duration 182°
Transfer 120°
Blowdown 31
The 40mm cylinder head arrives tomorrow.
 

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Discussion Starter #146 (Edited)
Need longer threaded rods.
159368

159369

Large squish area.
It's an minarelli vertical ac head. High compression 40mm.

I bought it from a local shop. 24 usd.
159370


I think it will be very good but how knows 😲
Got some thin head gaskets too.

159371
 

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Discussion Starter #148
On the 44mm engine? No i didn't cause i don't have an sparkplug adapter that's fit the 2 piece head. I've never seen such a small threaded sparkplug as those on the 2 piece cylinder heads.
 

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Discussion Starter #149
40mm engine.
I grinded the cylinder on a thick piece of glass.
I think it's flat enough. Don't have patience to wait for machining it right now.
New bearings and oil seals installed.
Soon ready for first start.
Lightened flywheel and rocket key, retarded ignition for more revs.
New sparkplug wire and cap.
159372
 

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There is no need to machine the cylinder top. Sanding it on top of glass will be completely fine :D
Hah, didn't know that the minarelli AC head bolt pattern fits straight. That combustion chamber is also surprising for a stock head. That's a huge squish area..
Get a precision syringe from a pharmacy and measure the combustion chamber volume. You can put grease around the piston rings, then put the piston at TDC, head on top and fill it with light oil from the plug hole until first 2 plug threads are covered. The cag piston dome is so tall that it will effect at least 0.5cc.
 

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Discussion Starter #151
I did sanding it only and seems to work. Not tried the engine more than with a drill, no fuel line attached.
Yeah the squishband is really big.
It's not a completely stock minarelli vertical ac head. It's a high compression version.

And yes the vertical minarelli ac engine share the same bolt pattern.
Also the old Yamaha fs1 and the old air-cooled yamaha dt50.
Horizontal minarelli are not the same.
 

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Discussion Starter #152
I've got the 40mm engine running.
Didn't start with the pumper carb.
I swapped to a modified stock carburetor.
Got it to start and run but is running lean.
Found out why it didn't start with the pumper carb. Worn out reeds.

My go-kart style exhaust is soon ready to be welded together. I really want it to be a good performance exhaust.
Will find out later.
I haven't seen any use a go-kart style exhaust.
I think it will make it rev higher than an traditional two-stroke exhaust.

Not my pipe on picture but it shows how I'm building my pipe with the perforated end cone and so on

159411
 

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Mostly what hits the brakes high RPM is lack of variable timing advance.

Programming a timing curve onto a Arduino then finding a CDI and HALL that can actually keep up with the task ( like a single box off like a Kawasaki H2 750) is one sure fire way to spin up the engine fast enough to grenade the flywheel

Back in the old days the old timers were cranking out 14k using some sort of ignition system for remote control quarter scale RC airplanes it was programmable as well and very expensive to the point that if you are not into quarter scale RC airplanes you would definitely not buy one.

N2O is the most inexpensive way to force these engines into the 14k category now you're going the other way where you're almost retarding the timing to keep it from detonating.

exploring any of these options definitely requires wrapping Kevlar around the pull starter to create a burst shield that prevents you from going to the hospital or worse if the cheap Chinese flywheel decides to come apart...
 

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I don't think that lack of variable ignition timing limits the rpm. In kart racing, they are forced to use fixed timing and those things can take upwards of 15000rpm (or so I have heard). The ignition is just a good suspect to blame for performance issues because the ignition system is usually not well understood.
RPM will be limited by the cylinder ports and poorly flowing transfer channels, nothing else. The faster the piston goes up and down, the less time there is that the ports are open. You need huge ports to keep the cylinder breathing at high rpms. And the transfer channels should be much better shaped, the cag open transfers with sharp angle will create turbulence and not flow well.
I think the cag engine should be able to take 13-14000rpm without changing the ports much. I look forward to prove this after I get the new cylinder and fix the cdi problem.

Yeah the flywheel bursting may be a real risk. I have thought to attach some flat steel on top of it, that should prevent the pieces flying onto the riders feet.
 

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Discussion Starter #155
Some go-karts only have like 170° exhaust timing, and they rev high like 14-15 000 rpm.
Part of reving that high with low exhaust durations is using that kind of exhaust I'm building. And very wide exhaust port i think.
The one with perforated rear cone.

It's really interesting with all people here sharing knowledge.
Sharing knowledge will get us to reach higher horsepower.

The exhaust I'm building is really a hit or miss. I have no idea if it will work out the way it should.

These CAG cylinders are really bad. I have thought many times to swap it for a morini ac cylinder or a vertical minarelli ac cylinder.
The vertical minarelli cylinder should work best i think.
Do some crankcase stuffing to plug the stock reed valve. Full circle crank. It should make it flow a lot better.

But it takes away the cheapness of this small two-stroke tuning. And i like it cheap 🤣


I don't think I will use nitrous oxide.
I don't like the idea of having the need for changing bottles all the time.
And tuning the carburetor for nitrous oxide will make it run too rich when the bottle is empty.
To avoid that i could use twin carburetor setup, or a fuel injection system with the nitrous oxide to get extra fuel when activating nitrous oxide shot.

But that's not my kind of thing. 🙂
 

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The biggest problem at the cag engine is the flow from the carburetor. At higher rpm the reeds will never close. It will be much better flow with the polini style reedcage in V Form and the next limiting is the way through the crankshaft.
A good build polini or BZM engine can Rev to 18k.
 
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