CY 29 problem

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Martin_

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Hi all, i need a little help here, I have a misterious problem with an engine, this is a CY 29CC billet head kit, 35mmBore/30mm stroke(stuffed crank), 14:1 dome, OBR reed kit with windowed piston, I cut 5th and 6th transfert port,
I widened the exhaust port at the maximum and increased to 190°.

That engine start to act weird somewhere after 4 tanks,(actually 8 to 9 tank on it) like if it's starve on fuel when I pull WOT and then let it back to idle, I tried everything, this is the 3rd time I took apart the engine, and everything seems visually fine, last time I opened it, I put new TFE bearing, new crank seal, sealed the head gasket, etc... there is absolutely no leak anywhere around the engine.

I tried 3 different carbs, I even put a modded carb with a lower pop of pressure..... nothing, it act exactly the same no matter what I do....... and no it's not the 90° elbow on the intake, I tried without and it changes nothing.

My guess is a crack somewhere on the head or crankcase, because when the engine is cold, it run alsmot normal in the first minutes, when it start to warm up, the problem increases, then at normal operating temp, I cannot run the engine for too long, It act very lean and I'm afraid of seizing the engine, and if I let it cool for 20 min, It runs fine again for a few minutes, then start to lean.

I don't know what to do more...........

Here are some videos , vid:1 cold engine, Vid:2 mid temp, Vid:3 operating temp.

Video talk by itself,
VID 1

VID 2

VID 3
 
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It sounds to me like the reeds are not closing off properly on deceleration.....

Check the reeds for splits and any other damage....also check them for tension when cold then warm them up with a hair dryer till they are about the temp you run the engine at and check the tension again......

They may be softening up when hot.....in turn staying open longer....

My MX bikes used to do the same thing when the reeds were fooked..
 
this is a CY 29CC billet head kit, 35mmBore/30mm stroke(stuffed crank),
Click to expand...

im betting the head is warped. the 35mm CY cylinders/topends are the WORST units. and the headkits (seperate heads) are even worse yet.

the 35m bores have the worst cylinder plating in all the CYs. over the years. i bet ive seen 2 times the cylinder plating issues with the 35mm bores, over any other cy bore out. add the poorly designed head, which warps when it gets hot and then leaks around the head gaskets. the 4 bolts realy needed to be 6 or better yet 8 bolts to hold the head onto the cylinder without warping. add the two together, and its sure disaster imo.

start it, get it warm then spray WD40 aorund the head/cylinder gasket, if you notice rpm changes, or the motor stalls out when you spray, you found your leak.


this is where i would start looking. you say its ok for a few, until it warms up, then it goes to poop. to me, this is a sing the head is sealing when cold, and when warm, warps a bit and starts to leak.

not sure the Reeds would cause the high idle after a run, atleast not like that, or for that long. but its very possible, if they arnt closing fully, then they are allowing more air in.
 
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I know about CY billet head kit, they are crap, but I got two of it almost new in a lot of motor deal for very cheap, I play with it to try different porting.

I'm mechanician and I have my own shop......... as for the cylinder base gasket I sealed the gasket with urethane and let it dry for a few days before strating the engine, believe me, there is no way to have a leak in this area.

I spray WD40 everywhere but the engine do nothing.

There is leak at the head button as usual for this billet kit, but I doubt that this is the cause.

It's hard to find, because after a few second the idle go back to normal, if there would have a permanent leak, the idle would be all time high.

Today I looked carefully the reed assembly, there is a petal not fully closed but I bought it like this, and used it like this on another engine whitout any problem, petal tension is still fine, but there is sign of wear on the petal, I think the problem is there.

I will buy new one and try it.

Next test if the new petal do nothing, I will try another cylinder.

I had planned to run this engine on alcohol with a 19:1 dome, but I think I'll forget this project.
 
i think the project is worth persuing, but you would be better off using the new zen 4 bolt 36mm topend, and just watch your squish area, to get it tight, so you get a high compression.

if you use a stock cy 2 bolt 36mm topend, it has around 17.5:1 compression ration in stock form, so may be much better suited to alchy. specially over the 35.

well, imo, if your aware its leaking a bit around the top head, i would say that certainly can do what it is doing. as you go higher in rpm, you create more pressure, heat, thus the leak opens up a bit, sucks air, as teh rpm and thus the compression goes down, the leak seals and back to normal.

maybe try doing the 8 headbolt mod to the topend to ensure its ALways tight and doesn't leak.

afterall not every CY 35m cylinder and topend kit is a big POS, just more then that are not. heh.
 
Martin_ said:
I know about CY billet head kit, they are crap, but I got two of it almost new in a lot of motor deal for very cheap, I play with it to try different porting.

I'm mechanician and I have my own shop......... as for the cylinder base gasket I sealed the gasket with urethane and let it dry for a few days before strating the engine, believe me, there is no way to have a leak in this area.

I spray WD40 everywhere but the engine do nothing.

There is leak at the head button as usual for this billet kit, but I doubt that this is the cause.

It's hard to find, because after a few second the idle go back to normal, if there would have a permanent leak, the idle would be all time high.

Today I looked carefully the reed assembly, there is a petal not fully closed but I bought it like this, and used it like this on another engine whitout any problem, petal tension is still fine, but there is sign of wear on the petal, I think the problem is there.

I will buy new one and try it.

Next test if the new petal do nothing, I will try another cylinder.

I had planned to run this engine on alcohol with a 19:1 dome, but I think I'll forget this project.
Click to expand...

Can you flip the petals over.......to use the other side......might be worth a try..
 
I already flip the petal when I built that engine, maybee I could flip back again to see if that change something, it's worth the try, this is the easiest thing I can do for now.

Or maybee installing longer tension plate, at the same time it wil add more top end and less buttom..... but the main goal is, ''I hope'' it will change something to the problem.

Timmahh, I already have a 36mm zenoah/+ 1mm crank in a Rampage TT, I love it, but I don't want to play with it further till I can make a reliable set-up on a CY guinea pig, anyway those CY engine are a real time consumming so that I do not even have time to play with the Rampage and his motor.

I also have a CY two bolt 36mm head, but I did not know he had 17.5:1 C/R, i'm surprised.
 
Ok, I see, you talk about the CY 29 40mm boss crankcase.

Btw... today I make the 8 bolt mod.
 

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Big thanks for all the suggestions, I tried all but unfortunately the problem is not resolved.

There is no insulator since it's a reed block, when the engine is running I can put my finger on the block and it's cold........ turn off the engine, I wait for 30 seconds, now the reed block gets very hot, but as soons as I start the engine, it become cold again after a few seconds.

I removed the block while the engine was hot and the petals are still stiff even when the petal holder is very hot.

I make some change since the last time, I spent today and two fuel tank trying to make it run properly, I modded the button head to put 4 more bolt (now 8 bolt), I put a new piston ring, I also change the piston and make different window on it, I make longer and stiffer tension plate for the reed petals, and finally I installed another 813 carb, absolutely no change.... the problem is still there.

The only thing left to change is the reed petals, but I have not yet received it, the actual petals are worn on both face where they seats and I suspect they can leak, I don't know.
 
ya it seems you ve covered the proper bases. Fab Mod on the 8bolt topend, nice work! that will atleast ensure your topend should not leak on you!

only thing i can think of is some of the port work itself is off.
Reed inductions can be kind of finiky to piston mods, and they just don't like some types of porting. But you may be on to something with the worn petals. they are the heart fo the reed induction setup, so if they are a bit chuffed, its very possible the issue.
 
New reed petal change nothing.

I also think that the porting may have somethings to harm, but what porting....? only the exhaust are widened and raised to 190 with a nice curve roof shape, there are windows on the piston and raised a little the skirt on the center on the intake side, transfert port are untouched.

As for the piston and windows on it, like I said earlier, I took another piston and make it different, I make larger and higher windows and raIsed the skirt differently.

What I forgot to mention is that I advanced the timing ignition by 6 to 8 deg, and make a thinner button head gasket to get a tighter squish, at .018 + or - 0.002, but I already run at .014 on another reed engine (28 str. non stuffed/35 bore) without problem.

Maybee there is too much heat on the combustion chamber and there is lots of detonation after a WOT..??

Or that stuffed +2mm crank creates too much crankcase pressure with the reed valves..??
 
Finally it seems that the compression ratio had something to do with the whole set-up, I put a very large squish band (only for testing) and the engine work well, it's not as powerfull as before........

I guess that I would have had a good result with high octane gas or alcohol.

Anyway, the plating is already gone in the cylinder, about 3mm almost all around where the ring is seat at TDC.

Edit: as for the timing, I slotted the hole in the coil, and take the read result with a timing light.
 
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