Big bore lacks top end power.

[Johnny 5]

After building our 183 big bore motor this summer for my wifes blue buggy I was very dissapointed on its lack of power.
62mm ceramic kit srp 3mm stroker crank ported head with the stock cam and 24mm stock carb with a 125 main jet. 11 gram sliders and a 39 tooth rear sprocket. The valve timing is dead on at top dead center.
It has great take off but lacks any upper power past 6500 rpms. I tried a smaller jet and the power was better but alot of pinging,went back to 125 jet.
We got to test it this weekend and my red buggy with a ported head and 58.5cc
srp piston kit 122 main jet dr2 varator has way more power and beat it every time we drag raced with my wife and son riding together and me by myself in the blue buggy
The compression on the 183cc motor is 155psi and the 155cc motor has 145psi.
I used 91 octane gas and the weather good about 78 degrees.
I took it easy for the first day and the last day we were going full speed for about 3 to four minuets and the motor clunked and clashed and I was sure it was junked. The rpms were 7600 and it sounded like it came apart. I think it was detonating and when the engine vacume went away the carb shut the slide and it had no power. I got out and it was still running and no oil coming out of the engine. We kept going and it would sputter at full throttle or high rpms. I had a spare cdi and put it in and it ran okay but still cant rpm like it used to with the 155cc motor.
I am thinking the compression is too high or the cam is too small to fill the cylinder or carb too small.
Tommorrow I am going to take the head off and make sure the piston rings arent all in pices. It ran the rest of the day just like before just no top end power. I was going to buy a oil cooler but now I may go back to the 58.5 piston kit that worked great before the 183cc build.
Any body have any Ideas?

[xlint89]

What head are you using, and who did the port work?

Stock cam is going to be small for a 5mm bigger bore and 3mm longer stroke. Might be good for bottom end, but it's going to starve for fuel on top

11 gram sliders are a bit small. I ran 12 gram on my 58.5mm engine and did really well. I would think that motor should pull 14 gram sliders with no problem.

Not sure what your internal gearing is, but my Yerf with the 39 tooth sprocket had no top end at all. It over-revved and I bent both valves in the head.

What type of terrain are you NORMALLY riding on?

I'm surprised to hear you say pinging. That's the first time I've ever read one of these motors doing that. 155 psi doesn't seem like enough compression for that to happen. Maybe check your timiing?

What CDI do you have?

What exhaust are you using?

And check your piston for possible contact with the valves while your head is off..

[BuggyMaster]

Which camshaft are you using? The stock one?

[xlint89]

Originally Posted by Johnny 5

ported head with the stock cam and 24mm stock carb with a 125 main jet.

Originally Posted by BuggyMaster

Which camshaft are you using? The stock one?

stock cam

[Johnny 5]

The head is stock 57.4 chamber with new valves and springs. The port work I did myself about two years ago and it made a huge top end performance gain on the stock 150 engine. I cleaned the port casting and matched the intake to the spacer, I didnt go to crazy on it just cleaned it up around the back side of the valves and opened up the intake and exhaust runners a bit. I polished the combustion chamber and the exhaust but left the intake roughed up.

The internal gearing is 13/40 3.07 ratio, I put the 39t sprocket on back in march of this year and I will say it did take away alot of top speed but helped when there are two people ridding. MY 19 year old boy Tyler who has down syndrome is about 180lbs and I weigh 200lbs so I thought the gearing would help. I do have a new 31t sprocket that I am going to put back on before the next trip.

We ride in soft dirt and sand washes alot of hills and open trails at top speed. The hill climbing is fun but I would rather go fast.The blue buggy gps at 29mph and last trip with the 155cc motor it gps with 10g sliders at 31mph same gearing 39t.

The noise from the engine is like a clattering under load, I did a full throttle run and pulled the plug and it was tanish brown with a 125 main. The red buggy with 155cc engine has a 122 main and the plug is light tan to almost clean and it dosnt clatter at all, but with a 120 main it would clatter at times. The next size jet I have is a 135 and that is way to big as far as my testing has proved on full throttle jet runs.

The origonal cdi was stock and after the motor got hot and would not acelerate I let it cool down and swaped out to the orange cdi and it ran without cuting out, but I was more aware of runnig it full throtle after it overheated for a long period. I then noticed it had better power at 3/4 throttle than full throttle.

The muffler is stock with a 3/4 inch tail pipe from the 5/8 tail pipe stock. I tried a straight pipe that I keep in our spare parts bag because a couple of trips ago the muffler broke at the flange and we got to try out our new 20ft tow strap that saved the day. It ran the same with the straight pipe at full throttle just very noisy and some hearing loss, made only two short passes before putting the stock 3/4 pipe back on.

I did put a new orange coil on and did not think of trying the old one untill we got home.

We did have a brand new voltage regulator fail the first 30 minuets we rode this trip when the head lights got dim and when the key was tuned off turned off they were still on. We rode back to camp and when the negative battery cable was removed it was sparking bad like a short in the system. I put the old regulator back in that I replaced because the conector is melted at the black terminal. I put new terminals on the conector before we rode this trip. It ran and was charging okay after replacing the regulator with the old regulator.

I forgot to mention the regulator melt down first that could be part of the problem, but when the regulator was replaced it was night and it seemed to run good in the cool air just top end lazy.

will the A12 cam be a good choice with the 31t sprocket?

[xlint89]

Hopefully Tom will chime in on this as he has a very good grasp on these GY6 engines.

But here's what I'm thinking is going on...

57.4mm (stock head) is quite a bit samller than a 62mm piston, so your compression should be much higher than 155 psi I would think. (150ish is normal on a stock motor)

The clattering may be your valves making contact with the piston, and causing a little loss of compression.

Have you verified that your CVT is in good standing order? meaning the sliders are installed correctly, and nothing is loose or damaged?

It's really easy to over rev the engine with a 13/40 internal gearing and a 39 tooth axle sprocket. (ask me how I know....) How many teeth are on the engine (drive) sprocket?

Your 11 gram sliders aren't really helping to bring the engine RPM down either. With that bore and stroke, you really should be able to load the engine down VS a stock engine much more. Meaning, you can run heavier weights to push the belt up higher on the variator and put a heavier load onto the engine, thus reducing RPM a little.

Do you have a way of reading how many RPM you're turning at WOT?

What grade gasoline are you running in that kart now? At least 91 or 92 octane I hope....

I think the A12 would be a very good cam to use with a 3mm stroker crank. That should put more power on the bottom end and give you the ability to turn the 31 tooth sprocket much better.

If that doesn't seem to work out too well for you, I found a 36 tooth sprocket at Tractor supply for $30 that can be made to work with little modification. That would boost the bottom end gearing a bit, but not hurt as much top speed as a 39 tooth sprocket.


Long story short, I think you have way too much power focused on the bottom end with your set up to get any top speed. (stroker crank, 11 gram sliders, 13/40 internal gearing, 39t sprocket, etc...) You should be able to climb mountains with that set up, but won't go very fast on the straight aways.

As I said, this is just my opinion, and hopefully others that do know much more about these will speak up and correct me if I'm wrong, or at least offer their side of experience.

[Johnny 5]

Thanks for your input xlint89 I appreciate the help.

The rear drive sprocket with reverse is 16t.

I am using 91 octane chevron gas, and the stock compression on the origonal 150 was 125psi. I bought the motor from a chineses atv store in 2009 and the combustion chamber is a large dome shape compared to the howhitt head chamber design/ more volume. And the 155psi was tested after 1 hour of run time so no valve damage.

We put a small tach/hour meter on and the max rpm was 7800 and it would sputter and cut out if you kept it wot for too long. If I kept the rpms at 7000/7200 it was okay.
It may be valve float? but kept running on our three day trip just had to keep in the throttle to go over 25mph probally due to the 39t.

As for putting the sliders in wrong I did that about two years ago and it takes off and slows down and takes off again, the cvt is in top condition as I tried 11g sliders then changed to 11g/10g combo then back to 11g then the last day put 10g in and ran the best for power and take off. I did run 12g sliders two trips ago on the 155cc engine and it was really slow on take off but cruised nice, thats why I got the 11gsliders to find the middle. My brother gave me a cool 24volt impact gun that I used to change the sliders out.
I use a 3/8 air gun at home on the #2 setting about 35/45lbft torque so I marked the variator pulley and nut to put it back in the same place because the 24volt gun is prety strong over 100lbft that would probally strip the shaft on my new crank.

Today my wife ordered a A12 cam 2 more voltage regulators for back up two more 65mm base gasket because we dont want oil leaks and a motorio dr2 that comes with 9g 10.5g and 12.5g, I havent had time to take the head off yet because Thanksgiving was more important and I ate too much and was lazy yesterday.
I hope to take it apart soon just need to was the dust off first.

Thanks for you replys xlint89, the problem solving is part of the buggy fever.

[x-bird]

sounds like you were running into valve float and the valve springs may have started to lose their tensile rating.

[xlint89]

It sounds exactly like my Yerfdog set up. I was able to over-rev mine with the 13/40 internal gearing, the 39t axle sprocket, and even with the 16t drive sprocket on the reverse gear box. I think Tom came up with a 37t axle would give me the same ratio as stock.

Even with mine screaming, I don't think we were going any faster than 30 MPH.

A 31t axle sprocket with the 16t engine sprocket might really bog you down. See if you can find a 36 or 37 tooth axle sprocket to get you back around stock gear ratio.

7800 RPM might be too fast for stock springs to handle. You'll only know if you're getting contact once you remove the head.

That A12 should do wonders for waking up the big bore.

I hope you get the problem resolved as I'm interested in seeing what the issue is.

[Johnny 5]

I put new valves and springs , stock replacement when the head was off for the rebuild.

I removed the head and the valves did not hit the piston, but there is a lot of carbon for a 8 hour motor.

The combustion chamber (57.4) I believe is too small for the bore causing a incomplete burn of the fuel.

So I am going to get a 62mm bore head but the problem is that all the 62mm heads are for egr style and I will have to get a egr valve cover. Will the cooling shrouds work or will they have to be changed also for the egr head?

Also am going to go back to the stock 31t gear to reduce heat and possible valve float.

Next trip will be in late december if all goes well.

[xlint89]

No contact is good.

All I did was carefully cut the EGR "ear" off the head to fit the cover on mine.

[Johnny 5]

Thanks for replying xlint89.

So you used the non egr valve cover on the egr head?

I am loking at the pictures of the head and it looks like I could use the non egr valve cover on the egr head, anybody done this?

[xlint89]

My NCY head had the EGR set up, so i had to cut it off in order for the plasteic covers to fit. No problem using my stock non-EGR head cover.