50cc topping at 25, low compression?
 

Hey everyone, Brand new to the forum here. Forgive me if what im posting here already has a thread on the forum, I've been trying to register for about a week and for some reason it JUST now let me through. Its Sunday, so i just wanna try to get an answer before this issue keeps me off the road any longer.

I've got a 50cc gy6, bought it new about a year ago and put about 10k miles on it going to and from work. It used to sit comfortably around 40, but now it usually tops around 25. I've replaced the carb, tuned the carb, Took the main jet to a #85, replaced everything outside of the motor and gearbox and there's still issues getting it kick-started(electric starter went out) and accelerating. Although i can feel a slight increase in power from the new carb and main jet, Theres really no increase in speed at all. I think it might be low compression, though I don't know how to test it.. So the question I wanted to ask; IF the problem is compression, Will properly installing a big bore kit fix that?
A new question was brought to mind while typing.. Does anyone think the engine could handle that kind of extra power? this is the kit i was looking at. Money is an issue, which is why i chose this specific kit.

http://www.ebay.com/itm/100cc-Big-Bo...56a5a7&vxp=mtr

I can't help ya. Just wanted to say.. Welcome to SD. :cheers:

Thanks!

Hi
Welcome aboard
Pleased to meet you
I would check and adjust valves the symptoms you have posted imply they are out of spec
.003 to .005 are good starting places to set them at
Try that first and if that dosent solve the problem then we can check out other possibilities
Take care and ride safely
Yours Hankj

When was the last time a valve adjustment was done?

It's entirely possible you have low compression too. Combine this with worn CVT components and you could easily get lowered top speed.

At 10k miles if the compression is low I would actually just replace the whole engine. You may be in for a costly rebuild. In order to return it to like new you would probably need to replace the rear pulley because those can wear out causing poor shifting, new roller weights, new CVT face, new belt. Also at 10k miles you'd likely have to split the case and replace the connecting rod because if you put a BBK it could lead to lower end problems on a well worn engine because as it is on a new engine it's tough to get 10K miles with a BBK. I'd wager putting in a BBK your engine would go no more than 5K miles after the rebuild.

New BBK is the cheaper more reliable route. About $375 from Scrappy's. So it's 100% complete and new so you'll get more reliable miles rather than trying to rebuild an already worn out engine with worn out transmission components on top of that.

Hi
To check your compression you will need a tester, I use one of these
http://www.harborfreight.com/quick-c...ter-95187.html
100-125 psi is good, 125-150 psi is very good, and 150+ psi is great
Take care and ride safely
Yours Hank

i agree with the valve adjustment first.

Okay guys, Sorry for taking so long I've had a busy few weeks. I'm going to start by sharing this. My bike was running low on gas, and i hit a steep hill. the gas level wasn't high enough to feed any into the engine, and i ran it dry because I didn't know that's what was going on.. It hasn't started since. BUT here's a list of things I've done to it since then, trying to get it to start:
Check compression by hand, felt decent
Checked spark, got spark although it looked weak
Cleaned carb
Checked fuel pump, seemed to be in working order

Below is a list of swaps/mods made
total engine rebuild, took it to 50mm with 64mm valves in the new head(after ran dry)
new carb, came with electric choke, #90 main jet
new(er) CDI, 'performance' blue box for 150cc, with a spare 50cc blue box
'Performance' clutch with 2K springs and 'performance' variator
'Performance' coil
'Performance' camshaft (Taiwanese)
swapped air box for uni filter(after ran dry)
Replaced starter clutch

Bike is still not working =/ Additional notes would be; mechanic didn't have feeler gauges for .003-.005 so he used the smallest he had and felt his way down. He says its an issue with the vacuum in the fuel line, but the line from the manifold wasn't connected. Once connected, the pump seemed to work fine. The spark plug seemed to have an excess of buildup, but when it was installed, I was having to start on a pretty high idle. Engine starting fluid hasnt worked, in the carb or on the plug........ Clutch cover broke, so i cant use the kick starter.

Any ideas?

You made a lot of changes so it's hard to say which one is the problem. Try switching back to the 50cc CDI maybe? The 150 could be different and not sparking correctly or at all. Also put your stock airbox back on until you can get it started, then mess with the uni. If the plug is fouled, get a new one. Double check that your valves are adjusted properly.

the LARGEST main jet your bike should use is #78, but I'd use a 74 or 75. Your mainjet is way too big.

Your second issue is an open element filter, as that is far too lean.

So basically, your carburation is off at BEST. So let's start there.

Close your mixture screw (BUT DO NOT TIGHTEN, only close). by turning screw clockwise until it stops. Then turn the screw COUNTERCLOCKWISE 2 and 1/2 turns. This should be BASE MIXTURE, your bike should run, you might have to turn up the IDLE SCREW to get a solid idle.

He's got a BBK on there now though, isn't 85-90 about what you want for a BBK?

No. My 100cc runs a #80. 85 to 90 is most likely far too rich.

also the gy6 CDI is THE SAME on 50ccs and 150cc.

And lastly, you cannot check compression accurately by hand. You MUST use a compression tester.