Problem with new build.

[Tyler6357]

Might want to check your crank seals, remove the case plate where the magneto is and see if any oil is in there, it should be clean. There are two rubber crank seals, one on the magneto side and one on the clutch side. If either of them are leaking you will see oil.

 

[Big Muddy]

Thank you. I hadn't checked the compression because it's brand new and the compression "feels" good, but on my gage it reads low, 55psi so that will be the problem. Gary55 had mentioned this too, "Often really weak performance on a incline is a compression issue." It isn't spitting out around the head and I went around it with a lighter while the engine was running to see if any pressure was escaping a few tests ago. I didn't expect any issues with compression since all looked and felt good, plus the engine is brand new but 55psi is low by anyone's standards.

I hope I'm not going to find the rings in upside down or anything like that! The plug looks real good. Why would my compression be low on a brand new engine? The head is torqued down.

 

[GreasyChris]

They use cheap aluminum head gaskets so it could just be damaged gasket. There's another gasket at the base if the cylinder as well some of the castings are known not to be very good. The seals is another area to check for leakage. Alot of guys take these engines apart as soon as they get them and clean up the castings and reseal the engine and replace the cheap Chinese bolts that hold it all together

 

[Big Muddy]

I'll definitely be looking at the crank seals if I can't get the compression up. At the same time, one would expect them to be good on a brand new engine. Saying that I understand that there are quality issues with these engines, however I've been running Chinese engines on generators and power equipment for years now. They are called "Honda Clones" but are the exact same made in the same plant as the Honda engines (some replacement parts come marked "Honda") and I haven't had any issues with them at all.

Well thanks for the guidance, everyone here has been a big help and I do appreciate it. I will be surprised if I have to tear the entire thing down and put it back together. If I go that far I'll replace the Chinese bearings as well while I'm in there. This is disappointing, however at the same time these engines are small enough that they really need to be running at peak for sufficient power.

There are also some things I want to do for the bike, upgrade the sprocket mount and use a 56 tooth sprocket and the engine mounted spring tensioned sprocket tensioner but I don't want to get involved with that until I know it's running good enough to bother with the needed accessories.

 

[Tyler6357]

Definitely a compression problem. If it's the crank seals you will have to remove the magnet and pull the old seal out or if it's the clutch side, remove the clutch. Note, it's possible the seal is not properly inserted, if it's not all torn up you can probably reinstall it to get it to seal, if it's all torn up make sure you get all the pieces out of the crank case before replacing it with a new one. They make 2 different sizes for the seals, you will have to check the ones you have to determine which one to get.
----> https://www.ebay.com/itm/2-Pcs-49cc-66cc-80cc-Motorized-Bicycle-Motor-Crankcase-Crank-Case-Seals-H/123766582204?_trkparms=aid=555018&algo=PL.SIM&ao=1&asc=20190212102350&meid=18d7d28a1a4c44d98df68dc7ac5aae64&pid=100012&rk=2&rkt=12&sd=123474468265&itm=123766582204&_trksid=p2047675.c100012.m1985

----> https://www.ebay.com/itm/49cc-66cc-...rankcase-Crank-Case-Seals-U-SL14/161197952513

If it's not the crank seals it is probably the head gasket seal. I've seen these cheap Chinese heads arrive brand new and way out of spec. Also, I've seen the cheap metal head actually warp from the heat and cause compression loss. It's best to get a high compression head with large fins because they really help the engine run much cooler. But if your head is bad you can try sanding it to get it flat or just get a stock replacement head for about $9.
---->https://www.ebay.com/itm/80cc-47mm-...m=123500540111&_trksid=p2047675.c100005.m1851

 

[gary55]

Crank seals don't cause low compression in the cylinder. To go anywhere from here the low compression must first be addressed. Usually if a seal is bad it causes a lean condition displayed by a run away high idle that can not be adjusted down with the idle screw. If you do tear it down you might take a look at the Thread called sport moped exhaust for some engine performance mods.

 

[Greg58]

FYI the ring groves have end gap pins, they are placed so the rings can't be put on the piston upside down. The pins are on each side of the intake port. You probably know this but crack the throttle when doing the compression test so the engine can draw air to compress.

 

[gary55]

Definitely a compression problem. If it's the crank seals you will have to remove the magnet and pull the old seal out or if it's the clutch side, remove the clutch. Note, it's possible the seal is not properly inserted, if it's not all torn up you can probably reinstall it to get it to seal, if it's all torn up make sure you get all the pieces out of the crank case before replacing it with a new one. They make 2 different sizes for the seals, you will have to check the ones you have to determine which one to get.
----> https://www.ebay.com/itm/2-Pcs-49cc-66cc-80cc-Motorized-Bicycle-Motor-Crankcase-Crank-Case-Seals-H/123766582204?_trkparms=aid=555018&algo=PL.SIM&ao=1&asc=20190212102350&meid=18d7d28a1a4c44d98df68dc7ac5aae64&pid=100012&rk=2&rkt=12&sd=123474468265&itm=123766582204&_trksid=p2047675.c100012.m1985

----> https://www.ebay.com/itm/49cc-66cc-...rankcase-Crank-Case-Seals-U-SL14/161197952513

If it's not the crank seals it is probably the head gasket seal. I've seen these cheap Chinese heads arrive brand new and way out of spec. Also, I've seen the cheap metal head actually warp from the heat and cause compression loss. It's best to get a high compression head with large fins because they really help the engine run much cooler. But if your head is bad you can try sanding it to get it flat or just get a stock replacement head for about $9.
---->https://www.ebay.com/itm/80cc-47mm-Gas-Motorized-Bicycle-Bike-Engine-Cylinder-Head-Gasket/123500540111?_trkparms=aid=555018&algo=PL.SIM&ao=1&asc=20131003132420&meid=8fbf63fe7f7a4d3394e4d0443b997229&pid=100005&rk=1&rkt=12&sd=162713920844&itm=123500540111&_trksid=p2047675.c100005.m1851

Crank seals won't help him with the compression. You don't have to pull the clutch to replace the clutch side seal. It's behind the small bevel gear. Agree on the after market head. A good investment for this engine and engines to come.

 

[Tyler6357]

Crank seals won't help him with the compression. You don't have to pull the clutch to replace the clutch side seal. It's behind the small bevel gear. Agree on the after market head. A good investment for this engine and engines to come.

You are correct sir. sorry, didn't mean to give faulty info. If his compression is low, it must mean there is an air leak somewhere, head gasket, base gasket, intake, carb, etc. correct??

 

[gary55]

You are correct sir. sorry, didn't mean to give faulty info. If his compression is low, it must mean there is an air leak somewhere, head gasket, base gasket, intake, carb, etc. correct??

Not a big deal. I gave a guy some screwed up info on trouble shooting a possible woodruff key prob. the other day. As far as compression, base gasket, intake, or carb. don't play a major roll. Head gasket, warped head, cylinder damage, or ring problems are the players.