48cc kit engine won't start

Hi all, I have a 48cc spitfire/starfire/grubee/skyhawk engine mounted on a mountain bike.

I've only put about 50 miles on it, I'm still breaking it in, but today it failed. I went about 2 miles on it and it started to loose power. I stopped the engine, and it won't start up again. I have spark, fuel flows freely into the carb. I believe the compression is OK (I can pull the plug, cap it with my finger and release the clutch, and when I push the bike forward I can feel the pressure).

The plug was very dirty, since I'm still breaking it in and it's using a higher oil:gas ratio, so I put another one in to test. It gets spark if I pull the plug and hold it against the cylinder head. How do I tell if the spark is weak?

I pulled apart the carb, it looked very clean, but I sprayed it out with an air can anyway. Spark was gapped to 0.038", then I tried it at about 0.032". I don't see any blockages in the exhaust, but there is fuel and oil practically dripping down the exhause tube, so I don't think it is combusting. It's hard to tell by the noise when I try to pedal-start.

I pulled the cylinder head off, and I don't see any damage to the cylinder wall or piston. The piston has a fairly thick layer of carbon/oil on it.

Any random tips would be appreciated.


Re: 48cc kit engine won't start

Disconnect the kill switch, the thin wire has a tendency to short out on the frame if you use tight tywraps on it.

Check all your wire connections.

Make sure the carb is not clogged by a randon peice of crud.


Re: 48cc kit engine won't start

No go. I disconnected the kill switch wire near the engine and same thing.

I cleaned off the cylinder head, it had a nice thick layer of baked-on crud on the top and just around the edge of the piston head. Oiled it all up.

How do I check the compression? It feels like there is pressure when I cap my finger over the spark plug hole, but I don't know if it's leaking.

I'll take apart the carb again and use carb cleaner this time, and spray it out with the air can.

I'm seriously frustrated at this point.

Any other suggestions?

Re: 48cc kit engine won't start

I asked someone knowledgable about engines, and he mentioned it could be a problem with the float in the carb, and to see if I can replace it. I wasn't able to ask for more detail at the time.

Does anyone know what would cause something like this? I'm pretty sure the float is.... floating. There was fuel in the resevoir with the float when I disassembled the carb. I do have a 2nd new carb that came in a box of spare parts I got a deal on, I may try that, but if it works, I'd like a direction to go to try to repair the old one.

What does the float in the carb actually do?

Re: 48cc kit engine won't start

Dubious deal on a kit and lots of spare parts?

Sometimes the float level needs to be adjusted and I have seen this cause them not to run.

There are 2 brass "fingers" that the float push up when the bowl is full and the fingers push the needle valve closed.

These fingers get bent in transit.

Try bending them down slightly to keep the carb from flooding.

Is there fuel running out of the carb?

You didn't perhaps get a spare CDI or MAG coils?

But you said you had spark....

Go here and ask.



Re: 48cc kit engine won't start

Thanks for the advice, I will look into that.

The kit and the parts were sepearate, from seperate dealers. I just happened to find someone clearing out their inventory, offering a full box of random parts. Everything that came in the spare parts box was new stock. The engine was in unused condition when I got it. although I could tell it had been sitting around for a while; there is some surface rust on the inside of the fuel tank. I used a wider-bore fuel line with an inline filter to avoid issues with that. Fuel flow is actually better than with the stock platic-y fuel line.

Nope, no CDI or MAG coils. Which one is which? There is the wire-wrapped core inside the engine, which is wired to the little black box outside the engine with the spark HV cable, and to the kill switch to ground it all.

Re: 48cc kit engine won't start

<snip>there is fuel and oil practically dripping down the exhaust tube<snip>

It's probably flooding.

I just looked at the picture and the engine seems to be tilted forward more the usual, this could throw off the fuel level in the carb and overflow it.

Try your float adjustment like I suggested.

The spark on these as usually thin and weak but sufficient on a healthy engine, a fuel rich mixture is harder to ignite.

You have eliminated all the other possible problems on a practically new engine.

The MAG coil is onside the engine cover and the CDI is the little black box.

You have what is commonly called the "round head" 48cc engine.

Whose name is on the little oval engine tag?


Re: 48cc kit engine won't start

It's a black oval that says

48cc engine

"Grade A"

2 stoke oil/mix

No actual name. At the time I purchased this, I hadn't really found any of the websites out there. I just bought it on ebay. I probably would have gone directly to a site had I known where to find them. Since then, I have found spookytoothcycles.com, simpsonemotorbikes.com, kingsmotobikes.com, etc. Oh well.

I will probably look into doing that little platform mount for the front motor mount, they don't fit too well over my frame as it is. If I do that, it will raise the front end of the engine up a little.

I'ma go try the carb adjustment.

Thanks for your help.

Re: 48cc kit engine won't start

No go with the carb adjustments. I made the little half oval spring a little longer, so the float doesn't have to be as high to cut off fuel flow, and it does the same thing.

I also tried the spare carb I have, and it didn't affect it. I'm starting to think I should be looking more at the engine itself.

As far as compression goes, I read that you can pull the spark plug off, cover the hole with your finger, and crank the engine over. I did this, and I can feel the pressure, but it isn't strong enough to blow my finger off (or maybe I am just pressing too hard?). If I go hook it up to a compression guage, what kind of pressure should I expect?

Re: 48cc kit engine won't start

to check this spray some carb cleaner or starting fluid into the carb and see if it'll start. If it does, then it's not getting fuel in the engine.

Re: 48cc kit engine won't start

No go.

I've been over just about everything. I replaced the gasket between the cylinder and the crank case, cuz the one on there was looking pretty crappy.

Just for shts and giggles, I checked for spark, and now I don't have spark. ARGH! I know I had spark whenI started troubleshooting this whole thing.

I have about 25V running from the mag to the CDI, but I guess nothing is getting to the plug. The reason I decided to re-check the plug in the first place was I noticed a little metal cap on top of the spark plug that wasn't there before, it came out of the spark cap. I screwed it back in, but I think the HV cable was soldered to the top of it, so I don't think it's making a good connection anymore. Fck.

Re: 48cc kit engine won't start

Compression should be over 100 psi. it should pop your thumb off the hole.

Perhaps your head nuts are loose, I think the torque is about 10 foot pounds.


Re: 48cc kit engine won't start

They are tight, I checked them with a torque wrench. about 10 ft lbs. I did replace one of the gaskets, the one that was on there seemed to have seperated the layers of cardboard or whatever crappy material that was.

I have a bigger problem now though, and I don't know exactly when it started. I know I had spark when I checked it a few days ago when this all started, but tonight a little cap came off from the inside of the HV cable when I pulled the spark plug. It was threaded, so I screwed it back in, but now I'm not getting spark. I think it may have been soldered to the HV wire, but I have no idea how they would do that, unless the plastic was somehow molded around the wire and metal cap that touches the plug.

I'm so screwed. Happen to know where I can pick up a new CDI? Or some way to replace just the spark cable portion? I noticed that it unscrews from the CDI. I guess all I need is the cap portion.

Re: 48cc kit engine won't start

Don't panic.

The plug wire "screws" into the CDI and the plug connector.

Inside the plug connector where the poug goes is a screw and under that screw should be a small spring.

Without the spring it will not have spark.

I had that problem with my first kit.

Make sense?


Re: 48cc kit engine won't start

It does make sense.

I checked outside with a flashlight and I did find the spring. Dropped it in, screwed the cap back in, and still no connectivity with the multimeter. If I drop the spring in, put one probe against that, the other on the screw post, I get connectivity. Just not when I screw in the cap that contacts the spark plug. I've taken it apart and re-assembled it about 6 times now, I have no idea what is going on inside that plug cap. I wish I had X-Ray vision. That would rule. I would spend all my time at clubs.

Some days, you just wake up and NOTHING goes right. I'm going to look into some kind of replacement, maybe from a lawnmower engine.

Re: 48cc kit engine won't start

See the attachment.

I made this diagram a long time ago when a vendor tried to tell me my CDI was good even tho I had no spark.

The missing spring was the cause of it all.


Re: 48cc kit engine won't start

You were posting as I was wrighting the last post.

The spring could have collapsed.

Lost of guys toss that cap out.

I use a ring terminal and brass thumb screw for the Vintage look that can't come apart.


Re: 48cc kit engine won't start

Ok, I figured out why there was no connectivity after I put the spring back in. It was one of those obvious solutions that don't tend to pop into your brain at 1am.

The spring was too short. I don't know how or why, but I had to stretch it a few mm to get it to contact, but now it has connectivity on the multimeter. Spring was only on the ground for a few hours, and it didn't get crushed or anything, so I have no idea why.

Thanks for the help, you are just a plethora of knowledge.

Re: 48cc kit engine won't start

Now see if it starts.


Re: 48cc kit engine won't start

that bike looks really neat can you but more pictures on how it mounted and all, cuz i have a motor that i was thinking of putting on a bike but was stumped on how to mount it, any pictures would be a great help

Re: 48cc kit engine won't start

Here's one.

What part are you looking for? I put some old bike tire around the seat tube and clamped it down on that. It is RIGHT on top of the front derailer, you can still shift, but not well. I never change the front gear anyway. The mount do not fit flush with the frame, my frame is a little too wide. You can also get small metal plates, drill 3 holes in a row, the outer 2 holes are for the bolts coming off the motor, the inner hole you drill a hole through your frame and put a bolt through that. I will probably do that at some point to level out the engine, assuming I can get it working again.


Re: 48cc kit engine won't start

well the motor that i have is 5 hp and is much wider so i dont know how to mount it because teh drive shaft is to the left and i dont think i can have the motor centerd on the from and have the gear from the motor be straight with the gear on the rear tire

Re: 48cc kit engine won't start

You would probably want to mount it over the rear tire then. I have seen some commercial solutions that do this.

Check this one:


The kit I am using was specifically designed to mount inside the frame, abover or near the bottom bracket. The mounts are designed to position the drive gear directly ahead of the left side of the hub, where the sprocket goes. The sprocket can be padded, or drilled out on a lathe to move in or out further to line up.

There are larger versions that require a widened crank set to fit. I think simpsonmotorbikes.com has one like this. Also some of them have pull-starters, and the pull start unit makes the engine wider.

Re: 48cc kit engine won't start

Why did i never think about mounting it over the rear tire, well the reason why i want to do this is because i want to go alot faster than the one huge gear that is on my mini bike, so how would i mount a gear to the other side of the bike since the drive shaft is on the left and the gears on a bike are on the right? with having the derailer still work?

Re: 48cc kit engine won't start

That's the hard part about building one. How do you implement a clutch.

The clutch on the kit I have is on the gear that attaches to the chain of the rear wheel. When the clutch is in, the friction clutch is not engaged to the gear.

The commercial above-read-wheel units I saw probably use a similar setup. The link I sent you was belt driven. You may need to make some kind of pulley system that mounts around the wheel hub. It won't be easy, but that's why you don't see a lot of custom made mopeds.

Re: 48cc kit engine won't start

well the thing is, my mini bike has a centirgual clutch that is attached to the drveshaft from teh motor, but the thing is that i need the gears from teh bike to be on the other side because the drive shaft is on the left, so can i just flip the tire around with the rear derailer, mount the derailer and run the chain from the motor down to that side of the bike to the gear?

Re: 48cc kit engine won't start

Blaine- The artist formerly known as Plumber Crack "(OFMC)" /

you will need a jack shaft to gear it down, which can also be used to get the power to the other side of the wheel.

Re: 48cc kit engine won't start

whats a jack shaft and why would it have to be geard down? even if it goes to the largest gear on teh rear tire of teh bike?

Re: 48cc kit engine won't start

If you flip the tire around, the rear cassette isn't going to be geared to catch in the right direction. Maybe if you put the tiny gear toward the tire instead of away from it, but then the big gear might be too close to the chain stay. That it a difficult problem.

You can actually get just the gear that I used on my kit, try simpsonmotorbikes.com or ebay. That way you can put a gear on the left side, and it mounts straight onto a standard hub, and you just bolt it to the spokes (with some rubber shims to protect them). I've seen them for like $20 on ebay with some other junk included.

You still have the problem of gear ratios though. In my engine, there is a small gear mated to a large gear, which goes through the clutch to a smaller cog that connects via chain to the rear sprocket. You have 2 levels of step down. The rear sprocket is like 46 teeth, the one on the clutch is like 8 or 10, and the ones inside the engine is probably another 8:1 ratio or so.

If you are going directly from the drive shaft, you will need a tiny tiny gear/pulley on the shaft, and something BIG on the wheel. There's a company revopower.com that is about to come out with a small 2 cycle engine that fits in the front wheel hub, based on the ifnormation on the site, I believe they use a 2-step gear-down. You have your work cut out for you.

Now does anyone have any idea what is going on with MY problem?? This is my thread.

How hard should it be to push the piston out of the cylinder? It isn't stuck, but I'm wondering if it is just too hard for the engine to keep it moving over after I stop pedalling. The entire top of the piston had a layer of black carbon on it, and I cleaned it so it shines again, but I think there may be some of that stuff on the rings. Could that cause the engine to stall out immediately after I stop pedaling or release the clutch? If so, what do I clean that with? There is no visible damage to the cylinder wall.

Re: 48cc kit engine won't start

Fernando go here for info, there some very talented and interesting people involved in these groups dedicated to the home built motorbike.



Now back to the problem at hand.

<snip>Compression should be over 100 psi. it should pop your thumb off the hole.<snip>

How about the compression?

I don't think the rings could be clogged and stuck this early on.

With the head off it should turn over prety easily but with some resistance.

How does the cylinder walls look? any "smears" or evidence of obvious damage?


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