VERY loud noise, need HELP PLEASE

Hi guys, I have a sound problem that I need to fix but don't know exactly how. I have a '77 Sachs Columbia Commuter moped that was running just fine, but then I lent it to a friend to ride and it ran fine for him but when he revved the moped in place (he was sitting on it with his feet down and gave it some throttle, but he held the moped in place, I don't know why he did that) just for fun it started to make a very loud sound, kind of like a harley or like firecrackers are constanly going off. It hasn't stopped making that sound still. It still runs just fine, but it makes that "chopper" or "firecracker" sound whenever I am going slow (about 10-15 mph) or if it is in idle, and sometimes when I am going faster (25 mph). But mostly it just makes the noise when I am going slow or in idle. The sound seems like it is coming from around the cylinder head. It stops making the noise when I hold this little rod (it goes up and down on the outside of the cylinder head when the engine is on, I don't know what it's called cuz I don't know much about mopeds) on the cylinder head and prevent it from moving- but I think it's supposed to move, I don't know though. It also seems like the exhaust is still attached fine to the cylinder, but I guess I may have to take it off and reattach it to be safe (I have no clue). Also, there's a little hole on the bottom left side of the cylinder head, but that also looks like it's supposed to be there. I'm thinking something popped off or got loose, but I don't know what. That's about all I know for now, but I really need someone's help in getting this moped to be a little more quiet like it used to be. Plus, I don't know if that noise is bad for the engine or anything so I don't want it to take a beating. Please help me. Thanks for your time.

Re: VERY loud noise, need HELP PLEASE

nice friend lol grrrrrrrrrrrr

Re: VERY loud noise, need HELP PLEASE

sounds like trouble with the decompression valve in the head..

There's this little valve in the head. Activating it (opening it) releases engine compression and allows the engine to start more easily. If the valve is leaking you might be hearing the 'pop' of normal combustion.

_when I hold this little rod (it goes up and down on the outside of the cylinder head_

Little rod is probably the decompression valve's stem. If so, it should not "go up and down" while the engine is running..

The decompression valve is held tightly closed by a strong spring. It would be forced open by pulling a lever on the handle bars.

Perhaps the decompresion cable, spring or lever has a problem.. or perhaps something, like a chunk of carbon, got under the valve and it's sticking open at all times.. gotta get in there and look. The valve should be closed and shouldnot leak.

To test the valve's sealing ability, remove the head, turn it upside down and pour a bit of gasoline around the valve. If any leaks the valve is leaking.

But it seems as though your valve will be found to be obviously open and this should be visible without any test.

Re: VERY loud noise, need HELP PLEASE

Thanks joew, I appreciate the help. Yeah, I will try to take the cylinder off and look at the valve and do the gasoline test you mentioned. But if it is leaking, do you know how I can fix it? Oh, and what cable on the handlebars can control this (you mentioned something about that in your post). Thanks.

Re: VERY loud noise, need HELP PLEASE

Actually, I was about to go in and fix the moped and was wondering whether or not it would be okay to just seal the valve shut at all times (or replace the hole that it would leave with a screw)? I heard that the decompression valve isn't really necessary to get a moped started. My moped is a pedal start, I don't know if it matters or not. If not, then I guess I'll take off the head and try to use a spring to hold it closed. Let me know any information when you get the chance.

Re: VERY loud noise, need HELP PLEASE

i dont use decopmpression to start the bike either.. i start the bike on the center stand and decompression isn't needed. But it does help when pedal-starting while actually pedaling the bike down or up the driveway.

i thought about permanantly closing the valve hole in the head .. maybe just welding it up nice and smooth. But the valve seals well and i prefer to leave well enough alone until i have a reason to change things..

Clean the valve's head and the valve seat of any carbon so it can't leak .. Attach a spring / lever or whatever to the valve stem to keep the valve pulled in closed. The spring should be strong. Look at any moped manual for a description of what the common decompression valve mechanism should look like.

A bolt or wire or other jury-rigging might loosen up.. and you don't want a compression leak. A spring would be best, imo.

Re: VERY loud noise, need HELP PLEASE

Alright, thanks. First, I wanted to ask you (or anyone else interested) if the valve needs to be pulled up towards the outside air to be closed or does it need to be pushed in towards the cylinder to be closed. Thanks. I think it has to be pulled out towards the air, but I'm not sure. By the way, do you know where I can buy a spring? Thanks.

Re: VERY loud noise, need HELP PLEASE

Also, would you know where online I can see a diagram of a decompression valve. I tried looking and didn't find anything too great. Thanks.

Re: VERY loud noise, need HELP PLEASE

Sorry, one more thing. I realized I've never had a spring on my moped (cuz I'm told the spring is external, and I don't see one, I'll attach a picture later). And the loud harley noise also stops when I plug up a hole in the left underside of the cylinder head. It seems like the hole is supposed to be there, but I don't know if it is supposed to be plugged with something, like a screw. Thanks again for your help.

Re: VERY loud noise, need HELP PLEASE

The hole is the vent to the decompression valve. It's pretty regular to have oil dripping out of it.

The valve, to be closed, needs to be pulled up.

Search here, and on the general forum, and I posted the size of the bolt that you would use to get rid of the valve. I think I've also posted pictures of the correct valve set-up.

It's been a few years.

Re: VERY loud noise, need HELP PLEASE

can't find a diagram.. look at the attached pic. Not great but it might give you an idea.

my bike's head has the little hole but the hole empties in into the exhaust port..

springs can be found at a hardware store..

Don't be afraid to remove the head and look..

It's your bike and you're gonna have to get familiar with it someday..

Mopeds are self inflicted, entry-level mechanical experience.. take it or leave it.

1124745846_decomp.jpg

Re: VERY loud noise, need HELP PLEASE

here it is... man this is an old pic!

Re: VERY loud noise, need HELP PLEASE

Some features to note- Sachs motors don't have heads- it's all one peice.

It's easier to remove the valve as a whole assemble than to even attempt to see it in the cylinder/head.

The starting clutch and the decompression lever work at the same time, from the same lever. The housing pushes the decomp valve open as it engages the clutch.

Re: VERY loud noise, need HELP PLEASE

Thanks so much man, I'm going to go and get a spring tomorrow and I'll let you guys know how it works out. Thanks so much, joew, your first diagram told me exactly what I needed to know. Thanks!

Re: VERY loud noise, need HELP PLEASE

Hey guys, thanks so much! I actually realized my decompression valve was more screwed up then I said, so I knew it would be hard to put a spring on. I ended up taking some key chain wire and shaping it to what I needed. It now works just like it's supposed to. The valve stays closed unless the lever is pressed. So no more harley noise!! Thanks, haha, and also my moped runs a little faster now. It has a lot less lag when starting from a standstill. I appreciate the help guys, thanks for sharing the knowledge.

Re: VERY loud noise, need HELP PLEASE

the first thing i would check would be the exhaust connected tightly and try a gasket...maybe some one already said this, but i didnt read all the post

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account