Spark plug choice, low compression

Just picked up a discarded Honda Express SR NX50M. Compression is 87 psi, want to try to get it running. Stock plug is a BP5HS. I see quite a range of plugs listed for this bike. Should I choose a cooler or hotter plug because of the low compression, or stick with stock?


Re: Spark plug choice, low compression

fix the compression, you cant really compensate for low compression

Re: Spark plug choice, low compression

Thanks for replying.

Am I correct in believing that the sources of low compression are pretty much limited to the rings and the head gasket? We found this bike in a trash pile last week, and it actually ran for about 3 seconds with a shot of starting fluid in the plug hole.

I'm told compression should be up around 120 psi. The odometer shows about 1900 miles. If it was 11,900, I guess I'd expect to be able to feel at least a slight ridge at the top of the cylinder wall, and I can't feel any ridge at all. Where else might the problem be?

I put up a picture of the cylinder wall at the bottom of this page:

I'd appreciate any advice.

Re: Spark plug choice, low compression

compression depends on a tight seal everywhere above the rings including the rings... spark plug gasket, head gasket, a decompression valve (if there is one) etc.

one likely and common problem is the rings can be stuck in their grooves, and so they don't seal against the cylinder wall.

Re: Spark plug choice, low compression

Hmmm... Maybe some Marvel Mystery Oil in the cylinder, stand the bike upright for a day or so, and see if it penetrates enough to free a potentially stuck ring? Or do they typically seize to the extent that they need to be persuaded to come out? Until I get the new head gasket I'm hesitant to take the head off.


Re: Spark plug choice, low compression

i wouldn't even bother oiling it. Just pull the cylinder off and take the piston off and get in there and do it right.

Examine the head and cylinder and piston for cracks.. maybe the piston has a little hole in it?

Ya neva know what you may find.. chances are nothing but lots of carbon and crap gumming up the rings..

The ring groves should be immaculate.. don't scratch or damage the grooves. Google for tips on how to carefully remove and reinstall piston rings.

Don't use force on anything involving rings. On reassembly, forcing the cylinder over the piston may mean you're about to snap a ring. Patience and care are the keys.

a head gasket can be traced and cut from thin aluminum. Pick something close to the original's thickness. Clamp the material between pieces of wood to drill clean bolt holes or use a suitable punch. Or cut everything with a Dremel MotoTool.

No reason to be shy about it .. a moped engine is the perfect engine to start on as far as gaining confidence. The bare minimum of mechanical skills and basic tools are needed.

Re: Spark plug choice, low compression

George Smith /

mopeds are very basic in design and the motors are incredibly simple, spark, compression, fuel, run.....the rest is kinda superficial :P just make sure all your gaskets are good to go

Re: Spark plug choice, low compression

i think it will run, and probably run well. did you ever try it?

Re: Spark plug choice, low compression

Just for a few seconds with a shot of starting fluid into the spark plug hole. It did run, but not long enough to draw any conclusions. I've bought an oil tank on ebay, which will take a while to get to me. Then I'll need to rig up a fuel tank of some sort, and a seat, and see how it does. Just picked it up (from a scrap pile) last week.

Ordered the manual today, as well as the head gaskets and a standard spark plug.

I thought that maybe with the low compression I should be using a plug with a different heat range. I was surprised by how many plugs listed for this bike. They show 12, and I wondered if it was a standard practice to shift to cooler or hotter plugs.

Re: Spark plug choice, low compression

after a complete tuneup, and after the bike is running well for about 50 miles or so, pull the (previously new) plug and see if it's clean. If it's got an insulator with solid deposits on it you could try a hotter plug.

All it needs is a plug-tip that stays hot enough to burn away low-speed deposits, so the plug will not be the cause of fouling/ignition trouble after 2, 3 or 400 miles.

Re: Spark plug choice, low compression

if you haven't done so yet, remove the air cleaner and muffler for diagnosis purposes. have you cleaned the carb yet? you have to eliminate all possible problems first.

Re: Spark plug choice, low compression

Yes, cleaned the carb. Haven't removed the exhaust, and it just doesn't have an air filter. Yet.

I figured I wouldn't try to run it until the oil tank arrives. I'm actually hoping that it does run fine with the low compression, just with diminished power. It's going to be used by my 8 year old boy and I don't mind him having to go a little slower.

From my point of view, I see it as a long term restoration project. My older brother had a Honda 50 when we were kids and we both loved it. It was probably around 1971.

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