HondaNU50 Urban Express popping noise

Captain Janeway /

I have a 70cc kit on a Honda NU50 Urban express. Spent the day cleaning carb, and upjetting to an 88 from an 85. I also put on a pod air filter rather than the stock filter.

It had been bleeding oil from both exhaust port and spark plug hole. Plug was sooty looking but not really oily looking. Lately it had been making an odd popping noise in the exhaust when I've been putting the bike away. It kind of sounds how metal does when it gets really hot and cools in the air. It had been running really slow.

After the changes above, I took it riding and it really goes fast. At WOT it's fine, but when I throttle down to about 10 mph I hear this putt, putt, putt noise. when stopped and on stand, it still made that popping sound like metal contracting.

I put the stock air box back on and will try WOT with that again to see how it feels. Any ideas of what else I should look for while I have it out? If the stock filter setup slows it back down, ideas? I figure I can try putting the new filter on, but I guess I'm concerned it will run too lean at that point with that strange noise.

Any help is appreciated.

Re: HondaNU50 Urban Express popping noise

88 to 85 jet? Wrong direction and either way both are far too small.

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

Had on an 85 and increased to 88. suggestion for size? I have a 90 and 92

Re: HondaNU50 Urban Express popping noise

You likely need bigger than the 92. I soft seized with a 92 not too long ago. If its the stock muffler you would likely be fine.

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

Thanks. I'll go tear everything open again and put in a new one. I wish the carbs weren't such a b___ to get into on this bike. Have a suggestion on jet size? I looked at the tuners spreadsheet and it looks like a lot of them were using a 92 or so jet. It's using the stock Keihin carb.

Any chance it would start seizing putting on the new air filter and the bigger jet? I'd rather take the stock box off because people have told me it might perform a little better if I let the engine breathe a little better.

Re: HondaNU50 Urban Express popping noise

Unlikely given the small carb size. The air box would do fine and give you some breathing room jet wise. Putting a different filter on is going to need a bigger jet still. With the stock box try the 92. I was using a small paper/mesh filter and it felt great, but after a minute of mid range cruise it ate it. In my case moving the needle up made it soggy and that was the best happy medium.

Re: HondaNU50 Urban Express popping noise

what pipe you using, stock or not? might be a clogged stock pipe bogging you down. you're trying to push a lot more air from the kit and hi flow filter out something the size of a straw.

as for bleeding oil, bad gaskets on plug/exhaust maybe? or you could have a leaky oil pump, unless you're running premix? need more details

get a head temperature gauge to help see if your jetting changes are high enough, keep it under 400.

if you remove the oil pump from the case, getting the carb in/out is pretty easy after taking the battery box off, plus you'll know you're getting the right amount of oil at all times because you mixed it yourself.

and the popping noise "It kind of sounds how metal does when it gets really hot and cools in the air" thats what youre hearing (edited)

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

Moving needle up? Doesn't that lean it out? I have it set on notch 2. Right now I still have the 88 jet in there. It shouldn't hurt anything should it? It's hitting top of speedometer at WOT.

The only thing I'm noticing now is a catching sound in the clutch when engaging and sometimes when it's sitting at idle.

I'm going to spray a bunch of carb cleaner around spark plug (with it in) as I'm trying to sort out if there's a leak in there. I hear what sounds like some sort of rubbing, scratchy sound when the engine's running at about 10 mph. I don't think I noticed it before because it was so loud with the exhaust being open a little.

If I change the air filter, I'll put in bigger jet.

I really appreciate all your help as I'm still learning. I just want to keep my baby running!

Re: HondaNU50 Urban Express popping noise

Yes it seriously can hurt things.

Moving the needle UP exposes the main jet sooner (moving the clip down), giving you more fuel in the mid range but having no affect wot or low throttle opening.

Dont spray carb cleaner on the plug, that is useless. Spray it around every gasket mating surface and the crank seals if you can get to them. Those sounds you are hearing are likely a lean mixture and lack of lubrication. With your needle leaned, AND a too small main jet, and cruising at low speed/throttle openings you are practically starving it for fuel/oil. You will hear the piston scraping around in there and the rings rubbing. None of that is good but there is always SOME cylinder noise.

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

@ Ryan,

No, I have the jet on 2, not #1 at the top. It's listed in the manual as standard, the number 1 as lean. spraying around cranks I'll take to mean spraying around the gasket attached to the crank cover. I have felt the kick starter stick a couple of times when I've used it. I'll try around gaskets as well.

Just figured I'd clean up around the plug port to see if it continues to leak oil. will try the head gasket.

I'll also make sure there's enough oil in outer reduction.

will upjet

@Chris, The pipe was modified to put in a bigger opening than the stock pipe had. I thought about a bigger pipe, but I really don't want this bike sounding like a small Harley.

The oil leak was coming out of exhaust port. Tried tightening it up better and it makes less noise and doesn't seem to be leaking. There is a lot of oil by spark plug opening. The plug was sooty/oily. Put in new one, and with the other changes will see what happens with it. Would you run a high speed plug in it instead of a standard?

Re: HondaNU50 Urban Express popping noise

Right. Looking at the needle, the first notch will be the leanest, because as the clip goes up, the needle goes down. I was saying that the needle may need to be raised (clip down).

No, spraying around the covers will not get it. The problem with the urban is that the stator is attached to the cover and you cant remove it while running. The kickstart also is required to start it normally and would be hazardous to try and remove while started. So getting carb cleaner directly to the spinning crank, at the crank seal is difficult if not impossible for most. Spray the intake joints and the base gasket area primarily, as well as the head gasket. Those are the things you have been playing with and stand a good chance to be the leak point if one exists.

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

It's actually an urban express deluxe, so it's electric start. the battery keeps dying (I'm told it may be the regulator, but I have a charger and don't worry about it), so that's the only time I use the kickstarter. I'll try spraying the other areas.

Giving the bike more air makes the mix leaner if I'm reading correctly right? So I shouldn't put on that other air filter (which I think lets it breath much better) until the bigger jet goes in correct? I'll put in that 92 and check for fourstroking. I think I understand how that works from reading Fred's page

Let's hope this helps some with gas mileage. It has truly sucked for mpg. I'm talking 20-30 mpg. that bigger cylinder really dropped the mpg.

Thanks.

Re: HondaNU50 Urban Express popping noise

Right, but welcome to having crap mileage. My bike gets 40-50 in a good day.

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

Thanks. I figured there was something wrong that the mileage was that crummy. I guess it's having the bigger cylinder.

Thank you for all of your help!

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

Tried doing a plug chop today, and it looks like bike might be running lean after putting in bigger jet. I either didn't do the chop properly, or I'm trying to figure out what I did wrong.

A bigger jet would make the fuel mix richer, correct? Putting back in the old plug and running it, it looks like even the old plug is getting some of the oily look burned off. The insulator tip is starting to look greyish, unlike the black mess it was before. I don't figure the ceramic color will burn back to white on the old plug.

The only thing I've changed is that it has a 92 jet instead of an 88, and I cleaned the carb. I tightened up the exhaust better so there's no oil leak there, and the bike does seem to run great (famous last words).

Until I sort some of it out, should I throw in premixed gas to be going along with the oil injector to help prevent possible seize?

Try a bigger jet or just try another plug chop?

Re: HondaNU50 Urban Express popping noise

Did you use a new plug and what is your method of doing the chop?

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

Yes, new plug. Got the bike good and warmed up, then stopped and put in new plug. Rode it around a fairly open empty mall parking lot at WOT off and on (had to slow for corners but didn't stop) for most of the way around it, then flipped engine kill and put on brakes. Pulled the plug and put back in the old one.

I want to find an area where I can really tear around on it without hitting a light or stop sign (the joys of the city).

Maybe I didn't run it far enough? At WOT I don't think I did more than 1/4 mile.

If anything, it seemed really oily on the old plug before I upjetted.

the other thing I notice is sound in the clutch area when the engine engages. Heck, it may have been there before, but I'm a little more attuned to the noises the bike's making right now.

Re: HondaNU50 Urban Express popping noise

That's not how you do it.

Get the bike warm, swap plugs. Get up to speed in one shot and ride WOT at least until you get into the power and build the temps a bit, 1/4 mi or so (less if unsure of safety). Then, WHILE holding WOT, flip the kill switch and close the throttle. Slow down as fast as safely possible to prevent the plug from self cleaning or being covered with fresh mix. Now check it.

Any time you allow the bike to slow down while running, or begin to close the throttle you are either allowing the plug to self clean or you are hitting it with unburnt mix.

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

OK. Sounds like I need to run it WOT a bit longer. It seems like I killed it properly though.

I'll give it another try.

thanks!

Re: HondaNU50 Urban Express popping noise

Captain Janeway /

BTW. Should I go to a bigger jet? I have the 92 on, but also have options for 94 and 96.

I have the stock Honda air box on which I think restricts the air some, so I was thinking of trying a pod type filter to let in more air.

If I understood one of your previous posts, I should upjet even further? Or possibly moving up a notch on the needle?

Re: HondaNU50 Urban Express popping noise

"It had been bleeding oil from both exhaust port and spark plug hole". leak around spark plug and exhaust. "I hear this putt, putt, putt noise". Leak around spark plug. This is your main problem. Just now read it.

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