Carburetor from hell

I got a "Revolution" carb (https://qt50.net/honda-nu50/honda-urban-express-revolution-carburetor/) the other day. It has become my worst enemy.

Installing on bone stock 1982 Honda NU50 Urban Express. With stock carb, was running about 25mph with 220lbs of fat ass rider but always a little rough and seemed like the autochoke and bystarter were always interfering with smooth fueling. So I got the Revolution to upgrade but it flat out won't work.

The low speed circuits seems to more or less work, but that's it.

Throttle cable is not too loose, not too tight, and I can see the throttle slide goes all the way down with no throttle, all the way up with full throttle. The jet needle slides into the needle jet just fine. Tried with the jet needle with the circlip in the second notch down as the carb came, then all the way to the top, and last with the circlip on the lowest notch. Made no difference. The needle jet is not bent and is held properly by the little v shaped clip thing.

Removed the exhaust and tried it that way just in case it was clogged. No help. It just won't get the RPMs up. Top speed is now 12.

The ONLY time I could get any real response is when I started it up with the chamber cap and throttle slide/needle jet assembly removed (ie, just open air on top of carb). Without the chamber cap on the low rpms about the same, but when I mostly covered the air inlet with my fingers or with the choke on at about 3/4, air would rush in through the top of the carb's open throttle chamber and rev right up, giving me that nice "zing a ding ding" you should hear from a two stroke when it's revved up.

So that's the problem. I own 11 carbureted engines, several of which are 2 strokes and am usually a pretty good wrench. I don't have any kinked fuel lines, the spark plug works and is gapped and installed correctly, the throttle cable and mechanism are just fine, compression is fine, carb is seated against intake manifold, no air leaks, etc. And like I said it more or less ran pretty good with the stock setup, just rough like a lot of old mopeds that spent most their life just sitting. Stock oil pump is out of the equation, routed return oil line to intake line and running about 40:1 premix.

Any ideas? Usually don't give up on a project until it's fixed but running out of patience and ideas with this carb.

This isn't something I can spend a ton of money on since I'm an out of work tour guide so just telling me I'm stupid for buying the revolution carb and that I should get a fancy italian carb, intake, pipe, reeds, big bore kit, etc. isn't gonna help here. I've got fast bikes, just want this one to run smooth.

Thanks for any ideas!!!

Re: Carburetor from hell

When you removed the exhaust did you look at the exhaust port on the cylinder? They can clog too on most mopeds.

Sounds like carb isn't compatible.

Re: Carburetor from hell

Yeah, exhaust is good.

Re: Carburetor from hell

And it's the exact same bike as the guy who sells these carbs. Same year even.

Forgot to mention, also checked float operation and made sure jets are clean and unobstructed.

Re: Carburetor from hell

Dirty30 Dillon /

This carb seems like a gimmick.

What main jets have you tried?

Re: Carburetor from hell

It didn't come with any extra jets. I got the carb 100% for the manual choke (I put a manual choke on everything). Dude says he jets it for your application and had decent reviews, and since he has exactly the same bike figured it would be ok.

Re: Carburetor from hell

Dirty30 Dillon /

> Real Deal Neil Wrote:

> -------------------------------------------------------

> It didn't come with any extra jets. I got the carb 100% for the manual

> choke (I put a manual choke on everything). Dude says he jets it for

> your application and had decent reviews, and since he has exactly the

> same bike figured it would be ok.

With a 30+ year old bike, I can't put much faith in a "pre-jetted" carb, given that compression weakens with wear, and he has no idea what your environmental operating conditions are.

If you're certain that the rest of your motor is tight, aka you've done a comp/leakdown test and your reeds/exh are all good, than that leaves carb tuning. In which case, buy some jets and do the thing.

Re: Carburetor from hell

Pushrod Fifty /

Its either flooding or has a severe air leak, Did you plug the oil line passage on the engine? Ive never deleted an oil pump before, I would research whether or not the mains will get properly lubricated by prefix and the correct way to do it. I would try the middle clip position and be sure the float operates correctly.

Re: Carburetor from hell

What do you mean "Stock oil pump is out of the equation, routed return oil line to intake line and running about 40:1 premix."

If the oil pump is deleted you should cap off the oil inlet nipple in the intake so there is no air leak there.

Are you still running the stock auto-choke and bimetal valve? You should remove all of the vacuum tubes for that as well and cap off any inlets left open now that you have a manual choke.(edited)

Re: Carburetor from hell

Jack Rutherford /

jetting is too rich

Re: Carburetor from hell

Hey guys, thanks for the input and ideas.

Just updating for closure, never had any luck with this and definitely not recommending anybody try the manual choke Revolution carb for sale at the website I listed above. The guy selling them had good prompt communication and tried to help. He was successful with the carb on his Urban Express NU50's which are exactly like mine and same year, so maybe I just got a bad one.

He sent me a smaller idle jet, which made a very slight difference, but still was never able to get the bike to rev up unless I completely unscrewed the top cap and pulled out the throttle slide completely. Tried with the #78 main jet that came on the carb (which worked the least terrible), the main jet from the stock NU50 carb which I believe is about a #85, and then with #71 and #75 main jets from treatland. No luck. With each jet combo, started with mixture screw out 1.5 turns, but with each jet didn't matter if turned all the way in or all the way out, zero effect.

All ports from the oil injection were capped off and I checked all around with starting fluid so there were no vacuum leaks. Float level and valve on carb operational. Plenty of fuel flow from newly cleaned tank and new fuel line and filter. Just bbq'd the exhaust and it's flowing great and reinstalled with a new gasket. Verified needle properly properly sliding into valve and tried with it the circlip at each height setting. Zero difference there. Verified no plugged jets and that throttle slide was going from all the way closed to all the way open. Again, no luck unless it was completely removed along with the top cap.

At some point you just gotta throw in the towel, so tomorrow just gonna put the stock carb back on minus all the autochoke/bystarter garbage and just go back to running the stock carb. Main reason for "upgrading" was to avoid the bimetal garbage autochoke system in the first place with the revolution's manual choke but to hell with it. Going from 25mph to 16mph isn't an upgrade and if I ever feel like I'm missing out with no choke, well that's why God created starting fluid.

Re: Carburetor from hell

Just to update, I mentioned to Andy the guy selling Revolution carbs (https://qt50.net/honda-nu50/honda-urban-express-revolution-carburetor/) the problems I was having and that I was just going back to the stock carb. Few days later out of the blue, a new carb body showed up. Swapped over the jets and everything, installed it, and it rocks.

I put it together the same exact way I tried with the first carb (so it wasn't just that I was doing something wrong) and the carb now rocks, running great with no problems. So apparently I did get a bad one. But with Andy sending me the new one and his great communication overall, gotta say that I do recommend this carb and it's simple manual choke for anybody looking to ditch the glitchy autochoke setup from their NU50 urban express.

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