Honda Express NX50 - need some help

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Honda Express NX50 - need some help

Postby Breaze » Thu Nov 20, 2008 7:08 pm

Just aquired 1981 Honda Express SR NX50 as a winter project having no experience at all in this area and could use some help. I did what I could reading the forums and using the on-line manuals for similar Expresses but I need some help specific to the NX50. I got the bike running by reading forums and doing all the basics:

- cleaned carburetor
- cleaned muffler
- changed transmission oil

It starts easily, runs ok, then starts to have problems when warmed up and won't re-start when hot - sounds like the auto choke/bystarter concerns I've read about.

Three problem areas I need help with:

1) Petcock: The upper fuel line flows freely even when the engine is not running and creating a vacuum through the bottom port

2) Carburetor: Last time I started and ran the bike, after turning it off, the fuel flooded through the carb and out (slow drip) as if the float and float needle weren't sealing off the fuel intake - not sure if this has to do with the petcock problem - seems like the float should shut down the flow anyway

3) Before doing too much trouble shooting I wanted to make sure I had the main lines mapped correctly (weren't all connected originally)

Line 1: runs from port on left side (bike left) of the intake manifold to the port on the left side of the control box (I'm assuming the auto choke control box feeds thru where it is attached to the cylinder head so not sure what this line does)
Line 2: runs from left side port on carb to middle port on control box (maybe feeds fuel mix to auto choke from carb)
Line 3: runs from right side port on control box to bystarter assembly (maybe feeds air to auto choke)
Line 4: runs from top port on carb to bystarter assembly (unsure but must feed air to carb to allow output from carb thru line 2)
Line 6: runs from upper petcock port to fuel intake of carb
There was another disconnected line which I am guessing runs from the bottom petcock port to the right side port on the intake manifold to create the vacuum to activate fuel line - it was unmarked - maybe line 5

Sorry to write a book - didn't realize I would get into this project the way I have. Any help would be appreciated.
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re: Honda Express NX50 - need some help

Postby Breaze » Sat Nov 22, 2008 8:30 am

Quick (maybe not so quick) update- I believe I have verified the mapping of the lines, although I am still not sure if the #1 line feeds the mix into the manifold or is just used to power/activate the mechanism by supplying the vacuum (suggested by Sean L. in another topic) and the feed is directly to the cylnder. I took the carburetor apart, cleaned again, and it is not leaking anymore - must have been sticking. I'm not worrying about the petcock - ordered a screen/filter for it and will put it in today and check out the vacuum valve workings. If it still flows freely afterwards, it shouldn't affect the performance - can find way to shut off later.

The next things on my list to address the lack of power and uneven acceleration:

1) I'm looking for a compression tester to see if I need new rings, head gasket, etc.

2) Probably need to check for vacuum leaks

3) Need to set carburetor mix and test the autochoke/bystarter mechanism - in order to do these items I plan to cap off (using vacuum port caps) the entire autochoke/bystarter assembly, start and warm up the engine, and then attempt to set the carburetor mix properly. If I can get it running well this way, I'll know I have good compression and no big vacuum leaks. Then I'll deal with the mechanism.

Anyone know the recommended starting point of the mix and throttle screws? When I got the bike they were set:

throttle (upper) screw: about 1 7/8 turns out
mix (lower) screw: about 1 1/8 out
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re: Honda Express NX50 - need some help

Postby Breaze » Sat Nov 22, 2008 4:12 pm

Latest - checked compression - almost 120 so I should be ok.

Capped off the autochoke/bystarter mechanism but still running weak - accelerates slow and choppy. Carburetor adjustments seem erratic at best. Would like to rule out the carb but just don't know enough - would really like to know factory/manual starting adjustments for screws.

One thing, when cleaning, I came up with a small silver washer that had dropped from somewhere - I checked the parts fiche and noticed that there seemed to be a washer and a gasket maybe on the air mix screw. That is where I used the washer but didn't seem to have the gasket. I am planning to order the gasket kit thru service Honda and see what comes with it.

Also am not using stock air filter element (using B&S sponge filter element) - ordering one to see if that makes a difference.

I'm really hesitant to get into the transmission because the weak acceleration and choppy running occur even with the rear wheel elevated.

Any suggestions out there?
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re: Honda Express NX50 - need some help

Postby Breaze » Sun Nov 23, 2008 2:55 pm

Well not sure anyone is following this saga, but here is the latest

Found the Urban Express NU50M '82 manual which seems to fit the NX5oM the best. This manual considers adequate compression to be 156 - 213 psi, I am getting slightly less than 120. Troubleshooting guide points to piston/rings or reed valve.

Piston/rings: removed cylinder head but can't budge cylinder - manual says don't pry or strike fins so I don't see any way to remove. Any advice here would be appreciated - maybe it takes more than just the 4 screws in the head for the NX50? (NU50 manual only mentions the 4).

Reed valve: pulled the reed valve and found actual pieces of straw (maybe thin pine straw wedged under the reeds - one side is pryed up slightly more than the other from the debris being underneath. I am planning to replace the whole unit ($22.00) since the manual suggests not dissassembling or bending.

So... I'll be placing the extra little ring gasket on the mixture screw in the carb, setting it initially at 2 turns based on the NU50 manual, changing out the reed valve, and holding off on the control box (bystarter/autochoke mechanism).

If I can't get the piston out (remove the cylinder), then I'll reassemble and check compression again.

I am also working on the starter motor which runs but is slipping where the motor engages the pinion. The plate between the motor and the pinion housing has a small opening for the rotator to push thru and engage the pinion. This hole has widened from wear and the rotor slips away from the pinion. I only need that plate but all I can find for purchase is the whole motor for $117.00. Does anyone have a used starter motor for sale?
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re: Honda Express NX50 - need some help

Postby John » Sun Nov 23, 2008 3:32 pm

Flip the reed petal over and try your compression test again. The jug should come off once you have removed the head. Spray some PB Blaster or another solvent around the base of the jug, has to be the gasket that is holding it tight.
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re: Honda Express NX50 - need some help

Postby Breaze » Mon Nov 24, 2008 9:24 am

Thanks I will, First though - a question risking igonorance exposure.

Reed valve: when I pulled the valves originally they were seated with the reeds up - seems like the reeds should be down to allow input to cylinder but not output. How are they supposed to be mounted so that I can 'turn them over'.
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re: Honda Express NX50 - need some help

Postby John » Mon Nov 24, 2008 9:37 am

Undo the bolts/screws and just flip the flapper material . They should be laying flat when not in use and open toward the motor by suction from the piston stroke.
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re: Honda Express NX50 - need some help

Postby Breaze » Mon Nov 24, 2008 8:38 pm

Once I got home and looked at it, I see what you're after - if it's slightly open from the debris, turn it over and it will be shut. I turned it over but will probably wait until I get my gaskets (carb and head) to put the carb and head back on to test compression.

Even if it doesn't help the compression, you probably saved me the $22.00 for a new reed valve.

Also, I read in an old ExpresslyMoped forum about a way to fix the starter. Because the bushing was worn back allowing the motor rotor to disengage from the pinion, it suggested tapping out the bushing, rotating it 180 degrees, putting it back in, and it should hold.

I did so tonight and it seems to work - another $117.00 saved from forum groups.
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re: Honda Express NX50 - need some help

Postby Breaze » Tue Nov 25, 2008 8:30 pm

Doh, One step forward, two back.

Started on the front fork and found that the fork was slightly bent - I had noticed that it required oversteering when I first got it started and went around the block. The bend is slight but noticeable when riding.

I plan to try and bend it back but I don't have much hope - anyone out there with a fork they would like to sell?

A new one is $72.00.
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re: Honda Express NX50 - need some help

Postby John » Wed Nov 26, 2008 6:19 am

Time to look for a donor bike.
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re: Honda Express NX50 - need some help

Postby Breaze » Wed Nov 26, 2008 8:38 am

Yeah, probably wasn't the most savvy buyer when I picked this one. Just chalk it up to lessons learned.

Not giving in though, even if I have to put some money into it, I'm enjoying the process and learning something - which is hard for an old f...

Hang in there with me, we'll set a record for longest topic. I've got all winter.
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re: Honda Express NX50 - need some help

Postby John » Wed Nov 26, 2008 8:47 am

Check with Bryan over at Mountain Moped in Jasper Ga. He may have a line on some parts.
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re: Honda Express NX50 - need some help

Postby Breaze » Thu Nov 27, 2008 11:15 am

Latest:

I was able to straighten the front forks - finally a positive (essentially used brute force)

Turned the reeds over as suggested, put everything together, checked compression, and still only 120

Used PBblaster as suggested and was able to remove cylinder. Piston and rings look good visually but I don't have micrometer or ID bore gage to perform measurements. Should I just replace rings (cheap) or get the equipment?

One comment exposing my ignorance again - I was running an oil/mix fuel along with the injected oil (until last compression test) and wondering why the plug was getting so oily. Would that effect compression?

OK, after compression woes solved, I'll be inspecting the transimission and the drive (clutch, etc.). I'm hoping everything will run well and I can get by just doing the specified maintenance.
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