'78 Honda Express NC50 : Modified : Engine Dies

All,

Here are some basics:

- Have kitted engine to 70cc.

- Upgraded carb, intake, exhaust

- Battery good charged and holding charge

I took the noped out yesterday for first run of the season after upgrading various parts of ped. Ran great for 30+ miles. I kept the speed around 30-34mph and didn't "open it up" or run it hard. Then, on my way home there was a good stretch of flat road... so I decided to test the new carb and open it up a little. Was probably doing 35-37mph for 1-2 minutes and felt a little intermitant hesitation in the engine. I backed off a little and then increase the speed again. All of a sudden it just died... like not bogged down or running rough... just engine quit all together and it came to complete stop.

I waited for 2-3 minutes and let the engine cool. While stopped, I turned the key on and tested the horn... nice and loud beep. So, electrical was working fine. I wound the starter spring and released... the engine started up NO problem. I ran again at around 35mph and same thing happened. Dead stop.

Ironically a neighbor of my was passing by and I got a ride home, rather and try this process for another 9 miles back.

Does the problem sound familar to anyone? I have some theories thus far, but could use some suggestions for diagnosing, advice, testing or disproving theories.

Theories:

- Electrical "grounded out" somehow by positive of battery touching metal of bike. (BUT, why would it happen when I was really pushing the ped?)

- Something is over heating... maybe Rectifier? The Magneto could be putting out way more than 6volts when going that fast and overheating components? (when I let the noped sit... it cools down and allows me to start it again)

- Just read another post (different brand moped) and someone suggested the piston might be seizing... when it cools... it is enought to free the piston and allow me to continue.

Any and all constructive advice would be appreciated.

Thanks,

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

Gas cap vent plugged? Or dirty filter strainer? I remember them having trouble with the vent lever on the cap,I believe. don-ohio (:^)

Re: '78 Honda Express NC50 : Modified : Engine Dies

I remember working on one years ago,and when you turned the gas cap lever to vent,ALL the way,it stopped the air.BUT,if you ALMOST turned it all the way to vent,it would vent then. don-ohio (:^)

Re: '78 Honda Express NC50 : Modified : Engine Dies

Andrew Squiggman /

maybe you ran your carb dry faster than it could refill

Re: '78 Honda Express NC50 : Modified : Engine Dies

I'm running a Mikuni 20 mm carb:

https://www.treatland.tv/mikuni-20mm-carburetor-p/mikuni-20mm-vm.htm

The reason I don't feel it's the carb is that I have run the noped out of gas before and it behaves different. When I run out of fuel I can feel a gradual decrease in engine power and then engine quits.

This was very quick. Like I hit the kill switch and cut the engine all together.

Know what I mean?

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

I know what you mean,but check the gas cap anyway. It does have the selector lever,right? don-ohio (:^)

Re: '78 Honda Express NC50 : Modified : Engine Dies

Test for spark when it craps out next time. No temp gauge?

Re: '78 Honda Express NC50 : Modified : Engine Dies

Quick update all on my honda express noped...

- I checked the gas cap and ensured that when in "OPEN" state air would easily travel through the pin-sized hole in cap. All good.

- I DID found a very poor connection that was soddered on the Ignition Coil (Spark Coil), green wire I believe. I removed connection, re-stripped wire, and crimped it back with a good solid connector.

- I DID replace the stock Petcock with a newer one, because the old one was leaking fuel. In an effort to get the old petcock to stop leaking I may have disturbed the flow of gas to engine. This could go along with the theories others mentioned about not enough fuel.

I will run some tests this weekend to see if that did the trick.

Thanks for the suggestions, and keep them coming.

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

Quick reply to you skuzz21...

I just installed a temp gauge that comes with the TrailTech Vapor. It was not on the ped when it initially died. I will watch temp in future for possible seize temp. (which I have no idea what that temperature would be).

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

All,

New quick question to enthusiasts out there. I have been reading up and exploring the notion of "carburetor stalling". I have found the following when researching the topic:

" It may be getting too much fuel once it’s warmed up. An engine needs more fuel when it’s cold than it does when it’s warm. Cold engines run inefficiently so they need more raw fuel to fire-up, and stay running. This is all a choke does. A choke plate closes-off the air going into the carb which makes the fuel mixture really rich so it can run. Less air = more fuel. You’ll also notice that cars with chokes also idle real high until they warm-up. This fast idle and the over rich mixture (due to lack of air), allows the engine to fire up and stay running. As the engine warms-up and starts burning fuel more efficiently, the choke plate begins to open, which slowly lets more air in and leans-out the air/fuel mixture. Once the engine is warmed-up, the choke plate will be wide open allowing the carb to run as it is supposed to with the correct metering of fuel and air..."

Soooo.... my ped may be running too rich and when the engine eventually heats up and I give more throttle... I am "stalling the carb"?

My question:

"What does stalling the carb feel and act like once the engine warms up to a higher temp?" (feel as in motion of bike or sounds, etc)

Mucho Thanko,

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

Brent Bublitz /

>"What does stalling the carb feel and act like once the engine warms

> up to a higher temp?" (feel as in motion of bike or sounds, etc)

It would 4 stroke.

Regular 4 stroke engines behave differently when running really rich than 2 stroke motors. Watch the video above, its a great demonstration of what an over-rich condition sounds like on a moped.

Re: '78 Honda Express NC50 : Modified : Engine Dies

Brent Bublitz /

-=JFK =- Wrote:

-------------------------------------------------------

> All,

>

> Here are some basics:

>

> - Have kitted engine to 70cc.

> - Upgraded carb, intake, exhaust

> - Battery good charged and holding charge

>

> I took the noped out yesterday for first run of

> the season after upgrading various parts of ped.

> Ran great for 30+ miles. I kept the speed around

> 30-34mph and didn't "open it up" or run it hard.

> Then, on my way home there was a good stretch of

> flat road... so I decided to test the new carb and

> open it up a little. Was probably doing 35-37mph

> for 1-2 minutes and felt a little intermitant

> hesitation in the engine. I backed off a little

> and then increase the speed again. All of a sudden

> it just died... like not bogged down or running

> rough... just engine quit all together and it came

> to complete stop.

After re-reading that, it sounds like you soft seized your kit. There is a break in procedure and it sounds like you were not following it. You have to run a richer oil mix, not go past half throttle for so many miles... You would have to look up the specifics for the kit you have, but I know going full throttle after only 30 miles is ... bad.

Re: '78 Honda Express NC50 : Modified : Engine Dies

yeah im getting the same on mine, its a softy. i just put a 21mm carb and a 70cc DR kit, you should run good at a 50:1. have you uped your jets? its a must.

Re: '78 Honda Express NC50 : Modified : Engine Dies

Hey,

I am running a 50:1 ratio with AMS Oil 2 cycle Dominator:

I am still running with the jets that came with the Mikuni 20mm side bolt carb.

Do you know what the stock carb jet size is... and what do you recommend for jets? (Jetting is a new arena for me)

Thanks,

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

Overpriced Parts /

Seized

Re: '78 Honda Express NC50 : Modified : Engine Dies

I ran the 70cc DR kit all last year. I upgraded the carb this year. Can't be full seize because I can go start it fine right now and the back tire has never locked up. Must be soft if anything. Now I just need help with what jets to use.

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

Oh... quick Shout Out to "Brent Bublitz" for posting that video.

MAN!!! That was the freakin' best demo of a moped issue I have ever seen! I am going to try to contact the guy who made the video and see if he can do some more repair and tuning videos.

Re: '78 Honda Express NC50 : Modified : Engine Dies

Yeah...

Soooo.... guess this was the cause.

Is it possible to remove save the cylinder?

-=JFK=-(edited)

image.jpg
image.jpg

Re: '78 Honda Express NC50 : Modified : Engine Dies

It's cast iron, You may be able to clean the aluminum smears with acid (search for technique) and then hone heavily to clean the bore. As long as there are no grooves or gouges in the iron, it is likely all aluminum smeared to the cylinder.

Re: '78 Honda Express NC50 : Modified : Engine Dies

Jerry Mansfield /

would like to know what jets I need to use in a mikuni 20mm carb for elevations of 3200ft and 900ft or if there is a chart posted somewhere that will help

Re: '78 Honda Express NC50 : Modified : Engine Dies

that is the worst DR kit seizure i he ever saw, mine lasted about 2400 miles or about 60~80 hours of use and i soft sized it a few times due to lean jetting and looks new compared to this. based on the pictures the piston is shot, the cylinder is probably warped,no longer circular and needs to be re bored in order for a new piston to fit properly. honing is only good if the cylinder is still true and square and is only made to brake the glaze so oil can stick to the walls. conclusive i think yore kit is shot and you should purchase a new one unless you know some one who can bore cylinders for free. a bore job cost about 80$ and a over bore piston is about 60 from treats, at that rate you might as well buy a whole new kit.

Re: '78 Honda Express NC50 : Modified : Engine Dies

All,

As mentioned by DPC Ryan, I performed a Honda Express Muriatic Acid Wash / Seized Cylinder Repair.

You will find a complete HOW TO for the process I did, along with video and catchy music.. ha!

Thanks for all your help. I'll let you know how it turns out.

Respects,

-=JFK=-

Re: '78 Honda Express NC50 : Modified : Engine Dies

Dirty30 Dillon /

The jet that comes in the vm20 is a 180 if I remember correctly. But that should be really rich on your setup, so I'm I'm guessing you have a bad airleak, probably around the crank seals or intake.

A 140~ on a vm18/DR/MLM peoples is 4-strokey my buddy's hobbit, so a 180 on yours should be stank.

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