Honda MB5

Just picked up a clean MB5 yesterday. Got it running today. The top end is fresh as is the crank and bearings. Stock Carb with a 108 jet. Running pre mix. I also have a Homette 50cc pipe on it. First ride it was bogging a lot. I went through and cleaned the carb a second time. Still having issues. Picked up a larger jet 112 and now its running better but still bogs throughout the power band.

I know this bike has limited power range but I expected it to pull after 7k. It pulls a little but if Im not careful with the throttle it will bog down.

Playing with the air adjustment just seems to change where it bogs down in the power band. The more I open the air adjustment the higher I have to rev it before I hit the power band. I have a stock pipe and may just re-jet and put the stock pipe back on it... and see what that does.

Any comments or info is greatly appreciated.

Thanks!!

Re: Honda MB5

Try raising your needle?

With the homoet 50cc pipe it will bog if under about 9k. But 9k-13k is awesome.

Re: Honda MB5

Michael Thomas /

Mine is bone stock and it pulls real good after 6K.

Re: Honda MB5

Mine pulls hard between 8-10k but at 10k it stalls again. I would be ok with a range of 9-13. But currently it isn't good enough to pull out in traffic.

I found a fuel filter was reversed and corrected that. Much better afterwards but not good enough.

I don't have a spark plug socket so Im going to wait until I can check/replace the spark plug.

I feel kinda stupid. Should have checked the plug first... My other bike is a four stroke and I have to remember the differences.

I will also try raising the needle. Took the bike to a parking lot tonight with some of my friends. One of them brought up raising the needle. I think its certainly worth a shot.

Thanks!

Re: Honda MB5

Mine does the same thing, but I have a mikuni vm20 carb. I've tried all kinds of combinations with jets as low as 125 all the way up to a 160, but it is always bogging somewhere. I'll continue to hunt for a solution. How much does needle position really affect performance? Just slightly? Or should I be able to tell a noticeable difference?

Good luck!

Re: Honda MB5

The guy I bought the bike from had mentioned this stall at 7k. I figured I could work it out. We'll see. He was nice enough to include an extra CDI that he had purchased in the process of deducing the problem. He didn't try the CDI. After I change the plug (which looks pretty dirty) I will give the CDI a go and see if that helps.

Thanks,

Ben

Re: Honda MB5

I am running a 112 jet on the stock carb with a pod filter and Homet 50cc.

I just raised the needle by putting the clip in the lowest position and it made a huge difference in power delivery. Much smoother even on the bottom end. The bike still isn't perfect but it revs much smoother and now I feel comfortable driving it to work.

Re: Honda MB5

That's great news! I'll try changing my needle position this weekend!

Hope things continue to work out for you! Keep us posted!

Re: Honda MB5

Its so difficult to deduce the performance of the bike because this is my firsf time riding a mb5. I have no baseline!! Anywho, ill keep you guys updated with my mods and progress....

Re: Honda MB5

Ok, found the carb breather hole was plugged. Also the main needle is sticking now...it was dirty at the points where it contacted the main jet. I tried to clean and polish with a copper brush. Wasn't very effective. Still not running correctly. Low end power is good, but the top end is notchy and I still cant rev from 7k to 10.5k without a stall or bog.

Does anyone know where I can get a #37b needle for the original Keihin carb??

Thanks,

Re: Honda MB5

I put the stock pipe back on much better powerband, pulls from 6k. Still notchy and stalling at 9k. Going to switch the CDI for the heck of it and see if that helps.

Anyone want to buy a 50cc homet pipe??

Re: Honda MB5

I tried moving my needle up, but it made no difference for me. I tried it with several different jets. Occasionally it will rev all the way up past 11k, but most of the time it's like my RPMs hit a brick wall around 8-9k. I've been tuning this for months now.....I can't get it. It's super frustrating when I just want to ride!

Re: Honda MB5

Swapped CDI...bike wouldn't rev past 3k and difficult to start. The original CDI looked nothing the "new" one.

Re: Honda MB5

why dont you get more jets? your not going to get it perfect first time around. and you can just slap shit on and expect it to work perfecto without tuning it first, and adjust your timing with the new cdi

Re: Honda MB5

John Joedicke /

Don't really know if it will help with scavaging but maybe by drilling the baffle holes bigger. there are 36 holes in the end of the baffle and I drilled 12 larger. I can't tell you if this works or not but that is what I will be going with when I finish this restoration sometime this summer.

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Re: Honda MB5

Anybody need an mb5 motor? Got one for a project that is on hold indef. lemme know...

Re: Honda MB5

A friend suggested to me that I try cleaning the screen in the petcock in case the carb was starving for fuel at high RPMS. I drained the tank and removed the petcock.no screen..threw a few bolts in the tank after I cleaned the petcock and shook the crap out of it. Tried to knock some of the crud and light rust of the interrior. Cleaned petcock and rinsed tank with gas. Back together...runs better but not quite right. Pulled all the way to 10K and thats a first. Getting closer....

I have a translucent green tub from my petcock to the carb with an inline filter.

The filter never really seems full of gas. I would think that if there were a large volume of gas a few inches above it the filter should be full....but its not.

I will be replacing the petcock soon....

I will probably drop my needle to the stock setting again also and see if that works....

When I pick up new plugs....Ill try a 110 jet

The previous owner replaced the reeds with boysen power reeds I believe... I think they were stock...not sure

Re: Honda MB5

The new CDI looks nothing like the original. Im not saying this as fact...but I think the original CDI...if there were issues with it. It would not function at all or very poorly. So, I think my original CDI is working the way it should. Your probably right about being able to tune the new CDI by adjusting the timing. But right now its at the stock setting in the service manual. The bike runs really well for the most part but seems like it starves for fuel at 10k or so

Re: Honda MB5

Replaced the petcock with an oem version. The screen wouldn't fit through the bottom of the tank so it was removed. Also replaced transmission seal. It was a bitch removing the old one. Glen also gave me a stock airbox! I installed that and it looks so much better. Before the battery was sitting on a piece of carpet padding on the frame!!

Right now with a 112.5 jet, it runs to about 46. It would do 50 before the airbox install. Going to try a needle that Glen gave me and open up the air screw a little. It's running great now! The petcock and cleaning the tank helped me the most. Just finished painting the exhaust with a 500 degree gloss black. Im attaching a pi of my modded airbox and a shot of the bike before the airbox. (edited)

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Re: Honda MB5

briny briny /

Cam we see a pic of the headlight/fairing? What setup is that? $? Does it work good?

Re: Honda MB5

Im assuming the previous owner installed this aftermarket headlight. I saw one on Ebay for $33. It doesn't work that great. The light shines a little low and its not very bright even on high beam. This could be a result of the scooter producing low amps/watts from the small battery and alternator.

I would like to go with a dual headlight setup that is much brighter.

Any suggestions??

Thanks

Re: Honda MB5

Glen Kuykendall /

Ben,

I've been tempted to experiment with some sort of LED headlight setup for my MB5 if and when the original bulb goes out. However I'm curious if there would be problems due to the wild fluctuation of voltage on these bike. There really isn't a regulator on the headlight circuit, and my concern would be the LEDs might totally cut off at idle and/or blow at full rpm. The original MB5 shop manual shows the headlight wattage at 30/31.5 so sticking with something close to 30 watts is probably best. That said I might try it some day.

Any luck with getting your bike to run wide open without cutting out? Cool air box mod by the way!

GK (edited)

Re: Honda MB5

are the cdi's rpm limited...

Re: Honda MB5

Glen,

Sorry, I didn't return your call. Still dont have a turn signal relay. I 've still been making little adjustments here and there. I dropped the main needle jet down(clip in top notch). Took a few mphs off of my top speed(46 orig 51). But no more bogging which was caused by a lack of fuel. The cause was an old/clogged petcock.

I have been very curious about LED lighting on these bikes. The price itself may prohibit such endeavors. Aftermarket LEDs seem to be pretty pricey.

It seems like you could rig something up using a simple circuit board and wiring in 30 1W LEDs. Maybe throw a capacitor or so in there to maintain a minimal level of voltage, during idle, etc. But I never researched how LED light refracts in comparison to other methods HID, etc. An HID light would be nice! Probably cheaper too. Slim ballast and get an aftermarket headlight that can handle one or two 35w HID bulbs. I dont really dig my aftermarket fairing... So eventually I will do something about it.

Have you checked on EBay.?...I saw some things that looked pretty cool. Just curious how they really work. Ive always been dissatisfied with my SVs lighting for the most part. But, dont want to blind people by using HIDs in a lens made for a halogen or normal auto bulb.

The air box mod just kind of worked out. I actually used a small bead of silicon around the internal rubber boot and aftermarket filter. I think it may have restricted the intake enough to make it lose a few mphs. But since I gutted the airbox(very carefully and saving the internals of course!) I can just removed those three screws and have a pod filter. So, when its road legal, im going to see which works better. Ill let you know about my progress and when i get a relay.

Keep me posted if you look into LED kits etc

Re: Honda MB5

I could be wrong but I don't think the CDI would cause the bike to stumble there, as far as functioning as a rev limiter. I think that's the engines feasible limits based on design(rotating mass, inertia and all that fun stuff) and the carb size. Well, probably most of all the carb size. Hopefully someone else will chime in with a more precise answer. I think with a lighter piston or decreasing any moving internal mass would provide a performance gain on some level. But its never really a good idea to paint such broad strokes in regards to engines. They are all very different. I think Glen has a better idea than me..

Re: Honda MB5

My mb5 makes me never want to ride a moped again. Is this normal?

Re: Honda MB5

It seems like a lot of mopeds might actually be faster than an mb5. It is technically a motorcycle with a moped/scooter power plant.

Nick...

What kind of jet are you using?

Stock intake or pod filter?

Using your oil injector or premix?

Oh, how much do you weigh and whats your bikes top speed...

Re: Honda MB5

Glen,

Have you broken in your bikes engine yet?

Re: Honda MB5

New pics... Painted muffler, oem airbox.

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Re: Honda MB5

Michael Thomas /

Looking good.

Stock forever:

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