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Honda mb5

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I have a Honda mb5 it idles great revs up fine but when g I ng down the street and I really need those extra revs it won't Rev past 10,000 rpm it didn't use to do that.

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I changed out the center gasket with a new one cause it was sucking In tranny fluid I had to retune the carb from what my friend did who is a lot better at this than I am . I got it as best I could but it just won't Rev any higher

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If that MB5 is the same as was the models sold here in Canada it's a 49cc 'motorcycle like' moped, manual gearbox, large wheels.

Teens with those got pulled over by the cops on a regular basis around here as they were very quick for a 49cc bike.

 

As with a dirtbike, going to a big bore vs stock usualy requires leaner carburation settings,

the larger engine displacement has more vaccum and sucks in more fuel vs the standard sized bore.

If the carburator still has the stock sized main jet it could now be jetted too rich which limits overev capabilities.

 

Also, perhaps some component in the induction tract (carb, reeds, air filter, port size etc.) or exhaust system

now isn't able to support the extra flow required for the increased displacement of the engine.

Edited by mlatour

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It's got a brand new big bore and the bike only had 500 miles when I got it

 

Right, and that 50cc motor last about 25 hours, or 500 miles.....or much less if you are running it up to 10k on a regular basis....which they don't do stock.

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I got it with 500 miles had original piston still ran like a champ! Threw in a arisal? 70cc big bore gave it a lot of compression I can do wheelies on that thing now. But you have to Rev the thing to the moon to go 45 mph I do 40 around town at around 7-9 rpm. You cant ride it another way

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I can get it to Rev up past 10k by reving till it starts to bog then slowly roll off the throttle just a Crack and it will Rev past so it must be a carb issue but I'm not so good with them. The idea of letting off the throttle and trying to think of what it needs to not bog confuses me.

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From 49cc to 70cc, the engine's displacement has been increased by about 40%,

Have the other components been upgraded to flow the require amount ?

Reeds, exhaust header, silencer, carburator

 

" Lot's of compression " on a 2 stroke often times also results in better low end grunt but less overev.

 

Don't expect a 49-70cc engine running 10K rpms to last 500 miles without some loss of performance.

Edited by mlatour

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Well I adjusted my oil injector cause it was running a bit lean. I had to retune it because I found the fuel screw on my garage floor and I put it back in and it wasn't enough so I just removed it entirely and it runs very strong now but still bogs past 9k at a quick Rev what could this be jetting? Not enough air. I have a 19 mm carb on at the moment I think I may have to upgrade to a bigger one

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Well just to put things in perspective, nothing too scientific but from what I can find

these are small bore KTM motocross carburator sizes versus engine cc's of displacement :

 

a KTM SX50 has a 19mm

a KTM SX65 has a 24mm

a KTM SX85 has a 28mm

 

so comparatively yes your 19mm carb is smallish for a 70cc engine if it has the ability to rev past 10K rpms.

Edited by mlatour

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Yes treatland has a 21 mm carb for 150$! Before I spend that much money on a carb if it is bogging when the throttle is held wide open is that too much fuel or air? I'd like to test out the cheaper option rather than go right into the bigger carb

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Bog is usually lack of fuel, or more likely in your case lack of total airflow (CFM's) because of a restrictive carb/inlet tract.

 

too much fuel (rich) feels like a stutter (engine cutting out) or like you're hitting a rev limiter.

 

Too big a carb size on an engine usualy creates poor throttle response at low speeds, but top end performance would still be ok.

 

I don't think going from a 19mm to a 21mm is a worthwhile change,

by my simple comparative above a 70cc engine should use a 24-26mm carb.

Edited by mlatour

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Now that you mention it it does sort of sound like a Rev limiter maybe too much oil from the injector? But there is still the question of that I have to run without the fuel screw in the bike and it runs great except for the top end bog

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Fuel screw or air screw?

on a 2-stroke carb an air screw is more common,

a fuel screw is more commonly found on 4 stroke engine/carb.

 

If the air screw is completely opened or even removed, meaning a lot of extra air is pulled into the carb from it's cavity.

If the engine somehow runs better that way it would indicate that the fuel metering is extremely rich at low rpms.

 

Unless you are fouling plugs or saturating the exhaust with unburnt oil, I don't think that any extra oil injected would not matter much.

That auto injector system is most likely limited in the quantity it can pump, probably never reaching high oil ratios like 20:1

Edited by mlatour

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Fuel screw or air screw?

on a 2-stroke carb an air screw is more common,

a fuel screw is more commonly found on 4 stroke engine/carb.

If the air screw is completely opened or even removed, meaning a lot of extra air is pulled into the carb from it's cavity.

If the engine somehow runs better that way it would indicate that the fuel metering is extremely rich at low rpms.

Unless you are fouling plugs or saturating the exhaust with unburnt oil, I don't think that any extra oil injected would not matter much.

That auto injector system is most likely limited in the quantity it can pump, probably never reaching high oil ratios like 20:1

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I think the solution to my problem could just be to take the 70 kit off and put a 50 back on for reliability what do you guys think?

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If reliability is more of a concern than performance,

 

there's no doubt with the 70cc kit, being about 40% more displacement than stock

the bottom end won't be as durable than with it's original 49cc set-up.

 

But keep in mind, if you have a carburator, oil injection or other tuning issue, that problem won't magicaly go away by switching top ends.

Edited by mlatour

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