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Top speed and revs problem!


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Hi guys,

I'm pretty sure somethings not right with my sons Motorhispania RX50.

It will not rev into the red and when it reaches between 35/40mph it feels really rough.

It splutters at the top end of every gear although it seems like it wants to on further?

It does blow a little blue smoke, but it is a 2 stroke engine.

Surely this isn't right?

Could it be the mixture being wrong or something else i could do to find out or cure this?

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Have you replaced the spark plug.......2-strokes can foul the plug causing it to run rough at the top end. The RD350 that I had years ago wouldn't get over 80mph if the plugs had been in there for a couple of months, but would hit 100 with new plugs........obviously when tested on a private road...... :wink:

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It had a brand new spark plug last week.

It was purchased as a none runner a few months back. It already had a replacement sports exhaust fitted, which was not fitted right. I put it in the garage for an MOT and after a few adjustments lights/wheels/New front disc/speedo cable etc, it passed.

Iv'e only been out on it a few times since buying it, and it has always seemed the same.

The only thing that i know is not right, is the choke cable comes out of the lever when i put it on/off, but seems to work. Could the choke being stuck on slightly cause loos of top end power?

Could the mixture screw have been adjusted wrong so the mixture is too rich or poor?

I am clueless with bikes, but would like to have this running nice for my son asap!

I am willing to listen and try any ideas?

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I haven't checked the air filter, the plug is still clean like new as the bike has only been out 2/3 times since it was changed. But it was running the same with the old plug. I will have a look at the mixture screw and adjust it to see if it makes any change.

Thanks to all so far!

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I have had a very similar issues with my bike, culprit was valve clearance.... It had none.

I went through all the normal checks too plug, filters, carb, cdi, coil etc etc it was just an off chance ckeck that found the problem

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Erm......this is a 50cc bike.......it's supposed to be limited to 30mph........even with the limiter removed (not recommended) I would have thought 40mph wasn't bad for a 50cc. Don't believe the stories you hear of people getting 70mph out of their 50cc bike..... :wink:

I think you're chasing moonbeams mate worrying about getting it to go faster......and possibly making the bike illegal for a 16 year old. Yes, I agree that limiting a bike to 30mph is crazy.......but that's the law...... :roll:

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I'm not trying to get the bike to go any faster, i just want it running right.

The problem is that the bike feels like it is choking/spluttering when it gets to around 35/38mph. But that is without the throttle opened all the way. It also doesn't want to rev into the red or anywhere near it, and i was told that it should reach the red?

It does feel like some sort of fuel blockage! As i understand that the faster you go the more the fuel is sucked through, but it doesn't seem to be getting through fast enough. Does that make any sense or am i way off the mark?

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Let's start from the top...

I take it that it didn't always splutter, so this is a change for the worse.

It sounds fuel related or ignition.

So first port of call, check the spark plug, and air filter.

If both are A-ok then put them back, if not, get new ones.

If they are ok you could give the carb a clean, and make sure it is set up correctly. Just give the jets a good clean, and ensure the needle is the correct height.

As you say it splutters a good portion up the rev range, so it's either the main jet, or needle jet at fault if this is carb related. I'd see what colour the plug is, if its showing it running lean, the needle needs to move up a couple of notches, and the reverse for if it is running rich.

If that doesn't solve your problem it could be the main jet is blocked. Give it a good blast and then put the bike back together.

If all that doesn't work and you do the cheap stuff, like spark plug, air filter, carb clean, clean the reeds, and so on then it could be expensive.

It could be a packed in ignition coil that is failing at higher output, or it could be the bikes crank case seals have failed and you have reduced fuel pressure.

That's about everything it could be 2-stroke related :lol:

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