Honda MTX 125r will not rev up

[buurt]

Hi!
I have a 1983 Honda MTX 125 witch problem.
It starts right up and idles well.
But when I try to rev it up, ut just reaches a certain rev and then stalls.

I have cleaned the carburator, new fuel, clean tank, checked plug, but not changed it.
I also tried sparaying start-gas in the intake, but then it dies right out.

I have also adjusted the float in the carb.

It had been standing still for some years when I got it home.

The first times I tried to start it, it only ran at full choke and full gas.
This problem was affected by moving the throttle needle setting for the lowest to the highest (or the other way).

I also once started it up and then it reved to max, over 9000 befor e I could stop it.

But now the problem is it will not rev and is wery weak.

I hope someone understand this, I am from Sweden:-)

I also uploaded a video of it running, hope it helps:
https://goo.gl/photos/nvPrpaUmsxiyMcYN6

Have a nice day!

Best regards Alex

[stirlinggaz]

hi,
i owned a mtx 125/200 a few years ago & done a complete strip down (including engine) so still have a vague memory of how it all goes together.
do you have an air leak? (actually an air ingress) is it sucking in air anywhere?
test by starting it & spraying easy-start or something similar around the rubber connection between carb & airbox & the connection between carb & engine.
do revs increase? (WITHOUT touching throttle)

have you done a compression test?
& done the usual basics? why not use a new spark plug?
when you take the old spark plug out, is it wet? black?
try a new plug, is it giving a good healthy blue spark when its earthed against a bit of the frame thats unpainted & when you give it a couple of kicks?

oh, does your bike have the ATAC system fitted?
in uk we didnt get this, which is why i fitted a 200cc engine

im just thinking of all places that could suck in air & make it rev high like it did before.
usuals are rubber bits around carb & airbox (easy fix) or seal on engine (not so easy fix) but atac might be another culprit.

you are using the standard airbox & filter, yes?
original exhaust? with no holes on the long thin bit that gets hot enough to burn skin, if heat shield isnt fitted?

after all that, i'd be going back to the carb & making sure its at the stock settings. (sorry, i cant help with this, i gave my paper copy of the wsm away when i sold the bike (seemingly they are quite rare
)

cheers,
GAZ

[mudcow007]

the "starting it an it ran full pelt", do you have the throttle slide int he correct way around?

That caught me out with my CG

[bypass2]

With out the engine running does the twistgrip/throttle cable work allright. throttle cable maybe rooted/seated wrong.throttle cable maybe damaged. good petrol supply to the carb

[RealRestore]

Does it have a vacuum pipe connected from the fuel tank and a petrol pipe?

[stinkwheel]

Change the plug.

No really, put a new one in there. Anything else is pointless. This is the very first step in 2-stroke fault diagnosis.

if that doesn't work. Put the float height back to where it was before. I've never know a bikes float height spontaneously alter. Don't mess with it.

Does it have points ignition? You just described classic symptoms of retarded timing. Fit new points and gap them.

Check your fuel filter and tap for blockages.

Still shite? Check your reed valves for damage.

Stil rubbish. Consider your crank seals could be shot, although this usually causes starting problems.
____________________
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[stevo as b4]

If it's really how you say it is, and is starting and idling well, it's fuel starvation that's causing your issue, and nothing to do with spark plugs or air leaks.

There's lots of things that can cause fuel starvation, but checking the fuel tank breather. And then all of the carb jets and circuits as well as the settings including the jet sizes, needle position and float heights would be the next step.

You should easily and quickly be able to find obvious problems like a kinked fuel hose or blocked filter as well.

[-]

If the float bowl is full, I'd be checking the main jet for obstruction if it idles OK. The pilot jet may not be all that clean either.
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[stinkwheel]

[stevo as b4]

Possibly, but we're all sat here trying to diagnose behind a keyboard which is also pretty sweeping and inaccurate at times too.

We are also assuming that OP knows what they are talking about and also how to correctly describe the problem they have? Won't rev could mean that the bike dies when you open the throttle, or that there's a rev limiting effect that's occuring. It could also mean that it just miss's under load when accelerating?

While an under load misfire can be often an electrical problem, usually a HT side issue, it's pretty unheard of for a two stroke to start and tick over perfectly IMO with a bad or faulty plug.

Oily combustion chambers at low temperatures at idle mean that two strokes nearly always foul plugs at low speed and under little or no load. If it sits there ticking over for 5mins without a hiccup, I do seriously doubt the spark plug is causing the issue?

All the talk of air leaks seems to be off on a tangent to me too, based on a previous owners recollection of the engine.

If I were OP I'd be having the carb off straight away for a full strip and clean out, if nothing more obvious like a fuel hose or tank breather is kinked or blocked. But this is only my opinion. Id come back when the tank, lines, carb and hoses and connections are all proved to be spot on.

[buurt]

First of all, thank you all for replies!

I started the day with changing the plug for a new one, with no difference.

When I was running it, I noticed the flow from the exhaust system and felt weak, like a moped.
I then removed the exhaust system at the cylinder head, and Yes!
That was it.
It was clogged in the pipe inside the system!
I cut it apart, cleaned it out and welded it back.

So this issue is now solved.

I thank you all again, and I have read all the answers. I really did all those things, carb, flow of fuel, air, starting spray for leaks.
And all this time it was the exhaust.
(Some pics attached)

(Also a film without exhaust, you can say it runs different now:-)
You can also see the ATAC system working)
https://goo.gl/photos/gdCwoZ2rCPhWoz4V9

Merry christmas to you all!