MTX125 spluttering/cutting out badly before hitting band

[grandad90]

hi
wondered if anyone has had any problem with this sort of bike

it splutters all the time really, only croaks up to 40
i can get it to stop by turning the fuel off while riding
wait a few seconds for it to stop spluttering and cutting out
then it will go ok if i leave the fuel barely on
maybe after a mile i can open the fuel tap fully,
then another mile it will cut right out
and slow back down to spluttering along at 40

ive had the carb to bits,
its as clean as a whistle,
float height right, fuel nowhere near overflowing
tried lowering the fuel with the float height but it still does it,

any ideas
i do like the old bike but it needs to be smooth on its throttle or its no use on green lanes

[grandad90]

pic

[Pol Anorl]

mixture screw?
air filter?

air jet?
fuel jet?

sounds like too much fuel not enough air.
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[temeluchus]

Is the carb original equipment?
Does the float float?
Is the float valve needle in good shape?
How does the spark plug look?
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[Teflon-Mike]

Turning fuel tap 'off' would imply some sort of over-fueling.... like as not a cluster-fuck of anomalous issues....

But starting with the stooopid, first... when you say you lowered the float height.. I recently had an argument with some-one over the float-valve.. they insisted it was the 'bit BELOW the float'... "No, it should sit ABOVE it" I said..." Which wasn't concluded, until I got them to show me their carburettor... and they were correct, on THEIR carb, sitting on the side, where they had cleaned it.. upside-down.. the float needle WAS 'below' the float. ... which does beg the question... via similar mind-fart with the carb on the bench being cleaned, when you say you 'dropped' the float-height... you didn't work upside down and actually raise it, did you?

As an asside of note; CMSL scematic shows carb, which does look suspiciously like the DelOrfuls, they sometimes fitted to the Italian built bikes, and I HATE... and have avoided best I can for a couple of decades until O/H got her Guzzi with a pair that have had me going insane, with a very similar problem.. that might offer insight... horendousely ritch running... dire fuel consumption, awful carburation, and 'random' fuel piss! Ultimately sorted ... (as best as a pair of Del-Orfuls can be short of fitting Mikunis!).. we hope! Replacing the choke plungers, float needle valves, AND I suspect crucially, the float-pivot pins...

However; Main-Jet... when you cleaned it, did you remove the jet from the emulsion tube? Did you clean the little holes in the emulsion tube, and when put back together.. is it all seating right?

AND are all the air-passages in the carb body similarly clean? There's probably a small air-way all the way through the carb, that feeds air to the emulsion tube, and it's likely that the choke, controls the upstream flow of air into that port, so that when 'closed', it chokes the flow of air into the emulsion tube, providing gross enrichment; also likely that the primary-air / misture crew is on the other end, down-stream of the emulsion tube, controling the amount of manifold vacuum sucking air into the emulsion chambers.

Offers three more potentialities for you; sticking/nonsealing/nonseating choke; broken/damaged/missing primary air screw; blocked pilot-air way... bit more thorough carb cleaning / inspection may reveal something, could be worth having the carb body ultrasonically cleaned too.

Back to the right sized jet... two-stroke.. needs total loss, crank-case lube.. on that bike, should be metered by positive pump, probably direct to the reed-block.... BUT common for posi-lube systems to be ditched, to run pre-mix, adding oil to the petrol in the fuel-tank. BUT, do that and the 5% or so oil that would have been pumped direct to the crank-case, has to go through the carb.. so you have to recal the carb to meter about 5% more 'pre-mix' into the air-stream so it gets the same amount of petrol and oil.. commonly done by up-jetting.. offering two possibilities; a) some-ones up-jetted too much for pre-mix... b) carbs been upjetted correctly for pre-mix, but without blanking the auto-lube.

Next, reed-valve / reed-stop. In theory a two-stroke shouldn't work.. sucking charge into the crank case when the piston rises, it ought to shove it all back out the way it came, when it falls again.. not shove it through the transfers.. and in fact, until possitive crank-case valving.. reeds and discs, came along, they mostly did.... and on piston-ported 2T's, it was common for them to be horendousely 'under' jetted, 'cos you could get a 'Tripple-Mix'.. air gets sucked through the carb on the 'suck' stroke, and has fuel added via the pitot effect... but, once in the engine, piston starts to come down, and shoved maybe half of what it had sucked through the carb back out.. through the carb.. adding more fuel, via the same pitot effect... and THEN.. if the inlet tract was long enough, that double enritched 'charge' got sucked BACK into the engine passing through the carb, getting enritched a third time, before some of it got stuck in the cylinder.....

SO.. if your reed-valve is broken, or the spring tension has gone, or there is shit under the seat; THEN it might not be doing its job and stopping this reverse flow out the crank-case, and you could be getting tripple enrichment of a significantly smaller volume of charge.

Random one to check, is the crank-case seals, that can go hard and stop sealing, or start being pigs, acting like a flap-valve, one way of the other, often randomly... would be worth checking gearbox oil level, see if its sucking any, also removing magneto cover and sniffing for petrol smell.

And onward... often find that many carburation issues, especially on two-strokes aren't.. relying so much on resonant harmonics to get charge into the cylinder and stop it buggering off before it can get burned.... exhausts can do strange things to them... and on this era dirt-bikes, that convoluted U-tube expansion chamber sat right in-front of the front wheel in the fling-zone, it's a known rot-trap, like to rust out where least obvious, either from the inside and condensation sitting in the bottom of the U-Bend or mud & shit collecting in the top of it, rusting through from the outside... other one is the tail-pipe getting bent 'in' in a drop, and the tyre rubbing a hole in it at full suspension compression, or the old glass packing getting burned out or clogged up... basically, look at the whole damn system for obvious or less than obvious faults! Possibly worth removing it and washing with caustic soda solution... if the caustic soda dripps out anywhere but the ends of the pipe.. could be a good indication of a pin-hole or crack or wot-not.

You also have, ISTR a mechanical ATAC exaust 'power-valve, offering further googlie... on the competition models the power-valve ISTR was actuated by some sort of centrifugal bob-weight mechanism, like a distributor, that offer lots of scope for grief, from the actual valve siezing in the barel, to the exposed linkage rusting or sticking, through the bob-weight springs snapping.... but I doint know whether the road going MTX has the same.... I just looked at the CMSL schematics, and am none the wiser.. it 'looks' like the UK model has the ATAC valve, but pegged in one position... B-U-T.. worth a bit of investigation.. is yours a UK model that may have an 'inert' pegged valve, or is it a Euro-spec import with operational ATAC, and in either instance is it all as it should be or has some teenager dicked with it, in the intervening quarter century since it was built?

Dont know if any of that helps, but that's where I'd start.. re-clean the carb, check jet sizes and posi-lube; check the reed-valve, clean the exhaust system and inspect the ATAC.
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If all jets are standard and the carb really is completely clean, I'd be guessing a worn needle valve.

Seen a similar thing before and the bloke sold the bike because he couldn't get to the bottom of it. He changed the carbs, but used the same slide and needle valve. The guy who bought it spotted the worn needle straight away.
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[grandad90]

First of all cheers for the replies,

i've had it in bits a few times:

the oil is original autolube on the reed housing

the float height should be 19 mm, ive got it at 26mm, it only just lets fuel in with the bowl on, and the float stays afloat for half hour in the removed carb bowl

it starts and ticks over perfectly, the idle circuit adjusts as normal

the jets are 48 and 118, the carb says pe65a va,

ive blown every passage in sight with gt85 and its straw
the choke seal looks normal, and ive blown down the passage, it only lets by with the choke open

ive had every part of the carb off (several times!) and it all looks like its doing what it should?

the needle, i really dont know how to check or where to get specs from, the markings on it have worn away so i dont know what needle it is, but it 56.3mm long, measures 0.81 on the bottom 7mm, tapers over 25mm to a width of 2.18mm, if that means anything i dont know

reed is intact, tho one of them does have a fraction of a mil gap?

the spark is a bit weak, not as bright as the one on my c90

the exhaust i think is aftermarket "big one", its not heavy or rusty but i've had coathanger wire down it to check its clear and even re-packed the silencer, which has made it a bit quieter

its a uk model with a non-existent atac valve, which is only a side passage on the exhaust port, a bit like the dead end pipe they used to restrict old peugeot speedfights, i do have plans to bung it up with jb weld or something once its running right again, hopefully

[zbraff]

[grandad90]

just checked and you're right
it does have a 200 carb fitted :o
its had 20 odd years of people messing with it i suppose

ive looked on cmnsl;
the 125 has 24mm carb with a #112 main jet
the 200 has 26mm carb with a #118 main jet

so, if i fit a thicker needle it might lean it off in the midrange enough to kill the splutter?

it did always feel richer in the midrange


but it ran ok when i got it a month ago??
could the pale spark or slight reed gap be doing it?

im from yorkshire and dont like to spend money before i know im not wasting it

[Pol Anorl]

gap in the reed valves will allow things to enter the cylinder.

weak spark is obviously not ideal.

change the jets to the correct size.
buy new plugs

ride.
____________________
GOOD GAME BODYGUARD: https://i.imgur.com/8WePGgf.jpg
20:30:37 Pyro.: I don't sort of like men, I take every advantage to choke on dick.
Jewlio Iglesias: You live in Liverpool - Chances are, the front door has already been kicked off the hinges

[temeluchus]

I would suggest finding a stock carb, if only for your sanity.
____________________
Some shite cruiser. Now with guns and FREEDOM!

[grandad90]

update:
spark is really weak looking, a fraction of the brightness of my mates kmx
but all the resistances, coils,windings,cdi,etc are within spec so im assuming thats not the issue.

measured the throttle needle
its supposed to be 2.6mm its actually 2.31
thats just at the top grooves where it (probably) wont even wear as much

its even shorter than it should be (56.34 to 57mm)
new needle on way, will keep thread updated for anyone else future reference

edit: got new needle ran sweet, tried stock carb as well, ran better
also binned the resistor in plug cap and used non resistor plug, spark a lot brighter