Yam RS125 running problems

I bought a 1982 model 5 days ago. Did 100 miles or so with no problems. Over the last couple of days I've had problems with it:

It's OK from cold but after a mile or too the engine starts to bog down. You can't rev your way out of this and evetually the engine dies. If you leave it a minute or two it starts first kick then runs for half a mile or so before bogging down and dying again. Repeat several times.

The bike had stood for a couple of years, most likely with an empty tank. I can see some rust in the tank. I took the carb off there were a fair few small rust particles in the float bowl but the jets were clear. The plan is to clean out the tank and fit an inline fuel filter.

Thought I'd run it by you guys though, see if it could be anything else, in addition to the obvious dirty fuel problem. I don't understand why it starts so easily shortly after dying. Is it maybe that the rust particles settle at the bottom of the bowl, get agitated by the fuel coming in when the engine is running, temporarilly block the jet, then drop back down when the fuel stops coming in? Or something else? Also, would appreciate any recommendations as to which fuel filter to get/ not to get.

Cheers.

There is probably not enough fuel coming in to keep it running but once it stops fuel will slowly fill the bowl up

also check the fuel tap for a blockage and the tank breathers too

Cheers I've already checked the fuel tap, it flows nicely, the fuel line between the tap and the carb is fine too.

check the carb float too

Checked that. I had an earlier problem with the carb overflowing that was due to the float needle having a bit of dirt on it. The float seems to be working ok, to my inexperienced eyes at least!

I presume you set the float height too?

I didn't touch the float height. Assumed it would be ok on its existing setting? Adjusting it is something I'd never done. The problem with the bogging down first happened having done around 15 miles on a dual carriageway at 60 odd, assume it would have happened earlier had the setting being wrong?

Just checked the Haynes manual, it says the settings are factory ones.

I mean set them to stock height or at least checked they where set right as they could have been touched before

I'm not sure really. When I put the carb back together the float was moving around at I expected it to, the pin was correctly located and the floatt needle seemed to be working well. I get your drift though, it does seem that everything is fine until a bit of petrol has been used from the float bowl, then things start going downhill.

Crap in the jets

I thought I'd sorted this but unfortunately not.

I cleaned the carb again then fitted an inline fuel filter and clear fuel pipe. I've done a few 4 mile journeys, mostly low speed, with no problems. Yesterday I set off on a 30 mile trip. After a few miles, I was doing 60 when the bike bogged down and died. Again, if I waited a couple of minutes the bike started fine. If I kept the speed and therefore revs low all was well. If I went faster with the corresponding higher revs the bike would bog down then die within a mile or two. Each time this happened I checked the filter and fuel pipe, they were full of fuel. I think this rules out any possible problem with the tank breather and/ or fuel tap.

I'm thinking it's a problem with the carb. It seems to work fine until it's asked to deliver more fuel, then fails. Does this maybe suggest that the float is set too low? If I measure it and find out this is the case, how do I adjust it? An other ideas?

Carb diaphrams are dried out and perished or have a split in them

As far as I know the carb doesn't have a diaphram? It's a Mikuni VM24SH (2A000) according to the manual, that's if it's still the original carb.

Is an incorrectly set float height likely to cause the fuel starvation issue I'm facing, what else could it be? Struggling with this!