Carb issues

[Nagasaki24]

Hey all, Really need some help here!

I cant get my bike (97' Yamaha SR125 50K miles on it) booked into a garage close to me anytime soon (Thanks summer!) so I am going to have to try and fix this my self!

I have been having issues with my carb for ages. Things like it not idling at all then it will be fine for a month or 2, Then it wont start only bump for a while then be fine etc.

Got fed up with it when it decided to start leaking fuel out the overflow a little bit and dropping my miles from 150+ per tank to around 100.

I got a 2nd hand replacement from eBay that was from the same year bike and in excellent condition. But it is not 100% the same as my current one. My current one has a heater and the replacement does not. Other than that they "Look" identical.

I put it on and tried it out. Bike started.. AND IT IDLES. Yes! I thought. Gave it a rev annnnnd no go. It would rev up a bit then just bounce about like it was hitting a rev limiter! (Around 4-5K revs by the sound of it) I tried to ride it and see how it ran but I could only get about 25-30 mph out of it.

I have had the carb off and blown it out with an air compresses so all the passages "should" be clear. Its clean as a whistle inside, and out!
Jets are clear etc etc.

One thing I did notice is that the filter is, well gone. It had disintegrated and left crap all inside the airbox. I have cleaned it out and ordered a new one. I cant see this being the whole issue as my old carb will work fine, ish if I bump the bike.

If anyone has any ideas or suggestions then please share them. I need to get the bike back up and going by Saturday for work.

TL;DR Carb is not working right. Hlp pls
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Yamaha SR-125

[A100man]

Carb could be a red herring. Adjust valve clearances (tappets) too. Clean carb (again) and fit new airfilter. Also new in-line fuel filter.

If after all this you still have trouble ebay the bike.
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[Teflon-Mike]

People dive in to the carburettor... probably one of the most intricate yet reliable mechanisms on a motorcycle, MOST often for the simple reason that it 'looks' less scary than things covered with oil inside the engine far more likely to be a problem..... and 'Av U cleaned Carb' lore.

As A100, unless you are sure its a carb faulty and ONLY a carb fault... leave the feck alone!

Drop in fuel consumption, and leak from overflow, does hint that the float needle isn't seating and or worn... but still....

DO YOU HAVE THE HAYNES MANUAL?

If not why not, and if so, have you done everything, and I mean everything, likely NOT done very often, if ever, on the 'Routine Maintenance' checklist?

- Check and set tappet clearances
- Replace that air filter you have already established is dumping crud to clog carb and make soot on valves!
- Charge /Check/ Replace battery... if it will bump-start, it will e-start! Only difference is the e-start is making it spin not your feet!
- Buy a new spark-plug.

Whist your there, what else is on the list NOT in the engine, like checking and adjusting brakes, replacing brake fluid, greasing swing arm bushes and head-race bearings..... wont make much of a difference to make the engine run, but motor is only half the word motorbike, and less than half the machine..... and old addage... before you try GO... make sure you can stop, and then still before you go, make sure you can go in the right direction! Find these things quite helpful personally.... especially after getting all exited and finding that a project bike runs as I wheelie it down the garden towards my back-door..... wondering why it dont slow down when I yank a brake lever!!!! Lol.

Oh Kay... do all the service checks and adjustments... base line the engine at some sort of known point, where if it still has 'problems' you at least know what is less likely to cause them.

Now, its an old bike.. more an old learner-bike... these tend to lead hard lives in the hands of learners who don't know much about riding, or mechanics, and make many many mistakes in both.

Put a compression tester on the thing. Good odds that the thing has done enough miles to need a rebore and new piston and rings cos the old ones is worn out... if not done enough miles..... probably even worse, it must have been layed up with piston rings getting corroded in the piston whilst valves rust... possibly more than once... so things dont seal when asked to work again, and they wear out tripple fast... so possibly STILL needs rebore, piston and rings!!!

You have effed around with the carb,.... one you have procured from e-bay isn't the same as the one fitted by the factory... or previous owner, as you say it doesn't have a heater....

Going to take a guess that your 'old' carb has an electric heater coil for the choke mechanism... replacement has a simple lever.... sounds the wrong way about to me, I would expect a later bike to maybe have an electric choke, older one a manual one... is the 'old' carb off an early YBR, by any chance? But check the haynes, that will say what you aught to have..... and how it works.

Meanwhile... gaskets... carbs are often fitted on rubber manifolds; these dont need a gasket between the carb and rubber, as the rubber is the seal... but that seal often accomplished with a rather flimsy pipe clip, often chewed up by numpties cleaning the carb... and then replaced by more diligent numpty with a jubilee clip from Halfords.. that often gets just as chewed not always on the screw/hex tightening but, but on the rack its worm trys to grip... A-N-D clip is usually thicker than the OE fit one, and doesn't sit in the rebate in the rubber maifold... so it doesn't tighten or tightens but rucks and doesn't seal.... IF manifold disturbed to get carb off, at the cylinder head end, theres another paper gasket there.... oft ignored, and ommitted on re-assembly, or re-used, or glooped up with 'instant gasket' crud (That often melts in contact with petrol BTW)... so no seal, no suck, carb does strange things, like not run unless the choke's on...

Go check, and eliminate making sure you have proper gaskets in place, rubber manifold is 'good' and not cracked or perished or split, AND you have the right sized hose clamp on it, and not just diameter, width too.

NOW... after all that..... we might be able to start some voodoo diagnostics by remote control.... but we can only make our guesses as good as you make your descriptions of symptoms..... and the Haynes manual CAN save you having to ask so many silly questions or describe so many symptoms before we start.....

Do the service checks, do a compression test; check gaskets, work from there....

Meanwhile, its Thursday, you need get to work on Saturday... go check buss time tables, book a taxi or phone a friend.... this is unlikely to be a five minute fix... or the only problem.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Nagasaki24]

"Meanwhile, its Thursday, you need get to work on Saturday... go check buss time tables, book a taxi or phone a friend.... this is unlikely to be a five minute fix... or the only problem."

If only it was that easy!

Anyway,

I will get to the rest of the bike in good time. Got new tyres to fit, chain etc etc. It's getting some good TLC over the coming weeks/months.

But for now I just need it to run. New piston rings (0.5m oversize) are on the shopping list as I know they will need doing. The bike has not sat for longer than a few days since I got it 4 years ago.

The clips on the carb for the intake etc are all fine and cleaned up and working as they should. The rubber is a little cracked up on the top. Added some tape for now as a "stop gap" in case it is leaking air.

I have taken the bottom from the replacement carb and added it to my existing carb as this part is clean and 100% identical as well as using the main jet and float from it as the existing ones are not in the best of condition. After doing this I could get it to start fine. Still will not idle. I can live with that for the time being. As long as it starts for now while I get more parts on order.

The inlet tappet clearance is fine as I did this just the other week as for the exhaust, I can not get to it as one of the bolts is rounded off. I will be seeking the help of a garage to get that sorted as I do not have the tools to get it removed.

Other upcoming things I will be doing is an oil change, new cam chain, oil filter. New front bearings and rear if they are needed. New pads/shoes. The head bearings need replacing as well not just a good greasing.

I will be touching up some of the frame as its getting rusty in places as well as the swing arm. Getting that off will be fun!

I know what I am doing with a lot of the bike but carbs, I just cant get my head round how they work :\
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Yamaha SR-125