Airhead

Might be an idea to attempt a start when you have this better compression due to the oil.

Good point Cynic but it doesnt mean the ring are good, the oil test suggest they arent so they may be part of the problem.

Does it make the ruby a mellow yellow?

For parts try advertisers of vintage yamaha parts in American classic bike magazines ebay bikebandit.com TS may advise on more You just might be lucky and only need a new set of rings and a top end gasket set.

Hey guys look what I found in the local paper today

Hi Chris and welcome to the forum, I have to admit that I'm not familiar with SR125 but why not just connect battery negative to a mounting point on the frame, it wont do any harm at all.

I think I might have an old magazine feature on building a harness, PM me you email and if i can find it I will scan it and email it to you tonight.

Hi that is in fact a common problem with new riders who put their bike into gear with the side stand down. Could this be the reason? Welcome to the YOC forum. It might be an intermittent fault on the stand switch though.

Assuming you did the test correctly with wide open throttle I would expect a compression figure or around 100psi for a two stroke in tip top condition and 60 psi as a minimum figure. try a teaspoon of oil into the plug hole and repeat the test, this will help to seal the rings and if you get a much better reading its time to take the barrel and piston off to do some checking, the most likely culprit is worn rings or possibly worn bore or maybe even a scored bore due to broken rings or partial seizure. Could even be crankcase seals are leaking.

You WILL have air in your pump and I recommend you bleed the pump by removing that screw before using the bike or you are very likely to undo all the good work you have done thus far. If you cant than ask someone to do it for you or get another pump.

The answer is yes a full pipe should be seen, i assume you have bled the air from the pump by removing the small screw from the top of it until bubbles disappear., you should have done this first before running with the pulley turned to max. Edited to say i had not read jims reply from earlier which states the same.

When the bike is running if you pull the cable outer away from the cable guide that goes into the engine case you will see that this pulls the pump pulley round to maximum flow and the pulses (slugs) get much bigger, Do this and hopefully all the air will be ejected into the carb in no time.

Are you using the choke when trying to start it? sorry to ask a dumb question.

These things can be difficult to find weve all been there, how long have you owned the bike? what do you know of its history, who put the fuel in...you?

Welcome to the forum Foxy

Hey...maybe it wasnt a sarcastic remark and as a noob you should try get to know people here before comments like that!

Oh God...dont do it Darren, I just wish I'd seen this earlier..Its a con mate, I sent mine last year and still havent received a penny

Not really, I've got six and no room for another!

Well youre £20 up on me so you must be doing something right.

Congratulations, like they say...money goes to money.

Look down the carb barrel from the manifold side towards the throttle slide, you will see one or two holes in the bottom of the barrel, how many can you see? if its just one then this is the pilot hole and if its two it will be pilot and bypass holes, both these need to be clear for no throttle and low throttle performance. Regarding the hole in the carb barrel nearest the manifold side...Many carbs have a two hole idle system which gives better low speed and mid range throttle response than the simpler one hole type. At very small throttle openings, the bypass hole and not the pilot hole actually provides the engine with fuel. Instead the pilot hole acts as a supplementary air bleed to further atomise the fuel airafter it passes through the idle jet. As the throttle opens wider, the pilot hole too begins to spray fuel. This serves to supply the engine with additional fuel until the air speed throgh the carb increases enough to start the main jet and needle valve flowing fuel into the engine

You say it dies when you apply the choke, this indicates that it is not running lean, however it might be running rich. Two things that can cause that are, clogged air filter,( take out the filter and try it without.) or float level in carb too high..to check if this is so switch off the petcock as you are riding, if the bike suddenly accelerates before running out of fuel in the carb then the fuel level is set too high.

That sounds like a. A blocked pilot jet...or b. A blocked bypass hole remove the pilot jet and hold it up to the light, you should see light through it if its not blocked. The symptoms you describe fit well with a blocked pilot jet, by operating the choke you are in effect doing what the pilot jet should do only more exagerated

Does it idle if you barely crack open the throttle? If so then you just need to adjust the throttle stop screw its the knurled one and turning it clockwise will increase idle speed, also check the mixture screw is set right it should be 1-1/4 to 1-1/2 turns out.

Hi and welcome to the YOC Thats a great looking bike there, did you do a lot of work to get to that stage, I see its off road only now, looks a very capable off roader almost like an old MX bike now. As for the 400 conversion we need to check things like part numbers for crankcases and crank etc if they are the same for both bikes it should be possible but I dont know for sure, also they may have different carbs too...