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I don't know if it's par for the course. My '73 DT250 doesn't get anything other than gas and oil and the the occasional basic/simple routine maintenance year after year. Now, I don't put a ton of miles on it, but it just starts and runs like these bikes have always ran for me; dependably. This RD is atypical. But I will get to the bottom of it and then it will be just all the other Yamahas I have had; a real joy. 
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Good idea. That way I could try the right hand carbs that I have. Got a best source for that line?

You can get them from john at economycycles

https://www.economycycle.com/product/yamah-oil-pump-line-pump-to-carbs-90445-05m07-00/

That seems like the way to go. Now, I just have to get enough for an order.

Thank You!

Probably not an issue here, but there is an increasing number of counterfeit spark plugs on the market, and they are getting really difficult to spot.

Buying plugs, and many other items, from eBay and Amazon can be rolling the dice these days.

Legitimate dealers and established parts stores tend to be a safer option.

Agreed! I have started using the NGK Iridiums. I get them from O'reilly's. Usually only about $3 more than the regular copper. The advantage for me being that the seem slower to foul and I can clean and re-use them several times as I experiment. The regular coppers are usually done the first fouling. And the Iridiums come with a 1 year warranty. They are no brainers for me in this scenario.

Good idea. That way I could try the right hand carbs that I have. Got a best source for that line?

If you want help, feel free to DM your phone number and maybe we can problem solve?

Super! Thank you. I have a thing or two to try including trying the hotter plug on that side. But we are getting some real winter weather tomorrow, so it may be a bit before I can test.

I just may take you up on that. 

As I suggested in another carb-related thread, if you can't end up finding a new carb, or one to swap for a test, contact Gordon at https://customcarbservice.com/

If he goes through the carb(s) you can be sure it is functioning properly.

I will keep that in mind. Thank you!

Did you check that the choke plunger is sealing ok?

I have tried to block it, but seems difficult and I am not sure that I actually had it completely blocked. Any suggestions on how to make sure it is blocked? I just stuffed some thin plastic under the plunger. I think it was blocked, but not for sure. And wouldn't the right get the same extra fuel as the left if it were leaking? I have ran it extensively both with and without the cross over. No difference.

Switching coils side for side is worth it too.

Done that. Then new coils. Then swapped them. I have done what I believe to be EVERYTHING possible to ensure that the left ignition is working as good as the right. Including; switching all ignition parts left to right and replacing them, too. Bypass the entire ignition wiring to eliminate the bike's wiring. Use a relay so that the ignition is getting full battery voltage. I use a dwell meter, timing light, and dial indicator to check, recheck, and check the readings again.

I have been using the NGK iridium plugs. They seem to be a bit slower to foul and more importantly, I can clean and reuse them several times. They aren't much more money and the BR8ES plugs are done after one or maybe two foulings. I just ordered a BR7HIX plug. I don't know if one heat range hotter in that cylinder will matter, but I don't think it can hurt. I will keep the 8 in the right as it is not giving me any problems.

swap the left and right carb, see if it follows it.  just plug the choke crossover fittings.

I would like to do that, but the old oil line does not seem like it will like being bent into the necessary new position. It is pretty stiff. I guess I can change it, but I am trying to not get too far afield with this experiment. But, that may yet be the best route.

Compression is good; 100 on both. I even installed a new Ebay cylinder kit just to be sure that there was nothing going on with the original std bore cylinders. That was another test that I didn't think was the problem and turned out it wasn't. It has the same compression as before and runs the same with the same fouling problem on the left cylinder.

There is a long thread on my chasing this issue with most feeling it is an ignition issue. And I guess that is still possible. But as I covered in the previous thread, I have addressed virtually every single piece and the wiring of the ignition.............repeatedly.

And again; I am not at all certain that the carb is my issue. But I have ran out of ideas and am kinda throwing darts.

When that left cylinder is firing, the bike starts and runs real good. I found a nice set of Bassani chambers. That really woke up the upper end and showed that the stock pipes weren't contributing to the left cylinder fouling. I am just out of ideas.

Everything looks good. I have had it apart several times, cleaned it very good including the emulsion tube, and even switched many parts from the right to the left. I have tried lowering the float a little. So, I am out of ideas on what else to try other than a completely different carb. But as I said; I am not sure that the carb is even the issue. Just trying anything and everything.

The left seems to foul most often during warm up. Sometimes, I can ride for quite a while if I can get past the warm up.

I wonder if stepping up a heat range on that left cylinder would help any without risking damage? BR7HS plugs are readily available.

'75 RD350. I am STILL having trouble with my left cylinder. I have something I would like to try: I have went through the left carb several times and have even swapped parts between the left and right carbs. I am not at all sure that the carb is my issue, but I have tried just about everything I can think of and some that you here have thought of too.

So, does anyone have a known good left carb for my bike that you would be interested in selling? I have a couple of spare right carbs, but the one spare left that I have is pretty much crap. I have thought about using one of the right carbs on the left, but am concerned about twisting that old stiff oil line that much. I would just like to try another carb on the left.

Anyone have one they would sell me? Thank you!

Oxygenated gas makes power; https://www.youtube.com/watch?v=ryqyTPfNj1Q

What is the usual amount of support?