hi there my fellow riders i just got my top end redone re nicklesild my cylinders to stock bore everything back to stock now the problem is im getting backfire through the exhaust on rear cylinder front seems fine to lean to rich air leak what do you guys think it is thanks for any help brent im so close to getting this baby on the road its killin me ahhhhhh
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ysr 50
90 mc21
91 mc21
and blata pocket rocket!
ride the moment!!

Do both cylinders have good compression?

NB. I know hes just had the cylinders replated, the question stands.
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If I have to take the carbs off once more...

hi again thanks for fast response it should have great commpression it feels like it does when i kick it over i just got it all rebult not even a mile on engine since the rebuild by a shop they just cant tune for sh*t but i have not checked the commpression myself yet
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ysr 50
90 mc21
91 mc21
and blata pocket rocket!
ride the moment!!

oh also another another note it seems to only happen in the mid range on the pipe seems fine hope that helps ty
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ysr 50
90 mc21
91 mc21
and blata pocket rocket!
ride the moment!!

If it has good compression on both cylinders I would check the pick up coils.

Check that their wires are ok and that the pick ups are clean.

You can also check the connectors on the ignition coils.
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If I have to take the carbs off once more...

ok i will do you think any of it has to do with tuning the carbs better
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ysr 50
90 mc21
91 mc21
and blata pocket rocket!
ride the moment!!

Is the bike stock, running stock carb setting or has it been altered in any way?
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If I have to take the carbs off once more...

its all stock im sure the settings have been messed with a bit but tried to get back im not sure
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ysr 50
90 mc21
91 mc21
and blata pocket rocket!
ride the moment!!

If the bike is fully stock then the best cause of action is to retune the carbs to their base settings and go from there.

If youre getting popping and back firing though the exhaust though it's more likely to be ignition to some degree though.
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If I have to take the carbs off once more...

ok thanks stephen for your help will try all the above mentioned and go from there ty long live 2 stroke ahhhhh
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ysr 50
90 mc21
91 mc21
and blata pocket rocket!
ride the moment!!

hi again also i found out they added premix to fuel with the injector working would this also make it run bad well rough with backfire?
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ysr 50
90 mc21
91 mc21
and blata pocket rocket!
ride the moment!!

do you know what brand and ratio oil they premixed? What is the brand and type of oil you are using in your oil pump too?

Having too much oil is not the best situation, as you'll be smoky all the time, if there is way too much, it may be stopping ignition (spark plug getting too fouled by oil).

Also, adding oil to the fuel will lean the mixture out.

An NSR with an oil pump doesn't need oil pre-mixed. Even with a new top end.

I'd go with Steve's suggestion. If all is 'out of the box' stock then set the carbs back to stock settings and go from there.

Matt, I'm trying to understand your statement in my simple brain....

I understand how it will run lean with respect to the air/fuel ratio, the higher (almost double) oil ratio will provide a very nasty environment for ignition, so you would not get such a clean burn, most likely gum up everything in sight. Is this most likely what will happen?

If this does happen, then it would be very hard to diagnose the jetting or running conditions as you're in a contradictory state.....lean but fouling plugs?

Yes, a very oil rich mixture could cause plug problems, even with an essentially lean mixture. The oily wet plug would come from the mixture refusing to ignite or incomplete ignition

Many moons ago we ran some tests for Castrol on a Yamaha 3XV. First on the brake and then on the track. Basically testing different oil mixtures (avgas and A747). Starting from 20:1 in steps of 5 up to 50:1.

On the dyno there was very little to choose between the ratios (jetting was optimized), but on the track you could feel the difference.

The 20:1 ratio kept cool, consistent engine temps, but seemed to lack midrange snap. The 50:1 was very crisp, but if used in a race situation we would have had to jet up in an effort to keep the engine cool on a long WOT straight, thus losing any benefits of the previous crisp response.

We ran around 30:1 at all races as this showed neglible engine wear with consistent performance. Hence why HRC recommend it I suppose!

I say around 30:1 as we would occasionally adjust the ratio if we needed half a jet size.