Air mixture screw settings

Technical questions and answers

Air mixture screw settings

Postby madmacksbikes » Tue Mar 22, 2016 11:58 am

Guys what is a good starting point for the air mixture screw setting on a TZR250 please? I was always of the assumption it was 1.5 turns out but is there any advances on that. Bike is running stock jets, air filter and pipes. Massive lack of power changing up into 5th gear but perfect up until this point, any ideas please?
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Re: Air mixture screw settings

Postby Higgsy » Tue Mar 22, 2016 5:16 pm

Hiya, what air screw? There isn't one, you've probably unscrewed the air jet I think

Check your powervalves for correct position and operation, also check that the joiner between the barrels hasn't come undone, stick your fingers up both the exhaust ports to check the position is the same for each

Possibly could be fuel starvation, check the float heights, have you got the early round float carbs or the later oblong floats. if you have the latter make sure the plastic extension is in place over the pilot jet. Sounds like the motor is running out of fuel by the time it gets to fith gear, pehaps the fuel tap isn't letting enough through or the lines are trying to run uphill?

Even if the TM28ss carbs had an airscrew it wouldn't cause the issue you have, but having a airjet unscrewed won't help the boggy feeling low down. Are the chokes working properly, they can jam in the on position. The boggy feeling and uneven running low down is usually signs of blocked pilot jets, TZR's are known for it :roll:
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Re: Air mixture screw settings

Postby madmacksbikes » Tue Mar 22, 2016 10:48 pm

I've got the later carbs and those plastic pieces were defiantly on when I put them back together. I will check the PV joiner but the operation of the valve is ok as I've checked that. New pilot jets and Venturi pipes fitted. I might not have screwed those air jets in tight. I did replace the seals in the fuel tap so might be that, routing of pipes look OK. Carb synchro was slightly out and I'd forgot to tighten the air box to carb clamps like a doughnut. What's the best way to check the float height?
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Re: Air mixture screw settings

Postby Higgsy » Tue Mar 22, 2016 11:20 pm

All sounds good then, what's the condition of the motor, has it been apert? And you say it's going well before 5th gear, what's it revving to, should be about 10,000 on stock pipes

There are various ways of checking the floats but i turn the carbs upside down and measure from the float bowl flange to the top of the float 17mm, as a guide the casting mark hlfway through the float should be parallel with the carb body
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Re: Air mixture screw settings

Postby madmacksbikes » Wed Mar 23, 2016 8:31 pm

Only the top end has been apart and inspected with new gaskets fitted but nothing else, yes it revs to 10,000rpm in 1st through to 4th fine. I shall take the carbs off on my next days off and check the float levels and make sure the air jets are screwed in properly
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Re: Air mixture screw settings

Postby madmacksbikes » Sat Mar 26, 2016 10:26 pm

GuysJust stripped the carbs off my 2MA and found that one of the pilot jets wasn't screwed quite home, therefore that plastic piece on top wasn't sitting right. Carb float levels were all over the place between 13mm and 18mm. I've set them at 16mm and obviously there's a massive difference. However at 9,000 rpm in 4th there's still a stutter, it's not all the time infact when it was bearly warm it was awsome, as the motor heated up the stutter seamed to get worse, however if I get past it, it's fine. Looking at the HT leads and plugs the leads are way too long really and they are obviously aftermarket along with the plug caps. Do u think the stutter could be electrical? as I say it's not all the time
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Re: Air mixture screw settings

Postby Higgsy » Sun Mar 27, 2016 9:01 am

Possibly electrical, although itermittent issues like that can be a bugger to isolate, what plugs are you running? Aftermarket leads are actually a good idea if it's been done properly, the originals are thirty years old now, the rubber NGK racing caps with B9EG plugs are the best combo I've found so far. The caps have a resistor in so you need a non resister plug. If you have the Iridium plugs bin them, they've caused me no end of grief!

Intermittent suggests ignition, it could be the start of a stator/pickup starting to play up, especially as it gets worse when it gets hot

Where are the needles set in the carbs?
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