Hi
I am in the middle of building up a pair of carbs to put on my bike (MC21 with full engine rebuild, HRC reed block stuffers, a small amount of 'cleaning up' of the inlet tracts etc (strictly no polishing) and Tyga pipes.
I am finding that the bike is a sod to start...lots of kicking on full choke and then it fires and you have to nurse it VERY gently on the throttle for a couple of moments until it runs ok...on one cylinder!! After 15 seconds or so of gentle revving the motor the other cylinder joins in the fun and it runs fine...except that it takes a little while for the revs to drop down. It is NOT an air leak, have checked fully, but after searching the forum I notice that other folks have had the same problem and it was down to the Slow Jets. I have a couple of new ones on order for the carb build but I am intrigued by the 'blanking off' of the power valves.
Could someone explain the benefits to be had from this mod. This is a road bike only.

Cheers in anticipation
Mike
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I don't mind dying...I just don't want to be there when it happens!

It's quite some time since I worked on my MC21 carbs with HRC settings.
If I recall correctly, standard road setting jet sizes were something like 128 mains, then with power jets coming in at high revs.

HRC SS/SP settings were increase main jets to something like 188, and blank the power jets. As the main jets are much larger than 'stock' the power jets are no longer needed.

I'm struggling to figure how slow jets could be casuing your running problems, as long as the slow jet is not blocked / clogged, this shouldn't present any other porblems.

I can say though, that you have 100% perfectly described my early starting / running problems with my MC21SP, and that was a knackered crank outer seal.

When you say it is definitely not an air leak, I take it you have done a full leakdown test on the crankcases, blanking off the exhaust and intake manifolds, and have tested pressure both ways?
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

Thanks Rich
I might be being a bit 'pretentious' saying it is no an air leak as I have not done a 'leak down' test yet as I don't think (?) that it is an air leak.
Maybe I'm wrong but I will build a set of carbs very carefully first and then run with them. I spent a lot of time carefully re-building the motor so I'll go with the carbs first as (I have to admit) they did not get my 'full attention' on the re-build.
When I originally bought the bike she ws a bit of a sod to start and never returned to idle as quick as I would have liked. I'm still using the original carbs!

I was just interested in what the blanking' off of the power jets would do and I'm grateful for your information. I have two sets of extra carbs (one on a spare motor) and am starting to strip them down. One has power jets that have obviously been soldered but then re-drilled.

One question, where is the best place to get Keihin jets?

Thanks
Mike
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I don't mind dying...I just don't want to be there when it happens!

...also, could someone confirm that number '1' cylinder (and carb) is the front facing barrel.

Mike
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I don't mind dying...I just don't want to be there when it happens!

#1 is the left hand (upright) cylinder.
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If I have to take the carbs off once more...

Hi Mike, if the Marketplace cant supply the jets you need, i have always got mine from Allens performance, just punch it in, if you have a spare set of carbs try rebuilding them.
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Proud Father of , 05 ktm 400exc supermoto 2018 honda crf rx supermoto

Thank you all.
I am in the throes of building a good pair of carbs so I'll keep you informed of my progress.

Mike
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I don't mind dying...I just don't want to be there when it happens!

Good luck Mike, let us know how you get on.
Out of interest, did you rebuild the engine from split crankcases?
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

RichG wrote:Good luck Mike, let us know how you get on.
Out of interest, did you rebuild the engine from split crankcases?

Hi
Yes I did. I fitted the new crank etc if that is what you mean.

Mike
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I don't mind dying...I just don't want to be there when it happens!

As a bit of an update....
I rebuilt the carbs with NEW slow runner jets as the originals seemed to have different sized holes. I didn't block the power jets (might try that later) but fitted 130 mains to keep on the safe side whilst running in!

I also checked my ignition coils, my pickup wiring by the rotor and all other wiring. Refitted everything and then switched ignition on, pulled on choke and gave it one kick and it fired then it started on the 2nd kick

I let her warm up a bit and then went out for a ride. Pulled really well but I left a trail of smoke behind me like a screen

She ticks over quite well at around 1500rpm but is hugely rich at up to 1/4 throttle openings (roughly)! Would you clever folks suggest the air jet settings. I have then set at 1 1/2 turns out.
If I adjust them without the tank and airbox on, as I can see no way to get at the far carb, will I need to do anything special with the pipe (plastic piece) that pushes into the bottom of the fuel tank (with the filter in it I believe).

Cheers
Mike

p.s. It now always starts on 1st or 2nd kick even from cold. (Smug Smiley face here!!!)
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I don't mind dying...I just don't want to be there when it happens!

Anyone??
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I don't mind dying...I just don't want to be there when it happens!