dyno results - nc23 projex

[The Novice]

was lean at tickover - Kev said it was actually off the scale ! which isnt good

Rich through the rest of the rev range but made 55bhp at the rear wheel and in his own words his dyno reads low - so all in all was well chuffed with it powerwise but i want it running properly even if that means less power - I cant use 55bhp properly anyway, i am a tittyprick around corners. The rich part makes sense as it is so snatchy/boggy when i put revs on, so much so i do fear round corners when i have to put throttle on! Sorting this out will really help my riding.

From memory he suggested i lower the float height about 1.5mm (or was it raise it ) . Turn the pilot screws out half a turn and reduce the main jet size. I am however open to suggestions on it - havent stripped it down again yet.

[amorti]

From my limited carb experience and understanding of how they work. Don't take this as gospel...

Best power (higher fuel usage) comes around 13:1 so actually your whole curve is a bit rich. For this reason the idea to reduce the main jet size is a good idea. This will mainly bring down the top end and give you more power and less fuel use, but also bring down the whole curve. It's not easy to estimate the complete effects though.

I am not well up on the effect of float heights, but I don't / didn't think they're adjustable on the Honda carbs anyway, short of bending the tangs out of spec, but even then aren't they made of plastic?

Definitely open up the air screws half a turn for more fuel at tickover. You will notice when starting that it need less choke less often by doing this, too. This is even more important with smaller mains.

The whole pilot jet area looks leaner to me, and opening the screw won't make much difference from about 3-5k. However by reducing the mains, the slow circuit will relatively be affected less, so that area may well level off. If that's still a problem, then maybe go up a pilot jet size and leave the fuel screw alone. Pilot jets are relatively expensive though so ideally you want to be sure they need changed before going there.

There's a weird lean spike about 5k. Is that not more or less where you get properly from the pilot jet onto the needle jet? I'd say if you're using clip needles then lift them one setting, or add another shim if you're not. Once you've done that, the top end will be a bit richer and the suggestion to go down one on the mains makes sense again. It might level itself off when you go down on the main anyway.

Remember though all these things are relative. Tune from the top down. First change the mains and open the idle screws, then go back on the dyno. This will require several visits, and a lot of faffing til you get to where you're happy with it.

What mods have you done, anyway?

[The Novice]

It's got dynojets on it, I would have to check through my old threads to know what size but back then it had a straight throughcarbon can.

I changed that for a quill end can as it was too loud for me. Apart from that, it's got stainless headers and everything else should be standard as far as I know.

I think the float tabs need to be bent to adjust the height so they can be adjusted.

Thanks for the input, you explained it really well and it all makes sense.

The dip in power at 13k was the clutch slipping.

[The Novice]

Just looked through old threads, i have 112 dyno's in it but as said its rich so need to sort what to put in

Sure there is a conversion chart in a recent thread i will have to have a look for.

Can someone move this thread to the CBR workshop please . I may have a crack at getting the carbs off later if the kids will go to bed!

EDIT - http://www.jetsrus.com/FAQs/FAQ_mikuni_ ... _sizes.htm

And I wouldnt have thought they should read so rich?as they are only one up from the standard jets. confusing me slightly and also as when i got it, it had 110's in it keihins which i swapped out for the dynos due to hesitation at higher revs.

Will have a look later, have a spare set of carbs i won off ebay a while back so might crack them and see whats inside.

[Wozza]

I assume you are running a standard air filter? 112DJs are best with a race filter. So you can either go down on the main jets or modify your filter. As amorti says start at the top and go down, but do reset your pilot screws if they are not at the standard setting, if they are I would check your pilot jets are clean and check the o-rings on the pilot screws - if they are all ok then turn them out.

Try to make only one change at a time then retest, with the exception of the above.

[The Novice]

yes - standard air filter and it looks a bit grubby. I think i am going to order some 115 keihins and a new air filter.

Q - How do i measure the float height - this is getting on my nerves now. Is there a point on the float i measure to?

I understand its the carb body, where the gasket would seat, but to where on the actual float. The float height i need to set to is only 7mm per the haynes so i am trying to measure what it is now without success.

whats best to measure with...........i am struggling here - i cant even get a photo on as the computer wont load windows.

[amorti]

If you've gone from 110 Keihin to 112 DJ jets, you've actually gone up 2 or 3 sizes as DJ come up a bit bigger. I can tell you 112DJ comes up around 115 Keihin, maybe 118. This is from my own experience.

Here's a lesson I learnt - if your dyno place stocks DJ jets, stick with DJ jets and don't confuse them, as anything else is not directly comparable.

If the air filter is grubby, then replace it, with a high flow while you're about tuning. Why tune the fuel to match a blocked air intake, you're hobbling yourself on both fronts and it'll be lean when you eventually put a new filter in.

[The Novice]

Ahh - but i only went to the 112 dj's on advice from graeme france.

It had 110 k's in it at that time but per the haynes, should have had 115 k's in it as standard - dont ask me why it didnt, I dont know the history, maybe it was restricted once?

The air filter is grubby but its really not too bad, just changing it in case. Can i even get a diff filter for it? just had a gander on wemoto and only see oem...........and they are nearly £30 ! bloody hell

[amorti]

Quill end can...

Hmm...

These are awesome in that they're very quiet at low revs and let loose at higher revs. About 5k rpm is where they get going. Could it be that lean ridge is where the can's opening up (some trick of resonance frequencies or similar withcraft) and the needles are a smidge bit behind that?

Bear in mind Quill cans are relatively restrictive for a race can, and the original filter grubbed up.. well, could end up that it was running rich even on original jetting and what you have is a bit bigger. However if you decide next to try 115 Keihins, you'll then be starting again. 115K are similar to 110DJ but it's NOT the same, just similar. If you went from a 112DJ and tried a 115K, but then needed to try the next size as DJ jets, etc etc etc. Just buy what the dyno shop has in, they will usually do you a part exchange trade on lightly used undamaged jets IMLE.

[The Novice]

changed my mind, going to go down a size on the dynojets first, as then i am comparing the same jets. I will also grudgingly get a new filter and then see how it runs.

Just changed the float heights. they were all way out hence lean and off the scale at idle, its made me feel better about the pain in the a55 of taking the carbs off and overcoming the impossible task I was making of measuring them with a tape measure.

After moaning i couldnt measure the carb floats and pondering it all day, i got home from work and made up a template - I suggest this is the best way to measure a float height, but i am a novice! Took me 5 minutes to make.

carbs



template I made, measured it up per haynes float height, 7mm, and cut it out carefully with a knife so it would fit the float/ gasket surface.



Here it is in action - you get the idea.



Go on, tell me that is pants and i have done it wrong!

Can anyone tell me if i will need to balance the carbs again after setting the floats, changing main jet and renewing the air filter?

Also - should i be looking at the needles for anything in particular, at the moment i havent opened up the other side of the carbs.