Guidance on Setting Up Carbs

pkay

pkay

GSX400S Rider
Not a spam email!!!

Reading Pete's email about setting up the carburation on his Kat as part of his restoration and also some issues I am having with some bikes I was interested to get perspective from the learned on base settings to start with for carbies when you put them back in the bike. I guess a sort of rule of thumb starting point rather than specific settings for max performance is what I was after. e.g. air needles at...., slide gap at....., etc. I've got two bikes that are giving me the shits so I'd like to be able to go through a check process to eliminate carburettor set up as the starting issue. Thanks for any advice.
 
If these are all stock setups with airboxes, air cleaners, stock pipes, etc. then all you should really have to do is make sure the filters are cleaned and oiled (if appropriate) then set the carbs up with stock settings. You need to verify things like float levels and fuel height of course. One thing I learnt recently is that just because your float heights are right doesn't mean your fuel level is right, especially with the older brass floats. If one carb's float is more bouyant than another carb's float and you set both float heights the same, you stand a good chance of having inconsistent fuel levels which means one will be slightly richer/leaner than the other.

The gotcha is when you start doing things like velocity stacks, pods, freer breathing pipes, etc. I did come across a rule of thumb guide at one point but haven't got the link handy.

The other gotcha is when you have things like worn emulsion tubes or needle jets, worn slides, dodgy torn or perishing diaphragms, older carbs that have sat and have clogged passages due to fuel going off, and all those fun things. The only way to get a good result with those is to sort the issues out before trying to get it running right. If any of those things are wrong, you're likely going to have a lot of wasted effort and you'll only get inconsistent results which will have you cursing every ride.

The best thing I've found so far after all that is an AFR gauge, it takes all that guesswork out and tells you exactly what's going on. I could really do with putting a bung in the 450's pipe to do that because I know it ain't right...
 
Hi Paul,
As Pete said get the basics sorted carbs clean and serviceable,

+1 on the AFR it takes any guess work out and gives you a real time indication of your fuel mixtures through all driving conditions.
It shows you idle/pilot/main jet mixtures.
I used it when I put the 38cv on and now for the EFI.

You can 'see' idle mixtures, for me 11:1 and now 12:1 with a change in fuel pressure.

And why you have acceleration surging/hesitation is it too lean/rich, I have a hesitation at constant throttle and in closed loop condition (when the ECU uses the factory o2 sensor to maintain 14.7:1) and when I give a gradual increase in acceleration it leans out to 15-16:1 and hesitates, but a quicker acceleration from closed loop and I can 'see' a mixture of 12-13:1 and a smooth acceleration. No guess work as to why it happens and you can 'see' the effect of changes good or bad.

Worth the peace of mind.

Hope this helps,

Cheers Andrew.
 
Another thing Paul is you need good new fuel.
Using shit in ya tank that has been sitting there for months does not help any.

Nige
 
nige501 said:
Another thing Paul is you need good new fuel.
Using shit in ya tank that has been sitting there for months does not help any.

Nige
Click to expand...

.....so Nige. Are you saying that left fuel is best avoided?
 
unless this in some kinda in joke about leftist fuel, old fuel is probably better in your old two stroke lawn mower, and not in your pride and joy
 
pez said:
unless this in some kinda in joke about leftist fuel, old fuel is probably better in your old two stroke lawn mower, and not in your pride and joy
Click to expand...

Hahaa Pez you have only been on hear for a couple of weeks and you have already sussed us out.
Derek knows im Right and he cant let his leftism go.
Nige
 
Pkay,
If you’re using old jets, make sure they haven’t been drilled out. Sometimes it’s cheaper just to buy some new ones.
Also, are the carbs CV’s?

I used the following links to tune mine
https://www.mikunioz.com/tuning-tips-and-manual-downloads/?v=6cc98ba2045f
http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

The Factory Pro guide (from memory) is the one that made more sense to me.
It boils down to testing the selected rev range cold and then hot, taking note of the changes and adjusting that jet/ needle etc. accordingly.
Later on I had the chance to put a bung in and check it with an AFR meter. It was within 1 size o; the pilot jet and the rest were spot on.
If they’re mechanical then I have no advice.

Cheers
Ben
 
I haven't seen that Factory Pro one before Ben, good link! Mind you, step 1 on a Kat around here could get interesting :D
 
the mikuni oz link does not relate to CV carbs.
the factory pro link is on the money.
 
Had another thought dawn on me this morning given my history of the wee beastie... make sure your ignition/electrical system is up to scratch too! Things like weak spark due to dodgy spark plug caps or low voltage will affect fueling, as will dodgy spark plugs etc. Just in case it hadn't occurred to you ;)
 
Thanks Guys, was informative (well except for Derek's high quality input of course)...
 
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