Carb Rejetting

Hi all,

I have some questions about basic carb tuning that I was hoping some people here could lend a hand with. I have a standard CB with an aftermarket Scorpion exhaust. This exhaust claims to not require any rejetting. It does pop slightly on overrun which leads me to the conclusion that it is close to OEM airflow but still slightly less restrictive. I also think it may slightly be down on power since changing it but that is very hard to objectively determine.

I am wondering is it possible to change main jets without removal of the carbs (i.e. remove the float bowls with them in situ and swap out the jets?) I am not interested in investing the money, effort and time off the road to install dynojet kits or the like. I am more concerned about not running lean and if that frees up a small power increase then that will be welcome as well. If rejetting in situ is possible, I may also switch to K&N air filter if its increase airflow can be matched by an increase in jet size.

If it's an easy few hours tinkering then I will order main jets 1 size larger, if not, I will not tear apart a bike that for the most part runs very well.

Thoughts would be awesome!

I dont know about carbs but i am also thinking of a k&n airfilter as i think it a waste to keep throwing away dirty airfilters... now i have a standard airfilter in my old fireblade&cb.. but i saw on a blade forum that (not my opinion)they suggested that k&n let in dirt and it didnt add any power to fireblade(92to98) true or not i do not know?

I use K&N for the car simply because it is reusable. I can't imagine it allows harmful amounts of particulate to pass through or else people would just go without filters.

Can anyone confirm or deny the plausibility of changing the jets with the carbs in situ?

Not a chance of doing them in situ I'm afraid.

What do you think of k&n airfilter   letting in more dirt than standard filter no extra power increase andy(according to a fireblade forum discussion)?

I have used them for many years and have had no problems at all with extra dirt. This problem is probably a result of people not oiling the filter after cleaning it.

Thanks andy your expert advice always appreciated

ANDYC wrote:I have used them for many years and have had no problems at all with extra dirt. This problem is probably a result of people not oiling the filter after cleaning it.

If you have a K&N you need the K&N cleaning kit like here.

It has a cleaning spray (that won't eat the filter) and the correct oil to re-oil the filter before refitting - given the small size of the filters on our bikes it will last for quite a long time.

I suspect that Andy is correct about the lack-of-oil thing.  People are quite lazy/uninformed when it comes to regular maintenance IME!

To answer the previous question: you need to take the carbs out to fit the new jets; you might be able to put another washer 'under the needle' (raise the needle hight a little bit) from the top and see if that makes any difference (I can't say either way).

If you replace the main & pilot jets do you need to replace the fuel bowl gasket?
I looked on Allens performance site but couldn't see fuel bowl gaskets.

Only if you damage them. I don't normally have to replace them.
Maybe I have just been lucky.

That's what I thought but it's been decades since I dealt with carbs & haynes manual said to replace.
I assume haynes manual eres on side of caution.

The OEM fuel bowl gaskets cost a fortune but my original ones were cracked when I cleaned my carbs so I took a chance on some non-OEM ones for relative peanuts off eBay and have not had any issues so far (only a couple of years ago but a good few thousand miles). (I too often feel Haynes eres on the side of caution but when you think about their market you can kind of understand why they would.)

With air filters I think it very much depends on environment. In the UK I'm fairly confident OEM style filters would be sufficient, however if you rode around a desert or a very dusty environment I reckon the smallest particles would get through including K&N filters. I found in Australia a foam filter spayed with an air filter oil performed best but the finest particles would still get through that too.

Is there a knack to getting the heater hose connector screws out except for luck & penetrating oil. Mine seem more likely to round off before they come undone.

Think I need better glasses but i'm assuming it's allen screw rather than torx.

Hi,

Make sure the head is clean and use proper allen key, tap the key gently into the screw head so it won't slip. Use new screws for assembly, you can actually buy stainless allen screw kit for the carbs from ebay, that's what I did when I rebuilt my carbs some months ago. The screws are standard size so if you have well equipped local shop for fasteners, you can of course buy everything locally.

BR,
Antti

You could heat them up with a hot air gun which might help - probably best to stay away from the blowtorch 'cos of the petrol...

Melitos wrote:Hi,

Make sure the head is clean and use proper allen key, tap the key gently into the screw head so it won't slip. Use new screws for assembly, you can actually buy stainless allen screw kit for the carbs from ebay, that's what I did when I rebuilt my carbs some months ago. The screws are standard size so if you have well equipped local shop for fasteners, you can of course buy everything locally.

BR,
Antti

Just ordered better allen key so I can tap it into screw head. I'll get some better screws as well to save time next time i do the carbs.

eternally_troubled wrote:You could heat them up with a hot air gun which might help - probably best to stay away from the blowtorch 'cos of the petrol...

Blow torch would certainly make it interesting.