Hi

I am in the process of collecting parts and cleaning for a conversion but i just want to know what carb combination setting (jets, needle position, how many turns out and what airbox or filters) people have on the carbs so i have a rough guide to what works smoothly throughout the rev range.

i am mainly interested in the 4JH and 4TV carbs but any suggestions are welcome

slowsport wrote:Hi

I am in the process of collecting parts and cleaning for a conversion but i just want to know what carb combination setting (jets, needle position, how many turns out and what airbox or filters) people have on the carbs so i have a rough guide to what works smoothly throughout the rev range.

i am mainly interested in the 4JH and 4TV carbs but any suggestions are welcome

I have standard (as they came off the bike) 4JH carbs with a modified 3TJ airbox.

I enlarged the holes that go over the carbs and 'sealed' them with pvc tape, then pressed the airbox over the carbs until they 'clicked' home over the lip on the bellmouth. I removed the intake trumpet (that holds the filter in place) because the tank won't fit with it there, and used gaffa tape to hold the filter down and to reduce the intake area of the filter to somewhere near that of the standard 4JH intake trumpet. It took a few test rides to get it right, but it runs really well and pulls right through in all the gears The gaffa tape was only ever meant to be a temporary fix/test, but it works...

thanks Kayla

but the carbs i had recieved with the 4JH engine had main jets of 108, and that can't be factory so i know someone fettled with them and i just want to get the carbs running right from the start so i need to know if factory settings work well even with little air feed, could someone please tell me the factory settings as the haynes i have is for the frame not engine lol

Or if anyone else has any variances from factory for the hybrid that seems to work like Kayla's airbox mod or different jets this information will be greatly appreciated!

I used 4JH carbs and found there to be a low down flat spot with teh standard #35 pilots, so upjetted the pilots to #42

viewtopic.php?f=37&t=16319&p=114141&hilit=pilot#p114141

4TV carbs seem to work OK as standard. Currently running these on my hybrid, which is now a track only bike.

I'm not sure if any configuration on these hybrids is great below about 6000 rpm. I always found it had to be kept in the power band to accept wide open throttle and actually have any shove to go with the airbox growl.

thanks for that plodder maybe that will help it to run decent, and if need be at a later stage to run to a dyno, any idea what main jet you used and any other tweeks that got you going?

any advice is greatly appreciated since i am a carberettor moron and a complete idiot in the ark of tuning carbs! lol.

plodder wrote:I'm not sure if any configuration on these hybrids is great below about 6000 rpm. I always found it had to be kept in the power band to accept wide open throttle and actually have any shove to go with the airbox growl.

I don't really agree with that, my bike's fine- it pulls smoothly right from 3000rpm or so with no stutters or flat spots. Obviously it runs better at higher revs cos that's where everything's working right, but there's enough shove to potter around under 6000rpm if you want.

I don't think the carbs tolerate being very far out of balance though, mine ran like a sack of sh*t until I did a council balance on them

thanks kayla,

i will bare in mind to bring out the carbtune after i got it all in then

Kayla850 wrote:I don't really agree with that, my bike's fine- it pulls smoothly right from 3000rpm or so with no stutters or flat spots. Obviously it runs better at higher revs cos that's where everything's working right, but there's enough shove to potter around under 6000rpm if you want.

Yeah, potter for sure but if you're in the wrong gear, forget the overtake or getting out of a corner well.

My hybrid keeps up with the proper 600 trackbikes on the straights and I was also on a par with or slightly faster than a YZF750 on the straight, so it can't be that bad.

I don't run a snorkel or any ducting on my airbox, so it's just an open air filter. I'm sure that's got a lot to do with it.

plodder wrote:I don't run a snorkel or any ducting on my airbox, so it's just an open air filter. I'm sure that's got a lot to do with it.

It probably is. Mine's hasn't got a snorkel fitted either, but I have closed up the entrance to the filter to the same area as the snorkel with tape which made a massive difference.

The engine probably relies on air velocity rather than just volume.