Carby help needed.

[Homer]

Hi all, well the Factory Pro tubes and needles arrived finally, and I fitted them, then booked the bike in for a dyno tune.

The previous owner had lowered the float level to 9mm and fitted No. 75 main jets. I reset the float to 14mm, and fitted 137.5 main jets before taking it to be dyno'd. I had to raise the needle to the second highest position to get it to run ok.

Spent the whole day at the dyno. Increased main jets to 142.5, dropped needle 1/2 a notch using 0.5mm washers. Pilot jet is No.45.

The dyno says the mixture is ok but the bike is still playing up below 3500 rpm. At peak revs its putting out 76hp so at higher revs it runs great, but below 3500-4000 it surges at constant throttle positions, but only at speeds above 40mph.

I'm wondering if the fact that I'm running foam pod filters instead of the airbox might have something to do with it. I read here somewhere that vacuum carbs like to breath in still air, and I'm wondering if the turbulent air flowing around the carbs at higher speeds may be the cause, though wouldn't this get worse as road speed increases?

Also thinking about trying original Yamaha needles with the Factory Pro tubes.

The dyno guy wants to increase the pilot jet size and possibly drill out some air passages in the carb to get the slides lifting quicker, but I'm a bit reluctant to let him near it if he's not sure what the problem is.

He also stated that the 2 into 1 exhaust could be the cause, so basically he's not sure, and he's an ex Yamaha GP team mechanic!

Any of the gurus on here have any suggestions as to what I can check, cos I'm just about out of ideas.

Thanks heaps.

[Rod.s]

Hi all, well the Factory Pro tubes and needles arrived finally, and I fitted them, then booked the bike in for a dyno tune.

The previous owner had lowered the float level to 9mm and fitted No. 75 main jets. I reset the float to 14mm, and fitted 137.5 main jets before taking it to be dyno'd. I had to raise the needle to the second highest position to get it to run ok.

Spent the whole day at the dyno. Increased main jets to 142.5, dropped needle 1/2 a notch using 0.5mm washers. Pilot jet is No.45.

The dyno says the mixture is ok but the bike is still playing up below 3500 rpm. At peak revs its putting out 76hp so at higher revs it runs great, but below 3500-4000 it surges at constant throttle positions, but only at speeds above 40mph.

I'm wondering if the fact that I'm running foam pod filters instead of the airbox might have something to do with it. I read here somewhere that vacuum carbs like to breath in still air, and I'm wondering if the turbulent air flowing around the carbs at higher speeds may be the cause, though wouldn't this get worse as road speed increases?

Also thinking about trying original Yamaha needles with the Factory Pro tubes.

The dyno guy wants to increase the pilot jet size and possibly drill out some air passages in the carb to get the slides lifting quicker, but I'm a bit reluctant to let him near it if he's not sure what the problem is.

He also stated that the 2 into 1 exhaust could be the cause, so basically he's not sure, and he's an ex Yamaha GP team mechanic!

Any of the gurus on here have any suggestions as to what I can check, cos I'm just about out of ideas.

Thanks heaps.

When in doubt always go back to the basics, refit the original parts and change one thing at a time. You can't change the exhaust but you can take the carbs back to basic, then mod one thing at a time, yep this is time consuming BUT at the moment you may have several different things effecting the performance and all you'll be doing is chasing your tail.

GP mechanics DON'T make wholesale changes they work towards a goal one step at a time, that's why it take a whole season to get a bike running perfect and that's with a team of very expensive people and lots of $$$$.

[Mincehead]

Surging is normally weak mixture, drilling jets is ok but air passageways are best left alone unless you DO know what you are doing and have some base settings to work from.
Drilling the slow running jets could well ease the problem.

[fung]

ok homer.
i can only recommend my settings :
three turns on the mixture.
std float height
middle groove with f/pro needles and tubes
std airbox with K&N filter
2-1 exhaust with a DB killer.
cant hurt to try this first before drilling up your carbies eh
cheers
fung

[Homer]

Thanks Fung, Mincehead,

I'm going to try a few things with the filters, airbox etc before moving on to anything else. Problem is I don't have an airbox.
And I doubt it would fit under the XJ6 tank anyway. I might try making some sort of an airbox to see if it makes a difference.

[M.V.]

Two into ones are reputedly hard to set up for low RPM's, coupled with no airbox, that's probably the problem.

Fungs advice sounds good to me, especially before doing major mods to the carbys.

(You should be able to drop your main jet too, I'm running 132.5 mains & it's still slightly rich... YMMV depending on a lot of factors of course.)

[Homer]

Thanks MV, thats sort of what the dyno guy said. Might have to look at going back to a 2 into 2 exhaust. Anyone know where I can borrow a set of standard pipes????

[fung]

a few more thoughts to ponder . . . .
do your carbies still have their( atmospheric balance) intake tubes hanging off the side/back with the little inline black strainers?
they haven't been plugged or looped??
cv's need atmospheric pressure to work correctly.
you may be able to mod a std trx or xj6 airbox to at least carry the pods.
the trx box splits in the centre horizontally, so there is some room for modding.
perhaps try some form of exhaust end cap restriction first.
cheers
fung

[Homer]

Well guys after searching the net for info on running Mikuni BDST38 carbys without an airbox, it seems likely that this is causing my problems. I've found there are blokes running 750/4 hondas without airboxes and are having identical problems to myself.

I'm happy to fit a stock trx airbox, but don't know if it will fit under my tank. Anyone got one lying around that they could measure for me???

[M.V.]

Doing some work on the bike tonight, should be able to do a quick measure for you.

[Homer]

Well I refitted the original Yamaha needles and took the bike for a ride. What a difference! Flat spot around 3000rpm almost (but not completely) gone. Surging below 4000rpm nearly gone, and bike pulls a lot better from 2000rpm up.

Still keen to try running the bike with the standard airbox if it will fit. I'd still like to get some measurements of the standard box if possible.

Thanks

[rbaron25]

Mate I have an airbox here that the previous owner had a go at ( read mangled) ... Could send that down for ya?
BTW had pods and put the standard airbox on ... Mmmmmmm lovely ... Like a dream. Stacks better low down more useable like everyone says on here.... Surprise surprise mincey and the boys are right

[Homer]

Thanks Rbaron, sounds like the pods are definitely the problem. I'll happily buy your airbox if your happy to part with it. Let me know how much, and calculate postage to 2263, and how you want the dough sent. I've got a Paypal account or could transfer to your bank account.

Thanks

Homer

[Homer]

Well I've tracked down a used airbox at Metro wreckers in Silverwater, just not sure if I can make it fit under the XJ6 tank. S'pose I'll have to ride down next week and try it out, unless someone can measure theirs for me

[rbaron25]

Sorry mate been out. Don't worry about cash. Have pm ya