jetting advice please?

[johnr]

im achingly close to getting my gsxr engined kat on the road, after a bit of a false start with the carbs that were on it not running well at all, i tried a second set that i bought for a different project. the bike is running a gsxr1100m engine with 36mm cv carbs on it, as far as i know theyr bandit carbs, the first set i tried were lumpy and it didnt run on all 4 so as a quick test i fitted my second set and it fired up on all four cylinders and ticks over as well as revving freely. so, im afteer a tip on jetting. the first set of carbs have 120 mains in them, and the second set have 115's. but i see different advice on jetting. the bandit 1200 carbs have 115/120 mains,  but this site, http://www.postdiluvian.org/~mason/moto/jetting.html     recommends a 150 main jet.  which seems a huge steep up. thee engine is stock with an afteermarket pipe and foam filters. also what needle settinggs can you rccomend? just as a base setting before i start ordering jets!

[wraith]

Ok, Mrs W, has gsxr1100k engine with b12 carbs in her bike, carbs are set with 150 main jets (dyna jets) needle in centre clip and pilot jets at 2.5 turns out.

Bike has Cobra 4-1 with one off end can and K&N dual filters.

Hope this helps.

[clivegto]

150/155 usauly works for my ones.

[johnr]

excellent advice, cheers mate i thought that 150 seems huge but the more i look, the more i see the same recommendations. but its always useful to hear from someone who has the same spec on their bike.

[wraith]

All these motor/carbs seem to be similar but all seem to have a slit different tweek, which seems to come down to zorst/end can.

Had to play about with my gsx1100f engine Kat because I change the end can, from running just right to running rich at bottom end. (Pilot screw)

[johnr]

aye, but its a starting point to get me running so i can get it out for the mot.

[nlovien]

got a set of 36mm - part dyna moded - set up on dyno for a std 1100L motor with a noisy exhaust and twin KN filters - if you want to play with them - see how it runs to help provide a bench mark - i'll not need them this yr given the pace my projects are not progressing so yir welcome for a "borrow"

[MeanBean49]

Best bet is to get it on a dyno.

Ive always found GSXR 36's need slighty bigger jets for same spec of motor than Bandit ones.

For a stock motor Ive always found 150's way too big on stock motor, unless you have paid a shed load of money for a dynojet kit and fitted air correctors.

115-125 is what i found the ballpark to be in. Think i only had 138's in my 1216 motor.

[Gixer1460]

Generally my 'rule of thumb' is, if you increase compression & breathing capacity the MJ will decrease as there is more 'draw' through the venturi - conversely poor airflow ie. Blandit lower compression needs stupidly large jets to compensate due to reduced draw. 

[Nik]

My setup; Powerscreen engine,  with R cams, 4-1 Yoshi, BST36 carbs with two vacuum holes in the throttles, dual V&H filters, 155 mains, up one or two steps on pilot jets, lowered the needle one groove. Ran very well with nice and even torque curve. The filters/airbox make a big difference on draw. My V&H filters are relatively worn/free flowing. If you use airbox or well-oiled foam filters, you need smaller main jet (my gut feeling says 135-145). Good luck!

[Nik]

Also, don't forget to check the float levels. It makes a lot more difference than most people realise. IIRC, BST36 should be set to 15 mm. 

[clivegto]

Ended up with 160 main jets on my last one 1127 motor mk1 b12 carbs.

[MeanBean49]

23 hours ago, Gixer1460 said:

Generally my 'rule of thumb' is, if you increase compression & breathing capacity the MJ will decrease as there is more 'draw' through the venturi - conversely poor airflow ie. Blandit lower compression needs stupidly large jets to compensate due to reduced draw. 

That makes no sense at all. Bore abd stroke dictate how much air is sucked in each stroke. Compression etc has no real effect at all on the airflow through the carb or the venturi. Its purely bore, stroke and rpm that alters the airflow speed through the carb

[Gixer1460]

Please explain why the Bandit - with more capacity (+4%) but lower compression (-10%) needs such large jets (+20%) then ?

It's my RoT - you don't have to follow it!

[MeanBean49]

1 hour ago, Gixer1460 said:

Please explain why the Bandit - with more capacity (+4%) but lower compression (-10%) needs such large jets (+20%) then ?

It's my RoT - you don't have to follow it!

Because bandit 36mm carbs are different to gsxr ones. Gsxr carbs need smaller jets than bandit ones do. Doesnt matter what motor they are on. 

If you mess with the size of the air hole (like the air correctors in sone DJ kits) it changes the size of main jet you need without making any real difference to performance. We did it with my mates fzr, couldnt get jets big enough so reduced size of the air hole, richer without changing the mains

Edited April 20, 2018 by MeanBean49

[johnr]

i should probably have just bought a box of all the sizes from 120 to 160 then..... lol