Carb jetting

[Tpot]

Gone from a 4into 2 exhaust to a 4into1, bikes lost most of its power, I thought it would be too rich but from what I've read I need to up jet it, anyone help? 

[Gixer1460]

A 4-2 is quite restrictive compared to a 4-1 therefore if it gets out quicker it also gets in faster and would automatically get leaner so usually 10-15% up jet is a starting point.

[Tpot]

Ah, I thought in my finite wisdumb that the 4 into 2 let the gasses escape quicker, more doors kinda thing, I'll try some bigger jets and go from there, thanks

Edited August 4, 2019 by Tpot

[Captain Chaos]

if it makes more noise, it's less restrictive.

[Dezza]

1 hour ago, Captain Chaos said:

if it makes more noise, it's less restrictive.

That principle did not work on my 1977 FS1E

[banoffee]

[Tpot]

Im sticking some 36mm carbs from a bandit on it (1127f) now, anyone have a ballpark figure for main jets please? Using a K3 gsxr1000 exhaust

[Gixer1460]

Ball park 138 - 145, if you read my bit above - still don't mean its going to run right!

Edited August 5, 2019 by Gixer1460

[MeanBean49]

1 hour ago, Gixer1460 said:

Ball park 138 - 145, if you read my bit above - still don't mean its going to run right!

I dont get what your bit above means?

The exhaust stroke pushing gas out easier doesnt make and engine suck more in on the inlet stroke?

The bore and stroke dictate the volume sucked in each stroke

[dupersunc]

1 hour ago, MeanBean49 said:

I dont get what your bit above means?

The exhaust stroke pushing gas out easier doesnt make and engine suck more in on the inlet stroke?

The bore and stroke dictate the volume sucked in each stroke

And cam timing, inlet length, exhaust length and diameter. etc etc.

To the OP.  These Suzuki engines aren't that sensitive, if the basic are right.

Make sure your carbs are in good working order, i.e. no  blocked pilot jets, pilot circuit, float heights set correctly, float needles sealing, good plugs.

If all that is correct the bike should run half decent. though pod filters will inevitable leave a fluffy area around 5k.

Modern fuels are a main with carbs though. The pilot jets quickly get blocked if left standing for any time and can be a pig to clean properly. Modern fuels also Don t leave much of a clue on plugs either so plug chops are a waste of time too.

Get the basics sorted, one that's right, by all means try the jets suggested but the only real way to sort it properly is a dyno and an operator that understands carbs.

[Gixer1460]

2 hours ago, MeanBean49 said:

I dont get what your bit above means?

The exhaust stroke pushing gas out easier doesnt make and engine suck more in on the inlet stroke?

The bore and stroke dictate the volume sucked in each stroke

stick a potato in your exhaust and see how much it affects the inlet ! My point was exhaust backpressure (usually greater in 4-2 systems) will necessitate lower jet #'s as cylinders will not be able to evacuate burnt gases as efficiently and conversely intake charge. Drag engines don't run silencers / mufflers for the sound - it's the easiest way of ensuring cylinders are empty for new charge.

[dupersunc]

11 minutes ago, Gixer1460 said:

stick a potato in your exhaust and see how much it affects the inlet ! My point was exhaust backpressure (usually greater in 4-2 systems) will necessitate lower jet #'s as cylinders will not be able to evacuate burnt gases as efficiently and conversely intake charge. Drag engines don't run silencers / mufflers for the sound - it's the easiest way of ensuring cylinders are empty for new charge.

Properly designed silencers don't cause any restriction.  They can be quite heavy though.

  Please stop using back pressure in reference to 4stroke exhausts. It makes my teeth itch.        Saying a 4-2 system has more resistance to flow ( I guess that's what you are saying) is such a generalisation as to have no relevance to the discussion.

[Tpot]

The thread for one of the pilot screws is knackered,  looking at new 36mm carbs, prices have gone right up, I've 33mm and 34mm tintops but no 36s, bugger, no chance of repairing the knackered pilot screw thread is there? 

[davecara]

Lots of engineering companies will sort the thread out for you and/remove the offending pilot screw if you're not confident doing it yourself.

Failing that, sell your tintops to one of the turbo boys once you've got 50 posts and buy some bandit 36's

[Tpot]

What's the difference between bandit 1200, gsxr1100  and the gsx750f  bst36mm carbs? Are they so similar I could up the pilot jets one size above what the 1100 should be to allow for the piper cross filters and exhaust, I know a dyno is best  (powerscreen 1100, looking at fitting bst36mm carbs from a gsx750f) 

Edited August 5, 2019 by Tpot

[manden]

On 8/4/2019 at 4:48 PM, Tpot said:

Gone from a 4into 2 exhaust to a 4into1, bikes lost most of its power, I thought it would be too rich but from what I've read I need to up jet it, anyone help? 

Try 140, prolly 40 on the slow jet

[Tpot]

I've put some 36mm from a 750f on it, needles not moved, 137.5 mains, pilot jets 17.5, screws 2 turns out but will try a half a turn back in as it starts up easy with no choke and seems a bit fluffy from tickover, goes like buggery and picks the front up easy in 1st now, sorted