Did you decide on the larger jets, main and pilot, from past posts? They sound large, don't remember having to change the pilot jet, simply opened it out to around 3 turns and upped the mains a couple.

Initial jet sizes & settings advice was given to me by Stripes1976 on a thread I started titled "K&N air filter advice".
I tried copying & pasting the thread link but it didn't work, (The thread is currently on page 3 of the workshop section), so I copied Stripes' post. Here's his advice below :-

"O.K. 117.5 is a good place to start with the main jet on the dyno but always start rich and come down.
I have always tried the 120 main as a base then usually drop to 117.5.
For every 3 main jet sizes you increase you will need to go up 1 in the pilot.
If you don't you have a huge flat spot and bad running in the middle and low down".

No offence intended, and I know nothing specific about the 400 Suzuki, but I strongly agree with Mr Variable and as I understand it he has quite some experience with these bikes.
Your immediate problem appears to be the pilot circuit and you have fitted larger pilot jets, the wrong thickness o-rings and do not appear to have a method to determine correct mixture. It also makes no sense to have to change anything based on a different brand of air filter. I would set everything back to standard and start from there.
cheers

Stripes certainly knew his stuff, probably knew more about the 76a than me as ive mostly ran 73a's but still sounds high to me.
My 73a with K&N needed the needles raising, the mains upping a couple and the pilots winding out 3 turns.
The two bikes are slightly different but carbs are basically the same regarding bore size, mains and pilot jets.
I found the airbox so restrictive that the jetting never moved much no matter what i did, same on the Bandit, its making 65hp and only uses 110's and stock pilots.
Guess the only way to find out will be to dyno it.

Thanks again for you input, gents, it's much appreciated. Once I've done a final carb balance &, hopefully, getting a steady idle the intention was always to have the bike dyno'ed to check the fueling through the rev range. VVS may be right about the size of the air box not affecting things too much as it's not the largest of airbox's on GK76's either.

But, the larger of the two rubber intake rings I've fitted that came with the filter fitting kit is about double the intake area of the OEM filter's though, hence why I've gone with Stripes advice for 117.5 main jets as a starting point. If I have to fit smaller mains & raise the needles again then so be it. I don't think Stripes gave me definitive settings, just safe starting points.

As for the bigger pilot jets & higher air screw setting, if it's any help, when the engine's stone cold it needs at least half choke to start & run smoothly &, after a minute or so, the chokes can be closed off completely.
Once up to normal temp the engine fires up straight away so it can't be too far off mixture settings/pilot jet wise at the very bottom end/idle.

Blair, I will get one of your internal seal kits eventually, honest!
(Let me get my money's worth out the OEM ones I bought & fitted )
But first I've got to repair my broken 3XV engine ... which ain't gonna be cheap!!

Cheers
Wullie.

Keep at it Wullie. You'll get there.
I think the infrared thermometer on the exhaust is a great idea and definitely worth a try. They are only $20 on eBay.

Thanks Blair. I'm hoping to get another balance run done tomorrow, if not then sometime this week.
Whether it's sorted or not ... you'll hear all about it!!

Definitely sounds like its close and I know stripes had a K&N fitted to his 76a so lets hope it works out, you will not need open up the pilot with a bigger jet fitted, tried larger on the bandit but it had a weird rich spot just above tick over that went away when I put stock size in.
Will be good to get a definite jet size for the 76a + K&N.

Hello again folks, I think I've finally cracked my "lumpy" idle problem. Last Friday I managed to reset all four carb's mixture screws to 2.5 turns out & do another carb balance run & there was an improvment !!
Cylinders 2,3 and now 4 were "pulling" virtually the same vacuum at 1100-1200 rpm ... but No.1 was still way worse than the other three ... even with a different coil, new ignition lead & plug cap fitted !!

I had a "play" with No. 1 carb's mixture screw to see if I could improve on the vacuum. Turning it out half a turn at a time improved things a bit, but not enough. Suspecting an air leak from somewhere in No.1 carb I started with the carb top & diaphragm first as they're the easiest. The diaphragm was still seated correctly & there was definitely no holes in it so next I checked the carb top again, this time with the aid of a X10 magnifier. Definitely no hairline cracks or damage of any kind anywhere on the cap. On checking the o ring seal between the carb top & carb body I wasn't happy with how far it protruded above its recess so I had a rumage around my spares stash & found & fitted a small o ring slightly thicker than the old one then refitted the carb top.

Unfortunately though it's taken 'til today to have the time (& the weather ) to give it a quick (static) run to see if there was an audible improvment which, thankfully, there was. With the temp gauge a third of the way into the white "Normal" zone the engine is definitely smoother at 1100-1200 rpm. If the carb top checks hadn't made an improvement next on the list to check for damage was my new OEM jet holder to carb body O section seal ... which would have meant taking the carb's off yet again !! I'm gonna do one more carb balance run just in case the carb's need a final tweak, then I'll have a play with all the mixture screws as suggested earlier in the thread by Blair.

Out of curiosity, while the engine was properly warmed up, on blipping the throttle from idle to 3-4 thou rpm the rev's rose & settled down nicely without hesitating, hanging or dipping below 1100 rpm. Between 3 thou & 6-7 thou though it sounds like it's hesitating/stuttering so I'll move the needle clips from the 2nd from top slot back to the stock middle position & see if that improves the throttle response in that region of the tacho. (Or could it be because the pilot jets are too big ??) If raising the needle's improves things then I definitely need to get the tank & seat on & see how it behaves whilst in motion.

So folks, slowly ... but very surely ... I'm getting there with the old git.
At long last I can see the proverbial light at the end of the tunnel.

Bye for now.

Wullie.

I knew it was too good to be true, my previous post was a false dawn. I'm still having trouble keeping the carb's balanced once set! When I blip the throttle the carb's go out of balance & I have to start all over again !!
It's flippin' annoying & bloody frustrating!! I must still have an air leak (or leaks) somewhere in the carb's.

Here's a video below to show what's happening ... (Apologies in advance for the odd expletive you might hear! ).

https://www.youtube.com/watch?v=I4gQdmkRZ0E

In case I have a rogue/damaged/undersized seal I've ordered a set of Blairs' LiteTek seals, (Kit A), to go along with the Kit B I've already fitted. Other than that I've no idea what could be causing the issue as I've checked the inlet rubbers & stubs for perishing/cracks & couldn't see any, the valve clearances were checked & adjusted, I eventually did a compression check (when I replaced the leads & L/H coil) & all four cylinders were between 10 & 10.5 bar so the rings & bores seem servicable. There was negligible wear on the butterfly valve shaft & its bushes when I had the carb's split so there's no excess play, so the new shaft seals shouldn't be letting in any air.
I also bought a can of WD40 today so I can spray around the carb's & the inlet rubbers to see if it's an external air leak or not. If a new set of internal seals doesn't cure the problem I may have to buy another set of carbs & swap the No.1 cylinder carb's over.

Does anyone have any thoughts/suggestions or had the same grief as me & cracked the problem ... eventually??!!
All ideas will be gratefully received & considered.

Yours frustratedly !!

Wullie.