El Gringo

Haha, that's very true Steve! In other news i may have found a suitable test spot - here's hoping for a bit of dry on Saturday

Yep, i could pull the carbs off my bandit 4 to try, and i may well do at some point, however i'm not sure how well they would work, the GK76a carbs are slightly more down draught and have the float bowls at an angle where as the Bandit carbs are pretty much flat but we can always give it a go

Me too! After all the hassle just to get it to run i was hoping for an easier time in getting it set up! Obviously not to be. I shall not be deterred. If it does turn out to be the CDI unit it just gives me an excuse to buy an ignitech so i can fiddle with the retard at the top end. I was going to save it for v2.0 of the engine but if it needs it sooner so be it I've already got plans for a big bore second motor with tuned head, probably a base spacer to get the compression down further, and maybe a bigger turbo, along with one of Ian Cross' lockup clutches - an ignitech makes perfect sense to top it off However, i'm getting way ahead of myself.... I also concur with your sentiments, this place is awesome, many thanks to everyone again for your help

I looked into the CDI limit idea, i can;t find any evidence to suggest that the CDI has any fail safe limit and the main CDI i'm running is an aftermarket unit so it probably wouldn't have been included. Also, having looked at the wiring diagrams, the GK76a does have an extra wire that goes to the CDI from the neutral switch which does something with the neutral circuit - however neither the aftermarket CDI nor the Bandit CDI have this input on the CDI - all the wire does is create an earth so if it's not running into the CDI i can't see that there's anything for it to affect. Plus, having looked at the dyno runs again, after it plays up at 6.5k rpms, it did carry on recording the rpm signal past this, which suggests, as the clip was on the plug lead, that it was still sparking (or at least receiving an HT pulse down the lead) after the point it stops pulling

I'd have to do it from a remote bottle, the tank has got a 1/2" feed, although i spose i could step it down to suit. I'll have a think. Need to try and test it without going back to the dyno, i can't afford to keep going over there on the off chance it's going to work

It's worth a go i think too, the only issue i can forsee is the fuel delivery, i haven't got anything other than the pump to run it on, can i just turn the regulator off by removing the reference to it and having it only pump the bare minimum or is that going to cause issues? We did try it without the turbo connected on the last dyno run, but didn't remove all of the air take offs on the carbs, and obviously it'd still got the holes open (although it would need them with the normal vents blanked off). Biggest issue currently is the weather and not being able to test it!

I think you're right on the messing around. I've only based the drilling on the guide on here, as the carbs would appear to be the same theory as the 40mm 11 carbs. As i say, it's not going to be difficult to block the holes back off to try it, plus it won't take long. I'm pretty sure i can now get the carbs off it blindfolded

I've had a look and there's definitely no atmospheric pressure above the diaphragms. It's completely sealed other than the slide holes. What i'm suggesting is the velocity of the inlet passing the slide holes creates the low pressure above the diaphragm which lifts the slide, as you say a pressure differential GK76a carbs are different, the Bandit carbs are the same as GK73a with the oval inlets with the port above the main bellmouth, which does away with the need for the holes. What i'm saying is if 1100,1200 and now GK73/Bandit carbs have the port that feeds under the diaphragm - be it atmospheric pressure in a NA engine or boosted pressure in a turbo engine and these work well, then surely i need to have the same pressure reference? The 76 carbs also sit at more of an angle i believe, slightly down draught where as the bandit ones are side draught - Thanks for looking at your carbs, much appreciated. I might try the carbs off my bandit 4 if this doesn't work.

Right, i've employed the mind of a clever bloke at work and he agrees that the slides lift as a result of a vacuum being drawn through the slide holes and the resulting low pressure above the diaphragm causes it to lift. Logically then as the underside of the diaphragm sees atmospheric pressure in an N/A engine, in order for it to be able to draw a vacuum on the slide and cause it to lift, in a boosted engine it would need to see the same pressure as passing in to the engine in order for the air velocity to create the depression that raises the slide. This means i was, in theory, going in the right direction but possibly the hole i drilled wasn't big enough to allow the pressure to equalise quickly enough as the boost increased I guess i could have closed off one slide hole to compensate as some mentioned above but i like drilling holes in things

In the case of turbo'd CV carbs i'd say it's a case of pressure equalisation above and below to allow it to function as it normally would *or not, i'm not really sure at this point

Or too much below preventing it from lifting? This is the theory i'm working on at present. To that end, we now have 5.5mm holes into the bellmounth as per the pictures i found above (5.5 as my 5mm drill is a bit tatered) It's all back together now so will give it a go, only thing i'm struggling with is a test location without having to take it back to the dyno Unfortunately living down a dead end in the middle of a sleepy English village isn't conducive to turbo testing, and also, I quite like my neighbours Plus it's still pissing it down here and freezing cold, and the idea of testing with an ancient slick on the front wheel doesn't strike me as being very brilliant

I've not explained that very well. The underside of the diaphragms is connected to the airbox so it's only an indirect atmospheric feed. Anyway, I will not be deterred! We're nearly there!

I'd also like to say a massive thank you again for everyone's help and thoughts on this

Yep, made quite a difference to the idle

I think that's the critical thing, on these there isn't a bleed to the underside of the diaphragms from a boosted or non boosted air point of view. The underside of the diaphragms vent to atmospheric pressure, but above will see boost through the slide holes. From reading the various guides the underside needs to see boost too, hence the holes. On an 11 or 12 carb set this is taken care of by the extra port above the main inlet mouth, hence leaving them stock as this pressure reference is already there. Next idea is to open the holes up further and try it. If it doesn't work i'll block them up and try again. At this point there isn't much to lose in trying both

Putting the electrical possibilities to one side for a minute and thinking a bit about the holes i've added, ages ago i found a thread on one of the FB groups about adding this hole to the 40mm GSXR carbs and they went with a 5mm diameter. I believe the guy who posted it was the bloke from Proboost in Finland (whether this is a good thing or not remains to be seen) I went with 3.2mm as i didn't want to over do it just in case it went wrong. However, comparing the area of the new inlet hole to the area of the slide holes would it makes sense that for it to be balanced they need to be at least the same cross sectional area? For example my 3.2mm holes is 8.02mm2 whereas the 2 slide holes add up to 9.82mm2 - not a huge difference but give the location and size of the extra hole, could the velocity of the intake air be causing a vacuum as it passes over the hole and holding the slide down as opposed to equalising the pressure?? Having looked at a set of Bandit 12 carbs the dyno guy had handy, you've got the top of the oval which feeds directly under the diaphragm so it sees equal air velocity to whats passing into the carb (and having checked my 36mm GSXR11 carbs they are the same) - working on the theory that when people turbo 11s and 12s they just add the silicones and this port sees the same boost as the main mouth of the carb logically (and as per the various guides for turboing CV carbs) the underside of the diaphragm needs to see boost and that the area/size of the port to get it there can be as big as it needs to be if you see what i mean. The obvious thing would be to use the 2 big air vent points between carbs 1&2 and 3&4 but i haven't got an easy way of adding a take off to the plenum to facilitate this. I suppose i could tee into the pitot tube line that feeds the float bowls and use the 2 main vents to test it, the addition of the small hole should affect it as it's doing the same job Sorry for rambling, this is me trying to get it straight in my head as much as anything Below are the pictures i saved from that FB thread Fortunately the 400 carbs don't appear to have the bit shown in the last photo where it breaks through into that tube that then needs re sealing

I did start wondering about swapping back to the original pickup wheel, but i'm 99% sure that with the new wheel matched to the CDI it should work. The holes are to get a boost reference under the diaphragms - unlike the 36 and 38mm GSXR and Bandit carbs these don't have the extra oval bit with the port running into the area under the diaphragms. That info was from Marco's article in the Vault about setting CVs up for turboing. There's also an article on the old turbobike forum about R1 carbs which are the same principle if i've interpreted it correctly I'm still liking the slide not lifting for the stop but will investigate the electrical possibility further. Thanks to everyone for the suggestions - gives me a few more ideas to chase down

The last run with the horrible line is with the turbo up pipe disconnected, so no boost

Hmmmm, i was under the impression that the pitot tube was there for dynamic boost compensation in the float bowls Rob? Hence connected to the float bowl vents The GK76a carbs are more like the 40mm mikunis - they have a vent to atmosphere from under each diaphragm. This is what i've got as it stands The little hole in the middle of each inlet mouth is 3.2mm and feeds into the void under the diaphragm. You can just about see where i've got the big vent between the pairs of carbs blanked off that leads to the same void. The theory being that the boost in the inlet should pressurise the underside of the diaphragm through the hole (rather than feeding it through the vents from the plenum which was going to be a ballache) Each slide has 2 x 2.5mm holes - maybe the hole in the carb body needs to be bigger to match or maybe one of the slide holes needs blocking?

Yep, this is what i'm thinking. We've got the holes in the slides to allow boost above the diaphragms and i've added a hole through the carb body which runs from the inlet of the carb mouth up into the pocketed area under the diaphragms. I saw this on a thread on a turbo bike group on FB, it seemed a better option than having to use the main under diaphragm vents between 1&2 and 3&4 - which i now have blocked off Maybe the hole i drilled isn't big enough to counter act the boost effect going through the slide? I dunno But the fact we changed a load of things and it's exactly the same is what makes me think it's something more fundamental. I'll pop some pictures up of what i've done in a bit and yesterdays dyno chart Up to the point where it goes wrong it's absolutely perfect and the A/F looks great

Cheers Boss. I'm thinking it something fundamental that i've got wrong in the plumbing or in the carbs. The fact it stops at the same point everytime suggests it's not just jetting

You Called? Funny you should shout, we've been back to the dyno today - it did not go well unfortunately........ Switched to 92.5 main jets (from 100s) and got a proper fuel pressure gauge and re set to 1.5psi base pressure when running. End result - a small improvement - 22.96hp! a whole half a HP!!!!! Yay!!!!!!!!!!!! Fuck and Indeed Bollocks!!!! We tried a second CDI - same, brick wall at 6,5k rpm We tried more fuel pressure - same, brick wall at 6,5k rpm With the smaller mains - same, brick wall at 6,5k rpm I'm now working on the theory that it's something that's not allowing the slides to lift Can anyone shout up on the theory for pressure above/below the diaphragms? We've got the holes in the slides to allow pressure above the diaphragms and i've got a small hole (3.2mm) drilled through the carb body to allow boost pressure to under the diaphragms - so it should be balanced. I'm running out of ideas now Good news is the fuel reg seems to work, at approx 1.5 psi boost the gauge was showing roughly 2.5 psi of fuel pressure, so around 1psi above boost which is nice. We also tried it with the up pipe disconnected and no fuel pressure and again it crapped out at 6,5K rpm but seemed to go rich as opposed to lean

The other thing to check is if the emulsion tubes are ovalled Mine was exhibiting exactly the same symptoms - new emulsion tubes cured it

No worries mate, had me panicking for a minute there! My mind turned to jelly too, and cake, and christmas pudding, and cheese, etc etc ad inifinitum

Aye, i thought that's where we were going with it, working on the theory it's too rich and blowing the sparks out?! You mentioned you went from 145s to 137.5s a bit further up the page Yeah, sorry about that, was delicious