Carb conundrum (help please)

[Otatts]

Thanks @Blubber mine already has that insert too but it’s still breathing too much hence making another restrictor to go inside that.

i might do as @Jaydeesaid an just get a stock filter and start from there.

i just want the bastard to run properly enough to get me to work. Don’t care about outright power.....got the other one for that

Edited May 2, 2020 by Otatts

[Otatts]

Ok so I managed to build up enough enthusiasm to face this thing again. So it’s time for an update

i found the old air filter so decided to see what that would do. Ran a bit better as in it would get through the flat spot at 5-6000rpm but it’s still very much there. 
 

popped down to the suzuki dealer and got some stock size 117.5 mainjets and a set of o rings for the carb rubbers on the head. Got home and my new air filter had turned up.

i decided to cancel out the original carbs from this bike having any vacuum leaks (though worn slides or worn throttle shafts) by installing the dynojet 1100 kit I’d been using in my 7/12 for the last two years and taking it for a run in the 7/12

 

bike ran perfect apart from some popping on closed throttle from the Smaller 750 pilot jets.......so there’s nothing wrong with the carbs. Which leads me to the conclusion it must issue between the needle/main jet Combo 

[Otatts]

Being that I’d half dismantled the bike to change the air filter it made the task seem a little less daunting

pulled the carbs out of both bikes. Got the original carbs for the 750 and put the OEM needles, emulsion tubes back in with the 112.5 mains 

decided to change the o rings and things went south quick as it does with this bike. Phillips head screws were seized in there and couldn’t get in there with an impact driver so the airbox had to come out....which meant the alternator had to come out which meant taking the sprocket cover off 

cracked most of them but had to drill a couple out that were already buggered from someone having a go at some point in its history 

[Otatts]

It was worth the effort after seeing the original o rings out

ports cleaned up and bike back together with new regular filter And ready to fire up

I’ll spare my neighbours a 10pm engine revving and wait till tomorrow to see if this is all in vain 

[Gixer1460]

Doesn't help you now but i've never found one of these screws able to resist the use of a fat crosshead bit in an impact driver and using a BIG club hammer! Copperslip the threads on replacement - threadlocker is really overkill for these!

[Otatts]

@Gixer1460No doubt I’ll have to take these bastard things out again so if I do I’ll pop the screws out and copperslip them. I replaced them all with hex head bolts from one of my blandit motors. Looks like suzuki finally got the message in later years that Phillips heads are useless for anything apart from ikea furniture 

[Otatts]

Haven’t built up enough backbone to start it up and face the prospect of having to pull these carbs out yet again but I have been rooting around on the Internet (not like that) and have read about how notorious the short stroke 750 is for being sensitive to the slightest mod (yay!)

I emailed Dynojet to ask if my 750 kit not coming with emulsion tubes and my 1100 kit did was a mistake and got this reply

 

 

[Otatts]

So I have a ‘euro’ kit in my australian 750 which I have no idea where that puts me. I’m guessing The Australian Bike’s are probably closer to the American ones than the euro.

Anyways I’m not buying another sodding jet kit for this bike. 
if I can’t get the OEM gear to work with this exhaust then I’ll stick the 1100 kit back in there as it ran ok like that and just live with it until I sell it and it’s someone else’s problem

[Blubber]

2 hours ago, Otatts said:

 

if I can’t get the OEM gear to work with this exhaust then I’ll stick the 1100 kit back in there as it ran ok like that and just live with it until I sell it and it’s someone else’s problem

You dont want to be that kind of previous owner... do you ? " put away running" or " just needs carbs cleaned"

It doesn't take much to go from P.O. to P.O.S. 

[Otatts]

@Blubberthe “just needs the carbs cleaned” is the line I got on this lemon. Spent WAAAAYYYY too much money on this thing as it is not to mention time!! 
it runs ok with the 1100 kit. Tiny flat spot at 8k but nothing major

they’d painted the motor in the bike. Motor was shagged, carbs were rooted, tank was rusty. I’ve fixed and replaced all that, Lucky I had a spare 748 engine. 

bike will be way better than I got it and they won’t spend as much as I have so I don’t feel Bad at all. I’ve just run out of patience with it.

rather put the time and energy into my 7/12. The lemon has made me sour 

[Otatts]

P.S I’d tell them exactly what’s going on with it......I’m not an asshole like the guy that sold me this 

 

just wanted to be clear on that......I feel bad now. Ha

[Dezza]

OSS is Euro based so we're all arseholes not assholes

 

What about a replacement set of secondhand carbs and then go from there?

Edited May 9, 2020 by Dezza

[Otatts]

Ha ha! We say arseholes too in Aus. My bad @Dezza

i think the issue is I don’t have an OEM exhaust and I’m going down the path of OEM parts for the carbs after the Dynojet kit didn’t work.

like the guy from Dynojet said if I got the right kit it ‘might’ work but there’s no guarantee. Shit thing is I got this kit through the AUS distributor for Dynojet so you’d think it would be the right one! 
 

might friend leant me his carbs set up 100% OEM and they worked.....again not perfect but better where I’m at now. So that’s the plan......go full OEM and see what happens
 

the bike had 117.5 mainjets in it when I got it so I thought it may have worked at one time with those but I’ve tried everything to no avail

Edited May 9, 2020 by Otatts

[icprey]

Here is an old article from Cycle Canada on a '88 Short Stroke exhaust set-up using stock airbox and filter.  You have a set-up that was proven at the time and track tested.  This is a Canadian model so you might want to check the differences if any from Aus.  I have that manual in the garage if ever you need me to check.  You know I too am going through some carb moments right now, but I feel at times I am struggling to get the carbs back perfect, meaning I have not completely disassembled every single piece and replaced, but rather cleaned the jets and passages hoping the rest is working ok.

Anyways for a free breathing exhaust all they needed was a 115 main up from 112.5 and lowered the needle one notch (did 36mm have adj needles like you show previously) to lean it out some and stretched the diaphram springs 6mm.  These were tuning recommendations from Yosh for free flow exhaust with stock carbs and filter.

Pretty small changes from stock to see the issues you felt when back at stock, but as you said it seems the 748 reacts more sensitive.  Only ever had one 748 in my youth (a few 749's) and stepping down 2-3 gears always solved everything.  I don't remember caring about driveability...hehe.  Oh how times have changed.  Have a few 748's lying around here for the last 20+yrs as well, always wondering what do to with (had a 815 short-long combo in mind).

Hoping the above can lock you into a setting and stick to fixing the issue happening with a known setting. Also just be sure you are not using 1200 36mm carbs as they are internally different and won't react to GSXR settings.  What is your carb stamp on the body side?

 

[Lachie04]

When I rebuilt my short stroke carbs I needed emissions tubes and needles couldn't get E24 so got matching uk E02 set as thought that would probably be the least restricted all the different countries had different set ups due to emissions regs as far as I know only the Cali models had all the stupid emission junk on them. Did take some effort running the float spot out but got it eventually after a good weekend on and off again and again

 

PS you should check and replace the slide guides for wear there's a place in the states that sells them as that's what's probably caused the wear to the top of the needle I try and find the US site guy was really helpful

 

Edited May 9, 2020 by Lachie04

[Otatts]

Wow thanks @icprey

i’ll look into that. Hopefully with 112.5 mains it’s close. Hoping maybe the richest setting on the needle will be enough with those mains....we’ll see

i have no idea what the can is as there’s no name or markings on it but it looks the part so I’m really trying to get that to work

It did have a decent 4-2-1 system on it which I will put back on at a later date if I can sort this issue out (and open up another can of worms ha ha!)

carbs are definitely 75036cv’s I have the same ones in my 7/12 with an 1100 DJ kit in them. Bandit 36mm have taller tops/different springs/needles

[Otatts]

@Lachie04flat spot you mean? 
were you running an airbox with aftermarket pipe? What changes did you make setting them up? It’s ok you can speak up....we’re all friends here ha ha!

 

also my carb slides only have one hole in them and it looks factory. Anyone else seen those before??

[Otatts]

Hey that was a great little article @icpreythanks for posting it

[Lachie04]

 

Yeah damn auto text lol

https://store.moto-lab.com/products/655/throttle-slide-guide/893/suzuki/gsx-r750/1988/usa

There's also a article on that site about the wear to the guide 

What worked for me may not work for you  I was running standard air filter and a home made exhaust

Here's something I wrote on some other site

So rechecked the Float heights and I don't think I must have done it right the first time and ended up adjusting them all to around 14.5 mm which was about a mm lower then were they were set so this should make the bike a little richer. Put the pilot down to 3 1/4 and was much better but needed more.

Down to 2.5 and was way worse so ended up on 3 turns out and pulls really nice when warmed up.Great

I ended up syncing the carbs at 2250 revs I had started out just doing it at about 1500 revs as I had read you should just do it just above idle but decided 2250 was probably a better place to do it. I don't know but anyway.

The Factory Pro tuning says you should tune the low end for smooth idle and 4k 2nd gear operation which it does, in fact it goes great at any acceleration.

So now it is just at 5th or 6th gear cruising it has a little lag at 4,500 to 5,500 revs roll on and if I slightly drop the throttle a little feels like it's starving which is more just annoying then anything.

Having spent so much off time trying to sort the tuning I've just decided to get it back on the road proper and ride it for a while maybe if it gets to me too much or when i get bored I'll rip it apart again and try the float at 14mm i'm thinking it maybe still be a little lean in that area or maybe play with the needle again to see if raising it back up cures the lag in the 5-6 gear range midrange cruise state, but is accelerating so great in all other ways, I don't want to stuff it up .....

Postscript:

cause it was quick and didn't take a lot of disassembly I lifted the needle back to the original middle position and it fixed the leaned out midrange but somehow the lowend was bogging down. So i relented and took it all apart again. Went over the float heights spent a bit of time on this and made sure they were all at 14.5mm turned the PA screw down to 2 3/4 and vacuum sycn'd the carbs at 1750 this time.

Now it pulls absolutely great through all the rev range and there is just the slightness hesitation @ 5000 in 5th and top at cruising speed which i can live with ATM. Some thoughts on this is either the pipe has a little flat spot around this range which is why it's been difficult to tune this bit out or maybe I could go one size up in the main as it's just struggling a bit just on the needle.

So the carb setup I've finally got is 5EZ61-3 needle jet, 105 Mains,37.5 Pilot jet

That's was on a teapot motor and the main jet could have gone bigger but I was starting of standard and working from there up hence the small main jet but got tired of tuning so when it started going well l left it alone

 

Edited May 9, 2020 by Lachie04

[Otatts]

Great info @Lachie04thank you

i want to get as close to standard setup and start from there. I think that’s the only way I’m going to make progress 

didn’t realize float height was so crucial 

[Lachie04]

These guides make the slides loose and let them wobble around causing accelerated wear 

https://store.moto-lab.com/articles/bst36-slide-guide-wear-indicators

If rebuilding old carbs I think is a must and worth checking

 

[Lachie04]

New vs old 

Wear to the slide face and at the bottom  we're ether slide lands

 

[Otatts]

Wow that site is a great find @Lachie04

those slide guides are impossible to get so it’s great having that as an option. Both my BST36cv’s are pretty flogged out there but it doesn’t seem to bother the 7/12 as I’ve tried both sets in that motor 

I’ll keep that site open for sure though!

[Otatts]

112.5 worse than before. Tried at middle Needle  position  Then richest still got a massive flat spot at 1/4 turn. Choke helps it get through. Same as always 

tried turning the pilot screws another 1/2 turn out (To 3 turns out in total) still the same (seeing as the choke helps it) Dies at 1/4 throttle. Same result on 112.5, 117.5, 120, 125, 130. Incidentally 130 was the point where the choke stopped helping 

im out of patience. This bike has broken me. 1100 DJ kit going in it and that’s it. 

[icprey]

Lachie04  great link!  Don't know if that is well shared and known, but I had never came by them.

Very curious about what will happen next here.  No popcorn emoji but let's see how this drama unfolds 

 

 

 

 

Edited May 10, 2020 by icprey