Carb conundrum (help please)

[Otatts]

Hey guys. I’m appealing to the brains trust here. I’ve been battling my second bike for over a year now (between kid, work etc) trying time get it to run right.

its a more or less bog stock ‘restored’ 1988 750j. Running a DNA high flow filter in airbox and 4 into 1 headers into an unknown can.

starts up and idles fine but when I rev it it cuts out and dies at 1/4 - 1/3 turn. If bring it up slowly it gets to about 5Krpm and quits. If I pull the choke on it gets through the hole and goes off it’s head once through it

currently has New stock suzuki 750 needles, new suzuki emulsion tubes, Mikuni 117.5 main jets, new pilot jets. 
 

 I’ ve tried  122.5, 125, 130 mainjets and top, middle and bottom clip positions for the needle for each mainjet size. Each time it just moves the hole around a bit but doesn’t get rid of it

carbs have been ultrasonically cleaned and balanced and new rubbers in the float bowls (gaskets and o rings) 

 

Edited May 1, 2020 by Otatts

[Otatts]

Bike barely ran when I got it but I thought it wouldn’t take much to fix it (how wrong I was!!)

carbs were an absolute mess when I got it

needles were a ‘touch’ worn

[Otatts]

Cleaned them up and put a dynojet kit in them and had a flat spot at around 8K so big that the bike would nearly stop. have been chasing the issue ever since. 
now I’m back to suzuki needles etc

but a short list of stuff done/changes chasing this issue:

tank cleaned, rust treated and sealed, new fuel tap (OEM), new fuel lines, fuel filters installed, needles and jets as mentioned. Swapped coils and leads from other bike, swapped CDI, swapped complete engine (original one had a knock in it) changed header from 4-2-1 to 4-1 (Made the flat spot at 8K wider but shallower)

tried every jet size in the dynojet kit (120, 124, 140 & 144) and top,middle,bottom needle positions all it did was change the size and position of the hole but never get rid of it.

went back to OEM and the flat spot is at 1/4 turn now

[Otatts]

I put the carbs from my 7-12 (36bst with dynojet 1100 kit) and put 124 dynojet mains and it ran good (very small hesitation at 8K)

thinking my 750 carbs must have wear in the throttle shafts I put all the OEM 750 internals (new) in the working carbs and I have the same problem again!! Gah!!

 

im at a loss...

Edited May 1, 2020 by Otatts

[Captain Chaos]

in the slides there are two small holes each side of the needle. How big are they? They affect the speed at which the slides go up when opening the throttle.

[Otatts]

The ones in it only had one hole. (2mm)

the slides in it Now have had the dynojet mod done to each hole (dill, tap and insert grub screws) grub screw holes are approx 2mm diameter....so 2 2mm holes

Same problem for either slide

[MadJack]

Let me get my head around all this swapping bits about. You have a set of carbs that don't work & a set that do. When you swap the internals from the set that do work into the set that don't work then they still don't work am I right???

[Otatts]

@MadJackyes correct.

sorry if this is confusing as I’ve been trying to find out what the issue is by a process of cancelling out.

both carbs are bst36cv’s one with dynojet 1100  kit and one with OEM 750

dyno jet carbs worked ok (With smaller mainjets to suit the 750) then put OEM 750 needles, emulsion tubes sand mainjets in and the flat spot is at 1/4 turn

Edited May 1, 2020 by Otatts

[Otatts]

This pic is to show the difference in needle profile between OEM and dynojet 

 

dynojet left. OEM right

the OEM ones are the fucked old ones hence the massive shoulder up top....new ones are in the bike at the moment 

 

flat spot seems to be where the taper changes on either needle......that’s me guessing

[Otatts]

Do you think maybe 117.5 main jet is too big for OEM needles??

stock is 112.5 

[Wee Man]

Have you checked the needle jet/emulsion tube for wear? If the original needles are worn, then I'm guessing the needle jet will be too.

[Otatts]

@Wee Manyeah when I bought the dynojet kit originally it didn’t come with emulsion tubes so I got a set of OEM ones from suzuki.

Now I have OEM needles, emulsion tubes, Pilot jets, float bowl gaskets and o rings. All new

Edited May 1, 2020 by Otatts

[Jaydee]

5 hours ago, Otatts said:

Running a DNA high flow filter

Loads of good info on the fueling but nothing said about the airflow.

A lot of these internal airbox filters are so free flowing, they're more suited to stage 3 jetting. I have a pipercross example, it's a foam filter which is far more free flowing than the K&N version (which comes with a velocity ring to choke the airflow more if needed)

I'd try a stock filter and then play with jetting. You'll end up chasing your tail of you have too many changes going on.

[Otatts]

Thanks @Jaydeegood info.

kept thinking as it has an airbox so it couldn’t be that free flowing!......could be wrong

 Restrictor o ring is already in......my next experiment was to put my hand over the opening as I got to the flat spot to see if it made a difference. It would at least tell me if it’s getting too much air

thanks for the input . I‘ll let you know home it goes 

[Gixer1460]

Are you sure the really worn needles are OEM as they look aluminium and very Dynojet in their profile? Worn that much one would assume the emulsions are also? With a freeflow air filter it won't flow like pods but will flow enough to be classed stg 3. The 1100 DJ kit has MJ's, needles, emulsion tube, slide lift hole jets - don't know what the 750 kit has so can't compare, The DJ kits usually just work straight off and need a dyno run to check MJ is correct - at least that's what i've found!

[Dezza]

My guess is there's still some cack in one of the fine drillings in one or more of the carb bodies. Sometimes more than one ultrasonic cycle is needed.

[Otatts]

@Gixer1460correct. The DJ 1100 kit had all those bits and worked straight off the bat for my 7/12 running pods. Even had it dynoed and the air/fuel mixture was pretty bang on. 

750 DJ kit didn’t have emulsion tubes so I just got new standard suzuki 750j tubes....Wrong move maybe? Also set it up as stage 1 not stage 3 like my B12

 although I did try all the MJ sizes (120-124-140) except the largest 144 because I thought there’s no way it could be pulling that much air......although I could be wrong. That was with the paper style airbox filter that was in the bike when I bought it so I replaced that with new high flow as the mechanic I took it too said it could be old and sucking bits into the motor (was never sure on that logic)

can’t be 100% sure the needles I pulled out were OEM but the profile is very similar to the new OEM ones I bought from suzuki (apart from the massive wear groove up top)...e.g rapid taper at the bottom of the needle. Both DJ needles (1100 & 750) have the same profile.....square bottom almost no taper

would have put a pic up of the OEM needles I bought but I can’t face taking those fackin’ carbs out again!! Done this about 20 times over the last year and it’s a pain in the arse everytime! 

Bike has OEM needles, emulsion tubes, pilot jets and Mikuni 117.5 MJ’s in it now so going to try choke the airbox by shoving some cardboard between the battery and airbox tomorrow so I don’t have to pull the entire bike apart again and see what happens 

[Otatts]

@dezza I put fine wire down all the holes last time they were out of the bike. No sign of any gunk.

I’ve put the DJ 1100 kit in them now and will try them in my 7/12 to see if it’s these carbs. Just waiting on one plastic needle ring from suzuki as I lost one with all this faffing around

if it still runs like shit then there must be still crap in them or they’re sucking air somewhere 

Edited May 1, 2020 by Otatts

[Otatts]

10 hours ago, Jaydee said:

Loads of good info on the fueling but nothing said about the airflow.

A lot of these internal airbox filters are so free flowing, they're more suited to stage 3 jetting. I have a pipercross example, it's a foam filter which is far more free flowing than the K&N version (which comes with a velocity ring to choke the airflow more if needed)

I'd try a stock filter and then play with jetting. You'll end up chasing your tail of you have too many changes going on.

@JaydeeOk so I warmed the bike up this morning then stuffed a sock down between the battery and filter opening to cover half the hole and it seems to have got rid of the flat spot!  
 

next question would be what would be the best standard filter to use? Ones from suzuki are $65 AUD  

guess I could make up another restrictor to go inside the existing one......save me buying another filter after spending countless $$ on this thing so far

Edited May 2, 2020 by Otatts

[icprey]

Hope your getting this figured out. Just know you are not alone. I believe there to be plenty of us trying to get 30yr old stuff running right!  I am so sick of pulling the carbs, I'm on Carb vacation right now

[Otatts]

Whipped this up while bubs was having her arvo nap. 
started it up and it still has a flat spot but if I bring the revs up gradual it makes it through the flat spot so that’s an improvement! 
bike also woke the baby up so I’m going to have to wait till later before making more adjustments 

[Otatts]

@icpreyi feel like I’m finally making head way here after chasing my tail for 12 months. 
I’m just pissed as my other bike was bang on first go running pods and this one has been a pain in the neck and it’s almost stock!

also swapped my K6 1000 track bike for this thing and it was suppose to be my daily Ride while I worked on my 7/12.....worked out the other way around!!

 

glad to hear I’m not the only one 

[icprey]

I've got two 7/12's kicking my butt right now with a variety of carb choices.  I've been coming in dejected and eyeing Brand Spanking New Mikuni's, but remain determined to use what I have and figure this out.  If Sept rolls around and I am still sucking my thumb, I might crack!!!

Keep us informed. Your plight may help mine or someone else's.

[Otatts]

@icpreyI just bit the bullet and bought a set of RS38’s for my 7/12 for when I put the DOT head on it as I was certain one of the two sets of bst36cv’s I have here was busted but now it look like they were both fine all along and it’s filter that’s creating the lean mix and not a vacuum leak. 
fuck it I needed an excuse to get the RS38’s anyway......can always sell one set 36’s to help cover the cost when all this is said and done.

 

anyways I hope this helps you solve your problem.

lemme know if you want my carb setting for my 7/12. It runs absolutely fine

[Blubber]

Some the aftermarket filters already have that insert in it.

 

This is just a quick search but you get the idea.