750 big bore carbs

triumph68

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ive done the searchs and carnt find a simple answer to will a set of vm 34mm round slide carbs be used on a 750 big bore kit if not what carbs are best .thank Andrew from down under
 

grizld1

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VM34 is a good choice on a limited budget. The best IMO would be Keihin CRF's--very serious money, and you'd be in very new territory, no user data for the XS650/750. Close in quality are the Mikuni 34 mm. dedicated 4-stroke pumper kits sold by Topham Mikuni in Germany; go to www.mikunitopham.de
 

gggGary

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Yes and jetting baselines are available for the VM34's on 750 kitted XS's Genuine Mikuni, no Chinese knock offs or snowmobile carbs with 2 stroke internal part$.
 

Signal

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It really comes down to what you want to achieve as to what carb you think is best (racing, everyday use, to look flash).

I think a good relatively inexpensive upgrade is the EX500 (GPZ500) carb conversion. I am about to do this to my road bike.
There is a good thread on this site about fitting them to standard displacement XS 650’s and 750’s as well.
 

triumph68

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im building a cafe racer for my son it will only be used for riding with his mates on the weekend . im puting the 750 kit on so the motor doesnt have to work so hard as well as the 5th over drive gear from mikesxs . i want it to have some grunt but not to silly . i put vm34 round slides on my 1979 750 bonniville and it made a hell of a difference. i hope this give you the general idea what im after thanks guys
 

scabber

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I have run BS 34 on my 750 no problem. I now have 34 mm Keihin PVK flat slides on the bike now. They perform better than my wildest dreams. Crack the throttle and go. Instant throttle response and be ready to shift. They are 4 stroke vacuum operated slides. Starts easy and good street carb. The VM 34 carbs will work lots of people run them. As far as carbs and Horsepower goes it would be interesting to see a comparison of the different carbs mule motor dyno runs. I think mrriggs has a dyno in the works. The PVK Keihin is on his current ride. He has run the gambit on carbs for these bikes. The flow rates, are all 34 mm carbs created equal. Great questions not many answers I know of.
 

650performance

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Properly jetting VM34mm carbs will support up to about 65 rear wheel horsepower on the XS650/750 engine. There is no need to go to 36 or 38mm carbs unless you have an engine built and tuned for even more power. 99.9% of the time this means a race engine.
 

triumph68

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thanks guys i got the answers i was looking for now i got to find the best place to buy them any ideas thanks andrew from down under
 

Mannyroad

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Yes and jetting baselines are available for the VM34's on 750 kitted XS's Genuine Mikuni, no Chinese knock offs or snowmobile carbs with 2 stroke internal part$.
I've been looking for such baselines but haven't come across any in my searches. Where can I find these, or what is the baseline? Additionally, are there timing modifications needed or is the stock 38 degrees still pretty much right?
Reason for the query is that my 750 big bore, 270 rephased motor is pretty gutless. I'm running 210 mains, upped from 190's. Plugs look perfect colour and timing seems close to stock at around 18ish idle, 38ish at 3500rpm. But the wife's 500 Rebel can leave me behind on top end, as my motor struggles to haul me up to 75mph!!!
 
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650Skull

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have you gone through the VM carb guide..........i cant say if they have the jetting for the XS650 as i have never read it
https://www.xs650.com/threads/carbs-carburetors.43/

this is what i was looking for............peoples jetting for VM's........(before you do anything on those no's, read the whole thread)..............Caution, there is no information on elevation the bike is running at nor the bike setup, as what exhaust ect................also note Griz1's post #75
https://www.xs650.com/threads/help-me-id-these-and-can-they-be-tuned-mikunis.6570/page-4#post-547589

the search list under "VM Jetting"
https://www.xs650.com/search/272614/?q=VM+jetting&o=relevance
 
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atom4488

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I've been looking for such baselines but haven't come across any in my searches. Where can I find these, or what is the baseline? Additionally, are there timing modifications needed or is the stock 38 degrees still pretty much right?
Reason for the query is that my 750 big bore, 270 rephased motor is pretty gutless. I'm running 210 mains, upped from 190's. Plugs look perfect colour and timing seems close to stock at around 18ish idle, 38ish at 3500rpm. But the wife's 500 Rebel can leave me behind on top end, as my motor struggles to haul me up to 75mph!!!
Hey Manny, without necessarily having any immediate BBK experience, if your wife's Rebel 500 is leaving you in the dust, there must be something significantly wrong with your setup. As you say you have clean plugs, have verified timing, I would start with checks of throttle displacement ( are you getting full lift at the slides?), air inlet restrictions, compression, .... must be something way off. A rephased 750 on VM 34's should fly. Keep us posted!
 

grizld1

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Sadly, Manny, there is no single "right" jetting. Carbs operate on compromise. Some combinations work, some work better, some don't work at all. Street riding demands are much more varied than race environments, so don't follow by racing setups unless you're setting up to race.

I don't know how many times this address has been posted, but here's a starting place: www.amckayltd.com/vm34-36.pdf . Print it off, read don't skim, and ask questions if there are parts you can't follow. I'll tell you right now that your mains are too fat, and you make no mention of the rest of the brass. Needle? Needle jet? Pilot jet? Float level?

And of course there's that weird tendency (don't think I'm immune) to start troubleshooting at the carbies, when everything else should be checked first. First step in your situation? I'd do a leakdown test. I have a suspicion that your motor has one or more valve train defects.

Good luck to you.
 
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gggGary

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I've been looking for such baselines but haven't come across any in my searches. Where can I find these, or what is the baseline? Additionally, are there timing modifications needed or is the stock 38 degrees still pretty much right?
Reason for the query is that my 750 big bore, 270 rephased motor is pretty gutless. I'm running 210 mains, upped from 190's. Plugs look perfect colour and timing seems close to stock at around 18ish idle, 38ish at 3500rpm. But the wife's 500 Rebel can leave me behind on top end, as my motor struggles to haul me up to 75mph!!!
210 mains in VM 34's seems big.
What pipes, mufflers?
a pic or two of the bike?
Have you done compression tests?
 

Mannyroad

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Thanks for all the info guys. Much appreciated. I did a mains change as a quick stab, to see if it made a difference on full throttle. I'm not too clued up with carbs. The motor was professionally rebuilt, big bored and rephased 1500 miles ago by Daryl Hutcheon in Oz, who made a living doing this to XS650s. I acquired the bike with only a handful.of miles on the clock. Now it's pretty much run in its disappointing to find it's so gutless. It has a Probe electronic ignition and my timing light suggests idle and 3500rpm up timing is about right. On my mate's rolling road it pulled through to 7000rpm no problem but yesterday on a ride with a few mates it wouldn't pull more than 4500 in top gear, and it took its time to get there. I so wanted to to get off and kick it's ass. I shall read through the techy info in the links and investigate what other brassware \needle info i can pull from my carbs and report back here for your views. Ive always thought setting up carbs a bit of a dark art tbh and have never been able to understand it like id like to. A pic of the bike is attached. I love it, but not the way it's performing, which is a shame after I've done so much to it.
 

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arcticXS

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VM34 is a good choice on a limited budget. The best IMO would be Keihin CRF's--very serious money, and you'd be in very new territory, no user data for the XS650/750. Close in quality are the Mikuni 34 mm. dedicated 4-stroke pumper kits sold by Topham Mikuni in Germany; go to www.mikunitopham.de
I suspect Grizld meant Keihin FCR flatslide pumpers. Those are indeed great carbs, I have a set of FCR 39 on my Ducati M600 with a 680 kit, and they work great. Better throttle response, better fuel mileage, and way better low rpm running than the OEM Mikuni 38 mm flatslide CV carbs.
On my XS, I have the 34 mm Mikuni TM/RS flatslide pumpers from Topham. These are great as well, and definitely the most convenient option for an XS.
If period appearance is important, roundslide Keihin CR carbs are a good alternative to Mikuni VM.
 

Signal

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Nice bike, something is wrong it should rev higher than that with stock gearing. Try a run without those air filters. Some filters that look like the ones in the photo are known to cause problems. The problem is with the mounting flange blocking air circuits. Better air filters are K&N or Uni pods. Unipods are ok but not as free flowing as K & N.
Does your mates dyno have a fuel air meter, you will be able quickly see what needs to be adjusted.
 
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