I need your VM34 Carb help

Zero333 · Posted: April 7, 2009, 1:54 pm Post subject: I need your VM34 Carb help

My carb tuning attempts have officially driven me wild. Please let me know what I should be doing because at this point I am at a loss. I recently picked up an XS650 with VM34s. The bike was not running when I got it. I have since cleaned the carbs, fuel system and changed the plugs. The thing is super clean. I can now get the bike to start up right away when the choke/starters are on and after a few minutes of warming up once switched off the bike idles fine. Once I try to give it the littlest throttle the bike sputters. If I slowly apply the throttle the bike with rine up the RPM range without a hitch. I read this www.mikuni.com/pdf/vmmanual.pdf manual cover to cover a few times but I am still not getting much if any progress. I have checked the intake runners/manifolds and they have no cracks. Below are my current settings

Pod intakes
Open pipes (I grinded off the mufflers, they were rusted to hell, plan to put a 2-1 Tracker set up on eventually)
Electronic ignition (came with the bike)
Bike has been rephased (done prior to my purchase)

Needle at middle setting
air screw 2 turns out

What else should I be adjusting? With the starters pulled up the bike runs pretty well, though I know I should not be reving the bike with the starters on. Please help me. I need to get this thing running hard before I move on to the rest of my projects. I contacted a local bike shop and they said they would fix it for 200 bux. I really want to avoid that because that would take a chunk out of my budget to finish this project.

Thank you in advance.

xs1961 · Posted: April 7, 2009, 3:34 pm Post subject: Re: I need your VM34 Carb help

Have you got the correct Needle jets & needles installed?
Are the carbs balanced?

Zero333 · Posted: April 7, 2009, 3:43 pm Post subject: Re: I need your VM34 Carb help

xs1961 wrote:

Have you got the correct Needle jets & needles installed?
Are the carbs balanced?

I only have the Needle Jets and Needles that came with the carbs. How can I tell what type/number I have. I am pretty sure the carbs are sync'd. I have both slides opening at the same exact time and the idles are set to identical hights.

Retiredgentleman · Posted: April 7, 2009, 3:48 pm Post subject: Re: I need your VM34 Carb help

Here is a link to good info on VM carbs;

xs650temp.proboards.co...read=10538

Zero333 · Posted: April 7, 2009, 3:53 pm Post subject: Re: I need your VM34 Carb help

Retiredgentleman wrote:

Here is a link to good info on VM carbs;

xs650temp.proboards.co...read=10538

Thank you. I will read up.

xs1961 · Posted: April 7, 2009, 3:55 pm Post subject: Re: I need your VM34 Carb help

Youll need to take the carbs off the bike , remove the float bowl, unscrew the main jet then tap/push out the needle jet upwards through the cab & read the tiny number & letter stamped on the side - it should be a P6 (mine are anyhow) .
Also , check the needles match the jets , they should be 6DH4 's .
Have you balanced the carbs by placing an approx 1/4" -1/2" drill-bit or steel rod etc. under the slide (from the air filter side)& adjusting the top nuts/screws so as the two cables are identically adjusted to lift together?

Zero333 · Posted: April 7, 2009, 4:23 pm Post subject: Re: I need your VM34 Carb help

xs1961 wrote:

Youll need to take the carbs off the bike , remove the float bowl, unscrew the main jet then tap/push out the needle jet upwards through the cab & read the tiny number & letter stamped on the side - it should be a P6 (mine are anyhow) .
Also , check the needles match the jets , they should be 6DH4 's .
Have you balanced the carbs by placing an approx 1/4" -1/2" drill-bit or steel rod etc. under the slide (from the air filter side)& adjusting the top nuts/screws so as the two cables are identically adjusted to lift together?

That is the method I used for balancing. I made sure they lifted the same, then measured the idle hight, then checked the lift again.

yamaman · Posted: April 7, 2009, 9:16 pm Post subject: Re: I need your VM34 Carb help

Start with the basics, enough fuel flowing into carbs?
Float level ok? Float needle valve ok?
If it runs better with the enricheners on, then it's obviously a lean issue.

Do you know if the bike ever ran with the re-phase? It could possibly be a timing issue?

Just re-read your post, I'm betting (assuming the carbs are fully cleaned) lean due to either poor flow into carb or float settings out!

Cheers

xsjohn · Posted: April 7, 2009, 10:14 pm Post subject: Re: I need your VM34 Carb help

Might also want to degree the cam on both sides........if the PO hosed the cam timing even a bit it may never tune..........had an advanced cam in mine for a bit and it was totally impossible to tune...to say the least........

Even 2 degrees from stock throws it into artificial leaness and the cylinder won't fill..........

xsjohn

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

When it comes up off idle the pilot jets take over until the main jets pick up the load. If it idles OK but don't come up off idle, you probably got pilot jet trouble.

Zero333 · Posted: April 8, 2009, 2:59 pm Post subject: Re: I need your VM34 Carb help

Thank you for all of your replys. I plan on taking the carbs off again asap and checking the numbers on all of the jests. I am then plan to buy a few different sizes of each to do a final tune. I am told by the previous owner that it was running fine for a long period of time. The owner then messed his knee up and the bike sat for over a year. Hopefully after a full break down, clean and jet swap this will be running strong as ever.

Zero333 · Posted: April 10, 2009, 9:23 am Post subject: Re: I need your VM34 Carb help

Hey guys How should/Do I check/adjust the float level? I want to make sure this is not the problem before I order a bunch of replacement jets. Beinth that the bike was running with this setup for some time I am guessing i just need to tune the current set up.

yamaman · Posted: April 10, 2009, 10:23 am Post subject: Re: I need your VM34 Carb help

First check you've got adequate flow out of the tank & lines into the carb.
Then see page 12 of the Mikuni tuning manual. Take the time to get this spot on!

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