VM34 R/S carb won't cooperate

Roto36

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Trying to dial in carbs using dead cylinder method. For some reason the right carb seems to not cooperate.

Background: this is a rebuilt engine with Mikuni VM34-168 carbs (using remote fuel bottle)
When I started tuning carbs the bike idled good, but was smoky appeared super rich. Checked the jets found a 40 pilot, 250 main and 6f5 jet needle. Plugs were wet and sooty. As a precaution I rechecked the typical stuff, timing, timing chain, then the valves. The valves were noisy to start with but when I checked them they were quite a bit off. I reset the intake to .003 and exhaust to .006, a lot smoother sounding now.

Figured the jet setup was to rich to start with. So I put in a 27.5 pilot, 240 main and 6DH4 jet needle, clip at mid position, reset the carb slide heights using 7/32 drill bit, air screw at 1.5 turns out, set the floats to 22mm and put in new plugs. For good measure I used compressed air on the jet openings before installing new ones.

No problem starting up and idles pretty good. Smoke is way reduced. So I let it warm up for about 2 minutes then start the dead cylinder test. Ground the right side plug to the head and unplugged the vacuum hose. Dialed the left carb down to 900rpm any lower and would stall.
Air screw had noticeable effect at 3/4 turn out and at 2 1/4 turns out so I figure 1.5 turns is my sweet spot. I figure this is my starting point for a good idle.

The right carb is where I'm having issues, will run smooth around 1000 - 1200 any lower and it'll putter out. So at this idle it runs smooth for about 10 -20 seconds then the idle starts to drop and finally stalls out. I get a little "Pfh-tt" spit back out the carb, not a backfire but more like it's spitting back a puff of air. I have clear inline fuel filters on each side so I can see the fuel. The right filter seems to run out just sitting there not running. I can pull the hose off and fuel flows fine, but I just don't see a fuel level like in the left side filter. But the bike will still start just fine. As it runs I can see fuel come back in the filter and maintain a level.

So am I running out of fuel in the right carb because it won't run continuously? Or do I need to pull the carb and check floats again and re-clean the carb?

Thanks
 
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I synched on the first go around with the richer jets. But when I changed the jets I know I have to start the process over again by performing dead cylinder method on both carbs then sync using a manometer. If I can get that right side carb to hold an idle my next step is to sync.
 
Its no use trying to tune the mixtures if your slides are out by a country mile....
Use the cable adjusters at the top of the carbs to adjust the slides to an even position @ W.O.T. then you might get a better result trying to dial in your mix.
 
hotdog, as mentioned in my first post I set both of the lift slides using a drill bit. They both lift at the same time. And yes I should and will follow up with manometer. But when performing dead cylinder test the engine is only running one carb at a time so I'm not sure how synchronizing applies to the question of why the right carb won't hold an idle on it's own.
My main concern is when running both carbs together the right carb could potentially peter out and the left would end up supporting idle for the engine.
 
your main is massivly massive, try a 180-190. also is the air jet still in? its on the intake bell side, bin that sucker. check the tech section
Thanks I'll check the air jet and see if that helps.

Haven't got the chance to work with mains yet, wanted to get the idle circuit worked out.
 
There is a jetting baseline for the VMs that works for the 650 but you don't appear to be using it. Why re-invent the wheel? With all the jetting options available for the VMs, it could take you a lifetime to work out your own baseline. You've made no mention of what needle jets you have. If you're not using a P4 or P5, you may not have much tuning luck. Your needle choice is wrong as well. Read this .....

http://xs650temp.proboards.com/index.cgi?board=carbfaq&action=display&thread=730
 
Hey 5twins, the rest of his jetting is generic, so the NJ's will be too--Q2's. P5 or P6 is the usual for street-going XS twins, you wouldn't want Q2's in anything but a full race motor under race conditions.

As far as I'm concerned the newbs can read 'em or weep. If they can't find what they need they ain't looking. You and I wrote this stuff once so we wouldn't have to keep writing it, and I'm tired of playing "Write it again for me."
 
I've been following this guide http://www.amckayltd.com/VM34-36.pdf step by step, but I wanted to see how the bike ran with what jets I have on hand. My original question of what would cause the right carb to act different from the left is obviously premature without the proper jetting.

No need to write it again, I'll start over with the baseline settings as noted in the guide. Didn't mean to anger anyone with my newby questions.
 
With the crap you have in there you'll have run the mix screws out past the max 3 turns before you get significant response. If you still don't get response, check floats, float level, and float valve (needle and seat).
 
5twins the needle jet is 159 P5

Ordered the jets suggested in the guide.

While waiting for order to arrive I wanted to see where my compression was at. This leads me to a question of high compression and is it related to carb issues or not. Does a high compression engine have an impact on jet configuration?

My test kinda shocked me as both cylinders tested north of 240psi. Performed this test 3 more times, warm and cold engine. Tested my compression tester against air compressor and was spot on. Rechecked the valves twice .003 intake .006 exhaust. Stopped all testing here and started reading about high compression engines.

My intention was to have a daily cruiser not a hotrod. I read this post by xsjohn about how a high compression engine will tear itself apart in the long run. So now I'm worried I have this high performance engine that might cost a bunch to dumb it down to a lower compression.

When I was shopping for a daily cruiser this thing landed in my lap. I did not build this bike, my in-law spent the last 4 years of his life building this thing before cancer got him. I know he spent a ton of dollars on it but I have no idea what he put in it. He once stated to me "I want a stock looking bike that would blow anything off the road".

So should I jet it like a stock bike and will it last? Or do I figure out how to lower the compression for a daily cruiser before worrying about the carbs right now?




 
John Underwood (XS John) shouldn't to be taken seriously on this issue. His paradigm for performance and longevity was the old John Deere 2-cylinder upright tractor motor. Remember that being noisy does not make a man right. John wasn't wrong in his goals, he just wrote as though anyone who didn't share them was a fool. That didn't keep him from asking for advice--always in PM's, never in the open.

Do a Google search on cc'ing out an engine to determine static compression ratio. As a rule of thumb, 10:1 is as high as you can reliably go for street use on high octane pump gas.

At 240 psi, your motor is higher than that. A number of inexpensive solutions are open to you. Thicker gaskets will drop the CR, but will require you to degree the cam. In most cases you can have enough material milled from the piston crowns to get down to 10:1. If I were you I'd start by removing .030", cc the cylinder again, and increase gasket thickness if needed. And yes, I've been there and done that.
 
John Underwood (XS John) shouldn't to be taken seriously on this issue. His paradigm for performance and longevity was the old John Deere 2-cylinder upright tractor motor. Remember that being noisy does not make a man right. John wasn't wrong in his goals, he just wrote as though anyone who didn't share them was a fool. That didn't keep him from asking for advice--always in PM's, never in the open.

John is dead, grizld1, and unable to respond to your usual vindictiveness.

You are not a "Senior Administrator" on this site, grizld1, you are just an ordinary member, so you should behave as such and not bully people around, especially John who had a lot of followers here.

For those who did not know John Underwood, also known as XSJohn, he held forth on the predecesor to this site as well as here in the early days. John was a passionate advocate for the XS650 and he offered a lot of ideas and thoughtful insight into the workings of our XS650's. John would spend his time on site helping others with a passion.

John died tragically in 2010 in an automobile accident. Here is the anouncement of his death:

http://www.xs650.com/forum/showthread.php?t=2289&highlight=john+underwood

May he rest in peace.
 
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I've been running vm34's on my 750 rephase past 4 years & have tinkerd heaps forget the mains for the time being they only come to play 3/4-full throtle,you can even run differnt piolts left to right as in all reality your playing with 2 singles,my right carb won't respond at all to mixture screw,never got that figerd,good luck with the new jets & make sure you have no vacume leaks
 
Who am I "bullying," Pete? John was dogmatic when it came to CR and high performance mods in general, and Roto has read his stuff and fears that his motor is a money pit/total loss. I'm trying to help here--got anything the man can use?
 
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