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Calling 5 Twins, need jetting help

Discussion in 'The Garage' started by garyr, Oct 10, 2021.

  1. garyr

    garyr XS650 Enthusiast

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    I'm not sure your still on this board. I need help with jetting.

    I'm stuck on a jetting problem. Have VM-28 mikuni carbs on a 1968 Honda CL 305cc

    The carbs are fine until wide open throttle. It dies, it either bucks and surges or bogs. Up to 3/4 throttle the bike rips. I went 15 sizes main jet up and down and it really does not matter much.

    The plugs are not wet but seem slight black soot. I can stall the bike if I keep the throttle wide open. Again not difference in 15 or so jet sizes.

    I did not want to use the stock 50 year old carbs so bought new mikunis. Something seems wrong mechanically with the carb, or something

    If you can shed any light would be much appreciated.
     

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    Last edited: Oct 10, 2021
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  2. thuban

    thuban Horse Scratcher XS650.com Supporter Top Contributor

    Ate you getting full smooth advance?
    Very nice bike!
     
    Paul Sutton likes this.
  3. garyr

    garyr XS650 Enthusiast

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    I don't know what you mean exactly about smooth advance. Advance to me is a term for engine timing.

    If you mean smooth throttle transition from idle, 1/4 throttle, half throttle to 3/4 throttle yes, very smooth and revs high and pulls hard.

    Because this carb does not belong on this bike and I had to make a spacer so the carb sits level something is causing the bog or stumble at full throttle, like not enough back pressure of something. It's there with every jet change. I'm not sure what would cause that.
     
    Paul Sutton likes this.
  4. 5twins

    5twins XS650 Guru Top Contributor

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    I have more experience with the XS650's stock CV carbs and not much with VMs. But I can make a few suggestions. Your plugs look very rich so I think I'd be down-sizing mains, not going up. Also, the generic VMs come set up for use on a 2 stroke dirt bike. The midrange/main circuit is set up richer by means of installing a small air feed jet for that circuit. When run on a 650, you take that air jet right out and run without it. You might try that on yours, you've got nothing to lose .....

    [​IMG]
     
  5. thuban

    thuban Horse Scratcher XS650.com Supporter Top Contributor

    Yes, I was talking engine advance. I'm working on a Honda now that the advance was not going full and had a problem like yours. However as clean as yours is, It was a shot in the dark because it looks mastered. 5T is the man in this area!
     
  6. Jan_P

    Jan_P XS650 Guru Top Contributor

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    Just mention it I can see there is a clear hose -- but can it be to little fuel ?
    Filter clogged or tank vent not open enough
     
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  7. garyr

    garyr XS650 Enthusiast

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    Dude, your the man.....never thought about that!!!!!!! Thanks man!!!
     
  8. grizld1

    grizld1 Grumpy old man Top Contributor

    Be sure slides are installed with the cutaway facing the rear. Troubleshoot the ignition. That includes not only gap, timing, and advance curve correction, but also making sure that sufficient power is reaching the ignition coils. One member here had similar grief on an XS650 to what you have on your Honda. He had the mains reduced till they were way out of range--lean as a winter coyote and plugs still turning black. Wouldn't believe me when I told him the problem might be electrical until I actually got my hands on his bike and ran a jumper around the defective main switch. Problem solved. According to a 1998 poll taken by the American Association of Shadetree Mechanics, 20% of carburetor problems were traced to compression issues, 5% to cam or valve train, 70% to the ignition system, and the rest to the carbies. You gotta have fire to burn the fuel!

    I almost never correct my old partner in crime, but the main air jet has to do with controlling atomization at full throttle at high rpms that are past your Honda's redline, not midrange tuning. But right, remove it. The N8 needle jet might be OK for 4-stroke applications. The #160 mains that came from the factory would be way rich for your motor, and so would the factory #40 pilot jets.

    BTW where did you get those VM28s? They're sold in Europe and maybe elsewhere, but Mikuni America doesn't import them. I had to go to Topham Mikuni (Germany) to find specs, they're not listed in the Sudco manual.
     
    Last edited: Oct 10, 2021
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  9. garyr

    garyr XS650 Enthusiast

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    We built two of these CL's and my friend got those carbs on eBay cheap. It is a mechanical circuit issue, I did not see those jets on the mouth of the carb, that is the problem. I will mess with it tomorrow.

    The ignition is electronic, I set the timing on the mark. The coils are new updated dyna's and they send a hell of a spark. The heads have been ported by someone that specializes in these bikes. And the cylinders were sleeved and punched to 350cc. We are going to race these bikes. The pilot is 70 now and it runs strong. The main is a 300 with the needle in the center. WOT only.. its starving for fuel. 5-Twins is right, those mid range jets are the issue. Once I work on them I'll get it shored. I'm sure once I remove that jet I will have to adjust the jets more but other than WOT the bike pulls and revs out.

    Oh.. the slides have a brass guide so they can only go in one way. I did at one time put the slides on backwards on my Kaw triple. They run so bad you can tell right away. I did not know these carbs were for two strokes.
     
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  10. garyr

    garyr XS650 Enthusiast

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    Sorry, the Carbs were bought on Amazon about 3-4 years ago when we started restoring them.
     

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    Paul Sutton likes this.
  11. grizld1

    grizld1 Grumpy old man Top Contributor

    Those aren't "midrange jets," but you'll find that out when you pull them and nothing clears up. I'm surprised the motor runs at all with pilots and mains that fat, it's much richer than what Mikuni delivered the carbs with and much richer than what's demanded by a big bore competition XS650 with VM34s. You can find the component list and float level specs for your carbs at https://www.mikuni-topham.de. Good luck with the tuning.
     
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  12. garyr

    garyr XS650 Enthusiast

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    I removed the air jet. Its a little better. but WOT its being fuel restricted still.

    I have to figure out why at wide open throttle only the bike bucks and surges. All the way up the throttle opening its runs strong and pulls hard. It doesn't matter the rpm I open the throttle fully, it acts the same and cuts off fuel. Whatever happens on that last circuit something is restricting fuel flow.
     

    Attached Files:

    Paul Sutton likes this.
  13. AjD

    AjD XS650 Member

    Just to add my 2 cents. I'm running vm34s on my xs650 my jetting is 22.5 pilot jet with a 170 main. Some people forget that the pilot jet starts everything and needs to be the right size to work everything else if that's wrong everything else is not right either.

    For example the guy who helped me set up my carbs had my pilot set at 11.5 and my main was a 190. Horrible fuel mileage didn't want to stay running at idle had to have the throttle at 3)4 to go at 60 MPH but it ripped in that mid range.

    I ended up using a wide band O2 sensor and a gauge to tune my bike properly because reading plugs is way too time consuming in the short-term.
     
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  14. grizld1

    grizld1 Grumpy old man Top Contributor

    Your symptoms are consistent with low fuel level. Check float level. It should be 15 mm., and if an unknowing person set it by specs for another VM carb it may be too high, lowering fuel level. Can't find a schematic for the VM28, but if you have independent floats and the tang of the float lever has been tampered with, it should be obvious because the tang will be far from flush. If that's the case, move the tang until the float lever arms are 15 mm. above the level of the gasket surface of the carb body, or 14 mm. if the gasket is in place. If the tang does not appear to have been tampered with, move the float arms to set level, not the tang. Then check fuel level with a sight tube.
     
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  15. garyr

    garyr XS650 Enthusiast

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    Appreciate the advice. Yes the floats are independent. They do work with the tang/arms. Last I remembered the tag was flat, I never measured anything.

    When I first put the carbs on, stock jetting that came with it the bike shut off mid range then again WOT. The bike would barley move. Larger jetting made it run almost prefect except WOT. When I drain the carbs before I take them apart I use a small cup. A lot of gas is till in the bowel when I drain them. I forgot one test ride to turn on the fuel and the bike ran for a while. I will check the float height but I'm pretty sure its not that. From idle when I put a load on the bike to WOT it will do that same, not reeving the engine not running the bowels out of fuel.

    I think its the slide, I have to see what's going on now and look at the operation. Whatever part of the circuit works at full throttle is the culprit. I know the main atomizes the fuel to the mouth body but something is being blocked.

    I will take them off tonight and check the float level with your specs. I have no specs on these carbs. I remember buying a float lever measuring gauges, looks like a L shape.

    It really doesn't matter what these carbs are on, they should work the same on all bikes regardless of the cc. Oh when I put the choke on while WOT it's worse.
     
  16. grizld1

    grizld1 Grumpy old man Top Contributor

    Motor falling on its face at WOT only is often a symptom of low fuel level. The level doesn't have to be off by much for that to happen (ask me how I know...). Could be at least part of the reason why your bike is asking for those huge pilots and mains. Be sure to confirm with a sight tube.

    Check the slide and needle jet numbers while you're in there. If you have a bigger cutaway than the #2.5 that Mikuni delivers in the VM28, it could be part of the problem; that would force bigger pilot jets. Needle as delivered is 5DP39. If somebody hung an overlength needle in the slides that could also cause grief, you never know what kind of monkey business the PO was up to. One more thing, height of the float lever arm is checked at the point on the arm that contacts the horizontal pin on the float. There's usually a light wear mark there. For the generic factory jet list, go to the Topham Mikuni site; they still get the VM28 in Europe.
     
    Last edited: Oct 11, 2021
  17. garyr

    garyr XS650 Enthusiast

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    So everyone is on the right track and I want to thank everyone trying to help.

    I got a hold of Mike Morse today @ 650 Central. He seemed to know right away the throttle valve, also referred to sometime as the secondary (main jet) is way to small. The throttle valve was a 2.5 and the needle itself was a 5F-21. So he is sending me "a kit" of 3 throttle valves and needles to try and it should sort it out plus different spark plugs :) . He said that set up in there now will make the bike shut right off at full throttle. That's why the jets are so large making up for a way undersized throttle valve and needle. Because its a 4 stroke it has no "air tube" so that air jet must come out.

    So the bike had to run way rich, pilot and main jets to make up for a small throttle valve mister, then at WOT it was starved for fuel.

    So I got talking to Mike and I'm not sure how much longer he will be doing his business, he has had about enough. He is a wealth of information that will soon be lost. There is no support knowledge at Mikes XS, I like them, I use them but your on your own. My advice is if your going to do a custom project, do it now and get Mike Morse involved to help sort it out. Mike has helped me SO MUCH. The new Omar's guy.. I had to buy a new exhaust, he had no idea what length to weld the inter pipe to make a power band. Mike M calls him and helps him so I get the right power band on my bike. He has also diagnosed many other problems and now my street tracker is flawless.

    Big shut out to 5 Twins who in the past has helped so much on jetting and using oval air pods to solve a 5k stumble that I would have never figured out. And Grz now I have time to measure my floats too while I'm waiting for parts. I think this tool measures flout height, not sure but I'm going to use my digital calipers.
     

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  18. grizld1

    grizld1 Grumpy old man Top Contributor

    "Throttle valve" is another term for the slide, and 2.5 indicates the cutaway. There is no secondary main jet in any Mikuni carburetor (although some Keihins use a secondary main). MMM was referring to the needle jet, which has an inverse relationship to the main jet (smaller NJ forces larger MJ, and conversely). Sounds right (Michael almost always is!) We seldom have to increase the size of the NJ in a VM carbie for 4 stroke use, but maybe the VM28 is an exception, or the PO stuck in a smaller NJ--the needle has been changed, so maybe the NJ was too.

    Lack of knowledge is nothing new at Omar's. Years ago Phil Little, the founder of the outfit, wanted to know if the OE lock tab on the swingarm axle nut had to be used (of course it doesn't if you use a nylock nut, conventional lock washer plus safety wire, etc., etc.) My response was that if he had to ask he shouldn't be in the business. He was all about cosmetics, ditto his successors.

    Michael has been talking retirement for some time, and he'll be sorely missed when he pulls the plug. We're fortunate to have Gary Hoos around to pick up some of the slack. He's another guy who does right and answers his own phone.
     
    Last edited: Oct 13, 2021
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  19. garyr

    garyr XS650 Enthusiast

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    The needle jet, also referred to as the secondary main jet slides right out after you remove the main. That's the problem with my application, its to small. So is the needle. I've never had to ever change these but I' fitting them to a custom application. I needed Mike Morse's help to match up what I need.

    Also the float arms are prefect.
     

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  20. grizld1

    grizld1 Grumpy old man Top Contributor

    Sounds like you're on the right track. Other terms for the needle jet are "emulsion tube" and "atomizer." The carbs would have come from Mikuni with N8 needle jets. Curious to know what size you found in there. Good that the float levers were unmolested. I'm sure the pic is meant to show the parts measured and the measurement taken, not the procedure you used, and that you didn't measure from the top of the flange instead of the gasket surface; we don't want to mislead the unknowing.
     
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