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Vm34 jetting help

Discussion in 'The Garage' started by 82xs400dude, Feb 26, 2021.

  1. 82xs400dude

    82xs400dude XS400 Member

    Haha yeah it didn't sound right so I took the carbs off and now I'm waiting on a new setup. I'm basically switching everything. I had a motorcycle mechanic order me the jets and all other parts that I would need to maybe get it to run. Also the petcock touches the choke so I have to raise the gas tank. And now the handlebars hit the gas tank so I need a set of higher handlebars. I'm thinking old school scrambler bars
     
  2. amc49

    amc49 XS400 Junkie

    You're lost already, jets alone are not going to fix the big gulp of fuel the needle will be set for at that application if it was right on the 250. You at some point will get it to likely start; run well? Never.

    Not trying to tell you what to do but getting you ready for it. Any 250 using 34 carbs will be a race one , the carb is way too big for a stock low dog street engine (normal is around 24) and the fuel requirement will be one for a sudden big hit somewhere on the needle/jet that will not run well at all on the 4 stroke needing a much more gradual but steady increase in fuel over a much wider ramp. Main jets alone can't touch that, it's a needle/jet needle thing and the pressed in air jets in carb main body are ones to match that action too.

    I've run 34 on 125 race stuff, it's a full race setup for like 7000-12000 rpm and a 250 is just two of them. No 400 4 stroke flows in that manner, they want smooth fuel increase all the way up instead of the almost nothing then fuel explodes to increase a super big amount all at once to accommodate a wild narrow race 2 stroke powerband. You may need to more than double the fuel output there in 2000 rpm, that type of fuel ramp will quickly foul plugs on a 4 stroke.
     
  3. 82xs400dude

    82xs400dude XS400 Member

    Well I put the carbs in and it idles. It ran for about 2 minutes before I hit the killswitch. It's actually sounding perfect at idle but I dont have throttle cables yet. I know the second I give it any throttle it will have way too much gas to run properly. The pilot jet seems pretty perfect though. I got the pilot jets but I got the wrong type of main jet. Sadly I have no idea what I'm doing but it's good experience
     
    Last edited: Mar 8, 2021
  4. amc49

    amc49 XS400 Junkie

    The difference in idle is remarkably the same over a wide range of engines, that is often close, it is the further ranging throttle that shows the greater difference. Take the Honda DOHC I commonly worked on, the idle jet is the same one from 750 to 1100. Idle fuel is such a small bite that it often shows no or not much difference over a wide range.

    Like I said, you'll get it to run, that's the danger that makes you think it is fine. Running means nothing, throw an O2 sensor on it and it may be off the chart, if plugs are new they will fool you for a while.
     
  5. 82xs400dude

    82xs400dude XS400 Member

    Where can I get an 02 sensor for my bike?
     
  6. amc49

    amc49 XS400 Junkie

    You are about to spend as much money as you likely did on the bike. Check out wideband O2 sensors and the readers for them. And then the skills that go with it can't be bought.
     
  7. 82xs400dude

    82xs400dude XS400 Member

    Well I got the bike for free. But I'll look into it
     
  8. amc49

    amc49 XS400 Junkie

    You need to weld a fitting into the exhaust pipe too.
     
  9. 82xs400dude

    82xs400dude XS400 Member

    So for getting one of those I would also need to do some dyno pulls? Or could i just watch the gauge with it bolted to the handle bars doing pulls on my road at various throttle positions?
     
  10. 82xs400dude

    82xs400dude XS400 Member

    Because I believe I need to put the engine under load
     
  11. amc49

    amc49 XS400 Junkie

    You don't need the dyno if you can think. You DO need to load the engine, in about every possible way you can think, slow tiddling up to wide open and everything in between. And you will be going nuts with those off kilter carbs, not being close in tuning parts means you will be getting conflicting numbers giving you no clear idea of which way to go in a lot of the case. But then you will find out. A dyno can only tell you a carb set is messed up, it even can't tell you how to necessarily fix it.
     
  12. 82xs400dude

    82xs400dude XS400 Member

    Alright so my best bet is to just go by feel then
     
  13. Alex650

    Alex650 XS400 Member

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    Couple things. Look up mc garage on youtube and they have videos that explain all about a carb and the parts. They talk about how to tune each system. Watch them a couple times and follow along when you're ready to work on it. When you get to main jet tuning mark your throttle position 1/16 to wide open. Then do plug chops in all positions starting with wide open. That should get you there. If it stumbles or bogs then is fine at wide open then start moving the needle up or down. The needle isn't as bad as they saw it is unless you have one that is drastically off. Like 2 inches shorter.

    I have been doing the same thing just take your time and make sure each step is fully done before moving on.
     
  14. amc49

    amc49 XS400 Junkie

    I disagree. The needle ramp and needle jet alone can utterly kill you and be the correct lengths, which has little to do with the ramp on such. Getting pure dead idle on along with absolute main jet are both easy too, it is the transitions off the dead idle and through the middle of throttle that most have fits with. Thinking any needle and jet 'matched' together will work is a recipe for disaster and carbs calibrated like the claim here of for small race 2 stroke will be among the worst if true.

    What does one do if the needle is fine at half throttle yet slightly more is too rich and slightly less is too lean? Since needle controls 1/4-3/4 of the throttle? You can hit an unsolvable morass there.

    I say that having tuned so many carbs I cannot count. 2 strokes have a 'gulp factor' for lack of a better word that has them suddenly want up to twice as much fuel instantly within say 50 rpm when any race tuning (porting) begins to line up, a 4 stroke gets nowhere near that demand change and the needle has to cover that and the ramp will be much more radical to cover it.

    Why there are a hundred needles covering each basic length, if it were that easy you wouldn't need more than 2 or 3 for every carb made. Take the extra needles that come in a late model Dynojet big Honda four DOHC kit, they are worthless, unless you have like pro stock cams in the engine. Put them in every possible clip location, they do NOT work and never will. But then Dynojet kits suck anyway, they on purpose stamp bigger numbers on smaller jet hole jets to make you think you are 'doing something' when you go bigger and you may have even jetted DOWN.

    I could tell a story of early Honda DOHC 4 and how Dynojet personally promised somebody a kit that would work with all stock Honda DOHC with simply changing from airbox to pods, it did not happen and despite much dyno tuning and eventually the owner of Dynojet admitted it 'could not be done' using various needle sets cut specifically to cure the problem. They wasted thousands chasing the issue to never solve it.

    Yeah, it's not as hard as some say it is. Go on believing that.

    Truth is not truth.
     
    Last edited: Mar 13, 2021
  15. amc49

    amc49 XS400 Junkie

    'When you get to main jet tuning mark your throttle position 1/16 to wide open. Then do plug chops in all positions starting with wide open.'

    Incorrect. When you tune for main jet you do wide open chops ONLY. You are wasting your time doing plug chops at any other throttle. Along with confusing yourself more. You get main jet choice FIRST, then and ONLY then you begin to do chops in other places, you should already have 1/16th throttle to about 1/4 already done by use of pilot jet, transition, and throttle cutaway. Needle jet choke will figure in that too and another reason why just any needle and jet will not work on all. Most of you have not a clue what the needle jet choke even does.

    AND, if the carbs are CV type you may be doing the second sentence all 100% wrong anyway, as you may not even be at wide open throttle when you THINK you are, a problem all by itself.
     
  16. Alex650

    Alex650 XS400 Member

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    Dallas
     
  17. Alex650

    Alex650 XS400 Member

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    Super sorry yes really poor wording on my part and got ahead of my self. Yes exactly that. The markings were for needle diagnosis. Thanks for the correction. Wouldn't CV carbs just take a little longer to get to wide open? I mean when you open the throttle its just a gradual increase of fuel rather then a flat slide typical "dump in fuel"?
     
  18. amc49

    amc49 XS400 Junkie

    You hope they do that. CV carb opening depends on the draw of the engine and older engines do not draw as much and then the slides open not only slower but slightly less, they may not be at full open ever. Why they self jet and flow to a certain extent when you mismatch them to engines, they do not give nearly the trouble as direct lift carbs but not as much power either.
     
  19. Alex650

    Alex650 XS400 Member

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    They are power restrictive? That seems kind of odd. Wouldn't you have people doing carb swaps if that was the case. I thought cv's we're just a more accurately fuel delivering system. Since the you up in the butterfly the draw the engine actually pulls up the diaphragm the diaphragm is really just a more smooth delivery system so yes it takes a little more time but ultimately it's the same result or what I thought was the same result. I haven't really studied the theory on constant velocity carbs.
     
  20. amc49

    amc49 XS400 Junkie

    'They are power restrictive? That seems kind of odd. Wouldn't you have people doing carb swaps if that was the case.'

    People DO do power swaps over it all day long. YES, they are restrictive, the slide requires up to 5 inches of vacuum to be open, a direct lift carb will flow all the way down to .5 inch and 1 is common. That is less vacuum or higher AIR DENSITY which is power since you are basically running an air pump there. On a 750 the CV type chokes about 10 hp. off the top and up to 20 if the motor is a full tilt hot rod.

    Take the 30, 32 and 33 mm. CV carbs I ran on the early Honda DOHC fours. If you put the 33 off the 1100 on the 750, it auto self adjusts and still runs fine, it just does not open the slide 100% at wide open throttle, there is not enough flow to do it. And that is regardless of you having the throttle butterflies wide open.
     

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