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Vm34 jetting help

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

  1. 82xs400dude

    82xs400dude XS400 Member

    Hello I'm new here and I just got a free xs400j maxim. I'm not very used to carburetors or motorcycles. So for this bike I got it with no spark at all and it came with no carburetors at all. One of the previous owners decided to start a modern mini fuse box design and then they gave up. So I just used some inline fuses and still no spark. New coils. No spark. New plugs and caps and wires and still no spark. Turns out it was the ignition relay (I'm still slapping myself in the head for this) and now my next step is new carburetors. So I ordered 2 mikuni vm 34 carbs and I just wanted to know if anyone else has done this. I have seen some forms of people trying vm 34 carbs. But I dont know what setup was getting the best result. So please any advice would be much appreciated. Like jetting and needles and all of the fun stuff. I kinda fell in love with this old bike and I want to ride it down the road. I'm an apprentice mechanic and I'm 19 years old. So motorcycles are something I dont quite understand yet. And I understand carburetors even less. Cars and fuel injection is my natural habitat
     
  2. amc49

    amc49 XS400 Junkie

    You better have ordered the carbs as pre-jetted close for that engine or a major mistake, there are like 1000 combinations there and most will be misery.
     
  3. 82xs400dude

    82xs400dude XS400 Member

    I guess I'm in for quite a lot of misery haha
     
  4. amc49

    amc49 XS400 Junkie

    You may not be ha ha ing for long, it's not easy to jet carbs from dead scratch and the needle choices can drive you utterly nuts. The needle alone can doom any choice of the other parts to fail over and over.

    Anybody can get one to just 'run' but running fine and at all positions of the throttle, that's wizard stuff. Most people settle for way less than they can be.
     
  5. 82xs400dude

    82xs400dude XS400 Member

    I appreciate what you are saying. I knew it was going to be a rough project but I love a challenge. No matter how long it takes I won't give up like the last 4 owners. This bike was the subject of hillbilly fixes over and over and I want to make it run and sound and look absolutely perfect. So do you have any idea what a good starting point for the jetting and needles would be?
     
  6. amc49

    amc49 XS400 Junkie

    Not a freaking clue, the idea is a waste of time until one even figures out what is in it to even begin with. Your needle jet/jet needle combo may well doom all effort. How well your basic engine runs will figure in there too.

    You won't get perfect without O2 sensor testing. Doesn't matter how much you want it.
     
  7. 82xs400dude

    82xs400dude XS400 Member

    Well thank you for your advice. I'm going to ignore what you said because I'm stubborn. But when tuning and jetting starts driving me insane I'll let you know so you can have the first laugh at my failures. But I promise you this. I will make this bike run perfect just because of how stubborn I am
     
  8. amc49

    amc49 XS400 Junkie

    You have plenty to mess with there like.......idle fuel feed, idle air and emulsion bleed(s), idle mixture and transition fuel mixture seesaw as relates to throttle cutaway, primary choke (NOT what you are thinking), needle jet emulsion air bleed, needle jet ID matched to needle itself and needle taper itself, Main jet, main air jet and emulsion jet(s), and maybe a choke air jet along with the fuel one. I may have missed a couple places.

    Often you can't get it right without messing with the air, I've used small pieces of wire as restrictions more than once when you cannot find jets as they don't exist. Often the fuel parts you can get are not the problem and will not fix it, you may well find yourself pressing or drilling out parts that were not intended to come out ever. BTDT way more than once.

    Some few of us have done that work. You still have entire power valve, economizer and accel pump systems yet too. CV carbs add a whole new realm of vacuum demand to all of that, engine condition then becomes one of your jetting tools.

    Have fun. I won't laugh, I know better, having been there. Why I tried to warn you.
     
    Last edited: Feb 28, 2021
  9. 82xs400dude

    82xs400dude XS400 Member

    I did see a vm 34 conversion kit for an xs650. I was just going to set up my new carbs exactly like those ones and just make fine tweaks from there
     
  10. amc49

    amc49 XS400 Junkie

    A 400 has individual 200 cc. cylinders, the 650 is 325. That is a whopping more than 100% displacement increase, the way the engine pulls on the carb will be markedly different. The 325 will induce much deeper vacuum to flow the carb totally different. Needless to say it will not likely meter correctly.
     
  11. 82xs400dude

    82xs400dude XS400 Member

    Even though it's a huge difference at least it's a good starting point. By the way what kinda bike you got?
     
  12. amc49

    amc49 XS400 Junkie

    It is NOT a good starting point. Thinking so shows things. You try to get carb prejetted for close to same size single cylinder within 25 cc. and even then it can be dicy. When you go bigger with direct lift (not CV) you will have fits with bogging and no accelerator pump setup, the jetting and needle have to be perfect to tune past it.

    I raced first gen Honda DOHC 750-1100 4 valve, still got a couple engines lying around and one bike. Getting old. Have 3 Kaw triples too. Kind of a waste with no decent fuel to run them on, 2 strokes choke hard on ethanol laced fuel. Beyond that I owned a bunch of them, used to race Yam twins too. Raced American Motors cars way down into the 9's 1/4 mile too at over 140 mph. 600+ hp. 395 inch engines that went around Chevy 454s like they were tied down. Dad had a hotrod shop garage.
     
  13. 82xs400dude

    82xs400dude XS400 Member

    My family is a bunch of construction workers that take every car to shops. I have the fastest car out of my whole family and it's a 2007 Buick with the 3.8. Now back to the bike. So what you are saying is that I'm probably not going to be able to get it to run properly on vm 34 carbs? And even if I do get it to run properly at all rpms it's going to be unpredictable due to the loss of the butterfly valves causing more airflow. Meaning that I need larger jets and needles than the bs34 carbs had. Damn i wish i could switch to fuel injection
     
  14. amc49

    amc49 XS400 Junkie

    I'm saying you don't just go and do it and done, the OEM spends literally millions with every single model just getting the carburetion right to the extent it is when you buy a new one. People think they will simply repeat that and it doesn't happen. They can't even tell you how to go about doing it.
     
    Last edited: Mar 1, 2021
  15. amc49

    amc49 XS400 Junkie

    http://www.superstockamx.com/page86.php

    What I grew up racing.........2 seater true AMX. We had two of them in the family and mine was one of the red white and blue painted ones.

    Go down to pic #6 of the crossram twin carbs. Think about jetting that. I did it blindfolded. I did carb mods like the cutaway choke housings on those carbs too.
     
    Last edited: Mar 3, 2021
  16. 82xs400dude

    82xs400dude XS400 Member

    I got the carbs for the bike now. I'm gonna try it with the jets already in it. I know it won't run properly but I just want to hear it fire
     
  17. amc49

    amc49 XS400 Junkie

    Luck, it may surprise you. Just bear in mind lots of setups appear to work fine until you run them long enough to then begin to get ragged around the edges. Too rich can do that, I've seen up to a month before they begin to run like crap. On setups close but not right it takes the engine a while to recarbon up to show a bad situation for what it is. That can go double for just coming off a CV carb setup, they are set up lean on purpose.
     
    Last edited: Mar 6, 2021
  18. 82xs400dude

    82xs400dude XS400 Member

    Haha I bought carbs jetted for a 250 2 stroke. The stock main jet in the bs34 carbs is 135. The main jet in the vm34 carbs is 260. Its gonna run like crap but I'll let you know what happens
     
  19. 82xs400dude

    82xs400dude XS400 Member

    I got the carbs in the bike and tried to start it. I had to cover the intake with my hand after switching the intake boots to the opposite sides because the carbs together are too wide. So now I need to get a 2 into 1 throttle cable. It sputtered for a little bit before it died. And now the battery is charging
     
  20. amc49

    amc49 XS400 Junkie

    Don't know why you bother, you will wash oil off the cylinder walls to damage if the plugs don't foul right off the bat. That's WWWAAAAAAYYY too much jet there.
     

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