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No Air Jets in the Carbs...what the ???....

Discussion in 'The Hokey Ass Message Board' started by bonez, Aug 10, 2009.

  1. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    HI guys, saturday after changing the valve cover gasket and giving a good clean to the engine bay i started tearin in to the carbs again, for those of you who know or remember me you know what im talkin about for the others i'll post a pic of the engine in the next few days.Anyway, i removed the mesh that i put in place of the air cleaners on the main bores and noticed no air jet in the 1st, then the 2nd the 3rd and the darn 4th!!!!!! I only removed the mesh when i syncronized them and never noticed, shame on me.The carbs are Mikuni VM 38s, installed w/ an esslinger intake on a Ford sohc 4, i had issues of floading and high consumption for ages, and tried pretty much everything, i just ordered smaller pilots and mains, and now i notice this? Now, i suppose the air jets serves the purpose of vaporizing the fuel and im sure that w/out them the suction sucks (excuse this) and where it supposed to flow air passes fuel, as the hole is a lot bigger than it should.What do you guys say? How did the previous owner ran them for years like this? or did he simply forget to mount them after cleanin them and never said so?Thanx a lot to those who will reply!
     
  2. oj
    Joined: Jul 27, 2008
    Posts: 6,113

    oj
    Member

    I've never messed with mikuni's but they are nice carbs, these air jets would they be the equivalent to holley air bleeds? If so, they are responsible for 'how much' air is available for emulsion. With them gone entirely the mixture would have been lean, but with 4 of them on a 2300(?) i would imagine you could have compensated. I work with those engines over here for circle track racing and can get 600 cfm to work really well, how much do those mikuni's flow?
     
  3. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    Hi OJ, yes theyre responsible for the "emulsion", my explaination was a little ruff, until a month ago, i had almost no clue on how this things worked, but now after opening them so many times and reading the manual as many i have a pretty clear idea on how they work, still, my explaining the same matter is kinda crappy. My banger is a 2.0, its a pinto not a lima, but this same "kit" was available for them too from Esslinger in the 70s till some time ago, its now discontinued, and the only way to get a hold of this is either evil-bay, swap meet or else. If you into circle track racing and 2.0 or 2.3 ford you should know Esslinger i'd say.About cfm, i seem to remember that a off the shelf VM38 should produce a total of 109 cfm, 436cfm for a bank of 4, as thunderace stated in my other carb thread, where he also claimed that my problem was "too much carbs" if you know what he meant. Anyway, back to biz, the mains and pilots bein too big could be an issue as the PO had a souped up motor, but now that i know that the air jets are missing that could cause trouble too. This is my daily people, suggestion to not blow it up until i got all o'the bits.
     
  4. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    Some news, and i hope that this time around i'll get sum feedback! as im gettin a little frustrated by gettin no results at all!!!!
    I drilled my own jets, by actually drilling some small screws the diameter of the air jet aperture,
    i did 1.5s and bought enuff drill bits to do 0.5, 1.0, and 2.0 if the 1.5 didnt go well, but guess what? No bloody change at all!
    I di a bit of homework and found out that on single carb snowmobile (the original veichle where the VM38s where mounted)
    there wasnt an actual air jet, but the 2.5 aperture where it would
    screw in, is the actual "jet", and also that in the bike world
    is not unusual to remove'em in some cases.
    Then, i went on the highway, normally i tend to avoid this due to overheatin caused
    by the poor tune of the carbs, but went anyway and pushed the pedal hard, i found
    out that from 4k rpm up the car goes like a train and the engine runs pretty damn
    smooth too, i topped over 160 km per hour and wasnt all the way down.....
    Then i slowly coasted and got out of the highway to avoid a blown motor...
    I suppose the mains are good, now i only need to wait for the pilots and should be on the ball park.
    Still consumption is skyhigh at 3 to 4 km x liter, and only at 1.5 psi on the regulator,
    what do you guys think is the cause of this high consumption of fuel?
    Am i right to think pilot jets are too big and flood the cylinders?
    Thanx, any response is well appreciated!
     

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  5. 97
    Joined: May 18, 2005
    Posts: 1,481

    97
    Member

    You are in Italy , why would you want Mikunis? Shit can them and put a pair of 45DCOE Webers on.
     
  6. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 4,668

    Ebbsspeed
    ALLIANCE MEMBER

    What do your spark plugs look like? Their appearance can tell a lot about the air/fuel mix. Is there any hint of black smoke in the exhaust at idle?
     
  7. oj
    Joined: Jul 27, 2008
    Posts: 6,113

    oj
    Member

    It almost sounds like you have raw fuel running into the engine? What do the plugs look like, seems to me if the fuel were vaporized and overly rich the plugs would show carbon foul but if the plugs are snow white, out of the box clean then they are being washed out by raw, unmeterd fuel. At low rpm the motor waould have a hard time running, at higher rpm it'll just throw it out the tailpipe.
    I am curious about the 'overheating' and 'avoid a blown motor' comments.
    The thing about 'too much carbs', you can't compare the 4 cyl cfm requirement to an 8cyl because of volumetric efficiency of 2 pulses per rpm compared to 4 pulses of the 8 cyl.
     
  8. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    So, thanx for some feedback, i need to start talkin this again really.
    1st thing 1st. 97, i want Mikunis cause they look kool as hell and they are good carbs.
    Everybody can put webers, but that wouldnt be half of the fun of experimenting to get more power outta it. as simple as this.
    Ebbsspeed, the plugs are dead black everytime i check'em, and i had black smoke outta the tailpipe only the 1st day i installed the set up, the tuned them and got only white smoke! :) ! from think fog to almost nothing (now) ...
    Then, yes OJ thats what it seems, from the sky high consumption and the fuel runnin from the tail pipe i always tought i had too much fuel goin in, but the plugs are black, they aint washed as you (and i also though so) say.
    Guess what? yeah what?:confused:!
    On a side note, the engine overheats mostly cause i still have to fit all of the water hoses, meanin that the previous intake had a port for the heater hose that kept it from gettin cold, but also gave a good almost 2 meters of extra hose to run on the engine bay and kept water kooler.
    now w/ the heater hoses cut off there is maybe 30 to 50 cm of hose and there is only the 7" fan coolin the rad (small, its of a 1.6). The 1st things you learn are: lean=overheating, at least in most cases, but lean it aint, so the problem is another.
    I just got the T-hose connector and will fit the heatin hoses and heater back on in days.
    I dont push it too hard also cause my front K member has been hit hard on the right side and if i take my hands off the 'wheel at speed the car will get violently on that side= dead on collision w/ whatever there is.
    Anyway, yesterday i went 100 kms in the highway to leave the car to the exhaust guy to have a custom dual sytem built and noticed better how the situation is.
    In traffic the car sucks big time, it starts in 2nd gar no problem but then it gets to 4k rpm b4 i shift and im only a few meters down the road, while at 4k i should get missile thrown!!!
    On the highway as said b4 at 4k pics up rather well, half pedal down at 160 km per hour and runnin smooth.40 minutes between 120 and 150 km per hour w/out a problem.
    As soon as i stopped i checked for engine temp and could bareley touch the valve cover, while b4 the carb swap i could touch it no problem.
    I think ive been a tad too long....but wanted also to say that the guy that said i was overcarbd, which by the way its a really nice person, has been told already that its bs, cause my 4 500cc cylinders can take a lot more than my single mikunis for exactley the reason you stated.
    Thanx to all.
    I'll be back here on monday hopeing to get you guys intrested and solve the bloody problem. oh one last thing: for some reason my card dont work and the jets i ordered havent been shipped, so i'll have to call the card company and get that sorted b4 any major step.
     
  9. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    Just a quick note:
    Friday afternoon i went to pick the car up w/ the new exhaust system installed.
    As i mentioned up here its a custom built system.
    As i would have guessed the previous evening i had to pop the hood and
    start twinkering w/ the carbs, as the 2 tail pipes where pumpin black smoke as they never did!????
    So, why in hell did the old fucked up exhaust system block the black smoke and pump white one????:confused::confused:
    I had to open the air screws like crazy and kept the thottle valve almost closed,
    as it was obvious didnt manage to drive like that for more that 30 minutes,
    and closed the air a bit and opened the valves, better, bt a bit smokey!
    WHy people! i know im gettin too much fuel, but why w/ the old system the car used to run better?
    Isnt it supposed to run better now w/ 2.5 inch tubes and open pipes??
    I have a small flat muffler on the side of the trans then open pipe up to
    the rear axle that split in two exits one each side exiting
    throug 2 triumph style coktail shaker tips which are empty w/ a small baffle that doest really serve a purpose
    other than if stopped by the law i can say its there..
    Any clues people? at least this is the final proof i need smaller jets...
    but aint this story weird?
     

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  10. Roupe
    Joined: Feb 11, 2006
    Posts: 719

    Roupe
    Member

    Its been about 15 years since I tuned Mikuni carbs on my dirt bikes and snowmobiles but they are good carbs. When tuning Mukunis always start with the main jet. The main jet controls 3/4 to full throttle, but big changes in the main jet affect the whole range. So get the main jet correct for wide open throttle first. The position of the needle on the slide affects mid range. The needle is held in place with a clip. There are 5 grooves in the needle to change the height of the needle in the slide. Raise the needle to make richer, lower the needle to make leaner mid range. The pilot jet controls idle to 1/8 or 1/4 throttle (I can't remember). There are 2 screws on the side of the carb. The screw on the side of the slide is the idle speed screw. Turning it in will raise the slide increasing the idle rpm. The smaller screw is a air bleed screw that helps tune the idle mixture (if I remember correctly). Turn it in less air (richer),turn it out more air (leaner).
    I have a Mikuni tuning book some where but I can't find it. All of the above is from memory. I hope it helps. I woud find a book that will help alot.
     
    Last edited: Sep 7, 2009
  11. Roupe
    Joined: Feb 11, 2006
    Posts: 719

    Roupe
    Member

    Check with your local motorcycle shops for a book and for different size jets. Don't try to drill your own jets they will not work correctly. Also the main jet is under the big plug on the bottom of the carb. Remove the big plug, gas will run out of the float bowl, the main jet is right there. About an 8mm wrench is required to remove it. There may also be a brass washer above the main jet so pay attention.
     
  12. truckerdaddy66
    Joined: Mar 12, 2009
    Posts: 10

    truckerdaddy66
    Member

    I've got a 1980 husqvarna cr 250 motocross bike. It uses a 40 mm mikcuni, you adjust needle on the slide where your cable hooks up ,.also with the main jet . FOor richness on fuel mixture.YOUR slide looks a little different from the pictures.Well anyway thats how I do it on my bike.

    Truckerdaddy66,New Caney, Tx.
     
  13. retromotors
    Joined: Dec 10, 2008
    Posts: 1,045

    retromotors
    Member

    I think a lot of your problem is throttle slides and slide needles.

    In the distant past (way distant!) I adapted a pair of Yamaha 2-stroke carbs to a BMW twin cylinder motorcycle. Initial results weren't good.

    After fiddling with it for a long time, I discovered that the slide cutaway and slide needle shape are the major differences in carburetion between 2-stroke and 4-stroke applications. The original BMW slides had a fairly large tapered cutaway on the leading edge, the 2-stroke carbs basically none at all (as looks to be the case in your pics), consequently running much richer at lower throttle openings. The 4-stroke needles had a much more gradual taper also, running much leaner in its entire range.

    I hand-ground the taper of the slide to approximate the stock carbs, swapped needles and the problem was pretty much solved. The rest of the tuning was done by standard jetting and adjustment.

    Good luck .... hope this helps.:D
     
  14. Harry Bergeron
    Joined: Feb 10, 2009
    Posts: 345

    Harry Bergeron
    Member
    from SoCal

    There are special jet reamers for that, to clean up after a drilling.
     
  15. Relic Stew
    Joined: Apr 17, 2005
    Posts: 1,107

    Relic Stew
    Member
    from Wisconsin

  16. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    Roupe: I know what youre talkin about, the needle is in its top groove,
    as theres lready too much fuel flowing i better leave it there.
    I opened them up a lot of times, and mean a lot!
    When i bought'em last year i had no clue how they worked,
    and now i know them fairly well, also thanx to this manual i linked,
    and yes you are right, the book/manual was a big help,
    w/out it i would be stuck..
    http://www.mikuni.com/pdf/vmmanual.pdf
    Oh, btw:
    Pilot jet controls closed to 1/4 throttle
    Throttle valve or slide 1/4 to 1/2 throttle
    Jet needle 1/2 to 3/4 throttle
    Needle jet, main jet and air jet/aperture 3/4 up to W.O.T.
    But you are not the 1st one that tells me that the main jet is gonna affect
    the whole range. Im really intrested in understanding this better.
    Truckerdaddy: I tried almost everything, either w/ the old exhaust,
    when all i did simply changed the intensity of the white smoke!
    and now w/ the new exhaust, where i either turn in the air bleeds
    to have pitch black smmoke or turn em out and have little black
    smoke, but still smoking! Seems to me i need jets.
    Retromotors: I think i know what youre talkin about, in my previous carb
    thread i did explain that these carbs are part of an old Esslinger
    engineering kit that sold from the 70s to early 90s w/ carbs and intake
    and the carbs are already modified to run on the 2 liter banger.
    Harry Bergeron: It turns out that they dont need air jets, but actually
    both you and Roupe are right.
    Relic Stew: The Sudco site has a lot of good info and stuff for sale, but i
    had a hard time gettin in contact w/ them so went w/ mfg supply, but then
    my credit card went hiwire and im still waiting for jets.
     
    Last edited: Sep 25, 2009
  17. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    A couple of weeks passed and i tried to gather the most information and clues about the carbs.
    One of these was askin to one of my colleagues to troubleshot my engine.
    Its a old restorer type, that restores old italian and euro bikes and mopeds, but
    seems to have a fair knowledge on old engines in general.
    What he said is that i had to connect the vacuum hose of the vacuum advance pump on
    the distributor to the manifold and that my brake booster vacuum hose, that was
    hooked to just one inlet port, had to be hooked to all four, cause the engine this
    way will breath more evenly.
    I trusted him and tried.
    The picture was taken a year ago, and the booster hose wasnt on yet.
    I hookt it in one of the plug holes plugged by the brass bolts.
    Now, to do what my collegue said i made 4 plugs, one for each inlet, that
    run a inch up to a piece of hose parallel to the valve cover and as long as
    the whole manifold. At one end i plugged the brake booster vacuum hose,
    at the other end the vacuum advance.
    Bottom line: car was un-tuneable!!!! I tried every possible combination of
    air and idle screw turns but no chance!
    Once i took vacuum advance off engine ran better.Better than ever that is.
    My collegue was right on one point, the brake booster vacuum hose hooked
    to all four inlets made the engine go smoother.

    Why i dont need vacuum advance? Or how should i set it up?
    Any help greatly appreciated as usual and as every once in a while i'd like to start talkin this again!:D
    Thanx to all, Nick.
     

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  18. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    Anyone? Please!!
     
  19. Old61
    Joined: Sep 20, 2008
    Posts: 268

    Old61
    Member
    from PA

    Is your base timing set right? Centrifugal weights working (if your distributor has them) & springs not weak? Is your vacuum advance working properly & is there an adjustment screw inside where the hose hooks up?
     
  20. bonez
    Joined: Jul 16, 2007
    Posts: 3,493

    bonez
    Member
    from Slow lane

    Base timing is supposedly right, but today im goin for mot (is that how you call it?) and will know for sure.
    Then, how do i know if vacuum advance works properly? is there a test i can do home?
    No adjustment screw were the hose hooks up, but i'll take a few pics of the dizzy
    and post them.
    Centrifugal weights? im not sure i know what they are? Maybe in older dizzys?
    This in the pic is exactley the bosh one i have, i know they sell non vacuum advance dizzys.
    Is that what i should do? buy a non vacuum dizzy? its a lot expensive
    for my funds now, so if i can do w/out it'd be better.
    Thanx
     

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