Technical Question w/carb

10 replies to this topic

Posted August 10, 2002 - 03:55 AM


I just stuck a yz 426 motor in my 400ex. I have a 2" gap between by carb and my rubber intake boot I need to somehow bridge. I want to know machine a sleeve and bolt it to the carb on the back side between the carb and the rubber inlet that hooks to my air intake boot. The rubber inlet on the back side of the carb is ported, the ports lead to 2 jets. If I machine my sleeve with the ports cut into the sleeve wall just like the inlet, will it effect how my motor runs?

  • John_Lorenz

Posted August 10, 2002 - 05:23 AM


Save yourself some time


  • Guest_mxrider426_*

Posted August 10, 2002 - 07:45 AM


Can't you just connect them with a plain old sleeve? Why do you need ports in it?

  • ejr

Posted August 10, 2002 - 09:35 PM


Yet another use for duct tape

  • Mark_Cantrell

Posted August 10, 2002 - 02:01 PM


I think the ports are because the slide creates a venturi effect resulting in lower pressure near the slide. The F16 looking scoop for the air jets is to get atmospheric pressure air away from the slide. Yes, I think you'll need it.


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  • jkcmotox

Posted August 11, 2002 - 06:59 AM


My vote goes for a 2" piece of PVC (schedule 40 or better should be fine) with three halves of some cigar tube thingies glued to the bottom to form the "triple venturi", then smother it all in your favorite epoxy-resin for a low parasitic drag coefficient.

Or duct tape.

  • yznvegas

Posted August 11, 2002 - 08:37 AM


It sounds like an improvised tunnel ram.

I used to have a jet boat with a 454 in it with a Weind tunnel ram. Basically it increased the length from the carb to the intake. I believe that will give you some torque but rob some horsepower, or it could be the other way around, I don't remember. But I definatley know the bike will run differantly with the carb further from the intake.

  • Vanilla_Gorilla

Posted August 12, 2002 - 03:22 PM


i would definately move the carb back to meet the intake boot and use a longer carb-engine boot. this would rob some response but increase torque. its a good trade off. either way, the 426 motor will make your 400ex a rocketship.

  • Wyatt

Posted August 12, 2002 - 04:26 PM


I didn't think he was talking about moving his actual carb back......just the boot between the airbox and the carb. (making this longer) Am I right? This should not have a large effect on how the motor runs as long as the ports are in place for the air jets. Moving the carb away from the motor definately would.

  • Grashopr

Posted August 13, 2002 - 05:24 PM


I am not 100% sure, but I think those two ports he is talking about (on the lip of the intake flange of the carb) are connected to the idle air circuit and some other air circuit (I could look it up, just don't have book in front of me). By building an extension flange WITH those ports in them, you probably aren't going to be screwing up too much as those two ports have removeable jets/air passages that you could change out if need be...however, I think that you could probably just make a 'regular' extension without spending the extra time to machine those ports in and be just as well off...they aren't as important as their machining on the carb makes them look. I could be completely wrong, take this with a grain of salt.

The 'Hopper' :)

  • Mark_Cantrell

Posted August 13, 2002 - 07:29 PM


I reread my previous post and it was very poorly written. I'll try to be better this time.

The slide when not fully open, creates a restriction in the bore of the carb. By Bernoulli's law, the increase in air velocity at and near the slide will cause a drop in air pressure near the slide. This localized vacuum is what sucks gas through the main/needle circuit.

The pilot and main air jets require air at atmospheric pressure, not at the venturi effect vacuum. To get this, they physically isolate the jets from the vacuum near the slide by creating a tunnel back to atmospheric pressure air.

I think if you don't do this, your air circuits will not function properly. Instead of a hard to machine tunnel or scoop, you could use a pair of tubes slightly larger than the jet back away from the slide. The obvious problem is that the bore of the joint would be larger (to include the air circuits) which would affect the air flow and vacuum created by the slide and reduced diameter.

A simple solution to both would be to create a joint back to the boot or air filter that has the same inside diameter as the current joint. Then, below the joint, run two tubes of substantial diameter back to the boot or air filter to get filtered atmospheric air to the air jets. The tubes could go all the way to the boot/filter or rejoin the tube by drilling steeply diagonal holes in the joint and cutting the tubes flush with the inside diameter.

Good luck and wish I could draw a picture,

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