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Carb intake question

Discussion in 'XJ Technical Chat' started by Maxwell Partridge, Mar 26, 2020.

  1. Maxwell Partridge

    Maxwell Partridge Member

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    So I'm going to be putting a single pod filter with a rigged up intake manifold on my XJ 750 with 4 carbs. I just took it through the church of clean, replaced jets with correct ones seeing as the air pilot and main were not even close to being the correct size (I only now literally just noticed it like 2 days ago) with my gas pilot jet being the stock (40 I believe) and the main gas jet having a small +002 on the jet sizing to compensate for lean running/age of motorcycle.

    My question I have (with pictures below) is on minibikes and old dirtbikes/quads rule of thumb my father always told me is that the inlet size (on the intake of the carb) as to be ATLEAST the same size as the outlet side (size connecting to engine) I wanted to know if it was the same principal with motorcycle engines of this size. I was planning on attempting to run the carbs off a single 38-39mm pod filter connected with PVC piping that has a chemical resistant coating on the inside for any back-flow from the carbs. Does that seem like it would be a bottleneck since the outlet of the carbs to the engine is practically 39 inches

    I really cannot find alot of information on this so I want your guyses opinion, should I jerry rig it into two air filters or see how it will run with the single 38-39mm one? I guess I should also say the reason I'm even replacing the airbox is because it's deteriorating and has multiple vacuum leaks in it.
     
  2. Maxwell Partridge

    Maxwell Partridge Member

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    [​IMG] [​IMG]

    The second picture here shows the outlet side of the carb (that goes into the engine) with the part of PVC where the air filter would be on, their the same exact size more or less, if not then the PVC is SLIGHTLY bigger. Would a single pod filter on this be cause for bottleneck of airflow? I dremeled out the inlet too, what I'm really asking I guess is should I try a single pod filter or put an adapter on the end of that inlet pvc to split it into two of the same size and do two 38-39mm inlet pods? Another option could be using two + connectors instead on the two middle carbs to double the intake size and just use two filters on that instead, but I figured that would create less vacuum and I've read all over these forums that these bikes HATE straight pods because they need a vacuum in the airbox to create proper air pressures.
     
  3. Maxwell Partridge

    Maxwell Partridge Member

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    I just noticed after looking at the picture.. I legit don't even need to buy two more + connectors, I can just run the two carbs on each side off their own individual pods and connect them how I have it shown in the picture... Wow I feel really stupid now for not noticing that earlier, but then it comes back to "will that make enough pressure."?
     
  4. Maxwell Partridge

    Maxwell Partridge Member

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    [​IMG]

    Yeah, this looks better and would double the total air flow while also making the inlet for the pod bigger, meaning more airflow ontop of the already doubling airflow from two pods instead of one. Doubling airflow for each carb since it's going from 1>4 to 1>2. But once again, Would this small amount of chambering create enough pressure within the PVC to keep the engine running optimally. I'm actually glad to be getting rid of that GOD AWEFULLY DESIGNED airbox, but I understand this will mess with the carbs/engine in unforseen ways, who knows it might make it even run better, I'm still having sputtering and stalling problems in low-gear low-rpm, I don't know if a messed up float level in the bowls (since i rebuilt the carbs using an aftermarket kit with longer fuel bowl cutoff thingies) I'm going to check that plus the shims under the valve cover when I get a chance, this virus and the lack of money due to layoffs is really messing with me, but at the end I'm hoping to have a nice running bike, it's super comfy, literally 6 dollars a month to insure with FULL coverage, and would look nice restored, so I really do not mind sinking some cash into it.
     
  5. Minimutly

    Minimutly Active Member

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    Congratulations! You've removed the "godamn awful airbox" (your words), and are in the process of replacing it with something even more godamn awful (my words).
    You are making so many assumptions and mistakes here it's not going to end well - firstly you have a low speed running problem - this is unlikely to be airbox related - more likely to be fuel level and or primary jets, carb ballancing, mixture screws and circuits.
    Then you are mixing plastic fitting sizes with airflow capability, and then confusing it further by teeing two carbs together, with one offset. This will affect mid and high speed running, so much that it is unlikely to work.
    Given all of this, you would be better advised to fix the carbs, then refit the airbox. Sorry, but that's the best advice I can give you.
     
  6. Timbox

    Timbox Well-Known Member

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    Max,
    If you look at how the OEM of the carbs goes into the airbox you will see you are causing air flow issues with this setup. Each carb will need to have it's own airflow and not be connected to each other. Each should have the same length inlet path as to keep continuous velocity (CV) to each carb the same. They are CV carbs and are engineered to have their own air inlet and not share it. I hope you can see from your design what you are stealing air from the carbs set up this way. You are also cutting the air flow to each in half doing this. The inside diameter of the PVC is smaller by half to each carb putting them together that way.
     
    Franz and Huntchuks like this.
  7. Maxwell Partridge

    Maxwell Partridge Member

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    "Minimutly"
    I meant god damn awful as in the design of it is horrendous for maintenance reasons, it takes me a solid 5-10 minutes alone getting the carb off, sometimes more putting it back on because there is no f***ing room and I end up crushing the hell out of the rubber connectors in the process, half the time I'm afraid I'm going to rip them. I'm replacing it more-so because I have to, the stock air box crumbles if you push in on it in certain spots and has a ton of leaks in it that bypass the filter altogether. I looked at the size of the PVC and compared it to the size of the rubber inlet boots and it's only afew mm smaller since I'm not using the 1>4 T anymore, and setting two carbs together (while obviously better than 4>1) I understand it could still cause problems, but I'm trying to do this without having to buy an overly expensive airbox replacement online. I've been messing with the carbs for the better portion of a solid 4-5 months, completely rebuilt them by taking them to church, broke the carbs completely apart and thoroughly cleaned with carb cleaner, used brand new jets for the air and gas pilot/main along with new gas inlets in each bowl, put new o-rings for the gasling connecting the carbs together plus new gaskets for all the bowls, tried using a gauge to get the carbs all synced using the vacuum ports on the rubber inlets to the engine by getting the primary idle jet to about 1250 rpm idling then syncing the rest of them to it and reset the air-mixture screws to (I believe, it's been awhile since I did it) 2 1/2 out from soft bottom under supervision from K-Moe.. Believe me when I say I've pretty much tried everything I possibly could have so far with my knowledge for the carbs, the only thing left I can try to get it fixed is getting the fuel bowls leveled to 3mm since their off, checking shims, and then inspecting the ignition system..

    "Timbox"
    So I should try and get two 90 degree angle pipes for the outside carbs with them pointing under the inner carbs pvc since I'm still trying to keep them in some sort of "airbox configuration" instead of just straight podding them from the carbs since I know that causes problems.
     
    Last edited: Mar 26, 2020
  8. SQLGuy

    SQLGuy Well-Known Member

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    Although I am not a fan of pods, I think they would be a much better solution than trying make an intake manifold for the carbs. Pods will give you the ease of removal and installation that you're seeking, and replace a potentially impossible design/tuning headache that nobody else has tried with a known tuning headache that at least some people have had some degree of success with.
     
  9. Minimutly

    Minimutly Active Member

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    There are numerous people on here that will tell you it takes practice to replace carbs with the original airbox - they are right, it does. But I've read 5 minutes, which is impressive.
    Sqlguy suggests pods would be a better option than the one your proposing - I agree with him, but then you will be in a world of pain trying to guess which jets/needles/air jets/float height you need to tweek to get it to run right. And many here will tell you they will never run as well as with the original airbox.
    Lastly there is your plan. It will not work. Air flow down the throat does not flow in a constant manner, the carb sees a pulse every time a valve opens. These pulses travell up the pipe, reflect back, do all kinds of clever things. One of a pair of your carbs has a straight path in, but it shares that with a flow demand from one side - the air won't know if its coming or going. Add to this your fittings have steps in them - what happens when air sees a step? It jumps out around it, effectively reducing the effective diameter/area to as much as half. It's a recipe for disaster. Put 4 straight tubes in there, with a pod filter on each if you must, but be prepared for some rejetting, probably, but not allways.
    Out of all your options, getting the carbs right, maybe even without the airbox, then fitting the airbox will be your best option. Buy a used box.
     
  10. Timbox

    Timbox Well-Known Member

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    +1^ on the used air box. If you use ratchet straps to hold the air box back away from the carbs. There are 2 or 3 10mm bolts that hold the air box in place, once the are out you can pull the straps tight and the air box will be out of the way. If you you 1 ratchet strap to pull the carbs onto the intake boots it should work. These boots are also old and they may be very stiff, using a hair drier or hot air gun will soften then up nicely. There is a little trick to putting them in, and once you take it slow and steady you will get it. Trying to put them in by hand and hold them in place without a ratchet starp, it is hard to do.

    If you want your bike to have a good top end speed and get there in a hurry, you will want to get a stock air box. PODS will work but they will not be as good as OEM. IMPO.
     

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