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Post by matt527 on Mar 29, 2009 14:51:33 GMT -5
Ok I know that there are other posts about this. I have no problem with the frame backbone getting in the way. As I am changing it around to fit a single carb and the manifold. But my question is what type of carb to use?
I have been searching and watched the videos on Youtube, but I am wondering what size mm carb to use etc. Any ideas would be a big help. Thanks everyone.
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Post by xsleo on Apr 4, 2009 1:22:19 GMT -5
I think a single 38mm would be enough, A Harley up to 1450cc uses a 40mm. Just jetted for the engine.
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Post by Burns on Apr 4, 2009 19:15:59 GMT -5
That's the way I'm going. I think a 38 constant velocity carb should be about perfect.
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Post by 5twins on Apr 4, 2009 19:55:13 GMT -5
I've seen this come up again and again too. I'm just wondering why? Why step backwards and gain less performance through what seems like a whole lot of effort? If I was to put this much time, money, and work into a particular mod, I would hope the bike would perform better and go faster afterward. But I guess that's just me, what the heck do I know. But here's a suggestion .... buy a single - they come with one carb. Granted they're only 100 to 250cc but your 650 probably won't be much faster with a home-made single carb conversion anyway.
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Post by Burns on Apr 4, 2009 22:07:02 GMT -5
One Mikuni replaced two Amals on the Nortons with no performance detriment. I believe that the reason most motorcycles have multiple carbs is due to intake plumbing geometry more than anything else. It's hard to get equal length intakes. My bet is that a single carb set up with equal length intake tracts on our twin will cost nothing or next to it on the top, will run cleaner on the bottom, have better roll-on and will get better gas mileage. It will always be "in synch."
Besides, we just like to tinker.
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Post by matt527 on Apr 4, 2009 22:30:49 GMT -5
5twins,
As Burns mentioned its more of just a tinkering addiction. I always like to do things differently. if they work...great, if not then back to where it was.
But I thank everyone for there help and ideas.
I picked up a great conditioned used carb from a XR650 I believe 38mm for $10 from my local Honda recreational dealer.
So we'll see if it works out.
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Post by tomterrific on Apr 5, 2009 6:20:33 GMT -5
Hi 5T,
I see the allure to one carb as well as understand where you are coming from. It's kind of funny, I brain storm single carb manifold designs that I think will work with limited space but I would never do it due to the stock carbs being so good. The stock carbs are linked well so are really similar to one carb in the balance department. That link is a fine addition to a multy carb bike.
Two carbs work fine if the pilot jet on one gets plugged. A single would stall and not idle.
Two carbs are responsive due to being close to the valve.
Two carbs can give more power on top.
Two carbs can be linked to operate as one at higher rpm for a big power increase using smaller venturi carbs. V-Max concept.
Two carbs look the business. Ever see flat tracker with one carb? (Okay, flathead V-Twins)
One carb is cool on an XS650 because hardly anyone has just one.
Tom Graham
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Post by matt527 on Apr 5, 2009 8:47:42 GMT -5
Yeah I understand that the 2 carbs would serve a better purpose with there setup and being so close to the valves. This is more of just a try-out thing. I'm not building a flat-tracker or else I would most certainly keep the dual carbs. Like you said it makes more sense for each cyclinder having there own fuel delivery right there at the valve for maximum performance. But this is just simply a cruising around town bike.
When I got this bike and the spare engine it didn't come with any carbs on it. So I am prepared to buy some stock carbs if this doesn't work.
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xsjohn
Junior Member
Posts: 83
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Post by xsjohn on Apr 5, 2009 10:51:26 GMT -5
Matt.....you won't get much help building a cruiser on the borads....most are into hopping them up instead of slicking them out........here is what I did........now I have a cruiser that will still pull 18/29 even double all over the mountains in NC and without a complaint or engine noise........
I would keep the 2 carbs so you can adjust the right side cylinder temp down a bit to match the left…….then I would use 2 head gaskets bonded in the center to reduce the compression to 8-1…..then retard the cam 2 degrees to put it back to stock position…(.020- .025 thousands measured where the cam meets the boss on the notch left side…..moving the cam clockwise as seen on the notched left side)..raising the head rotates the cam forward……."have "to put it back to stock position……then retard the timing about 1/8” from the F mark at 1200….then you will then have a real cruiser…….
xsjohn
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Post by matt527 on Apr 5, 2009 17:50:35 GMT -5
John,
Thanks for the help, I am always open to suggestions as I am new to these types of bikes.
But I'm just wondering what increasing the compression to 8-1 will do for the engine, and noise etc.
So just making sure I understand your terminology; bond 2 gaskets together between the cylinders and the head. Now by doing that will lift the head, so must then re-align the cam back to the stock position with the notch facing up, and the "T" mark lined up on the alternator???
I am understanding this correctly?
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xsjohn
Junior Member
Posts: 83
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Post by xsjohn on Apr 5, 2009 18:21:18 GMT -5
Didn't realize this was your first build.....reason I suggested the 8-1 is I thought you were looking for a cruiser.....with the single carb thing alnd all.....actually 8-1 is reducing the compression from 8.7-1 or 8.5-1 depending what year...........and the 2 headgaskets are bonded together in between them then put on the engine...not bonded to the engine.....and the cam has to be removed from the chainwheel on a press and physically moved and repressed into the chainwheel....(as described above)....for a first timer this might be a bit out there.......the engine will then run with a bit less power but a lot smoother ............for a cruiser........
And for the guys that think I am a total nut bag.....was 85 degrees today and it ran "considerably" cooler that ever......and because of this I think in hot weather it produces about the same power.....and is a "lot" more fun to be on in the city.....and on the express for that matter.............
xsjohn
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darrenlindy
New Member
ride or die!!! 72 chopper 75 brat style bobber
Posts: 8
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Post by darrenlindy on Apr 11, 2009 21:28:52 GMT -5
why would the intakes have to be the same length?
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Post by matt527 on Apr 11, 2009 23:19:54 GMT -5
XSjohn,
This is my first build. I used to race and repair my own dirt bikes, but nothing more than simply replacing for rebuilds every few races.
DARREN,
I think they have to be the same length because the fuel deposit from your carb to your engine is split between the manifold to both sides. And have to have the exact length so the fuel/air can travel the same distance making for a smoother running engine...I THINK this is why. There may be more to it that I just don't know about.
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xsjohn
Junior Member
Posts: 83
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Post by xsjohn on Apr 11, 2009 23:45:51 GMT -5
Spent a couple of hours doin some online research........and from what I know......
In order to create more horsepower the runners become shorter in length allowing the air a more direct path, and when torque is desired the pipe length is extended
The extra length of the intake tubing to the right cylinder acts as a larger plenum resulting in uneven air distribution between the left and right cylinder…."A smaller plenum gives you more torque, while a bigger plenum makes more power at high rpm."
Smaller volume gives faster air flow for power.
Larger volume gives better torque.
MY TAKE………since the right cylinder already runs 25 degrees hotter than the left any intake modifications that would increase this situation would "not" be good at all….
I read for a couple of hours on this subject………seems the air filter may have to be on the right to reduce the plenum ……not much room there with the kick start and all….and I don't know that this is even true....you would needl to research it to understand what effects the extra plemum has on mixtures....if you leaned the right any don't do it....if you richened the right that would be good ...........
I couldn’t find much information on intake length and plenum size and its effects on mixtures….but air speed and vacuum pressure on these carbs would have an effect on drawing fuel up and out of the carbs I would think….lower atmospheric pressures due to the slower moving air at lower rpms with a larger (uneven) plenum WILL EFFECT SOMETHING…..and if it effects the right cylinder temp anymore than stock it wouldn’t go on mine….and you will loose the ability to directly effect each cylinders mixtures in order to balance cylinder temperatures…….And I do know something about that on these....
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Post by tomterrific on Apr 12, 2009 7:08:44 GMT -5
Hi John,
I suggest reading the excellent book by Philip Smith, "The Scientific Design of Intake and Exhaust Systems". This book tells of a study of why designed systems work and some work better than others. It is an English engineering book so is easily readable by the layman. The first copyright date is 1960 or something close and the way the different systems are tested is fun to read about. It's a good read for someone interested in how and why pipes work.
Tom Graham
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