Alternate Carb

[Glyn Pickering]

Not knowing about flow rates etc with the carbs, according to my calcs you would need a 50mm carb to get the same cross sectional area as 2 36mm carbs. Would a 40mm carb would be sufficient iro cross sectional area?  Is 40mm the typical size of a Harley carb? I've seen a 38mm advertised locally - but may be a bit small? The older Nortons often had an SU conversion done which seemed to work OK - but they would have been lower reving than the XZ. Are car carbs  - specifically the SU's (loads off mini's) not up to feeding gas to an XZ?

Cheers

Glyn

[Blake]

Glyn,

Im in the same boat as you wishing i could know the flow rates of carbs..  the crosssectional way is exactly how i was fiddling with the exhaust pipe diameter (1.5" is the way to go :) )...but i dont think it would work for the carbs that way..  my whole thing, is you see these 100,125" v twin engines that MAYBE have 44mm carbs.  now i definately know the suck more air in with each stroke, and i could do the math for it (too early in the morning for that), but id be interested to know the different volumetric amounts of air (per minute lets say) of a 125" v twin at say(little more than 2,000cc), 3,000rpm (cruising) aas compared to the visions 550cc engine running at 8,000, or even 6k.  The way ive been looking at carbs, i always thought that MAYBE 38mm could be used (modern day 600cc sportbikes are using those exact sizes, and theyre cylinders are smaller, but even a 1000cc bike would have roughly the same sized cylinders as ours, and they use 38mm).

But the one thing you have to remember (which just popped into my mind) the cylinders are never sucking at the same time.  Thast why i think at 40, or 42 at most would be suffice.  because the engine is turning so fast at higher rpms, the cylinders essentially would be sucking the mixture at the same time (as far as we could tell-a constant flow), but in reality they arent.  now my logic may be way off, but i figure MAYBE 1.5x bigger than the size of the stock carb would be sufficient in supporting both cylinders.


OH.. i just rememberd.  one of the XS650 sites.  those bikes are 2 cylinder 650cc high reving bikes (12k redline like ours)..  hmm..let me find that site.

http://www.650motorcycles.com/UJ.html

there we go. :)

"a friend gave me a big honda 650 single carb off a 4 stroke dirt bike. I went to home depot and bought some "t's" of copper plumbing. I brazed it into the configuration of a single intake manifold based on the design of a chevy six's intake. I attatched the carb with radiator clamps and a rubber sleeve, just like a harley carb. It started on the first kick and has run mint ever since. It also is really quick"

i used to know the size of that carb when i looked it up before but it slipped my mind, but im pretty sure its 42 (think thats why i keep saying to use that side)..but man, i completely forgot about that bike...maybe now im going to have to go buy some copper pipe and see about rigging up my own intake :)

as for the car carbs, id steer away from them (unless its a weber or the like, then that might be made to work and people use them all the time)  i took the rochester carb on my caddy apart last month and was amazed at how relatively simple it was with the jetting (two needles that raise and lower together and a single main jet.)..but then that engine never actually goes above 4,500rpm..


hope this helps some.


Blake

[Walt_M.]

Just thought I'd ad another 2 cents here, some of the talk about putting on a side draft single carb and changing the intake position, adding an elbow fitting, whatever. This is going to have an adverse effect of performance. The whole idea of the downdraft carbs is to give as short and straight an intake path as possible for better performance. Don't give up on the downdraft carbs.
My '83 is still running great with the stock carbs, I haven't even adjusted the idle screws and I have NO stumble. It did stumble when I put it back on the road last summer but that was cured with a carb synch.
I'm thinking about the FCRs only for possible future need.

[Lucky]

Blake aren't those 4bbl Q-jets Great?, especially when you flip that big old air cleaner cover upside down, mash the gas & listen to those back 2 barells open up! BWWAAAAA!, lol

[Blake]

haha Lucky you read my mind!

Im actually about to head out to the local 1/8 mile drag strip with the caddy :)    and was exactly thinking of the "performance air filter flip"..hehe...  should be an interesting time out there with those rice burners.haha..


definately interesting carbs :)



Blake

[Glyn Pickering]

Just been looking on e bay at all the diff types of Harley carbs.
Great variety re years / models etc. Would it matter which one to go for - as long as it was a 40 - 42mm bore and you can still get (moderately) priced parts for them.?  What's an S&S carb compared to a Mikuni? The jetting would most likely have to be played with anyway.
Pretty keen on this project since I've got a couple of spares bikes I can use as dummy's  for building up a manifold.

Anyone got a cheap Harley carb out there???

Cheers

Glyn

[Bernard Lajoie]

Yo,Rick G.come on down! to Tempe.AZ. that is,call me at the shop (480)921-3038 M-Sat. Hopefully your not as flakey as other R.O.V.'s I've met over the years! Anybody is welcome,
yea I may seem cranky,but then when you read some the
most redundent questions that have been asked here it makes your head hurt! Also the wack-job who is a so-called
"Weber expert" should have talked Stu Cooper and his mech. a few years ago(83'-84') about the B-S they went thru trying to get to work correctly all the time i.e.different tracks.
 Bottom line they still belong on cars,not bikes.By the way it took one needle change from dyno-jet to make the FZ carbs to work!Cheers!Frenchthewrench.

[Glyn Pickering]

Hi Bernard

I understood about 10% of that. Are you the guy that put 2
FZ 750 carbs on a Vision? Any picures of the finished job? Did you have them both on the same side or seperate? How did you run the cables if seperate? KN's? What jet change did you need?

Cheers

Glyn

[rick_nowak]

hi, from the "whack job"  weber expert.  Bernie, you are such an opinionated jerk.
I don't hear Bernie denying anything i said, actually supporting it pretty much.  i never said that it was easy to tune webers out of the factory dyno rooms, just that it was possible, unlike most carb designs, and some people had enough understanding of what they were doing with them to make them work better than any other option available at the time.
lets face it, compared to fuel injection that you can tune with a laptop, carbs are just dinosaurs, any carbs.  i have fond memories of the carb on my dodge minivan, one of the final generation of carbs.  i'll bet any of the professional mechanics on this board will agree with me that the last generation of carbs were just one problem after another, many never worked well at all.not only that, they were (are) expensive.
it all comes  down to $$, doesn't it?  yamaha experimented on the visions carbs and it turned out that they work really good if you keep them clean, and, set them up correctly.  then they give us a gas box with lower corners that collect water, rust the tank out, and send invisable rust particles into the pretty good carbs to clog them up!
carbs are a bundle of compromises, sort of like tires.  for most riders most of the time the carbs on the vision are too big, and too big carbs can and do cause ridability problems like the ones that come stock with the vision. things like bogging and flat spots.  these can for the most part be tuned out, but it isn't what the factory had in mind for them to be so fussy.  too much warranty cost.
carbs work ok if you can make them run a little bit rich all the time everywhere.  the problem with this approach is that it doesn't produce the best power and it has a negative effect on exhaust emissions.

[silicon_toad2000]

Appologies Bernard for my rant on the other post. I had you pegged all wrong. I thought you were just some psycho that didn't know what he was talking about, being all negative about someone having a go.
I have been told that you do know what you are talking about.
So again, my appologies.

[Glyn Pickering]

Hi Blake,

Drop me a mail on  pickteam@xtra.co.nz  if you want to swop
ideas on the single carb conversion. There seems to be loads of stock 40mm CV Harley carbs on e bay with manifolds etc.
Were these not a good carb - or are they all just upsizing - like Macdonalds fries? to 42mm or larger? Rec ::)kon a 40mm would be OK for the Vision? I'm about to throw in a bid for a 40mm carb.

Cheers

Glyn.

[Bernard Lajoie]

     S.Toad,Thank you but it's not nessary,when you go to this site to read whats new some things never change. Had a nice chat with R.Nowak(he E-mailed me,feel free to do the same)I think he understands were I come from now.Had a kid in FLA.whom I got started into road racing that went by the name of "Team Toad" so "Silicone Toad" got my attn.Glyn,the reason your seeing so many harly c.v. carbs for sale is they either wasted their $ on an S&S carb(lawn mower tech) or  they spent it on an HSR 42 or 45(depends how big the motor is) which is light years ahead of the S&S! I would go down to your friendly HD dealer or independent shop(HD oriented) and pick one up there.$50 should do it.your next trick will be fab.of an intake manifold,card board mock-up will a good starting point,don't use the rubber manifolds,you want it to bolt to the heads,runners cannot be flat(fuel will"puddle")think of it as a tunnel ram.At this point I hope you find some one that is good at fabing with alum. and a very open mind,pull your motor or mock-up a junker and build it on the motor(out of the frame)after you work all the problems with the card board model in the bike.The only other problem I see is the carb "shutting off" since this carb works on a motor that turns 5,500rpm's on a good day when the V will turn 10-11K IF it's working right at the time.Then I could be wrong(I know it's hard to believe but I can admit it)anyway good luck keep me(us)posted on the project,you will receive input like it or not.P.S.don't use one of those f*#king "hyper charger"air cleaners cause they"look cool" they are junk!!cheers frenchthewrench

[Brian Moffet]

The only other problem I see is the carb "shutting off" since this carb works on a motor that turns 5,500rpm's on a good day when the V will turn 10-11K IF it's working right at the time.

I don't think this would matter much.  The vision has a cubic displacement of about 1/2 a harley, but spins at twice the speed, so the cubic feet per minute of air being pulled through the carb will be about the same.  

There may be a matter of the pulses of inrushing air being different, but I my hunch is that won't make much of a difference.  The Harley engine would be more pulse prone since it has to pull in a larger volume 1/2 the time.  The Vision should be more like a constant pull from the carb as compared to the Harley.

It is an interesting question though.  

Just pondering this with an eye to the physics...

Brian

[wolfman]

I don't think this would matter much. ?The vision has a cubic displacement of about 1/2 a harley, but spins at twice the speed, so the cubic feet per minute of air being pulled through the carb will be about the same. ?

There may be a matter of the pulses of inrushing air being different, but I my hunch is that won't make much of a difference. ?The Harley engine would be more pulse prone since it has to pull in a larger volume 1/2 the time. ?The Vision should be more like a constant pull from the carb as compared to the Harley.

It is an interesting question though. ?

Just pondering this with an eye to the physics...

Brian

Brian is of course correct.  A four stroke engine has to turn 2 complete revolutions to complete 1 combustion cycle, so.. a 1100 cc engine turning at 5000 rpm actually is moving (using) the same amount of air/fuel mix as a 550 cc engine turning 10k rpm.  So, a carb that can keep up with said 1100 cc engine would have no problems keeping up with the needs of a 550 turning at twice it's average rpm.  While a vacuum meter hooked to a running Vision would tend to disagree, the vacuum being drawn through the intakes/carb bores is constant but somewhat variable when the engine is running. (It never reverts to "pressure" just highs and lows in Hg) so I likewise doubt the "pulses" are of any concern.  Since the fuel pump of a "V" functions independant of the carbs, it could also be used along with the YICS canister to enhance fuel delivery and combustion if needed.

[Glyn Pickering]

Thanks for all the input everyone.

I've decided to go for a Mikuni flatslide carb, 40mm since they appear to give a more direct respose than a CV, also no diaphram etc to give potential problems and loadsof tuning info / jets avalable. Will use a Harley airfilter.

My homemade Yikes box worked very well but really showed up the stumble nicely. No I havn't tried to synch the carbs etc, anyway it seems to be an ongoing issue worthy trying to eliminate. I bought some 40mm copper pipe fittings today, two right angle bends and a T piece which seem to be just
right size for the carb.  I'm doing the keep it simple approach
and not spending too time or money intially. Hey if the work on Harleys ( a high tech machine if there ever was one) it should work on a low tech Vision. We'll see...

I'm not a racey type so doubt my bike will get above 5000rpm
at any stage.

Cheers
Glyn