Technical Discussion Topics related to Technical Issues

So I just GOTTA ask - why TWO carbs?

Old 10-14-2007, 04:14 PM
  #1  
Senior Member
Back Marker
Thread Starter
 
Slim's Avatar
 
Join Date: Sep 2007
Location: Toronto, Ontario, Canada
Posts: 203
Slim is on a distinguished road
So I just GOTTA ask - why TWO carbs?

Yeah yeah, i'm new. Give me a towel for behind my ears and all that. LOL!

Yes, I tried searching. Didn't find anything so .....

- has anyone tried it?

- results?

- if no one has tried it then why not?

I remember reading a while back some guys tried altering their stock configuration on a Virago (v-twin, two carb) with a different carb and intake manifold with varied results. Really wondering what might happen if one did this with a VTR.

Anyone care to provide input ..... or laugh in my face?

Lemme know. PATIA!
Slim is offline  
Old 10-14-2007, 04:30 PM
  #2  
Senior Member
SuperBike
 
Gregw's Avatar
 
Join Date: Jun 2007
Location: Los Angeles ish
Posts: 1,031
Gregw is on a distinguished road
I guess if you're striving for that Harley power you could try one carb. Seriously though, the straighter the passage for the fuel charge into the combustion chamber, the better. No kinks, bends or branching to slow things down.
Gregw is offline  
Old 10-14-2007, 06:07 PM
  #3  
Banned
MotoGP
 
Join Date: Aug 2007
Location: Lake View Terrace, CA
Posts: 5,942
8541Hawk will become famous soon enough
'Cuz three won't fit.......
8541Hawk is offline  
Old 10-14-2007, 06:18 PM
  #4  
Banned
MotoGP
 
Join Date: Aug 2007
Location: Lake View Terrace, CA
Posts: 5,942
8541Hawk will become famous soon enough
Now for the real reason..... it allows you to tune each cyl. to maximum efficiency and proper A/F ratio. On a VTR the rear cyl. needs to run richer than the front due to the fact it runs hotter. So with a single carb you could jet it for the rear and then the front would be running rich or jet it for the front and thr rear would be lean.
8541Hawk is offline  
Old 10-14-2007, 07:59 PM
  #5  
RK1
Senior Member
MotoGP
 
Join Date: Aug 2007
Location: Way Out West
Posts: 2,547
RK1 is on a distinguished road
I've been wondering about the richer rear cylinder thing for a couple of reasons.

My '83 V45 Interceptor came with the same size main jets in all four cylinders. So did all the later carbed VFRs. You would think, if anything, the rear bank of a Vee-four would run hotter than the rear cylinder of a V-twin.

I believe VTRs shipped for sale in the UK came with same mains front and rear, and I know the Dynojet UK kits call for same mains front and rear.

Maybe the cooling system on VTRs is considered more marginal than the one on the VF/VFRs? Maybe it only matters when VTRs are likely to be run in the much higher summer temperatures encountered in some parts of the USA, temps much higher than those encountered in the UK?

Does anyone out there actually know how much hotter the rear cylinder actually gets on a VTR?
RK1 is offline  
Old 10-14-2007, 08:15 PM
  #6  
Senior Member
Back Marker
Thread Starter
 
Slim's Avatar
 
Join Date: Sep 2007
Location: Toronto, Ontario, Canada
Posts: 203
Slim is on a distinguished road
Originally Posted by RK1 View Post
I've been wondering about the richer rear cylinder thing for a couple of reasons.

My '83 V45 Interceptor came with the same size main jets in all four cylinders. So did all the later carbed VFRs. You would think, if anything, the rear bank of a Vee-four would run hotter than the rear cylinder of a V-twin.

I believe VTRs shipped for sale in the UK came with same mains front and rear, and I know the Dynojet UK kits call for same mains front and rear.

Maybe the cooling system on VTRs is considered more marginal than the one on the VF/VFRs? Maybe it only matters when VTRs are likely to be run in the much higher summer temperatures encountered in some parts of the USA, temps much higher than those encountered in the UK?

Does anyone out there actually know how much hotter the rear cylinder actually gets on a VTR?
No, but I was gonna ask. Actually, I wouldn't mind getting an infared temperature gun and doing some testing. I can understand it being hotter because of less cooling, but how much ..... I dunno. I think between that and independantly monitoring afr using a WB02 on each cylinder they would be neat tests/observational reports.

Always wondered about the overseas DJ kits.
Slim is offline  
Old 10-14-2007, 08:18 PM
  #7  
Senior Member
Back Marker
Thread Starter
 
Slim's Avatar
 
Join Date: Sep 2007
Location: Toronto, Ontario, Canada
Posts: 203
Slim is on a distinguished road
Originally Posted by 8541Hawk View Post
Now for the real reason..... it allows you to tune each cyl. to maximum efficiency and proper A/F ratio. On a VTR the rear cyl. needs to run richer than the front due to the fact it runs hotter. So with a single carb you could jet it for the rear and then the front would be running rich or jet it for the front and thr rear would be lean.
Y'know, I always miss the simple things. (shrug)

So I guess the biggest difference this can be done on cruisers is ..... airflow over the engine? But it's not like a VTR is fully clad anyway.

EDIT - Hawk, is this also part of the reason that we don't see more riders switching to twin filter elements (a la the RC51)? *sorry, haven't searched that one yet, only just thought of it now


I'm with RK1 - anyone got any numbers? How much difference between the two cylinders?

Last edited by Slim; 10-14-2007 at 08:38 PM.
Slim is offline  
Old 10-17-2007, 03:06 PM
  #8  
Senior Member
Superstock
 
Join Date: Mar 2007
Location: San Diego
Posts: 268
Anto is on a distinguished road
So, no one here has tried the same rear & front main jet?

sounds like you guys might be on to something
Anto is offline  
Old 10-17-2007, 08:04 PM
  #9  
Moderator
MotoGP
 
superbling's Avatar
 
Join Date: Dec 2004
Location: South Texas
Posts: 2,553
superbling is on a distinguished road
I would think jet kit companies would have done it if it worked.

Flat track harleys turn the rear cylinder around 180 and run twin carbs. That also puts the rear exhaust facing forward for max cooling.
superbling is offline  
Old 10-17-2007, 09:15 PM
  #10  
Banned
MotoGP
 
Join Date: Aug 2007
Location: Lake View Terrace, CA
Posts: 5,942
8541Hawk will become famous soon enough
I've never heard of doing it. You would end up with either a rich front or a lean rear as the rear runs hotter. As for the VFR question, I believe that the cly's on the VFR run at a closer temp to each other due to the placement of the radiator. The incoming air passes through the rad. before it hits the front cly's so it doesn't cool them near as much as the front cyl. on a VTR which is exposed diectly to the air stream. but thats just my opinion and we all know what thats worht.
8541Hawk is offline  
Old 10-17-2007, 09:24 PM
  #11  
Banned
MotoGP
 
Join Date: Aug 2007
Location: Lake View Terrace, CA
Posts: 5,942
8541Hawk will become famous soon enough
Originally Posted by Slim View Post
EDIT - Hawk, is this also part of the reason that we don't see more riders switching to twin filter elements (a la the RC51)? *sorry, haven't searched that one yet, only just thought of it now
Another opinion question....damn
Well the RC51 would be easy to tune for the filter chage as it's a fuel injected bike. The CV carbs on the VTR like a nice smooth air flow to work well. So if you don't want to spend a bunch of time chasing lean and rich spots through your power band (caused by fluctuations in air pressure) you really wan't to leave your air box alone, maybe run a aftermarket filter, but that's it unless you are really comfortable tuning CV carbs.
That and the bigger the CV carb, the more touchie it is to get it just right.
8541Hawk is offline  
Old 10-17-2007, 09:51 PM
  #12  
RK1
Senior Member
MotoGP
 
Join Date: Aug 2007
Location: Way Out West
Posts: 2,547
RK1 is on a distinguished road
I've never run the same mains front and rear 'cause where I live summer temps run way over 100 degrees F many days. With coolant temps running near 230 in traffic, I'm happy to have the rear jug a bit richer. If I lived where it never got over 75 or 80 degrees I don't know if it would matter.

Again, other V engined bikes, including Hondas, run same jets F/R, but Honda engineers decided VTRs shipped to hot places (but not more moderate climes) should run richer in the rear. I think Honda considers the VTR cooling system a bit marginal in really hot ambient temps.

It must be concern only about rear cylinder damage when it is really hot out. Otherwise why ship bikes to the UK with same jets front and rear?

Last edited by RK1; 10-17-2007 at 09:52 PM. Reason: mis-spelled "marginal"
RK1 is offline  
Old 10-17-2007, 10:22 PM
  #13  
Banned
MotoGP
 
Join Date: Aug 2007
Location: Lake View Terrace, CA
Posts: 5,942
8541Hawk will become famous soon enough
Can't answer as I don't have the specs for a Euro bike.
Do you have a like to somewhere that shows them?
8541Hawk is offline  
Old 10-17-2007, 11:17 PM
  #14  
RK1
Senior Member
MotoGP
 
Join Date: Aug 2007
Location: Way Out West
Posts: 2,547
RK1 is on a distinguished road
8541Hawk;

Don't have a link. Just remember reading multiple posts on Firestorm UK forum of guys mentioning their bikes came with same jetting F/R and bitching about the UK Dynojet kits coming with 3 matching pairs of mains instead of the six different sizes in the U.S. kit.

It is a retorical and purely academic question for me 'cause 1). My bike runs great with the +1 jet in the rear. 2). It gets really hot here. and 3). I assume both the Honda and Dynojet engineers know what they're doing.

I guessing they shipped VTRs to cooler places with same/same jets and hotter places with +1 rears because they were concerned about rear jug heat in the hot places. I don't know that for a fact, I just can't think of why else they'd do it that way.
RK1 is offline  
Old 10-18-2007, 04:51 PM
  #15  
Remember stock is BAD!
SuperSport
 
divingindaytona's Avatar
 
Join Date: Jun 2004
Location: Jacksonville FL
Posts: 760
divingindaytona is an unknown quantity at this point
Does anyone out there actually know how much hotter the rear cylinder actually gets on a VTR?

I know on ducs that I have worked on there can be a 20-30 degree temp difference between the two cyl heads. I found this on the 996 and 998s.

I would imagine that the hawks run close to that
divingindaytona is offline  
Old 10-18-2007, 05:22 PM
  #16  
Senior Member
Back Marker
Thread Starter
 
Slim's Avatar
 
Join Date: Sep 2007
Location: Toronto, Ontario, Canada
Posts: 203
Slim is on a distinguished road
Wow, great stuff guys! Noticing i'm missing a few more easy, simple things and then a few that I didn't think of.

So since the rear runs warmer then why not get rid of the dust shield infront of the carbs and make some ducts to funnel air over the rear cylinder? Maybe even use those dummy air vents up front to guide some air through too?!?!

Just thinking out loud. I'm not an engineer, but even the big corporations have to build compromises that I don't necessarily have to be forced to live with. We have aftermarket parts because ..... ? That's right - choice.


So to throw a curve ball into the pattern now - if the rear carb is re-jetted to compensate from the heat build up, then why are people bothering with using a different venturi (both short or both tall)? My limited knowledge on this tells me people are asking for tuning issue(s) to insue.
Slim is offline  
Old 01-24-2008, 08:02 PM
  #17  
Remember stock is BAD!
SuperSport
 
divingindaytona's Avatar
 
Join Date: Jun 2004
Location: Jacksonville FL
Posts: 760
divingindaytona is an unknown quantity at this point
Originally Posted by Slim View Post
why are people bothering with using a different venturi (both short or both tall)? My limited knowledge on this tells me people are asking for tuning issue(s) to insue.
The reason why there are two different size intake runners is tuning. The short intake runner vs. the long intake runner produces a different airflow.

The way the airbox is set up - the short runner is optimized for horsepower. The longer intake runner is tuned for torque. That is just the way that intake plenums are set up.

The inlines now a days use a valve to achieve this, this valve is controlled by a servo.
divingindaytona is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
Circuit_Burner
Knowledge Base
15
10-14-2013 06:44 PM
FTL900
General Discussion
15
08-17-2013 02:02 AM
tomzxt
Everything Else
9
04-16-2012 12:45 PM
bigpat123
General Discussion
39
12-12-2005 07:48 PM
Dan S.
General Discussion
9
03-25-2005 07:53 AM


Thread Tools
Search this Thread
Quick Reply: So I just GOTTA ask - why TWO carbs?


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.

All times are GMT -7. The time now is 02:30 AM.


© 2019 MH Sub I, LLC dba Internet Brands