How do 'you' know which cyl must be richer?

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John Orchard
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How do 'you' know which cyl must be richer?

Post by John Orchard »

Forgetting what Honda have done originally, how does anyone here 'know' why front or rear cylinders require different jetting? Has anyone run on a dyno with two completely separate exhaust systems? I'm talking with same length intake trumpets.

Some say "the rear cylinder runs hotter (which I could agree with) and that it needs extra fuel to cool it", surely the the designed-in factors of the cooling system would control that?

Some say "the hotter (rear) cyl causes a richer mixture", a cold engine needs a richer mixture (choke) and less fuel when it warms-up. So I could agree that a rear cyl, if running hotter (but not dangerously over the limit) might require 'less' fuel.

If nobody here can offer tested proof, then we throw all previous suggestions out the window, and stick with quoting same size: pilot jets, emulsion tubes, needles and main jets between cylinders, if running same size intake trumpets?

Your thoughts?
jchesshyre
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Re: How do 'you' know which cyl must be richer?

Post by jchesshyre »

My assumption was that it is because in stock form the rear cylinder has a longer intake trumpet, meaning that the rear cylinder produces more torque at lower revs than the front cylinder does, and vice versa at higher rpm, and that this particular tune likes a slightly richer mixture.

I've always thought that the cooling difference is negligble because it's liquid cooled.

All of the above might be entirely wrong - it's just half-baked musing based on some knowledge and some guessing!
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8541Hawk
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Re: How do 'you' know which cyl must be richer?

Post by 8541Hawk »

John Orchard wrote: Tue Oct 29, 2019 6:47 am
Some say "the rear cylinder runs hotter (which I could agree with) and that it needs extra fuel to cool it", surely the the designed-in factors of the cooling system would control that?
Some say this because it is the truth and has a simple reason behind it. The front cylinder is hanging out in the breeze while the rear cylinder is buried in the bike.
The added air cooling of the front cylinder is what causes the "issue" and why the cooling system design can not control this phenomenon.

I guess you could shroud the front cylinder to address the issue but it is so much easier and lighter weight to just jet each cylinder for where it lives.

When you really get into tuning V twins you actually port each head slightly different but that is another topic something you might want to talk with Roger about :thumbup:
Loud pipes don't save lives, knowing how to ride your bike will save your life.
John Orchard
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Re: How do 'you' know which cyl must be richer?

Post by John Orchard »

Yep I can agree that the rear cyl is running hotter than the front at certain times BUT how should carburation be changed to suit that temp increase? I have my own ideas on this (see my background below), I'm interested in proof of testing from anyone else in the world.

Is extra richness to cool the cyl? If extra fuel is used to cool it, then the mixture is not correct for best performance. I guess most of us have removed the radiator expansion tank and modified the ducting around the cylinders to increase cool airflow to the rear cyl, so temp variation between cyl's would be minimal.

Considering different length intake trumpets, yes different jetting could be required, although altering the trumpet length on some of my older GSXR750/1100's (carb models) didn't require it. Generally a longer intake trumpet will richen the mixtures, as the intake charge creates a slightly lower atmospheric pressure in the venturi, hence pulling (pushing) more fuel out.

Unless you run the bike on a dyno/rolling brake road with a lambda sensor in a separate exhaust system for each cyl, or a lambda probe in each header, no one knows for sure. This is why I ask the question :-)

I have been working in the industry as a motorcycle mechanic for 45 years, raced Superbikes in England, America & Australia for 25 years. I spent many hours on the dyno turning the first VTR that came into Australia into a racebike.
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fabiostar
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Re: How do 'you' know which cyl must be richer?

Post by fabiostar »

on the subject of trumpets on these, when i changed mine to shorter trumpets, both front and back it caused the bike to run rich, iv just been through a major ballache with mine that came down to simple airflow causing all sorts of hassle, iv two engines, a standard one that likes the rear pot richer on the jetting and a tuned motor that uses the same jetting front and rear pot,

these huge carbs on these bikes are SO touchy and dont behave like most bikes fitted with CV carbs. for a standard motor, as in just maybe pipes but no internal mods honds done a really smart job on the intake side of things to balance them out. even changing the emulsion tubes front and back can cause upset on a lot of motors. :thumbup:
the older i get,the faster i was :lol:
tony.mon
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Re: How do 'you' know which cyl must be richer?

Post by tony.mon »

I ended up doing as you suggest, running a wide and lambda sensor in each downpipe linked to a real time display on the dash.
It's not that expensive, and saved me lots of dyno time.
And if you run a flo commander circuit you can spoof smaller or larger mains, and with a second circuit, pilots, to adjust fuelling without having to strip and reject every time.
I found that to get the stoich ratio right I needed one size up on mains on the rear.
But that was on an engine with lots of mods, so a standard one might behave differently.
It's not falling off, it's an upgrade opportunity.
John Orchard
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Re: How do 'you' know which cyl must be richer?

Post by John Orchard »

tony.mon wrote: Wed Oct 30, 2019 11:01 pm I found that to get the stoich ratio right I needed one size up on mains on the rear.
But that was on an engine with lots of mods, so a standard one might behave differently.
Ah cool, thanks. :-)
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8541Hawk
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Re: How do 'you' know which cyl must be richer?

Post by 8541Hawk »

John Orchard wrote: Wed Oct 30, 2019 12:36 am Yep I can agree that the rear cyl is running hotter than the front at certain times BUT how should carburation be changed to suit that temp increase? I have my own ideas on this (see my background below), I'm interested in proof of testing from anyone else in the world.
Well I only have over 20 yrs of real world testing on a VTR plus more than I want to admit before on other bikes but no worries....
John Orchard wrote: Wed Oct 30, 2019 12:36 amIs extra richness to cool the cyl? If extra fuel is used to cool it, then the mixture is not correct for best performance. I guess most of us have removed the radiator expansion tank and modified the ducting around the cylinders to increase cool airflow to the rear cyl, so temp variation between cyl's would be minimal.
No the extra fuel is not to"cool" the rear cylinder. Let me ask you this, if you had engine "A" that runs at 200 C and engine "B" that runs at 250 C would they be jetted any different?
Wouldn't the hotter running engine need to be run a touch richer just because it runs hotter?
John Orchard wrote: Wed Oct 30, 2019 12:36 amConsidering different length intake trumpets, yes different jetting could be required, although altering the trumpet length on some of my older GSXR750/1100's (carb models) didn't require it. Generally a longer intake trumpet will richen the mixtures, as the intake charge creates a slightly lower atmospheric pressure in the venturi, hence pulling (pushing) more fuel out.
Been running a set of billet, equal length, stacks for around 10yrs.....while this requires re-jetting you still end up with the staggered jetting for best performance.
So the "real world" testing has shown the different stack lengths make no difference to the jetting requirements on this bike.
John Orchard wrote: Wed Oct 30, 2019 12:36 amUnless you run the bike on a dyno/rolling brake road with a lambda sensor in a separate exhaust system for each cyl, or a lambda probe in each header, no one knows for sure. This is why I ask the question :-)

I have been working in the industry as a motorcycle mechanic for 45 years, raced Superbikes in England, America & Australia for 25 years. I spent many hours on the dyno turning the first VTR that came into Australia into a racebike.
Sorry you don't believe running the bike in real world conditions rather than strapped to a rolling road can give usable information but then again mine has also been on a dyno a few times...so I guess I can draw from both.
Also I am a bit confused as with all the dyno time you have spent on a VTR that this question still bothers you. It's just how V-twins work.
As I stated before, per Roger Ditchfield (who has forgotten more about VTRs than any of us will ever know) you even need to port the heads differently for the front and rear cylinder for best performance.
Which can be extrapolated to the cylinders flow differently from each other, which could cause the need for staggered jetting.
Plus the economic aspect of the issue. If there was an easy "fix" wouldn't the manufacturers be all over it but instead they need to stock 2 different main jets, 2 different needles and 2 different emulsion tubes for each VTR.

While it would seem that IMHO you are trying to reinvent the wheel, still wish you the best of luck with your endeavor :thumbup:
Loud pipes don't save lives, knowing how to ride your bike will save your life.
jchesshyre
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Re: How do 'you' know which cyl must be richer?

Post by jchesshyre »

8541Hawk wrote: Fri Nov 01, 2019 4:52 pm
Been running a set of billet, equal length, stacks for around 10yrs.....while this requires re-jetting you still end up with the staggered jetting for best performance.
So the "real world" testing has shown the different stack lengths make no difference to the jetting requirements on this bike.
Interesting to know this, thanks :thumbup:

Something I've just thought - if the front cylinder running cooler is due to airflow over it while the bike is moving, presumably this means that this airflow needs to be fairly accurately replicated when on a dyno?
tony.mon
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Re: How do 'you' know which cyl must be richer?

Post by tony.mon »

All dynos will have a fan, to blow air over the rads.
But a good dyno will have a fan system capable of blowing air in at a sufficient flow rate to stimulate a ram Air effect if the bike has intakes designed to make use of this.
It's not falling off, it's an upgrade opportunity.
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fabiostar
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Re: How do 'you' know which cyl must be richer?

Post by fabiostar »

i just found out the hard way that a dyno can lie, as you say tony they need and high airflow.

i had mine strapped to a dyno weeks ago and it showed up no problems as such, rode the bike down the road 5 mins later and it run like a bag of sh1t :lol: ..

and the problem was down to simple airflow. the dyno couldnt reproduce what real world riding/airflow showed up, it only took me 8 months to find this out :whatever
the older i get,the faster i was :lol:
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