Cafe Racer Forum banner

41 - 60 of 60 Posts

·
Registered
Joined
·
30 Posts
Discussion Starter #41
No the bike is not rideable now. What I am trying to do is get the mixtures close enough to ride and then do a road test to get it right. I know the settings will not be just right but close enough to ride. The rich mixture did not have a strong smell but I am using non ethenol gas. But was obviously a wet mixture.
 

·
Registered
Joined
·
5,602 Posts
I honestly think you are trying to tune out a symptom that might not even exist, if the motor starts well and the idle screw makes the idle go up and down between ~600 and 1200 rpm but the idle remains steady, I think you will find that pilot settings are good to go and you can't really do much more carburetor tuning until you test it under load and at normal operating temperatures.
Maybe try holding a clean white cotton cloth over the exhaust outlets and see if you can catch some of what you think is unburnt fuel.

Observing all obvious fire safety considerations, if that rag does become soaked with wet gasoline it will burn like a sob,
if it becomes wet with dirty water condensation and carbon it won't burn worth a crap.
 

·
Registered
Joined
·
2,941 Posts
:cool: ............... Adding an O2 sensor and gauge is still the best easy way to test your fuel to air ratio.

... what happens when you apply the choke? (which is actually a fuel enrichment circuit and not a choke plate)


... son of a gun lookie there they show all these gizmos reading 14.7 ;) I wonder where they came up with that number.
No it is not. Unless you know what air:fuel ratio you re looking for and the answer is rarely 14.7:1 in case anyone skipped that part.

And just out of interest, who here has fitted an A:F gauge and found it to be a useful tool on a street bike? That would be no one probably because you have no frame of reference. By all means use a gas analysis on a dyno where the throttle is fairly steady for long enough to get a steady state reading but on the average street, that ain't going to happen very often for very long. Add a TPS and accelerometer and it's starts to become more normalized.

For sure, riding down the highway at steady throttle is possible but that's rarely what we do when trying to get the jetting close enough to ride the thing safely. Even then it's only useful if the technician understands that a steady 14.7 is only useful at cruise and even then it's not what you need, but it has to go rich when the load is increased or else you will melt something expensive.
 

·
Registered
Joined
·
5,602 Posts
No it is not. Unless you know what air:fuel ratio you re looking for and the answer is rarely 14.7:1 in case anyone skipped that part.

And just out of interest, who here has fitted an A:F gauge and found it to be a useful tool on a street bike? That would be no one probably because you have no frame of reference. By all means use a gas analysis on a dyno where the throttle is fairly steady for long enough to get a steady state reading but on the average street, that ain't going to happen very often for very long. Add a TPS and accelerometer and it's starts to become more normalized.

For sure, riding down the highway at steady throttle is possible but that's rarely what we do when trying to get the jetting close enough to ride the thing safely. Even then it's only useful if the technician understands that a steady 14.7 is only useful at cruise and even then it's not what you need, but it has to go rich when the load is increased or else you will melt something expensive.
Mike for one:

On the CB250 I race, I have 30mm VM Mikunis. Most people I know racing CB350 have 32mm VM.
Regarding Air-Fuel Ratio. If you search Wikipedia you'll get a fairly accurate explanation. From a couple of other sources, I've concluded the following.
A lean mixture (high A/F ratio) will give higher top end temperature. No good in an aircooled engine.
Maximum power will be obtained with A/F around 12.5 to 13 on ethanol free petrol.
My CB250 has moderate tuning, only a fairly hot cam but still standard springs. Redline a mere 12500rpm. Still capable for podium places.
For last practice I fitted a wideband lambda sensor to it. Readings in race pace mostly 12.5-13, so it seems jetting is OK.
Happy with that.
Plus every manufacturer of modern motorcycles and gas powered cars in this century :/
The A:F gauge reads from an O2 sensor in the exhaust path just as your fuel injection system does, it's how the fuel injection computer knows how much fuel to deliver to the injector and how to time the ignition. The frame of reference is the stoichiometric ratio of the fuel you are attempting to burn. It is a scientific proven fact that for gasoline the fuel to air ratio that provides the most complete combustion of gasoline is around 14.7:1 it is also a race proven fact that a more rich ratio will perform better on the track.

I'm not the guy telling him to lean out his pilot jet until he doesn't see smoke exiting the exhaust at idle and risk melting his pistons :/ I'm the guy telling him to stop fuking-around with the fuel jets that came on his carburetors until he test rides it.

If the OP leans out his VM carburetor to the point that it performs as lean as the original CV carbs that were on the 350 twin motor, I wager that he will introduce a significant hesitation in the bikes ability to accelerate smoothly from a dead stop. That's one of the concepts behind the CV carb design, they use the motors intake vacuum to operate instead of the throttle cable, you can haul onto the throttle all you want with a CV carb and the throttle slide and main jet needle will only open as fast as the vacuum allows it, with a VM carb you can easy exceed that F:A ratio and starve the motor for fuel.

Does he need to fit a sensor to tune his carb, absolutely not! But he does need to test ride it at more then just idle and sitting on the centre stand and from my read that is what he has been doing so far.
Putting it on a Dyno puts the bike in the situation of operating under a load except without the added resistance of wind that would be typical from riding the bike, that's what a Dyno is! It is a machine that emulates the conditions of actually riding the motorcycle.

Does he need to set the bike to operate at this optimum stoichiometric ratio throughout its operation, absolutely not! But pounding more fuel through it at any point in its operation to cool the motor will result in raw fuel exiting his tail pipe because his old bike lacks one more thing, it has no catalytic converter that if fitted would attempt to burn off any unburnt fuel and turn it into wasted heat.
 

·
Registered
Joined
·
2,941 Posts
Mike for one:



Plus every manufacturer of modern motorcycles and gas powered cars in this century :/
The A:F gauge reads from an O2 sensor in the exhaust path just as your fuel injection system does, it's how the fuel injection computer knows how much fuel to deliver to the injector and how to time the ignition. The frame of reference is the stoichiometric ratio of the fuel you are attempting to burn. It is a scientific proven fact that for gasoline the fuel to air ratio that provides the most complete combustion of gasoline is around 14.7:1 it is also a race proven fact that a more rich ratio will perform better on the track.

That's almost right but not quite. The fallacy that many people seem to believe is that 14.7: is some sort of magic number and if they get that, everything will be perfect and that's just not accurate. Forget about tracks or racing. Max power is with a surplus of fuel and max economy is with a surplus of oxygen. In other words, to get maximum power, all the oxygen must be used and for various reasons that takes more fuel that 14.7:1. It's easier to understand that for maximum economy, all the fuel must be burned and to achieve that there has to be a surplus of oxygen.

Starting and slow speed running needs to be slightly rich. One reason that slow speed has to be rich is that to accelerate it has to be closer to 12:1 and opening the throttle with a 14.7:1 idle leads to more air being pulled in before the heavier fuel starts to move, so it goes really lean for a second or so and hesitates.

With any fuel injection system, that's not an issue because the engineering team link up a TPS so that as the throttle is opened, the mixture can be richened to compensate. If you look at a 3D fuel map on almost any vehicle you can see the values cover a huge range and only at small throttle openings and moderate (cruising) revs is the target A:F at 14.7 or higher.

You probably understand that already but many people don't and they read things telling them to aim for 14.7 and their bikes will not run well. And that doesn't begin to address the fact that different fuel have a range of A:F from 14.4 to 15.0 depending on the composition. And of course cool dense air contains more oxygen and will read "lean" and high altitudes, or hot air have less oxygen will read "rich" just to add to the confusion.

On the street an A:F gauge is at best a distraction. On the race track it has value to ensure consistency as air conditions change. They are a tool and like any other tool require training. We jet our race motors and any modified motor on the dyno and the A:F is useful to tell us where we are but it's just another set of data points. If a particular motor makes most power at a certain ratio with a certain fuel, it's useful to know what jetting gave that result so it can be changed as weather changes to maintain max power for the changed conditions, but on the street it's one more thing to distract me.

If the OP understands all that and wants to use a gauge, all power to him, but I wouldn't recommend it.

Get idle right and try to run it. Tweak it until idle is OK and pick up is snappy then get the mains right. After that get the middle right. We agree on most of that.
 

·
Registered
Joined
·
5,602 Posts
Hey man all I'm trying to do is help him get his motorcycle working good and to that end he has already made progress, he now has his bike firing on both cylinders. Is all good :cool:
 

·
Registered
Joined
·
30 Posts
Discussion Starter #47
Well I didn't mean to start a big thing. What seems to have gotten lost in this is: I am fitting a new set of carbs that I know 100% is jetted completely wrong for this bike, the bike is not now idling correctly and is completely not rideable so all I can do is get it idling close enough to go on and get the mains close enough to ride and then get the jetting correct for street use.
 

·
Registered
Joined
·
30 Posts
Discussion Starter #50
Well I'm making progress. Got the 22.5 pilot jets in and got a good clean idle with the air screws turned out about 1 1/2 turns. Now I can go to the mains. They are 180's right now and appear to be way too rich yet. Probably will be a couple of days before I get to play with it any more.
 

·
Registered
Joined
·
1,680 Posts
And just out of interest, who here has fitted an A:F gauge and found it to be a useful tool on a street bike? That would be no one probably because you have no frame of reference. By all means use a gas analysis on a dyno where the throttle is fairly steady for long enough to get a steady state reading but on the average street, that ain't going to happen very often for very long. Add a TPS and accelerometer and it's starts to become more normalized.
I have! But I used it with data logging rpm, tps and tuned for best acceleration. A/F measurements were used to help determine initial direction of tuning and to add a small factor of safety.

I agree, a simple a/f meter that is accurate only at stiochemetry is of little or no use for performance tuning. An ultimately tuning to an a/f ratio doesn’t necessarily mean the fastest accelerating bike.

This was tuning a “modern” EFI bike with a piggyback ecu tuner (2006 bmw k1200r with rapidbike module and innovate wideband o2 sensor and data logger). I also agree with you, tuning a carb’d bike for the street can be done without it.
 

·
Registered
Joined
·
1,680 Posts
One thing I learned from the project was that a motorcycle will run on a WIDE range of a/f ratios. Even stock from the factory, my bmw had a HUGE lean hole purposely tuned into the WOT map. We were talkin 15:1 ratio lean.
In other words, engines will run almost despite what you do to it. And there is no real magic a/f ratio.
 

·
Registered
Joined
·
2,941 Posts
One thing I learned from the project was that a motorcycle will run on a WIDE range of a/f ratios. Even stock from the factory, my bmw had a HUGE lean hole purposely tuned into the WOT map. We were talkin 15:1 ratio lean.
In other words, engines will run almost despite what you do to it. And there is no real magic a/f ratio.
And with wide band sensors at around 90 bucks a time as they die prematurely, it's cheaper on a dyno..
 

·
Registered
Joined
·
5,602 Posts
On my 2-stroke TY trials bikes I had to adjust the cir-clip on the main jet needle once every spring and again in the fall because of the weather changes. But I do live in a climate that see's drastic swings in temperature and humidity and they sell winter gas up here throughout our winter months, ymmv. It is -20C here today.

Dyno, lol city people, I think the closest dyno to where I live would be 3 hours away and 30 minutes before a competition starts when you are 6hours away from home that would never be an option. Tuning a VM Mikuni is suppose to be easy, that's one of the big reasons to run one.

My Montesa bikes came with 2 header pipes, one with an O2 sensor and one without, no extra charge, needless to say the O2 sensors are just like new and still will be in 20 years time.

Oh and can you set up a carburetor without all the sensor technology :/ well of course you can, my grandfather was scratch building 2 and 4-stroke gasoline powered motors at the turn of the previous century and long before the sensor technology even existed.

OP have you got that thing running right yet? It's probably nice down there, I envy your riding season :cool:
 

·
Registered
Joined
·
5,602 Posts
Well I'm making progress. Got the 22.5 pilot jets in and got a good clean idle with the air screws turned out about 1 1/2 turns. Now I can go to the mains. They are 180's right now and appear to be way too rich yet. Probably will be a couple of days before I get to play with it any more.
You can be a pioneer and document your final setup for the next guy to use :cool:
 

·
Registered
Joined
·
1,680 Posts
You can be a pioneer and document your final setup for the next guy to use :cool:
Yeah! (I want to see someone document a rewiring job, too!)


IMHO, don't bust your hump making pilot jet changes.
For some reason, large main jet changes almost always changes that partial throttle fueling to the extent that you need to change the pilot again. Just get the bike to start and idle and get in the ballpark with the pilot. Then go straight to WOT main jet tuning. Once you get that dialed in, go back and figure out the pilot you need. Pilot changes will be smaller than main jets. Sometimes you can skip main jet sizes. pilots, no so much.
Buy a range of jets, 1 or 2 or 3 usually won't do it. Nice if you have someone that can put you in the ballpark or even better a friend that has the same bike and some spare jets.

I transplanted some more "modern" triumph triple carbs to my yamaha xs750. Stock the triumph carbs had mains in the 90's. really digging through triumph forum posts from riders with your standard mild performance modified engines, I started with mains in the 130's. I ended up with 150 mains (!!!). NUTS! And the triumph legend was a larger displacement triple. 885cc vs 826cc.
 

·
Registered
Joined
·
30 Posts
Discussion Starter #57
Well at the moment I am waiting on more jet needles and tubes. They should be here tomorrow and If it will stop raining long enough I should make some more progress. At any rate I will post all my jetting info when I get the bike running.
 

·
Registered
Joined
·
3,445 Posts
Your never going to get a consistent 14.7:1 with carbs.
EPA were trying to get a constant ratio in 2010, it's why majority of bikes are now fuel injected.
Personally, I would adjust float level at least 1mm higher (1.5mm if your using E10 fuel) VM carbs were designed in the 1950's(maybe earlier, made under licence from AMAL since 1920's) .
Even though main jet isn't in operation at idle, there is always slight 'leakage'. it's worse if fuel is too high in float bowl
Fuel is completely different today compared to even 30 yrs ago so you need a fundamental change in thinking. TThe carb 'manuals' and tutorials were mainly written in 1930's then added to as time prprogressed. I really don't think anyone has done published research on 'new' old technology.
 

·
Registered
Joined
·
2,941 Posts
"I transplanted some more "modern" triumph triple carbs to my yamaha xs750. Stock the triumph carbs had mains in the 90's. really digging through triumph forum posts from riders with your standard mild performance modified engines, I started with mains in the 130's. I ended up with 150 mains (!!!). NUTS! And the triumph legend was a larger displacement triple. 885cc vs 826cc. "

That sounds right in as much as a larger engine will create higher air velocity through the carbs which will pull more fuel in. A smaller motor in lower state of tune with generate a lower gas velocity and will lift less fuel proportionately and is typically harder to jet.

Small carbs have higher gas velocity than large carbs - on the same motor - and tend to be easier to jet and tend to use smaller jets.
 

·
Registered
Joined
·
2,941 Posts
Without a dyno, an A:F gauge with wideband sensor would at least indicate if the mix is lean or rich at all mid throttle openings and loads. It won't tell you what the bike needs but will at least show if it tips from rich to lean or if the fuel curve is lumpy or skewed.
 
41 - 60 of 60 Posts
Top