The Concept of Back Pressure...

Vince Lupo · Post by **Vince Lupo** » Mon Sep 20, 2010 4:28 pm

So based on that restrictive flow at higher revs, would it logically follow that you'd need less gas at higher revs, and not more?

But if that's the case, then why would the power on my bike be the same from 3/4 throttle to full throttle, and therefore point to the need for MORE fuel instead? Or am I reaching the wrong conclusion?

brewsky · Post by **brewsky** » Mon Sep 20, 2010 5:59 pm

Vince, you can try holding wide open throttle under load and then slowly close the choke to see if it picks up any power.
If so, that would point to needing bigger mains.
If not, the mains are rich or OK.
Most of the bikes I have had displayed the same symptom of not feeling any difference in power between 3/4 to wide open throttle in high gear.........until they approached max rpm also.......which doesn't happen very often in high gear on the street.
I put a stick pin in my throttle grip and marked the 1/4, 1/2, 3/4 and WOT positions and rode for a month or so fine tuning my Dream.
I was amazed how much of the time I was at below 1/4 throttle even when going at speds of 50 MPH+.
A plug check without targeting the specific throttle position in question, just gives some average of idle, mid range, and WOT.
To check the mains, new plugs should be installed while the engine is warm, immediately started and taken on a WOT run under load for some distance, then engine killed and drift to stop.
That way the plug color is not influenced by other throttle positions.

jensen · Post by **jensen** » Tue Sep 21, 2010 1:11 am

Hi,

In headlines you're right, but don't forget that buy closing the choke valve, you also alter the venturi of the carb. And that the choke is not designed to work at full throttle.

The back pressure thing is just to complicated to predict behavior from it in a theoretical way. Believe me, (and I'm a physics guy), a discussion about that would be pointless. I read a few books about the subject because of my interest in strokers (I owned a Yamaha RD500 LC4, a Yamaha RD350 UPVS and still own a Kawasaki KR1-s), and I lost track.

There are just too many parameters to take in account.

You know probably what my advice would be, and if you have the money to spend, take the bike to a good dyno place, and sort things out when chancing something on the exhaust system.

A seat in the pants feeling is more influenced by the sound in you ear and how the torque curve is altered (maybe you can read my thread in the "other twins" section about the CB450 bomber). What you could do is do some full acceleration runs with someone who times the runs (or yourself).

I do that sometimes too, just find yourself a nice piece of empty straight road, mark the start and finish, and time yourself a few times with the different set ups. It doesn't say everything, but it will give you data to compare.

Jensen

Vince Lupo · Post by **Vince Lupo** » Tue Sep 21, 2010 5:01 am

Only problem with the suggestion to use the choke is that my Amals do not have chokes (since they have the 'ticklers', there's really no need for them). And, I have in fact had the bike wide open for a stretch, and as I say, the power (speed) is no different than at 3/4 throttle (though I haven't done the 'kill the bike and let it drift to a stop' method). It doesn't feel like it's 'bogging down' at this point, but just that there's no difference. As well, I tried even backing off the throttle to see if maybe it picked up speed (and therefore concluding that my main jet was too big), but that didn't happen either.

Actually a dyno test would be a great idea, if I could find someone in my neck of the woods who in fact had one, and who really knew what they were doing. I'd be more than willing to give that a try!

jensen · Post by **jensen** » Tue Sep 21, 2010 5:39 am

Hi,

Oeps, forgot that halve of your bike is British. I can't help you with that at the distance we are apart.
But if you find a dyno place, (the guy is really the most important part), make sure it has a gas analyzer. The good thing about having two separated mufflers is having the opportunity two balance everything out.

Don't go to dyno day's or quick power run day's, it' not worth your money, and probably the guy doesn't know what he's doing. Make sure the engine works, has good spark, and the timing is spot on, otherwise you will measure what you don't want to know.

Throw in some new spark plugs for the test, with the right distance between the electrodes., make sure there are no air leaks in the inlet trajectory, make sure you air filters are clean.

If the dyno guy have to do that, it's you who has to pay for it.

Start with analyzing the low and midrange (no more then 6000-6500 rom), and analyze the results with him, including the gas analyses. I can forward you dyno charts of my standard CB72, the curves should be more or less the same (at least in scaling / power output). It's an air cooled bike, make sure the ventilators in front of the dyno are working.

Before you let him proceed to the high rev region, make sure the gas mixture is within the save era. It's your bike, your engine, your money.

It's best to visit a few guys and see for yourself how they are working, pick the guy who can explane the curves to other people clearly and who are not getting irritated due to simple questions.

Take your own jets with you, and your own tools to change the jets. Take some extra plugs with you too, plus packings for the carburetor. Go when it is not crowded, it isn't a show, it's better that you and the dyno guy are alone, so you can have a good conversation about the curves.

Make clear that you changed the mufflers and carbs and that you want to know if the bike is running good, not in demand for the highest power output. If the guy takes you serious on that it's good.

I have seen many dyno places, 10 % is serious about what they are doing the rest is crap. Don't take a official dealer of any brand motorbikes, these guys want to sell not measure. Avoid places with showrooms, well I think you get the point, I guess,

Jensen

brewsky · Post by **brewsky** » Tue Sep 21, 2010 6:42 am

Vince Lupo wrote:Only problem with the suggestion to use the choke is that my Amals do not have chokes (since they have the 'ticklers', there's really no need for them). And, I have in fact had the bike wide open for a stretch, and as I say, the power (speed) is no different than at 3/4 throttle (though I haven't done the 'kill the bike and let it drift to a stop' method). It doesn't feel like it's 'bogging down' at this point, but just that there's no difference. As well, I tried even backing off the throttle to see if maybe it picked up speed (and therefore concluding that my main jet was too big), but that didn't happen either.

Actually a dyno test would be a great idea, if I could find someone in my neck of the woods who in fact had one, and who really knew what they were doing. I'd be more than willing to give that a try!

Sorry, I forgot you were running Amals!
Did your bike pick up speed between 3/4 and WOT with the straight thru mufflers?
If so, you may be able to make some straight thru perforated baffels yourself and more closely duplicate them with less noise.
The "plug chop" is the best method to read main jet size absent a sniffer as far as I know.
Maybe someone can chime in on the 3/4 to WOT throttle phenomenon?

Vince Lupo · Post by **Vince Lupo** » Tue Sep 21, 2010 7:58 am

I never actually ever got to WOT with the straight through -- but, the bike just kept on gaining speed. I usually stopped once the speedo got into the 90's (indicated), but it wasn't WOT at that point (I have actually hit 105mph indicated on the speedo). Now, at either 3/4 or WOT the speedo is only hitting about 85 (indicated).