Diogenes

Your reply seems to make a lot of sense. I am still not 100% sure it is correct though(I'm not 100% sure of my theory either.)

I am planning on testing my theory using the stock exhaust with and without the spark arrestor(cheaper than buying aftermarket exhaust system).

Instead of doing this by feel and/or plug color I will be using a wide band A/F sensor and a small data acquisition system.

I think you would agree that a change in exhaust system has a much smaller affect on jetting than a restrictive air filter system due to the air density affect.

By the way, My username is nothing more than the name of a band I like. Try seaching them on CDNow.

 

There is a way to test this pipe theory using your obvious equipment abundance.

Throw your inductance lead over
the plug wire to get your tach in the loop. Put your exhaust sniffer in the tail pipe.
Install a temperature probe on each exhaust pipe a couple of inches from the head.
After the motor warms up, check your left and right exhaust pipe temp's, at idle, and
every 1000 rpm until apx. 9000 rpm. Be sure and let the pipe temperature stabilize
at every rpm level, should only take 5-6 seconds.

After you remove your spark arrestor, do the same procedure again.

You should see increased air/fuel ratio's, and
hotter exhuast pipe temp readings through-out the rpm range with no spark arrestor
in place. This means a leaner burn condition in the combustion chamber. Time to rejet.

The way you are talking, you have all the equipment for a first clas test. Good Luck!

 

The channels that I have available and that I am planning to acquire are:

1)Vehicle Speed
2)RPM
3)Throttle position
4)A/F Ratio

I have had problems with thermocouples with the Pi data acquisition system in the past so I cannot use them now.

Besides, with the Air/Fuel Ratio sensor it is not necessary to measure exhaust temperature. The output is a number between 10 and 20 A/F. For gasoline anything leaner than 14.7 is considered lean. But to make the best power at WOT the A/F ratio should be approx 12.

Exhaust temperature is relatively inaccurate but cost effective way to measure A/F ratio. Exhaust temperature will actually drop if you get to a point of being too lean. The hottest exhaust temp will be somewhere near 14.7 I believe.

I plan on going out tomorrow morning to finally jet my bike with the new K&N filter I put on it. Right now I have 182.5/177.5 in the carbs and I have the stock exhaust on the bike. 160.155 jets were way too lean.

If anyone can help me out I'll be glad to post screen captures of the data graphs. I could not post any of the data I have taken so far on this board and I don't have a website.

 

Well, I went out today to try to finally get my bike jetted with the custom K&N Filter and the new jets I received from Sudco but......
I STILL don't have rich enough main jets yet!!!

I am currently running 187.5/192.5 and it still is a little lean here in Phoenix. I can't believe I need to give this thing so much fuel. My riding range at the dunes is really going to change from stock.

I did find some strange interactions(rich) with the needle height on engine speed ranges below 3000rpm. I am thinking that I may need a new needle with these large main jets.

Does anyone know anything about needle jets for this carb? The part number I have does not correspond with anything in the Sudco catalog.

By the way, I did not get a chance to take data with and without the spark arrestor at the same carb settings due to a minor problem with the data acquisition system. Maybe next time.

 

My only evidence supporting my theory is experience racing and tuning motorcycles & ATV's, the corroboration of others with similar experience, and the documentation such as I provided in my previous post (from the Motorcycle Carburetor Manual).

Fooling with the spark arrestor alone is a grossly poor platform for a valid test; dyno charts show stock ATV exhausts perform MORE effictively with spark arrestors installed than removed over a considerable portion of the rpm ranges.

The fact that main jets lean out under high aiflow is verifiable and demonstrable. Higher airflow is facilitated by installing a completely engineered low-backpressure scavenging exhaust, not simply messing with the spark arrestor.

Anyway, what's your goal? Proving the conventional wisdom of the entire internal combustion engine performance tuning community wrong? Demonstrating, somehow, that main jets do not lean out under higher airflow (published documentation to the contrary), and that a stock main jet can accommodate ANY volume of air per unit time, always providing optimum air/fuel ratio? Good luck.

Diogenes

 

ive recently purchased a 1999 Yamaha warrior, got it rode it a little but tented to idle high and low randomly and when I'd ride it and it dies and bogs after a minute or two of riding . It has fmf titanium 4 exhuast, k&n filter and converted twist throttle, so i bought bigger jets because it was popping out the exhuast i went a size bigger 45 pilot and 147.5 main jet. Cleaned carb now hard starting and idles very low when it starts and still pops out exhaust. I was wondering if it's because i need to adjust the idle screw to be ideal with the new jets.

 

This may be seen if you were to start a new thread.

 

Yes, reading through long 17 year old posts to find a new one which has little to do with what has gone before is a bit annoying.

However to answer the question, yes you will need to alter idle settings, you may also need to change jets again. Knowing how to read the air/fuel ratio by the colour of the plug nose after a long run, is also necessary if you modify an engine, unless you have an exhaust gas analyzer or one of those Colourtune plugs. An engine which "dies and bogs after a minute or two of riding" may have other problems (electrical) too.

 

Sorry I didn't know post was old im brand new to this website. And when it dies after riding a few minutes it will start right back up or if i pull choke out fast it will stay idling almost seems like it's starving for gas .

 

I can only guess that valve lash may be out of adjustment.