carb tuning around free'er flowing exhaust questions.

[urbandork]

I recently changed my exhaust system to a free'er flowing system. Everything is fine while twisting the throttle up until i get to the half to full throttle area. At this point the bike loses power and starts to sputter IF i continue to open the throtle at an agressive rate.

Im pretty sure what is happening is that the mixture leans out. This is what is to be expected with free'er flowing exhaust right? or is it to rich?

In order to richen it up i need to change the position of the clip on the jet neddle right?

In order to richen it up i would have to lower the clip, thus raising the needle right?

Just double checking.

NOTE: im using dellorto phf36 carbs from mikesxs

Thanks in advance

 

[azman857]

Check your petcock and fuel filter to see if it's flowing enough. Rust in the tank can clog the petcock and the filters in the tank. The tank vent too.

 

[urbandork]

yup already done that... forgot to mention that i have/done

gas cap vent works fine
non vacum petcocks
Free flowing petcocks and filters
already cleaned the carbs out
pamco ignition so no points problem here
everything is timed right and the auto advance unit works/has been checked/timed
I also have filter pods but the bike was running fine with those when the stock mufflers w/ baffles knocked out and mikexs single walled pipes where on the bike

 

[littlebill31]

I'm no carb guy, but from reading the "carb guide" it would appear that you are correct. Needle down=rich. Up=lean.
Pilot circuit is from idle to 1/4. Slide valve is 1/8 to 1/2. Needle jet and jet needle. The jet needle as you said, is the one with the clip. This from 1/8 to 3/4. Finally the main jet, which is 3/4 to full. Here's a link that better explains it. http://www.iwt.com.au/mikunicarb.htm

Testing and time. It's a little pain, but worth it.

carb guide: http://www.amckayltd.com/carbguide.pdf
Read it, learn it, live it.

 

[grizld1]

5twins, my co-conspirator on the Carb Guide project, covered this very well indeed, but I'll recap for you. There are strong secondary effects involved whenever you change a component. Changing the main jet, for example, will have strong upstream effects, influencing needle position selection. Changing needle position will impact pilot jet selection.

So here's what to do. It seems you've checked everything else, so inspect valve lash and cam chain tension (inspect and adjust everything that influences engine function before you mess with a carburetor!), then check diaphragm function by lifting each slide and blocking the vacuum chamber port with your thumb. The slide should fall slowly. If it drops quickly, the diaphragm is bad. Next: in 2nd or 3rd gear, get rolling with the motor turning around 4500 rpm, and roll (don't snap) the throttle wide open so that you're on the mains. If the engine hesitates or falls on its face, start stepping up the mains until it runs clean. Then check your mid-throttle operation. If you've had to step the mains up more than 3 sizes, you may have to drop the needle to lean the mid-throttle mix.

Once you have clean running at mid-throttle and wide-open throttle, fine-tune with roll-off testing. In 4th or 5th, find yourself some open road and run up to 5K rpm or higher with the throttle wide open, then back off slightly to 7/8 throttle. If you accelerate harder, step your mains up one and go again. If not, go to WOT again and back off a little more to 3/4 throttle. If the engine sputters, that's a rich symptom. Work mains and clips until you eliminate it.

 

[TwoJugs]

I am getting into fine tuning my carbs too. Similar situation. I have 1 3/4 open pipes and air pods (ebay cheap ones) on 38 carbs. Motor is stock except for 1st over bore. I know it is running lean because it ran okay during our hot summer days of 90 plus temps but when I got into cooler weather in the 40s or 50s it would stumble during acceleration and pop at high rmps. I know the combo of those pipes and fliters will never give me solid low end performance but it is just a local hopper ride wirh a 1 gallon tank and not made to go far or fast. I just want it to open up without popping and getting attention and comments.

I see the main jet now is a 135 and the air jet a 27.5 which seems stock. Is there anyplace that sells a jet kit or what sizes should I go over 135 if I am buying seperate pairs and playing swap out? I was thinking going to a 140 then moving the needle if needed.

 

[grizld1]

Jet kits only give you a baseline, and if your engine was running well in warmer weather, you're already close. One more time (I sure get tired of writing this): you can't jet by recipe. You have to do what your motor tells you to do, and to do that you have to learn to listen to what your motor tells you. Those pipes certainly can deliver solid low end performance: before you start jetting, think about installing Michael Morse's EPO's and TPO's. If those pods are the cone type (Emgo or Emgo clone) with thin foam filter medium under a metal screen, you'd be well advised to get rid of them. Apart from what they do to carb function, they won't keep anything smaller than pea gravel out of your motor.

 

[5twins]

From the jet sizes you've mentioned, it appears you're dealing with a '78-'79 carb set. This carb set pretty much always requires leaning the needle a step if you increase the main jet size. When you lean the needle, you will probably need a larger pilot as well. So .... I would get a #30 pilot and #137.5 or #140 mains. Install the new jets and lower the needles one step.

Your cheap pods may impede your ability to tune the carbs. The problems they cause have been well documented. When you get tired of dicking around, go out and buy some UNIs.

 

[littlebill31]

Thanks to both of you for writing the "carb guide". Carbs were something I just walked away from, but now I have a better understanding. That being said I have been over messing with mine. Every little thing made me change something to no avail. Stupid. Chasing a ghost. Now I'm slowly dialing them in. Still boggles my mind though. I'm at sea level, they run perfect when it's around 65+ degrees and the engine is operating temp, but any colder...spits and spudders. I've become a much better listener now.
Thanks so much again guys.