Hesitation from low rpm / full throttle
in [Motorcycles Tips]
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From: . on 22 Sep 2008 09:30 On Sep 21, 10:00�pm, "Dennis" <cctden(a)sbc_global.net> wrote: > I currently curretly have them set at 2 1/2 turns out. �If I open them to 3, > the idle is rougher and the hesitation is reduce. �At 3, the bike idles at > ~900 �RPM a couple minutes after it is started and �~1200 when hot. If you open the idle mixture screws too far, the exhaust note will become dull and thudding and the engine will run rougher. You may smell raw gasoline in the exhaust. I don't happen to know what the recommended number of turns out from lightly seated is, and, when someone says, "Oh, you should start with x number of turns," my question is, "But what size are your idle jets?" The jet sizing system is based upon a #100 jet having a hole that is 1.00 millimeters. So an idle jet that has a hole that is 0.30 millimeters in diameter is a #30 idle jet. #30 is a fairly small idle jet, and a #45 idle jet is *huge*. If you work out the area using the pi X radius squared formula you learned in high school, you'll see that a #45 idle jet has a much larger area than a #30 idle jet. So a carb with a #45 idle jet might need to have the idle mixture screws set at only 1/8th of a turn out from lightly seated, while the carb with #30 idle mixture screws might be set at 3.0 turns out. I looked on the fiche and I didn't see any > diagrapham.http://www.bikebandit.com/houseofmotorcycles/showschematic/m2877sch3877�Is > the diagraphram part of 13? That link didn't work for me. #13 is probably the vacuum chamber cap. Honda only supplies the diaphragm as part of the vacuum piston. But there is a company in England that makes cheaper aftermarket diaphragms. > I currently have the float level at .3" ( float bowl mating surface to the > bottom of the float ). Prior to rebuilding it was .17" You're not measuring the float height correctly. If you have the carbs upside down on the bench, the floats are upside down too, so the bottom of the float is on top, and your correct float height is something around 14 to 17 millimeters. I dunno what the exact height is, you can google for it. > Would raising the float height possibly help? �I'm thinking .25? Yes, but google up the correct float height dimension so you won't have to guess. Adjusting the float height so the float valve shuts off later raises the level of in the float bowl and your engine can more easily suck gasoline through the idle jets when the level is higher. Would doing > this make the bike run richer throughout the throttle range? Your engine will draw some fuel through the idle jets at all throttle settings up to wide open, but the engine vacuum decreases as you open the throttle further. The most fuel that will flow through the idle jets is at about 1/4 throttle. Fuel flow through the idle jets stops when the throttle is fully open because there isn't enough engine vacuum to suck gas through the tiny orifice in the idle jet.
From: Dennis on 22 Sep 2008 17:59 I have checked and the diaphragm's and they have no problems. I adjusted the float level 1 mm, and everything is pretty much set by the book (Clymer's): Main jet: 115 Slow jet: 40 Float Level: 7.5mm ( just changed to 6.5 mm ) Pilot screw setting: 2 3/4 ( changed to 2 1/2 ) I also synced the carbs while I had the tank off. Its currently 75 deg F and when the bike is fully warmed up, it's running *perfect*. Until it is warmed up, it still has a very slight hesitation. I think I'm just going to leave everything as is for now and spend some time riding and enjoying the weather before it gets too cold :) With these carbs, I don't see any midrange adjustment clip? I used to race dirt bikes and the midrange was adjusted by needle clip position. I am also confused about the whole purpose of using a diagrapham/piston setup to open and close the jet needle? All the bikes I had previously it was much simpler, and the jet needle was moved by throttle position? Is there a performance improvement by using vacuum vs. throttle position to control the mix?
From: . on 22 Sep 2008 18:52 On Sep 22, 2:59�pm, "Dennis" <cctden(a)sbc_global.net> wrote: > Slow jet: 40 > Pilot screw setting: 2 3/4 ( changed to 2 1/2 ) That sounds like a lot of turns on the pilot screws. My Yamaha FZR1000 has #40's and the pilot screws are turned out less than 1/4 of a turn. > With these carbs, I don't see any midrange adjustment clip? Nope. If you think you can improve the midrange mixture, run down to the hardware store and buy a bunch of 1/2 millimeter thick steel washers with a 3 millimeter hole, and stack as many as you want underneath the head of the needle. If you look at the needle in a street bike carburetor, it doesn't have much taper, so raising it a millimeter or so isn't going to do much. The constant vacuum carburetor is designed to keep the Earth's atmosphere cleaner by taking direct control of the mid range away from the rider's twitchy wrist. Back in the 1950's and 1960's, British Amal carburetors were crude, nasty, dirty-running things that couldn't produce a clean-burning mixture. Nothing would flow through the jets until the throttle was opened quite a bit, and then the engine got a big nasty slug of almost raw gasoline all at once and bogged down. Then it would clear out and the engine would run well on large throttle openings. Along came Mikuni and they added cross-drilled holes to the idle jet and the main nozzle (needle jet holder), and the extra air allowed motorcycle engines to run cleaner on an air-emulsified fuel/air mixture. IOW, the air bubbles in the gasoline coming through the idle jet and main nozzle allowed the gasoline to break up into finer drops. The British had the classic SU carburetors on their 1960's cars, and the SU's were even cruder than the Amal motorcycle carbs. An early SU carburetor had a main jet with a 1/8th inch hole and vaccum raised and lowered a vacuum piston with a tapered needle. If the owner wanted to tweak the mixture, he could loose a set screw and move the needle up into the piston about 1/8th of an inch or he could screw the main jet up and down in the bottom of the carburetor. The SU's were too crude and dirty running, so the Bendix diaphragm carb was invented for British cars in the late 1960. Keihin produced much improved vacuum piston carbs for Hondas in the late 1960's, and Mikuni built rubber diaphragm constant vacuum carbs for Suzuki and Yamaha. >I am > also confused about the whole purpose of using a diagrapham/piston setup to > open and close the jet needle? Cleaner air. �
From: Dennis on 22 Sep 2008 22:17 Thanks for all your help!!! I have alot better understanding of CV carbs and how to tweak them :) Greatly appreciated!
From: SAMMMM on 25 Sep 2008 13:13
from past experience, the float level controls the idle mixture. that is to say, when i couldn't get the idle to be right at 1 1/2 turns out, it could be adjusted by the float level change. as to the hesitation, i've found many jap carbs set lean from the factory. it was always a PIA to get them right, if there was a way. the fact that it idles better when hot, suggests a lean condition. are the needles adjustable? sam "Dennis" <cctden(a)sbc_global.net> wrote in message news:FDFBk.980$as4.879(a)nlpi069.nbdc.sbc.com... > Thanks alot... I have a much greater understanding of how these carbs > work. > >>If you have adjusted the idle mixture screws too far open, and then >>adjusted the idle speed screw to compensate for an overly rich >>mixture, the idle speed will race up to 3000 or 4000 RPM, especially >>when the engine gets hot. > > I currently curretly have them set at 2 1/2 turns out. If I open them to > 3, the idle is rougher and the hesitation is reduce. At 3, the bike idles > at ~900 RPM a couple minutes after it is started and ~1200 when hot. > > >>If I'm wrong, and all those passages are pefectly from your previous >>efforts, you may have a tiny tear or pinhole leak in one of the vacuum >>diaphragms. > > > I was pretty vigerous in the cleaning. Soaked, sprayed, let sit, sprayed > again, compessed air... I looked on the fiche and I didn't see any > diagrapham. > http://www.bikebandit.com/houseofmotorcycles/showschematic/m2877sch3877 > Is the diagraphram part of 13? > > Prior to rebuilding, the float level was to the point it was leaking gas > when on the sidestand. The idle screws were set at 2. The bike seemed > to have a little more power, and the idle was rough and speed varied quite > a bit based on how hot the engine was. At the time it did not have the > hesitation. > > I currently have the float level at .3" ( float bowl mating surface to the > bottom of the float ). Prior to rebuilding it was .17" > > Would raising the float height possibly help? I'm thinking .25? Would > doing this make the bike run richer throughout the throttle range? > > > > > > > > |