Underfueling after carb rebuild
Posted: Sat Mar 13, 2021 2:31 am
Hi All,
Firstly, thanks to the administrators and contributors who have made this forum such a great resource. I am in Melbourne and have a '97 green TRX that I have been slowly rebuilding over the last 12 months. It was in fairly rough condition, including bent forks, broken fender, mice in the tail section, a glued on fleece seat cover and homemade brake rotors! I have no mechanical experience, so have been reading the forum extensively and so far it has the usual recommended mods/upgrades: changed to an on/of fuel tap, new running gear bearings and seals, rebuilt suspension by MPE, Metalgear rotors and rebuilt blue spots/14mm MC, new OEM plug caps and inlet manifold rubbers, new spark plugs, Kedo right air cut cover, new air filter, and Ignitech ignition. Most recently I have attempted rebuilding the carbs with Keyster kits and LiteTek seals. I have also blocked off the left inlet manifold vacuum hose closed to the manifold.
Prior to the rebuild (and with the old manifold rubbers) the bike was snatchy at low RPM and would pop on overrun. On opening the carburetors, the emulsion tubes and needles were worn and many seals perished. I put the Keyster needles and emulsion tubes in, and also modified the teflon washer on the needle by dremmeling an edge down so that the needle does not rub on the emulsion tube as much. Visually, the Keyster needle appeared to have the same taper as the Mikuni ones and also had the same number of clip positions - 3. All jets were switched out to the standard factory sizes provided in the Keyster kit, i.e. 142.5 mains. I set the needle clip to the middle position, pilot screws to 2.5 turns out. So now I have the same needle clip position as before, but with the needle sitting deeper due to the modified washer.
It started and idles well, however when revving up to 4,500-5,000 rpm it seems to run out of fuel and the rpm drops, accompanied by the odd pop and bang. When riding the bike, it completely dies if I open the throttle past 3,000 rpm, and even lower if there is more load on it. When cracking the choke one click: it runs like a dogs breakfast (surging) with the choke at low rpm, happy around 3,000, and stalls at idle. I have opened the carbs again and flushed all the jets with minimal improvement. I am 100% certain fuel is reaching the carbs (my first fuel line would kink when it got warm), and I do not see how it could be leaking air in with all the new seals and rubbers in place, so is it most likely to be an issue with my needle/clip position? I have not had a chance to try the lowest clip position yet, but my gut feeling is that changing one needle clip position would not resolve such severe symptoms.
Other thoughts I have had:
-could it be an issue with the TPS? I did try checking position the other day, but stupidly (I think!) had the Ignitech hooked up and so the tacho never moved. Resistance seems within spec on the multimeter though.
-could there be an air leak somewhere else causing these symptoms, for example the other vacuum hose from the fuel pump to the right manifold?
-Have I just completely buggered up the carbs or forgotten to connect something?!
Thank in advance for any thoughts or suggestions. I realise that this is a "lazy" post because there are still things I can try, but the bike is kept over an hour away and I have very limited time to work on it - so being able to direct my efforts efficiently would be great.
Cheers,
Duncan
Firstly, thanks to the administrators and contributors who have made this forum such a great resource. I am in Melbourne and have a '97 green TRX that I have been slowly rebuilding over the last 12 months. It was in fairly rough condition, including bent forks, broken fender, mice in the tail section, a glued on fleece seat cover and homemade brake rotors! I have no mechanical experience, so have been reading the forum extensively and so far it has the usual recommended mods/upgrades: changed to an on/of fuel tap, new running gear bearings and seals, rebuilt suspension by MPE, Metalgear rotors and rebuilt blue spots/14mm MC, new OEM plug caps and inlet manifold rubbers, new spark plugs, Kedo right air cut cover, new air filter, and Ignitech ignition. Most recently I have attempted rebuilding the carbs with Keyster kits and LiteTek seals. I have also blocked off the left inlet manifold vacuum hose closed to the manifold.
Prior to the rebuild (and with the old manifold rubbers) the bike was snatchy at low RPM and would pop on overrun. On opening the carburetors, the emulsion tubes and needles were worn and many seals perished. I put the Keyster needles and emulsion tubes in, and also modified the teflon washer on the needle by dremmeling an edge down so that the needle does not rub on the emulsion tube as much. Visually, the Keyster needle appeared to have the same taper as the Mikuni ones and also had the same number of clip positions - 3. All jets were switched out to the standard factory sizes provided in the Keyster kit, i.e. 142.5 mains. I set the needle clip to the middle position, pilot screws to 2.5 turns out. So now I have the same needle clip position as before, but with the needle sitting deeper due to the modified washer.
It started and idles well, however when revving up to 4,500-5,000 rpm it seems to run out of fuel and the rpm drops, accompanied by the odd pop and bang. When riding the bike, it completely dies if I open the throttle past 3,000 rpm, and even lower if there is more load on it. When cracking the choke one click: it runs like a dogs breakfast (surging) with the choke at low rpm, happy around 3,000, and stalls at idle. I have opened the carbs again and flushed all the jets with minimal improvement. I am 100% certain fuel is reaching the carbs (my first fuel line would kink when it got warm), and I do not see how it could be leaking air in with all the new seals and rubbers in place, so is it most likely to be an issue with my needle/clip position? I have not had a chance to try the lowest clip position yet, but my gut feeling is that changing one needle clip position would not resolve such severe symptoms.
Other thoughts I have had:
-could it be an issue with the TPS? I did try checking position the other day, but stupidly (I think!) had the Ignitech hooked up and so the tacho never moved. Resistance seems within spec on the multimeter though.
-could there be an air leak somewhere else causing these symptoms, for example the other vacuum hose from the fuel pump to the right manifold?
-Have I just completely buggered up the carbs or forgotten to connect something?!
Thank in advance for any thoughts or suggestions. I realise that this is a "lazy" post because there are still things I can try, but the bike is kept over an hour away and I have very limited time to work on it - so being able to direct my efforts efficiently would be great.
Cheers,
Duncan