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Hey guys I’m stumped here. I’ll give you the TLDR first: R&R’d carbs, unclogged one pilot jet, revs will not drop below 5000 RMP when started.
Bike is a 1982 GS 450 TXZ, with the BS34SS (CV) carbs. Running pods and straight pipes, 135 mains (vs 115 stock), and needles at stock height. Idle air screws are both at 1.5 turns out. Valves were adjusted weeks ago. With this configuration it runs great at higher RPM/high load and had a really low lumpy Harley-ish idle. It has always started easy, hot or cold. But it had a pretty loud afterfire when you closed the throttle at high RPMS, and I was under the impression that upping the pilot jet by a size would help or cure the afterfire. It also had a little kind of a burble or flat spot that came and went at sustained speed, but I think that’s due to a questionable plug boot.
So I pulled the carbs (I had forgotten that the pilot is located in the bowl of these carbs) and removed the pilots, and discovered that the cylinder 2 pilot was completely blocked. Air tight. The other pilot was fine, and with a little cleaning the blocked one was good to go. I replaced the old pilots with the upsize 47.5 pilots, put the bike back together, pods and all (expecting to take it on a test ride), and it fired right up as usual with choke, then revs immediately climbed to 5000. Turned off the choke and it falls flat on its face like it usually does when first started and very cold. Turned it off and backed the idle adjuster until it lost contact with the butterfly shaft, confirmed that the throttle cable was slacked, and visually verified that the butterflies were closed. Slides rise and fall nice and smooth just like always. Tried different combination of choke, throttle, mouth holding, etc., but nothing changed. Twist the throttle it revs right up, and falls right back to 5,000 RPM.
Going back to the last good configuration (except without a blocked pilot), I put the old pilots back in. Fired her up and the exact same thing happens. Revs straight to 5k. Acts just like you were holding the throttle, nice and smooth, both cylinders running fine, just free revving. It seemed to me that a vacuum leak would usually not affect both cylinders and probably wouldn’t run so nicely, but the ether squirting around the intake pipe trick didn’t turn up any leaks. Just to be sure I hadn’t screwed something up seating the carbs, I pulled the carbs back out, warmed and lubed up the boots and re-installed, once again no change.
I pulled the carbs back out and opened up the diaphragms, thinking maybe the needle had come unclipped or there was some mechanical interference but nothing was out of the ordinary.
I left it there yesterday, I think my next step will be to put the dry carbset back on, and while cranking with the ignition on spray ether around the boots and see if it tries to fire. Could it be anything else going wrong? I’m really stumped at this point, it was running like a top Monday night when I brought it to the shop, before I tore it down.
Sorry for the long post, I was trying to get all the details in there. What the heck is going on here?
Bike is a 1982 GS 450 TXZ, with the BS34SS (CV) carbs. Running pods and straight pipes, 135 mains (vs 115 stock), and needles at stock height. Idle air screws are both at 1.5 turns out. Valves were adjusted weeks ago. With this configuration it runs great at higher RPM/high load and had a really low lumpy Harley-ish idle. It has always started easy, hot or cold. But it had a pretty loud afterfire when you closed the throttle at high RPMS, and I was under the impression that upping the pilot jet by a size would help or cure the afterfire. It also had a little kind of a burble or flat spot that came and went at sustained speed, but I think that’s due to a questionable plug boot.
So I pulled the carbs (I had forgotten that the pilot is located in the bowl of these carbs) and removed the pilots, and discovered that the cylinder 2 pilot was completely blocked. Air tight. The other pilot was fine, and with a little cleaning the blocked one was good to go. I replaced the old pilots with the upsize 47.5 pilots, put the bike back together, pods and all (expecting to take it on a test ride), and it fired right up as usual with choke, then revs immediately climbed to 5000. Turned off the choke and it falls flat on its face like it usually does when first started and very cold. Turned it off and backed the idle adjuster until it lost contact with the butterfly shaft, confirmed that the throttle cable was slacked, and visually verified that the butterflies were closed. Slides rise and fall nice and smooth just like always. Tried different combination of choke, throttle, mouth holding, etc., but nothing changed. Twist the throttle it revs right up, and falls right back to 5,000 RPM.
Going back to the last good configuration (except without a blocked pilot), I put the old pilots back in. Fired her up and the exact same thing happens. Revs straight to 5k. Acts just like you were holding the throttle, nice and smooth, both cylinders running fine, just free revving. It seemed to me that a vacuum leak would usually not affect both cylinders and probably wouldn’t run so nicely, but the ether squirting around the intake pipe trick didn’t turn up any leaks. Just to be sure I hadn’t screwed something up seating the carbs, I pulled the carbs back out, warmed and lubed up the boots and re-installed, once again no change.
I pulled the carbs back out and opened up the diaphragms, thinking maybe the needle had come unclipped or there was some mechanical interference but nothing was out of the ordinary.
I left it there yesterday, I think my next step will be to put the dry carbset back on, and while cranking with the ignition on spray ether around the boots and see if it tries to fire. Could it be anything else going wrong? I’m really stumped at this point, it was running like a top Monday night when I brought it to the shop, before I tore it down.
Sorry for the long post, I was trying to get all the details in there. What the heck is going on here?
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