85 GPz 750 Cafe Project

Reassembled the carbs, installed them and the K&Ns. Hooked up fuel, third spin it lit and took off. Before it would chug and just barely keep running for several seconds. It hasn't been started in 4 or 5 days IIRC. I'll try it again tomorrow morning to verify fix but the short term results say major improvement. Booyah!

Second cold start this morning. It takes three, three second pulls to get it to kick over but it comes up. This suggests to me the carb jet should be opened a bit more but I'm taking the win and running with it.

Several cold starts later I tried to take the bike out for a test ride. It won't throttle up under load. Pushed it back into the garage to do a jet change (too rich I believe) and had a soul crushing moment. A tank bubble near the repair up top. Going to see a TIG welder tomorrow. I left gas in it for 5 days prior to painting. 1 hour on the bike and I get this.

I sanded down the tank and took it to a radiator shop that does lead repairs. Interesting how few radiator shops there are anymore. The first three I called said "Oh we don't fix them, we just replace them". Criminy, what a disposable world in which we live. That should be ready Friday. Interesting talking to the old bird that owns the shop. First comment was "We use that stuff on the outside of tanks all the time, it is great, but we never put it on the inside. Someday it will come off, period." Second item when I brought it bank sanded was "I would have bet money I could see the crack" after he put an LED flashlight on the area.

I blew apart the carbs again. One screen had come off of a fuel bowl inlet valve. No doubt it was tumbling and causing an obstruction. All other parts installed properly. Second time that has happened. After the first time I cleaned everything with alcohol and snapped it back on which it did with a seemingly proper snap. Then picked at it with my fingernail. Twice bitten, I replaced it with an older original unit I salvaged from a previous carb rebuild. I moved over the new o-ring and noticed it was nicked. I measured the installed dimensions and they measure the same. I'll recheck fuel levels once reassembled later today.


The battle wth the anti-christ bike continues.

Carbs reinstalled with 115 main jets. I checked the fuel bowl levels, had to adjust the one where I replaced the float valve. Brought it up, exact same symptoms. It is a bit reluctant to throttle up and if I double care the throttle (rev, rev) it bogs severely. Second rev won't come up at all. I checked dynamic timing, spot on throughout the rpm range. Got the tank back from the radiator shop. Pics below. I'd like to pressure test the tank it but I'm struggling with how to seal it and then get air pumped into it. I could buy a Pingel 44mm adapter but that is a very expensive solution. Pinger is quite proud of their stuff. I've got another set of BS34 carbs ready to reassemble and install on the 82 project in waiting. I think I'll reassemble and install them just to take the carbs completely out of the equation. I'm trying to open my mind to other possibilities, what if something built a nest in one of the mufflers? I'll get behind the bike and try to get a sense of the exhaust flow to see if one is distinctly different than the other. I've got air, I've got spark, I've got fuel, I've got compression.

Looks like a nice solder job if nothing else. Glass is half full...

Fascinating day in the garage. I finished the rebuild on the 82 GPz 750 carbs, a slightly earlier version of the BS34s, set the fuel levels, installed...it runs like a scalded dog. This alternate set of GPz carbs has 110 mains, and 37.5 pilots. I did have to open the pilots all the way up, bout 3 and 1/4 turns to get them to idle properly.
Next step, put 110 mains in the original set of carbs, I left the 42.5 pilots in place at 2 turns, reinstalled and fired it up. It runs rough and absolutely will not throttle up. Unbelievable to me. The original set has been ultrasonic cleaned twice at high temp for 45 minutes each time. 1.5 hours total. Carb fluid blasts freely through all passages. Something is up with this bank of carbs.

Noteworthy, the alternate set of GPz carbs would not cold start. I think the drilled out choke jets will still be required. I suspect the 42.5 pilot jet helps there as well.

Let's start here: The bike runs perfectly now. A bit of a journey to get there just in the past 36 hours. For reference and clarity a) first set of carbs came with the bike, I broke the float piller on carb #1 b) second set of carbs is from another GPz I have in the shop 3) third set of carbs I procured to address the #1 broken carb of the first set.
I rebuild set #3 and installed them this morning, same problem; I moved #1 carb from the second set to the third set and it runs perfectly. There is something wrong with #1 carb from the third set. I'll tear it down at some point to see if I can locate the bug (again) but I'm going to complete this bike with the set on it now. At loooooooong last there is a light at the end of the tunnel. Hopefully it isn't another train.

I have gotten in the habit of pressure testing any tank I'm going to paint. Make a block off plate for the percock and a strip of rubber to seal under it. Then get one of those expanding rubber plugs with a bolt through the middle. Get one bigger than the filler neck and take it apart so you can taper it with a razor blade. Jam it in the filler neck and put an air sprayer in the hole where the bolt was. It doesn't need a perfect seal. I set the regulator to 5 psi and fill it until I can hear air escaping. Then do all of this in a big tub of water so when you submerge it you can see where air bubbles are coming from. Works great. You only need a couple pounds of pressure in the tank any more and you run the risk of distorting or splitting the tank

Great idea DoctoRot. I bought one of the plugs you described but it fits inside. I'll buy the next size up. The bike is running on the bottle, doesn't run for shit from the tank. Won't run wth the choke off. Replaced the Motion Pro fuel line with the pancake filter with a straight piece of 5/16 clear fuel line and it runs much much better, but still doesn't like to have the throttle cracked twice in a row. Let it idle for 10 seconds and, crack the throttle, runs right up. It is like the bowls aren't filling fast enough. I can barely see any fuel at all on the clear line while running. Seems like a vent issue but I have verified the vents and vent tubes are clear multiple times.

While I had some significant success this is not an update I'm proud to write. I took out the caliper and checked the mark I made for fuel level. It shows at 6.7 mm below the edge of the fuel bowl, not the prescribed 4.0mm I intended to make. Meaning the fuel level was 1.7mm lower than the minimum level of spec. I fixed that issue and the bike behaves much better. I took it out for a ride and it runs well up to 5K rpm and it hits a wall. It bogs and absolutely will not surpass 5K. After a 6 mile ride the once black and sooty spark plugs were white. That makes me think my 112.5 main jets may not be sufficient. The c-clip on the needles are one down from the top. My inclination is to move up two main jet sizes since it isn't just lean, it won't function above 5K.

Contradictory opinions are invited.

Aftermarket fuel tees? I'm starting to wonder if mine aren't partially blocking some of the inlet ports on my carburetors. Could be what's happening to yours above 5K.