I bought a CJ750 sidecar rig a couple of weeks ago. It has a decent running transplanted R75/5 Airhead engine that needed some carb work. The carbs needed to be synched, and the left cylinder was leaking fuel out of the overflow, so I figured I might as well do a thorough cleaning of both carbs since the rig had been sitting for a few years.
I pulled the carbs, one at a time, disassembled, cleaned in an ultrasonic bath, and re-assembled with new o-rings, floats, and bowl gaskets. I set the floats so that the seam in the pontoons was level with the bottom of the carb base. I've done this project a few times in the past with other airheads, so I'm familiar with the carbs, and also watched all the youtube vids just as a reminder.
Put everything back together and back on the bike, to find that both carbs overflow when the fuel is turned on.
No problem, I thought, I'll just adjust the floats a bit and good to go...
With the carbs on the bike, and the float bowl off of the carb, when I turn the fuel on, the fuel flows as it should. When I raise the float, it shuts off the fuel just when the seam in the float is level with the bottom of the carb as it should. But with the bowl on the carb, fuel runs out of the overflow tube.
I spent an hour fiddling with the left carb trying various float heights, and when I get the float low enough to stop the overflow, it also stops the fuel all together. Any higher and it overflows. Finally threw in the towel and called it a night.
The carbs are clean, the needle and seats look good and work fine without the bowl by raising the float to shut off the fuel.
I'm missing something, just not sure what... The only thing I can think of is maybe the brass vent tubes are cracked or something, letting the fuel run out when there is fuel in the bowl, but can't see anything obvious.
Thoughts?
Thanks,
Pete
Even with the Triumph/BSA front wheel, that's a very nice looking bike ! Congrats on the find. (If you don't know about the dual leading brake shoe adjustment on that front brake, then I can help.)
In my thinking, you either have mixed parts, wrong parts, or some trash on the float seats. Inspection order...
• If both sides are acting the same (leaking), then the evidence points to mixed parts, and I would first look at the float needles themselves. There are steel tipped versions and Viton (rubber) tipped versions. Which do you have? Can you compare their tip-to-neck length to other R75 needles ?
• Then we have to consider that the new floats themselves measure too high above the mold line. That there is not enough room for them to rise (under/inside the main carb body), which in turn holds the valve open. You might measure that mold-to-top distance with calipers and then pass the depth-measuring-end of your caliper along the carb body's gasket surface looking for obstructions (low casting points which hang down from the carb body).
• Finally we come to the float bowl gasket itself. If the gasket is not OEM (3rd party), then a thinner gasket would allow the float to sit higher in the carb body and the (say for instance) missing 0.5mm thickness is allowing the float's top to bump the carb body unnaturally. One way to test for this is to install 2 gaskets per bowl and see what happens. If the leaking stops, then you'd know that the float, needle and seat are fully capable of shutting off the flow of fuel. That is to say, it's a purely a clearance issue and not a faulty parts issue.
(This is a permissible installation because the normal fuel level in the bowl is far below the gasket surface. The gasket is there only for insurance against "splash" and is not submerged (sealing liquid.)
• Lastly, but not assumed due to the stated "one carb at a time", we must check that the bowls have not been swapped Left to Right.
Really hope this helps.
An after thought on correct mold line position...
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
I knew the front wheel was Triumph/BSA, but it's new to me so any advice on adjustment would be great.
The bike has a "barn find" level of patina with rusty bolt heads and lousy Chinese chrome, a dent in the sidecar fender, etc. But I'm ok with that, I prefer bikes that aren't perfect so I don't have to worry about them too much. I wanted this one because it has the R75/5 engine. It also has the CJ reverse gearbox. It's a bit of a hodgepodge, but seems to be well done. I rode it around for a few days, and it ran pretty good other than needing the carbs to be balanced and it didn't want to idle.
The carbs have been worked on be previous owners as evidenced by allen screws in the carb tops. I didn't mix any of the parts between the carbs, doing them one at a time, but who knows if they were mixed up before I got them or if they were assembled with incorrect parts.
The float needles are steel tipped, and have the spring loaded ball instead of the hairpin clip, and were in the carbs. I don't have any other needles to compare, but could measure their length for reference. Wondering if I should just by new rubber tipped needles and seats.
I tried both the new floats that came in the kit from Euro Motoelectrics and the original floats, with the same result. I'm wondering if maybe they are hanging up on the sides of the bowl and need to be tweaked a bit? Not sure. Odd that both the new and old floats are doing the same thing.
I installed new cork bowl gaskets that came with the kit, which seem to be a tad thicker than the old gaskets that were flattened out from age. I can try adding one of the old gaskets too, to see if it makes any difference.
I don't think the bowls have been swapped left to right, but I'll double check to be sure.
Thanks for help and quick reply,
Pete
Even with the Triumph/BSA front wheel, that's a very nice looking bike ! Congrats on the find. (If you don't know about the dual leading brake shoe adjustment on that front brake, then I can help.)
In my thinking, you either have mixed parts, wrong parts, or some trash on the float seats. Inspection order...
• If both sides are acting the same (leaking), then the evidence points to mixed parts, and I would first look at the float needles themselves. There are steel tipped versions and Viton (rubber) tipped versions. Which do you have? Can you compare their tip-to-neck length to other R75 needles ?
• Then we have to consider that the new floats themselves measure too high above the mold line. That there is not enough room for them to rise (under/inside the main carb body), which in turn holds the valve open. You might measure that mold-to-top distance with calipers and then pass the depth-measuring-end of your caliper along the carb body's gasket surface looking for obstructions (low casting points which hang down from the carb body).
• Finally we come to the float bowl gasket itself. If the gasket is not OEM (3rd party), then a thinner gasket would allow the float to sit higher in the carb body and the (say for instance) missing 0.5mm thickness is allowing the float's top to bump the carb body unnaturally. One way to test for this is to install 2 gaskets per bowl and see what happens. If the leaking stops, then you'd know that the float, needle and seat are fully capable of shutting off the flow of fuel. That is to say, it's a purely a clearance issue and not a faulty parts issue.
(This is a permissible installation because the normal fuel level in the bowl is far below the gasket surface. The gasket is there only for insurance against "splash" and is not submerged (sealing liquid.)
Upon closer inspection with a magnifying glass, the copper overflow tube was cracked.
I decided to try soldering the tube, and it seems to be holding for now.
I re-set the floats again, installed the bowls, and no leaking. The left side had the cracked tube, the right side must have just been stuck.
All good for now.
Thanks for the help.
Pete
https://www.youtube.com/watch?v=UgeecrykGs8&list=PLg9JiekSBQcAcf_tsAxWLcb9bt9bCbR1B&index=15
Great news on the carbs! I hope it holds.
• One more thing you might check... One time I saw 3rd party float bowl gaskets that were too wide. The extra gasket width protruded into the float area and barely touched the side of the float, keeping it from moving freely.
• As for the annual front brake adjustment... Lift the front wheel off the pavement and fully release the brake cable at lever. Remove the black rubber plug on the conical section of the hub. Use a light through this hole to find a large, straight-slot adjuster screw. Then insert a hefty screwdriver into the adjuster. Fully tighten (CW) the adjuster, then back off (CCW) the adjuster 2 clicks. Rotate the hub 180° and find the second adjuster. Do the same. Apply rubber lubricant and reinsert the rubber plug. Reset the lever adjustment. Do this once a year.
This video shows it at about 15:10.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
• As for the annual front brake adjustment...
Thanks for the tip, no idea when the front brake was last adjusted, if ever...
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