Topic: Cb750a '77 carburetor woes

[LadyTano]

Alright so to start, this bike currently has pods. I have a (generously) donated airbox coming Wednesday.


So Friday my bike is running kinda, well garbage. Stalling. Generally being grumpy. Etc.

I rebuilt the motor with help from a friend. That all seems well and good, that was finished a couple weeks ago. Timed according to oem Manuel, and ran pretty not terrible (pods,. Carb needed work, etc )



So Friday, running poopy. Popping sounds, hitting throttle makes her kinda "glug" and maybe with babying get speed up. But, very annoying to say the least. Also noticed a pop noise, but from air pod #3 (same one like to piss gas.....)

I check idle adjustment, seems ok.

Anyway, so I contact a local gent who knows these bikes really well. I spend Saturday with him as we go through teaching me these carbs, and cleaning them. All looks good after the work. Everything adjusted and checked. All good?


No, finally get her back together, just now.

Fire her up, runs.... Well. Kinda crap still. Still pees gas out #3, idles rough, and the #3 still randomly got the pop noise, which was the carb spraying fuel back into the pod through (I think) the right side brass tube.


Discouraged doesn't quite cover it.


So, I guess I need some seal kits, but any recommendations? The bike has already murdered my budget. But I'd rather do right. I've heard good thing about 4into1?


Anyway, beyond that, what would cause fuel to spray back? That seems exceedingly weird.


Thanks for any and all advice. I've been out of riding season because of the motor and I'm sitting her bashing my head against anything solid trying to figure it all out.

🖤

[scottly]

Also noticed a pop noise, but from air pod #3 (same one like to piss gas.....)

Pissing gas is usually caused by float needles/float adjustments that don't shut off the flow of gas once the fuel level in the bowl is at a certain level, which when checked with the clear-tube method should be about 3mm below the top of the bowl/carb body joint.
Is the #3 spark plug black? If yes, is it wet, or dry and sooty?

[LadyTano]

Scottly:


So update! I pulled the bowl. Seems during cleaning we missed a few tiny, tiny cracks in the float bowl siphon tube (the brass tube).

I spent a bit trying to find a good solution aside from buying a replacement.



I soldered the siphon tube. Carefully, with higher temp electrics solder. Rosin core.


Testing right now with fuel in bowl, and paper towel under it. Seems to be sealing. Fingers crossed.


I'm really hoping this was also causing the popping noise. But what causes cracks? O.o


I still gotta do a compression test, but figured carb pissing gas was a slightly higher priority.

Thank you for your input! I hope this was the problem.

[stocky]

Ditch the pods till you get er running right.

If you haven't pulled the carbs apart to clean and inspect everything, now would probably be a good time to do it while its off. I just did a full rebuild on PD41A carbs on a 77 CB750K, and while it was a bit of struggle at times, it was rewarding to get it done and not have to worry. There are several good videos that people have online and picture tutorials to be able to get it done.

Definitely need to get the bowl levels as close as possible like Scottly mentioned. It can be tricky for the PD41 series lot of people including me had a tough time getting the bowl level correct. After the rebuild, bench test everything and make sure everything flows well and there are no obstructions, bench sync the carbs, then get it back on and do an actual carb sync while its running.

Would definitely benefit from a full tune up including intake exhaust valve clearances, cam chain tensioner, timing, new air filter, points, plugs, coils, plug wire, etc.

I had a hairline crack on the brass overflow tube as well and used solder to fix it, seems like its holding up but it was also pissing fuel before. No idea how it develops cracks besides the old age, vibration and abuse it gets on a daily basis.

I have used rebuild kits off ebay and 4into1 and both were pretty good. I just know one of the kit's carb bowl gaskets were slightly bigger and it was a PITA to get the gasket to stay in the groove while trying to screw it back on.

[scottly]

Scottly:


So update! I pulled the bowl. Seems during cleaning we missed a few tiny, tiny cracks in the float bowl siphon tube (the brass tube).

I spent a bit trying to find a good solution aside from buying a replacement.



I soldered the siphon tube. Carefully, with higher temp electrics solder. Rosin core.


Testing right now with fuel in bowl, and paper towel under it. Seems to be sealing. Fingers crossed.


I'm really hoping this was also causing the popping noise. But what causes cracks? O.o


I still gotta do a compression test, but figured carb pissing gas was a slightly higher priority.

Thank you for your input! I hope this was the problem.

A leaking vent tube would not affect the amount of fuel entering the combustion chamber, only pissing on the floor. What does the #3 spark plug look like?

[LadyTano]

*Ditch the pods till you get er running right.
- can't till get new airbox. I get it Wednesday




*If you haven't pulled the carbs apart to clean and inspect everything, now would probably be a good time to do it while its off.
-yep, did Saturday. I think it was skipped over in the post.



*Definitely need to get the bowl levels as close as possible like Scottly mentioned. It can be tricky for the PD41 series lot of people including me had a tough time getting the bowl level correct. After the rebuild, bench test everything and make sure everything flows well and there are no obstructions, bench sync the carbs, then get it back on and do an actual carb sync while its running.
- yep. Again, did that with a local gent Saturday. All seemed well

*Would definitely benefit from a full tune up including intake exhaust valve clearances, cam chain tensioner, timing, new air filter, points, plugs, coils, plug wire, etc.
-oh absolutely. I checked clearances already, seems good to my untrained eye/hand. Can chain tensioner seems well set but gonna give it a third look, airfilter (see above), already new plugs, plug wires-isnt that a while new unit?

*I have used rebuild kits off ebay and 4into1 and both were pretty good. I just know one of the kit's carb bowl gaskets were slightly bigger and it was a PITA to get the gasket to stay in the groove while trying to screw it back on.
-perdect! That's the one I have in my shopping cart as we speak.

Scottly: I didn't pull it yet, but I'll give the plug a look this evening and post a pic.

Thanks yall!

[bryanj]

PD carbs are a nightmare, the pilot jets block for a pastime and they look like they dont come out--they do but you have to grip them carefully with vice grips then pull and twist at same time, spray cleaner is not enough they need to come out. The reason the drain tubes crack is they get water in that freezes.
Only place i know off that does replacement pilot jets is Sirius Consolidated in Canada

[Stev-o]

A little bit of info from first hand experience:

- Popping noise on deceleration can be an exhaust leak.

- Compression checks are fine but a leak down test is much better.  My best running bikes have had valve jobs.

- Aftermarket carb brass can make it near impossible to properly tune.

Since you are getting an airbox tomorrow, I'd slap that on and further evaluate at that time.

Good luck.

[LadyTano]

Another update (and thanks guys for the info!)


Got carbs to stop pissing gas. My little fix seemed to work. It also stopped the popping noise.



Did compression test. Ran way lower than should have (had to rig an adapter to actually kinda seal. Motor was warm but not "warmed up" I'd say.

Cylinder numbers, avg test of 3)
1) 70 psi
2) 65 psi
3) 70 psi
4) 67 psi (kept jumping between 70/65)

This was using a loaner kit from auto store.

But, as always. New problem. One of the spark plugs wire/boots cracked off on removal (cylinder 3... Looking at boot, the wire attachment spot had a TON of rust).


So now I'm hunting for a wire/boot kit. Preferably something I can pick up in store tomorrow (local places are oreilly and AutoZone). I know how to solder and do some repair work. No biggie.


I'm just not sure on wire gauge to get. Or resistance. Trying to search forums isn't turning much up for me.


So if you happen to have a few min and have a recommendation, or a wire set you like.... Please post a link

🖤 Cassie

[scottly]

The stock coils don't have replaceable wires? The stock boots are just screwed on, and are easily replaceable.
PS, your compression test was probably flawed.

[bryanj]

Did you do comp test with throttle wide open and choke off?

[Stev-o]

There are few [correct] parts for your bike at Autozone, buy the proper part here...

https://www.ebay.com/itm/NEW-CB750-CB750K-CB750F-CB750A-sohc-NGK-Spark-plug-caps-Plug-wire-ends-Resistor-/201604286193?_trksid=p2349526.m4383.l4275.c10#viTabs_0

Your wires should be ok, trim them 1/8" before installing the new caps.

[LadyTano]

Scottly: oh absolutely. Only consistent part was the deal I made (I think)  and that's it. But yeah, wide open throttle.

Needs a redo. After other things.

Wires read all not so hot. Non replaceable without solder work and that route. Totally in realm of things I can do, so that's what I'm gonna do.

As I explored the wiring one was corroded to hell (not at coil).
So I don't trust em.at all. Sadly.