New to the forum and my first bike so still learning. I have a 82 nighthawk 750 that was getting 20mpg and running very rich. I bought a rebuild kit for my carbs and just installed then back on the bike. The carbs, werevery clean before hand but I did find a missing o-ring. All taken care of.
Upon initial start, the bike. Wouldn't hold an idle. After feathering the throttle to get the engine warm , I then adjusted the idle via the black knob carb 2. I have a steady idle at 1100 rpm.
The problems:
-Popping from carbs,
-Choke has no effect after I adjusted the idle
-If I rev the bike via the throttle, the rpm it will occasionally stay high and progressively get higher until I turn the bike off and back on. I checked and the throttle itself and it is closing so it's not sticking open.
Typical vacuum leak. Check the rubber boots, and clamps that connect the carbs to the head.
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No fire? No explosions? So whats the point of your story?
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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
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1973 CB 350
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Checked the boots, took them off to make sure they wernt backwards, no rips, clamps are fine and tight, the rubber nipple on carb 2 is in tact with no leaks.
Now every time I blip the throttle it revs to 4-5000 and stays. Choke doesn't affect it but if I twist the idle knob ccw to close it and the revs slowly go down but the bike stall out.
It will hold an idle though until I blip the throttle. Are there any other places I could be having a vacuum leaks? (And checked the 4 screws by the boots on the head)
Alright, so with a steady idle at 1k, I sprayed starting fluid ALL over the boots and carbs with no change in rpm. Only after I blip the throttle (with the cables attached or not) will rpms go up and stay up. The only way to get it back down is to turn the idle screw ccw to lower it BUT then the bike will stall out unless I turn it cw.
I did find that if I spray starting fluid in the intake box, even as little as 2 small sprays, the bike will bog down to a near stall.
Also found that at a steady 1k idle, the coke has NO effect and it is hooked up right.
I took the stock jeys off, they looked great, I cleaned then well and made sure all passages were cleared on the 3 jets. I couldn't have mixed then up on the install right? There all threaded different or different sizes right?
I'm 95% sure that the nighthawk carbs are the same as the DOHC cb750 carbs, and if they are then you could have easily reversed the primary and secondary main jets. Stock on my 1981 CB750K were 68 and 102 mains.
They are in different immulsion tubes though. Idle jet is under a rubber plug.
Just took off carbs, all primary are 68, and all secondary mains are 102. The slow jets are 72 and have all new rubber plugs on them. Floats are all good and under the cap is all good. I re-bench synced the throttle plates and idle.
I put the carbs back on the bike without any cables or the airbox hooked up and fired up the bike. Same problem. If I could get a steady idle by messing with the idle knob.. as soon as I turned the throttle plate (by hand), the rpms hung at 4-5k until I turned the idle knob to basically all the way closed.
My slow jets are pressed in so I've never taken them out, and I don't see them listed in the manual, but 72 sounds huge for a slow jet.
What is the number on the side of your carbs? Are they VB42A? VB42B?
It is also possible for these carbs to have vacuum leaks around the body of the carbs (e.g. at the throttle linkage). Checking there with a flamable aerosol would be prudent.
Some other stuff it might be...
Did you remove the SOS, and if you did, is your vacuum nipple sealed off tight? It's in between carbs 2-3 on the head side of the carbs.
You say under the caps is all good. Did you verify that that slides rise and fall smoothly like in this video?
It ran fine before the rebuild with the same slow jets do I wouldn't think that would cause this now, not sure though.
They are 42A.
The slides move perfect to the video.
SOS isn't installed, the fuel vent is open but the vacuum cap on carb 2 is installed. I messed with it thinking that might be it, but it's not the cause. Mine is between 1-2 on the head side.
If I had a vacuum leak, and I sprayed starting fluid looking for it, wouldn't the engine rev higher? Not bog down to the point of stall?
I also checked my slow jets, there all k72. It seems there supposed to be 35 like you said.
Someone did rebuild this before me, I could tell by the new screws in certain places. Could that be causing this problem? And I'd so who want I having this problem before I rebuild it myself? I used all the same jets.
Maybe they were partially clogged prior to you cleaning them?
Is there a k72 stamped into them or are you measuring them? A 35 jet should be .35mm or 0.13 thousands of an inch (this is why a .10 guitar string is perfect for cleaning them).
Theoretically here is what I think could happen with massively over sized idle jets:
- When first starting, bike will run very rich. It will allow lots of fuel, but not much air will get in because it is being restricted by the throttle plates and the CV slides stay down.
- For it to run at all, you need to turn the high idle screw up a lot so enough air can get in around the throttle plate
- When you blip the throttle air rushes in (along with some more fuel) and you finally start to get decent vacuum coming from the pistons.
- The vacuum coming from the engine keeps the CV slides up, allowing for more complete combustion of all of the fuel the big idle jets are dumping into the engine.
- The revs stay up because they're getting lots of fuel, and some of the air they need (I still suspect it'll run rich because the throttle plate is mostly closed).
- Because the high idle is so high to start with you are getting enough air for it to stay at 4K rpms.
This also explains why the choke has no effect (you're already running super rich)
First of all, thanks for all your help, really appreciated. When you lay out all out like that it world make seems that that's happening. I haven't measured them but will when I get home to double check. They did have k72 stamped in them though.
I went ahead and ordered 4 new #35 slow jets this morning so I will be installing them and try again here in a few days. I will post will an update hopefully by this weekend, I'm getting bike withdraws!
I got the new pilot jets in and installed, fired the bike up with the Bernice sync and it as almost spot on! Bike runs great now! Little issue with the throttle linkage by gonna fix that tomorrow.
Whoever rebuild the carbs before me installed the wrong jet... Thanks for all your help!
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