1980 CB650 Custom power issues

thwrightstuff said:
Gee thanks, fortunately, I've got that detail ironed out. I ran into yet ANOTHER problem last night. My gas tank bolt wouldn't come off so I have to cut it off. Turns out there's this floating nut inside the subframe that the gas tank bolts to. So I'll have to JB Weld that and pray that it holds nicely for awhile. I don't understand why they didn't just weld a threaded nut directly to the frame like they did with the seat bolts, seems pretty stupid to me.

I think maybe my lifters are a little too tight like you said, I'll go through and readjust tonight or tomorrow.
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yeah, bummer about the nut but at least you have 4 cylinders again . . keep us posted.
 
thwrightstuff said:
I just discovered the strangest thing. Apparently, the positioning of my throttle cable makes a huge difference on the rpms. Generally, if my front wheel is turned to the left, my engine revs up like crazy, but when I straighten it out, the rpms drop and the bike eventually cuts out.

I've narrowed it down to the throttle cable and I believe it has an issue at the twist grip. Why on earth would this be making such a difference? It's not like it's electrical and the damn thing doesn't seem to pull any on the throttle body of the carbs. How is this happening?
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Simple. turn the bars slowly and look at the throttle cable(s) where the go round the headlamp and under the tank. At some point the outer is being caught and the inner pulls the carbs open as you push on the bars. Cables may be too short of badly routed or just pinched.
 
So I haven't made any progress because I don't have a welder and the JB Weld didn't work for shit. I thought that stuff was supposed to be strong? It ripped apart like dried playdough! I'll get back to you after I finish a few of my other projects.
 
thwrightstuff said:
I just discovered the strangest thing. Apparently, the positioning of my throttle cable makes a huge difference on the rpms. Generally, if my front wheel is turned to the left, my engine revs up like crazy, but when I straighten it out, the rpms drop and the bike eventually cuts out.

I've narrowed it down to the throttle cable and I believe it has an issue at the twist grip. Why on earth would this be making such a difference? It's not like it's electrical and the damn thing doesn't seem to pull any on the throttle body of the carbs. How is this happening?
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In your video you can see throttle cables are routed wrong. They shouldn't be behind fork leg. (2 seconds into first vid)
 
They should be running down through the same position as brake hose then around steering head to carbs. I think the brake hose goes to front (on top) of throttle cables.
I haven't set up a stock CB650C since around 1980 though so can't remember
 
So I changed my pilots to 42 and the mains to 115. it runs a lot better. IO'm at least able to get somewhere, but my bike runs smoother with the choke engaged, especially at less than 1/4 throttle. The other main issue I'm noticing now is irregular power. I know it's likely due to the K&N filters, but I recall the bike not having that issue before all of this happened. Btw, the surges go away when I engage the choke a little, so I'm thinking go up another jet size? Pilots and mains?
 
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drive along at around 3000 rpm in 3rd gear then open the throttle quickly . . if it accelerates properly, that will confirm the your problem is in the pilot and or small main jet and one or both are too lean.

if your needles are adjustab;e and you dont currently have any jets, you could rsise them all the way and see if that helps.
 
I have a bunch of jet sizes, so I don't need to fiddle with the needle just yet. I'm not sure this will solve my power surging issue at higher throttle as well though
 
Leave the needles alone until the main and pilot jets are right. Try one size larger pilot jets to see if it cleans up the idle/off idle lean condition and see what that also does to the rest of the rev range.
 
thwrightstuff said:
I have a bunch of jet sizes, so I don't need to fiddle with the needle just yet. I'm not sure this will solve my power surging issue at higher throttle as well though
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you never know until you try, so get off the internet and start stuffin jets in it.
 
I took your advice and finally got off my fat lazy ass and did some minor tweaking. I don't have any larger jets for pilots, though. I tweaked the screws and it's a little bit better, but I'll probably need to go to 45s. The mains are still feeling crappy, power surges mainly. Is there anything else I can tweak on the bike that might increase the performance of the pilot circuit before dropping over $40 for 4 pilot jets? Could the performance of the mains or needle effect the performance of the pilot circuit at all?
 
Great news guys. I know you've been just DYING to hear from me. Aside from the stupid gas tank bolt, the bike seems to be running pretty damn well, save for a couple of minor things.

1. When idling, in neutral or any other gear, the rpms seem to run very high. I'm running 42 slows right now and just installed 120 mains. The response is good, very little surges. Should I turn the pilots so that the rpms are lower or is it more likely a throttle cable positioned wrong? It seems like a pretty consistent problem right now. This after the bike is properly warmed up, I went riding for 5-10 minutes up and down the street.

2. I think fine tuning everything will smooth things out a lot. Do you think it's necessary to do a plug chop when I'm this close? I've always been going based by power response, I've only done plug chop once and it confirmed exactly what I already knew.

3. Any suggestions as to what I should work on first with this carb issue?
 
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the carbs should be synchronized

the air screws should all be turned the same amount.

the idle should be a cable or stop screw adjustment . . do not use the air screws to lower the idle.
 
I rerouted my cables, they were a little tight and that cleaned things up a lot.

I guess I should tune my pilots so that I get the highest rpms possible at idle? I definitely need to retune the entire carb before changing it again. I am very close, it's slightly boggy after altering the cables, so I'm pretty close.
 
High idle rpm usually means your running lean or have air leaks ;D
Idling and running better with choke tells you it's too lean so needs bigger pilot jets and probably main jets as well
 
I changed my pilots to 45s and I have to say, it's cleaned it up a great deal. The power isn't completely steady until the bike has properly warmed up, which is typical. I also changed the mains to 125s and that helped a lot. Now I just need to tune the rest. It appears that when I yank on the throttle hard, the engine bogs a little briefly. Is this as a result of the jet needle and throttle slides not being tuned properly?
 
I had a similar problem on a cb650z (same engine) 120/42 jets is the right size. Mine had the same problem you describe and it turned out to be a loose coil wire in the plug. It had just one tiny string attached. Just enough to keep the paired cilinder firing and on idle, fire all 4, but when revs go up the tiny string of wire couldn't handle the voltage. changed the coil wire and BRAAAPP
 
So another update. I synced all 4 slide valves to an accuracy of +/- 1psi. I swapped out the nonadjustable jet needles for the adjustable ones that come with carb rebuild kits and then richened the needle by two clips. This seems to have drastically cleared up the performance of my bike. I think the one thing left to do is go from 125 to 130 mains on my bike. I think this exhaust and filters are leaning my mixtures way out. It's almost perfect and this has really cleaned up the lag when I hit the throttle hard.
 
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xlnt, just keep in mind that if the plugs are dry and black, it is too rich, if they are clean and the precelain is nearly white or white, it is lean.
 
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