Topic: '76 750 will not start

[rewcashion]

hi guys, this is my first post on here. you guys have helped me immensely, so i hope you can help again.

i recently purchased a 76 CB750K which has proven to be riddled with more problems than i foresaw ( i guess thats just how things work). whenever i purchased it, it was running wonderfully. the fuel mix seemed to be a little rich, but it roared like a beast and wouldn't skip a beat at high or low revvs. the PO said he'd had problems with the jets sticking, but i wouldn't know what that means for the engine or how to resolve it.

so, my father and i get it home to start work on it. first was the wiring system, now fully functional. next was the bent rear wheel. we replaced that, sprockets and chain, adjusted the throttle cables (it would stick past 3/4 throttle which made for an interesting test ride), also adjusted the clutch cable.

we are now getting back to the engine about a month later, and she won't start. checked the plugs, the coils, the points. everything checks out there. checked the floats which are holding gas. so, now i'm kind of confused. theres gas, and theres spark, and its turning, but it wont start. i figure its something to do with the carbs, but i have no earthly idea where to start.

its my first time working with a carb, so be gentle!
thanks in advance for any insight you can give in the matter. 

[ekpent]

Usually a carb stick would be more related to the floats and float needle.
 For starters is your battery well charged. Have you tested all 4 spark plugs for spark as in laying them on the head and actually seeing sparks in them with the engine turning over?
   Have you removed any of the plugs to smell or look for gas getting to them. You can also drop a float bowl after turning off the petcock to see how much gas is in them.
  Have you changed,set or altered the timing at all.
  If it ran before it can't be to serious and also make sure your kill switch is set to on or run 

[rewcashion]

the battery is indeed charged. ive tested all the plugs in that fashion. need to replace one, so replaced all. they are not getting gas. ive dropped the float bowl on the outside carbs and there is fuel in them. about a quarter inch in cyl 1, and a little more in cyl 4. the only thing ive messed with is the spacing on the plugs.
i figure its not exactly serious and thats why its so frustrating.

and yes, the kill switch is set to on :p

[ekpent]

Those bowls when you drop them should be almost full. Sounds like your float needles may be restricting flow or your petcock for that matter.Take off a float bowl put a catch pan under it and turn on the petcock to test and see the flow. Rattle the floats up and down to try and dislodge the needle if necessary.Is there any sign of rust in the fuel tank or float bowls also.  Have the carbs been through yet. All 4 carbs having that same issue would be a little strange though.

[Gordon]

Is there gas in the tank?  If there is, then most likely the petcock isn't allowing any past, since none of the bowls have gas in them.  Is the petcock filter clogged, or if there's an in-line filter is it clogged? 

[Hondawggie]

HIGHLY IMPORTANT -- pull your spark plugs, are they wet or dry?
Man oh man the spark plugs will TELL a tale.


1) try to start the bike as you did previously

2) then pull the spark plugs

3) are they wet or dry and what color are they?
    P.S. I realize you just replaced them but do step #1 above FIRST before you pull the plugs -- try to start it THEN what do you see on the plugs? 

[bjatwood]

I think the problem is the screen is plugged in the petcock. I can see one or two of the needles sticking but all four at once, seemingly impossible. So unscrew the petcock bowl and check things out. Let us know.

[Spanner 1]

With respect, are you sure there is 'enough' fuel in the tank and that the petcock is turned to 'reserve' ?
Fuel level that low in the carbs wont 'reach' either jet ( slow and main ) = no start. +1 on the petcock being gummed-up as a possibility too .

[Roach Carver]

I agree. make sure there is PLENTY of gas in that tank. I dont mean, 'you can see it sloshing around down in there'. Make sure there's a lot. If that is not it drain the tank and pull the petcock and clean the screen. probably needs it anyway.

[Don R]

I am always suprised when my bike is empty and I pull the tank, how much fuel is in there.  Just to agree with what roach carver said. I usually pull off the fuel line and then open the petcock and check fuel flow into a container. 
 If it is flooded you may need to leave the plugs out to dry the cylinders a bit too.
 Good luck.

[rewcashion]

thanks guys very much. got ol girl running again. had a half full tank (optimism!). pulled the floats separately to check fuel levels. cyl 2 was lacking and appeared to be clogged, so i gave the float some good jiggles until the fuel began to flow. reassembled and kicked it over! just like that!

thanks again. im sure you'll see more of me on here requiring n00b tips.

she's now running, but the idling is very strange. without me touching the throttle, she'll start revving up to about 5grand. i try to ease it back down with the throttle, but its not very effective most times. i checked the throttle cable linkages and tension and they aren't moving, so its gotta be internal. if thats not happening, i notice some spitting and sputtering and misfiring. i blame ghosts. that, or a rich fuel/air mix, which seems to be a more tangible and easier target to tackle. im currently in the process of cleaning the carbs. ive got my rebuild kit on order as we speak.