Fuels issue

[Djeric]

Hi all, thanks for having me in the forum. I'm in a bit of trouble with my 1974 cb750 I just bought. It was fine the first couple day when I took it out for a quick ride, but in the third day, during a semi fast cruise, I felt the lost of power, then as I slow down it died. Buddy and I managed to get it home and this is what we noticed:
1. We have to prime the engine using the kick start to get fuel into the carbs, then we can electric start the bike.
2. After the bike started, we realize the fuel is not flowing at all into the carbs from the tank even when we give throttle, but it does flow when we use the kick start.
3. We know what the issue is for lose of power, simply the carbs doesn't get fuel. But we don't know why that is. The fuel lines are not plugged because we can clearly see fuel moving when we kick start it.
4. So basically we kick start to get fuel in, the start it, and the bike will run for a bit until it uses up all the fuel in the carb then die.

Please help!! Thank you so much

 

[dirtdigger]

Kick starting has absolutely nothing to do with getting fuel to the carbs. So if you take the fuel lines off the carbs does fuel flow? Have you checked for a plugged screen in the petcock? Have you opened the fuel cap and see if fuel flows with the cap open? When you kick the bike over I think you are just jostling the fuel around enough that you get a minute amount of fuel to flow. Kicking the motor over or turning it with the starter has nothing to do with the fuel because it is all gravity flow. You either have a vacuum lock in the fuel tank(plugged vent in cap) or mostly plugged screen in the petcock or you have a ton of trash floating around in the tank that moves when you kick it.

 

[amc49]

X2 all of that. Your shaking of the bike while kick-starting is what is feeding the fuel. Partially blocked something or another. Shaking breaks the liquid tension of the fuel at the congested location and then it flows.

The petcocks are really just about too small for a 750, it has virtually the same size passages as the ones used on the much smaller engines. The same exact one on a CB550 I've had issues with priming even with a full tank once the bowls run out, but zero once they fill up. Some of it like said is tied to the tank vent, and the particular line setup you have to carbs will most definitely affect the gravity priming there. I pulled the tank filter on the 550 to add a better bigger one down low and the extra pressure resistance just to soak and fill it was causing my specific issue. The later first couple years of DOHC used the same petcock and then Honda realized they were too small in flow and began to enlarge them through the next few years. The 900 and 1100 can flat run the carbs dry at full throttle load if you use an early tank with the smaller petcock on the bigger motor.

 

[bamkiiute]

I found this thread while searching for "prime using kick starter". I was looking to see whether this was a "thing" because it actually says in the K2 Owner's Handbook, under "Starting in Extreme Cold Weather. Prime the engine before starting by cranking several times with the kick starter." This should be done with the bike switched off.
But you are saying that kick starting has nothing to do with getting fuel to the carbs.
So what does this priming method achieve?

 

[dirtdigger]

All you are doing is making the fuel mixture in the engine extra rich by not letting it fire with the bike off....no fire=extra unburnt fuel in the engine making it extra rich. Has nothing to do with getting fuel to the carbs. Its all about getting an extra rich mixture in the engine.

 

[amc49]

'Prime the engine before starting by cranking several times with the kick starter.'

Like said, the handbook is assuming that fuel is already AT the carbs to be able to go into the engine. A misunderstanding there, the carbs are gravity fed only, there is really no way to fill them other than by that. And doing that 'prime using kick' thing with ignition off is a GREAT way to foul plugs out with wet fuel if one is not sharp enough to know when to stop. How so many end up posting here with 'one pipe is cold while the others are hot' type complaints, you can't count 'em.