fuel issue?

having a problem getting gas to carb. bike has a 26 mm mikuni with inlet on side. have brand new 3 way petcock mounted on frame just above carb. looked down in tank with light appears clean? if line pulled off carb will flow no problem?took carb off to look for stuck float. seems to be ok.line coming from petcock is slightly higher than petcock itself but below tank? bike has been together for awhile now but sitting waiting on new pipe to come in. gas was visible thru clear inline filter before? now no?bike would start up just didnt want to run it long without exhaust. any ideas????

If it flows before the carb try this. To verify fuel flow thru the carb, turn the gas on and open the bowl drain screw. If it doesn't flow, it must be the carb.

mark from florida said:

having a problem getting gas to carb. bike has a 26 mm mikuni with inlet on side. have brand new 3 way petcock mounted on frame just above carb. looked down in tank with light appears clean? if line pulled off carb will flow no problem?took carb off to look for stuck float. seems to be ok.line coming from petcock is slightly higher than petcock itself but below tank? bike has been together for awhile now but sitting waiting on new pipe to come in. gas was visible thru clear inline filter before? now no?bike would start up just didnt want to run it long without exhaust. any ideas????

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I would think float jet, probably the kind with a rubber tip, which are notorious for sticking, especially if you are running regular pump gas, switch to ethanol free REC90 or whatever ethanol free octane you can get as the ethanol will first expand all of your rubber components before just eating them. Meantime, try tapping or rapping the float bowl with a brass mallet to see if it allows flow.
Also, could be a metal tang on the float to float jet needs a little adjustment as well. I usually use a piece of hose and adjust it by blowing air through Inlet and make sure the float is cutting off when it is horizontal.

These are simple as fuel systems get, really basic setups. Yet, they have a hair-trigger element, i.e. one minor imbalance can cause what seems to be a huge problem.

There's not much head pressure to sustain fuel flow and that's the biggest, single, issue. From my experience, locating an external petcock using the upper airbox mount on the frame places it just low enough to allow the tank to empty completely. The routing of the fuel lines, especially the outlet side, can be a make/break proposition. If you're getting a dynamic fuel starvation problem, i.e. the bowl drains under full power, then refills during shutdown, look at float level & line routing as the solution. This, of course, assumes a clean fuel filter, proper tank venting and no pinched lines.

Next biggest variable is bowl inlet location + carb mounting angle. With Mikuni carbs in particular, it is sometimes necessary to tweak the float height if the rear of the bowl (where the main jet well is located) sits higher than it would if the carb sat level. Mikunis also have a main jet well O-ring that can be damaged...or shrink; remember all the work kirbby did trying to chase down the optimal jetting on his VM22? That turned out to be a perished main jet well O-ring. That model and the VM26 are really temperamental to dial-in.

I'd wait and see how it acts after you get it done/running. I've had intermittent issues with this same problem. Use clear fuel line so you can see what's goin on. Sometimes it seems a big air bubble can keep fuel from passing. When this happens, I pull the fuel line off of the carb just enough to let the air escape, and fill the line with fuel.
I've used heat to put a 90 degree bend in poly hose...so the hose leaves the carb at a better/lower angle. There are 90 degree brass hose barbs available too.

thanks for the suggestions. ill give all of them a shot. carb is brand new. petcock is mounted on upper air box tab.routing from petcock to carb is kinda bad due to side bowl vertical feed line.will look for 90 degree elbow fitting which should lower line giving it more of a gravity feed.will aslo look for clear line.thanks again for all the feed back

When it comes to actual fuel flow rates of these motors, hardly anyone has a workable & realistic mental picture. Hint: it's way less than seems possible. We generally agree that a free-flowing fuel system will allow a CT70 tank to drain completely in under 5 minutes. That's workable but somewhat random. And...it's more than good enough. Fact is, that rate is enough (mathematically) to feed 10mpg fuel consumption. Consider a really thirsty 150-ish displacement motor, inhaling fuel at 60mpg; at 60mph, it'd take 35-40 minutes to empty a full tank. Most bikes deliver 80mpg+. A "typical" 110-140cc tune, fitted with a VM20, 22 or 26 carb can cover 1-1.5 miles just on the fuel inside the float bowl. The point of my ramblings is simple...the minuscule real-world fuel flow rates make it very difficult to determine much of anything by simply observing clear fuel lines. Yes, transparent fuel lines & filters are a good idea; you can see if they're completely empty, full, or contain dirt. That said, air pockets are to be expected; completely filled lines are the exception, not the rule.

Best test, imo, is observing initial fuel flow, starting with an empty float bowl. That allows maximum flow rate that the system can support. Should take under 30 seconds to fill the float bowl. After that, the inlet needle & seat assembly stops additional fuel from entering the carb, until bowl level drops normally as the engine runs. From that stage, fuel flow is going to be minimal and intermittent...also nearly impossible to observe while riding. And that's where most owners get confused. Fact is, once fuel level inside the bowl is sufficient to cover both jets, it's enough; if you can see fuel inside the main feed line, just above the point where it attaches to the bowl fitting, the carb is "full". Best to learn what normal fuel flow looks like when the petcock is initially switched "on" for the first cold start of the day...and when the lines are truly drained dry while riding. Those two will tell you when there's a fueling issue whether it's a restriction, or it's time to refill the tank.

Spot on Bob, my 140 can go exactly 1 mile on a bowl full.

finally got a chance to go out and putts around on the bike. decided to remove carb and check float level. that looked ok but i noticed pointed piece that fits up into jet was not moving freely. it would stick?pulled it out and inspected. looked ok? {new carb] blew everything all out and reattached lines. bike fired right up.got it to idle and has good response off idle.still waiting on ordered pipe.got bike titled with tag. gettin kind of anxious.only been 2 months????thanks again for all the tech support.