76 KZ900 FUEL STARVING

FIRST POST!!
So I just cleaned my carbs and leveled my floats. Also installed KN filters and about to install MAC 4into1's. I understand that betwen the filters and the exhaust improbably looking at rejetting. However, as of right now and before I installed the KN filters I have been having trouble with starving for fuel. I cleaned my gas tank and replaced the petcock with a simple on/off/reserve, put new fuel line on and installed an inline filter. I have changed filters several times and have a fresh one in place now.

I will be driving along and sporadically will starve for fuel. So I pull over, turn off the fuel, and remove the fuel line from the inlet side of the filter, turn the fuel on, it spills all over, then I reconnect it to the filter and wait while the bowls fill and away I go.... then it all happens again in a while when the bike decides....

Its like an air lock somehow between the carbs and the filter. Any suggestions please?

Next time it happens, slowly open the gas cap and listen for air moving (most likely a vacuum sound). You could have a poorly vented gas cap and it's vacuum locking the fuel in the tank. I can't say that's it, but at least it's an easy check.

Later, Doug

SevKZ9 wrote: ... just cleaned my carbs and leveled my floats. Also installed KN filters and about to install MAC 4into1's. I understand that betwen the filters and the exhaust improbably looking at rejetting. However, as of right now and before I installed the KN filters I have been having trouble with starving for fuel. I cleaned my gas tank and replaced the petcock with a simple on/off/reserve... I will be driving along and sporadically will starve for fuel. So I pull over, turn off the fuel, and remove the fuel line from the inlet side of the filter, turn the fuel on, it spills all over, then I reconnect it to the filter and wait while the bowls fill and away I go.... then it all happens again in a while when the bike decides....

Its like an air lock somehow between the carbs and the filter. Any suggestions please?

Hello, SevKZ9. and WELCOME to KZr!

If not already done, would perform the clear tube test on each carb to assure fuel levels are within specs.

May diagnose possibility of malfunctioning vent in the fuel tank cap by test running while leaving the cap ajar (partially open and not snapped down). During the test run with fuel cap ajar, the fuel starvation issue will either persist or disappear.
If issue disappears, the cap vent is likely obstructed.
If issue persists, may rule out the cap vent as a suspect.

Would also assure that both carb float bowl vents are exiting to open air.
Which is usually via a hose attached to each vent nipple.
The hoses may be routed to exit underneath the bike, or as in the KZX900's stock routing shown here:



Would perform spark plug readings after high speed throttle chop before enlarging the main jets.
The stock KZ900 carbs were designed before the EPA regulations, and aren't inherently lean as are the later EPA designed carbs.

The stock pilot jets are imo even more likely to be perfectly okay with the K&N pods and freer breathing 4>1 exhaust system.

The KZ900 came from the factory with a manual petcock.

If the original air box is still at hand, would hang on to it, as they are now a rarity, hard to find, and getting increasingly expensive.

Good Fortune!

right on I'll check it out and keep it posted

Also make sure there is not a sharp bend or kink in your fuel line as that can pinch off the fuel flow. Ed

I checked it out last night and found several issues. Rust in the filter so I tumbled the tank and now it sits with evaporust all day. Collapsed air route in the cardboard "gasket" in the cap so I repressed it as best I could. Also cleaned the petcock for this evenings reassembly.
Tonight I'm putting on a new filter and going to try it again!

Patton: please explain the clear tube test??

Also you mention two vent lines. In that location on my carbs there were caps when I got it. Last night I damaged one while I was removing the filter so I just taped over it until I get more caps but maybe I need vent hose instead? Also I have 3 of those vent nipples not just two?!

Pic for reference: you can see my tape lol

The clear tube test (see Patton's link below) is used to measure the fuel level inside the carb bowls. If the fuel level is set too high the carbs can overflow. If it is set too low the engine may starve for fuel.

kzrider.com/forum/2-engine/580335-homead...at-level-tool#580344

Your set of carbs should have 2 vent ports, and they must not be capped. They are intended to prevent the carb bowls from building pressure or vacuum as fuel flows into them or is drawn from them. One vent port should be on carb #2 and one on carb #4. They should have a vent hose going to the open air. The fact that you have a vent port on carb #3 tells me that a previous owner replaced that carb with the wrong one. That may not hurt anything, but that vent port should not be capped either. Ed

If you're using Ethanol in the gas(most of us are, and don't know it), it can affect all part of the fuel system.

Ethanol in today’s Gasoline

Today's gasoline(for the most part)contains Ethanol, not friendly to carb parts(floats, jets, and rubber fittings, including diaphrams on the CV carbs, and the vacuum operated petcock). Read this:

Ethanol In Gasoline Problems (E-10, E-15, E-20, E-30, & E-85)

Certain materials commonly used with gasoline may be incompatible with high-level ethanol blends, causing them to degrade and contaminate the fuel. Metals that have been shown to degrade over time in the presence of high-level alcohol blends include brass,(floats & jets) lead, zinc(carb bodies) and lead-based solder.

Nonmetallic materials that degrade when in contact with ethanol include natural rubber, polyurethane, cork gasket material, leather, polyvinyl chloride (PVC) polyamides, and certain thermoplastic or thermoset polymers.

On the other hand, unplated steel, nickel-plated steel, stainless steel, black iron and bronze have shown resistance to ethanol corrosion, with nonmetallic materials like reinforced fiberglass, Buna-N, Neoprene rubber, polypropylene, nitrile rubber, Viton and Teflon meeting acceptable usage standards with E85.

1. Ethanol can break down resins and fillers in fiberglass gas tanks, causing them to leak.

2. Resins leached from fiberglass tanks can go through the fuel system, sticking to valves and other internal engine parts.

3.These deposits have caused bent pushrods and have clogged intake valves.

4.The alcohol attracts water, leading to increased corrosion in metal gas tanks.

5.Water in the fuel affects the octane and leads to knocking and decreased performance.

6.Ethanol acts as an efficient solvent, gradually cleaning out the accumulated gunk in fuel tanks and lines, and clogging carburetors.

7. Certain rubber gaskets and fuel lines are weakened by ethanol. Some rubber fuel lines may develop internal swelling, restricting the flow of gasoline. My understanding is the Silicone fuel lines resist the effects of Ethanol in gas.

8. The Ethanol in the gas has been reported to attack the glue used in gas filters, rendering them useless, as the paper filter medium is now coated with glue. It also softened the filter hose connection ends, causing possible failure.

It's possible that carb float(s) may have a leak, letting in gasoline, and having the excess go into the engine's crankcase, diluting the oil. To check, open up the oil filler cap, and give it a sniff, if you smell gas, do not operate the engine, as the oil may be contaminated. Fix the leak, and change the oil before using the bike.

Well here in Montana we got 15" of snow... Needless to say, not a lot of riding or testing.... However I did make a cool new kickstand.... So that's something. With the tank cap closed I get no fuel,with the cap open, I get no fuel. If I remove the fuel from the inlet on the filter, I get fuel. Then when I replace the line on the filter, the fuel drizzles out and stops. Again, no fuel.

I'm hoping to do that clear tube test tomo when I can get places

Nice rugged stand you got there.Are you using one of those lawn mower fuel filters?I Just installed one on mine.Mine is a cone shaped one with a copper filter in it.Are you running your pet cock on the ON position.I only pick crap up on reserve,I guess cause when on reserve its picking up fuel from the bottem,& when ON it is an inch off the bottem.It sounds like your filter is to small of a micron or its clogged.

thanks about the stand. I am using an inline mower filter, I noticed both inlet and outlet orrifices were very small which would create a low flow system so I trimmed the nipples to a larger ID along the taper. Also i started trying to check the starving isuue agin but on the reserve setting and decided to wait and try again with a full tank so i can be assured a more accurate test.

Also after reading several other threads I suspect my main problem was coming from having the vent lines "capped". The caps were sooooo old and worn that I'm sure they were leaking which would explain some of the erratic tendencies of the starving then running fine. So now that I have put vent lines on each nipple it should really change things!
Tonight I'm going to fill the tank, do a clear tube test, adjust if needed, then take her out for a spin. My suspicion is that after richening the mixture by adjusting the needle when my vents were capped, I will have to lean it out this evening since there wont be any vacuum on the float bowls from the caps which should have prevented the fuel from consistently being drawn thru the carbs.

Also, can anyone confirm that these are certainly vm 29's? I measured the throat and it came at about 28.6mm if I remember correctly. So how does that compare to their appearance?

Here's a couples more pics.... I think they are 29's with the wrong bowl on them?