R75/5 only running on one cylinder

Hmmm First off, I would mark your bowls. See my build thread here. Couple of ways to adjust floats but the bottom line is the height of the fuel in the bowl. This method addresses that directly. if a float isn't floating as much, it won't matter. The fuel height is the fuel height. period. I got fancy with a couple tiny pits made with a dental burr in the dremel. Any mark will work. The sharpie splotch is just to give me something to scribe a line in with my calipers. I was surprised it took gas. Haven't looked recently but it will work short term it seems.


If the level in the bowl is low either enough gas isn't getting in or more is being used. A LOT more. this would be a massive carb misadjustment. Not likely. So I'd go with it not getting in for the moment.

Adjust the floats for correct fuel height per your mark. Don't worry about what's in the lines. Give the bowls a full minute or more to fill to account for a flow issue if there is one. You are just after float adjustment for the moment.

Unless the fuel lines are less than a year old, replace them. Basically you want to remove the line from the crossover to the carb and examine it as well as the fuel inlet at the carb to the bowl. You should be able to look through the line. But for the effort of getting that line off, may as well put a fresh one back. And for that, do them all. 3' of 1/4 fuel line from the auto parts store works. Cheap. Put a nice stainless washer on the petcock nipple ahead of the line. 1/4" will be tight. Makes it easy to get off. Also clean the screen in the petcock nipple while you are there.

if you have never used 1/4" the experience is valuable. You might want the looser cloth braided next time. I generally replace lines every other year. They get hard.

Putting hardened lines back risks a ling chip of rubber getting in the line from the lip as it is pushed on. This will be downstream of your filter. bad.

You might do a fuel flow test from each petcock, at the petcock, no line, on both main and reserve. Tank full. Flow data on Snowbums site.You're looks for "same-both-sides" for the moment. This isn't your problem but check. if the flow is low order two petcock to tank gaskets. Any time the petccocks come off the tank, replace these unless they are very new. Re-used old ones weep and your petcocks get all varnished up and crummy looking. leave the petcocks on the tank until those come in.

With the new lines on do a flow check at the end of the line where it connects to the carb. Then replace the fuel filters and repeat. If no difference put the old filters back on. Spray some (very little!) carb cleaner through the new ones and put them on the shelf to wait their time.


The float valve is simple. The inlet pipe goes to a passage that ends in a very small hole. Below that hole is a larger bore and the float needle rides up and down in this bore. The float needle (hardly a needle, it's a chubby little thing) has a conical rubber tip. When it's all the way up in its bore the tip plugs the little hole from below. The float pivots on a rod and a little tab pushes the float needle up it's bore. you bend that tab very slightly to adjust the position of the float so that it has the needle all the way up and plugging the hole when the fuel level is just right.

read Snowbum on this adjustment.

if you do a flow test from the carb do not have the bowl in place. Have fuel flow for a measured time into a clean jar or something and see how much you get per unit time. Check and compare both sides.

If the lines are new and clear, the float needle is descending and you have weak flow into your jar, then I would say there is bit of crud blocking the tiny hole in the top of the float needle bore. There could also be crud in the bore. The needle is triangular so the fuel can flow around it.

Remove the carb. remove the float needle pivot pin. Must do this in the correct rirection. The knurled end does NOT get pushed through the supports trashing the bores. You push from the smooth end so the knurl comes out the other side first. The float then lifts away along with needle which is attached by it's clip wire. Shoot some carb cleaner up through that little hole to clear it. If you spray so what comes out the inlet pipe goes onto a fresh paper towel, you might spot the villian. Examine the tip of the float needle. A ridge in the conical tip suggests replacement. not having a rubber tip really suggest replacement. if there is a ball in the tab end, poke it with a very small screwdriver. It should be spring loaded. Replacing it is obvious. Dealing with the little clip is an untold joy. No beers before this operation. Unfortunately the needles are costly. if the carb hasn't been leaking, leave it alone. You know how to do it anytime now.

The classic problem is the needle tip gets worn or hardened, or there is a bit of crud on the needle side, or the needle won't go up it's bore because it doesn't feel like it or has a headache or whatever, or the float sticks on the pivot rod...whatever...but the float needle doesn't seal the hole and the carb overflows on your boot---or the parking lot. Thus you always turn the petcock off when your boot isn't handy to soak up the spilled gas.

Your measured fuel flow rates are ok .. .3l/min = 18l an hour .. ok.

The low fuel level on the right side is suspicious .. the fact that it varies is even more suspicious...

What is the fuel level in the right side carby .. just let it sit for a while motor not running. If this level is 'normal' then I'd suspect a blockage just before the float needle.. The blockage is disturbed from time to time by the actions of the needle bumping up and down .. thus a variable flow rate..

I am thinking that you are wasting your time with the fuel solution. Check all of your connections on the Starter relay under the tank. I had a situation very similar to this that was driving me nuts. Finally I accidentally knocked the starter relay and it started running on both cylinders, then died. I tried cleaning all of the connections, but eventually I wound up replacing the relay and it has run perfectly since then. Give it a shot, at this point it is only a few minutes and you really have nothing to lose.

More work and more strange results...

Today I took apart the problem side carb, removed the float and the float needle (there was a frantic 10 minutes when I dropped it and lost the little ball bearing in the parking lot) and sprayed carb cleaner up through where the needle was. A clean paper towel on the fuel inlet revealed a little bit of gunk was blown out...


I blasted the rest of the carb with carb cleaner while it was apart and then wiped it down and reassembled everything. Fuel flow to the right side seemed improved so I took it for a ride.

It rode well for the first mile but then started to stutter again and I had to ride it back on one cylinder.

At this point I decided to try the starter relay and removed the tank, labeled the relay, and cleaned all of the contacts before replacing everything. Started it up again and it was running on both cylinders, so I went for another ride.

This time it ran great for quite a while, including getting up to highway speeds. It stuttered once or twice during a 10 mile trip between at 45 and 70mph but recovered quickly.

I turned around and rode it home without issue until I got into stop and go 20mph traffic. It went from idling normally to idling at approx 2k rpm and then cutting out. By the time I made it home about 1/2 a mile later, it was again running on one cylinder.

It seems so strange that the issue would be intermittent like this, but I am hoping that all this detail might point one of you guys in the right direction.

The ball bearing in the float needle should NOT fall out ! Get a new float needle, or two, for starters.... Did you adjust the float level ? Also, squirting UP through the jet orifice only pushes gunk up where it can come down and cause more problems. Try pulling the fuel line to the carb and spraying down through it, with the jet needle out of the way, of course.

I am thinking that this should tell you something. Starter relays can be pricey. There is a way to use a GM starter relay, and it is probably in the dark secrets thread. My starter relay had gotten doused with brake fluid when the master cylinder blew up. I had thought that I had cleaned it all off when I fixed the master cylinder but it had worked its way in. Interestingly mine would idle perfect and then not run on both cylinders when traveling at speed. Good luck.

I think he had the carb off when he blew the carb cleaner thru the inlet.

Yes to new float needles. There is also a small spring under the ball. Did that fall out also?

Not sure from that photo but is the tip of that needle rubber or solid metal?

Yes, the carb was off when I blew the carb cleaner through it.

The ball and spring didn't so much as fall out as were knocked out when I clumsily dropped the float needle from carb-height onto asphalt. I put it back in and it seemed to be okay? The tip of the needle is solid metal.

I admit I don't exactly follow how the starter relay could/should affect running on one side... does someone with a better understanding of the electrical system want to clue me in here?

The ball is spring loaded in the needle.

The old metal tipped needles should be replaced. But it if isn't leaking, whatever. They are not cheap and you know how to do it anytime.

Clean up the bosy of the needle. Fuel has to be able to flow around it and it must move freely in it's bore.

Finding an empty carb bowl when one side isn't running points to a fuel issue.

The starter relay won't affect running on one side only. The main electrical feed stops by there and drops off some power to the relay. That connection can b flaky and affect power to the whole bike.

Sorting a one side only ignition issue is quite simple. get the fuel feed to both carbs right first, then power balance them.

You anywhere near Canojoharie?

BTW, for parking lot work, score an old white pillowcase from the thrift store for a buck. Spread this under the working area to catch dropped bits. You can also lie on it, put stuff in it, etc. handy item to have, good size and they don't unravel.

Did you ever take the petcocks off and clean them and the gas tank out?

You would think so, but it doesn't work that way. There are two connections from the relay to the coils, one for each, hence issues with one cylinder. It is one of the mystery items that drive people nuts. It is definitely worth pursuing, the worst that happens is you have a spare one. Not a bad thing to have a round.

This incorrect information for a /5. There is one 12 V feed to the left coil, and the coils are then wired in series and on to the points.

I haven't - I ordered petcock rebuilt kits with new filters and gaskets and I'm waiting for those to come in.

Expensive little buggers aren't they? I would suggest leaving the plastic filters off for now. After problems solved you can add them for grins. A lot of big time Heads amongst the Airheads do want you to use them but it's no problem to leave them off till fuel flow has been solved. (if fuel flow is your problem)

There is a new type screen and aluminum washer for inside the tank. It's better than the older skinny screen and red fiber washer. You are probably going to find a red fiber washer. Use what ever you have new for now but the old red thingies can sometimes be saved for spares. Don't use the large screen with aluminum washer AND the red washer.

I hope you got new fuel line.

So today I drained the tank and took off the petcocks. When I got the large filters off they looked like this:



Pretty cruddy! I cleaned them off.


Then I took apart the petcocks, which looked pretty good inside with soft o-rings, no visible crud, and the newer 5 hole rubber strainer.



Now I'm trying to put them back together but I'm a little confused as to where some parts go...

The small metal filters I assume go over the shorter reserve straws (there were no small filters on them when I took them apart)
but where do the red washers go? I didn't find any other washers to replace.

Thanks again!

if you didn't take red washers off, don't put any back.
Both straws go in the tall screens, the short screens are for petcocks with screens in the nipples below the petcock.

When you have the advantage of working on two of something, you can do one at a time so you have the other still assembled for reference.

Makes sense.

Now lets see if I can get them back together... these springs are really stiff!

So, I got the petcocks back together, reinstalled the tank, and filled it with 3 fresh gallons of non-ethanol premium.

I also replaced the fuel lines with new 1/4 tubing and removed the inline filters.

Net result: Fuel flow to bad side seems significantly improved. I noticed this because it came overflowing out of the bottom of the carb and all over my foot. "Off" on the right petcock seems to be "Mostly off" now. The ball fell out of the right float needle again, so I've just ordered two new ones.

Left side petcock is weird too now - Reserve works fine, but On doesn't flow at all. I guess I have to take it apart again.

I tried starting it up and... still not happy. No change to the one-side-only problem.

The small filter goes inside the tap, but on the exit side of the tap. That's only if you can detach the exit side, which yours can't. The washer goes between the tank and tap but i'm not sure if it'll fit with the large filter (I don't have that on mine).

I'm usually a calm man but screwing down the tap-handle cap brought forward a whole load of expletives when I rebuilt my tap.

I haven't read everyone's comments but have you checked that your carbs aren't massively out of sync e.g. a cable is caught on the lip of the adjuster? That happened to me once after fitting an inline fuel filter and threw me a loop for a while.