I am stumped...

Hi Everyone,

I've been lurking on here since the summer. I am new to Commando's having bought two (a '74 & a '73 basket) last spring. After replacing and paying attention to a few things on the '74 I was able to get it running, but the dang beast stalls out after a few minutes. At first I thought it was something with the gas tank not venting, but leaving the cap open doesn't keep it from happening. I replaced the fuel filters & it seemed to work but then it started stalling out again...

Once it starts stalling I have to let it sit for a bit before it will fire up again. It will run long enough to go about a tenth of a mile before it dies again.

One cylinder will stall before the other (maybe 10 seconds) - able to verify by activating the kill switch.

To top it off it has dual Mikunis, which I prefer Amal Concentrics as all of my motorcycling experience has been with the Amal, those choke levers are driving me nuts!

Any chance that this is something else other than a fuel flow issue?.?.?.?. Perchance something easy :cry:

Thanks,
Geoff

Have you removed the float bowls, opened the petcocks and observe the fuel flow rate through the float needle system?

I would have to agree that you are running out of fuel...if it isn't the venting then it almost has to be a restriction in allowing the bowls to fill, as it sounds like you are running the bowls dry and then having to wait for them to fill. If there are no restrictions in the fuel line, at the taps, or as they enter the carbs then I would be looking at the float needles to make sure they aren't sticking, or that the floats aren't hanging up. First check might be the fine screens on the taps. Do you have a fiberglass or steel tank?

Russ

Absolutely not. I have not really done anything with the carbs except look at them from a far distance, Japanese carbs scare me .

It is something I would be willing to knock out this weekend though, its suppossed to be a balmy 32F tomorrow.

rvich said:

I would have to agree that you are running out of fuel...if it isn't the venting then it almost has to be a restriction in allowing the bowls to fill, as it sounds like you are running the bowls dry and then having to wait for them to fill. If there are no restrictions in the fuel line, at the taps, or as they enter the carbs then I would be looking at the float needles to make sure they aren't sticking, or that the floats aren't hanging up. First check might be the fine screens on the taps. Do you have a fiberglass or steel tank?

Russ

Click to expand...

Its a steel tank that I cleaned and coated with Caswells this summer. Taps are new. I will compare the fuel flow rate out of the tank with my Bonneville to confirm the lines are ok. After that I guess I will have to overcome my trepadation of Mikuni carbs.

Thanks alot you guys, this board is awesome!

Geoff

Mikuni carbs are easy. 4 phillips screws to remnove the bowls.

That's where it gets a bit squirrely, as the floats are contained in the bowls, riding up and down thier own little pins.

Still, just remove the bowls, turn on the taps and see if you have free flow. THAT'S the test.

I have not really done anything with the carbs except look at them from a far distance, Japanese carbs scare me

Click to expand...

The Mikuni's are very simple, if you want to check the fuels flowing just take off the drain bolt from the bottom of the float bowls, even easier than Amals!! I recently had a problem and it transpired the rubber in the taps had swelled restricting the flow, they weren't that old, put a couple of old scruffy ones on and hey presto no more probs

Mikuni Manual here http://www.mikuni.com/pdf/vmmanual.pdf

One of the things I do not like about Mikuni VM carbs is the little pins the floats ride on. I have noticed these get "gummy" when the carbs sit with gas in the float bowls for an extended period of time. It is possible that this is not allowing the float valve to open beyond a trickle.

At least drop the float bowls and thoroughyl clean the bowls and the float pins. It's easy and could solve your problem.

This may sound simplistic, but I would unhook your two fuel lines from the petcocks, but something under them, and open the petcock to full flow.
They should be POURING gas with no restrictions.

I had the same problem you had just last week and ran this test, pathetic flow, replaced both petcocks with new, and now she runs great.

Worth verifying your petcocks are in really good flow condition, if you have not done already.

The fact that both your cylinders are crapping out at the same time seems to indicate that it's something they share in common, maybe gas supply. Even new petcocks have been known to be way down on flow.

A lot of aftermarket petcocks are rubbish and don't seem designed to run with the latest mixes of fuel that seems to be available theses days. The internal rubbers swell and don't pass fuel. I agree with the advice that you should simply disconnect the fuel line and allow the fuel to flow into a bottle for 30 seconds or so.

Just an updat and another question -

This is the second time in my life I have been bit by a non-venting gas cap! Where does the cap vent? (Triumphs have that easy hole in the top!)

Thanks for all the help!

Geoff

So it WAS or WAS NOT the gas cap venting?

geoff71 said:

Where does the cap vent?

Click to expand...

There should be a small vent hole in the pressed "rib" of each of the two circular plates that sandwich the cap gasket?

Both plate vent holes are out of alignment with each other, so the "circular" gallery formed by the two parts being fitted together back to back acts as a baffle to prevent fuel escaping. The area above the upper plate and below the cap itself is open to atmosphere.

grandpaul said:

So it WAS or WAS NOT the gas cap venting?

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Hi Grandpaul - It is the gas cap vent. The previous - previous - (maybe even further back) owner did a really bad POR-15 job, which included the gas cap - rubber gasket and all. I never cleaned the metal parts of the cap of the offending silver goo - and I quickly realized that was the culprit on last Saturday as I was starting to troubleshoot the petcocks, but I could not figure out where the vent actually was.

Now I will be digging out POR15 and hopefully it only plugged the one vent.

Thanks!
Geoff

There should be a small vent hole in the pressed "rib" of each of the two circular plates that sandwich the cap gasket?

Both plate vent holes are out of alignment with each other, so the "circular" gallery formed by the two parts being fitted together back to back acts as a baffle to prevent fuel escaping. The area above the upper plate and below the cap itself is open to atmosphere.

Click to expand...

Good description.