PMO carb starting issue

[PMNorris]

Hi,
I have a starting issue with my car. It starts fine if I restart it fairly soon after its last run, even if its cold. I think my issue is that my 46mm carbs are draining slowly.

After leaving my car for a while the windows on the carbs show that the chambers are empty




After turning the car over many times, pumping the gas and finally getting the car to fire, the chambers are around 50% full, which is correct.




Is it expected that the chambers drain when the car is not used. I would expect them to stay half full. Any idea what could cause this?

Many thanks
Paul

[PMNorris]

I might have answered my own question as this is listed on the PMO technical FAQ


2. Float Needle Valves

We started out with viton float needle valves which have a somewhat blunt viton tip. Historically, these have been the choice for off-road cars.
We changed to the Weber float needle valves when we found quality control problems with the viton valves. After that, we went to the double-ball type. Both didn’t work as well as the viton ones for holding fuel level. Consequently, we went back to viton, but we now individually inspect every valve.

As a footnote, not even the viton valves will hold the fuel level in some cases where solid motor and transmission mounts are used. For race cars, we recommend the Clubsport mounts which can be stiffened by modification. WEVO recently developed urethane motor mounts which should be the best of both worlds.

PMO has different thickness float needle valve gaskets available to set the float height. Ideally, the tang remains parallel to the rest of the metal stamping with the float set at the correct height. The float tang should be at 90 degrees to the float needle valve when adjustment is completed. The gaskets are available in .010, .020, .030, .040 and .060 thicknesses.

[edmayo]

Can't you just turn on the ignition before cranking to let the electric pump fill the carbs?

[1QuickS]

If the air correction jets for the idle jets are clogged then you will siphon all gas from float bowls into the cylinders.

[tom1394racing]

If the car sits for a week or so, won’t the fuel in the float bowls evaporate?

I always turn on the key and let the fuel pump run for 10 sec or so before pumping the pedal and starting.

[Frank Beck]

As a footnote, not even the viton valves will hold the fuel level in some cases where solid motor and transmission mounts are used. For race cars, we recommend the Clubsport mounts which can be stiffened by modification.

I'm curious who "we" is?

You guys sound amazing but I would love to know how you arrived at your conclusions about "solid mounts"? Besides Richard Parr's proselytizing?



It's a load of hooey that solid mounts cause problems like you describe.


Richard, bless his heart would blame the world-wide hunger problem on solid mounts if he could.

[PMNorris]

I'm curious who "we" is?

You guys sound amazing but I would love to know how you arrived at your conclusions about "solid mounts"? Besides Richard Parr's proselytizing?



It's a load of hooey that solid mounts cause problems like you describe.


Richard, bless his heart would blame the world-wide hunger problem on solid mounts if he could.

That came straight off the PMO website. However, Richard has given me a few pointers too which I will try over the weekend. He also recommended leaving the fuel pump on for 30 secs to fill the float bowls prior to cranking the engine as a result of evaporation. My own impression of living in Switzerland for the last few years after having lived in the UK for most of my life is that the humidity level is much lower in Switzerland, which would result in faster evaporation. However, I've checked a website (Its on the internet, so must be 100% accurate) which tells me that the humidity levels in Surrey, England and Schwyz, Switzerland are pretty similar.
I will check the air correction jets for the idle jets at the same time.

Thanks for your help

Paul

[Frank Beck]

When you say "After leaving my car for a while"...

What's "a while"?

Here in Phoenix where Mother Nature sucks the moisture out of everything it's not uncommon for fuel in bowls to evaporate within 5-6 weeks.

[sithot]

If the car sits for a week or so, won’t the fuel in the float bowls evaporate?

I always turn on the key and let the fuel pump run for 10 sec or so before pumping the pedal and starting.

Agree. You should hear the pump hitting "resistance" when the bowls are full.

[PMNorris]

When you say "After leaving my car for a while"...

What's "a while"?

Here in Phoenix where Mother Nature sucks the moisture out of everything it's not uncommon for fuel in bowls to evaporate within 5-6 weeks.

The bowls are empty in less than a week for sure. I've not been checking it daily though.

Agree. You should hear the pump hitting "resistance" when the bowls are full.

I've not tried that, but I'll give it a go.