Carb questions/starting procedure questions

[mike the snake]

My race engine is a twin plug 3.5, 13-to-1, GE 80 cams, 50mm PMO carbs.

I got the car needing ignitions and induction, which I installed exactly as the car had been set up before I owned it.

When I first started the car, the fuel pressure was like 7 pounds (no idea why it was that high, PO doesn't remember anything). I have adjusted the regulator down as low as it goes to 5 lbs.

I've driven the car in a few DE events, and it runs perfectly, but I still have some questions.

When cold starting, when I first turn the fuel pump on, it appears the float bowls are overfilling. Once started and running the sight windows show proper float levels, but when I first hit the fuel pump the carbs fill up completely.
Would it be a good idea to just fill the bowls and then turn the pump off until the engine fires-attempt to keep the bowls from intially overfilling?
Could the high-ish fuel pressure do this?
Could the floats or needles be sticking initially? This was my first thought because the car only gets started/run every few months so far.

When cold starting, I usually get one big shotgun backfire before the car fires, and I'm wondering if the overfilled bowls are contributing to this?

I always get the car started, but I'm also wondering if there's a standard procedure for starting, like 2 pumps and hold quarter throttle til it fires, or is it just a case to case situation.

The car is new to me, no real issues, just observations at this point, but I Would like to get rid of the huge backfire that usually happens.

Thanks for any input.

[Steve@Rennsport]

Mike,

Some basics about carburetors,....

Your fuel pressure is much too high and driving the needles off the seats. Get it down to 3.5-4.0 psi. All of the jetting is based on this so this is a very critical step.

Once you get this dialed in, it will start & run much better,...........

[mike the snake]

Thanks Steve, I Am aware that the Webers and PMO's are supposed to run the pressures you are recomending.

It's funny, because as I ask around, I get different answers from everyone.

When I asked the same questions one race shop told me the PMO's liked the higher pressure, like 5 or 6, and to leave things at 5lbs.

The car was run in this configuration by the PO for a long time, and it runs perfectly on-track. The regulator is set as low as it goes to 5lbs.

I have limited time on the car, and it runs sweet, so I'm hesitant to change anything.

I have the PMO regulator, but I would need to run the return line. If I decide to change anything, I'll go to that setup and drop the pressure to the correct 3.5-4lbs.

[lindy 911]

If the needles are being forced off their seats by too much pressure, the overflow has nowhere to go but into the cylinders. One of the exhaust valves is going to be open and raw fuel will get into the header. When you fire the engine the fuel in the header will ignite and BOOM, shotgun backfire.

I'm pretty sure the needles and seats from the PMO are the same as Webers which like about 3.5 psi. I use a rotary pump that makes just at 3.5 psi without a return line on my 46 Webers.

If you decide to stay with your current set-up try starting the car right away instead of giving the pump enough time to overflow the bowls. In other words, don't turn the key to the on position and then wait. Wait until you're ready to fire the motor and start it right away. Might help. Standard procedure for starting is going to be an individual thing; find what your motor likes and use it.

Lindy

[mike the snake]

Thanks Lindy.

I'm not sure as to exactly what's happening, but what you describe sounds like what may be happening.

Once the car is running, the float levels are fine, it's just the initial fill that overfills, so maybe the pressure IS too high for that reason.

What I had in mind to do was to just blip the fuel pump switch and physically watch the bowls fill to the proper level, and then turn the pump on only after the car fires.

I'm firing the car up tomorrow, so we'll see what happens.

Thanks for the input.

[Walt Fricke]

Can't say as I have seen advice for Webers which said you could run more than 3.5-4 psi. So I would wonder where this conflicting advice is coming from. Sounds like the PMOs (which are based on Weber principles) are the same. Just get a regulator which will reduce the pressure to that. Why put up with the hassle of watching your float bowls while starting up each day.

Seven pounds sounds like a reasonable unregulated pressure from a pump suitable for carbs, like the Holley Red. Dropping that at the other end of the car from the pump to 3.5 is just not that difficult. Have you discussed this with PMO?

I run a vacuum modulated pressure regulator - a Cagle. Their pitch is that you want your fuel level in the bowl to be as high as it can be short of overflowing. To do this at idle you want lower pressure, so as not to overflow the float bowls by overpowering the needle, as Steve points out. But at WOT you can have higher pressure, as the flow through the carb is high enough to work well with a high float level and available needle function.

I don't have the nice PMOs, so seldom do I check float level. But mine is set at the very top of the upper line on the glass tube you use for Webers. Has worked fine.

But you don't want overflowing float bowls. My cold startup routine, especially after a couple of days, is to run the pump until I hear the pitch change (indicating the bowls are full), then start. One day, after a carb rebuild, this played me false. I got tired of waiting for the pitch change to happen, and after a decent interval started the car. It caught on fire. One of the floats had stuck open. The fuel sat on a closed butterfly, and ran out the throttle shaft (well used 46s, no nice ball bearings and shaft seals), and found a ready ignition source somewhere. Gave a whole new meaning to "fire it up."

Not a good thing. Instead of racing that weekend I was cleaning things up.

Walt

[mike the snake]

Could the floats/needles be sticking because of the car sitting between runs? So far I've run the car every couple months.

The overfilling only happens on the first cold start after sitting, after that it behaves and starts normally.

The car runs perfectly.

The PO doesn't remember the settings he used, and the race shop I asked about pressure said "the PMO's like 5 or 6 lbs". This shop was recomended by the PO. I am SCARed about GOing with their advice any more as my experiences so far with them have not been satisfactory, but that is what he told me.

I will research this more.

I intend to dyno the car and dial things in perfectly soon. Plug check and slightly grey pipe residue tells me I'm running a tad rich, so dropping the fuel pressure will probably make things right.

I will run the car and observe things more closely, it is new to me so I don't have a lot of personal data yet. Putting the car back exactly as the PO had it was my starting point.

I'm firing the car up later today. I will observe carefully and report back.

Thanks very much for the input everyone.

[mike the snake]

I fired the car up today. It's been a few months, and it was cold. It cranked over a bit slowly because it was colder than usual, I was worried it might not pop over with the light 5 pound flywheel, but it fired right up.

This time I blipped the fuel pump switch, filled the bowls, and started the car with no fuel pressure. Once it fired I immediately turned the fuel pump on.

No Backfire! It had plenty of chances (it took a bit of cranking) and never once popped, so I think I have the backfire issue dealt with.

I also think now I have a good routine for starting.