Mystery Solved - PMO Float Bowls Overfilling
 

Why, out of the blue, would PMO float bowls start overfilling? The answer for this episode was vibrations at certain RPMs caused the needles/balls to stick open. And it turns out that the vibrations were caused by the new fancy engine crossbar. When we returned to the stock crossbar all was well.

But that wasn’t the first place we looked. The first thing we tried was to go back to the previous carburetor filters and covers, I had made a set of snap on prototype covers and we thought that perhaps the hardware was vibrating the top cover, but that wasn’t it. No pictures because they are top secret.

Ian decided the next thing to do was to clean and reset the float levels, we did that, 4 times, didn’t work.

Then we replaced the needles and inlet valves, that didn’t work.

Then we replaced the stock fuel pump with a new low volume pump;

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This is a L shaped bracket I had on hand, just drilled a couple holes and it mounted in the stock location perfectly.

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Changing the pump didn’t solve the float issues either, but it’s a nice pump and it’s doing a great job.

It wasn’t until we changed the crossbar back to stock that the vibrations ceased.

All is good now and it’s back to fine performance tuning.


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Plenty of torque and horsepower but still running rich at light cruise.

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SmileWavy

Resonance is a mysterious thing. Congratulations on figuring it out.

Thanks! What an adventure, the answer is always at the last place you look. ;) I think it’s worth mentioning that I’m using the stock motor mounts.

Fuel line sizes. Tank to pump. Pump to feed.
 

It was also an adventure finding the right size fuel lines, ended up with 7/16” ID for the 6” piece from the tank to the pump, and 3/8” ID from the pump to the feed.

Strange ... any chance the new crossbar was causing metal to metal contact between the crossbar and the chassis? Richard always says to avoid solid engine mounts when running PMOs. Perhaps errant metal to metal contact could cause similar issues?

No contact with any other metal. The new crossbar wasn’t threaded, and the bolt holes were shorter, the stock bolts passed through, so I used locknuts. I’ve read Richard’s tech bulletins regarding mounts. I’m guess it’s mostly to do with carburetors, and just because it happens on one car doesn’t mean it’s going to happen on another. Could be a combination of the crossbar and the cams, or something else.

"Plenty of torque and horsepower but still running lean at light cruise. "

oops, running rich at light cruise, 12.2 to 11.8 AFR

I have had 4 cyl engines do this Float disruption in the past .

The cross bar is too stiff for the Carbs , the TQ, and the RPM , and wicked horsepower .
All things together are what caused this behavior .
Ian

I also need to apologize to the crowd of followers for the GT2 Evo 108 cams .
I had never driven a car with a well tuned 3.2 running these cams .
Now that I have tuned this car , the cams are fantastic.
Like a Tesla car !!! smooth and great power . I think Shane will end up with 265 hp at the crank.
3.2 ss, stock 78 big port heads , 9.5 JE pistons measured to 9.0, GT2 Evo cams , MK 1.5/8 headers , 46 PMO carbs

Have More Fun !!!

Interesting. Additive effect causing some resonance.

Ian, I have the 2.8 Fancy holes Titanium engine cross bar. My 46mm PMOs float bowels do not over fill. I also run Rebel Racing stiff motor and trans mounts. I'll keep this thread in mind.

It would be interesting to put accelerometers on the carbs and test both bars to see what frequency causes the issues and the rpm range it is being excited.

We think it’s closely tied to HP and RPM’s. Ian’s ButtDyno (c) has this 3.2SS at 255-260 and it is purposely driven well over 6000 and hits 7200 frequently.

I told Ian that I thought the stock crossbar was a concert pitch 440’hz F#. He didn’t seem to find that amusing.

I’m just glad to be back to where we started. Now we’re back to experimenting with the fourth transition holes, drilling jets and emulsion tubes… and all the normal stuff that all the cool kids are doing.

Out of curiosity, are your carbs sitting on insulators? They're for heat of course, but they may provide some attenuation of vibration.

Can you please share which model Fuel pump that is?

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Edelbrock Universal Micro Electric Fuel Pumps 17301

I hope this helps
Ian

ps . yes the carbs are sitting on insulators, Pmo, gasket on both sides

That's wild. Happy you solved it.

Yes, they are on the insulators.

So crazy. Glad you solved that mystery!

Re-sizing progression holes is rather dangerous, recovery to oversized holes is an extensive project. Better to adjust light throttle AFR via air correction jets for the idle circuit. I found on my 2.3 liter, twin-plugged engine that with the main circuit completely disabled I could cruise at 5500 RPM on the idle/progression circuit alone.

Not many people are aware that the upper progression holes lean fuel mixture at small throttle openings. Air at atmospheric pressure leans fuel that is sucked into the intake tract below edge of throttle valve which adds to the air delivered to the fuel from the idle air correction jet. So, enlarging upper progression holes will richen fuel mixture at light throttle cruise since their leaning effect will stop when throttles are opened above top holes.

Thanks Paul. Ian has shared his theory that seems to line up with what you are saying. That being, if the upper hole was smaller it could lean out light cruise. I think PMO went to 3 holes on later carburetors, mine has four holes.

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I tested contribution of progression holes emulsification of fuel being delivered to intake tract on my test engine. With the engine warmed and adjusted for Lean Best idling I then plugged the top progression hole. This resulted in a VERY rich idle mixture as my AFR gauge indicated. It confirmed that air above throttle valves emulsifies fuel drawn into intake tract below throttle valves.

So, increasing size of upper progression hole size will lean mixture at idle. This is opposite of what you are seeking. Also, increasing upper progression hole diameter will richen mixture when throttle valve is above the top hole. Again, the opposite of what you want.

I have found that adjusting idle air correction jet size is very powerful for tweaking transition mixture strength without relying on idle jet as the sole means of adjustment. A larger idle air correction jet will lean out contribution of slow speed circuit at light cruise/transition. It will also upset idle jet size and Lean Best adjustment if the air jet is increased dramatically in size.

The idle air correction jet has much the same effect on the slow speed circuit as does the main air correction jet has on the main circuit, a larger jet will lean out top end mixture.

Yes, Richard changed from four hole to three hole progression pattern for the 46mm PMO after I provided consultation in June, 2014. Richard had some customer issues with the 46s and sent them to me to review. I know Richard implemented the revised pattern soon afterwards. In fact, his original 46s had a three hole pattern with a divot for locating and drilling the fourth hole. He also had some sort of "fix" for blocking off the same hole that required epoxying a plug to block it off.

It was also at that time that the "hatchet" accelerator cam-lever was incorporated by Richard, I alerted him to them as a result of my efforts. They were actually OEM for Weber carbs for 1969 911T and 914/6 Webers and provided a rapid squirt response to throttle actuation.

As far as modification of the 4th hole I believe you are referring to the following from PMO:

4th TRANSITION PORT ISSUE 6-14
Regarding:
Leanness in the 2,000 - 3,000 RPM range. This may be caused by not enough transition port feed in this area.
Note that we used to have four transition ports but the top one sometimes would give too much overlap on the main circuit so we eliminated the fourth transition port. This helped cars on cruise in the 3,000+ RPM range but seems to have hurt at the 2,000 - 3,000 RPM range. The 50mm carbs still have the original 4 hole pattern.
We are now experimenting by putting the fourth hole back in, but smaller. The fourth hole was .043 and the hole we are putting back is .032 This helps. We are also experimenting with fourth holes up to .037
We have also made a different pump cam to give a quicker accelerator squirt. But most engines don’t need a quicker squirt.
The .032 dia. is a #67 drill or .8mm .035 is a #65 drill or .9mm .037 is a #63 drill or .95mm
Please let me know if you are going to open up the fourth transition port hole in your carbs. There is a drill dimple at that location.
Best Regards,
Richard Parr @ PMO
Phone 310 394-0088

I recommend that before re-sizing any progression hole diameter you first exhaust tuning efforts via idle air correction jets sizing.

Is a four hole progression better than a three hole? Definitely. How do you optimize the diameters and spatial arrangement? Research & development through dyno and in-car testing.

As a side note: Richard told me on several occasions that he used the Weber DNF as design basis for his PMOs. During my investigation of his 40s, 46s and 50s I found the progression pattern was mis-located for the 40s. The 46s were good and I think the 50s were acceptable. I did revise spatial arrangement adjustment at that time which he confirmed that he implemented. The DNFs were designed around small venturis much like the IDTPs were and as such the progression patterns were not well located for use on our engines with larger venturis.