Strange PMO problem

mreid · 02-21-2016, 11:50 AM

I recently broke in the cams on my 3.2ss engine. It ran fine, blew some smoke, but nothing out of the ordinary. Today I tried to tune the carbs. First, I had zero fuel pressure on the PMO regulator. Fixed that and adjusted to 3.0 psi. Engine started right up and the fuel came about 1/2 way up on each of the four windows. Initial mixture set at 2 turns out for each carb. Seemed to be rich so I reduced mixture to 1 1/2 turns each. RPM went up slightly, so I reduced another 1/2 turn. Long story short, I turned all the mixture screws all the way in and the engine hit its highest RPM and ran great.

That seems to be saying that fuel is leaking into the engine, but how? The air cleaners are installed properly and everything else seems fine. Any ideas?

patz · 02-21-2016, 11:57 AM

Turn the fuel pressure don to 2.5 psi. See if that helps.

Gordo2 · Gordo2's Garage

If you are still idling with the mixture screws closed, this Indicates your butterflies/throttle was open/not at idle while you were adjusting - the carbs are drawing fuel from the idle progression holes...

Adjust per PMO instructions ensuring you adjust the idle speed before working on idle mixture.

Good luck, Gordo

mreid · 02-21-2016, 12:34 PM

Thanks, folks. I tried the fuel pressure at several settings, even 1/2 psi and still had the same problem. Gordo, that sounds right, but the plates were closed, idle at 1000 RPM.

I turned the fuel pump on with the engine off and noticed fuel droplets forming on the throttle plates. Normally, I would think fuel pressure too high, but no. Perhaps idle air inlets blocked? No, they are fine. Somehow fuel is leaking into all six carb barrels the same. Very strange.

snbush67 · snbush67's Garage

Are the PMOs new? sounds like the float levels may be off but you said the Windows filled to half way mark, double check when the motor is running.

bgyglfr · 02-21-2016, 01:04 PM

Odd problem. Float levels too high maybe? I guess that would be obvious though with PMOs.

Edit: Shane beat me to it.

Steve@Rennsport · 02-21-2016, 02:04 PM

JMHO,

Whenever starting an install on new set of PMO's , I ALWAYS check the float levels on the bench before putting them on the engine. In most cases, things have changed during shipping and this extra effort has saved me many headaches later on.

"Trust, but verify" is the operative word for almost everything.

JohnJL · 02-21-2016, 02:56 PM

Check you have the right mix needles? I found one cylinder wouldnt behave like the others, turns out someone put an odd mixture screw in.

mreid · 02-21-2016, 03:50 PM

All cylinders are behaving the same and these are brand new PMOs. Very strange problem. I can't get back to it for two weeks, so please keep the ideas coming!

patz · 02-21-2016, 03:52 PM

Contacting PMO would be a good idea. Since they are new and I assume the original buyer they will support the product.

Gordo2 · Gordo2's Garage

Keep it simple - I think you are chasing demons.

Fuel enters the throttle bodies in 3 places:

(1) Idle/idle progression circuit via throttle body holes. Requires vacuum to draw the fuel in, therefore there should not be any fuel flow through this circuit (even with the pump running) unless the engine's running...

(2) Main circuit - requires significantly more vacuum (than idle) to draw fuel in (engine not running).

(3) Accellerator pump - pushes fuel through the nozzles at the top of the throttle bodies when you press the gas pedal & actuate the pump linkage & pump.

Your float height indicates your floats and fuel delivery is working/setup correctly (or at least within reason). If your float needles weren't seating - your float bowls would overfill (to the top vs. 1/2 high fuel in the windows). Additionally if the fuel pressure was too high - at worst the pressure would overcome the needle and again overfill the fuel bowl (when not running).

I recommend starting with the setup to confirm your linkage is correct and that your throttles plates are completely closed (on both sides) with the idle speed screws turned out.

Try this:
- Disconnect the linkage (via vertical ball joints at the ends of each carb).
- Back the idle speed screws out all the way.
- Set mixture screws at 2 turns out.
- Start engine & let idle.
- Close the mixture screws one at a time.

If the engine still runs with all of the mixture screws closed - something is keeping your throttle plates from closing completely. Bottom line, the fuel has to be metered for a decent idle - which means it has to be coming through your idle / idle progression circuit.

Gordo