carbueration issue

[joetiii]

Over the past month or so, I have noticed two things on acceleration. One happens at low RPM and the other at high. Both issues were very apparent at Sundays AutoX.

First is an off idle and low RPM stumble. at 2000 RPM+, the car does not want take all the gas I give it. I have to hold back on the pedal till it catches. It just bogs down.

The second issue is quite the opposite. When I'm at 4k holding it steady round a corner and then get on it hard, it doesn't seem to have the same power as if I was going strait thru the gears in that RPM range. Are these issues something I can tackle myself, or should I leave this to the PMO gurus?

[pope]

Don't rule out the possibility of it being an electrical problem.

[2.7RACER]

Hi Joe,
Sounds like you have a couple of readily solved carb issues.
To understand what could be happening we need to know a few things.

First what type and size carbs? Jet sizes if you know? Fuel pressure?

Have you AX'd previously without these problems?

Weber and PMO carbs are similar, with the PMO's being the improved newest model.
The PMO's being the improved model, handling float issues and fuel metering a bit better.

Plus most Weber carbs these days are twenty or more years old and it may be time for a little tender love and care.

Things like float levels, fuel filters, fuel pressure and the like.

Anyway, if you can answer the above questions, we can get you started in the right direction.

[joetiii]

Originally Posted by 2.7RACER

Hi Joe,
Sounds like you have a couple of readily solved carb issues.
To understand what could be happening we need to know a few things.

First what type and size carbs? Jet sizes if you know? Fuel pressure?

Have you AX'd previously without these problems?

Weber and PMO carbs are similar, with the PMO's being the improved newest model.
The PMO's being the improved model, handling float issues and fuel metering a bit better.

Plus most Weber carbs these days are twenty or more years old and it may be time for a little tender love and care.

Things like float levels, fuel filters, fuel pressure and the like.

Anyway, if you can answer the above questions, we can get you started in the right direction.

These are a set of PMO 40 Carbs. I do not know the jet sizes off hand, but I think they are in the install manual and order card.

I have autox'd with this car once last October and this past April, May and June.
The problems were slightly noticeable in May. In April, I sucked oil through the Oil Tank breather into the carbs (and allover the engine compartment as well). I changed out the plugs and cleaned up the air filters but did ntothing to the carbs. Plugs didn't look fouled out.

I have a fuel pressure meter in the back so I can check. I imagine we need to know what the pressure is at different RPMs correct?

[911Etarga]

If PMO's have the little vacuum nipples on the sides of the carbs, like Zeniths do, I would check that all the plugs are there. I had the same issues with my '71, turned out that one of the vacuum plugs had come off.

[911pcars]

"I have autox'd with this car once last October and this past April, May and June.
The problems were slightly noticeable in May. In April, I sucked oil through the Oil Tank breather into the carbs (and allover the engine compartment as well). I changed out the plugs and cleaned up the air filters but did ntothing to the carbs. Plugs didn't look fouled out."

So these symptoms have been gradual instead of right off the bat.

At the most basic level, the accelerator pump circuit allows fuel to play catch up with intake air when the throttle plates suddenly open (fuel has more mass than air). Thus, I suggest removing the air filter housings and look down each of the throttle bores while working the linkage (engine OFF please). You should see a definite squirt of fuel in each throttle bore.

As for high rpm stumbling, whether the vehicle is going straight or not, I would suspect fuel pump pressure/volume. If the stumble is during high G loading, there's a chance the fuel level is centrifuging in the float bowls, but PMO's are supposed to be less susceptible than Webers in that regard.

"The problems were slightly noticeable in May. In April, I sucked oil through the Oil Tank breather into the carbs (and allover the engine compartment as well)."

What was the prognosis for puking oil? You might see excess oil either in the filter housings or in the engine compartment, depending on how the oil tank breather hose terminates, but not both. An oil dump in the air filter AND into the engine compartment from a single oil tank breather hose is curious. With PMO's you must have individual air filter housings (K&N, ITG, etc.). Describe or show us your setup.

In autocrossing, momentarily high G-loads could force oil up the oil tank wall. In addition to normal crankcase blowby pressure, this might push a small quantity of liquid oil into the breather hose. You may want to consider an intermediate catch can in the breather hose between the oil tank and air filter to improve the separation between oil and oil vapors.

Hope this helps,
Sherwood

[joetiii]

Originally Posted by pope

Don't rule out the possibility of it being an electrical problem.

Well I have an Electomotive Crankfire ignition that doesn't have any moving parts to speak of that can go out of whack. The timing was set last summer and has run good since. What else to check in electrical to check? wires?

[pope]

Make sure all cylinders are firing through the whole range of rpm. Make sure wires are ok and plugs aren't fouled.