Crankcase vent tube on carburated engines

Jon T. Nelson · 07-03-2004, 06:31 AM

I have a '74 2 liter with Weber carbs. I have run the crankcase vent tube into a plastic bottle in the engine compartment. This stinks up the rear trunk and the environment. What have others with carburated engines done about the breather tube?

balthazar · 07-03-2004, 05:21 PM

Check this link...

PCV/oil breather and carb linkage ?s

Air_Cooled_Nut · 07-03-2004, 10:47 PM

That thread isn't too helpful on the breather side of things. This is something I want to know as well.

Wouldn't the vacuum from the engine pull on the PCV, thus in combination with even minimal pressure from the crankcase, the engine would be sucking in too much case vapor? And what effects would happen with too much [oil] vapor being burned in one cylinder (or simply one side of the engine)?

balthazar · 07-04-2004, 08:47 AM

I thought the point was to dispose of the crankcase gas by recirculating it back into the combustion chamber? There are no nasty fumes in the engine compartment any more. Any oil that is "spit" out of the breather is contained in the hose. This makes for a very clean engine bay!

Seems to work fine. After installation, the carbs needed to be balanced again. The car has a mildly different feel to it now. The acceleration is roughly the same. On decceleration, the engine "snaps" back to idle. Before it was more of a steady ramp down. Other than that, there are no other concerns.

When accelerating, there is a steady positive pressure coming out of the breather. I simply adjusted that carb (left) to compensate for the extra air. Any suction from the crankcase pulls the PCV shut. Therefore, there is no air "stolen" on that side of the engine.

I think that when the PCV is shut, the left side is deprived of that extra air. This helps to kill the RPM's when you let off on the throttle. The situation would be worse w/o the PCV! It would prolly kill the engine. Any time you add air, you go lean. Any time you take air away, you go rich.

Ultimately, I would like to have the same thing happening to both carbs at the same time. That way I could have them equally balanced. Here in a coupla months, the PCV will go before a T, which will have a section connected to each carb.

Air_Cooled_Nut · 07-04-2004, 09:28 AM

What I've seen is either a breather hose aimed down at the ground OR a mini air filter...which I would think would become oil soaked and messy...?

I know the PCV is spring loaded but if the carb is sucking in air there would be vacuum on the discharge side of the valve and thus the spring would be 'weaker', relatively speaking. Making it a 'T' would simply increase the vacuum pulses against the valve (four total instead of just two). I'm probably over-thinking this, right?

How do you adjust the carb for the extra air? And would a 'T' be better engineering/tuning-wise or is it easy to compensate one side?

The "snap" back to idle suddenly makes sense! I disconnected my breather from my carb and I noticed that the idle down takes longer. Since there seems to be no difference in the sneezing/backfiring I'm having I'm going to hook my breather back up to the carb (during testing I simply plugged it).

What PCV do you recommend (year, make, model, part no., etc)?

I appreciate the input! You've given me more to think about

balthazar · 10:17 AM

"I know the PCV is spring loaded"

Actually, this one is not spring loaded. If you blow into one side, it pushes the stopper away and allows flow. If you blow on the other side, it pushes the stopper into its closed position, stopping flow. (heheh...he said blow...heh he heh)

"if the carb is sucking in air there would be vacuum on the discharge side of the valve and thus the spring would be 'weaker', relatively speaking."

Any suction coming from the crankcase closes off the system completely. The stopper rests against a hard limit in this case. No fear of losing spring tension!

"How do you adjust the carb for the extra air?"

Adjust idle setting, air mix screws and perhaps air bypass screws. Tune the engine at 2500 rpm. A Unisyn gauge is invaluable for measuring airflow. I have come to the conclussion that an air/fuel meter is necessary. I am running a 1.7 with Weber 40IDF carbs. My margin for error is miniscule. It should be easier to find the "sweet spot" on your 2.0, especially with an a/f gauge.

"would a 'T' be better engineering/tuning-wise or is it easy to compensate one side?"

Maybe....I am guessing that if you are going to do something to one carb, it will be better if you do it to the other as well. It seems to me that this will keep the engine balanced better.

"Since there seems to be no difference in the sneezing/backfiring I'm having"

I had these same symptoms a coupla weeks back. Do you have a temp gauge hooked up? If you are running hot, you are prolly a little lean. I was running hot, backfiring, etc...and it turned out I was lean. Once I adjusted the AF mix screws OUT, the temps went WAY down. No more backfires. I am running a little rich now though...

"What PCV do you recommend"

The one I used was off the shelf at Autozone.

balthazar · 07-04-2004, 01:28 PM

Well, I just had some more ideas as to what is happening with this setup. My Father and I took the car out for a little spin just now and we made some observations that may explain a little better what is going on here. The pertronix konked out, but that is another story...

The effects are magnified because of the size of my engine. The 1.7 is almost too small for the 40IDF carbs. Even running 115 mains (thanks Jake), there is a very narrow band where AF is correct. I am either too lean or too rich.

Taking the above into account, the added air from the breather on the left side is causing a lean condition at high revs. I was really into the throttle today, and I got a little hesitation at around 5k rpm.

When at lower rpms, say 1500 to 2k, the AF is richer, and the carbs will load up. This is what causes the drop in revs. At idle, the air from the breather drops off, and has no effect on the AF, allowing for a text book idle...

Maybe I'm running headlong into a cam issue here? Perhaps the added lift and duration of a "carb cam" will fix this or at least smooth it out a little, so the "sweet spot" is broader?

I would tend to think the 2.0 would have a broader area where AF is correct. What carbs are you running? What size mains, which venturis? If that is the case, then this sort of lean/rich yoyo condition will not effect performance nearly as much.

Air_Cooled_Nut · 07-04-2004, 10:34 PM

The situation I'm working on is actually for my air-cooled VW (1.6L bored and stroked to 2007cc) but the engines are similar enough. I'm not carbed lean.

I do have a CB Performance A/F gauge, just gotta get an O2 sensor. And I have the Unisyn and the [I think better] flow meter; you're right, I should have the A/F rigged up then tune in the rpm range above idle.

There are a NUMBER of PCV valves out there. Hmm, I'll see what I can get from a similar sized engine.

Thanks