Inliners International Forums General Discussion Engines PCV Retrofit to a 235

I have a 235 with a Rochester B carb that I'm trying to retrofit with PCV instead of the road draft tube.
I tried a Fram FV184 PCV valve because it's for the Chevy 194/250.
I'm using 3/8 rubber hose from the valve to the fitting on the intake manifold
It seems as though the model B carb can't provide enough fuel through the idle jet to support the extra air coming in through the PCV valve. I've got it almost all the way out and it still runs rough as if it were lean. Any comments? Do you think I should restrict the flow through the PCV some?
Anybody done this successfully on a similar engine/carb combo?

regards,
Leon

Yes. 261 with Progressive triple B's (from 235's) with PCV drawing from the crankcase and a breather in the valvecover tied to the middle air cleaner.
I hooked up an air/fuel gauge and found that I'm running a little lean at idle. I am running a 54 main jet in the middle carb. I have a small amount of air leaking past the throttle plates on the end carbs, if I could stop that air leaking by, it would be fine. My carb is responsive to idle screw adjustments and I only have about 11" of vac due to the camshaft I'm running.

If you decrease the size of the hose too much, then the system serves no purpose.
What size jet are you running?
Have you verified that you have no other vac leaks?
What is the condition of the carb?
Is the engine completely sealed, or can fresh air enter the engine via a breather or slots in the valve cover?

I haven't opened the carb. So whatever the stock jetting is, I suppose is what's in there.

I don't think there's any other vacuum leaks. Without the PCV hook-up, idle mixture seemed right at about 1 1/2 turns open, now I've got it 3 turns open.

The carb seems to be ok, an occasional backfire on throttle tip-in, that was there before I installed the PCV. I haven't checked yet, but i expect the accelerator pump is not working.

Plenty of air into the motor through slots in the valve cover and a breather cap. I have the pcv valve stuck in the road draft tube hole with a rubber grommet.

Well, without looking at it or getting more info, I'm not sure what else to suggest at this point. I get the impression that you are assuming you have a problem, and that you have not done anything like using a tailpipe sniffer for example, to verify your hunch?? Have you tried using a vac gauge to set the idle mixture?
It sounds like a $12 carb kit is in your future, at the very least.
Do you have more than one vac port in your intake?

I haven't had time to do more detailed diagnostics.
I just got the truck 3 weeks ago.
Wihtin the next couple of weeks I'll try to diagnose the backfire. If the accel pump is worn, I'll probably put a kit in it. I may find some goop the idle circuit that is contributing to the other oddity.

I'm using the stock intake manifold.
It has a single 1/2" IPT hole in the side for the wipers.
I used a bushing to reduce it to 3/8 IPT, then an elbow to connect it to the 3/8" hose.

What size does your setup use?

regards,
Leon

This is how I did a PCV on my 50 PU.This 235 has a Holley #1920 carb off a 60's Mopar slant 6,but notice the vacuum line is attached to a PCV port on the carb base? The several other PCV set ups I did were similar,attached to a port on the carb. Perhaps on your engine running the vaccum off the intake manifold causes mixture problems? But I'm thinking the Rochester has no vacuum port?
235 PCV

I run dual mopar slant 6 carbs on my 235. I don't have any major mixture problems. I made a housing for my pcv and shoved it in where the road draft tube was. Then I routed the line back to the intake manifold like it was on a stock 56 - 235 and havent had trouble since.

I'm using 3/8" for the PCV also, and its run up to the center connection on the intake where the wipers usually go.
Rason i asked about mor connections is that i found if you tie the wipers and PCV together, the wipers may no longer work. Depends on your vac signal. Either add another hole elsewhere in the intake, or convert to electric wipers.
When you set your idle with a vac gauge, I would suggest that it not be tied into the same spot as the PCV either for similar reasons.
Drilling a hole in the carb insulator and pressing in a tube is one way to provide a source for the vac gauge. Just get a rubber boot to cover it when you're done using it.

My truck will be at the convention in June.

Tom do you have a picture of your setup?
John

It's interesting that everyone who has satisfactory PCV systems is either running multiple carbs or running a carb from the 60's that supported PCV in its original home. Gearhead, do you have the idle circuits on your 2 end carbs closed off? I'm still trying to rationalize that a single Rochester B may not be able to supply enough fuel to offset the "vacuum leak" caused by the PCV valve. But then again, Gearhead, you said you only had 11" at idle. I expect my carb is seeing more than that.

Hmmm.

Well,looking through my old GM manuals shows that PCV systems were available for Chevy and GMC 6's in trucks back in the late 1940's.Looks like the PCV piping was from the valve cover to the intake manifold where the "PCV" was located.The manuals mention if a PCV system is retro fitted,it may be necessary to increase the main jet by one number,no mention of idle mixture issues.
With a proper operating PCV system there should be a slight vacuum where outside air is taken into the engine.A bad valve cover gasket may not leak much oil,but air could be drawn in upsetting the carb mixture.
Mybe you can use a restricter in the vacuum hose or another type of PVC valve?

Yes, the end carbs are dumpers, just like factory tripower 2GC's. I plugged the idle circuit, removed the power valve and the spring loaded ball that it pushes on and epoxied the hole shut. I wrapped the inside of the throttle bore with viton rubber and used a throttle plate from a 216 version carb.

Everyone that is running a PCV system is running leaner than they did before they installed it, unless they made changes to there jetting, etc... to account for it.
Remember that the jet size doesn't affect the idle circuit. If the idle circuit can't richen it up enough at idle by turning out the screw, then, I believe, you would need to drill out the idle circuit hole to a larger size?? in order to supply the proper amount of fuel. I'm thinking about trying this myself, as it would be alot easier to do than getting a perfect seal on my end carbs.

At one time I tried installing a steel slug int the PCV line that had about an 1/8" hole in it, just to see what it would do. The valve still worked, but the reading on my air/fuel gauge didn't really change. As I said before, too small of an orifice, and the system is useless anyway, so I removed it.

Worst case, if it really bothers you, you could always go back to the road draft tube, or perhaps pull the guts out of the valve and then run the line up to the air cleaner instead.

You have to access the idle feed restriction,easy on some carbs,near impossible on others.On some situations ,a switch to a carb know for the same engine but had an original PCV might be the answer.
I think a proper PCV sustem is worth the trouble,more likely to get rid of internal water vapor and maybe even more important,1/2 the pollution of an older engine comes from the road draft tube.

Thanks for all the feedback, guys.
It frustrating to not be able to actually work on the truck due to other tasks and priorities. But discussing it at least provides the illusion of progress.

I found that my local Autozone is wiling to order me a carb kit.
I'm going to try to check the accelerator pump tonight and see if I need to do that to address the occasional backfire.

Leon

I agree. Just to clarify, when I said "I removed it", I meant the restrictor slug, not the PCV setup.
I have not seen a Rochester 1bbl with provision for a PCV line.

37 Jimmy I do not know how to post a picture here, but there are several on http://www.wixski.com - you will see how I made it and there is a crummy picture of the pcv cannister houosing as I mounted it on my 56 - not the best photos but you will get the idea.

It seems to me that the major factor is how much air is allowed into the intake system by the vacuum "leak" the pcv provides. That has a big effect on whether or not ir runs rich or lean depending on carburetion.

I found a couple pictures I have on my home computer if you want them 37Jimmy. Send me an email or you are welcome to post them here - they are better than the website pictures.

Leon,

How does it run with the PCV tube disconnected and the intake manifold port plugged? This is not a solution, just a diagnostic, of course. I'm running a PCV on my 1954 261 similar to yours, but with five Strombergs - two primaries and three secondaries. It idles fairly well with about 16" of vacuum. Has the ignition timing been checked and set? I've seen incorrect timing cause problems that you describe.

That stock 235 with the Holley one barrel photo I showed earlier pulls 19-20 inches of vacuum at idle.I also have a modified 302 GM,fairly hot 3/4 cam,it has a pCV and idles at 16 inches of vacuum.
In my opinion,the PCV isn't so much a vacuum leak,the air through the PCV just replaces the air normally metered by the throttle plates at idle.Otherwords,pull off the PCV hose,plug the port,ya need to turn in the idle throttle stop screw a turn or two to get the idle speed back where it was.Generally,then you make a small adjustement to the idle mixture screws,not like you're screwing then in or out 3 turns.

Hoyt,
Until I added the PCV, the engine idled well with the mixture screw about 1 1/2 turns out. Now its about 3 turns out and seems to still have a lean misfire. I haven't checked the timing yet. I did check to see that the vacuum advance is working. I thought that might be contributing to the occasional backfire on throttle tip-in.

I recently installed PCV valves to my 302 GMC running 3 Weber 32/36 carbs. One valve is on the breather and the other is on the rocker cover. They both connect to a common manifold that feeds into the bottom of the 3 air cleaners which were designed with plastic fittings for this puropse.

This setup does not affect the A/F mixture as far as I can tell. Before the installation, the car used to fill up with engine blow-buy fumes to the point it was uncomfortable to drive. Now the smell is completely gone.

Mr Hainley. Love the article on your car in the 12 Port News. Is there a thread here somewhere on how you built your manifold (or perhaps do you have pictures of theat side whowing the manifold)? I have a 228 that I'm going to be overhauling in the future and am interested in your carb setup.
Sorry for the Hijack folks.

Here's an update.
I have a qualified success.
I checked the timing it was about 5 deg retarded,
assuming that the BB in the flywheel is TDC.
I reset it to the BB (0?) and the idle sped up.
I slowed it down and adjusted the mixture for smooth idle.
This also cured the sneezing on throttle tip-in.
I still have a slight hesitation on throttle tip in, but smooth idle and much more torque just off-idle, making for smoother starts.
I'm using a Motormite #42054 grommet in the vent hole on the block, a Fram FV184 PCV valve, and running the hose around behind the engine to the manifold with a tee to feed the vacuum line to the wiper.

Thanks for all the input along the way, guys.

regards,
Leon

Leon,

Me thinks you are too lean on the center carb because somewhere above I saw you had the air fuel screw 3 turns out. As you know you want no more than 1+1/2. Richen up the idle fuel delevery some and try to readjust back to 1+1/2. Keep going til you do. Just like Holley Webers richen up the ilde circuit. Get them end carbs to completely close on idle.

52pu,

I'm running a single Rochester B.
Someone else had a "tri-power"

The Rochester B idle screw is a fuel control.


Leon

Leon,

I does not matter what you are running a holley weber, rochester etc, one carb or 5 the fact is if you are 3 turns out your too lean on the idle circuit and you must richen up the idle circuit and re-adjust the air fuel til you get 1+1/2. The connection with the Holley weber is the same; if you are more than 1+1/2 out means the idle circuit is too lean and you must step up in ilde jets til you can adjust to 1+1/2 out.

I don't mean to sound confrontational, but why do you think that the number of turns of the mixture screw is critical? Based on the diagram and description in the Model B service manual, it appears as though the mixture screw controls the amount of fuel available to the engine when the throttle plate is nearly closed, at minimum idle speed. As the throttles is opened, 3 more ports are uncovered to add fuel for the transition to part throttle operation. I'm willing to believe that the additional air coming through my PCV system is affecting the operation of these 3 ports by causing the throttle plate to be too far closed for 600RPM idle, and thus is partly repsonsible for my slight hesitation coming off idle. but I just don't see how the mixture screw plays into this. It seems like anywhere in the adjustment range should be fine, as long as you can get the right mixture for the minimum idle position of the throttle plate.

regards,
Leon

It has been my experience with retro fitting pcv that the most important part has been to get the manifold feed as near to the base of the carb as possible. If you were to look at the PCV manifold port on a 250 for instance, the port appears to be in the center port of the manifold. However, in actuality, there is a tube that goes into the manifold directly under the carb.
If the PCV inlet is off center, it will lean the closest end cylinders more than the other end cylinders, requiring you to richen the idle to mask it. I've seen many bent 8s have a similar problem when the PCV was hooked up to the back of the carb rather than around to the front where it would enter at the primary throats.