All Small Six Which cylinder head?

[Otto]

Before I start making carb changes, there are a couple of things that I want to iron out.

The first is the Mercarb has no provision for ported vacuum, so the distributor is connected to manifold vacuum. It’s either this or rely on the mechanical advance only. There is much debate on ported vacuum vs. manifold on the internet, so I’m curious about this. This situation only has one choice so maybe it’s a moot point.

The second and maybe more important is the PCV system. The van’s original engine had a road draft tube and that is gone. The new engine has a late model valve cover with the PCV valve in the front and the oil filler/breather in the back.The breather has a hose on the top that leads to the air cleaner. The PCV valve is connected to the intake manifold.
I put this together from photos on the internet.

If I pull out the PCV valve and put my thumb over it, the engine runs quite a bit smoother and obviously richer. The engine has always had a rough idle so maybe there is a fundamental flaw here. The 2V carb masks this a bit but it’s still there.

Do I have the PCV system correct?

 

[Frank]

Struck me as odd too. The next time I have both carbs apart, I'll compare and see for sure.

Please do, thanks. That's important info when considering a purchase.

 

[Frank]

Before I start making carb changes, there are a couple of things that I want to iron out.

The first is the Mercarb has no provision for ported vacuum, so the distributor is connected to manifold vacuum. It’s either this or rely on the mechanical advance only. There is much debate on ported vacuum vs. manifold on the internet, so I’m curious about this. This situation only has one choice so maybe it’s a moot point.

The second and maybe more important is the PCV system. The van’s original engine had a road draft tube and that is gone. The new engine has a late model valve cover with the PCV valve in the front and the oil filler/breather in the back.The breather has a hose on the top that leads to the air cleaner. The PCV valve is connected to the intake manifold.
I put this together from photos on the internet.

If I pull out the PCV valve and put my thumb over it, the engine runs quite a bit smoother and obviously richer. The engine has always had a rough idle so maybe there is a fundamental flaw here. The 2V carb masks this a bit but it’s still there.

Do I have the PCV system correct?

View attachment 24179

The PCV valve is flowing too much volume. I've found that the new overseas valves are often wrong for the application, they flow too much. Each PN is designed to bleed air volume relative to engine size. Yours is letting too much air pass at idle, causing a lean condition. I'd replace the valve, hopefully you'll get a correct one. On an engine in good internal condition, I've bought a valve for a smaller displacement application to help idle quality. You have the system correct, assuming the PCV inlet to the intake is in the center plenum area.

The only place affected by manifold vs ported vacuum advance is base-idle. The ported vacuum port is exposed to intake vacuum as soo as the throttle is open a few degrees. At base idle when the ported port is above the edge of the throttle there is 0 vacuum. As soon as it's exposed to the intake side it spikes up to intake vacuum. From there on as throttle opening increases, ported and intake vacuum are the same. . . Excessive advance at idle can cause an intermittent miss, heard most clearly at the exhaust pipe. If your engine idles to your liking with the vacuum advanced at idle, good-to-go. Deceleration will be diminished also, since the timing remains advanced when letting off the pedal. That's about the only two side effects I can think of. .

Very nice clean engine bay!

 

[Otto]

Thanks Frank,

These are excellent points. I looked up what a 1978 Ford Fairmont w/200 I-6 would have for a PCV valve and it looks different than what is in there now. The correct valve is on the way.

I also won't worry about the vacuum advance anymore. The engine is seeing 35 degrees of total advance- I'll leave that alone for now.

The engine compartment has stayed clean because the van isn't driven much and it doesn't leak yet! (knock on wood) All in good time!

 

[gb500]

Vacuum advance gives better fuel economy

 

[Otto]

This weekend I installed the PCV valve spec'd for this engine; it really didn't change how the engine ran substantially but I'll stick with it. I increased the base initial timing from around 8* to 12*. The engine seemed to like this.

The engine idles at 18" Hg with the idle mixture screw at 1 3/4 turns out. This seems to be where the engine is happiest, but it still has a rough idle in my opinion. It's better, but still not as smooth as other inline 6s that I've had.

I took the van for a drive today and collected some data:

Idle= AFR @ ~ 12.5-13.0
Light cruse= AFR @ ~ 13.5- 15.5 with some incidents of 16s and 17s climbing a slight grade & light throttle
WOT= ~ 13.0- 14.0

Driving impressions were a lot more go! Putting a boot into it from 2nd to 2nd OD was impressive compared to before! The AFR meter is distracting and tough to reach a conclusion as it's always changing, but the above data is a general average of what I saw. My initial thoughts are that it is lean in the mid-range, but also seems rich at idle and light throttle, and off-throttle too.

My first time with one of these meters so if you have experience doing this, please share.

 

[TrickSix]

Just thinking... So, are your PCV hose and distributor vacuum line both connected to the same tee fitting on your intake manifold? Normally the PCV flow is introduced at the base of the carb through the carb spacer, and so is fairly evenly mixed and distributed through the intake manifold. By short-cutting straight into the manifold through that vacuum port, it looks like most of the PCV flow dilution would go to the front three cylinders, maybe mostly just into the number three cylinder. Plus the variable PCV flow being so closely coupled to your distributor vacuum signal on the tee fitting there it might influence it. Ford probably put the PCV line on the carb spacer for a reason. Your nifty homemade spacer/adapter looks thick enough to tap for a hose barb to receive the PCV line. Or maybe it doesn't matter, I'm just venting some hot air myself here...

 

[Frank]

Just thinking... So, are your PCV hose and distributor vacuum line both connected to the same tee fitting on your intake manifold? Normally the PCV flow is introduced at the base of the carb through the carb spacer, and so is fairly evenly mixed and distributed through the intake manifold. By short-cutting straight into the manifold through that vacuum port, it looks like most of the PCV flow dilution would go to the front three cylinders, maybe mostly just into the number three cylinder. Plus the variable PCV flow being so closely coupled to your distributor vacuum signal on the tee fitting there it might influence it. Ford probably put the PCV line on the carb spacer for a reason. Your nifty homemade spacer/adapter looks thick enough to tap for a hose barb to receive the PCV line. Or maybe it doesn't matter, I'm just venting some hot air myself here...

I agree. The crankcase vacuum leak needs to be centered in the plenum, if it can't be on the carb base.

This weekend I installed the PCV valve spec'd for this engine; it really didn't change how the engine ran substantially but I'll stick with it. I increased the base initial timing from around 8* to 12*. The engine seemed to like this.

The engine idles at 18" Hg with the idle mixture screw at 1 3/4 turns out. This seems to be where the engine is happiest, but it still has a rough idle in my opinion. It's better, but still not as smooth as other inline 6s that I've had.

I took the van for a drive today and collected some data:

Idle= AFR @ ~ 12.5-13.0
Light cruse= AFR @ ~ 13.5- 15.5 with some incidents of 16s and 17s climbing a slight grade & light throttle
WOT= ~ 13.0- 14.0

Driving impressions were a lot more go! Putting a boot into it from 2nd to 2nd OD was impressive compared to before! The AFR meter is distracting and tough to reach a conclusion as it's always changing, but the above data is a general average of what I saw. My initial thoughts are that it is lean in the mid-range, but also seems rich at idle and light throttle, and off-throttle too.

My first time with one of these meters so if you have experience doing this, please share.

Your idle mixture is fine, they tend rich. tune the screw until it idles best, forget the gauge, use your ear and maybe a handheld tach.
Tweaking the PCV system can probably correct the irregular idle. I drilled/tapped/installed a PCV fitting on the back of a 2 barrel adapter for the 240. Not hard.

 

[Otto]

The PCV is plumbed to the factory vacuum tree I found on my spare engine.


The PCV is connected to the large threaded end of this, and the boost/vacuum gauge and distributor are here too. I’m not sure how this was configured at the factory, but this seemed like a simple way to go. The new valve has two hose barbs- a large and small. I connected the valve to the large barb and sealed off the smaller. The new valve did have some effect, the engine ran richer and I was able to run the idle mixture screw in a bit.

I could try and move the PCV connection to the adapter- I really want the idle smoother. The carb needs to come off anyway to re-jet.

 

[TrickSix]

It's 1/4 pipe thread in the intake manifold for the crusty old factory vacuum tree. You could replace it with a nice brass fitting configured just as you wish if you want. Also, that exhaust manifold has emission control pipe connections that need to be plugged and might cause minor flow restrictions inside too. A late 60s/early 70s manifold is a better choice.

 

[Frank]

The PCV is plumbed to the factory vacuum tree I found on my spare engine.
View attachment 24213

The PCV is connected to the large threaded end of this, and the boost/vacuum gauge and distributor are here too. I’m not sure how this was configured at the factory, but this seemed like a simple way to go. The new valve has two hose barbs- a large and small. I connected the valve to the large barb and sealed off the smaller. The new valve did have some effect, the engine ran richer and I was able to run the idle mixture screw in a bit.

I could try and move the PCV connection to the adapter- I really want the idle smoother. The carb needs to come off anyway to re-jet.

Thanks for the specifics! There are a couple of problems with this configuration. First, PCV should not be shared with anything else. It's oily, and the oil will find it's way all the way to the far end of any other connection. In a few months there will be oil residue in your vacuum advance can/ brake booster or anything else connected to that source. 2) Nothing else connected to that source is getting full manifold vacuum, it's reduced by the large flow of the PCV. Note that on the EFI 300- the only engine I'm familiar with that uses a vacuum tree like you have above- everything that needs vacuum is connected to that one tree, except PCV which has it's own separate port. No factory design coupled PCV with anything else.
I like the wooden spacer! they work well. Fabricating a fitting for the crankcase would probably be a worthy project. . The carb is 180* off from a better orientation: the bottom of the throttle blades face the valves. There is a high rearward thrusting of the air/fuel mix from the angled throttle blades at all but WOT. This applies to all carbs or throttle bodies. They all shove a higher % of the mixture toward their rear, the air is not coming straight down into the plenum, except WOT when the blades are parallel to the airflow. Cylinder distribution is better with that thrust moved toward the outside of the plenum where it has time to dissipate/mix with the lower pressure more turbulent mix that makes it past the front of the blades, rather than being shoved directly at the #3 & #4 intake valves. . . It's probably not worth the hassle to redo your linkage, etc, this is just FYI. But it also affects where you put your PCV fitting into your spacer. The best location is at the rear of the carb, because of the thrust described above. In the rear the PCV vacuum leak transfers the least signal onto the base of the carb, and is drawn into the strong airstream, dissipating quicker. The carb base is where critical mixture metering is judged, if a PCV system influences that, not good. So with your carb orientation the best location would be on the valve cover side- not great access or ideal relative to the plenum. It can be put in the front of the carb. Needs to be in the center, for both carb metering and even distribution.

 

[Otto]

Thanks for the information! I learned today! I was under the impression that all the PCV needed was a source of vacuum so I gave it one. I need to stop thinking of it as a vacuum device instead of a source of dirty air.

Anyway, I can locate a tap for the PCV on the valve cover side of the adapter, there's just enough room. The carb doesn't work in the other orientation as it hits the valve cover and the mixture screw would be inaccessible. If I understand your diagram and the theory, the PCV port should be on the high velocity side of the throttle blades. Is this always opposite the mixture screw(s)?

It could also go on the outboard side if that's better.

 

[Kritas]

To support Frank's point and expand on it a bit - I've bought a number of cars where the PCV and vacuum systems were cobbled together. Oil will indeed seep into anything sharing the vacuum source. It's well worth researching the factory setup as you are doing. If you change components or make modifications to a vehicle, thinking through systems pays benefits. Many POs don't seem to understand the difference between ported and manifold vacuum and the mess of connections looks to be a matter of convenience rather than science.

 

[Frank]

Thanks for the information! I learned today! I was under the impression that all the PCV needed was a source of vacuum so I gave it one. I need to stop thinking of it as a vacuum device instead of a source of dirty air.

Anyway, I can locate a tap for the PCV on the valve cover side of the adapter, there's just enough room. The carb doesn't work in the other orientation as it hits the valve cover and the mixture screw would be inaccessible. If I understand your diagram and the theory, the PCV port should be on the high velocity side of the throttle blades. Is this always opposite the mixture screw(s)?

It could also go on the outboard side if that's better.

Thanks Otto. Good reason to have the carb that way. I'll add an addendum to the airflow direction at the base of a part-throttle carb: research found that the effects of the disproportional flow front-to-rear is neutralized @ two inches below the throttles. Any spacer, plus the depth of the plenum before it reaches the runners all count. So with your spacer + intake carb stack, your distribution effects from carb orientation are minimal.
I owe all this carb distribution info to Doug Roe, the engineer of the Rochester Q'jet, and his excellent book released in the 1970's.

It is not overly critical which side the PCV is plumbed, as long as it's front or rear in the center of the carb. Would be a cleaner looking system in the rear. Could use a 90* fitting and a relatively short hose.

Some carbs like the Carter (Edelbrock) 4 barrel have the PCV ported in the front. Their internal metering system is designed to isolate PCV from metering passages which transfer vacuum to the power pistons for mixture control. On automotive 2G's it's in the rear, on the high speed side of the blades. The reason your marine carb does not have a port is, marine gas engines do not use a PCV system. The constant high loads to keep a boat moving = low vacuum. A PCV valve would be ineffective. Marine gas engines run an open hose from a valve cover breather to the outside of the Flame Arrester style air filter. The fumes are drawn in with the incoming air and pass through the carb from the top. There's no paper element, the "filter" is horizonal layers of steel mesh, it's all of steel. It does not filter much, but will snuff out a carb backfire before it reaches the outside of the element. There's not much dust in open water, air filters are a low priority.

 

[Otto]

The PCV valve’s hose has been relocated as recommended to the adapter. I had high hopes for this, but I’m not seeing any real improvement in idle quality.


I checked for vacuum leaks but haven’t found any- it’s still idling at 18” Hg. There is 12V to the coil and to the hot side of the HEI ignition module. There seems to be adequate spark; it will jump a ¾” gap. The plugs are gapped at 0.045”. The engine idled in a similar fashion when it had the 1bbl carb (w/PCV under the carb); the difference now is it will take full throttle and rev where it wouldn’t like to do that before.

A little background on the engine: it has approx 300 miles from a full rebuild- it does have an unknown stock profile camshaft. So it’s disappointing and frustrating that it runs this way.

 

[Frank]

The PCV valve’s hose has been relocated as recommended to the adapter. I had high hopes for this, but I’m not seeing any real improvement in idle quality.
View attachment 24249

I checked for vacuum leaks but haven’t found any- it’s still idling at 18” Hg. There is 12V to the coil and to the hot side of the HEI ignition module. There seems to be adequate spark; it will jump a ¾” gap. The plugs are gapped at 0.045”. The engine idled in a similar fashion when it had the 1bbl carb (w/PCV under the carb); the difference now is it will take full throttle and rev where it wouldn’t like to do that before.

A little background on the engine: it has approx 300 miles from a full rebuild- it does have an unknown stock profile camshaft. So it’s disappointing and frustrating that it runs this way.

Yes that's frustrating. It still may have too much flow even with the new valve. Check the flow from the inlet side, the hose on back. Idling hot, put thumb over it to block. It should take at least 2 seconds for vacuum to develop. If it's quicker or there is any appreciable intake of air there, the valve is too big. The system is a near break-even design, just enough pull to inhale the blow by plus a small % of fresh air entering. there should be only a small volume of air entering the rear, close to neutral pressure.
I like the clean layout, looks good on the inside.

I noticed your comment about plug gap. Some engines do not want extra gap- I have two right now. converted from points to electronic, opening the gap caused intermittent miss at idle on one, and the other with EFI era plugs, and a wide gap, did not run as well as the specified plugs at specified gap. Even though you've got hotter electronic ignition, try the same plug called for with points, at the same factory gap.

 

[awasson]

@Otto Have you attempted to see if the PCV is the problem by temporarily removing it and inserting a breather (and blocking the vacuum) to see if the idle can be dialed in smoothly? Long term, I think that PCV is important to get right but if it were my engine, I would want to take it out of the equation to make sure that I am chasing the right problem.

I quite like that carb adapter you came up with along with the tidy cable setup. Nicely done.

Cheers,
Andrew

 

[Otto]

Today I tried some things. I re-gapped the plugs to 0.035” as Frank suggested with no real joy. I did check the rear valve cover breather for vacuum signal and there was a little, I’m hesitant to call it vacuum, but it would draw a piece of paper towards it- not very strong at all. The brass fittings in the adapter have an ID of ~0.25” at most and in some areas it appears less.

Everything I try seems to have no effect or the engine runs worse, so I had to step away and just crack a beer.

I again checked for vacuum leaks with propane and found none. Putting propane into the carb would pick up a little RPM, but not smooth out the engine. The additional fuel was read by the AFR meter.

The engine just has a miserable idle and what sounds like a constant misfire well into the RPM range, so I can’t help thinking there is some underlying issue here, but don’t know what it is. All components seem to function properly, but the engine is not happy. Ignition?, thinking of substituting a TFI coil on the shelf.
Another thought I have is when I assembled the engine, I degreed the assumed stock camshaft to specs in the Falcon Performance Handbook. It was off about 12 degrees with the assumed stock timing chain set the engine came with and I installed the CI adjustable double roller set to match the timing events in the handbook. I’ve degreed camshafts before, but the thought that I F’d this up somehow sticks in the back of my head.

I guess I failed to mention earlier that I bought this “rebuilt” engine at an auction and went through it when I got it home. So all the parts are new, but I didn’t purchase them.

I owned an early Mustang quite some time ago that had a rebuilt 200, and it idled so smoothly you could set a wine glass on the air cleaner. I realize this thread has gone from a simple question of cylinder heads to where it is now, but I do appreciate the help.

 

[Otto]

@Otto Have you attempted to see if the PCV is the problem by temporarily removing it and inserting a breather (and blocking the vacuum) to see if the idle can be dialed in smoothly? Long term, I think that PCV is important to get right but if it were my engine, I would want to take it out of the equation to make sure that I am chasing the right problem.

I quite like that carb adapter you came up with along with the tidy cable setup. Nicely done.

Cheers,
Andrew

Thanks Andrew, the adapter wasn't that hard to make.

I would pull the PCV valve out and place my thumb over the inlet to eliminate any flow through it and it didn't run much better. I had mentioned doing this improved things with the previous valve, but not now.

 

[awasson]

Another thought I have is when I assembled the engine, I degreed the assumed stock camshaft to specs in the Falcon Performance Handbook. It was off about 12 degrees with the assumed stock timing chain set the engine came with and I installed the CI adjustable double roller set to match the timing events in the handbook. I’ve degreed camshafts before, but the thought that I F’d this up somehow sticks in the back of my head.

I installed the cam in my engine a few months ago and the dot-to-dot cam timing was off by about +10 degrees compared to the cam card. I checked and checked again and there was no mistaking it, the cam timing has that much variance. I used a solid lifter and pushrod with my dial gauge to measure.

I have the adjustable gear set and when I retarded the cam by the full 8 degrees, it was within 3/4 of a degree to the card. I backed it off 2 degrees at a time and took all of my measurements with my dial gauge just to make sure. The guy I talked to at Clay Smith didn’t seem particularly surprised.