5S276 Fuel distribution problem?

[dockbay]

HI guys!
I have my car back up and running after a fresh engine, tranny and rear end rebuild. Here's the short story. I just pulled off my headers to take them back to Sanderson who did a crappy job ceramic coating them black. What I noticed when I removed them (I have run the car for about 50 miles and it seemed to run great) is that the most rearward cylinder on each header is clearly running ultra lean. There is consistently similar carbon distribution on the first 3 ports and almost NO visible carbon on the last port on each side. The motor seemed to idle fine so I don't suspect that there is a vacuum leak on both sides (I will check)  So I'm thinking that I might have a fuel distribution issue? My thought is to try a phenolic spacer under the carb and see what that does. Any other thoughts you all have would be helpful. Something amiss in the manifold? Anybody running a spacer on an otherwise completely stock setup? Will it fit under the hood? Truth is I'd really like to keep the car completely stock without a spacer but clearly something is amiss....
Cheers and thanks in advance,
John

[6s1802]

The rear tubes on 6S1802 run hotter too, especially the right side. I suspect it might be the original intake manifold. I run a 1" phenolic carb spacer and run the PCV hose into the back of the spacer not the rear vacuum port on the manifold.

[s2ms]

+1.

Check your spark plugs, the rear 2 cylinders, especially #4, get the huge bulk of the PCV vapors due to the design of the Cobra intake. I suspect the carbon buildup is from the oil in the PCV vapors and your plugs should reflect that. Running a thin spacer and attached PCV hose, like the Boss 302, will balance out the PCV vapors. But it won't look totally stock...

Dave

[doublemyv]

SFM5s444 has had the 289 completely rebuilt to original specs by Craig Conley @ Paradise Wheels.   One thing he mentioned when we were breaking in the motor on the test stand is that " we put a restrictor inside the pcv line between the pcv valve and the manifold, to prevent the rear cylinders from running too lean, but maintaining a stock look ".   He did not mention the size of the hole drilled in the center of the plug/restrictor, but others on this forum may have some info on that subject.  Good Luck, Mark

[dockbay]

Thanks for the feedback! I just checked in my records to see how old the PCV valve was as I thought it might be involved somehow. It's pretty new. I like the idea of the restrictor in the line as that would be hidden. I'll give Craig a call in the morning to determine the size of the hole in the plug..
Cheers,
John

[Rickmustang]

Please let us know what you find out

[zray]

Yes the rear cylinders will run slightly more lean due to the. PCV intake location. However this is not to be considered extreme or damaging in ANY way., there is absolutely no problem in using the stock Cobra Hi-rise intake with the rear PCV hose location.  Additionally, there is no performance advantage to be changing the stock Shelby arrangement to an under carb spacer for more even distribution. Any theoretical advantage does not translate into actual measurable improvement .  I've experimented with every possible variation in PCV / intake hose plumbing using the the Cobra Hi-Rise manifold and found no benefit to deviating from the stock arrangement.

Z

[6s1802]

I also restricted the PCV hose for less flow

[shelbydoug]

A .100" restrictor seems to work better but a lot of the issue is that the pcv valve itself is basically cheap junk and doesn't do what it is supposed to do. They are supposed to completely close but don't.

If you decide to try to re-engineer the mess, go immediately to the ME Wagner unit. It is about 95% efficient rather then 5% like an over the counter Chinese valve is.

It's the best that you can do at this time.

[zray]

".........What I noticed when I removed them (I have run the car for about 50 miles and it seemed to run great) is that the most rearward cylinder on each header is clearly running ultra lean. ...."


"...... The motor seemed to idle fine......."

OK, so the car is running "great". as described by the OP

additionally, it "....idles fine..."

You don't have a problem. If there was a lean condition so bad it was causing engine damage, the car wouldn't be running great and idling fine. With two cylinders REALLY running extremely lean the idle would be hunting all over the map,  and you would be down on power. Just because the carbon residue is lighter colored in the two rear cylinders is not reason to think the sky is falling and something must be done.

You will never get all the cylinders to show the same exact burn color with a Cobra Hi-rise manifold.

So put on your headers when they are done (again) and drive the car. When you have a problem you won't need to be analyzing the inside of the headers to discover it. It will be obvious and manifest itself by an unstable idle, massive bog off idle, and poor acceleration.  Until you have those symptoms, you don't have a problem.

Z

[2112]

Yeah, lean cylinders are not a problem.........until they are.

Ever blow an engine up on a few lean cylinders? I have. It sucks.

It was the non-tunnel ram, un-corrected intake that caused my meltdown.

I can understand not wanting to touch an original intake (equalizing flow) but if you want to run the engine hard on a regular basis, an option would be a flow equalized repro (Art Francis) intake manifold.

Easy bolt-on/bolt-off proposition.

[zray]

".......Ever blow an engine up on a few lean cylinders? ......."

No I haven't . Either I've been lucky for 55 years, or I know better. You decide.

I'm just some guy that cleans up the mistakes of others. A nobody.

There's a difference between inconsequential lean, and lean that matters.  A Cobra Hi-rise manifold is the former.

Z

[2112]

Until it has been singing at 6,700-7,000 rpm for a while.

Not my engine, go for it, right?

[2112]

If intake modification or swapping intakes isn't desirable, at the very least, I would retard the timing and/or jet the secondaries to richen the leanest cylinders.

IMO, all non-tunnel ram/Webber intakes have a lean runner(s) somewhere. If you run hard, you will find out which one eventually.

[zray]

Until it has been singing at 6,700-7,000 rpm for a while.

Not my engine, go for it, right?

I think the OP has a more or less stock engine, right ?   And it runs great and idles fine, no lean hunting at all.   Doesn't sound like a problem to me. For example, with a vintage Paxton sitting on top of the Hi -Rise manifold in my old '66 GT350, I ran the course of the BBORR many many times (off season) at full throttle for the entire 69 miles. The rear cylinders always showed a little lean, but nary a sign of  damage.  If a 289 w/ a Paxton and the suspect Hi-rise manifold can make the Big Bend course look like a walk in the park, over and over, again, then I'm not worried about telling someone they don't have a problem with a well known minor lean issue on the rear cylinders.  After all, this issue isn't something new. It's been observed since these cars were new, and Hasn't been the cause of engine failure yet.  I fussed with it myself for a couple a years, and never found a way to eliminate the anomaly without creating other problems. Eventually coming to the same conclusion everyone else has since the days of the '65 championship season; minor lean running can be tolerated in the rear cylinders without causing damage

As pointed out, most dual plane manifolds have some irregularities in fuel charge distribution. If that's worrisome, just put on a nice set of 48 IDA Webers and problem solved.