V6 Right Bank Running Rich

I have investigated and posted on this here before...still fighting very different plug color on each bank. For the most part, the rebuilt v6 has been running really well for the last year. I got a bit of an idle stumble/misfire on the way home today - first really warm (80) day here and I was stuck in stop and go traffic with quite a bit of idling. To compound matters, the throttle screw has been just a bit high from an wheeling trip a few weeks ago (I tend to bump the throttle up just a bit to help keep it from choking out - no hand throttle) and seemingly never turned it back down (we are talking maybe an extra 200 rpm - so maybe 800-850). Rechecked initial timing as well - all good.

Between the hot day and extra idle I figured I might be running a bit rich - and sure enough the right bank plugs are carbon sooted. But the left bank are perfect grey. I turned the throttle screw back down and checked the idle mix screws - they are set just out from lean mis-fire - there is no room to lean the idle out on either screw. IIRC from posts on here before - the right side jet and idle passage doesn't only feed the right, so any right-left bank differences are probably not from the carb?

So what would affect right vs left bank? Could I have leak around the intake manifold gasket on one bank that isn't affecting the other side? I am stumped on this for the last year.

 

Carb gaskets are all new - well from a year ago when I rebuilt it. Intake gasket is a question - might see how to test.

Manifolds going to y-pipe into single exhaust - built by local shop. It does bring up an interesting question...I got the manifolds after the POs fenderwell headers rusted out). IIRC, the manifolds are not a matched pair - I think one side is an older vintage and may have an slightly smaller port. Ok - off to the photo files to find pics and see if that one might be on right bank.

 

I've been giving this some thought (uh-oh, here it comes...)

Does having a single plenum really mean that the streams coming down through the barrels are going the thoroughly mix?

DISCLAIMER- Figuring the induction pulses for an 8 is easy, I'm afraid envisioning them for a V6 is a bit more than my head can handle this morning. ​

Certainly there will be some mixing but in my slightly addled mind it seem to me that the mixture entering the manifold from the right barrel will be mostly sucked to the right cylinder bank while the left barrel's out put will be mostly headed left.



Enquiring Minds Want to Know....

 

no - one of the reasons the two manifolds don't match is I was trying to get ones without any emissions pieces on them to keep it simple and not have to plug the tubing ports.

Pertronix in a delco style distributor with their flamethrower coil. This system ran before the engine rebuild - although I did use a new/rebuilt cardone distributor with the pertronix system after the rebuild.

I checked that I am getting spark to the right bank plugs with my induction timing light on each wire - obviously it doesn't tell if the timing of the spark is correct, but at least tells me I am getting conistent current pulse. Given how well/smooth the engine runs - I don't think its a spark issue. I guess it could be firing a bit off timing on the entire right bank - but its not causing any type of misfire or loss of power until the plugs foul when it idles a lot. When I first tackled this problem right after the rebuilt, I replaced the plugs to ensure the plugs were good - even switched them from one bank to another. Same result.

My gut is telling me its the difference in exhaust manifolds (one being a bit more restricted) as that is truly a right vs left bank different - but I ran these manifolds for 3+ years before the rebuild without issue. Of course, the old tired engine with stock cam might not have cared. fresh engine with 252 comp cam might be a bit more sensitive to it - especially at low rpm idle? It definitely doesn't like to idle as low as it used to. I am not ready to jump into manifold changes just yet - but how easy would it be to open up the manifold ports just a bit? I could also look for a matching large port manifold - but I think the collector neck is a bit different and then it won't match up to the exhaust pipe.

 

Going to start here - but is there a better way than carb cleaner? It likes to eat the (reasonably) fresh paint on the engine.

Heads and block surfaces were checked and lightly resurfaced by the machine shop but not significantly shaved or decked such that cylinder volume decreased or compression would have changed.

 

Just to ask - how would one go about opening up manifold ports? Guessing I would need to find a machine/speed shop that could do it as I would guess I don't have the tools for any precision work. Of course, I do have hand held die-grinder but that would be a bit imprecise to say the least - although that appears to be pretty standard way using gasket or template of exhaust port on engine.

 

Allright - not sure that I am very good at this (as trivial as it seems) - but spraying carb cleaner along the base of the carb, along the intake manifold gasket and into the valve covers doesn't seem to change anything in terms of rpm. I guess I will pull the manifolds and see what I can make of that. So here is the quiz question...If I have two manifolds with slightly different size ports (larger vs smaller) - which one will be on the rich side and which one will be on the lean side?

 

I took pics of the ditributor caps - my newer one and my older one that was fine and is now on my spare distributor - unfortunately I forgot to upload them. But it all looks the same and correct...I actually think the 'newer' distributor cap was run on the engine prior to the rebuild when I first installed the pertronix and before I killed the bottom end.

Anyway - since I was messing around with the distributor cap, carb and and doing all the manifold lead test with carb spray...I thought I was hearing a bit of a misfire at idle - not a consistent misfire but every few cylces there seemed to be a hiccup and a small puff of smoke out the tail pipe. With further investigation while operating through a range of throttle and acceleration - there seemed to be an inconsistent hiccup every once in a while. So I backed my initial timing off just a bit - I had pushed it all the way up to 12BTDC - and now moved it back to 10BTDC. That required just a bit of opening of the idle screws to avoid lean misfire, but it seems good and don't seem. I think I don't hear the idle hiccup anymore and it seems to move up through the rpm range a little smoother. Again - I am not 100% sure I really know what I am listening to with regard to engine sounds, but its probably safer in the long run to back the initial timing off. I set it at 12BTDC back in the winter when it was cold, and maybe with the warmer, less dense summer air it more prone to pre-detonation.

I have no idea if this could be the cause of any of the right bank rich indicators, but seems that in addition to the damage caused by pre-detonation, it would also only partially burn the air-fuel mix and could be the cause of richness. Why it would only happen on the right bank? I am going to hold off on pulling the exhaust manifolds for a little bit and check the plugs again after some driving around.