old school v6 tuning question

[FINOCJ]

I am always trying to get my engine tuned as well as possible - and that includes the combination of tuning/jetting the carb as well as adjusting timing. Overall, the engine has been running fine - really darn well from the best I can tell, but I got a few questions anyway. The main symptom that has my attention is that the engine seems to lug more than I would expect in the 1500rpm range (lets say up to about 1800rpm). When it gets up close to 2000rpm, it just hums away nicely. It cruises at 2500rpm so nicely (to my untrained ear and and seat of the pants feel). 2000-2600rpm is where I DD it mostly - and that seems to be a good sweet spot for a v6, but I wish it would pull a bit better at the lower end. With a 3 speed, its a pretty big rpm drop when shifting up a gear, and it seems I really have to push it up to close to 3000 rpm to keep the next gear close to 2000rpm. Obviously, this is exaggerated here in the mountains, but even a slight uphill incline really seems to require 2000rpm for the engine to avoid lugging. I don't know how much of this is just this engine (and getting accustomed to my modern Toyota v6 that pulls well down to 1000rpm) vs what is due to my tuning, and how much might be due to the comp cam 252 grind - which is slightly higher rpm focused than a stock cam. Additionally, I feel like my gas mileage since the rebuild has just been worse than previously (12mpg these days) and I think it might indicate I am not in the optimal range for the engine's performance. Keep in mind of course that I am running 4.88s and 33s - so that seems reasonably appropriate (although I think 5.38s would be perfect).

I feel like it used to pull quite a bit better at the lower end, but I have messed with the tuning over the last couple years and guessing I have caused this issue myself. Right after the rebuild, I was running more advance on the initial timing (12BTDC), and also running a larger carb jet (49). Over time, I have slowly backed off the initial timing (8BTDC) as I was worried I was getting a pre-detonation misfire and I leaned out the carb (all the way to a 47 jet) as it was fouling plugs at elevation. At the same time, it drove ridiculously strong around town (but smelled rich) with the starting set-up. Given that I go from 5000' to 13,000' in the summer, I realize tuning the carb is a bit of a compromise. As I leaned out the carb it runs better at elevation, but its probably a bit lean around town. I tried running 48 jets as a compromise, but was getting plug fouling and rough running engine when wheeling at 8000-1000' so went back to 47s and never had a problem all the way to 13,000'.

So this is just a long winded way to ask if my concerns around low end/rpm performance is probably due to me leaning it out and under-timing too much. I feel like I can't push the initial timing too far advanced given the lean fuel mixture? Seems like a richer fuel mixture allows for more timing - or is that just my mis-interpretation? And what would be the best way to bring back some of that performance given my conditions? thanks

[Brian]

I always tried to time to the highest vacuum. It's been a while but I believe that's a good setting to at least start with on the timing. In other words jetting and timing can be optimized independently. It sounds like you have the jetting at a happy compromise, so I'd say spin the dizzy to the highest vacuum and take it for a spin.

[FINOCJ]

I originally tried to set timing based on vacuum but it wanted to much advance and pinging...so I backed it off. I use ported vacuum and not manifold...sometimes I think manifold vac would allow for the desired idle vac but wouldn't cause as much advance during hard acceleration and reduce pinging. But that's another discussion. Thinking of trying to push the timing back up just a degree or two. You might be right but my gut feeling is that I am actually timed about right...but under jetted. Some ecj5 members and know some of the work I have done are suggesting exhaust back pressure issues are causing the fouling and not a rich mixture. I am starting to think they might be right...the fouling is only on the passenger bank and the exhaust on that side has some flow impedence.

[Jim]

Is your distributor advance working as it should be - both vacuum and mechanical (flyweights)? Does it have vacuum advance? If yes, is it plumbed correctly?

I had a 68 Ford wagon - purchased on the west coast and drove back to Illinois. It idled fine. It ran the highway fine. City driving, off the line, it was a dog. Come to find out the vacuum advance circuit in the distributor wasn't working. Fixed that and bingo - great all-around performance.

I found this article interesting:
www.superchevy.com/how-to/additional-tech/1601-everything-you-wanted-to-know-about-vacuum-advance-and-ignition-timing/ (http://www.superchevy.com/how-to/additional-tech/1601-everything-you-wanted-to-know-about-vacuum-advance-and-ignition-timing/)

[Jim]

I use ported vacuum and not manifold...sometimes I think manifold vac would allow for the desired idle vac but wouldn't cause as much advance during hard acceleration and reduce pinging.

The article talks about ported vs direct. Use what the OEM specifies for your motor. It also talks about timing being RETARDED (err - not having vacuum advancement in addition to mechanical advancement) during WOT. WOT = low vacuum condition.

[FINOCJ]

The article talks about ported vs direct. Use what the OEM specifies for your motor. It also talks about timing being RETARDED (err - not having vacuum advancement in addition to mechanical advancement) during WOT. WOT = low vacuum condition.

I use ported vac as that was OEM - although many suggest the manufacturer switch to ported vac was more to do with emissions and pcv than performance....I am fine with ported vac...I think I have a plan - sometimes I have to write my thoughts out and then re-read it a number of times as I read all yall's comments to help my stagnant brain start functioning.
1) I am going to deliberately and little by little bump my initial timing back up - probably going from 8BTDC (where it is now) up to 10-12BTDC (assuming no pre-detonation issues). Manifold vacuum readings suggests it likes a bit more initial timing (actually it likes a lot more 12-14BTDC, but that is what made me nervous when timing advance went all in).
2) I will begin to address the exhaust issues as best as I can given I don't think I can rebuild the entire system at the moment ($). I will probably start with trying to grind and clean up the manifold ports - I've heard this isn't such a fun job and I am probably going to need some special hard grinding bit for my die-grinder to work on the iron manifolds. I will also get some new manifold gaskets - I don't like the fel-pro ones I can get from my local FLAPS as they are individual for each port and I feel like they don't fit or stay in place all that well. I have seen some recommended on here that look way better - don't need the gasket impeding exhaust flow.
https://www.summitracing.com/parts/rfl-13-004/overview/make/buick
3) I may even be able to cut-out my T-junction and replace with a y-junction....but have to learn a bit about connecting or splicing in a section of exhaust pipe. I see a lot of exhaust junk at FLAPS that slip fit together with an expanded end - but then I assume I need to weld it solid. Probably can't do that in place on the jeep and would need a lift to get the entire system in and out as a single piece.

Another thought along Jim's ideas of timing - how would changing the mechanical advance weights affect things. I don't really have a good way to map out how much and when the advance comes in (graph of rpm vs advance - I would probably need to get an adjustable timing light), but wondering if a softer spring would allow the mechanical advance to come in a bit earlier would help? Or maybe a softer vac advance canister - the high elevation up here limits how much vacuum can be achieved. Might be worth trying to investigate?

Oh - and somewhere along the way I am hoping to get an eaton locker installed before the Rubicon this summer!

[Jim]

I would not touch the distributor weights/springs/adjustments unless you have a method to "dyno" the distributor on a bench to be able to watch/document the weights' advancement with regard to RPM with regard to OEM advance curve.

My position on the distributor / spark advance systems is to ensure it's working as the MFGR intended it to work. You could take it to a distributor shop and have them verify operation. I'm wondering if the dist/advance system is not working properly and you're trying to compensate with base timing adjustments.

[FINOCJ]

Don't worry Jim, the distributor isn't that nice. You can easily test each function on the distributor with a timing light and plugging the vac line and or removing the weights to test vac can operation etc. I did all function testing when I cobbled it together from 3 other distributors using all the best parts - as some of my used parts were better than the rebuilt junk units I bought. I'd like to curve it properly - and a distributor dyno would be best, but not sure who or where you can do that anymore - and I think it can be done in the engine if you have a proper variable timing light and tachometer. Disconnect and plug the the vac can hose and adjust the variable timing light to mathematically 'subtract' the initial timing at idle - light will appear to flash on the 0 of the crank timing scale (but it really firing on whatever your initial timing is). Then increase the rpm by steps of 100 and record the advance - on a variable timing light you just keep turning the dial back to keep the light flashing on the zero and record how much the dial is 'subtracting'. This should give a decent graph of when and how the mechanical advance comes in....or maybe I am just crazy. Thinking aloud here...but this isn't about getting the engine to run 'normal' - it already does that just fine - this is about squeezing a bit more performance out of it, especially at low end here at high elevation. The OEM factory distributor with weights and vac can (at least what you cab buy as replacement today) was not curved for high elevation. Increasing initial timing is probably step 1 - then its all about the dist curve. Or maybe just HEI and a vac can....

[Jim]

Thanks for putt'n up with my "it _must_ be the distributor" angle.

Any progress on the issue?

[FINOCJ]

Right now I am in Montana working my skiing job...I began upping the the timing a bit last week before I left but I found a puddle of coolant from a heater core leak so everything is on hold...have to either temporarily bypass the heater or pull it and try to solder it before moving ahead with timing concerns. Old jeeps never die...just in a perpetual state of repair.