Ben,
It sounds like you've got pratically the same setup as me. My carb is set to the stock settings (idle air bleed was specified in the manual as 4 1/2 turns out from fully seated, the rest looks exactly as Clem specified), with a couple of minor changes. The idle mixture screws are 1/2 turn richer & the idle air bleed valve is quite a way off from the stock setting (in). If it's any help, I set my lean stop 5 1/4 turns out from fully seated & later, after making a gauge, found it to be correct. It ran well with this setup with duals, cam, & intake changed. On fitting Headers & OD tranny the car ran hotter, as I said. I suspect that the secondaries are running slightly weak, but the primaries seem correct (runs well, good idle, good pickup, nice plug color after normal road speed running).
After reading this post I'm starting to suspect that my increase in coolant temp might have nothing to do with the tranny but be due to the Headers heating the tranny/cooler lines (they fried my starter motor, so heating tranny fluid is very likely).
Have you ever had the carb apart? If not, it could be worth removing the air horn & checking for visible damage. The solenoid plunger has a "wing" on the top which pushes the primary needles down. The wing on mine was bent so that one of the needles was always pushed slightly down (lean) on the p side. The idle mixture screw for that side had been set to a very rich condition to compensate. And because it was on the p.side the O2 sensor was never aware of the problem. It's a long shot, but you never know what's been done in the past.

Justin, thanks for the reminder on the carb :) With all your experience of measuring the O2 sensor readings, etc, etc, have you got any idea of how "forgiving" the ECU is of incorrect mixtures, ie how bad does it have to be before an error is stored?
:cheers:

 

It seems to me that the O2 sensor needs to be reading at the extremes of it's range to set a code, and probably for an extended length of time. Mine never set a code back in the days when I had a monster cam in it with no vacuum at all and it was running rich due to it puddling gas in the intake and not sucking it into the motor, then when I would rev it up it would suck it all in and flood it out real bad. However, when my ECM failed and I lost control of my MC selenoid I did get an O2 rich code and check engine light. By checking the O2 sensor with a multimeter it may not be needed to tune for air/fuel ratio, but it would at least let you know that your sensor is reading within range. I found that I was set up right within range by using the gauge setup and setting it to Doug Roe's specs.


-Justin

 

Justin,
Thanks for the info :thumbs:
Well, it's a bit of a coincidence that both mine & Ben's run hotter after fitting Headers. Fitting a tranny cooler just hid the problem so, assuming everything else on both cars is working correctly, the question has to be: are they running hotter due to a weakened mixture, or due to the Headers heating stuff up (or a combination of both). I'd think a tweak of the rich stop adjustment could solve the problem by richening the mixture. But one thing I've found out by playing with air cooled engines is that it's easy to mask a heat problem (eg due to too much advance) by making the mixture richer.
What I'll do before adjusting anything is to get it on a mates exhaust gas analyser to verify whether it's running lean, or was getting hot due to radiated heat from the Headers. I'd guess that it's running lean, but it's always best to prove it. This could take a while (I've got to catch him working late, or on a Sunday), but I'll post what I find when I've done it.
:cheers:

 

It's possible that both are going on. I imagine you are flowing a lot better with your new exhaust, causing the engine to need more fuel, and you are probably radiatiing heat off of your headers as well. The exhaust testing will tell a lot. It'll be interesting to know.

-Justin

 

I checked my rich and lean stops according to Doug Roe's book, and it appears that my rich stop was aleady set higher than it should be. Now I'm thinking that my heat problems are probably something other than my carb.


[Modified by Ben Taylor, 7:51 PM 9/16/2003]

 

Ben,
Is your's an Auto?? My tranny cooler lines are closer to the Headers than the starter motor, so I'm 90% sure that the tranny fluid was getting heated on its way to the rad & also on it's way back. This would cause the tranny fluid to run to hot (which it was) & additionally the hot fluid would heat the coolant in the rad. It's to much of a coincidence that a tranny cooler dropped the engine coolant temp to what it should be. I can't help wondering if I would have got the same result by wrapping the cooler lines (or p.side collectors) with heat insulation? If you haven't already, I'd strongly recommend fitting a starter heat shield.

Justin,
I think it's a combination of heat radiated from the Headers & also improved exhaust scavenging causing the problems, so I'll chuck a couple of ideas in here! Swapping the stock system for duals gave a big power boost, but swapping the stock manifolds (2" restriction) for decent Headers would have dramatically improved scavenging as the restriction in the most crucial place (next to the exhaust ports) was removed. That's bound to give a better scavenging improvement than freeing up resistance further down the system, so it's highly likely that it's running to lean now.
Due to our exceptional weather there's no way I'm taking the Vette off the road at the moment &, as I haven't got a spare carb, I want to throw an idea at you :)
The lean & rich stops are factory set to give the leanest & richest conditions that a stock motor would ever require & the ECU just adjusts between these 2 limits. After your info on the O2 sensor readings showing the wide error margin the ECU gives, it looks like the mixture can be way off b4 the ECU throws an error. Now, have you got any idea how much adjustment/variation the primary needles will give beyond the factory set lean & rich stops? ie. is there much scope for going leaner or, more importantly, richer???? What I'm thinking is that the primary needles give a range between the richest & leanest mixtures that they can physically supply & any adjustment beyond these limits won't have any effect. Are the factory lean & rich stops set near these limits of physical adjustment? If not, then surely we could just move the "window" of adjustment into a richer position? eg, If both the lean & rich stops were raised 1 or 2 turns then the range of adjustment would be richer than the stock "window". Of course this will depend on how good the ECU is at keeping the solenoid plunger at the correct average position but, after seeing your O2 sensor info & error margins, I'm taking the view that it probably ain't to accurate :D
There could be some hysterisis(sp) in the system to prevent the ECU continually "hunting" either side of the ideal position. This could mean that it allows for very slightly rich or lean to an extent, but if it goes out of limits it would then drive the plunger to adjust the mixture. Now at some point, if it drives to the fully lean position, the mixture will be to lean & if it drives to the fully rich position it will still be to lean, giving an overall effect of a lean mixture. So making the rich stop give a richer mixture doesn't address the lean stop being to lean. Does this sound logical, or just a bunch of cobblers?!
:cheers:

 

I do think that we could definitely richen the mixture by raising both stops, but that really depends on the ECM working correctly and running it in that range. Assuming the ECM is doing its thing, I don't think that these would be set up at the physical end of the metering range, because then there would be no real need for such a wide range of adjustment on the rich and lean stops.

Ben, The tranny cooler line thing is interesting. Sounds like that could be the problem, since your carb is setup correctly.


-Justin