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Discussion Starter #1
I wish I could post videos, but I haven't figured out how yet...

Motor 400, unknown cam ~12" vacuum (so I suspect something), rebuilt tri-power, Doug's headers, and full exhaust with crossover.

So my cold start idle AFR is ~11:1.

Warm idle is not much leaner ~11.4:1

Cruising AFR in 3rd gear is ~13.x:1

WOT in 2nd gear ~13 - 14:1

I'd be interested if others are logging AFR and if so, what you see and please provide details of motor / setup.
 

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Premium Member
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679 Posts
Discussion Starter #3
I wish I had a afr meter. I would like to see what I am at.

What meter and setup do you use? Thanks
I have the Innovate LM-2 and have a bung welded on each side of the exhaust (just after the headers).
 

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I put one of these in my car this go-round:

https://www.summitracing.com/parts/avm-30-0300

I like it - a lot. It has been a HUGE help as I've been struggling to learn how to deal with this newfangled carburetor.

I also bought this book. Yes, it's focused on Holley but don't let that throw you. The author, David Vizard, is one of THE top authorities on engine air flow dynamics and even though so far I've just been cherry-picking passages out of the book to work on specific problems I've been having, I can already tell that there is a TON of information in there that applies to any setup including tri-power and even weird esoteric stuff like SU and Weber. What I like about Vizard is that he not only will make brand-specific recommendations but also tends to spend a lot more time getting into the details and theory/principles behind them, and that's what applies to any type of carburetor set up.

https://www.amazon.com/David-Vizards-Holley-Carburetors-Modify-ebook/dp/B00E257X72/ref=sr_1_1_twi_kin_1?ie=UTF8&qid=1536673618&sr=8-1&keywords=vizard+holley

I bought the Kindle version because of the "instant gratification" factor of having immediate access to it, and I don't have to have a Kindle device - there are Kindle apps for smart phones, PC's, MAC's, and even a cloud-based Kindle reader you can access directly from any web browser without having to install any software at all.

What I've learned so far from the book **DISCLAIMER** None of what follows is anything more than what I >think< I've learned from the book so far combined with what I >think< I know from past experience. Your mileage may vary, void where prohibited by law, past performance is not a guarantee of future return - so please keep that in mind:

* At idle, the leaner the better as long as the engine seems to be happy and doesn't stumble on tip in. Hot engines with big race cams might have to go into the low 13's or high 12's because of low air flow and high reversion at idle. Right now I've got my 461 (moderate solid roller with about 250 degrees duration at 0.050) idling in the high 13's low 14's and I think I can go leaner still - I'm still learning.

* Idle mixture and settings have a lot more effect on the transition from the idle system to the main system than I previously thought.

* Idle SPEED SCREW setting is very critical. It's very common for people to crank open that screw to get their cars to idle, especially with bigger cams, but that can cause all kinds of problems. What that causes is it opens the throttle plates too far so that they expose too much of the transfer slots at idle (transfer slots are there to help ease the transition from idle to off-idle), and that causes the car to be idling mostly on the transfer slots instead of the main idle circuit. This is 'bad' because now the car is pulling most of the fuel from the transfer slots instead of the idle circuit (which is why your idle mixture screws don't seem to do anything - they're not), and then when you do move off idle there's "not enough slot left" to ease that transition so you get a stumble - even with the accelerator pump shot. There are ways to correct this problem depending on what kind of carb the engine has.

* Best power/full throttle. Usually very close to 12.5:1 Going to the lean side of that can be dangerous the the engine if you go "too far". Lean mixtures burn very hot and can promote detonation and other unhealthy things. Also going towards lean makes the power drop off a lot quicker than going to rich side does. HOWEVER - there is also a danger to being too rich, if it's a lot too rich (low 11's, high 10's for example): at this point you're probably getting fuel wash - dumping enough raw gasoline into the cylinders that it's going to leak past the rings, wash all the oil off the cylinders, get into your pan, and dilute the oil making it thinner. There goes your oil pressure and the protection for your bearings. Pull your dipstick and check it often for the smell of gasoline. If it's obviously present, time to change all the oil and filter and correct the mixture problem that's causing it. (I have yet to run the first full tank of gas through my car since I got it running again, and I've already changed the oil and filter >3 times< because of this. :surprise:)

* Everywhere else, including part/light throttle cruise. Should NOT be richer than the best power mixture - ever (see fuel wash problem above). Like idle, you generally want things to be as lean as your engine tolerates here without pinging/rattling or overheating. If your engine is "upside down" (richer at part throttle than it is at full throttle) but ok at idle, suspect a problem with the power valve / power enrichment circuit. These most often use manifold vacuum to sense engine load and will add fuel when vacuum drops. Very often they depend on manifold vacuum to hold them shut and a spring to force them open when vacuum goes away. If that spring is too strong (or vacuum signal too weak as can happen with a 'bigger' cam), then the valve/enrichment circuit will be open when it shouldn't be and will be dumping more fuel into the engine than it needs.

* Accelerator pump. I'm still struggling with this one myself. It's job is to supply a momentary shot of fuel during the period when the throttle plates are first opened but before air flow has started to make the main carburetor circuits start working. There are several variables to be tuned: how much fuel, when, and delivered over how long of a time interval. If any of these are "wrong" it can make the car stumble/bog and it's very hard (for me) to tell which one needs tweaking and in which direction. If any are "too much" or "too little" then you'll get a stumble when you hit the go pedal.

* Fuel level/float level. Has to be right. Too high and it might be puking fuel out the vents, causing a safety hazard and making it run like crap because it's being flooded with fuel. Too low and all the circuits in the carb won't be working right because fuel level in the bowl(s) affects liquid pressure at the various orifices (like the main jets) and thus affects flow volume and rate.

*Fuel pressure/volume. Also has to be right. High enough to keep fuel in the bowls but not so high that it forces the needles off their seats and causes flooding. (I actually fought a nasty problem for two solid days earlier this week that turned out to be too much pressure. I got fooled because it had previously been running "OK" so I just "knew" it had to be something else.)

* When adjusting things, do them in this order and make sure each one is perfect FIRST before you move on to the next one.
1) Float level/fuel level
2) Fuel pressure
3) Float level/fuel level (again - after the pressure is right)
4) Idle speed screw setting (remove the carb, turn it upside down, measure how much of the transfer slot(s) is/are exposed. Start with NO MORE than 0.040.)
5) Idle mixture and idle speed together - use a good vacuum gauge. Adjust for highest vacuum at lowest possible steady, sustainable, workable RPM
6) Check transfer slot(s) exposure again. If more than 0.060 is needed to get it to idle then this has to be fixed - there are ways.
7) Idle mixture/idle speed - again - after you've solved any transfer slot(s) exposure problem that cropped up.
8) Transfer slot(s) - check one more time. You MUST be able to get a good idle with no more than 0.060 exposed before you move on. Less is better.
9) At this point unless you have a seriously nasty, long duration, high overlap cam your idle A/F mixture should be at least in the upper 13's low 14's or higher.

This is about where I'm at with mine right now, so I'll stop here for now :)

("Get a Holley" they said, "They're easier to tune than a QJet" they said....)

Bear
 

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Discussion Starter #5
So I adjusted the idle screws a bit (clockwise / tightened) and my idle AFR is now ~14.x....Not sure if I should tune the idle for more like 18-20:1 like modern cars....

Thoughts?
 

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64-67 Expert
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8,561 Posts
New cars idle at 14.7:1. It's the ideal ratio for the cleanest overall burn. Go into the 16:1 range like Chrysler did in the '70's with Lean Burn, and you get high hydrocarbons due to lean misfire. Been driving my tripower '65 GTO since 1982, and it has a cam that produces 12" vacuum at idle. You do need to change the power valve spring in the center carb when running a big cam in these cars due to the false signal the low vacuum will give the carb. Less vacuum makes the carb richen up too much at idle. It's been too long for me, but there was some recent posts on this very subject on the tripower section of the PY/Ames forums.
Bottom line, don't run your car leaner than 15:1 at idle. It'll run rougher and hotter.
 

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Premium Member
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Discussion Starter #7
New cars idle at 14.7:1. It's the ideal ratio for the cleanest overall burn. Go into the 16:1 range like Chrysler did in the '70's with Lean Burn, and you get high hydrocarbons due to lean misfire. Been driving my tripower '65 GTO since 1982, and it has a cam that produces 12" vacuum at idle. You do need to change the power valve spring in the center carb when running a big cam in these cars due to the false signal the low vacuum will give the carb. Less vacuum makes the carb richen up too much at idle. It's been too long for me, but there was some recent posts on this very subject on the tripower section of the PY/Ames forums.
Bottom line, don't run your car leaner than 15:1 at idle. It'll run rougher and hotter.
Thanks for the reference, I will try to find the article. The nice thing about having a wideband is one can watch their AFRs in a variety of settings (WOT, partial throttle, etc.).
 
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