I had some time this weekend to play in the garage and try to fine tune my carb. Because of the cam and stroker of my motor, it has some definite quirks that make tuning correctly a lot of fun but a challenge. It is very responsive to trying different things (some good, some bad).

Let me start by saying that I am tuning using an Innovate Motorsports LM2 and a good ol' vacuum gauge.

I know that theoretically, the best AFR is 14.7 (neutral). The best power comes from running 15-20% rich (i.e. 11.75-12.5). I also know that the best economy is...oh who cares! I am trying to get the best bang out of the engine while not fouling the plugs.

I have had the AFR all over the map, but here comes my question. What is the best idle AFR given that this is a street motor with .65 lift and 242/248 duration @ .05? It seams to like idling at 850 with an AFR in the mid 12s. When I try to lean it into the 13-13.5 range, I have to really kick up the accelerator pump to fend off the off idle lean stumble (still working on this). I can crack the secondaries a little more and get the AFR stable in the 13s, but then there is more air moving through the secondaries and the choke is for crap. So, what I have right now is a balanced air flow between the primaries and secondaries with an idle rpm of about 850 and what I would think is too rich of an AFR at idle of mid 12s. The choke works good in the config, but when I lean out the AFR at idle, I get off idle problems. When it is more rich at idle, I have less off idle problems, but man that seams like too much from what I have read.

Thoughts from the tuning gurus?

 

if your not getting eye burning smoke from the exhaust @ mid 12's while idling and it is helping your off idle stumble stick with what you got.

otherwise you could try a accel pump cam that brings em in a little faster, or maybe a larger discharge nozzle size, or maybe both.

btw do you have a 30cc or 50cc pump?

 

I have a 30cc pump. It cam stock with a 31 nozzle and a Pink Cam loaded in position 1 for both the primaries and secondaries. Early on when we were tracing the crazy rich condition, we changed the primary nozzle to a 28. Although the Pink cam produces a bunch of fuel, it is slow to do so, so I installed a Blue cam in position 1 and then position 2 this weekend. I still have a stumble and noticed that when pushing the throttle down, the nozzle continues to squirt after the throttle has stopped moving...so that tells me that I need a larger nozzle to keep up with that cam, so that is on my list for sure. How big to go kind of depends at this point on how lean I make the idle. The more lean I go, the more accelerator charge I need. I may also need to go with a larger power valve to help with this stumble as well, but I am trying to just do one or maybe two things at a time.

Currently, I do a fare amount of idle as this mostly a hopped up street car, so I do not want to load the plugs up every morning on my way to work if you know what I mean. The exhaust is not burning, but I definitely smell like a hotrodder when I get done in the garage.

 

Urban, remind me what carb and intake you have and are the primary plates drilled? Also, what is the idle vacuum (best you can get)? How much of the transfer slot is exposed at idle? It sounds like you are running more off the main jets than the idle circuit.

You have a cam that is going to hate idling and 850 sounds a bit low. You need a 50cc acc pump as in your case it is more about the size than duration of the squirt (did I just type that?!). I'll give you some more thoughts with the answers to my questions above.

 

It is a 4150 double pumper 750cfm. I jetted down the front from 73 to 68, I think. That puts me in the 14 range while cruising, although weather and how I have the idle tuned that day affect that to a certain extent. I have to look at my notes on the vacuum at idle...if I remember right is in the 6in of manifold vacuum at idle. I need a longer hose to check the vacuum at cruise. The primary and secondary idle transfer is not very exposed at all. Maybe a few thousandths. I know because I have read a bunch on that and check it early. I balanced the primary and secondary based on the transfer slot (secondary was more than primary going into yesterday). Based on my understanding and my current values, that means that I am still running mostly on the idle circuit at idle...right? I had not thought about the 50cc pump. At what point, size of nozzle, would I need to move up to a 50cc pump?

Also, the secondaries are basically stock configuration with a 68 jet instead of the stock 78 jet (I think). The power valve is currently a 2.5 (replaced the 6.5 PV early on).

 

i know but this might be a stupid question but why would one care much about AFR in idle. this is completely down to idle circuits which are out of equation once you are cruising or accellerating.
I'd be more interested to hear what AFR you have when you are floorin git or am I completely off my books here

 

Maybe larger IAB's? Should allow the mixture screws to do more. I'm not sure that 12.5 is all that bad if the vacuum is up. Its been a while since I've fooled with a carb though. Good luck.

 

After another tuning session today, here where I am and what the results are. It is amazing how much it changed from one day to another.

Kalli, the reason idle AFR is just as important as cruising or WOT AFR is it does not take long at all for plugs to get fouled, carbon to build up, and detonation issues to ensue. I went through this early when I was running way too rich at idle. I will never ignore idle AFR again.

Primaries are slightly more open than secondaries but still just a few thousandths of the transfer slot open. Jetting same as yesterday 68 up front and 67 in the back. 2.5 power valve. 28 nozzle up front and 31 out back. Blue #2 cam up front and Pink #1 cam out back. Idle mixture at about 7/8 of a turn open front and back.

I ended up with about 11in vacuum at idle at 800 rpm. The quality was not great, but ok. Higher idle was smoother, but then I have dieseling issues when I shut it off. Idle AFR was about 13.5. Cruise AFR was about 13.0, which is strange because it was more like 14 before I started working on the idle. I still see a lean condition when I pop the throttle, but I cannot feel it, which is good. When I let off the gas like coming to a stop light, the rpms drop way down and it goes lean for a while before it recovers. I think a higher vacuum PV will help that. WOT was about 12:1. I did one 3000 rpm launch and it got it...no stumble. Off idle was better too. It liked the AFR around 12.5, but I am too chicken to run the idle that rich.

I think my next step is to put in a 5.5 power valve. I had a 6.5 in there originally, but had terrible tuning issues and took it way down before I discovered the art of adjusting the secondary butterflies! The rule is half the idle vacuum right? With that, I may need to lower the cam some so it does not get too rich, but I will see what happens. There are no cams or PV anywhere around and I have borrowed everything they have, so I have some PVs and other cams on order from Summit.

Tuning this car always makes me scratch my head. I had 14AFR at cruise and 13.5 at idle, but the quality was bad and the choke sucked due to the secondaries being open more than the primaries. Now the primaries are open a little more than the secondaries and the thing is running more rich at cruise but the same at idle. The cam might be some of that, but you would think it would go back after the charge was burned. Strange...It is getting warmer and a little more humid here these days, so I may need to drop the jets a point or two to get the cruise right. I am super pumped about getting the idle stumble gone (still there on the computer, but not in the seat of my pants)! I just hope the plugs will be happy, because the engine feels like it gained 25hp.

The moral of my story so far, is man the Holley's are customizable. It also does not take much change to make a big tuning change, at least on my car.

 

i was going to say try the 31 nozzle again up front, and use either a 5.5 or 6.5 PV. now that you've dialed in other areas a bit these changes may get you really close.

btw, i really need to get a a/f ratio gauge. they are invaluable when tuning.

 

Hate to tell you, but you are tuning it backwards. The changes you are making, are changing something else down the fuel curve. Work backwards from WOT. Get that where you want it, 12.4-12.8, and make sure it's flat. This is where you do the HSAB's. If you change later, like trying to activate the mains sooner, it'll play with the curve you already perfected. What ever change you make to the front in WOT, make it to the back. Don't worry about the cruise AFR until WOT is set. From there and how far your cruise AFR is, will depend if you make a LSAB change or jetting change up front. If you make a jet change up front, you have to drill the PVCR's to compensate. 12.5 for idle is just fine is that's what the motor wants. The way I tell if it's pump or idle to lean it to put it in gear(all your idle adjustments are made in gear), and ever so slightly ease of the pedal. Keep the pump shot out of the pick. If it's not quite rich enough it'll start to jump around and almost stumble, rather than slightly raising in idle speed. Your front blade position will effect your cruise AFR. If you have it all but closed like it should be, you will actually cruise on the idle circuit when not loaded. When it's like this the pump shot can be finicky to get right. This is the shortest version I can possibly type out. try working backwards, see if that works for you a little better. I will only go from idle to WOT in tuning with blo thru. You have to bridge fuel curve gaps a bit differently, and their PVCR's are adjustable, like jets which makes it possible to add more fuel without changing a jet. If you had to change a jet, it screws it all up.