Need help with DIY tuning.

Symptom - Car is a PITA to restart when it's at full temp (runs around 170 - 180*)

When cold it fires almost instantly with little more than a bump of the key...once its warmed up there is a 50/50 chance is going to fire up, or give me grief. I will crank fast but with minor backfires through the exhaust as well as the carb. Eventually it seems to end up flooded - I just stand on the throttle while cranking for like 10 seconds before it finally sputters back to life.

Some basic info...

LS1 with cam, Victor Jr, Holley 750HP, MSD 6010 (no MAP sensor yet). I don't have my laptop handy but my home brewed timing map looks something like this...(pill removed)

0-500rpm - 12*
500-1000rpm - 24*
1000-2500 - 28*
2500-7000 - 32*

 

I have the same problem. Unfortunately im in for answers, as I dont have the seat time to have fixed it yet. I was just assuming that id hadnt found the hot start proceedure (every car has one) that this engine likes. Guess we will see what the rest of the crew has to say about it. Eric L

 

Funny you mention the hot start procedure...I thought the same thing but I've tried everything w/o success.

 

Ramp the timing up to 20* initial and all in to 32* by 2000 and see if it does any better.

 

Adding vacuum advance sure made my car easier to start. Your timing curve doesn't look too bad. Too much timing when cranking overworks the starter, but then you want to bump the timing up once it catches. That is where a vac advance comes in nice.

My guess is you are running too rich off idle. Most cars are hard to start when cold cause the carb just naturally goes lean. The fact yours starts well cold but then needs fully open blades to start suggests to me you might be too rich. HP so I assume no sticking choke. Also assume you have checked the basics like float level, good carb gaskets, no blocked air bleeds, nothing leaking in the carb, etc

How are you setting your AFR's? Do you have a wideband? Make sure you set the idle mixture screws when fully hot.

 

I just completely cleaned and inspected the carb (due to allowing trash in my fuel system and sticking the float). Everything is clean and in good shape. You are correct...it's running rich especially at idle. As for setting my AFR's, I'm torn. I kind of want to take it for a dyno session, but lately I've been getting a little better understanding of the carb and think I might be better off just buying a wideband set-up and doing my own. Map sensor is coming soon and my timing curve is honestly my first attempt based on stuff I've read on this site. My guess is there is more left in it both from a power and drive-ability standpoint.

Can you explain what the air-bleeds do? Not sure it's part of my problem, but since you mentioned it I thought I'd ask.

 

Definitely get the mixture right. If that does not solve your problem. Put a carb insulator spacer on it. You might have a fuel overheating/percolation problem.

 

So just warm up the car then adjust the air screws? Is there anything else that affects idle & low speed mixture?

If I had a wideband - what would be the desirable range for idle/light throttle?

Thanks for the help!

Anybody have any good links, or reads for carb tuning? I'd love to be giving advice one day instead of asking for it

 

Make sure your throttle blades are only showing .020 transfer slot.

Get the car up to temp.

Then play with the idle air mix screws to dial it in.

 

If it has an electric choke, check it before starting hot to see if the choke closed back. They need constant +12V to heat up and open but sometimes when the car is shut off they cool down too fast and close back when the engine is still hot. Holley makes a choke delay kit that is supposed to keep it from closing so fast on a warm engine.

 

I've read a couple of different explanations but air bleeds keep the AFR from getting too rich as the airflow increases within a circuit.

Not counting the power valve and accel pump a carb has 3 different circuits. Each one works over a certain RPM range. The main circuit comes in somewhere between 2 and 3 thousand RPM. The main jet meters the amount of fuel feeding the circuit. The main airbleed meters the amount of air getting mixed with the gas in the emulsion tube. If you didn’t have airbleeds, the carb would start sucking more and more gas as the air flow increases. So my understanding is you adjust the size of the jet to get the AFR right at the low end of the RPM band, then adjust the air bleed to keep it flat throughout the range.

The intermediate circuit covers idle to the point where the main jets take over. It also has a separate gas restriction and air bleed, but in this case it is called the idle feed restrictor or IFR. With most carbs this isn’t adjustable like a main jet. It is pretty easy to drill these out and replace them with set screws that you drill yourself or even wedge a piece of wire in there to change the orifice size.

So from what I have read the idle mixture screws adjust the AFR at idle only. Right off of idle you use the IFR to adjust the AFR. The idle air bleed can be adjusted to keep the AFR flat through this intermediate range until the main jets take over. The intermediate circuit is what you use for 90% of street driving and unfortunately it seems to be the most difficult circuit to get right.

A long winded explanation but I don’t know it well enough to make it simple. Of course float level or leaky gaskets can throw everything off. So you need to be methodical in tuning a carb.

For AFR, I have my idle set right at 14.7:1, cruise just a touch richer and WOT it goes to 13:1 or so. You can cheat on those, but this is working well for me. I would strongly recommend buying a wideband over paying for a dyno session. It takes a bit of patience but just keep good records of what you do (so you can back off changes if needed) and the wideband will let you really dial in your carb. I don’t know how the old timers got their carbs right before widebands. That is probably why carbs have such a bad reputation. So many guys just got use to driving cars with mistuned carbs back in the day.

 

I really appreciate you taking the time to write that...I'm certain I'm not the only one who just learned something.

 

Your idle mixture will affect light load and part throttle. You size your main jets for the AFR you need at cruising RPM when fully on the jets, and then adjust the idle mixture as needed to correct AFR at light throttle (up to around 1800 RPM) and idle. If it can't be nailed down with the idle mix alone or you can get a good idle but have issues at low speed (1000-1800 or so or whatever the car pulls with very little throttle) then you start in with the low speed air bleeds.

You can think of the air bleeds kinda like if you had a sealed bucket of water with a pipe coming out of the bottom. You can adjust the spigot on the pipe open or closed and it will only do so much and the water will kinda glug out because air needs to get in to replace the water that's coming out. That's your air bleed. You now punch a hole in the bucket and the water will flow out of the pipe more freely at all ranges of the spigot. Say you need more water out even with the spigot all the way open. Punch a bigger hole. Say you got your water flow about right but under some conditions it's still a little too much water coming out (too rich at light throttle). Leave your spigot alone but close the hole up some. I've tuned light throttle before by using pieces of wire stuck down in the air bleeds to effectively reduce their size.

 

I had the EXACT same issue. Pulled around to make a pass at the track and had to turn car off in burnout due to a broken car in front of me. My car wouldn't crank again. Let it cool off a little and was good.

I know its a stab in the dark, but I chased mine for weeks and turned out to be the crank sensor. I had an extra engine on the stand and took it off and put it in for the hell of it and it's been fine for a year and a half now.

Good Luck