Need Quick Fuel carb tuning assistance
- Thread starter duster360
- Start date
-
I know that the emulsion ports in the metering block bring air into the fuel. Also the IFR jet in the upper bowl side of the block restricts the amount of fuel going to the idle circuit. BUT, if I were to close off one of the 3 emulsion holes, what would that do? I assume it would put less air in the fuel and it would run richer? Am I even close.
It might have no effect on the idle circuit, but yes it should go richer in the upper rpms.
It may also delay the start of the mains, creating a lean transition.
The air bleeding into the main well lowers the overall density of the fluid mixture. So sometimes enlarging the main bleed brings in more fuel earlier - which shows up as richer AFR.
Going from very small or no bleeds to ideal size bleeds will do the same.
That's what this fig 11-11 from Obert shows.
Yellow highlights the start up with too little or no e-holes, and then that would also mean the main air bleed can't compensate for increasing draw at highest flows.
Emulsion Tuning
yellow rose
Overnight Sensation
Just a few thoughts after watching your video.
I couldn't see what the timing was at idle in the video.
Have you VERIFIED that your tach is correct? I have 3 different AM tachs and every single one of them reads different at idle. By 2000 RPM they are all correct. I can check this because my ignition box will do that. I can set the box at say 800 RPM and the box will send an 800 RPM signal. The tach I'm using now at 800 RPM is really about 750 so that's not bad.
The tach that I took out of the car (which was purchased new) was off 130 RPM at 800 RPM. That means if the tach was reading 800, the engine was really at 670 RPM.
Your idle sounds slow to me. Turn it up to 900ish. There is nothing wrong with that. With a stick it's even less an issue.
I don't let my stuff idle under 1000 RPM. It's easier on the lifters, the oil flow and pressure are a bit higher and if you tune the idle circuit to match, you'd be surprised how much cleaner they will idle.
If you are running a bunch of overlap, or a lot of seat timing, you may want to move the idle feed restriction to the upper hole. My cam is 281/281 seat timing and 255/255 at .050 and my idle is much better with the IFR up. Which is going against current wisdom. You need to give it what it wants so try anything you can think of.
Just some random thoughts.
I couldn't see what the timing was at idle in the video.
Have you VERIFIED that your tach is correct? I have 3 different AM tachs and every single one of them reads different at idle. By 2000 RPM they are all correct. I can check this because my ignition box will do that. I can set the box at say 800 RPM and the box will send an 800 RPM signal. The tach I'm using now at 800 RPM is really about 750 so that's not bad.
The tach that I took out of the car (which was purchased new) was off 130 RPM at 800 RPM. That means if the tach was reading 800, the engine was really at 670 RPM.
Your idle sounds slow to me. Turn it up to 900ish. There is nothing wrong with that. With a stick it's even less an issue.
I don't let my stuff idle under 1000 RPM. It's easier on the lifters, the oil flow and pressure are a bit higher and if you tune the idle circuit to match, you'd be surprised how much cleaner they will idle.
If you are running a bunch of overlap, or a lot of seat timing, you may want to move the idle feed restriction to the upper hole. My cam is 281/281 seat timing and 255/255 at .050 and my idle is much better with the IFR up. Which is going against current wisdom. You need to give it what it wants so try anything you can think of.
Just some random thoughts.
This is the cam I have.
20-227-4 - Xtreme Hi-lift Hydraulic Flat Tappet Camshafts
I will round up another tach and verify idle speed.
Timing is at 20* at idle.
I bought that tach 25 years ago and never got to use it until I put it in this car.
Hard to find a tach with both the red pointer and shift light. These days they have one or the other.
20-227-4 - Xtreme Hi-lift Hydraulic Flat Tappet Camshafts
I will round up another tach and verify idle speed.
Timing is at 20* at idle.
I bought that tach 25 years ago and never got to use it until I put it in this car.
Hard to find a tach with both the red pointer and shift light. These days they have one or the other.
Last edited:
That was a good read. Now it is making more sense. The books I have just don't go into any real detail about emulsion. Thanks.
Dropped fuel level to bottom of sight glass (has the large sight glasses). Cleaned up idle AFR just a touch 13.5ish, engine also idled up just a hair with this adjustment. I went down to 68 jets and it does not like it. Lots of surging, going back to #70 (stock jet) and go from there. Bought 8' of hose today, so I can hook up the vacuum gauge and check cruise vacuum.
I think dropping jets size and fuel level at the same time may have caused this bad surge. <---- Need opinion on this.
Also went from #76 to #78 jets in the rear to fatten up the WOT. Last test with the #76 ( stock jet) AFR went to 13:1 at 5000 rpm. If this change helped WOT, I will adjust with the PVCR from here on out.
Am I doing this correctly or am I running off in the wrong direction.
I think dropping jets size and fuel level at the same time may have caused this bad surge. <---- Need opinion on this.
Also went from #76 to #78 jets in the rear to fatten up the WOT. Last test with the #76 ( stock jet) AFR went to 13:1 at 5000 rpm. If this change helped WOT, I will adjust with the PVCR from here on out.
Am I doing this correctly or am I running off in the wrong direction.
I am wondering since I am having to fatten up the carb, if I should have went to a 750 cfm. Power wise, I am right in that gray area (411 hp at 5700 rpm) where 650 cfm runs out and 750 cfm picks up. But this is not a drag car either. Street only with occasional spirited throttle testing.
Put the #70 jets back in ( surging went away) the primary’s and at 3000 rpm in high gear I am around 12.5 ratio. It did not like the #68 jet in the primary’s.
I also went from #76 to #78 jet in the rear and the AFR is now at 12.8 ish at 5000 rpm. Should I leave it there or rejet the PVCR and fatten it up more. More jet? Engine makes full power at 5700 rpm.
I also went from #76 to #78 jet in the rear and the AFR is now at 12.8 ish at 5000 rpm. Should I leave it there or rejet the PVCR and fatten it up more. More jet? Engine makes full power at 5700 rpm.
This probably causing more headaches than you realize.
Discussion here:Holley/Demon Carb stumble
Fuel mix should be steady throughout the full load, wide open throttle. In other words, 3 (or 4th gear) pedal to the metal.
Smaller carb will help fuel distribution. Larger carb has potential for higher top speed (more hp) at the 1/4 mile, and a fraction off the e.t. if fuel distribution is good.
See above comments.
For now, it seems you must use 70 PMJs to drive on the highway regardless of what the WBO2 says.
For me, the easiest is to jet for steady conditions and jet for WOT in top gear (dyno and track).
If WOT at track wants richer jetting than interstate highway driving, then adjust PVCR.
cawcislo
Well-Known Member
Bringing up an older post. I’m having quick fuel carb issues. Did you ever get yours dialed in?
512Stroker
We are all here because we are not all there.
Videos are now working
should have left the Holley on that worked - if it isn't broke don't fix it.
Read the plugs instead of your AFR they will tell you the story.
Fuel gauge on the line or regulator or a external gauge? From what you described it does sound like too much pressure.
-
