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1970 Mustang Fastback
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Discussion Starter #1
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-Timing is set at 13 BTDC
-MSD distributor is set up with 21 degree blue bushing and 2 blue light springs
-Idle at 1200 - 1300 rpms
-Vacuum at 18 - 19 Hg

Does the idle seem a bit high? I can’t seem to dial it down (idle speed screw) without losing vacuum. See video for details.
 

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Idle seems high to me. What are the specs of your cam? It doesn’t sound too radical. I think you’re supposed to remove the vacuum advance on the distributor when setting timing. But don’t take my word for it, wait for the real mechanics to weigh in.
 
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Cam specs?
 
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Discussion Starter #4
Cam specs?
I believe this is the cam COMP Cams Xtreme Energy Cam and Lifter Kits CL31-242-3.

Idle seems high to me. What are the specs of your cam? It doesn’t sound too radical. I think you’re supposed to remove the vacuum advance on the distributor when setting timing. But don’t take my word for it, wait for the real mechanics to weigh in.
Yea that’s what I’m thinking. With the stock distributor, I was idling around 700. When I set the timing, the vacuum advance was plugged, no worries there. Timing is at 13 BTDC.
 

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Assuming you are not misspeaking,:rolleyes: Idle screw should be OFF! Mostly, some need more or less of the slot exposed for good transition but flaps closed, THEN adjust mixture screws for max vacuum.
 

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Discussion Starter #7
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As of right now, the car is still set at 12 BTDC. I turned the idle speed screw for the engine to idle at around 750 rpms and then adjusted the mixture screw for maximum vacuum. However, I am in the late timing zone, 15 Hg. I can’t seem to get past that. See Video, any other advice would be appreciated.

Assuming you are not misspeaking,:rolleyes: Idle screw should be OFF! Mostly, some need more or less of the slot exposed for good transition but flaps closed, THEN adjust mixture screws for max vacuum.
Not sure what you mean by off. I don’t think that is possible.

Set the initial to 16. Adjust the idle mixture for max vacuum. Turn the curb idle down to 750-&00.
I will attempt this tomorrow. Should I change out the advance stop bushing to a lower degree as well?
 

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Not sure what you mean by off. I don’t think that is possible.
OFF=Zero throttle=Butterflies closed=car running on carburetor's IDLE circuit alone. If you can't make that happen then other compromises and expectations need to be made.
 

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Camshaft overlap (both valves open at the same time) kills vacuum. So an engine with a performance camshaft is not going to register in the green arc of the typical vacuum gauge. If your engine only "pulls" 14"-15" of vacuum at idle there is nothing you can do about it. You adjust the mixture screws for maximum vacuum and if 14" is your max vacuum then it is what it is.
 

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I have a 347 tuned up quite well and vacuum at idle is around 7. It will vary depending on your camshaft specs. I have the idle screw all the way off for the primaries and adjust idle speed with the secondary screw. First thing is to get the four idle mixture screws the same and with best air/fuel, or idle sound and speed, or vacuum-however you do it. Actual idle speed can be fine tuned after with either the primary screw, or secondary (in my case). 1200 RPM seems very high unless you have a pretty crazy cam. I can idle at around 800 RPM and don't have an extreme cam, but it does have some personality :)
 

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Yes, idle too high (primaries open too far). Also suggest you find a different place to hook up your vacuum gauge (rear manifold tap) rather than a carburetor port.
 

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Discussion Starter #13
Probably not. The engine should be ok to 36-38. 21 and 16 is 37. Should be perfect.
I will give it a try tomorrow and see how the car likes it. Thanks for the feedback.

OFF=Zero throttle=Butterflies closed=car running on carburetor's IDLE circuit alone. If you can't make that happen then other compromises and expectations need to be made.
How do I adjust this? I have a Holley 650 CFM Double Pumper carb

Camshaft overlap (both valves open at the same time) kills vacuum. So an engine with a performance camshaft is not going to register in the green arc of the typical vacuum gauge. If your engine only "pulls" 14"-15" of vacuum at idle there is nothing you can do about it. You adjust the mixture screws for maximum vacuum and if 14" is your max vacuum then it is what it is.
Okay gotcha, this answers pretty much everything then. Thanks alot! I was over here scratching my head why I can only adjust it so far when everything was right.

I have a 347 tuned up quite well and vacuum at idle is around 7. It will vary depending on your camshaft specs. I have the idle screw all the way off for the primaries and adjust idle speed with the secondary screw. First thing is to get the four idle mixture screws the same and with best air/fuel, or idle sound and speed, or vacuum-however you do it. Actual idle speed can be fine tuned after with either the primary screw, or secondary (in my case). 1200 RPM seems very high unless you have a pretty crazy cam. I can idle at around 800 RPM and don't have an extreme cam, but it does have some personality :)
Wow, that is low. I guess my tune is fine then, I’m just overthinking it. What is the correct way to turn off the primary idle screw?

Yes, idle too high (primaries open too far). Also suggest you find a different place to hook up your vacuum gauge (rear manifold tap) rather than a carburetor port.
I was plugged into the port that gives full vacuum on the carburetor. Thanks for the tip, I’ll give this a try and see if it’s easier to read.
 

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Correct. There are little square slots in the primary butterfly's that allow air and if they are too big/open it screws the air fuel mixture up. I have had best results with the curb idle screw "off" for the primaries and then if I need to adjust, using the screw on the other side for the secondaries. This is just for curb idle speed, after the 4 mixture screws are tuned in. It's really hard to tell what is going on without a wideband AF gauge though.
 

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Discussion Starter #17
Correct. There are little square slots in the primary butterfly's that allow air and if they are too big/open it screws the air fuel mixture up. I have had best results with the curb idle screw "off" for the primaries and then if I need to adjust, using the screw on the other side for the secondaries. This is just for curb idle speed, after the 4 mixture screws are tuned in. It's really hard to tell what is going on without a wideband AF gauge though.
Thank you for the clarification Chip. This helps me out alot. I’ll test it out. And yea I agree, I’m tuning with just a vacuum gauge and by ear, it’s takes time to dial everything in just right.
 

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Discussion Starter #18
Question: Is the secondary idle screw the same thing as a fast idle screw/cold idle screw? I'm getting a little mixed up with the terminology. I did more research, I will back out the primaries today until they are square. Thanks for all the advice so far, you guys are great~
 

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Discussion Starter #19 (Edited)
Disregard my last post. I figured out that they are the same thing, I thought I was going to have to pop off the carb to get to that secondary screw. I have one on the passenger side of the carb.

So here are my latest results, everything seems to be running great now. Primary idle screw is off, timing is set to 16 BTDC, idle is around 750- 800, and vacuum is stable at 15 Hg.
See video

I went for a test drive, but now I am running into a different issue. The moment I press the throttle too much (about halfway), the carb pops and PCV plugs shoot off. Car loses vacuum and shuts off instantly. What can be causing this?
 

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You need to back the idle mixture screws out about 1/4 to 1/2 of a turn to richen the mixture a little. In your other thread I mentioned that this would happen if you left the mixture screws at max vacuum.
 
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