I'm having idle issues with my 91 notch. Its a stock 91 short block that has a summit dual plane intake, Holley 600cfm 4150 HP, gt40p heads, Anderson n-21 cam, and full MSD ignition. I've researched my problem many times but haven't found anything. I have my timing @14 degrees initial and 23 degrees mechanical all in by 2500rpm. I set the idle mixture screws a turn and a half out like Holley recommends and the car cranks and runs and drives perfect (around my yard) but is very rich with smoke coming from the exhaust. My floats are adjusted properly according to what Holley says and I have 6psi of fuel pressure. So I hook my vacuum gauge up to tune the idle mixture for best vacuum and I can get it to idle fine with 13 inches of vacuum and at 850 to 900 rpm. But when I go to put the car in first or reverse it pretty much dies and runs like crap if I don't give it gas. It doesn't do this when its rich, the rpm barely drops when it's rich. I've sprayed all around the intake manifold and carb flange with carb cleaner looking for vacuum leaks. I can't find any and I only have one vacuum fitting on the manifold and it's for my brakes and it's not leaking. Can anyone please help me diagnose my problem, I keep starting from the top of the list and I always end up back here. Also my pcv system consists of a dash 10 bung welded to my passenger side valve cover with a catch can and a filter/vent on top and the drivers side has just an oil cap, if any of this matters. Thanks for any help Casey.
Idle Problems
- Thread starter Roller1991
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mikestang63
SN Certified Technician
1. what size jets are in the cab. Sounds like you may need to swap them for smaller size jets.
2. you may have too small an idle air bleed screw
3. have you adjusted your secondaries to let more air in at idle.
2. you may have too small an idle air bleed screw
3. have you adjusted your secondaries to let more air in at idle.
Did you buy the car with the carb conversion on it or did you convert it after you bought it?
Start by capping off every vacuum fitting on the intake manifold except the one that the vacuum gauge connects to. That includes the power brake booster and PVC or whatever you are using to vent the crankcase.Then start the car an see what vacuum you get - 13 " of vacuum is very low.
Start by capping off every vacuum fitting on the intake manifold except the one that the vacuum gauge connects to. That includes the power brake booster and PVC or whatever you are using to vent the crankcase.Then start the car an see what vacuum you get - 13 " of vacuum is very low.
Gear grabber
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- May 23, 2016
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Start by confirming all fuel screws are set near the same. (example 1 1/4 out from lightly seated)
With the car warmed up and vac. gauge connected,turn all fuel (2 or 4)screws very slightly richer,and note vacuum reading. If the vac gauge did not drop,continue richer in tiny increments. When the vac. drops slightly try that.
With the car warmed up and vac. gauge connected,turn all fuel (2 or 4)screws very slightly richer,and note vacuum reading. If the vac gauge did not drop,continue richer in tiny increments. When the vac. drops slightly try that.
I only have one vacuum fitting on the intake and it's for the booster and that is where I put my vacuum gauge. @Gear grabber ive done this multiple times and I always come out with the same reading. I make sure my timing is correct and then I let the car warm up with the mixture screws 1 and 1/2 turns out, it's very rich but runs perfect this way. Once it's up to temperature I proceed to tune it with the vacuum gauge, I get it to 13" of vacuum and it idles at 850 to 900, but if I put it in gear and let off of the clutch then it instantly drops 250 to 300 rpm and will die if I don't give it gas. If I try to get more out of it then the curb idle screw becomes relatively useless and won't change the idle whatsoever.
Gear grabber
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If i read that right.
You are releasing the clutch without giving it any gas,and wonder why the revs are dropping ?
You are releasing the clutch without giving it any gas,and wonder why the revs are dropping ?
I know that you have to give it gas for it to get going that's not what I'm referring to, I'm talking about the rpms falling way down below where I feel they are supposed to when put in gear. Am I expecting to much forgiveness like it was stock, also could it be the combo of my cam and rearend gears (2.73s), they will be 4.10s soon, I don't know if it has anything to do with it or not. Also when I hook up my vacuum gauge should it be steady or can it bounce around cause mine is pretty sporadic between 12 and 14 inches.
Gear grabber
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With a mild/moderate cam it should be steady. I am 244/252 @.050 gauge is steady.
You mentioned not being able to raise the idle. Any chance you have the idle screw turned very far in,uncovering the transfer slot ?
You mentioned not being able to raise the idle. Any chance you have the idle screw turned very far in,uncovering the transfer slot ?
My specs are only 219/229 so my should be stable. I took the carb off and revealed the transfer slots so they look like "squares" as I read on another forum to do and this seemed to work until I had to mess with the idle curb screw again after I got the mixture screws "right". How do I keep from messing with the curb idle screw to much as to mess up the transfer slots and get the idle down from it Reving up after I get the mixture screws adjusted
Gear grabber
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It is a ''dance'' to get it all right.
A question. In your first post you say,14 initial,23 mechanical,37* total ? A lot imo
And all in at 2500 may be asking for trouble. Do you all so have vacuum advance ?
And if so,ported or manifold ?
A question. In your first post you say,14 initial,23 mechanical,37* total ? A lot imo
And all in at 2500 may be asking for trouble. Do you all so have vacuum advance ?
And if so,ported or manifold ?
Last edited:
No it's mechanical advance only. Should I back the timing down some and recurve when my advance comes in. From what I understand is 12 to 14 is a good starting place for a cam like mine then 36 to 38 total, I might be wrong and need to bring it down some and move it to be all in at like 3000 rpm. What are your thoughts on this?
Gear grabber
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I don't say it's the problem,but i would back it off some until it is more sorted.
With no vac advance you want a fair amount of initial,but i would limit total to 32-33*
All in 3k+ (i am 30*,427 tfs heads)
With no vac advance you want a fair amount of initial,but i would limit total to 32-33*
All in 3k+ (i am 30*,427 tfs heads)
Gear grabber
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There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and every hose and connection.
Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-725 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.
Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as starting fluid, propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...
Vacuum leak due to slipped lower intake manifold gasket...
Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.
Determining if you have a leak due to a slipped intake gasket as shown above. This test is only good if you can get the engine to run somewhere in the 1000-1700 RPM range
If your valve cover oil filler & PVC systems are still in the original configuration, try this:
Cap or plug the hose from the intake manifold to the PVC valve with a bolt.
Cap or plug the PVC valve with a piece of hose with a plug or bolt in it.
At that point the only vent for the crankcase is the tube from the oil filler neck to the throttle body.
Disconnect the tube that runs from the oil filler neck to the throttle body. Make sure the oil filler cap is on securely. Start the engine and put your thumb over the end of the tube that comes from the oil filler cap. If you feel suction, there is a leak. Another thing to do is to extend the tubing from the filler neck so that there is enough to stick the end in a jar or cup filled with motor oil. If it sucks up the oil, you definitely have a leak at the underside of intake manifold.
This isn't necessarily the definitive test, but it is the best thing I could come up with on short notice. If there is a lot of blow by, this obviously won't be of much help.
Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-725 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.
Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as starting fluid, propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...
Vacuum leak due to slipped lower intake manifold gasket...
Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.
Determining if you have a leak due to a slipped intake gasket as shown above. This test is only good if you can get the engine to run somewhere in the 1000-1700 RPM range
If your valve cover oil filler & PVC systems are still in the original configuration, try this:
Cap or plug the hose from the intake manifold to the PVC valve with a bolt.
Cap or plug the PVC valve with a piece of hose with a plug or bolt in it.
At that point the only vent for the crankcase is the tube from the oil filler neck to the throttle body.
Disconnect the tube that runs from the oil filler neck to the throttle body. Make sure the oil filler cap is on securely. Start the engine and put your thumb over the end of the tube that comes from the oil filler cap. If you feel suction, there is a leak. Another thing to do is to extend the tubing from the filler neck so that there is enough to stick the end in a jar or cup filled with motor oil. If it sucks up the oil, you definitely have a leak at the underside of intake manifold.
This isn't necessarily the definitive test, but it is the best thing I could come up with on short notice. If there is a lot of blow by, this obviously won't be of much help.
I've looked at them and they are very black from it running rich but I don't see any specs of aluminum on them @jrichker i keep getting oil in the bottom of my intake manifold, right below where the air fuel mixture actually goes in. Could this be something that could cause my problem or is there some other problems that are causing this one?
