irregular idle while in gear (vacuum leak?)

[74sundancekid]

The engine a fresh rebuild is a 1974 318 la and currently it is idling at around 1000 to 1500 in neutral and then when put into gear (A904) the idle drops to 500 and then dies when the accelerator pedal is pushed.

compression in the cylinders ranges from 125-150 psi
the compression ratio is around 9:1
Carb is a Holley 1850 rebuilt 1 year ago and currently has new jets (64) and squirter port has been cleaned
spark plugs and wires are new but are covered in black soot already
No power brakes
cam is a comp cams xe268
converter is stock
in idle it pulls around 15 inches of mercury
inital timing is 18 total is 32
electronic ignition

Anyone have any ideas why it is acting this way? I'm stumped and could you some help from the experts

Thanks!

 

[khryslerkid]

spark plugs and wires are new but are covered in black soot already

Running rich? Choke not opening?

 

[74sundancekid]

Running rich? Choke not opening?

manual choke - it seems to be rich but I already dropped the jet sizes from 67 to 64. idle screws are 1 1/2 turns out. Could a clog in the idle circuit cause it to run rich perhaps?

 

[khryslerkid]

manual choke - it seems to be rich but I already dropped the jet sizes from 67 to 64. idle screws are 1 1/2 turns out. Could a clog in the idle circuit cause it to run rich perhaps?

Did you try to get the highest rpm out of the idle screws or highest vacuum? Then recheck the timing. Not much difference in idle screw adjustments can mean a vacuum leak.

If you think there might be dirt in the carb try revving it up and close the choke or put your hand over the carb till it almost dies and rev it up again. Sometimes this can dislodge a spec of dirt. Otherwise check carb bowls for dirt.

 

[74sundancekid]

Did you try to get the highest rpm out of the idle screws or highest vacuum? Then recheck the timing. Not much difference in idle screw adjustments can mean a vacuum leak.

If you think there might be dirt in the carb try revving it up and close the choke or put your hand over the carb till it almost dies and rev it up again. Sometimes this can dislodge a spec of dirt. Otherwise check carb bowls for dirt.

Highest vacuum I could get was 15 unless I moved the initial timing up to around 30 which seemed way too high but I'm not sure. could it be transmission related if it is not that? the trans has not been rebuilt

 

[khryslerkid]

Highest vacuum I could get was 15 unless I moved the initial timing up to around 30 which seemed way too high but I'm not sure. could it be transmission related if it is not that? the trans has not been rebuilt

Think simple things first. Timing and carb adjustments work hand in hand. Close each screw, one at a time, until it wants to die and back it out to the highest rpm or best vacuum. Do one then the other. Then recheck timing. Need to see if the carb is responding to adjustments.

 

[Geoff 2]

Not surprised that engine liked 30* at idle, but should run reasonably well with 18*.
Fresh build? I would looking for air leaks, intake & carb gaskets, etc.
The centri weights in the dist might also be advancing with such a high idle speed in N; then drops the timing once in gear. To check: timing in N should be the same as in D.

 

[Wietse]

See what your vacuum does when you drop it in gear, vacuum will likely drop a lot.
Would also double check your throttle blade position, changing jet size does not affect idle mixture as it only runs on the idle circuit and not through the main jets.
If your engine idle does not get affected by turning the idle screws in/out it means the throttle is too far open and you are sucking a ton of fuel through the transition slot, which will flood the engine once you put the trans in gear.
If needed, take the carb off and check the current transition slot exposure, it should only be exposed a little bit. (make sure the choke mechanism is fully off to ensure you are seeing the throttle blade position at curb idle without choke.)
Transition slot below the throttle blade should be more wide than tall (rectangular), if that is set correct you need to play with the idle screws to make it idle as best as you can and observe by vacuum gauge and listening to the engine.

 

[PurpleBeeper]

manual choke - it seems to be rich but I already dropped the jet sizes from 67 to 64. idle screws are 1 1/2 turns out. Could a clog in the idle circuit cause it to run rich perhaps?

Not likely, but yes, it's possible. It happened to me before

 

[RemCharger]

Some of those carbs have a center idle mixture control that works in conjunction with the side mixture screws. Pretty sure it was a 68 440 carb that I ran into that.

 

[Canadian1968]

You said the car is idling around 1000- 1500. That is a pretty big jump. Does it not idle smoothly even in neutral ?

As others have explained , set your timing then hook up a vacuum gauge to manifold vacuum port. Aim for 800-900 rpm. Adjust the idle mixture screws for your max vac.

How new is this build? Are you trying to drive it for the first time? Or have you already driven the car?

If you haven't been able to drive the car yet there are a few other things I would check as well.

A fresh motor will also require a lot of tuning as it breaks in BTW.

 

[74sundancekid]

You said the car is idling around 1000- 1500. That is a pretty big jump. Does it not idle smoothly even in neutral ?

As others have explained , set your timing then hook up a vacuum gauge to manifold vacuum port. Aim for 800-900 rpm. Adjust the idle mixture screws for your max vac.

How new is this build? Are you trying to drive it for the first time? Or have you already driven the car?

If you haven't been able to drive the car yet there are a few other things I would check as well.

A fresh motor will also require a lot of tuning as it breaks in BTW.

Havent drove it yet so still in the brake in period, im going to check all these things you all have suggested and report back.

 

[74sundancekid]

See what your vacuum does when you drop it in gear, vacuum will likely drop a lot.
Would also double check your throttle blade position, changing jet size does not affect idle mixture as it only runs on the idle circuit and not through the main jets.
If your engine idle does not get affected by turning the idle screws in/out it means the throttle is too far open and you are sucking a ton of fuel through the transition slot, which will flood the engine once you put the trans in gear.
If needed, take the carb off and check the current transition slot exposure, it should only be exposed a little bit. (make sure the choke mechanism is fully off to ensure you are seeing the throttle blade position at curb idle without choke.)
Transition slot below the throttle blade should be more wide than tall (rectangular), if that is set correct you need to play with the idle screws to make it idle as best as you can and observe by vacuum gauge and listening to the engine.

In neutral - 14hg
in reverse - bounces between 5-10hg
Timing stayed the same at 18degrees and idle in neutral is now 900 and in reverse dropped to 500 with a lil variation

Do you happen to have a pic of what the transition slot looks like? I have the idle speed screw turn in only about a half turn and the mixture screws seem to do nothing

 

[Wietse]

Since you said it is a fresh build i want to ask if the cam has had its break-in already or not?
If not you need to do that first, regardless of idle quality because idling and doing these adjustments are not good for the camshaft and tappets and can result in damaging them.

The transition slot looks like a fine vertical cut in the carb bore.
Only a small part of it should be exposed below the throttle plate, see below picture from the web i found as example, the exposure shown there is already too much.
As you describe that the idle mixture screws currently have no effect on engine idle i think you need to close the throttle a little at the time by the idle screw.
If there is too much exposure of the transition slot there is a lot of fuel being fed through it and causing it to run very rich.
Let the idle fall to 750-800 by closing the idle screw, after you can try opening/closing the idle mixture screws to find it needs more fuel or less.
Engine idle rpm will increase as you get close to the correct adjustment, i guess the current 1,5 turns you have at the moment is too much already so i would try closing them first a quarter turn at the time with the engine idling.
Have a vacuum gauge connected on the manifold port and use it as a guide, getting towards the right adjustment, the vacuum will increase.
Have the engine warmed up a bit before you start and give it a little time between adjustments to see what it does.

 

[74sundancekid]

Since you said it is a fresh build i want to ask if the cam has had its break-in already or not?
If not you need to do that first, regardless of idle quality because idling and doing these adjustments are not good for the camshaft and tappets and can result in damaging them.

The transition slot looks like a fine vertical cut in the carb bore.
Only a small part of it should be exposed below the throttle plate, see below picture from the web i found as example, the exposure shown there is already too much.
As you describe that the idle mixture screws currently have no effect on engine idle i think you need to close the throttle a little at the time by the idle screw.
If there is too much exposure of the transition slot there is a lot of fuel being fed through it and causing it to run very rich.
Let the idle fall to 750-800 by closing the idle screw, after you can try opening/closing the idle mixture screws to find it needs more fuel or less.
Engine idle rpm will increase as you get close to the correct adjustment, i guess the current 1,5 turns you have at the moment is too much already so i would try closing them first a quarter turn at the time with the engine idling.
Have a vacuum gauge connected on the manifold port and use it as a guide, getting towards the right adjustment, the vacuum will increase.
Have the engine warmed up a bit before you start and give it a little time between adjustments to see what it does.



View attachment 1386806

Since you said it is a fresh build i want to ask if the cam has had its break-in already or not?
If not you need to do that first, regardless of idle quality because idling and doing these adjustments are not good for the camshaft and tappets and can result in damaging them.

The transition slot looks like a fine vertical cut in the carb bore.
Only a small part of it should be exposed below the throttle plate, see below picture from the web i found as example, the exposure shown there is already too much.
As you describe that the idle mixture screws currently have no effect on engine idle i think you need to close the throttle a little at the time by the idle screw.
If there is too much exposure of the transition slot there is a lot of fuel being fed through it and causing it to run very rich.
Let the idle fall to 750-800 by closing the idle screw, after you can try opening/closing the idle mixture screws to find it needs more fuel or less.
Engine idle rpm will increase as you get close to the correct adjustment, i guess the current 1,5 turns you have at the moment is too much already so i would try closing them first a quarter turn at the time with the engine idling.
Have a vacuum gauge connected on the manifold port and use it as a guide, getting towards the right adjustment, the vacuum will increase.
Have the engine warmed up a bit before you start and give it a little time between adjustments to see what it does.



View attachment 1386806

The cam has been broken in.

I followed what u said and the results are better. Car now starts immediately without any throttle pumping, got the neutral idle to about 750 and in gear its 450-500 bouncing. Vacuum stays around 14 in neutral. I am getting some popping in the exhaust when it starts to warm up and the car dies when u press the gas fast. Is that related to the squirters? Thanks for the help I believe im closer to finally taking this thing for a test ride.

 

[Canadian1968]

sounds like your making progress.

450 - 500 is to low. Going to need to bump that up a bit. You want at least 750 in gear or a bit more if the motor needs it. In earlier post you were questioning if the problem could be transmission related? Is it a hard shift into drive at all? Does the car really seem to surge when you drop into Drive? The converter is stock to the car?? or aftermarket replacement. You could jack up the rear tires off the ground, support well on axle stands , block the front tires. Start the car and drop it into gear, let off the brake slowly and see the rear wheels spin. Give it just slightest bit of gas and make sure it doesn't stall, Eliminates

The accelerator plump is adjusted correctly on carb correctly?? Did the car smell like it was running rich or did you just think it was rich from the plugs?? Don't look at the plugs at all for a few hundred KM , untill the car has actually be driven you can't read the plugs yet.

 

[74sundancekid]

sounds like your making progress.

450 - 500 is to low. Going to need to bump that up a bit. You want at least 750 in gear or a bit more if the motor needs it. In earlier post you were questioning if the problem could be transmission related? Is it a hard shift into drive at all? Does the car really seem to surge when you drop into Drive? The converter is stock to the car?? or aftermarket replacement. You could jack up the rear tires off the ground, support well on axle stands , block the front tires. Start the car and drop it into gear, let off the brake slowly and see the rear wheels spin. Give it just slightest bit of gas and make sure it doesn't stall, Eliminates

The accelerator plump is adjusted correctly on carb correctly?? Did the car smell like it was running rich or did you just think it was rich from the plugs?? Don't look at the plugs at all for a few hundred KM , untill the car has actually be driven you can't read the plugs yet.

Trans was a lil low on fluid so i topped it off. No surge when switching gears. Converter is stock. Accelerator pump currently has a 36 jet size and has .02 gap. Im not sure what rich exhaust smells like, there is mosture when i stick my hand to it (not sure if its gas or just that its 65 degrees in AL. Plugs looked rich but thanks for the tip ill check em again when im on the road. Should i raise the idle with the idle screw until i get 750 in gear?

 

[Black_Sheep]

Do you know what power valve is in the carb? If vacuum @ idle is lower than the power valve rating it will open and your AF ratio will be really rich. Are your float levels correct? When you assembled the engine did you verify the timing marks on the damper and front cover line up with #1 @ TDC? Is the vacuum advance disconnected when you are setting the timing?

 

[74sundancekid]

Do you know what power valve is in the carb? If vacuum @ idle is lower than the power valve rating it will open and your AF ratio will be really rich. Are your float levels correct? When you assembled the engine did you verify the timing marks on the damper and front cover line up with #1 @ TDC? Is the vacuum advance disconnected when you are setting the timing?

Not sure about power valve it was just the one that came in the rebuild kit from summit it did originally have a 25 jet on it so should i change it back to that? Floats have been set and yes damper is lined up, also vacuum advance is disconnected currently

 

[Black_Sheep]

This write up on the Holley website explains power valve tuning a lot better than I can...

How To Tune The Power Valve In A Holley Carburetor