Holley 750 DP idle question

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Walker434

Walker434

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I've got a 71 Dart Street/Strip car with a 414 C.I. stroker and a procharger with a Holley HP converted to blow through carb. Been running this set up since 2019 with great results and doing dragweek a lot. The timing is locked at 20 degrees.

Typically this setup idles great in park at 900 and pulls down to about 750-800 when in reverse or drive. Been like that for years and runs great.

Lately the idle is still good at 900 but when you pull it in drive it pulls down to like 550-600 RPM and wants to die. I'm not sure what changed. I recently pulled the carburator and gave it a thorough cleaning. That didn't change anything.

The other day I put on the vacuum gauge, and it only read 5 lbs of vacuum at idle in park. I thought that was odd, so I started adjusting the 4 corner Idle screws until I ended up at around 10 or 11 lbs of vacuum which is more normal for this cam. after revving the engine a few times, I noticed it was back down to 5 or 6 at idle.

Any ideas? It doesn't seem to have a vacum leak and runs great street driving and at the track. It's just idle in gear that is a problem at stop signs.

Any help is appreciated, I've attached a few pics for inspiration.

Dart 8.jpg


Dart 4.JPG
 
A couple years ago, I was quite surprised to find how much difference a valve body can make to your cars idling manners.....and a 750dp to boot. I think "clean neutral" makes a big difference. The biggest issue with the old VB was dropping into reverse....it died very easily...no more.
 
I'm no expert on setups like yours, and I won't pretend to be... BUT- it seems to me that pulling idle vacuum numbers at idle, with a Procharger, could effectively mask a small vacuum leak. Possibly a sticky blowoff valve not seating?
 
Is the timing still locked? Have you verified with a timing light? The first thing I would suspect otherwise would be a vacuum leak.
 
RustyRatRod said:
Is the timing still locked? Have you verified with a timing light? The first thing I would suspect otherwise would be a vacuum leak.
Click to expand...
Yes, I did verify 20 degrees locked timing the same day I did the vacuum gauge. Yes, I'm curious about a vacuum leak as well.
 
I verified timing becuase it almost acts like one that doesn't have enough initial timing when you put it into gear.
 
1st off, love the car. 2nd, i have no first hand knowledge of how a blow thru carb operates, so i'm just spit ballin' here.

When i saw that you played with the 4 corner idle and saw some temporary results it had me thinking it may have been acting rich? If so, is there a chance your fuel pressure has increased for some reason. Maybe a faulty regulator if your using one?
 
OldmanRick said:
1st off, love the car. 2nd, i have no first hand knowledge of how a blow thru carb operates, so i'm just spit ballin' here.

When i saw that you played with the 4 corner idle and saw some temporary results it had me thinking it may have been acting rich? If so, is there a chance your fuel pressure has increased for some reason. Maybe a faulty regulator if your using one?
Click to expand...
That's actually a purdy dang good thought.
 
OldmanRick said:
1st off, love the car. 2nd, i have no first hand knowledge of how a blow thru carb operates, so i'm just spit ballin' here.

When i saw that you played with the 4 corner idle and saw some temporary results it had me thinking it may have been acting rich? If so, is there a chance your fuel pressure has increased for some reason. Maybe a faulty regulator if your using one?
Click to expand...
No, I’ve got a fuel pressure gauge on the cowl and an air fuel gauge. Pressure is normal and air fuel ratios are in line. Thanks for all the ideas though. This is what I was looking for.
 
The other day I put on the vacuum gauge, and it only read 5 lbs of vacuum at idle in park. I thought that was odd, so I started adjusting the 4 corner Idle screws until I ended up at around 10 or 11 lbs of vacuum which is more normal for this cam. after revving the engine a few times, I noticed it was back down to 5 or 6 at idle.

When you rev the motor you clean the intake floor of fuel puddles and the motor goes lean until the puddles return.
Depending on cam duration 20* initial sounds low to me. I have no idea what timing control you are using to pull or control timing under boost so this is just a thought. All my small blocks like a lot of initial timing with lots of cam duration. As an example I run my [email protected] 340 with 28* initial.
Take a pencil eraser and block off each idle air bleed one at a time with the motor at idle. Motor should go rich each time and that tells if all four idle circuits are working. You can also have someone hold the brakes and adjust the idle mixture in gear. On a street car I always favor the primary idle mixture more than the rear. Example 1 turn out primary, 3/4 turn secondary. It helps the street manners.
All my cars are NA pump gas and .067 idle air bleeds work best for me at sea level.
If your metering block gaskets are old and you've done multiple jet changes you may want to replace them. you can have a vacuum leak there that is hard to find. I also assume your power valve is good.
 
mderoy340 said:
The other day I put on the vacuum gauge, and it only read 5 lbs of vacuum at idle in park. I thought that was odd, so I started adjusting the 4 corner Idle screws until I ended up at around 10 or 11 lbs of vacuum which is more normal for this cam. after revving the engine a few times, I noticed it was back down to 5 or 6 at idle.

When you rev the motor you clean the intake floor of fuel puddles and the motor goes lean until the puddles return.
Depending on cam duration 20* initial sounds low to me. I have no idea what timing control you are using to pull or control timing under boost so this is just a thought. All my small blocks like a lot of initial timing with lots of cam duration. As an example I run my [email protected] 340 with 28* initial.
Take a pencil eraser and block off each idle air bleed one at a time with the motor at idle. Motor should go rich each time and that tells if all four idle circuits are working. You can also have someone hold the brakes and adjust the idle mixture in gear. On a street car I always favor the primary idle mixture more than the rear. Example 1 turn out primary, 3/4 turn secondary. It helps the street manners.
All my cars are NA pump gas and .067 idle air bleeds work best for me at sea level.
If your metering block gaskets are old and you've done multiple jet changes you may want to replace them. you can have a vacuum leak there that is hard to find. I also assume your power valve is good.
Click to expand...


Yep, they run off the puddle. If you don’t account for that you can get in the weeds right quick.
 
mderoy340 said:
The other day I put on the vacuum gauge, and it only read 5 lbs of vacuum at idle in park. I thought that was odd, so I started adjusting the 4 corner Idle screws until I ended up at around 10 or 11 lbs of vacuum which is more normal for this cam. after revving the engine a few times, I noticed it was back down to 5 or 6 at idle.

When you rev the motor you clean the intake floor of fuel puddles and the motor goes lean until the puddles return.
Depending on cam duration 20* initial sounds low to me. I have no idea what timing control you are using to pull or control timing under boost so this is just a thought. All my small blocks like a lot of initial timing with lots of cam duration. As an example I run my [email protected] 340 with 28* initial.
Take a pencil eraser and block off each idle air bleed one at a time with the motor at idle. Motor should go rich each time and that tells if all four idle circuits are working. You can also have someone hold the brakes and adjust the idle mixture in gear. On a street car I always favor the primary idle mixture more than the rear. Example 1 turn out primary, 3/4 turn secondary. It helps the street manners.
All my cars are NA pump gas and .067 idle air bleeds work best for me at sea level.
If your metering block gaskets are old and you've done multiple jet changes you may want to replace them. you can have a vacuum leak there that is hard to find. I also assume your power valve is good.
Click to expand...
Thanks for the reply. I don’t think it’s a timing issue because this car has always liked 20 degrees of initial timing. Thanks for the tip on the idle adjustment and I’ll try the eraser trick. Maybe those metering block gaskets do need replaced.
 
I think I figured it out. Went for a drive. At a stop sign I foot braked the car it would only stall to about 1,900. Floored it and it was a total dog, didn’t even spin the tires until we hit about 3,000 rpm. Torque converter is shot. Probably needs freshened up
 
Walker434 said:
I think I figured it out. Went for a drive. At a stop sign I foot braked the car it would only stall to about 1,900. Floored it and it was a total dog, didn’t even spin the tires until we hit about 3,000 rpm. Torque converter is shot. Probably needs freshened up
Click to expand...
That $ucks. 727 or 904?
 
mderoy340 said:
That $ucks. 727 or 904?
Click to expand...
904. Lenny at Ultimate built the converter for this car in 2018 so I suppose it's time for a freshen up anyway. Does anybody know if he still owns Ultimate? Last time I called them I couldn't get in touch with Lenny.
 
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