Big RPM drop from park to drive???
 

I have an RPM drop of around 550 when I put my truck in drive. It seems like a lot to me. To be able to idle with the ac on in drive I have to set the idle at around 1350 RPM. How many RPM should I drop from P to D? What 2-3 things should I check out first?
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Re: Big RPM drop from park to drive???
 

recheck vacuum lines and all of your adjustments. should idle about 750 drop to maybe 650 in gear

Re: Big RPM drop from park to drive???
 

double post

Re: Big RPM drop from park to drive???
 

Stock cam, lumpy cam?

Re: Big RPM drop from park to drive???
 

tight converter?

Re: Big RPM drop from park to drive???
 

Vacuum advance to full manifold or ported? If hooked to full manifold, it could be droping timing with the load in gear.

Re: Big RPM drop from park to drive???
 

I would check for a vacuum leak. You can use carb clean in a spray can. Spray it around all the vacuum lines and intake gasket as well as the carb (make sure to leave the air cleaner in place). If you spay it with carb clean and the rpm's drop, you have a vacuum leak.

1350 is to high for a stock engine. You should get no more than 200 - 300 rpm drop from park to drive.

Re: Big RPM drop from park to drive???
 

Stock Cam

Re: Big RPM drop from park to drive???
 

Ported

Re: Big RPM drop from park to drive???
 

I took some readings today if these help at all -

Idle in park - currently - 1380 RPM (if I go much lower it stalls in drive w/ AC)
Idle in Drive - 790 RPM
Idle w/ AC on, in Park - 1080 RPM
Idle w/ AC on in Drive - 690

Convertor stalls at 1750

Any thoughts?

I will check for vacuum leaks tomorrow.

Re: Big RPM drop from park to drive???
 

What's your timing set at?

Re: Big RPM drop from park to drive???
 

If you have never done so replace the grommet to the fitting going to the power booster, it gets hard and stops sealing well with age.

Re: Big RPM drop from park to drive???
 

I have not, but it feels pretty pliable. It looks like the PO probably changed it. I will check for a leak around it tomorrow.

Re: Big RPM drop from park to drive???
 

Here are some new numbers:

Timing:

-12* initial set at 700 RPM
-20* initial set at 1350 RPM
-28* connected to VAC at 1350
-There is no change when connected to the VAC at 700 because there is no ported vacuum. Should there be some? Is this indicative of a leak somewhere?

Vacuum:

-At 700RPM manifold vacuum is 12" HG
-At 1350 RPM ported vacuum is 6" HG
-At 1350 RPM manifold vacuum is 19-20" HG

Re: Big RPM drop from park to drive???
 

I'm thinking you shouldn't have vacuum at ported source in idle therefore you've opened your throttle plate too much to get the 135 idle and are on transfer circuit. This makes tuning hard. Your RPM drops are likely related to the drop in timing you found. Your mechanical advance is dropping 8 degrees which will give a big drop and I'm not sure about your vacuum advance because your wrote there is no change because there is no ported vacuum, then reported 6" ported vacuum...
I think you need to better stabilize the timing to reduce the in gear drop.
Also, 12" in gear seems a little low for a stock cam, so leak possible. Is your engine older? Possible valve train wear.