Big RPM drop from park to drive??? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1972Blaze · 09-13-2013, 10:41 AM

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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**kwmech** · 09-13-2013, 12:16 PM

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

**kwmech** · 09-13-2013, 12:54 PM

double post

**Ol Blue K20** · 09-13-2013, 01:16 PM

Stock cam, lumpy cam?

Longhorn 70 · 09-13-2013, 03:00 PM

tight converter?

msgdsrf · 09-13-2013, 06:13 PM

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

**TBONE1964** · 09-13-2013, 06:19 PM

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.

1972Blaze · 09-13-2013, 06:37 PM

Quote:

Originally Posted by Ol Blue K20

Stock cam, lumpy cam?

Stock Cam

1972Blaze · 09-13-2013, 06:37 PM

Quote:

Originally Posted by msgdsrf

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

Ported

1972Blaze · 09-13-2013, 06:40 PM

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.

geezer#99 · 09-13-2013, 07:45 PM

What's your timing set at?

swamp rat · 09-13-2013, 09:17 PM

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.

1972Blaze · 09-13-2013, 09:56 PM

Quote:

Originally Posted by swamp rat

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.

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.

1972Blaze · 09-13-2013, 10:03 PM

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

msgdsrf · 09-14-2013, 10:22 AM

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.

09-13-2013, 10:41 AM	#1
1972Blaze Registered User Join Date: May 2013 Location: Covington, Louisiana Posts: 243	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? Posted via Mobile Device Posted via Mobile Device __________________ 1972 K5 Blazer Cheyenne 1966 L72 Corvette Convertible 2016 GMC Yukon Denali

09-13-2013, 12:16 PM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,413	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

09-13-2013, 12:54 PM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,413	Re: Big RPM drop from park to drive??? double post Last edited by kwmech; 09-13-2013 at 01:02 PM.

09-13-2013, 01:16 PM	#4
Ol Blue K20 Proprietor of Dale's Corner Join Date: Nov 2012 Location: Vacaville , CA Posts: 15,888	Re: Big RPM drop from park to drive??? Stock cam, lumpy cam? __________________ "Some Days Chickens And Some Days Feathers" Dale XNGH ECV Sam Brannan 1004 R.I.P. 67ChevyRedneck R.I.P. Grumpy Old Man

09-13-2013, 03:00 PM	#5
Longhorn 70 Registered User Join Date: Dec 2012 Location: Chandler AZ Posts: 1,149	Re: Big RPM drop from park to drive??? tight converter? __________________ 1970 Longhorn, Front Disc, 350/4 bolt, 882 heads, HEI, Edelbrock, 700R4, HO-52/4.11. 1996 Corvette, Collector Roadster, LT4, 396, 450RWHP, 6sp, 4.11/Dana44/posi 5 point roll bar And let us not grow weary of doing good, for in due season we will reap, if we do not give up. Gal 6:9

09-13-2013, 06:13 PM	#6
msgdsrf Registered User Join Date: Jul 2012 Location: White Salmon, WA Posts: 263	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.

09-13-2013, 06:19 PM	#7
TBONE1964 Cluster King Join Date: Nov 2012 Location: Junction City, OR Posts: 5,263	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. __________________ Contact me on all of your gauge cluster needs. I specialize in restoration, repair and parts sales for 67-72 Chevy and GMC trucks. email me at tbonegarris@yahoo.com I am also a dealer for Counterpart for gauge cluster parts only. Also see my facebook page, CG&C

09-13-2013, 06:40 PM	#10
1972Blaze Registered User Join Date: May 2013 Location: Covington, Louisiana Posts: 243	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. __________________ 1972 K5 Blazer Cheyenne 1966 L72 Corvette Convertible 2016 GMC Yukon Denali

09-13-2013, 07:45 PM	#11
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,522	Re: Big RPM drop from park to drive??? What's your timing set at?

09-13-2013, 09:17 PM	#12
swamp rat Registered User Join Date: Oct 2007 Location: Spanaway Posts: 8,451	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. __________________ Mike. Swamp Rat build thread : http://67-72chevytrucks.com/vboard/s...d.php?t=595019 72 3/4T 4X4 4" BDS Lift 33" BFG's

09-13-2013, 10:03 PM	#14
1972Blaze Registered User Join Date: May 2013 Location: Covington, Louisiana Posts: 243	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 __________________ 1972 K5 Blazer Cheyenne 1966 L72 Corvette Convertible 2016 GMC Yukon Denali

09-14-2013, 10:22 AM	#15
msgdsrf Registered User Join Date: Jul 2012 Location: White Salmon, WA Posts: 263	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.



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