rpm drop when going into gear

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rottenralph

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My Car
1972 Bright Lime green Mach 1. Has 351c 4v heads with Holley manifold and summit 650 cfm carb, power steering, Power disc front brakes, FMX 3 speed auto tranny, and A/C. The car sits on 17 inch Eleanor wheels with 225/45 front and 245/45 rear. Car is nice bust needs some love under the hood.
What causes the big drop in RPM when shifting into gear and what can cause the drop to be greater than it should be. I am having to increase my idle speed in order not to stall them= engine when I go into gear. What causes the drop and how do you address it?

 
An incorrect or bad torque converter, a bad one-way clutch in the torque converter.

An aggressive camshaft will need a higher base rpm.

Idle circuits in the carburetor dirty or idle adjustment incorrect.

Carb calibration.

Secondary throttle plate adjustment.

Vacuum leak.

Bad pcv valve.

 
I was thinking vacuum leak myself. Carb is new and needs some minor tuning. Car idles fine in park but it degrades in gear as car warms up. PCV is cheap so I will change it out for sure, I will also check lines carefully and make sure they are not damaged as well.

 
Last edited by a moderator:
+1 on Don C's post. Given the new carb I'd start there, if it didn't display the problem before the carburetor swap. http://www.holley.com/data/Products/Technical/199R7948-5rev6.pdf

Some drop in rpm is normal with automatic transmission equipped cars of this vintage (extra load on engine when transmission is in drive or reverse). If the carb adjustment doesn't fix the problem you might check the dwell (point gap) and timing, in that order. Chuck

 
I just changed the carb and it did the same thing before the change. I am working my way thru the car trying to get it right. The car runs much better but it is still not quite right.

 
Since it happened with the old carb too, I'm thinking cam related. Vacuum leaks are strong possibility also.

 
I'd look for a vacuum leak or a weak ignition as my first two thoughts. A vacuum gauge will be helpful to see what is going on too. this is a great

page

http://www.secondchancegarage.com/public/186.cfm

How rough is the cam at idle? If it hits a good lick, it will open the power valves in a Holley. The solution is usually to jet up about 4 steps on both the primary and secondary and plug the power valves.

 
Hi,

we got the same problem here with a C4 from a `68 coupe.

Shift in gear the motor drop around 500 rpm.

We rebuild the transmission complete.

The clutch plates are stick or bind together as one unit,I thing that`s the Problem.

A few bronze washer where badly worn.

My buddy like to burn the tires,thats the next problem....:dodgy:

Claus

 
Removing the primary side power valve and jetting up 6 to 10 numbers is a serious drag race only "modification" that I don't think is a good idea even in a drag race environment. Holley makes power valves that have closing points from 2.5 to 10.5 inches of vacuum and a selection of 2 stage power valves for economy and RV purposes. http://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning

The power valve is there to provide additional fuel when a heavy load demands it and not provide too much fuel when it is not needed. Removing the primary power valve and jetting up means you are constantly dumping more fuel than is needed. Mileage suffers, plugs foul, oil is fuel contaminated at a faster rate, and piston ring and cylinder wall wear are accelerated. Most vacuum secondary Holleys do not use a power valve in the secondary side because, unlike a double pumper Holley, the secondary side is only used under heavy load conditions so the problems described above do not occur.

Go to the Holley site and look up you carb by list number. It will tell you most of the calibration settings including the power valve. http://www.holley.com/data/TechService/Technical/Carb%20Numerical%20Listing.pdf As before, check the point gap and timing before opening the carburetor. Good luck. Chuck

 
I'll explain-while removing the power valve is not the first or second choice for tuning-there are times when the power valve opening will not work with a particular camshaft due not only to low vacuum but other factors including overlap. If you choose one that will stay closed during the lopey part of the idle, you may have a lean condition on WOT.

I did not recommend this be the cure, UNLESS the camshaft was one which had an overly aggressive idle

 
I'll explain-while removing the power valve is not the first or second choice for tuning-there are times when the power valve opening will not work with a particular camshaft due not only to low vacuum but other factors including overlap. If you choose one that will stay closed during the lopey part of the idle, you may have a lean condition on WOT.

I did not recommend this be the cure, UNLESS the camshaft was one which had an overly aggressive idle
Click to expand...
I am going to respectfully disagree. The power valve has no effect on idle circuit fuel delivery. Open or closed at idle makes no difference as long as the main booster isn't flowing.

 
From the Holley web site: "Problems:

The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle. If you suspect that your carburetor has a blown-out power valve, you can perform this simple test.

1) Check the manufacture date of your Holley carb.

Click image for larger version.

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Performance Holley carburetors come with a power valve blow-out check valve built in. It prevents damage to the power valve in case of backfire. Holley carbs older than 1992, however, may not have this check valve built in.

2) Test it using the idle mixture screws

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If you still suspect the power valve is blown out, start your engine and allow it to idle and get to normal operating temperature. Then, turn the idle mixture screws all the way in. If the engine dies the power valve is not blown."

The idle circuit never quits. The idle and power circuits are both fed fuel by the main jet. The power valve circuit is in parallel with the main jet. If the power valve is open at idle, additional fuel is feed before the emulsion tubes causing an overly rich mixture at idle. Chuck

 
From the Holley web site: "Problems:

The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle. If you suspect that your carburetor has a blown-out power valve, you can perform this simple test.

1) Check the manufacture date of your Holley carb.

Click image for larger version.

Name: PwrValve-9.jpg

Views: 1241

Size: 19.8 KB

ID: 268

Performance Holley carburetors come with a power valve blow-out check valve built in. It prevents damage to the power valve in case of backfire. Holley carbs older than 1992, however, may not have this check valve built in.

2) Test it using the idle mixture screws

Click image for larger version.

Name: PwrValve-10.jpg

Views: 840

Size: 24.3 KB

ID: 269

If you still suspect the power valve is blown out, start your engine and allow it to idle and get to normal operating temperature. Then, turn the idle mixture screws all the way in. If the engine dies the power valve is not blown."

The idle circuit never quits. The idle and power circuits are both fed fuel by the main jet. The power valve circuit is in parallel with the main jet. If the power valve is open at idle, additional fuel is feed before the emulsion tubes causing an overly rich mixture at idle. Chuck
Click to expand...
Don't want to muddy up this thread but I think we may have to agree to disagree on this one.:D

To be clear my statements regarding the effects (or none effect) of the power valve opening on idle is in the context of a stationary vehicle with the throttle blades closed with a proper amount of t-slot exposure i.e. a properly set up carb at idle. I agree that a blown power valve or a damaged or missing power valve gasket will dump fuel into the intake manifold at idle. An OPEN power valve can't because it only flows when the main flows. The main jet is not the last restriction in the idle circuit. The mixture screws are. You can take the main jets completely out of the block and it will idle just fine.

While I agree wholeheartedly that an incorrectly spec'd PV opening point can cause all sorts of drive-ability and performance issues, it will only do so when the main circuit is active.

 

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