# RPMs in gear



## nuggets (Apr 27, 2011)

Can't seem to find anywhere in the forum where people have asked this before. What is the average RPM dip while switching into gear? 69 gto, 400, turbo400 auto trans, original. When switching into drive or reverse, the engine will drop roughly 400 RPMs. It's enough to make it a pretty violent shake in the car. I have to keep the engine idling around 800-900 in park or else the engine will die with the drop into gear. This does not seem normal to me and I was wondering if anyone else have had this problem. Fluid is full, replaced with new filter around 10k miles ago. 

The tranny is oringal and as far as I know, has never been rebuilt. (We got the car at 50k miles) Rear diff has never been touched either.


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## Sick467 (Oct 29, 2019)

Do you have AC? If so and the AC is on, the extra drag of the compressor will drop the RPM's significantly IF you don't have an idle-up solenoid.

I have also known what you are seeing to happen when the engine is tired (bad compression) or if you have a miss in the ignition (one or more cylinders miss firing or not firing at all).

Vacuum leaks could contribute to this as well.


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## nuggets (Apr 27, 2011)

No a/c car, engine has roughly 60-70k on it, and has been in 3 different cars, so tired engine is a possibility.


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## armyadarkness (Dec 7, 2020)

Could be several things.

Mine did the same thing for well over a year. 

Cam? Did you (or the previous owner) put a radical cam in?

Timing is not advanced enough? Advancing the timing will raise and smoothe out your idle.

Vacuum leak?

Misadjusted rocker arms?


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## armyadarkness (Dec 7, 2020)

My car used to die whenever I hit the brakes. The rockers were too tight.

It also had to have a high idle, but I I was able to advance the timing and lower the idle... it kept the same RPM but completely different effect and result.

If your timing isnt right, and you raise the idle, then yes, youre just taking an engine that's not running right and speeding it up. 

What is your timing set at?


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## nuggets (Apr 27, 2011)

Timing is set at 13. I can prob bump it up some more but any higher and I'm hitting roughly 40 at 3k RPM. This is of course all assuming the timing marks are correct on the balancer.

It does have a Crane cam in it. Have no idea what the specs are since it was installed by previous owner. But it's not a radical cam, has a fairly smooth idle.

I actually just recently went through the rockers and pushrods to verify that same question, all seemed fine. No unusual wear or drifting, push rods were all fine, and arms were torqued back down to spec. 

Vaccum leak will be an interesting attempt to find.


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## nuggets (Apr 27, 2011)

I should note, it currently has a Holley 750, old quad was on its last leg so i switched it out to get a rebuild. However, I did have this same issue with the quad. I decided to ask this question after the switch cause I though the quad was always the cause of it


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## armyadarkness (Dec 7, 2020)

nuggets said:


> torqued back down to spec.


This might be an area of concern. Not all rockers get torked to spec. I had been adjusting rockers for 35 years, the same way, and yet I still had a tight one... which caused A CRAP TON of issues.

I then adjusted them with the engine running and a vacuum gauge hooked up, and I gained another 8hg of vacuum!

As for cam timing, dont pick an arbitrary number. Give the engine the timing that it needs. Im at almost 20 base timing and almost 40 all in.


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## armyadarkness (Dec 7, 2020)

vacuum leaks are easy to find. What is your vacuum?


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## nuggets (Apr 27, 2011)

I'll have to get a vaccum guage to confirm. 

As for the rockers and timing I'll try and give it a shot in the next few days. Valve cover gaskets need replaced anyway so might as well while I am in there.

Thanks for the help with this.


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## armyadarkness (Dec 7, 2020)

The gauge wont lie. Grab one cheap, hook it up directly to the carb, and record your reading. 

Then if you do adjustments, you'll know right away if that was your issue. My vacuum jumped from 12 to 20! 

But, do you have stock rockers?


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## nuggets (Apr 27, 2011)

Rockers are stock, yes


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## armyadarkness (Dec 7, 2020)

When you torqued them down, were the valves closed?


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## armyadarkness (Dec 7, 2020)

I have been using roller rockers on everything since the 90's, but I know you should have the cylinder on TDC when torquing down the old bottle necks and stamps. I couldnt say how much it would matter if you didnt, but I can say that the issue youre describing is exactly what I went through when my rockers were too tight.


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## BearGFR (Aug 25, 2008)

First thing, get yourself a good vacuum gauge and find out 1) how much vacuum you have at idle in neutral and 2) what it drops to in gear. Use the gauge to adjust your idle mixture (you'll be adjusting while you're looking for the highest vacuum reading possible). The idle circuits that feed fuel in any carburetor are extremely sensitive to vacuum, such that a significant decrease will also decrease fuel flow and can cause an overly lean condition. 

Are you running a 100% stock ignition system (coil, points, condenser) or has it bee changed?

Bear


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## nuggets (Apr 27, 2011)

Pertronix. 12v from battery with pertronix coil. All plugs have good spark


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## lust4speed (Jul 5, 2019)

Some engines really dislike full manifold vacuum for a stable idle. An engine really likes vacuum advance and with full vacuum in neutral you will be turning down the idle screw to get an acceptable RPM. Then drop it in gear and the idle vacuum goes south along with all that additional timing put in by the vacuum advance. Instead of a small 200 RPM or so decrease you end up dropping 400. Then figure that at 900 RPM idle you are probably supplying a few degrees of mechanical advance that will also go away when the idle drops off. So a double whammy that has the engine idle dropping off a cliff.

Going to ported vacuum advance removes the possibility of dropping additional vacuum supplied RPM because it takes the throttle to be partially opened before adding advance. My engines with larger cams have a much more stable idle with ported advance. Next thing to check is to disconnect vacuum and record your timing at 900 RPM, then remaining in neutral drop the idle RPM down to 600 and see if you haven't lost a few degrees of timing. Most of the distributors I set up start adding timing about 750/800 RPM -- so this will also cause additional RPM drop when those few degrees of timing go away.

Next, any possibility that a larger cam was installed? If so, it might require a higher idle and that in turn would have the engine much happier with a looser converter. On the extreme end my drag car idled at 1,250 and didn't move when dropped in gear, but that converter would be miserable on the street. A stock converter that stalls out as low as 1,600 isn't going to be happy with engine RPM over about 750 without the converter wanting to move the car when dropped in gear.


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## PontiacJim (Dec 29, 2012)

nuggets said:


> Can't seem to find anywhere in the forum where people have asked this before. What is the average RPM dip while switching into gear? 69 gto, 400, turbo400 auto trans, original. When switching into drive or reverse, the engine will drop roughly 400 RPMs. It's enough to make it a pretty violent shake in the car. I have to keep the engine idling around 800-900 in park or else the engine will die with the drop into gear. This does not seem normal to me and I was wondering if anyone else have had this problem. Fluid is full, replaced with new filter around 10k miles ago.
> 
> The tranny is oringal and as far as I know, has never been rebuilt. (We got the car at 50k miles) Rear diff has never been touched either.


First, the idle RPM should be set in Drive - which should be 650-675RPM's. When in neutral or park, the RPM's will be higher because you do not have any load being applied to the torque converter. Once you put it in gear, RPM's will be pulled down.

So what is your RPM's in Drive, NOT Park/Neutral.

Second, if you have AC, Idle RPM's will be more AND if you have an electric solenoid on the carb, it has to be adjusted correctly to get your idle adjusted.

My experience with the "violent shake" or "clunk" when you go from Park/Neutral into Drive/Reverse is an indication that you may have worn out U-joints and they have a lot of play OR the ring/pinion gear has a lot of play/slop. Easy enough to check by grabbing the driveshaft and trying to rotate it back and forth/up & down by hand with the car in Park. Check the rear end by setting the Parking Brake/Chocking the tires so the car will not roll/move, then put the trans in neutral so the driveshaft can be rotated by hand. Then simply rock/rotate the driveshaft back and forth by hand and see how much the driveshaft will rotate back and forth. If the pinion gear/bearing/ring gear are worn, you will get some good movement out of the driveshaft.

So if you find worn U-joints (which at 50K are most likely shot) or the pinion/ring gear are worn from hard driving, the slop will cause a violent bang/shudder when you go from Park/Neutral into gear as the power being sent through the trans has to take up all the slack/slop in the worn pieces the instant you drop it into gear and you get a harsh engagement.

When/if you have to install new U-joints, mark the postion of the transmission yoke as related to the driveshaft, and the rear of the driveshaft as related to the pinion yoke. The driveshafts are sometimes specifically balanced in position and if you do not install the trans pinion shaft in the same position as orientated on its driveshaft, you can experience a vibration as the assembly is now out of balance. I usually just use some white spray paint to make a mark/line on the trans pinion and the along the driveshaft so I can reassemble it in its original orientation. The back is the same just in case the rear end's pinion gets moved/rotated - I can rotate the rear axle to get my marks line up.

Not a big job, but get ready for trans fluid to leak out the tailshaft of the trans once you pull the driveshaft IF you do the job yourself or take the driveshaft to a shop to have the U-joints pressed out and back in.









Pontiac Tune-Up Specs 1955-70


Purchased a 1970 Tune-up Guide for both cars and trucks. Neat little handy booklet. Here are the specs related to Pontiac which spans the 1955-1970 models. Just click on each of the 5 documents to see any of them, then you can expand those for easier viewing.




www.gtoforum.com


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## nuggets (Apr 27, 2011)

Thanks for the help everyone. Quick update:

Stock rockers so not adjustable, but got them torqued down to 20 while each valve was closed. 

Secondly I was able to get my vacuum up. Previously it was 15 at idle in neutral and dropped to 12 in gear. I was able to adjust to 22-18 respectively. Still a little low in my opinion but that was the highest I could get at 800 rpm in neutral and 650 in gear. It increases to 25 at 1000rpm. No longer dropping 400 rpm by switching into gear. Idle is smoother as well. 

Jim hit it on the head too, there is some slack in the driveshaft. It's not terrible but I can deff rotate it back and forth maybe a few cm. Project for another day.


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## PontiacJim (Dec 29, 2012)

nuggets said:


> Thanks for the help everyone. Quick update:
> 
> Stock rockers so not adjustable, but got them torqued down to 20 while each valve was closed.
> 
> ...



Your vacuum numbers are right in line with factory, not going to get too much more.

Idle RPM in gear is right on the money.

You will always have a little slack in the drive line simply due to wear/play - most is normal. However, most factory U-joints don't have grease/Zerk fitting so you can grease the needle bearings inside the U-joint on a regular/maintenance schedule. Once the grease hardens (the older grease is a waxy base) and "goes away," the bearings no longer rotate on the inner trunnion cross and simply wear down the trunnion or the needle bearings go flat. They will still work, but are on their way out and can eventually wear to the point that the U-joint gets real sloppy even as the end caps remain tight in the output shaft/driveshaft ears. The excessive play can eventually cause the U-joint caps to break as the U-joint is excercised to its angular limits by the up/down travel of the suspension changing rear axle/driveshaft angles. Then one day, when you least want it to, (Category 10 Twister taking place in a Category 15 Hurricane while you have your wife's mother-in-law in the car who just noted she had to use the ladie's room while on the longest stretch of road to Vegas where there are no rest areas) the U-joint will fail and you will be on the side of the road ad hopefully did not drop the driveshaft, tear up the floor, and bust the pinion yoke as it twisted free of the u-bolt straps.

So what I am saying is, you may want to add to the to-do list to R & R the u-joints IF they are the originals and have not already been replaced.


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## Sick467 (Oct 29, 2019)

nuggets said:


> ...Jim hit it on the head too, there is some slack in the driveshaft. It's not terrible but I can deff rotate it back and forth maybe a few cm. Project for another day.


I'm not sure how you are measuring the slop in the drive line, but a few centimeters strikes me as "terrible". Maybe you meant millimeters?

My old wore out single-peel-junk-yard rear diff has maybe a 1/2 centimeter of slop (measured at the outer portion of the input yoke) and I consider that bad.



PontiacJim said:


> ...Then one day, when you least want it to, (Category 10 Twister taking place in a Category 15 Hurricane while you have your wife's mother-in-law in the car who just noted she had to use the ladie's room while on the longest stretch of road to Vegas where there are no rest areas)...


Holy Moley, Jim's imagination is HOT today OR he's been there! lol!


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## Sick467 (Oct 29, 2019)

Jim hit on a lot of wear concepts with respect to the drive line (good info Jim)...I will add...any noticeable slop in the drive line just adds to the speed in which it all wears. Every time you clunk into gear, that clunk is hard on the joints and rear end. It's cheap maintenance to service sloppy U-joints sooner than later since the rear end and transmission output shaft takes the brunt of the energy being jolted through the drive line during the "clunk".


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## PontiacJim (Dec 29, 2012)

Sick467 said:


> I'm not sure how you are measuring the slop in the drive line, but a few centimeters strikes me as "terrible". Maybe you meant millimeters?
> 
> My old wore out single-peel-junk-yard rear diff has maybe a 1/2 centimeter of slop (measured at the outer portion of the input yoke) and I consider that bad.
> 
> ...



First, a few centimeters isn't too bad in the scheme of old cars with mileage. You may have crapped your pants if you saw the amount of play in my '68 Lemans 10-bolt pinion gear when I grabbed it to check. An automatic is much easier on the driveline than a manual trans due in part to the "cushioning effect" of the torque converter. Even with all the slop I had in the rear end, it did not stop me from dropping that clutch and smoking tires (non-posi). After a good hard 20K plus miles, it never popped, but I did expect it to at some point. If you have more HP than stock, a posi rear end that works, and wide sticky tires, you should have more than the factory 10-bolt and are only asking for trouble - one reason Pontiac went with the 12-bolt on the 455 GTO's.

Second - nope, no imagination at all. When my brother flies in for a weekend stay, we go to one of the local buffets. He just sits there and watches the show and cannot believe some of the sizes of the women attending. Some can barely walk and shuffle along or hobble to and fro like a weeble. You know a gal is big when she has to sit on 2 chairs - 1 chair for each "cheek," - no lie. It just amazes me at the average size of the woman here in my area. I've seen hanging arm fat that is larger than my head just flapping away in the breeze. If you like "thick" women and BIG butts, come on down. I can handle a proportionate medium build gal whose bone structure is aimed at her size - that gal maybe big, but there is a difference between a big gal who is designed big, and a small boned gal designed for skinny and has an extra 200-300 pounds of sausage biscuits and pulled pork sandwiches that the Creator never designed her for. LOL People in my neck of the woods eat dinner between 8-10PM and then go to bed, wake up in the morning to eat a breakfast and repeat. Does not take many years of that routine to chunk up way past the limits of your bone structures. I always get a tickle when I see mom and younger daughter of early 20's walking together in a mall or store. Here is this attractive, curvy, hot young gal next to the wide glide butt, the waist line fat roll spilling over her pants, chubby chest with barely any boobs because they got overtaken by the layers, and the double chinneck (the chin and neck somehow seem to merge as one, thus the chinneck). I just shake my head and think, that was what mom must have looked like when younger as well, then she suckered in some poor young horny guy and got married, took up eating and cooking as a hobby, had 3 kids, watched soap operas, and never exercised a day I her life - as she grabs a drag off her Salem Light cigarette.

Gotta be honest, when I visit my brother up in New England, I have to take my place at the local mall (or casino we frequent) and just sit and watch and note how much prettier, attractive, and slimmer those gals are. So, I've been there! LOL


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