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Discussion Starter #1
Hello guys,
my Le Mans (1968, 400 / 350/ Tripower) runs well, but:
idling at P or N is in my opinion a little bit too fast (tachy: 1200 rpm, didn't know if the display is correct.)
When switching to D or N, sometimes engine stops because of reducing idling, even when power-steering is in action.
It seems to me, that the tranny takes too much power away....
I think, the carb(s) work well, vacuum sems to be o.k., choke is o.k.
Any idea what I can do ? (maybe a problem with the modulator ?)
Regards: Peter
 

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You might want to adjust your fast idle screw, that will be on the choke side and keeps the idle up when the choke is on. Your car should drop Rpms from P & N to drive because it picks up the load from the torque converter. You should let it warm up a couple of minutes in park, then with foot firmly on the break move to drive or reverse.

but not fast as you want a tad warm up, but if the fast idle screw is set too high, the drop to load can be too much. 1200 is not really too high, but you said it may not be accurate. try 1100 or 1000 on the fast idle screw and see if that helps.
 

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It could be too low an idle, but if you don't know if your tach is accurate, that could be an issue if you are going by it.

Air cleaners good? A dirty or plugged air filter can act just as having the choke "on". Those small filters need regular replacing in my book.

Crappy gas from sitting too long or water in the gas?

New fuel filter?

Check the nuts at the carb bases to make sure none have loosened up and you are sucking in air under a gasket.

Bad intake gasket sucking in air.

Make sure you have a working PCV valve and that there is no vacuum leaks. Ensure you have the correct one for the Pontiac as there are a couple different types. What has always been said about PCV valves is that if you shake it and it rattles, it's good. It might be good, but that doesn't mean it's right for your application. There should be enough engine vacuum at idle to suck the valve closed, and then under conditions of more throttle/less vacuum the check valve opens/drops allowing the crankcase to vent to the carb. If the PCV valve is not working, the carb will be drawing vacuum and any adjustments you do to the carb (idle speed screw or idle mixture screws) may have little to no effect on your high idle. You may want to do a simple test and plug the PCV line coming from the carb (thus closing off the vacuum leak) and see if you can drop the idle speed. If it does, take the car around the block and see if it improves things. If it does, the PCV valve is bad or you need the correct one for the car - don't trust the parts store guys to give you the correct one as they often seem to have the line "this should work" down pat when in fact it does not work for your application.

Breather on the valve cover working? Not plugged up with oily grime blocking air flow needed for PCV valve operation.

I would first want to check my engine vacuum, both in Park and then in Drive. Compare the 2 numbers and see what kind of drop you see.

If power brakes, the check valve going into the brake booster could be defective or even the the brake booster. You would typically know this as you would feel it in your pedal action. Take a look at all hoses. I might check engine vacuum while in Park and then depress on the power brakes and hold and see if you get any kind of radical change which could indicate you are sucking air.

Adjust your idle mixture screws in the front of the carb to make sure they are not running too lean. Using the standard Rochester 2 Bbl specs, turn the idle mixture screws in until lightly seated. Then back out 5 turns to make your preliminary adjustment and then go from there following the idle speed/mixture adjustment procedures.

It could be the trans modulator, but I would think you would also be experiencing poor shifting of the automatic. It would not hurt to check/replace all vacuum lines and make sure none are cracked/split or someone simply used a little too large an ID hose and it fits loosely and is sucking in a little air.

Engine timing can also be the problem. Initial timing (9 degrees BTDC with vacuum advance to distributor unplugged and line from carb capped - for '68) could be too low. You could have a bad vacuum advance - reconnect and initial timing should change as engine vacuum adds additional advance. Check points/point gap/dwell (.019" gap/30 degree dwell) if you still have points. Condensors also go bad/weak, as do coils. So engine timing/distributor/spark issues could be suspect as well.

Unless you have a radical cam, which the 400/350 HP really isn't (should be the "066" cam for an automatic), you should not have a 1200 RPM idle. Factory for automatic cars is 500-600 RPM's in Drive, while Manual Trans cars are 650-850 RPM's in neutral.

These are a few things I would first check. It could be several other things, but I would begin with these above. :thumbsup:
 

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Discussion Starter #4
Hello Jim,
Thanks a lot, I'll check out.
Before two weeks, I fixed the 4 nuts on the middle carb, I could easily turn them a half rotation !
After that, the RPM slows down.
I also checked the gas filter, gas should be o.k.

Next time, I'll first lower the idle at the screw, maybe the torque converter sucks too much load at the high rpm...


Then I'll check the PCV Valve ( I had it out several weeks ago, and I remember, that it rattles...) by deplugging and closing the Vacuum-hose to the Valve
-then the check valve at the valve-cover and the brake-booster.


Where ( between which vacuum hose) should I check the engine vacuum and what is the value ?
Thanks, Peter
 

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You may have a “Throttle return Check Valve”,....looks like a little dashpot on the side of the carb. I say may because often they are removed.

But their original design was.....From Factory manual..

.”The throttle return check is mounted on the carburetor and is designed to open the throttle valves to increase engine speed slightly and prevent stalling when engine vacumn drops. It also acts to slow throttle closing when the driver suddenly takes his foot off the accelerator pedal. The throttle return check is standard on most V-8 engines with automatic transmission”

Adjustment...

1) set hot idle and mixture to specs (In drive 600 RPM I think it is)

2) place trans in Neutral

3) With engine running adjust diaphragm plunger screw so that when hose is disconnected and plugged, engine speed of 1050 RPM is obtained

Note: It is very important to hold adjustment screw plunger to prevent it from turning while adjusting contact screw so as not to damage diaphragm inside return check”

So if you are at 1200 in P&N you are close,..150 too high, but that slight adjustment may fix it if you have the throttle return check.:nerd:

:nerd::nerd::nerd::nerd:
 

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"Where ( between which vacuum hose) should I check the engine vacuum and what is the value ?"

Any place you can plug into. You may be able to use the fitting going to the distributor vacuum advance as long as it is a direct vacuum source and not a "ported" vacuum fitting that does not draw vacuum until the throttle blades are opened slightly. Easy enough to test - put your finger over it.

You may have a vacuum fitting on the manifold itself - maybe where your power brake hose attaches. You may have to fashion up a "T" adapter of sorts to give you a fitting to tap your gauge into. Some of the Pontiac manifolds also used a hollow carb stud with a vacuum fitting that a vacuum hose could be run off of.

So you should have some where you can hook a vacuum gauge to.

As far as value, depends on the cam, but seeing you have a stock cam, I am guessing above 15"Hg and up to around 18"Hg at idle. The needle should be steady. Changes in the needle as to whether it remains steady or bounces/swings can indicate engine problems as well. Several guides on the internet and YouTube has a few good videos showing how this is done. :thumbsup:
 

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Discussion Starter #7
Thanks a lot.
I'll check this out, soon, but maybe not before net saturday...too much work.
Regards: Peter
 

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Discussion Starter #8 (Edited)
Hello, Throttle return check valve is installed and it works.
I checked the tachy, it works correct.

I'm not really shure, if the PCV valve work correct, I read many about it.
( it rattles when shaking, when sucking smoothly at the manifold-side, its open, when sucking stronger, it's more open...)
So I'll order a new one and a new grommet.

The net days (when it stops raining...), I'll check out for vacuum leaks by squeezing the vacuum hoses...
(Modualtor and timing advance have no leaks)
Here's a picture with my setup. If there's something wrong, please let me know...
Do I need a PCV valve when having a valve cover breathing cap ???

Regards: Peter
 

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Discussion Starter #9
Some more infos:
I checked the dwell angel, it's about 31°
Ignition timing at idle (without vacuum advance) is about 15° BTDC.
Is this too much advance timing ?
(Motor setup: 400 cui engine from 1970, heads 62 from 1968. Tripower carb, Cam: don't know...).
Vacuum at idle: 7 inHG (needle steadily), at higher rpm: >20 inHG, no vacuum leaks recognizable.

Hope, someone can help....
regards: Peter
 

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Some more infos:
I checked the dwell angel, it's about 31°
Ignition timing at idle (without vacuum advance) is about 15° BTDC.
Is this too much advance timing ?
(Motor setup: 400 cui engine from 1970, heads 62 from 1968. Tripower carb, Cam: don't know...).
Vacuum at idle: 7 inHG (needle steadily), at higher rpm: >20 inHG, no vacuum leaks recognizable.

Hope, someone can help....
regards: Peter
OK, more info. Dwell is OK, factory is 30.

Advance is a funny thing. Each engine can be different, especially once you deviate from stock. If it fires right up and doesn't struggle, it should be OK. Too much and the engine will turn over hard.

First off, if you are running the #62 heads along with stock pistons, you have high compression - probably 10.5.

10.5 compression/iron heads typically requires race gas and/or octane booster - UNLESS you have dished pistons to lower compression in the 9.0-9.3 range.

7" of HG (at what idle speed? 1,200 RPM?) is low. The best I can do is quote from my diagnosis testing from my Chilton's Manual on the 1968-73 A-Body series. Tghis is for a stock engine just for reference. Big cam changes everything. 17-22" HG is normal at idle. Between 10-15" HG, steady but low - late ignition or valve timing, or low compression. 4-9" HG - vacuum leak.

If you are not experiencing "pinging" and you know you have 10.5 compression, then you may have a big cam with a lot of overlap to reduce the "Dynamic Compression," ie cylinder pressure. Big overlap cams reduce engine vacuum.

Big cams will idle higher than stock (Ram Air IV idle was a factory 1,000 RPM's) and will need a higher stall converter so it does not cause you to have to stand on the brake pedal to keep the car from moving when you put it in Drive at the higher idle RPM's. Trying to drop the idle too low for the cam will cause the engine to quit when you put it in Drive.

Next, you probably have a few carb issues that go with the big cam. The carb should have a "power valve" that has a spring designed to open it based on a vacuum signal. If the vacuum is too low, it is not pulling the "power valve" closed as it should and the carb is running rich adding more fuel to the engine. You can get a lighter spring. I think if you check around with carb shops you can find one.

Then, I suspect you probably have to adjust your idle screw to keep the engine running at the higher RPM's so it does not stall out. Doing this opens the throttle blades and exposes the idle circuit slots more than they should be, so adding more gas and more air, and in turn raises the idle.

There are also idle tubes in the carb that may need to be enlarged. Read through this post as it may help: https://www.gtoforum.com/f170/tri-power-timing-questions-120865/


So if you have the high compression and the engine doesn't "ping" when you stand on it, and if you have the big cam that only produces 7" HG at idle, then you may have to focus on the carb and see if you can make a few adjustments/modifications to get the engine to idle lower.

That's my guess from where I am seated. :thumbsup:
 

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Discussion Starter #11
Hello Jim,
thanks a lot.
I read many about the modifications on the tripower carb.
First of all, I'll check all other possibilities out, and maybe examine and modificate the carb next winter.
I have been crewing for more than 30 years, but I'm a rooky in the V8 sceen.
It's hard to find a specialist here in germany (next to my village).
So thanks for your help, all of you.
Regards: Peter
 
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