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Old 04-02-2010, 01:42 PM
Borat drives a Pontiac, U.S.&A
 
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Pontiac 400 Idles to fast

I need some ideas on this problem. According to the tach the engine is idling at 2000 rpm. The tach is good as the engine is roaring at idle. The throttle plate is fully closed and there is a gap between the idle speed screw and throttle stop. Moving the fast idle cam does not help. The idle mixture and fast idle mixture screws are set at 4 turns out. It's a 700 cfm 4 barrel Q-jet. I haven't done any internal work on the carb.

What should I try next?

Thanks,
BT
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Old 04-02-2010, 01:58 PM
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How strong is the spring you are using on the throttle? Mine is doing the same thing i have been trying different springs it's doing better now.

JMO
Cole
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Old 04-02-2010, 02:13 PM
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Quote:
Originally Posted by beertracker
I need some ideas on this problem. According to the tach the engine is idling at 2000 rpm. The tach is good as the engine is roaring at idle. The throttle plate is fully closed and there is a gap between the idle speed screw and throttle stop. Moving the fast idle cam does not help. The idle mixture and fast idle mixture screws are set at 4 turns out. It's a 700 cfm 4 barrel Q-jet. I haven't done any internal work on the carb.

What should I try next?

Thanks,
BT
If I understood you post right, the throttle blades will not close any more than they are now- that is, the throttle blades are fully touching the throttle bores of the carb, but it's still idling high?

Look it over very closely for a vacuum leak would be the next step.

It could be losing vacuum at the carb base, the vacuum hoses coming from the carb or intake. The carb top or base can be loose from the carb body- this will cause vacuum leakage internally in the carb.

If the brake booster is shot, it can be leaking vacuum.

Is the car shifting OK (if AT)? Is the vacuum advance working?

Has the engine or the carb been sitting for a long time? Did it just start this, or?
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Old 04-02-2010, 02:26 PM
Borat drives a Pontiac, U.S.&A
 
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I am using the two stock return springs that came with the car. I applied hand pressure to the throttle to slow the engine down but it didn't slow down.

I will look for a vacuum leak next. Last Fall the engine always idled around 1000 rpm and I never fixed it. This Spring I had the carburetor off and after I replaced the carburetor I noticed the engine idling at 1500-2000 rpm. So this may be related to vacuum leak. I haven't check the vacuum advance or taken the car for a test drive. It is a automatic trans. The car has power brakes so how can I tell if the booster is leaking? Just do a brake check and see if power brakes work? The booster is original to the car 1970.

Thanks for the ideas.

BT
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Old 04-02-2010, 02:36 PM
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Quote:
Originally Posted by beertracker
I am using the two stock return springs that came with the car. I applied hand pressure to the throttle to slow the engine down but it didn't slow down.

I will look for a vacuum leak next. Last Fall the engine always idled around 1000 rpm and I never fixed it. This Spring I had the carburetor off and after I replaced the carburetor I noticed the engine idling at 1500-2000 rpm. So this may be related to vacuum leak. I haven't check the vacuum advance or taken the car for a test drive. It is a automatic trans. The car has power brakes so how can I tell if the booster is leaking? Just do a brake check and see if power brakes work? The booster is original to the car 1970.

Thanks for the ideas.

BT
Usually, just pumping the brakes will give an indication of a bad diaphragm, there will be a "whooshing" sound sometimes, sometimes the idle speed will change, and the brakes won't work as they should- but you'd have to drive it to know that last part. If you DO drive it, be REAL careful that you have plenty of room to stop in the event the brakes give out on you.

I'd tend to think the base gasket might be the fault, or one of the vacuum lines is off or broken/split- that sort of thing.

If you have a vacuum gage, you could hook it up to see if you're getting a low reading.

More on interpreting vacuum gages: http://www.secondchancegarage.com/public/186.cfm
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Old 04-02-2010, 03:52 PM
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Most likley cause is a manifold/carb/vacuum hose vacuum leak.

Pinch off the vacuum hose to the booster with vise grips to see if its leaking.

Stuck distributor advance weights. Broken/jambed vacuum advance.

On some Q jets there is a hot idle AFR conpensation air valve (bi-metal flat spring covering a air port) at the back of the carb body that opens up to allow more idle airflow to increase the engine idle speed and lean the idle AFR when the carb body gets too hot and over rich. (prevents hot engine over rich stalling) If this spring valve is stuck or broke off the port will be uncovered and the idle will be high even with closed throttles.

On Qjet carbs the choke and fast idle system is controled by the secondary vacuum brake pull off diaphram (it also controls the sec air valve tip in delay)
If the diaphram is shot the choke/fast idle pull off will be inconsistent.
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Old 05-31-2010, 02:45 PM
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Quote:
Originally Posted by F-BIRD'88

On some Q jets there is a hot idle AFR conpensation air valve (bi-metal flat spring covering a air port) at the back of the carb body that opens up to allow more idle airflow to increase the engine idle speed and lean the idle AFR when the carb body gets too hot and over rich. (prevents hot engine over rich stalling) If this spring valve is stuck or broke off the port will be uncovered and the idle will be high even with closed throttles.
My Q Jet is a 1977 model. I don't know if it has AFR compensation air valve or not. Does any one have a photo or link so I can see what this looks like?

I did notice two V cuts about 1\16" deep on each primary butterfly. It looks like some one took a triangle file and made these cuts. This would increase air flow and raise idle speed even with butterflies completely closed. Maybe this is what is causing my problem? Why would some one make V cuts in butterflies????

So how do I fix the V cuts; install new butterflies, fill the V cuts with solder and file smooth? Any one know where I can get replacement butterflies? I checked Summit Racing and Edelbrock and they do not list them. I suppose I could get a parts carb at the next swap meet?

BT
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Old 05-31-2010, 02:52 PM
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You can try spraying wd-40 or something similar around the suspected areas...carb, carb/intake, vacuum lines/ports, intake gasket area. That'll give you an idea of where the leak is at.

You "replaced" the carb, or rebuilt the Qjet and put it back on?
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Old 05-31-2010, 04:53 PM
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Quote:
Originally Posted by a1supersport
You can try spraying wd-40 or something similar around the suspected areas...carb, carb/intake, vacuum lines/ports, intake gasket area. That'll give you an idea of where the leak is at.

You "replaced" the carb, or rebuilt the Qjet and put it back on?
I tried spraying WD40 all around the carb and throttle shaft as you mention above but the leak continued.

The engine has always idled too fast since I bought it. I installed a rebuild kit but it still idles too fast and it idled too fast before the rebuild kit was installed.

bt
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Old 05-31-2010, 05:16 PM
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The V cuts in the throttle blades were for increased idle air flow with a big duration cam. Replace the carb.
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Old 06-01-2010, 10:00 AM
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Quote:
Originally Posted by beertracker
Any one know where I can get replacement butterflies?
This carb may have been fubar'ed to the point that it will never tune in for you- there are so many things that may have been done to it, besides just the "V"-cuts, that I just don't know if it's worth pursuing.

Choice is yours, however. If you think all that's been modded on it is just the primary blades, at least you can swap them out.

Get the butterflies or the complete baseplate from an old core carb or the butterflies from Gesslers. You will also need new screws.

To get the old screws out, the back side has to be ground smooth to the throttle shaft because the screws are peened to prevent them from coming loose while in use. They will then not be reusable. A dremel makes this job easy.

If you replace the butterflies, this would be a good time to bush the throttle shaft. Cliffs has the kit for this, as well as just the bushings.
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Old 08-13-2010, 04:49 PM
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still idles too fast

It still idles too fast. I checked all the vacuum hoses, brake booster, sprayed WD40 around the carb base plate gasket and intake manifold gasket and no luck. I was wrong on the V cuts in throttle plates; my mistake.

I have installed a rebuild kit and bushings from Cliff and they worked well. The engine idles great at 1800 rpm and revs nicely. All I need is a stall converter .

I am running the stock smog intake manifold. I wonder if I have a runner crack or leak and spraying WD40 on the intake gasket won't reach the underneath of the intake gasket?

I checked the distributor vacuum advance hose and vacuum advance is working.

When running the engine I run it with out a air filter in the garage. Would the lack of a air filter cause too fast an idle?

I will be in the garage.

BT
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Old 08-14-2010, 04:42 PM
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Is the initial timing too high? If there's a combination of excessive initial advance and the vacuum advance is connected to manifold vacuum, this could be a cause.

Is the mechanical advance working correctly, with the springs able to positively return the weights to closed position at idle? If the springs are too weak, or the advance plate is sticking, there will be too much advance.

Try disconnecting the vacuum advance and plug the hose to see if the idle drops.

What is the condition of the PCV valve and hose and connections? The PCV hose will become hard as a rock over time and will leak at the valve and/or carb end, this is a massive vacuum leak, as well as if the PCV valve itself is bad.
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Old 09-08-2010, 09:51 PM
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I think I have solved the problem!

First, I removed all vacuum hoses and plugged the vacuum source at the carb and all vacuum lines. Next I ran the engine and one at a time I reconnected the vacuum hoses looking for a change in rpm; result no change in rpm.

Next I connected my new digital timing light and measured initial advance at 18 degrees BTDC. Centrifugal advance measured 21.3 - 18 degrees = 3.3 degrees @ 2500 rpm. Both of these measurements seem wrong. What do you guys think? Maybe the weights need a little clean up & lube? These measurements are with stock 6x-8 heads, stock exhaust & intake manifolds, stock HEI, dual exhaust. I don't have a vacuum pump so I can't measure vacuum advance but I know vacuum advance works since I applied mouth suction (Bill Clinton style, sorry) and the idle speed increased. LOL!

Eventually the engine will have 4 tube headers, Summit 2802 cam (2200-5500 rpm),stock converter, 2.78 gears for now, 6x-4 heads milled .040", 8.9:1 CR. What do you guys think as far as timing? I am more concerned about this paragraph setup.

Forgot to mention, the vacuum source is ported (above the throttle plates). When moving vacuum source to intake manifold the idle speed increases.

One last question, why is it necessary to go change to a floating valve train when going to a after market cam & milled heads on a Pontiac block? I thought the hydraulic lifters would compensate for changes in head milling height and after market cam lift??

This forum is great! I have learned a great deal about hot rodding!!

Thanks,
BT

Last edited by beertracker; 09-08-2010 at 10:10 PM. Reason: vacuum source is ported vacuum (above throttle plate)
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Old 09-08-2010, 10:09 PM
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If you are saying you have 18 at idle and only 21.3 w/the engine running at or above 3000 RPM, this is pointing to a non operational mechanical advance mechanism.

This isn't at all uncommon. The ionization of the air inside the distributor cap causes rust to form on all the steel parts, this can cause sticking.

But more likely, is the advance plate bushing is dry and this will cause the plate to become almost unable to turn on the distributor shaft by the force of the weights and/or vacuum advance alone- and if it DOES turn, it will tend to stick.

This would be a good case for a thorough cleaning.
HERE'S an exploded view, along w/THIS description will get you through it, most likely. You might do a search for better instructions, I don't have a "go to" site for that.
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