|09-08-2010 11:22 PM|
The 18 initial is a starting point, the exact number will depend on the cam specs mainly, to a lesser degree the CR (if higher than stock).
The amount of mechanical advance available depends on the configuration of the advance weights and the "cam" that bears on the weights. The RATE of mechanical advance is controlled by the springs. You want all the mech. timing in by at the most 3000 RPM and less if the engine will allow it w/o detonation. That will depend on the weight, use, rear gears, etc. of the vehicle.
I would use about 34 degrees of total timing. With 18 degrees coming from initial, that leaves 16 degrees that has to come from the mech. advance.
What you will need to do, is clean it up, get the mechanical advance operational and recheck how many degrees it is allowing, and tune it from there.
There are different ways to go about adding or limiting advance from the mechanical advance, but see what you have first. You may not need to actually modify the mech. advance- there's leeway in what the initial timing can be, and that may be all you need to work with to get it dialed in.
Also you will want the vacuum advance to add about an additional 10 degrees when at idle and/or light throttle cruise conditions. You can cross that bridge after determining where you are w/the mechanical advance.
|09-08-2010 10:48 PM|
Correct Timing Info
What are the proper timing parameters as far as initial and centrifugal? I will clean up the weights and re-measure.
|09-08-2010 10:09 PM|
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.
|09-08-2010 09:51 PM|
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!!
|08-14-2010 04:42 PM|
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.
|08-13-2010 04:49 PM|
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.
|06-01-2010 10:00 AM|
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.
|05-31-2010 05:16 PM|
|F-BIRD'88||The V cuts in the throttle blades were for increased idle air flow with a big duration cam. Replace the carb.|
|05-31-2010 04:53 PM|
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.
|05-31-2010 02:52 PM|
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?
|05-31-2010 02:45 PM|
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?
|04-02-2010 03:52 PM|
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.
|04-02-2010 02:36 PM|
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
|04-02-2010 02:26 PM|
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.
|04-02-2010 02:13 PM|
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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