'65/389 idle way to high? - Pontiac GTO Forum
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post #1 of 12 (permalink) Old 02-24-2017, 03:29 PM Thread Starter
 
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'65/389 idle way to high?

I finally fixed my water pump leak and fired up the GTO for the first time in 2-3 weeks. Idle was off the charts at 1500rpm! Vacuum advance on HEI was blocked off. Backed off the idle screw completely, no change, adjusted timing-no change except when going way clockwise (retard) and still too high, adjusted air mixture screws all the way in and out one turn increments out to 3 turns-no change. I'm thinking at 1500 rpm centrifugal advance has already kicked in and carb is beyond the idle circuit. The 389 has been bored 30 over, a mid range power cam 2000-4600, Edelbrock Performer intake, restored '68 Q-Jet, Hooker Headers, TH400.
Could it be weak advance springs or vacuum lines incorrectly attached? A good vacuum diagram for '68 Q-Jet would be good. Any advice would be greatly appreciated! Thanks!
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post #2 of 12 (permalink) Old 02-24-2017, 07:23 PM
 
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How is your throttle linkage? The throttle spring. Check for vacuum leaks. I can't see how changing a water pump would have anything to do with the idle speed.
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post #3 of 12 (permalink) Old 02-24-2017, 07:52 PM
 
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Garage
Sounds like a carb problem, not a distributor problem. First, make sure your choke is wide open and that the choke linkage is not set on the high idle side. You should see a screw adjustment for this as well as the fast idle screw - 2 different screws & adjustments if you hav this on your carb. Make sure the cam/arm is not hanging up or not dropping down like it should to kick down the choke and lower your idle speed. Here is a You Tube video:

Sometimes the throttle cable can be too tight and keep the primaries on the carb from fully closing. Quick check would be to undo the cable and fire it back up. If it idles down, then adjust the cable to give it a little more slack.

Wrong carb gasket on the base allowing either the carb to not fully close or allow a vacuum leak. There are several gaskets that fit and you have to be sure you have the correct one. There could also be a vacuum leak somewhere in the intake system, ie the carb, a vacuum line, even a bad intake gasket - all can let more air in and raise idle speeds. I have never done this, but it is said you can use a small hand held propane torch to run it along these various areas to find any vacuum leaks. If the engine speeds up while running and doing this test, you found the vacuum leak.

Bigger cam will sometimes affect the metering rods within the carb. There is a spring found under the power valve - which is controlled by engine vacuum. Less vacuum will allow the spring/rods to be in the raised position allowing a richer mixture, where they would normally be pulled down into their lowered position for a factory cam/strong vacuum signal. They make weaker springs that can be swapped out to match the lower vacuum signal and drop the metering rods into position to run leaner at idle. The top of the carb has to be pulled off to access the spring.

Is the carb flooding? Too high a float setting could be pushing some extra fuel into the carb causing it to run richer/faster.

Retarding your timing should drop idle speed, but retarding it too much will make your engine run hot. Set your timing to factory specs and then do some testing with the carb as it does sound more like a carb/vacuum issue rather than timing at this point.
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post #4 of 12 (permalink) Old 02-25-2017, 02:54 PM Thread Starter
 
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Good points. I thought at first it could be a sticky throttle linkage but it seems to be ok. The engine was showing 20" of vacuum and have not yet found a vacuum leak but will use propane to confirm. A couple things I did find today was when I took the distributor cap off the rotor button was 180 degrees off... don't know how that happened since the HEI w/vac canister will only allow about 20-30 degrees of rotation. Also, the mechanical advance springs are very weak and do not fully close the arms. I've ordered new springs and will put them in next week and reset distributor to #1 cylinder. I'm still scratching my on this one. Thanks, Roqetman.
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post #5 of 12 (permalink) Old 02-25-2017, 03:00 PM Thread Starter
 
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PontiacJim, Good advice, thanks for the videos. I'll check choke fast idle cam and vacuum leaks using propane. As I indicated in my reply to Extinctmake, I found the rotary button off 180 degrees when I pulled the cap this morning, don't know how that happened??? Also, the mech advance springs are weak and do not fully close the arms. I'm going to fix both these issues next week and post my findings. Thanks, Roqetman.
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post #6 of 12 (permalink) Old 02-27-2017, 04:30 PM
 
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Roquetman:

Sound advice from PontiacJim as always. I agree that first thing to check is Choke fast idle cam and choke operation. Once that ok then look elsewhere. Vacumn leaks are possible, but with a steady reading of 20 hg, not the first concern

your Distributor weak springs and losse waits are giving you too much centrifigal timing at idle. You should have no Centrifigal timing at idle and springs should fully retract weights. this clearly additional timing can make the engine run faster.

Try this, remove dist cap and just take off weights and springs. then replace cap and set timing at 10 BTDC or 8 if you like, see what the idle is there if it is better. You need those replaced. Clearly that is wrong.

Weights should only come in above idle? your dist vac can should be disconnected and vac to manifold plugged to check timing?...do you run full manifold vac or Ported to Distributor?

it matters because timing is pulled in there as well depending on vac can...

you will get it!!! hang in there
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post #7 of 12 (permalink) Old 02-27-2017, 04:35 PM
 
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PS a rotor off 180 degrees will change your timing about 13 degrees.........so that matters!
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post #8 of 12 (permalink) Old 02-27-2017, 05:22 PM Thread Starter
 
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I'm embarrassed to say this but... I apparently set the dist in the engine at top of exhaust stroke instead of TDC, hence 180 off. I'm installing new springs tomorrow and of course be sure to set dist at TDC of compression stroke and adjust to 10 BTDC. From there I can check for vac leaks and fast idle cam. I'm running ported vacuum for vacuum advance, connected to driver side of Qjet above the throttle plates. As for other vacuum lines. The PB, PCV are connected to directly to manifold vacuum. The transmission vac is connected to rear base of the carb, below throttle plates, is this an appropriate port for the Trans? And the AC vac line is connected to the front passenger side at base of carb. Any advice on vacuum line placement with Qjet would be appreciated. Thanks for everyone's advice! I'll keep ya'll posted.
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post #9 of 12 (permalink) Old 02-27-2017, 07:02 PM
 
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Roqetman; that will all work ok. The easy xl for manifold vac is just to pull line at carb base at idle and see if it is pulling vac,....then it is full.

I think your Dist being 180 off giving 13 degrees off on timing and loose springs and weights could have been giving lots of idle advance making it run real fast......

Using ported vac assures you will have no vac advance at idle, so for now that is good.

I like full manifold vac to the vac can and pull in more timing at idle, but that is a topic for another day.

Right now you want to get a normal idle and spark, and get her running. Don't be embarrassed we have all done much worse stuff!

Make sure when you set the Dist in that the "dimple" on the rotor gear on the bottom of Dist is in straight up orientation to the rotor tip, sometimes this is missed and also throws timing off. It is an easy visual check if the Dist is out of the engine.

Also make sure you trace at least twice to verify where you think #1 plug terminal is on the cap, that is easy to mix up as well, just make sure.

I know you will get it and your car looks awesome!
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post #10 of 12 (permalink) Old 02-27-2017, 07:07 PM
 
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Also I wanted to tell you that sometimes weak throttle return spring messes you up and won't close the throttle enough, just make sure that is correct and in good shape, should close throttle strong back to idle....
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