# NEW MEMBER: Rough idle after changing dwell angle on 389cu inch. 1966 Pontiac



## Maczuga (Sep 16, 2020)

Hello guys,

I am new here and I apologize if this topic was already solved (tough I couldn't find my exact problem in the forum).
So to let's start with, I am owner of what looks like to be quite a survivor, a 1966 Pontiac Star Chief Executive 4-Door HT with the 389cu in engine and TH400 transmission.
The car was shipped to me (Poland) from the State of Kansas and has probably original 36k miles on it (I searched back all the previous owners, who happend to be two older women and a man). The interior looks very good (tough the dash pad is multiple times cracked), car had one repaint (recent paint is still on lead basis so I assume the job was done at least in the 80s or earlier) and only two corrsion holes, one in the trunk and right quarter panel. As far I can consider (I am no mechanic) the car seems to be all stock and had even original sparkplugs and wires. So no modifications are noticeable.

Now to get to the problem of mine, the car was a runner since the first start I have made, so there are no start up issues. But the problem occures after warming up and leaving the car idling. At the very first time the idle rpm's sat at 1600 and it was very hard to lower them to ~1350 (lowest possible before stalling). This realted to a higher vacuum and resulted in heavy transmission jerks while putting into D or R gear. That's why I gave the car to a mechanic and he did the following things:

1) disassembly, cleaning and changing gaskets of the carb (two barrel Rochester) it turned out there was a vacuum leak right at the base of the carb, at firewall's side,
2) disassembly, cleaning, repainting of intake manifold + valve covers, replacing gaskets,
3) full fluid flush of engine, transmission, powerbrakes, powersteering and cooling system + chaning filters,
4) replacing spark plugs (AC 45RS put in),
5) disassembly, cleaning and reapainting of fuel tank,
6) changing of some cracked vacuum lines around carb,
7) enigne tuning.

The day I picked up the car and drove back home (~20 miles), idling was good at 650 rpms (measured whilst put in D, dashpot unhooked and sealed). I wanted to make sure that the mechanic did everything alright, so I checked my Pontiac manual and looked up for all other things to look after and stumbled upon "engine dwell". I have the "check" window in my distributor cap so I just hooked up my car multimeter and begun to measure while having the engine idling. It clearly stated 14° and had very little play when I changed the motors rpm's (like 13,9 - 14,2°). The manual says that the dwell should be placed at around 30° +/- 2 so I took my alan screw and increased the dwell to exact 30° by steps of 5°- adjusting each time timing back to 6° BTDC . Doing this maneuver I had hooked up all the time my dwell meter and disconnected distributor vaccum as well as cabin+dashpot, all sealed. After reaching the 30° I checked back timing, and after that I looked up the rpm's - and surprisingly they were way off at 1.100 with waves of ~150 up and down.
Readjusting any of the carb screws (fast idling screw or both fuel mixture screws) did nothing. Going back with the dwell also didn't work. Installing a vacuum gauge clearly showed a vaccum fluctuation between 17,5 - 14,0 ins Hg on idle. Tranny still kicks like a horse into D/R
The engine parameters are now as follows:
Timing: 6 BTDC
Dwell: 30°
I have on my own tried to look for some vacuum leaks but found none.
Changed now also all spark wires with distributor cap and rotor.
Engine starts with set choke at once, almost instantly, doesn't even take a full turn of the starter.

Do you have any suggestions what might happened to the engine?

P.s.: I did not check yet the compression of each cylinder (36k miles).
I noticed strange knocking coming off the right side of the engine, maybe below the block, only after warming up and while stronger accelerating the car while driving. The frequency of the noise is constant and does not relay to the engine's rpm. Might that be a loose exhaust manifold heat valve? This noise was present ever since I first drove the car.
I fill the tank with Shell's V-Power Racing (100 octane) + Kleen Flo Nitro Additive
Below: Picture of the vacuum modulator of the TH400, Engine Bay and my first trip with the car

Thank you for any replies and greetings from Poland 

Artur


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