Engine: Avanti 327 trouble

[X-she]

Hello, since many month i've trouble with my 1967 avanti II, she's got SBC 327 Hi Perf 300 hp, holley 3810 and standart Automatic trans; in fact when ther car is running and when i do acceleration, i 've got misfire like a hole not a back fire carburetor ; so i've rebuild the Holley with a basic rebuild kit and nothing change....
Thanks for your help.
David from France

[christophe]

[TDC71]

Say what?

[Pat Dilling]

Have you replaced the basic tune up items, that would be spark plugs, points, condenser, distributor cap and spark plug wires? Have you done a compression test? If I understand correctly the misfire is there under all running conditions is that right?

[Mike Van Veghten]

When you replace the points/condensor...make sure that the plate the points attach to, is clean, dry and most of all...tight, not sloppy and loose.

Also...by your description...the timing chain/upper sprocket (gear!) might be trash. This causes a big change in cam to crankshaft timing and will cause a backfire at any given time.
After you do the basic tunup that Pat mentioned, with a timing light connected, raise the rpm to about 2000 and and let it settle back down. The timing mark as shown on the damper...should move very smoothly up and down with the rpm change.
It jumps around and the point plate is tight....it's time for a new cam timing set.

Mike

[christophe]

Say what?

Sorry. I thought it was more convenient to use French for this reply. I told him to check the fuel pump pressure, the float level of the carb and the function of the accelerator pump. I told him too that a tired distributor cap can give the same symptoms.

[X-she]

[Gunslinger]

Just throwing this out...check for a cracked distributor cap or one that's carbon-tracked between towers. If a full tune-up is done, the cap would be replaced anyway.

[55coupe]

Hi here is something else to look for. About a hundred years ago my little 265 chevy did the same kind of thing drove me crazy. After much replacement of tune up parts and no improvement I was ready to quit when a friend said have you checked the wire under the advance plate in the distributor? That little guy can crack and only be a problem under vacuum advance. It goes form the points to coil if I remember , later the same thing happen to mom's vega. Good luck

[christophe]

Thanks for your kind words David.
One more thing that I should check is that the advance weights of the distributor move freely and progressively. I suppose you have a window distributor, so it should be easy as the weights are at the top. Of course, you can also use the timing light for this.
Nice day to all.

[Hawklover]

This reminds me of a situation about 40 years ago.

The R-1 Stude would not start.......looked everywhere tried everything (so I thought)...........no start.
Just for the heck of it as an after thought I started playing with the small wire riveted to the breaker plate.
Long story shortened..........the copper conductors under the cambric insulation were separated, not all but a good number.....when I played with the wire she would fire, then quit........as a fast down and dirty I cut the wire in two, skinned the cambric back, took a short piece of copper wire and twisted it onto both sides of the breaker plate wire.........a miricle!...........that lasted until I pulled out the dist and put in a new breaker plate with wire.

[Gunslinger]

Does the car still have the ignition shielding in place? If so, as unlikely as this sounds, it could be a similar problem I had with my '69 Corvette 427. The coil wire between the coil and distributor tower had a pinhole in it. The spark started jumping to ground by arcing to the shielding cover. When I took the shield off the car started and ran fine...when I placed my hand near the coil wire a spark jumped to my hand...I think I needed to go in a dark room and see if my gonads had turned to night lights! That was some spark!

I replaced the coil wire and no more problem. Check to see if your spark plug wires are allowing a spark to jump to ground...you'll definitely get a misfire.

[1962larksedan]

Sorry. I thought it was more convenient to use French for this reply. I told him to check the fuel pump pressure, the float level of the carb and the function of the accelerator pump. I told him too that a tired distributor cap can give the same symptoms.

No apology necessary if it means keeping another Stude type on the road overseas.

[Ed Gray]

I've have a somewhat different situation on my 1980 Avanti II, the engine misses at idle after warming up. I replaced the CA smog SBC with a new Goodwrench engine around 15 years ago and it has all new plugs, a newly rebuilt carb, and a new EGR valve. The indicated vacuum is OK. Any likely causes?

[Pat Dilling]

Ed, a compression check is always a good diagnostic method. Also, using an insulated set of pliers, pull one plug wire at a time and watch the tach to see if there is a cylinder that does not show a drop in RPM when the wire is pulled. When you changed the plugs did you notice if any looked as though it had not been firing? Is it a steady miss or intermittent?