Vance,
You're gonna hate to hear this, but check the compression. Sometimes a
slightly weak valve will cause the type miss you describe - or maybe a
tight one.
HTH
Bill '70 6
Navarrette, Vance wrote:
> Folks:
>
> Got a mystery on my hands. I think I can figure it out, but I
>would like some brainstorming help from the list to make sure I cover
>all the bases.
>
> As you know, I got my car put back together ( mostly. =:-o )
>after a complete teardown last winter for paint and a suspension
>rebuild.
> It ran purrfectly before the teardown, but has had a low speed
>miss since being reassembled. The engine, ignition, and carburetors were
>not disassembled or serviced, they were merely removed and reinstalled.
> I can recreate the misfire at will; Put it in 4th gear,
>decelerate to around 1500 RPM, then use light to moderate throttle to
>accelerate. It will misfire 2-3 times and then accelerate smoothly. It
>misfires at other times while accelerating in any gear, but always below
>2,000 RPM. Under these conditions it will not always misfire, rather it
>is occasional. Cruising is smooth and powerful, but in 4th gear at
>around 1,500 RPM it will do it every time. High RPM is fine - no breakup
>or misfiring.
> Items I have done to date: Regapped the champion plugs to 0.025"
>(they are almost new). New points and condenser. Replaced a frayed
>breaker point mounting plate ground wire, reset the timing. Verified
>vacuum retard is working properly, replaced the spark plug wires with
>new ones, topped off the dashpots, resynchronized the carbs, adjusted
>the idle, and reset the bypass valves. The plugs are a nice even tan
>color, and the "lift the slide test" works properly, indicating the
>mixture is fine (?). The diaphragms were replaced when I rebuilt the
>carbs last year, as were all the o-rings and gaskets in the carbs. The
>float level was checked and reset at that time.
> By the way, I am running B1AR needles, not the stock B1AF
>needles, which are about 10% richer than stock. I had to change them
>after I shaved the head, installed a 270 cam, and ported the head. The
>stock needles were just too lean after all the modifications.
> I am now suspicious of the carb dampers. The low RPM
>acceleration misfire has me thinking that the air valves are rising too
>quickly, and the engine goes lean briefly. I am running 10W-30 motor oil
>in the dampers. I am thinking about experimenting with 80WT to test this
>theory. If it is going lean, then 80wt should push it to the other
>extreme and it should go rich, but not misfire. If that works, then I
>can fiddle until I find the right viscosity for the dampers.
> Am I on the right track here? I have wasted a lot of time
>pooping around with the ignition, and now am thinking carbs. What else
>should I be checking? The car looks so sweet with the new paint, and
>it is frustrating that the motoring experience has a fly in the
>ointment. Any comments or thoughts are welcome.
>
> Thanks everyone!
>
> Vance
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