Timing Question

[Larry S.]

I have a '66 427-425HP coupe that I purchased recently and I think that the timing may need resetting as the engine backfires on accelleration and also at idle when revving up. It starts okay but idles a bit rough. I had the Holley carb rebuilt professionally and so I think that is okay (it was backfiring thru the carb before rebuilding it). The jets I have adjusted open about 1-1 1/4 turns.
The distributor is the original as is the carburetor and the engine was recently rebuilt.
Here is my question: The distributor is the breakerless point one that was used on the 425HP engine. When I hook up the timing light, do I hook it up the same as I would on say, a 327-365 engine? What is a realistic advance to initially set the timing at?

Thanks in advance,
Larry

[Timothy B.]

Re: Timing Question

Larry,
The timing light is connected the same as regular point distributor cars. Check your point dwell and set at 30* then set timing at 12BTDC, make sure vacuum advance is disconnected and plugged and idle speed is as low as can be so engine is not idling on the centrifugal curve.

A friend of mine had a 66 425 car and to this day it is the fastest car I have ever been in.

[Duke W.]

Re: Timing Question

Assuming you mean it's backfiring through the carburetor as opposed to afterfiring through the exhaust, or even if that's the case, it could be a minor or a serious problem, so a compression/leakdown test is in order.

Procure a '66 CSM, which has all the service data and a COM if anything needs overhaul. The AMA specs are also very useful and are part of the no charge package available from GM.

You should also search the archives for the L-72/71 conversion from ported to full time vacuum advance. You will surely appreciate the 10 percent improvement in around town fuel economy in addition to the better idle quality and cooler running.

Duke

[Michael H.]

Re: Timing Question

I have a '66 427-425HP coupe that I purchased recently and I think that the timing may need resetting as the engine backfires on accelleration and also at idle when revving up. It starts okay but idles a bit rough. I had the Holley carb rebuilt professionally and so I think that is okay (it was backfiring thru the carb before rebuilding it). The jets I have adjusted open about 1-1 1/4 turns.
The distributor is the original as is the carburetor and the engine was recently rebuilt.
Here is my question: The distributor is the breakerless point one that was used on the 425HP engine. When I hook up the timing light, do I hook it up the same as I would on say, a 327-365 engine? What is a realistic advance to initially set the timing at?

Thanks in advance,
Larry

Larry, one of the first, and easiest, things to check/replace are the spark plugs. It's also the most common cause of misfire/backfire on any high compression big block.
If the carburetor has been rebuilt, correctly, and is functioning correctly, a set of standard heat range plugs should last a long time IF the car is driven at least occasionally.
If the new spark plugs correct the problem, at least temporarily, you have found the results of the original problem.
That could be a too rich fuel mixture or excessive oil consumption on one or more cylinders.
Instect the spark plugs after removing and keep them in order so you can tell which cyl is causing the problem.

Beyond this, there could be several other reasons for the problem but spark plugs will tell a story.

[Joe C.]

Re: Timing Question

I have a '66 427-425HP coupe that I purchased recently and I think that the timing may need resetting as the engine backfires on accelleration and also at idle when revving up. It starts okay but idles a bit rough. I had the Holley carb rebuilt professionally and so I think that is okay (it was backfiring thru the carb before rebuilding it). The jets I have adjusted open about 1-1 1/4 turns.
The distributor is the original as is the carburetor and the engine was recently rebuilt.
Here is my question: The distributor is the breakerless point one that was used on the 425HP engine. When I hook up the timing light, do I hook it up the same as I would on say, a 327-365 engine? What is a realistic advance to initially set the timing at?

Thanks in advance,
Larry

A little more background is in order. Specifically, the exact circumstances under which it spits/backfires,
What is your idle vacuum @ what RPM?
My vote is for "tight valves". Since the motor was just rebuilt, that is the most likely suspect. That is, assuming that the carb was rebuilt properly. If the float level is set too low, and/or the Holley's primary butterflies are exposing too much of the idle transfer slots, then you will have hesitation/backfire during "tip-in".

Joe

[Larry S.]

Re: Timing Question

Okay, thankyou gentlemen, I will investigate further. The car has been a "trailer-queen" after the engine etc., was rebuilt about 6 years ago and has only gone maybe 100 miles since then. I had the carb completely done over as upon inspection, it was not done correctly. I havent had time to go further yet with it, but Thanks again for all your suggestions.

[Ted K.]

Re: Timing Question

Assuming you mean it's backfiring through the carburetor as opposed to afterfiring through the exhaust, or even if that's the case, it could be a minor or a serious problem, so a compression/leakdown test is in order.

Procure a '66 CSM, which has all the service data and a COM if anything needs overhaul. The AMA specs are also very useful and are part of the no charge package available from GM.

You should also search the archives for the L-72/71 conversion from ported to full time vacuum advance. You will surely appreciate the 10 percent improvement in around town fuel economy in addition to the better idle quality and cooler running.

Duke

Can't find anything on the conversion for ported to full time vacuum advance in the archives. What would it be under?
Ted

[Jim T.]

Re: Timing Question

Larry with all the back firing, this can rupture the power valve. A ruptured power valve could be contributing to the engine running rough.
A quick check of the front power valve can easily be done by removing the air cleaner lid and starting the engine. Let it idle and put a finger over the front verticle bowl vent. If the engine dies as soon as you put your finger over the verticle bowl vent the power valve is ruptured and will require replacement.
Easy replacement. A Holley power valve is available in a blister pack. The one in your Holley will have a number on it that will identify which one you need. Since your carb is newly rebuilt you may not need new fuel bowl and metering block gaskets.
Holley also sells a kit to install a check ball to help prevent backfires from rupturing the power valve. The carb has to be removed to make the modification. Newer and current Holley carbs already have the check valve.

[Duke W.]

Re: Timing Question

Can't find anything on the conversion for ported to full time vacuum advance in the archives. What would it be under?
Ted

Do an advanced search, keywords L-71 and VAC under my user name Duke Williams (22045)

Search both the current board and archives.

Duke

[Bill M.]

Re: Timing Question

Larry: Do new plugs first. My '65 396 backfires when it needs plugs.

[John H.]

Re: Timing Question

The jets I have adjusted open about 1-1 1/4 turns. Larry

Larry -

Those are the idle mixture screws, not jets, and they aren't adjusted based on "turns" - they should be set using a vacuum gauge connected to full manifold vacuum, for highest steady vacuum.

[Larry S.]

Re: Timing Question

THanks John,
What kind of vacuum should I expect to get with proper adjustment at idle?

Larry

[John H.]

Re: Timing Question

THanks John,
What kind of vacuum should I expect to get with proper adjustment at idle?

Larry

Larry -

You should see around 12"-14" Hg. or so at 800-850 rpm. I highly recommend changing the vacuum advance source to full manifold vacuum - just tee the line into the existing hose from the front of the carb base to the choke pull-off diaphragm and re-set the idle speed. The added idle timing will also allow you to close down the throttle plates at idle some more, which will help stop the dieseling.

[Stuart F.]

Re: Timing Question

Larry;

The only time you talk "turns" on idle screws is when you do an intial setting after replacing them during carb cleaning/rebuilding. Once you have it running, then you go to the vacuum gauge to fine tune it.

Stu fox