Stovebolt.com Forums The Shop Area The Engine Shop ? About timing marks

It’s been awhile since I’ve set timing on anything other than my old BMW motorcycle. My 250 online 6 in my c10 has an A at the top and an O at the bottom of the timing plate attached to the block. I’m guessing the A is advanced and the O is for top dead center. I can’t find the answer in my Haynes manual.
Thanks in advance.

I can't think of anything else it would be. That's pretty typical of most GM cars.

If there are no other hash marks, I'd bet A is where it should be set when dizzy vac advance is disconnected and idle speed is set. That's probably 5* BTDC or whatever the engine is spec'd for.

Yes. A is the advance side & 0 is top dead center. Each hash mark is 2 degrees.

George

Thanks, timing is way off. It’s 2 marks past A not 2 marks before O. Could explain why it’s backfiring through the carb.

How many hash marks from 0 to A?

It's probably advanced to far, especially if you're using 87 octane. Mr Google says '67 Chevy 250 should be set at 4* BTDC.

So I’ve got the timing set at 4 degree’s before TDC, point dwell is in between 31&32 degrees, the idle is set at 650rpm. The truck idles great but when I give it gas it will about 1/2 the time back fire through the carb and sometimes through the exhaust. The temp light will also light up about 10 min after starting it. Any ideas what to look for? I was told by someone here in town it could be a stuck valve which I’m assuming a compression check will help figure that out. There is no smoke coming out of the tail pipe.

Sounds like your carburetor may be dumping fuel/leaking fuel bowl/bad accelerator pump.

Assuming your points, cap , rotor, condenser, etc are all in order.

Check your vacuum advance to make sure it's working.

Is the truck actually overheating when the light comes on? Have you checked it with an accurate thermometer or gauge?

Carb is a rebuilt unit I got through a local parts store, it isn’t leaking, the points condenser and cap are all about 1 year old. The fuel pump is only about 1 year old. When the vacuum line is unhooked from the vacuum advance the truck back fires pretty much every time I give it gas and improves a lot when it’s attached. A little over a year ago it all of a sudden started doing this. That’s why the all the parts are about one year old. Started with a basic tune up which didn’t help then I put on the fuel pump. Tried rebuilding my old carb but it needed expert help so I bought one already rebuilt and installed it. Other things in life got in the way and now I have some free time to work on it. But I am getting pretty frustrated with it. I’ve even talked about selling it but my wife says no.

Take a step back and double check the easy stuff.

Is the sparkplug firing order right?

Are the spark plug wires new?

Are you capping the vacuum line when you check the timing?

Confirm the distributor centrifugal weights are working, when you rev the engine does the timing mark advance while aiming the timing light on it?

Condensers can be bad right out of the box...replace it and see if anything changes.

Mike B

That's a lot to unpack. Don't give up, though.

Like Mike said, don't rely on new parts to function as "new". They are often junk right out of the box. That goes for ignition parts, fuel pumps and carburetors. I've had two brand new fuel pumps fail on me, one wouldn't pump right out of the box. Both made in China.

I bought a new carburetor for my Mustang before I realized it was a Chinese-made piece of junk. I should have rebuilt the original. An NGK condenser failed after only 1500 miles.

Only check/test/change one thing at a time or you'll throw in too many variables to narrow down the problem.

Once you have all the basics Mike mentioned confirmed - not assumed - to be functioning perfectly, then we can better move forward with further tests.

Your timing is too slow. Advance it about 4-6 degrees & try it. If it pings on heavy throttle set it back a little.
Listen to your wife. Don't give up yet. It idles smooth so the valves are probably ok & the wires are not crossed.

George

Find the vacuum leak and fix it. You're running very lean, possibly into "imminent piston meltdown" territory.
Jerry

Could I possibly have the idle mixture screw adjusted wrong. Ive never been great with adjusting carbs.

After today I won’t have much time to work on it for a couple of weeks.

Not likely. That only controls the mixture at idle not during regular driving

HRL makes a good point about checking for vacuum leaks.

While the engine is running, take a can of carburetor and choke cleaner and spray it around the base of the carburetor and any other potential area for a vacuum leak.

Spray it around the joints in the carburetor, both ends of any vacuum lines connected to the carburetor or manifold, etc.

You can also use an unlit propane bottle to do the same thing as the carb cleaner. If you pass over a vacuum leak, the engine rpm will change as the propane or carb cleaner is drawn into the intake manifold.

I agree with Mike. Check the plug wires. Mine sounded like it was idling perfect but I would get some back fires on acceleration. I had a couple wires crossed. I got the corrected and problem solved.

I believe the problem has been found. The vacuum hose from the vacuum advance to the carb was leaking. Replaced it and seems to cleared up my problems. It still will backfire every once and awhile but not often.

It should never backfire. You may have found the main problem but not all of them.

Keep searching for more vacuum leaks and check all the other stuff we mentioned.

Otto is right. I still think you need to try advancing the timing a little.

George