Hello all, I recently rebuild my engine and put a mild cam in.I approximated my timing, started the engine and broke in the cam. Once It started I adjusted the timing by vacuum. Later I got a timing light I picked up at harbor freight and to my surprise found that it's firing at about 25° Btdc @ 900 rpm. It seems to be most happy there. The engine has some shuddering and shakiness to it and I do get a fluter in vaccum, not all the time though, sometimes it runs extremely smooth. Is it more likely that my light is defective or am I having another issue. I couldn't seem to find much troubleshooting advice on a slant firing that early at idle.
Thank you

The first thing I would suspect would be the harmonic balancer. Quick check would be to bring #1 cylinder to TDC (Top Dead Center) and see if the timing marks line up with your timing tab.
If the marks don't look correct, I'd replace the balancer.

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-MattMan
LEANED & MEAN

I'm sorry I forgot to mention that. I did check my marks its TDC is correctly marked.

This sounds more like a misfire, than a symptom of excessive ignition advance..

Having 25* of ignition advance may be compensating for something else being out of tune..

Vacuum leak?

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Official Cookie and Mater Tormentor.

Interesting

I ran a test with propane and didn't seem to get any reaction from The engine. So I believe I do not have a vaccume leak.

Be sure to Set that Initial timing with the Advance can Disconnected.

Also Check to be sure the advance Can on the Distributor works.. Many times I have fund them to be defective.

Have you adjusted the Valves properly hot before setting the timing. Trying to set timing with unadjusted valves can make the process not very effective or productive.


Greg

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http://www.youtube.com/hyperpack

I'm going back and readjusting the valves. I did a hot lash while the engine was off but I don't think I did it very well. I'm new to valve adjustment and I feel I might have done it poorly.

It is a "feel" thing. On my race / hot cams I will get the engine hot and then bump it over with the starter relay or pull the engine over with the mechanical fan when setting Lash.

On a stock engine / cam that will idle 900-500 RPM I will lash them hot / running.... I recently found that a long handled 3/8 box wrench is much easier to use than a ratchet and socket.

Greg

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http://www.youtube.com/hyperpack

I'm still somewhat new at it as well, having only performed the dance with a flathead 6 cylinder, and the slant six a few times each. What I have gleaned from reading about it is this: if you are going to err, err on the side of slightly too loose rather than too tight. I believe that goes for performance camshafts as well as the stock ones.. but others may chime in with the truth.

I adjusted the valves and it seemed to run a little smoother but didn't solve the timing issue. I have also noticed that my mark fluctuates by a few degrees randomly but frequently.

What you might be experiencing is what happens when you run counter to what all the magazines and hot rodders consider gospel.

On my V8's, I run at least 20 degrees advance at idle. I do this because it produces excellent vacuum (or at least vacuum that is as good as it's gonna get) and a smooth idle. That's similar to what you're reporting, though 25 degrees might be a tad much.

The trick, of course, is to retard the timing as soon as you load the engine so as to avoid detonation. You can drop back to around 5-10 degrees then begin building advance back as you gain RPM. Because this is not the way igntion systems engines built in the 1950's and 1960's were designed, people pooh-pooh it.

But....if you start to look at an ignition map on a modern EFI engine, you'll see that is exactly what they do. So why people think you shouldn't do it on an older engine escapes me.

D

Interesting, well it is a mid 70s truck. I rebuild the motor but I kept the old timing chain. It was an aftermarket gear and chain to begin with an the slop was within spec. I read somewhere that end play in the distributor shaft can cause variance in timing, but that was a GM forum. What about those hi po ignition kits they sell on eBay? Worth looking into or garbage?

Static timing is 25 BTDC or atl east that is what the gun is reading.

So I am fairly certain my timing is bouncing because of my mechanical advance weights. Is 900 rpm too early for them to begin advancing?