Now I hve read a few of the past posts and I am still scratching my head. The marks on my harmonic balancer are from like 10* bTDC to 20 or so aTDC. I have read in previous posts that people are using 38*. Some how this does not correlate to the shade tree mechanic with a timing light. Am I missing something?

The process as I have always understood it goes some thing like this. loosen the bolt holding the distributor snug. unhook the vacuum advance to the dist and temporarily plug the vacuum line. I like to leave the key off and out of gear for the next part. I climb underneath the truck with a piece of soapstone and mark TDC with a narrow mark and 10* before and after with the soap stone.

Start the truck hook up the timing light to the battery and #1 plug wire. Rotate the distributor until the idle begins to bog down a little and you should see your marks appearing near where they need to be.

How am I doing so far?

So this is the part of the discussion a shade tree mechanic might find useful. 38* is not a possibility as I can measure. First the marks don't go back that far on the harmonic balancer. On this particular engine I am working on now, the cam is unknown I get close to 8* bTDC and I can feel it start to miss. 10* bTDC kills it. So I am experimenting around 4* bTDC. What is the timing for most 360's?

 

Also, at 4* bTDC it does not pass my launch test. This is my semi quantitative test where I take my bald tires on the concrete parking lot outside the shop and stomp on it. As it works through the RPM range it is coughing, maybe ever 1/2 seconds or so.

No coughing earlier with the exhaust manifolds off in the shop. So maybe I am lean? carb out of adjustment or timing?

What happens if I run it a 0* or 5* after where it idles more free?

 

You need to check to make sure that TDC on the balancer is actually TDC for the engine. Pull number on spark plug, pull the coil wire, now put your finger into the plug hole and start bumping the starter. When you get compressed air past your finger stop cranking. Now, by hand continue rolling the engine over with a dowel or a long screwdriver in the plug hole. When the piston quits rising, check the damp to see if pointer is pointing to TDC. From your description, your damper has slipped or your timing chain has jumped. I've run 20° initial advance and there should never be the problems you describe.

Now the 38° issue, You need either a adjustable timing light, which is the easiest route or you can measure the distance between TDC and 10° and the add that to the 20° mark and then measure for 8° and add that to your new mark. Another option is to go to the auto parts store and buy a timing tape and last you can purchase a timing wheel and use it. The 38° number is for TOTAL mechanical advance, that's no vacuum connected and the engine revved up until the timing no longer advances. With a stock dist. that will be as much as 4500 rpm.

 

Checked the TDC and with my screwdriver in the hole method. It is pretty close to the 0 mark at TDC. While doing this I got a closer look and realized I made my marks wrong. DOH! Made new soap stone marks at the right spot. So now is starts missing down around 10* before. I backed it off to around 16. When I plug in my vacuum it sucks it well past 30* at idle. Sound closer to normal now?

Now to tune the carb I guess. What is this Holley stamp correlate and how do i tune it?

1850-5
0890

 

NO! that is not right. You are plugging your vacuum advance into manifold vacuum. You need to plug it into ported vacuum which at idle will have no vacuum on it. What carb do you have?

 

Holly 4 bbl with the stamp 1850-5 and 0890 stamped below it. Manual choke with vacuum secondaries.

The vacuum line hooked up to the dist is coming off the bottom of the carb at the front.

 

Connect the vacuum to the fitting on the right side (passenger) on the front fuel metering block of the carb. It is about half way up the side of the carb. On a Holley any fittings on the carb base are manifold vacuum.

 

Thanks, found it right on the passenger side of the fuel bowl. After I hooked it up & it still sucked it back about the same amount as mentioned earlier. It runs better now, less sputter when I stomp it.

 

Something isn't right. If that port has vacuum on it at idle, you have the carb throttle plate stop on the other side of the carb too far open (screwed in to far). The ported vacuum port should have no vacuum on it at idle as the port in the carb is above the throttle plates and if the throttle plates are closed like they are suppose to be at idle, there can be no vacuum.

 

If it has vacuum on it at idle and it is hooked to the side of the fuel bowl as you indicated. could that meant the carb has an issue?

 

I looks like I posted just after your response, sorry. Can the carb throttle stop plate adjust independant of the idle? If not just dial the idle down?

 

The idle should be set at about 550 for a manual trans and 650 for an auto trans out of gear. What are the idle mixture screws set at. 1 1/2 turns + or - a quarter turn, out from lightly seated is normal.

 

So I adjusted the carb as instructed and it improve the idle some. BUt the timing is still way off the marks on the harmonic balancer. I have checked the tdc and also made sure that I am not 180* off time.

I have read some posts about 390 timing adjust on the cam. I have never seen it but prolly some aftermarket bolt on thing maybe. I have never seen it. The guy who sold me the engine said it was a 390. the stroke and crank stamps say otherwise.

It is 30 over on a recent rebuild. What if they put in the 390 cam advance (which I know nothing about) in on a 360? eh? Maybe? Tell me more about the 390 cam timing thing please. I think they called it retarded cam timing

The reason I suspect this is because I had a new comp cam timing set that I was going to put on while replacing the gaskets, but the gear would not bolt up to the cam. looked way different than the new gear, The pin and holes would not line up. Odd.

either that or something is wrong with timing gear alignment to the crank or harmonic balancer is not true.

It runs right now a gentle touch to the pedal and cough - cough. Although a patient and gradual pressure on the pedal you can gently wind it up and make it sing.

Your thoughts?

 

The only way to check the cam timing is with a dial indicator and the right hand valve cover off. In the truck engines the 360 and the 390 used the same cam so no that is not an issue. The cam timing adjustment you are talking about is accomplished with a crank timing sprocket that has several different key ways in it. Once it is in any given key way the timing is fixed and can't slip except when the chain jumps. When the engine was last rebuilt was the timing gear on the cam all metal or were the teeth covered with nylon?

 

I've been working on a '71 F250 with a 390. It has a Holley 670 Truck avenger carb.
it likes to backfire when deccelerating. I rejetted the carb down 2 sizes (4,300 ft. elev.)
Replace carb manifold and spacer gaskets. When I set the timing, I used a vaccum guage and advanced it until I got the best reading and it idled the smoothest. ( Same process was used when I adjusted the the carb idle mixture screws.)
But the timing, at idle is like 25* advanced and still will backfire on decell. Not nearly as bad as before.
Any ideas?

(sorry for hi-jacking this thread)