I am getting ready to rebuild my 350 sb. It is a 300 hp from my 69. It had one rebuild on it when I tore it down. It has been bored 0.30 over.

It ran good before I tore it down but I noticed the valves rattling when it was under a load. And alot of times it would continue to run on after the key was shut off. I would have to put it in gear and then shut it off while in gear.I played with the ignition timing with no luck.

It had a pretty much stock cam. With silvolite flat tops. The pistons look like new so I plan on using them again. I had the heads resurfaced. And am installing a comp cams 275 deh cam. Can I expect the same when I get it back running?

What could I do now to help this problem?

your dieseling problems is because the rpm was to hight.get the rpm down to 700/800rpm and it will fix that
Nick

So if I drop the engine idle RPM that will fix my shut off problem? What about the pinging under load conditions?

In all honesty, this sounds to me like your ignition timing is a bit too far advanced. Often, using the high-test fuel will compensate for a long initial timing advance (and allow better performance, naturally). Try using the high octane stuff and see what happens. If that doesn't jive, then just retard your ignition timing a bit and that should cure your problem. Worst case is that you'll have to adjust your valve lash again, but no biggy.

I would definitely be checking the timing, but are you using vacuum advance? I bet so, and if it's an HEI then usually these make too much timing under light and moderate loads. A Crane limiter is a good bet if this is the case.

You also said the pistons look like new....I would suspect with a statement like this that the mixture is a tad lean. A too lean mixture will also add to the "run-on" and pinging. Read your plugs.

Yeah i would agree adjust your idle timing and total timing make sure its good and check the mixture... if you're running big proformance try to keep the idle below a grand that will help as well (some big horsepower cars have trouble idling at what most would consider normal idle (700-900 rpm)

set valve lash to specs.

set ignition timing to specs
then set your idle
the problem will go away

Thanks everyone. I am running a stock GM electronic ignition with vacuum advance. The pistons looked like new on the skirts, the tops actually were a little cruddy before I cleaned them up. No big carbon build up, but I think my carb was running a little rich if anything.

I am running a hydraulic cam also. What would you recommend I shoot for on my timing at idle?

Sometimes, cams that are milder will actually contribute to detonation and run-on. As you increase overlap (with a larger cam), cylinder pressure at low speeds will bleed off. I would start at 8degrees initial and try to get it to 12.

Got to work on that sig file.

I would shoot for initial timing in the 10-12* range (more if it will take it). Initial is not really the concern here, you need to know what your curve is. If the timing is advancing to fast or to much this will cause your pinging. I think the run-on is a result of that. Once the pistons/chambers get hot spots then the car will want to run on. I would look to have a total timing of 32-34* all in by 3000 (if it will take it that soon). These hot spots can be caused by incorrect timing. Reading your plugs is a good idea as well.

I assume you are running the highest octane pump gas you can and that your compression is "streetable"?

I assume you are running the highest octane pump gas you can and that your compression is "streetable"?
Yes I am running the highest octane pump gas. I am going to put my motor back together this weekend.
As for my compression, I don't know what it is. All I know is I am running 0.30 over flat tops with valve reliefs,and a set of 69 300 hp heads. It will have a bigger cam now than what it had.

Cam specs.
Duration at .050 Intake(219) Exhaust(229)
Lobe lift .3077 .3210
LSA 110

Now would be a good time to figure out your compression ratio. If it's too high it won't matter what you do it's going to ping/detonate.

Measure how far in the hole the piston is at TDC, measure (CC) you combustion chambers on your heads and the valve reliefs. If you can't do that at least take the heads to a shop and have them measured. You will be able to get fairly close with that. Since the heads are iron you don't have a lot of room to play around with compression on pump gas.

The cam you have is small and has a fairly wide LSA this will add to the problem if your compression is too much or borderline.

Now is the time to fix it right, not after you put it back together and break a piston or beat the bearings out of it. Detonation is VERY distructive.

If all else fails you can run race fuel, that just makes road trip stay within a tank full of home and fill-ups more expensive.

What is the best way to measure how far down my piston is at TDC? What CC heads did they use on a 69 300 hp 350? I have ran the casting #'s and that's what these heads are. They have been resurfaced.

This is a stock motor other than the flat tops and cam. What type pistons did the 300hp 350 have in 69? And what was it's compression ratio? I just don't see where my compression would be much more than it was from the factory, but I am just guessing.

What can I do short of changing pistons or heads to lower the compression if I find it's too high?

Thanks again for all the help. I want to put this thing together this weekend so I need to figure this out.

If you provide the casting numbers we can figure out what CC the heads started life as. Or you can go to www.mortec.com and look up the numbers.

I like to use a dial indicator to measure how far in the hole the piston is. If you have a dial caliper you can use the rod as a depth gauge as well. If neither are available to you stack some feeler gauges until you are dead level with the deck, you can use a straight edge accross the deck/bore to find the sweet spot.

I don;t know what pistons you have 2 or 4 valve relief, nor do I know how big the reliefs are.

I'm sure some of the guys here know all about that engine. The problem is you don't know how much has be shaved off the heads, and are you sure the block hasn't been decked?

If you weren't already fighting a pinging problem I wouldn't be such a stickler but, a little extra time and effort now will save you headaches down the road. I'm sure your ocal machine shop will CC those heads for you and not charge much at all. It is simple to do and doesn't take much time. You don't have to CC every chamber but I would do the four corners at least.

I don't know the stock compression of the engine you mentioned but, you have to remember back in 1969 you could still buy "good" gas so compression wasn't an issue.

If the 300HP 350 was a 9:1 engine I wouldn't worry too much about it but, I bet it was closer to 9.5 or 10:1 if that's the case then a little shave off the heads and or deck and switching from 4 to 2 valve relief pistons could push you over the top.

My casting #'s are 3947041. Here is a pic of the pistons I have. http://img.photobucket.com/albums/v212/ck-2/2005_0414piston0007.jpg

Those heads started life as 64cc heads. The pistons are 4 valve relief and look a lot like TRW pistons. They are .030 over so you will have 355 cubic inches. You should be in the 9.5 - 9.7:1 range but, without knowing the current head cc and how far in the hole the piston is, it's only and educated guess. Could be as low as 9.3:1 or as high as 10.5:1 (maybe higher maybe lower). I am not sure if you can pull off 10:1 with those heads and cam on pump gas. If it is 9.5:1 or lower you shouldn't have a problem at all. still a few variables that would have to be answered if you want exact numbers.

I looked up a set of Sealed Power pistons with 4 reliefs and they are advertised as 9.72:1 in a 350 with 64cc heads. Typically advertised compression is assuming zero deck. I doubt your block is zero deck but, your heads are shaved (we just don't know how much). That would probably put you pretty close to this advertised ratio.

If that is the case the proper cam and tune-up and you should be able to run on pump gas.

Thanks for helping me, I was told by the guy at the machine shop these pistons are silvolite.