Thread: Rattling something

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Hello,
I have very many problems :-) Here's another one:

I just rebuilt my first 350 SBC. Nothing very wrong, was rebuilt a year ago, but not moved since, so I replaced one headgasket (something had fallen into the cylinder), left the block as it is and swapped the stock manifold and Rochester Quadrashit for Edelbrock Performers, Air cleaner also. everything else is still stock. the engine ran well, idled nicely (but at 34° BTDC, but don't care about that now). Here is my Problem:

When sitting in the garage and idling, it will gladly take the accelerator right up to WOT, without any audible valve train noise or anything. BUT as soon as it's under load (acceleration of the vehicle) it will start rattling at I guess 2500 RPM. It's a rattle as if something was loose. It doesn't lose power, it just rattles, guite loudly though. I don't think its the valves, what else can be lose which only makes a noise once the engine is under load? Does the Carb have any bits that might rattle when vacuum sinks?
THX!

Mad, make sure that "rattling noise" is not really premature detonation (AKA: ping or spark knock). With that much advance on your distributor, you may be causing the fuel to ignite too early and you're causing detonation before the piston has reached TDC. If you have a vacuum advance distributor, pull off and plug the vacuum line, run the engine to about 2500 rmps, then adjust your timing to about 28 degrees. If you're running less than 10:1 compression, regular pump gas and that advance should be sufficient. Some things to try first is simply retard the distributor a little and add some high-octane fuel (92 octane will do). If the "rattling" stops, you know its due to the advance on your distributor. Also, put a timing light on and change the rpm of the engine (don't forget to remove the vacuum line to the distributor). If the timing fluctuates more than 2-4 degrees, you may have a worn distributor gear or timing chain.

Why you ask would this be different under load vs revving it up in the driveway. Well, a little complicated, but here goes... In a free spinning engine, there are different dynamics on the rotating parts. The rings, pistons, rods, crank flex, are all contributing factors. Under no load, they are free to spin and are generally in the same position throughout the rpm band. When under load, the parts position themselves a little differently and may be just enough for the combustion to act differently. For example, when under load, the crank flexes a little causing the piston to be a little further down in the bore when the spark plug fires (as opposed to when you revved it up with no load). Thus, the timing has changed...you now have more "effective" advance because the piston if further from TDC when the spark fires. Maybe giving you the equivilant of 35 degrees advance instead of 34. Maybe just enough to cause ping.

Another option is to take the hood off the car and strap your neighbor to the fender and have him put his ear in the engine compartment to see what is rattling. Just kidding

Best of luck...

check the flexplate , i have had them cracked and sound like that
kenny

I was also heading towards the flex plate as a problem, but take a look at your torque converter bolts, they can make a nasty noise if loose under load also, but do usually make noise at idle.
A question on your timing numbers, is 34 degrees total timing or at idle?

I agree with TJ, your timing is way advanced.

Total timing in a race car is about 36 deg Max (good starting point anyway). That burning 103 octain fuel. If you are at 34 deg and still have you vacuum and weight systems still intact you could be pushing 40 deg total time. That my friend will burn a hole in you stock pistons real fast.

The reason your engine does not ping at idle or when just reving is pressure. Higher octane fuel resist detonation but increase pressure promotes detonation. A engine with little or no load has much less pressure in the cyclinder because nothing is resisting the piston from moving like the weight of the car.

Flex plate, could be, the sound are very differnt but I have no idea how to explain the differnts. The flex plate has more of a "Tinny" sound to it. Both are simple possible problems and are easy to test. Pop of the dust cover on the torque converter and look for cracks. MOST of the times the crack heads for one of the torgue converter bolts and are easy to spot. After that back off the time like 6 degree and see if there is any change. Dealing with any setup thats not factory you should alway check you timing at the RPM you will push the engine to.