Have a 360 Auto rebuild with the only modification from stock being a Holley 4160, a Holley Dominator intake manifold, dual exhausts and an RV cam. The only thing that I am not happy with is a bad vibration. VERY noticable at light throttle to braking in gear. A little better in park and noticable at different highway speeds. Is this the nature of the RV cam or is there something I can do about it? It just feels wrong. Could timing help? I have a Jacobs Ignition coil, regapped the plugs to .052 and am running a pertronix module in place of the points. What should timing be?
74 F100, 360,PB,PS,AC, AT C6, great shape, stock w/ 82K original miles

Scott

 

Did you do the rebuild? If so, did you change anything in the bottom end of the engine, such as pistons, rods, or crank? If so, it could be out of balance. Also, if you moved any rods or pistons around during the rebuild, that can throw it out of balance too. Not saying that definately is what it is, but that's what it sounds like to me. I usually run 10-12 degrees btdc for the timing.

 

Thanks for the reply. No. Complete motor professional rebuild. Speed shop in FL. Kept my old motor - plan on rebuilding it myself. What RPM do you set your timing at? I am at 16 BTC @ 800 RPM. No hint of a ping. I am going to continue to roll it back until it starts to rattle.

Scott

 

I set mine at 500rpm. If I set mine at any more than 12 degrees BTDC it'll ping at wide open throttle, just after shifting into second. Not where I want it to ping. Anyway, you're sure you got the vacuum advance disconnected and the hose plugged when you set it, right? Otherwise, it'll look like it's advanced a lot farther than it really is.

Edit: I'd contact the speed shop, and make sure they balanced it. If they go "huh?" I'd ask for my money back, or have them fix it and put in a new set of main and rod bearings.

 

Hose all plugged up! I'm going to check it again to see where I am. Advanced it a little more and still no ping.
I'm sure the shop would not admit a mistake even if they made one. I am hoping it is something else. However, if it is an imbalance issue what problems might it cause?

Scott

 

Perform a compression test. It is possible to have a weak cylinder and it can feel like a vibration. A 360 is internally balanced and if the wrong flexplate is used it will vibrate also.

 

If it is imballanced to the point that you can feel it, it'll ruin your main bearings in short order. It could be a wrong flexplate, however they should have taken this into consideration when they balanced it, suggesting that they did not. This too will ruin the main bearings. It is also possible that it's an ignition problem, however the engine power output would be severely reduced if that were the case. On a new rebuild I dont see how it could be a weak cylinder, everything should be sealing properly.

Did they break in the cam or did you? If neither did, it is possible that you wiped a cam lobe, and that's causing a weak cylinder. However, I dont see how any compotent engine builder would send out an engine without breaking in the cam first, or at least giving instructions to do so. Also, power would be down a lot if this were the case. How is the power from the engine, anyway?

 

Power is great. 0 - 40 it has moderate pickup I'm guessing that is in the gearing, 40 -80 runs like a scalded cat. Changed out the 2-in 1-out vacuum modulator and the shifting got much better. Not that it shifted BAD before but it seemed to get into 3rd too soon and didn't have that feeling of being in control.
I had been told by a few people that a different cam in this motor (like the RV that is in it now) may cause more of a vibration since it is working the motor differently. I have no experience with RV cams so I don't know what to think. I am just keeping myself aware of it. I can say that since I regapped the plugs to .052 to take advantage of the Jacobs coil I notice it more - but I also notice the increase in compression and power. Runs great, sounds great, idles great, accelerates great but has that vibration - especially at low speed or at stop.

Scott

 

My brothers had a chibby 350 that they had in a blazer. They didn't have it balanced before putting it together. It too had a vibration at lower rpm's and idle. We sold it though, so I dont know the ultimate fate of that motor. However, what you are describing sounds, to me anyway, like imballance in the engine, either because of changed bottom end parts or wrong flexplate. This will ruin main bearings eventually. How quickly depends on how out of balance it is. I'd take it back to the shop and tell 'em they screwed up and to fix it. Sure, it'll run good for now, until one of the mains goes, and it throws a rod. Not trying to scare you, but this needs to be taken care of.

 

Thanks for the advice!

 

Start simple first and make sure that your motor mounts are tight.

 

In response to the weak cylinder can't be the problem:

Yes new engines can have a weak cylinder if any of the cam lobes go flat or a pushrod gets bent during break in. Also any new parts can fail to seal if not properly installed or the break in isn't done properly. Improperly performed valve jobs are a common cause of a weak cylinder in a newly rebuilt engine.

Did you make sure the pushrods are spinning during intake and exhaust cycles for each cylinder while the engine is running? If any of them don't then you have a problem with a flat cam lobe.

Always perform a valve to seat seal test before installing any rebuilt head. With the head upside down where the gasket sealing surface is level, fill the combustion chambers with kerosene and allow to set for a few of hours. If there is a problem you will notice the level of kerosene will have dropped a considerable amount in the combustion chambers of the poor sealing valves and seats. If your heads don't pass this test and you just paid a machine shop for a rebuild on the heads then go pay a visit to the machinist/machine shop.

Have you performed a vacuum test to see what vacuum your engine is capable of? This will also show a pulsating loss of vacuum if there is a more serious problem. Don't just assum that since your engine is rebuilt that you don't need to perform any compression and vacuum tests.

If any cylinder doesn't deliver the power as the other seven are because of loss of compression then you will feel what seems like fairly consistent vibration because it is essentially a pulsating loss of power. This may not be the problem here but it is a possibility. Hopefully you have a miss caused by a loss of spark and will be caused by only a bad sparkplug or wire.

 

Thanks again for all of the great advice. I have a feeling that I will be a busy boy thgs weekend.

Scott

 

could be the cam is the cam u put in is not stock, i put a modified cam in my 360 and it vibrates on idle and never did it before i put it in

 

Sounds to me like he's got a vacuum leak. An RV cam will not have enough of a "lope" to shake the motor and will smooth out the litttle lope it does have by 2000 rpms. Check all vacuum sources for leaks, if there's none then your intake to head seal could be the culprit. Pull the plugs and see if there's any evidence of oil fouling, a vacuum leak at the head surface should suck in oil from the lifter valley. I really think also that you need to gap your plugs a little tighter ( around .040-045) too