[updated:LAST EDITED ON 02-May-02 AT 10:25 PM (EST)]I purchased my truck used with a rv cam already installed. 6-8 degress timing is stock. How do i figure out the new timing?
76 ford f100 390 4x4 4bl headers / intake
gary

 

i would suggest 10-14 with vac plugged to dist

 

[updated:LAST EDITED ON 03-May-02 AT 06:47 PM (EST)]An engine is basically an air pump. The more air pumped in the more power it makes. This is why using a higher lift/longer duration cam makes more power. The engine can move more air so it makes more power. With this in mind it tends to reason that the best setup for timing and fuel mixture is the one that yields the most vacuum at idle.

There are two ways to find the optimum start setting and spark range (or initial and advance timing). The first is to use a vacuum gauge and tachometer. Warm the engine to operating temperature and plug the vacuum line that runs to the distributor housing.

Now set the engine idle rpm’s to the corrected sitting listed by the cam manufacturer. You said you bought the engine with the cam installed so I am guessing you don’t have the cam card or the relevant information necessary for this step. That’s ok though because we know it’s an RV cam and these are normally just a little more aggressive then stock profiles. Setting the idle rpm for this cam should be very close to stock. So 600 to 650 rpm’s for an auto or 650 to 700 for a manual should be just fine for your engine.

Now on to the vacuum gauge, there are two types of vacuum sources in a carbureted engine, venturi and manifold. Some will say carb and manifold others might call it port and manifold- it’s all ta-may-to/ta-ma-toe. The difference between the two is where the engines vacuum is measured. Venturi vacuum is measured above the throttle blades, while (you guessed it) manifold vacuum is measured below them or from the manifold. This is important because as engine rpm’s go up the venturi vacuum increases and the manifold vacuum decreases. Because we want to increase the spark advance as the engine speed increases we want to use the venturi vacuum for the distributor connection.

At idle there is very low or no venturi vacuum and high manifold so it’s manifold vacuum that we will want to deal with for setting the initial timing. Find a source of manifold vacuum that you can hook your gauge up to. Usually there is a tree or other source that you can uncap a port from. The idea is to move the distributor clockwise (that’s the direction for advancing the timing) while watching both the rpm’s and vacuum gauge. There will come a point (usually around 20-25psi for a stock engine) that the engine will pull max vacuum at X rpm’s. This is where you want to stop and lock the distributor.

Even though you can advance the distributor another 2 or 4* without the rpm’s falling and still hold max vacuum, you don’t want to do that because you will run into “ping” even at low rpm’s. The second method I mentioned is called the “ping” method. It takes a lot more time and requires the use of a good hill but it works.

The idea here (using the hill) is to give your engine an working load by going up the hill, and not just a straight line load by driving down the street. Start at the bottom of the hill and set your timing at 18* with the vacuum connected to the distributor. Go up the hill and when you’re in second gear give the gas a hard push to the floor and listen for ping. If you don’t hear anything reset your timing up 2* and try it again. Repeat these steps till you hear the engine pinging and then retard the timing 2* and try it once more. If the engine doesn’t ping after you retarded it, and retested it, lock the distributor down. If it does ping chances are that the engine got a little hot. Let it sit for a couple of minutes and retry it, it shouldn’t ping after its cooled back down to the normal temp range.


There ya go, two completely different methods of achieving the same goal. In all honesty the second method (hill climbing) is the least scientific but it is a lot closer to real world driving and probably will give the best results for performance and economy

 

Thanks...it turned out doing a vacuum timing im right at 14 ...but on a vacuum testing the needle doesnt stay steady..is this a proplem?
gary

 

 

. Um maybe, maybe not lol, I’ll explain. Most RV cams are what is knows as dual profile (No they don't need a shrink to feel right, just talk to your engine once a day and it will love ya).

What dual profile means is that the intake is ground one way and the exhaust is ground another. The cam profile card might say something like 270* duration Intake (advertised) and 280* duration exhaust (advertised) with .494 valve lift Intake with 1.71 rockers and .510 valve lift exhaust with 1.71 rockers 110* cam centerline.

As you can see the intake and the exhaust are not ground equally. The intake as a little less duration and lift compared to the exhaust. The reason why they (those crafty cam manufacturers) do this is to help the engine exhale and breathe.

If you think about it the engine only has to draw threw a paper filter, the carb, the manifold and finally the intake port. All told that's about 15 to 20 inches of total travel. In contrast the exhaust must go almost the full length of the car before it's out in the open so to speak. This is why most (some people just can't take a hint) hot rodders and racers will tell you the fastest way to increase power is to work on the exhaust.

It's because of this dual profile that you will see a slight fluctuation in the vacuum gauge. That's the good news lol, now the bad news.

You didn't say just how much your gauge is bouncing around? As shown above this could be the nature of the beast because of a dual profile cam, but it could mean a few bad things too. It could mean, a stuck valve, bad lifter, bent push rod or a broken spring. All of these are (has Martha Stewart would say) "Are a bad thing.”


Here is a chart on what vacuum readings can mean.

PLEASE understand this. I Honestly feel your readings are from the dual profile cam. I say this because the engine was just rebuilt and you only said you have a small fluctuation (I am assuming by small you mean 1 or 2 psi). If they did a good job it’s hard to believe that one of these might be a problem.

Smooth and steady idle(800 to 1200 RPM) Between 17 to 21 inches Engine is in Good Condition, but perform next testto be sure.

Open and close throttle quickly Jumps from 2 to about 25 inches Engine is in Good Condition.

Smooth and steady idle Steady, but lower than normal reading Worn rings, but perform next testto be sure.

Open and close throttle quickly Jumps from 0 to 22 inches Confirms worn rings.

Steady idle Intermittent dropping back 3 or 5divisions and returns to normal Sticky Valves. If injection of penetrating oil intointake manifold temporarily stops pointer fromdropping back, it's certain the valves are sticking.

Steady 3000 RPM Pointer fluctuates rapidly, faster enginespeed causes more pointer swing Weak valve springs.

Steady idle Fast fluctuation between 14 to 19points Worn intake valve stem guides. Excessive pointervibration at all speeds indicates a leaky head gasket.

Steady idle Constant drop Burnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally missfiring.

Steady idle Steady 8 to 14 inches Incorrect valve timing. It must also be rememberedthat vacuum leaks and/or poor compression canresult in a low vacuum reading.

Steady idle Steady 14 to 16 inches Incorrect ignition timing.
Steady idle Drifting from 14 to 16 inches Plug gaps too close or points not synchronized.

Steady idle Drifting 5 to 19 inches Compression leak between cylinders.

Steady idle Steady below 5 inches Leaky manifold or carburetor gasket, or stuckmanifold heat control valve.

Steady idle Floats slowly between 12 and 16 inches Carburetor out of adjustment.

Blipping engine speed Quick drop to zero then return tonormal reading Muffler is clear.

Blipping engine speed Slow drop of pointer then slow returnto normal reading Muffler is choked or blocked.


The only other thing that I would suggest is to be sure you carb is set correctly.

Hope this helps you

 

how would you set it if you have efi and the electronic ignition? I have unplugged it to set the timing but once I plug it back in it goes to the 10° that it's been told to... so how do I get around this? I have a brand new motor with around 100 miles on it with an rv cam and im starting to get a ping.

 

I'm not sure what you're working on or what you're doing but it sounds like you're talking about your vacuum advance. it will pull in and raise your advance if you're hooking it to manifold vacuum, if you don't want this hook it to your ported vacuum fitting that I assume is available on your throttle body. if not then you'll have to go by what works with manifold vacuum and forget the factory specs which don't mean mush anyway.

Then check your total timing curve. detonation will destroy your engine.

 

I've used recurve kits that used lighter weights to reach full mechanical advance early, and as I recall, a limiter is added too, so mechanical advance is both shorter over base timing, and it's quicker getting there. Mine has been like that since '86, I check timing at 2,000 or more rpm, as mechanical advance is all in by then, but I do it with the vacuum line undone at the distributor & plugged. Then, when set at 35* total, I remove my light and hook the vacuum back up. I added timing marks to 45* BTDC.
Cam is an old grind, Crane called it a "Fireball", but it is made / copied, whatever by others. NAPA used to offer it as a RV cam too, maybe they still do? Same numbers as Edelbrock 2172 ... Operating RPM Range: Idle-5,500 (but I seek to keep it Under 4k) ... Adv. Duration: 282* I / 292* E ... but Dur. 204* I / 214* E @ 0.050" lift ... Valve Lift 0.484" int./0.510" exh. stock rockers ... Lobe Separation 112*.