OK. I have a 1970 F100 running a rebuilt 351W. Rebuild included a Cat main girdle cap, high volume oil pan, mellling oil pump,KB silvolight hyperutechtic flat top pistion,new Comp Extream energy 262 dual patter grind,hyd. flat tappets(Crane), pushrods(Crane), Comp 1.6 ratio roller rocker Energizer rockers, Comp springs, Heads are a 1985 351 head that I ported and polished, with 69 cc chambers.Long Tube hooker headers Intake is a Edelbrock preformer RPM air gap, Carb is a 650 road demon Vaccume secondaries manual choke, jetted with a 64 primary and 72 secondary, standard vaccume spring. Ignition is MSD pro billet distributor, MSD 9mm wires Blster Coil, and 6al contoller with limiter. I run a manual tranny 3 speed, and an open diff 3.55 with bfgoodrich radial TA/ 295 65 r15" tires.
I have an issue with poor mileage, and poor driveability below 2500. It typically lags off the line until I let up and stomp it again, then it will start to accellerate ata decent pace until all hell brakes loose at about 3500 rpm and it just gives er until 6000 rpm where my limiter stops it. A couple of streetlights wil cost me a quarter tank of gas, which is fine with me if it didnt bog of the line (unless I clutch drop at 3000) and could get 16 mpg highway at a modest 65 mph. I have heard things like advancing the timing untill it cranks hard and then backing it off a bit is a good way to set initial timing, and curve your timing untill you hear a ping, and back it off 5 degrees.
Is this a legit way to tune or is it a load of bull?

I have also heard cam advance can play a major role in preformance anre there any good guidelines to follow with a cam timing change?

All opinions welcome on this one
Scott

 

Did you gasket match the heads and intake?

 

Set the initial timing at 12 degrees BTDC. Sounds like you've got it retarded too much.Start it and check with a timing light and see how far the timing advances as you rev it. Total timing should be about 35-38 degrees BTDC. Might need a lighter vacuum spring too, but I'd bet the timing's off. With that cam and the rest of the combo, it ought to pull from idle to 6500. Only part I'd have left off is the main girdle. You didn't need that.

 

I suppose I didnt need it but it was a 300 dollar girdle for 100 bucks, and I do drive hard so bottom end longevity was in mind. The heads and manifold were gasket matched and the heads ported, thermactor bumps ground out and filled. I beleive I have the cam timing set at 12, the top end might be out but it pulls good up there. This is where I get into cam timing issues, I am also wondering if there are variations of balancers that may have differant indications of timing based on keyseat position?

I run a vaccume gauge full time and my vaccume is at about 15 in Hg at 3000 rpm my power valve is a 6.5 should I try a differant power valve?

 

Did you install the cam "straight up" ? ( timing marks aligned) The marks on the balancers for the 335 series engines are all in the same position, there was never ( that I know of) any different timing pointer position used that would require anything else. It's also posible that your balancer has slipped, that would play hell with ignition timing.

 

The machinist that assembled the bottom end installed the cam, and he made no mention of advancing or retarding the cam. As for the balancer slipping, I dont think so.. wouldnt that require breaking a keyway? I installed the thing so over a year ago so I dont remember. I will look into this. If I installed the balancer wrong I am going to be so pissed... Thanks for the tips. Anyone with guidelines at to what advancing the cam would do to an engine?
Scott

 

Two things, might not have anything to do with it, but the RPM intake is designed for farther up the scale (1500-6500 vs. idle-5500 for the performer), and IMO that's not enough carb for that combo, especially if that head work has them flowing the way it should. JMHO.-P.

 

Well, I cant disagree with your points pete, but the issues that would arise from these parts selections are somewhat differant than the issues I am having. Yes, the RPM is designed for 1500 to 6500 rpm. My issues involve a bog at 2000 RPM, poor idle, poor mileage. I decided on this intake because of 1) the air gap design, cooler intake charge to help supply the ported heads, tuned for power above 1500 rpm- I drive a long box half ton with totally stock suspension and an open diff. Needless to say I have massive traction issues already so I dont need brute torque right off the line, it will cost me nothing but tires, and I dont tow with this truck.
Yes, the carb is on the edge of too small, and considering I live at 3000ft above sea level, more cfm would be benificial for top end, but I run a stock crank so 6000 is my redline. I have read from many sources that a carb a little on the smaller size will afford greater driveability and throttle response untill it reaches its airflow limit. Right now my issue is again driveability in the lower rpm ranges where this carb is supposed to shine. That said the vaccume gauge I run full time... if I put my foot on the floor vaccume drops to zero and I have hit 5500 rpm in third gear and still no vaccume reading at which point I am doing about 120 miles per hour. Same thing in second at 70mph. Did I mention this was in a 36 year old truck with stock suspension and brakes?
I really dont need to go alot faster untill I make upgrades in other departments. I sure would like to be able to pull up to a stop light without having to baby the throttle to keep her from stalling. Especially in stop start traffic.
Im not trying to jump on anyone here, I am just trying to paint a clear picture of whats going on, and what I want to do.
Thanks for your inputs guys, every one of them keeps me thinking.
Scott

 

The outer ring of the balancer can slip out of line with the inner ring. You can't install it wrong. Easiest way to tell the ring has slipped is it will develop a vibration from being out of balance plus the ignition timing will be thrown off when checking it with a light. You might want to check the vacuum advance to see if it's working and check for vacuum leaks.

 

Your power valve might also be the wrong size for your application.

 

OK, did some tweaking over the weekend. I swapped out the Vaccume secondary spring from the standard spring to the 2nd lightest (long yellow) which is supposed to open at about 1900 rpm. The bog seems to have gone. Accelleration is better, but there is still a distinct surge a little after you floor it. Is this the power valve's department or should I be looking at an even lighter spring?
I checked the timing, it is set with 10 degrees initial and 25 degrees advance. I left the total timing alone but put the lightest springs in for the centerfugal advance.
While I had the cap off I noticed that the Tip of the rotor was somewhat burnt on the leading edge. I looked at the electrodes on the cap and they were quite blackened, and there was a very fine black powder on the inside of the cap. I used some fine grit sandpaper to clean them up again, but this seems to suggest arcing between the rotor and distributor no?
I also checked my MSD manual and it suggests setting initial timing as far ahead as possible without encountering excessive starter load. So I start advancing the timing and before I know it I am running about 18 degrees initial advance and still not seeing any added load. I am rather bewildered by this. Anyone have any insight?
Thanks, Scott

 

Your initial timing will be limited by the engine's resistence to pinging not the starter load. If it's pinging at total advance then you need to either back off the timing or run a higher octane fuel. Total advance is usually around 35-38 degrees BTDC.