81 F-150, 4.9L, 1BBL carb, 188,000 miles, C-6 auto trans.
Sticker under the hood says to set timing at 10 degrees BTDC. Checked the timing yesterday and it was set at 8 degrees BTDC. The timing hasn't been changed in at least 7 years. So i advanced it 2 degrees to bring it up to 10. I noticed two things:
A very slight increase in spark knock at highway
speeds and just a tad more of acceleration. This engine only likes 93 octane. I've been told that the compression ratio is probably more because of carbon build-up. (how do i eliminate carbon build-up if i'm using 93 octane?)
Would i be better off retarding the timing back to 8 degrees or leave it at 10? What's to gain or loose with either setting? MPG's are around 14-15.
The 300 seems to be very susceptible to carbon build up on the tops of the pistons . It doesnt seem to take much to cause spark knock problems using using lower octane fuels . What I occasionally do is run some Marvel Mystery Oil Fuel treatment & that seems to cure the knocking problem & I can use 89 octane with no trouble . Run the tank down to 1/4 full , pour in the fuel treatment before you fill the tank & drive it . It may take a 2nd treatment , but you will usually notice a difference after burning the treated fuel .
I could swear the tag under my hood says timing at 6 degrees BTDC; maybe I'm wrong, can't look at it right now. Since original post is 81 F-150, 4.9L, we should have same engine -- ??.
What would be benefit of going to 8 or 10 degrees BTDC? Any drawbacks? Is it safe to think that if it runs OK, you are not damaging anything or does one take a risk by advancing beyond suggested setting?
Pardon the simple nature of my questions - I just thought that you are supposed to set timing where you are told to. My chance to learn different.
w/ 81 F-100, I6, SROD, Carter 1V non-feedback carb, no A/C, no PS, no PB, but it gets me where I need to go
The reason i had the timing set at 8*BTDC was to eliminate some of the spark knock (and it does)....but retarding the timing also works against you with performance. Loose some performance and some knock? Or gain back some performance and have a slight knock? Thats the issue. Spark knock or pre-ignition is not good by any means, you just don't want it to knock constantly. If i use 87 octane thats exactly what it would do. When i use 89 octane the knocking is much less but still there. Using 93 octane i only get very slight knocking when climbing uphill at speeds of 65-75mph. I can live with that. And of course the timing should be correct. My sticker says 10*BTDC, yours should be the same unless it varies from state to state because of emission laws.
I put back in my motorcraft plugs which i side-gapped on the bench grinder and set the gap to .044" I was using a set of new bosch platniums. The side gapped motorcrafts have alot more punch. Gas mileage stayed the same with both plugs. But acceleration was noticeably better with the side-gapped plugs. Removed tailgate for unrestricted air flow out of the bed, and have all tires holding 35 PSI.
Got off the subject of timing, but thats what i'm running here in michigan. The 4.9L is the best and EASIEST engine i have ever had to work on.
My next mission is to consider the removal of my catalaytic converter which is almost 20 yrs old and probably not worth crap as far as any usefullness goes and besides we have no sniffer testing here in the motor city. Maybe the 4.9 will gain a little more performance without the restriction of the cat....who knows!
Other cars in my household:
96 Mercury Cougar XR-7 4.6L V8
94 Pontiac Grand Am 2.3L Quad-4
91 Ranger 3.0L V6
89 Merc Grand Marq 5.0L V8
81 F-150 4.9L
73 Plymouth Cuda 340 CU.IN. V8 (original owner)
I keep pretty busy doing tune ups and oil changes. Guess i better shut up and get to bed.
My emissions sticker has 6 degrees BTDC for the timing, my 84 donor truck was marked 10 degrees, and the F600's are 6 degrees BTDC.
82 F100 FlareSide
HD 300-6, 240 head
2.73 Geared 9"
Blue & Silver
Something else you might want to consider is the harmonic balancer (crankshaft pulley). My experience is these are a weak link on this engine. The way these things are made is a strip of rubber gets sandwiched between the inner hub and the outer ring of the balancer where the belts are. The mass of the outer ring and the rubber smooths out the jerky motion of the crank each time a cylinder fires so the engine rotates smoothly. The problem is, time and motor oil wreak havoc on this rubber strip causing it to crack and/or swell and eventually the outer ring "slips" in relation to the inner hub. This won't harm the engine but the problem is, the timing mark is on the outer ring. The next time you go to check the timing, it appears to be off. If you adjust it back so it indicates correctly, your engine is now not timed correctly. Instead of putting it back to 10 like you thought, you may actually be at 12. Since you said you got more knock after you did this, you may actually be having this problem I am describing. Worth a check anyway. The only way I know to tell if you might have this problem is to look at the balancer's rubber strip to see if it is poking out.