So I purchased a 81 f100 with the 300 With the intention of pulling the motor and replacing it. I am currently building a 408 for it, but I love to drive it as is. I replaced the intake with the offy intake and put on a new edelbrock 600 carb. I jetted down the carb because it was too rich but for the life of me I cannot dial in the timing on it. It runs well enough to puts around town but it's just not right yet.
Besides the above, I have done plugs and wires and the cap and rotor have about 400 miles on them. Since I'm not keeping the motor except for another month, I really don't want to replace the ignition system unless I can find one in the junkyard.
I've ran the motor from 9* to 18* and have had no luck dialing it in. I still get a small diesel when I shut it off hot, still get either a knock at rpm under load or it falls down when I peg it.
Anyone have a suggestion or observation in how to fix this thing so it runs like it should?
Thanks, Paul

Edit: idle rpm is around 750

 

Check for vacuum leak(s) then go to the next step...

I've found it best for me to use a vacuum gauge to set the timing. Unplug the vac advance, hook up the gauge to a manifold vac source, turn the dizzy up to the highest steady reading and back off 1-2 hg. That is your starting point. Then adjust the carb idle mix the same way. Then adjust the idle speed if necessary. Drive and try.

Changing the jets is quite often not necessary. Be sure the ignition system is operating properly before tearing into the carb.

 

Plumbing some kind of heat to the bottom of the intake will help a ton if it's not bolted to a stock log exhaust manifold.

If you're getting a knock it's running too lean. Try some stiffer step-up springs to get to the rich part of the rod sooner. Or maybe go back to the stock set-up and try that.

My truck runs better with the vacuum advance hooked to manifold vacuum vs. the ported side. The truck will run cooler, too. It should be able to idle ~600 rpm. Make sure the choke is opening all the way and none of the linkages are binding. Set the idle to 650, plug the vacuum advance, and set the timing to 10 degrees. Hook the advance can to the manifold side of the carb then you'll probably need to back down the idle. Drive test for performance, advance 2 degrees, repeat until it feels right.

 

I will try the vacuum method tomorrow. I spent a lot of time with the timing prior to re jetting the carb, but was always running rich with the 600 at factory carb/ metering rods. Do you think I should re jet the carb prior to trying the timing again? 750 is high, but it diesels less at this Idle and if I bring it down too much I almost stall when I shift into gear.
Thanks for the quick reply by the way.

 

I already switched to a full vacuum port and it helped quite a bit. I think with both of you telling me jetting the carb was no good, I will go back to factory. Are you sure the 600 won't dump too much gas? Don't most run the 390? Choke works well, that took awhile go get right.
I read in a post most guys with the offy and a bigger carb run about 16* is that too much?
I just remember the carb at original jets/rods was too rich, but I don't know what else to try.

Side note I run ethanol free gas and all my plugs come out a nice clean brownish.

 

The 600 is not too big since it will only run on the primaries (think 2 bbl) under normal operating conditions. The "too big" part comes into play when the secondaries are not adjusted properly usually opening too soon when mashing the gas at low speeds and creating a bog condition...just too much air all of a sudden for the engine to handle. There are probably more threads on this Forum about problems getting a 390 to work than any other carb. The Edelbrock 500 or 600 appear to be the easiest to dial in...usually just the idle mix and speed adjustments. Re-jetting is the very last operation to do when dialing in a carb.

"Plumbing some kind of heat to the bottom of the intake will help a ton..." X2 on that.

 

Thanks I'll check that out today weather permitting. Ill re jet it prior to timing it.

 

Pinging is usually caused by too much advance for the octane of the fuel.
You may have gotten a bad batch of gas or it is old or the timing is just too far advanced. You may be all carboned up increasing the compression ratio requiring less advance. Engine running too hot also helps pinging.
Dieseling or run on is usually caused by hot spots (carbon), too high an idle, open butterfly at idle (instead of idle circuit), engine that runs too hot. Has nothing to do with spark since there should be none with key off.

I would first set the timing properly around 10° and check then the advance at about 2000rpm. Manifold vacuum always runs cooler at idle.
You may want to run some seafoam or water thru the intake to decarbon the engine.
Check the actual running temperature with an IR gun.

Always time the engine properly first, the carb is always last, whether you are rejetting or just making idle adjustments.

 

"Always time the engine properly first, the carb is always last, whether you are rejetting or just making idle adjustments." X2

 

I spent a good week trying to adjust the timing for the new carb before installing the smaller jets, so it wasn't an initial reaction, but at that point I would try anything.
This is also the second tank of gas through it, its quite clean and the fuel filters are clean as well. It's had a few cans of sea foam through it, the first can (used over 3 days) did help, and it did burn a lot of crap out of it, but the second didn't get much. I made the note of dieseling just due to the overall condition of the motor.
So today I spent 3 hours with a torch to get the cat out to find out that it had been gutted out completely, probably years ago. It had a hole in it, so I replaced it with a pipe I had laying around. It has a pretty restrictive muffler on it right now so I'm not too worried about back pressure. I put the carb back to factory, timed it to 10*. Ran only slightly worse than before, smells rich too. Timed it to 19hg vacuum, that's all I could hold steady (I'm at sea level). Ran exactly the same cold or hot. Starts right up and purrs at idle and at revs with no load, as soon as load is applied it gets a bit of a pop in the exhaust and has a hard time staying in it after about 1/2 throttle. I also think I have a small leak in my exhaust manifold now from tugging on the exhaust.
Don't have an IR gun, don't really have the money to sink buying one. Dieseling has stopped more or less, just still runs like poop.
Unless there is anything left I think she might just run like poop until I pull the motor (short block should be here in about 2 weeks)
Anyone in SC want a 300 that runs ok? I can have it out in a month or so!

 

"It has a pretty restrictive muffler on it right now so I'm not too worried about back pressure." ??? That's not making sense. The "needs back pressure" myth has been debunked many times over going clear back into the last century.

 

Should've ran it for a minute without the test pipe in place that would've told you if the muffler was plugged or not. My trucks muffler fouled out within weeks of putting my intake/carter 500 on it and caused similar symptoms, however so can a improperly tuned carb. If you've set the timing with a vacuum gauge drive it around for a little while then check the plugs.
As far as back pressure goes I ran efi manifolds with a walker y pipe that I cut just after the O2 sensor bung and then I ran 2.5" pipe that I then left pointing at a 45* angle towards my rear axle and I ran it like that without a muffler up until maybe 3 weeks ago so close to a year with no problems.

 

I apologize for not knowing that, but the muffler is new, and is not just a whole for which my exhaust to fall through. A few threads about cutting out the cat read something about back pressure, so I didn't just put a pipe down from the exhaust manifold to a 90*
To be honest, I am not spending a whole lot of time learning this 300 top down because I am pulling the brick out of the truck soon. I just wanted it to run well enough i can put it around town, because I do really enjoy the truck.

Definitely started pulling the cat prior to putting the gave on it, and it didn't look nearly as bad as it was to get off. That problem is resolved now though.
The carb is now just a factory edelbrock 600, and the floats are set properly. ill drive it a bit and see what the new plugs tell me.