I have recently put on a new Offenhauser C-Series intake matched with a Holley quick Fuel 450 CFM vacuum secondary carb, with some Efi exhaust manifoldS and am having some issues. (Forgot to mention the truck is a 1981 F150 with a straight 6) I put everything on after I came across a sheared distributor gear and figured while I was down why not upgrade a few things. I got the truck all put back together with the new distributor and carb and all, and went to set my timing with a borrowed timing light from a freind. This was the first time I have ever done that so I didn’t entirely understand vacuum advance and mechanical advance for the timing. Anyway, I can get the engine to idle perfectly, better then it ever has, and can also slowly rev it up as high as I want, but as soon as I put the engine under any kind of load it just bogs, occasionally backfires, and if I don’t let of the throttle, it will die.

Now when I say putting the engine under load I don’t mean when flooring it or when drivinG up a hill. When I put the engine under more then about what I’m guessing to be maybe a 10 percent load, it does this. So I called Holley and they told me it’s most likely getting to much fuel, and that I need to check me plugs. My plugs were pretty black, so I ordered some smaller main jets and a power valve and slapped it on there.
No difference

So now here I am not to sure what my issue is, but something is telling me it’s not carb related. From what I understand, this carb should run well enough, to atleast go down the road, right out of the box, followed by some minor tuning. Well I’ve messed with everything I can think of on the carb and can’t get it to have any change, so my next suspect is timing. What I’m trying to find out is:
Can mechanical advancement of timing “go bad”?
Would this be a reasonable culprit for the symptoms I’m having?
And most importantly, How do I fix it?

If I’m way off here for thinking that this is the issue, could someone please lead me in a direction as to what it might be? I’m really stumped at this point.
P.S I have the initial timing set at about 8 degrees BTDC

 

Make sure ignition timing is straight, then make sure carburetor is setup and adjusted. A mechanic's vacuum gauge is a very useful tool to quickly determine how well an engine is tuned. It is a good check on whether the ignition timing is set correctly, and a couple dozen other engine problems.

One of the problems with older engines, and setting the base ignition timing, is that the numbers on the crankshaft balancer are no longer accurately located. The balancer is actually made of two separate pieces, a heavy steel outer weight bonded with rubber to the basket. With time and age it will slip. So it's possible in this kind of situation to set the timing to say 12° BTDC, when in reality, the engine is really at 2° ATDC, and runs like &@$&!!#% and everyone is scratching their head.

So the recommendation now, before trying to time an older engine, is first verify the pointer is aligned with the "0" or TDC mark when the #1 piston is known to be at the exact TDC.

The way you do that is with an inexpensive tool called a piston stop. To your other question. Setting the base igntion timing is a matter of disconnecting and plugging the vacuum advance connection, and keeping the engine RPM low or at factory idle.

It is also important that the mechanical advance timing is checked throughout the RPM range for proper advance at different RPM. It should move smoothly to somewhere in the mid 30s and then back down. With vacuum advance connected 50 to 60 degrees indicated while in neutral at high RPM is typical.

It is possible for both mechanical and vacuum advance to be defective.

 

Don;t worry about the vac advance right now just disconnect it.

First lets check to see if the mech advance is even working.

Step 1
Remove your dist cap from the spacer no need to unhook the wires.
Step 2
Grab the rotor and turn it counterclockwise it should turn from 1/4" to a 3/8", let it go and it should snap back. If it does this the mech advance is at least functional, That does not say it is working as intended but it is functional.

If the rotor refuses to move the advance mechanism is seized if it moves and kinda slops back and forth the springs have failed.
Since you have the cap off now is a good time to lube the advance mech remove the rotor and put a couple 3 drops of oil done the hole in the shaft the rotor sits on.
If the mech advance seems functional you can likely rule that out.

You also need to verify if the damper has slipped. (you can get it close enough with a pencil to check for damper slippage)

Pull the plug on cylinder #1 Set the engine to TDC #1 or 0° on the damper .
Put the pencil in the spark plug hole (Eraser side out)
Turn the engine clockwise an 1/8th a revolution the pencil should go further into the cylinder.
Remove the pencil.
Put the engine back to TDC #1 or 0° on the damper.
Put the pencil back in the spark plug hole (Eraser side out again)
Turn the engine counterclockwise an 1/8th a revolution the pencil should go further into the cylinder.

If at any time you turn the engine and the pencil comes out of the spark hole instead of going in the damper has slipped and you will need a new one.

Set your base timing to 10° BTDC this a good all-round setting to start to dial in from.

Let us know how these tests turn out and we can go from there...

 

I'm almost certain that the damper hasn't slipped because when I put in my new distributor, I used that method to get the cylinder to TDC so i could atleast have the timing be close for initial start up. Also, Is is really possible that the mechanical advance could be malfunctioning on a brand new distributor? Could I possibly be chasing the wrong issue here?
I will double check these things when I get home, but I don't think that the damper has slipped.

 

What new dist. Did you get?
Post up a picture of it.
I would throw a timing light on it to see if it is advancing as a start even if the ring has slipped.
Could adjust it with vacuum gauge. Adjust for highest vacuum reading and test drive if you can to see if it pings or hot start issue and back off a little.

I don't think it is a dist. Issue but a carb issue.
I take it you have not driven it yet because of this.
Jets are high speed, read anything off idle, and if you can't get above that the Jets will not help.
What jets did you have and go to?
Same for the power valve?
Dave ----

 

Is this a brand new imported knock off distributor, brand new as in NOS factory, or a "rebuilt" factory, or?

It is important to set/adjust/verify ignition timing at idle and throughout the RPM range before blaming the carburetor. You mention it "idles perfectly", maybe it does but a vacuum gauge is a better objective measure of engine tune and carb adjustment. Most engines will idle reasonably well even with the ignition timing way, way off, but then fall on its face as soon as any load is applied.

 

I just went to Napa and told them I need a stock distributor replacement. It was like a $60 part. The Jets the carb came with were #55 jets and a 6.5 power valve. When I called Holley, they told me it sounds like too big a power valve for my application. When I pulled the plugs, they were pretty black in color (They are brand new), so I went and ordered a size 5 power valve, and #52 jets and saw no change. Maybe I'm heading in the wrong direction. When I put a vacuum gauge on, I got about 18inches for my reading so this would lead me to believe that I should need a #9 power valve, but didn't go with it due to the black plugs.

The truck hasn't been ran more then just in my neighborhood because It just runs so poorly. Could I possibly be headed in the wrong direction and maybe the plugs are black due to a rich idle mixture? Should I try a larger power valve and jets?

I'm just not quite sure that the carb is the issue because there is a guy with a 79 F-250 on youtube, who's setup I have almost made an identical copy of, and he made no more then a few minor adjustments on his carb and it runs great.

 

If the 18" of manifold vacuum at factory idle RPM is accurate, a 6.5 power valve would be fine, it certainly would (should) not be open at idle or cruise in any case.

This "divide by 2" formula business for selecting a power valve so often mentioned is more geared towards performance engines that pull a lot less vacuum than is typical. A factory engine in good tune and stock form will pull 19" to 21" of vacuum at sea level and were spec'd by the factory to use 7.5 thru 5.5 power valves depending on elevation. The power valve should never be adding any fuel except when accelerating or under heavy load.

Jetting doesn't factor at all in the idle mixture. Make sure fuel pump pressure output is in spec, and float height is set correctly. Spark plugs should burn very cleanly with modern gasoline. Idle mixture should be fairly lean, or plugs will tend to "load up".

 

Tedster did hit on something that I and others have over looked ..... the float level.
In shipping that box gets thrown around so I would make that a first check before any adjusting of the carb as the rest will not go good if the level is off.

Holley carbs will take the normal fuel pump psi unlike the Eldel carbs where they need a lower psi but would not hurt to check the psi for S&G.
You sure the choke is opening all the way? If it is not opening it could be choking for more air and why it runs bad when giving it throttle and the black plugs.
The vacuum of 18hg sounds good and I would have gone with the #9 power valve as a start but that's me.

Because I have been tuning old motors for a little bit, 60 years young, I do the fist tuning be ear.
So I would get the motor up to temp and adjust the dist. so it gets the best (fast) idle then back it off just a hair till I can get a light on it later if I feel a need.
I also adjust the idle speed and then the idle mix by ear. If I have the tools, meter for RPM and timing light out I may use them but if I am not getting any pinging or kickback on hot start I leave the timing where it is.

BTW what works for 1 guy and his motor may not work for yours but here's hoping it will run good with a little tuning.
I am also not a big fan of a v4 carb on a straight six as I have seen to many stories like yours of not running right.
A 390 CFM might have been a better carb over that 450 but I would have gone with a Holley 2300 in the 350 CFM size, 500 CFM is to large even for a 258 AMC six but is what we had on the shelf.
The large ports of the intake also don't help as the air flow is low and the gas tends to fall out of the air and pools on the intake floor making the motor run rich.
Dave ----

 

Want to try a quick experiment? When you get it warmed up and idling, pull the vacuum hose off the distributor. Did it stall out or almost stall out? If so, that is your problem. Leave that hose off, take your timing light and time it, re-adjust the idle and the idle mixture and get it idling good with the hose off and plugged. Then take it out on the road and see if it will run ok. If it does, you need to work on your vacuum advance hose location.

 

Well everyone, at this point I’m totally lost. No idea where to head next. Anybody with any possible idea as to what my issue could be that hasn’t yet been listed?

 

Vacuum leak? Gasket, some port left unconnected and open, leaking brake master, bad pcv, transmission modulator line.....

Have you run down all those possibilities?

 

Well when everybody is done farting around and running around in circles maybe we can actually resolve this.

 

Well I don't think we seen him put a timing light to it yet doing as DaveF said or even checked if the ring has moved so he needs to check that first.

Then carb float level and idle mix adjust and tell us what happens then.
Dave ----

 

I went to start my truck and now it wont even hold idle once its warm. Something is weird for sure. The damper has not slipped, and I adjusted the float bowl level slightly, but it was just about dead on to begin with. I also completely reset my idle mix by turning the screws all the way in and then backing them out about 1 and a quarter to one and a half turns and no change. I'm going to try and get it to hold idle and then try what Franklin2 has said with the vacuum advance hose. If this is in fact my issue, (I had someone else tell me to check this too) what can I do to resolve it? Use a different port on the carb? Use A shorter Vacuum Line? Don't really know too much about it.

Also, it was to my understanding that your vacuum advance only does anything when your cruising at a constant speed and your AFR is lean so it advances the timing even more to give the fuel more time to burn completely, thus making you motor run more efficiently. I thought it didn't do anything really at idle or under acceleration and that the motor will run fine without it, just not be as efficient. Please correct me if this is wrong.