Hey amigos,
I have a problem that i beyond my limited experience and need help. At Idle around 800rpm my 390 is surging, or at least i think that's what it is, and the idle fluctuates when it does this. Best way i can describe it is when sitting in the vehicle it feels like a T-rex stepped close by becuse the truck kind of shakes and makes a sound. Also at rest, if I increase the rpm seems to smooth out and run well. When i drive, sometimes the power seems to fluctuate as it moves through the rpms and i get backfires out the exhaust every so often. I have looked at the timing its at 8 BTDC and the plugs, wires, and distributor cap are all around 6months old. I can't find a vacuum leak and messing with idle mixture screws doesnt affect anything so i dont know where to look. Any help is great because i dont know a whole lot but i am trying to learn.

Ian

 

You have a vacuum leak. If your truck is an automatic, you may have a leak all the way down at the modulator. If you have power brakes, you may also have overlooked the vacuum booster.
Either way you may have overlooked a vacuum leak somewhere. The FE is difficult to isolate vacuum leaks, but you do have a leak.

 

I know you said you can't find one, but the fact that changing the idle mixture setting does nothing indicates a vacuum leak. Check for vacuum leaks by spraying a can of carburetor cleaner around suspect areas and listening for a jump in idle speed. Common trouble spots are the carberutor base, power brake booster (if equipped), or dryrotted vacuum lines.

EDIT: Sorry 75F350, I left this up for a few minutes before I submitted and didn't see your post.

 

Thanks guys ill have to dig into it more and see if i can find what is leaking, in your opinions does this sound like a small leak, fairly large, or does it matter?

 

Dont sweat the small stuff. We just verified and agree on the fact that the OP has a vacuum leak. We dont call it redunancy, we call it confirmation.

 

If you can get it to idle at 800 rpm, and it does not die when you do it or put it into gear if you have an auto, then I would venture to say that it is a small leak, not a large one, but the small ones turn into large ones and can cause problems all the same.
Remember that a leak is air getting into the engine, and this means that fuel is not being mixed, so this "lean mixture" can cause problems. Just because the engine appears to be running fine at higher RPM's, it does not mean that it is not lean. Lean conditions can cause detonation, higher engine temps, as well as some other damaging things.
Be sure to look carefully and find that leak. Your truck will thank you with good reliable service when it is running properly.

 

Thanks for the advice and help

 

Nice work as usual 75

 

GDay Mates,

I had the same surge when the truck was warmed up 73 F100 auto Motocraft 2100 2 bbl.


The idle surge fix worked. I rebuilt the carb with a NAPA kit and surging is gone .It runs great!! YAY!!

The other day in troubleshooting the surge problem:
1. Vacuum leak check with carb cleaner = No change
2. Pinched off all vacuum hoses = No Change
3. Pinched off fuel line for 10 seconds = No change
4. screwed in left jet = No Change reset to 1.5 turns out
5. screwed in right jet = No change. left it screwed in bottomed out
6. Pulled off PCV SMOG hose to back of carburetor = Engine RPMs shot up to about 1700 RPM. I quickly screwed idle down to ~700. = ran smoothly.

I suspect a sticky economizer valve or leaky float valve. Of course the carb had all sorts of carbon and crud I soaked it overnight in "Oil Eater" and water then sprayed it out with carb clean. I used old Guitar strings to poke though all the tiny holes.

I did remove the original Carb Spacer with the Vacuum diaphragm valve looks like a flying saucer, and replaced it with a spacer off my old 72 302, However it left a 3/16 x 1/2 in intake manifold gap (exhaust leakage) for the exhaust to come up and heat the stock carb spacer.

I plugged the gap temporarily with red hi-temp RTV. I will mod the old stock spacer by cutting off the Vacuum diaphragm or mod the new spacer by aluminum welding or JB welding on a lip to close the gap.

My truck is a 73 F100 Ranger 360 auto Motocraft 2100 2 bbl California SMOG truck made in San Jose CA so, the carb spacer with the crazy vacuum diaphragm may be unique to California 360s.

Just checking in

Cheers

 

The "crazy vacuum diaphragm" is an EGR valve. It directs a certain amount of exhaust flow back into the intake at some operating conditions to reduce NOx emissions. Common in all 50 states.


If you pulled the PCV line off and didn't plug it, the reason the engine revved up is that you created a massive vacuum leak by doing so, and it was getting a lot more air flow.

 

I only drive this truck under 200 miles a year.

 

Modern gasoline blends are formulated to reduce evaporative emissions in the summer (different volatility for summer blends), and also to reduce hydrocarbon and CO emissions from the engine (ethanol contains oxygen and helps further complete combustion in the engine). Overall, they're much better than anything we've had in the past. What's an issue is using high-ethanol blends in marine engine applications - ethanol is more corrosive (not usually a big deal except in like weed-eaters with really cheap rubber seals), and is hygroscopic (i.e., it attracts water - that can be a big deal in boats).

For < 200 miles a year I wouldn't be terribly worried about emissions either.

 

Amen Brother A neighbor told me He buys the Alcohol free 2 cycle premix in a can from Home Depot $5.99 a can. He said his weed eater and blower starts now much easier, even over the winter storage. Less carb cleanouts and frustration. So I bought some so we'll see.