I've read through 50 or 60 pages of threads and have gotten a lot of helpful info, but would like some definitive direction on where to go next. Thanks in advance!

Bought a 1985 F-150 4.9 with granny 4-speed, 106k miles, no A/C, no cruise, no power windows/locks. Drove fine for a couple of weeks, but the idle gradually worsened to the non-driveable point. Seemed like it was sucking air somewhere and I knew it had an exhaust gasket leak, so this weekend I replaced:

• intake/exhaust gasket
• carb mounting gasket
• valve cover gasket
• fuel filter
• all vaccuum hoses (w/silicone), check valves and caps
• PCV Valve
• plugs (autolite)
• wires
• dist. cap
• rotor
• air filter

Things I found:

• the big vac hose off the front of the carb was off (the nipple had come loose from the carb)
• the rotor was not screwed down tightly (?!?) and had some pretty nasty carbon
• carb was kinda dirty, but not terrible
• check valves (in big carb vac hose and air cleaner hose) did not hold vaccuum)
• Idle RPM starts at about 700 for maybe 15 seconds, then drops to an erratic 500-600 swing
• intake manifold vac at idle holds pretty steady around 15hg
• above 1000 rpm smooths out and seems to run ok, but doesn't like load
• spark plugs looked identically perfect

Based on my studies here, I'm planning to:

• Check EEC-IV computer codes (I have the rotunda tester and breakout box but haven't learned to use it yet)
• Check timing
• Check EGR valve/mounting for leaks
• Check gasket between intake/exhaust manifolds

I'm seriously considering converting from TFI to Duraspark II and buying a new (or rebuilding my) carb. I'm not looking for power mods or anything, just want decent power and gas mileage. It won't be inspected for emissions, so if the smog stuff ends up being an issue, I don't mind taking it off. I also wouldn't even mind having a manual choke, so an ancient-style carb is not an issue if it would get me out of the clutches of all that primitive "computer control" junk.

I've worked a fair amount on old 289/302/351's (Mustangs, Fairlanes, Econolines) as well as mid-80's turbo saabs and a 72 BMW R50/5 (world's easiest to rebuild carbs!), so have a fairly decent background. Any guidance would be greatly appreciated!

Best regards,
Scott

 

my 85 idles at 550 where i set her just fine.

DT

 

Do you have power brakes? Could have a leaking power booster. Disconect the power boosters vacuume hose and plug to test.

Carbon build up can make an engine run funny. Put some Marvel Mystery oil into the gas and oil, and it will help. Eliminates sticky valves too.

Did you replace the check valves? Could send vaccume where it's not meant to go. EGR valve for instance. Is the egr operating properly? Is it stuck in a closed or open position due to carbon build up? A partially open EGR valve can cause eratic idle. It should be closed at idle.

All I Can think of at the moment.

Hope this helps

 

Thanks for the reply, 81-F-150-Explorer. Check valves are fresh and clean and holding vacuum all day long. I'll check the brake booster and EGR valve and do a little Seafoam/MMO/Snake Oil messing around tomorrow before I head up to the junkyard. Man, if it's that easy, I will be a happy camper.

Anybody else want to weigh in?

 

We have the same truck, but mine's been stripped of many of the emissions components. Trying to get it running really nice has been interesting to say the least. All these systems are inter-related, tracking down what's messing with the computer settings can be a challenge. My manual doesn't properly cover it, and there's too much missing to expect it to be right anyway.

Rich-running was the worst condition on mine, a carb overhaul did the most good of anything I've tried and really settled the idle. There's a plunger/diaphragm in the carb that had a tear, so it might be worth peeking into yours.

Despite the problems, mine's actually running pretty good. Hope you fix yours!

-Smoky

 

Thanks, Smoky. I read back through some of your posts; sounds like it's been an adventure!

Hopefully my testing/junkyard fun day will result in a bunch of new things to play around with. If it doesn't help I'll tear that carb down. I've only done bike carbs before, but if that Carter is even half as simple as the Bing slides on my Beemer it'll be a walk in the park...

Anybody know what a loose rotor could do to an engine? That's pretty much the scariest thing I've seen so far, other than the fact that the truck only got driven about 2,000 miles in two years.

-Scott

 

Sure sounds like a vacium leak. Can't remember if your engine has a vac tree but if it does that is a good place to start. It will tell you what system (other than pcv) to start checking. 15 in. of vac sounds low to me.

 

This may or may not bare any help to you rotor button information. you are looking for but, here it gos, i have an 88 300-6 and i put new plugs, cap, rotor and such in mine. After pulling the dizzy cap and pulling out the rotor and getting ready for the new cap and rotor i found out that my distributors holes for the dizzy cap were all wollered out.

When i put it back together the 1 screw wasn't even grabbing and the other was just barely holding. For some reason at the moment i didn't think it would be a big problem of any sort. I went to turn it on and heard my buddy yell to turn it off after hearing a realy wierd noise like a knocking sound. With my infinant bad luck i figured i had blown the motor or such but, come to find out my dizzy cap was spinning in circles lol. Kinda amusing after all.

Well when i figured this out i added teflon tape to the screws (ghetto i know) but, it hasn't done it since glad to say. What i was trying to get through though was the fact that i still "seemed" to run great with no real difference other than the noise even while the rotor was spinning the cap in circles lol.

 

It's been suggested by others here to spray WD-40, alcohol, or some mildly ignitable fluid around the base of the carb, etc. If it leaks, the engine will speed up as it burns the volatiles. I'd start with the engine cold and test areas separately with time between for evaporation to avoid a buildup of vapors.

The manual suggests spraying soap and water.

I have a YF with a loose throttle shaft that looks like an easy leaker.

I think certain failures of the egr system could cause it to annoy the idle and suck from the pcv and/or exhaust at too low a throttle, or leaks. Or failure in the idle solenoid and popper valve could make the idle "hunt." I haven't figured out the fuel solenoid yet on the side of the carb bowl, so let's blame it too! That'll learn it.

Advance is controlled by the pcm so uncertain sensor action could make the computer set timing all over and wreck idle that way.

A few days ago I placed some Carter carb exploded views on my gallery page, click my forum name to go there. It was my first rebuild, and very easy, just don't invert it without watching for the checkball and weight to fall out!

A loose rotor would probably run flakier with revs, it might do nothing bad other than that, or ruin itself and the cap or crash more parts if it flew apart. Don't ask how I know this...

Do you hsppen to have any pics of your motor with air cleaner off? Hoping to clear up the missing parts and vacuum hose routing on my engine.

Good luck!
-Smoky

 

my 85 bronco is carbed 4speed(np435) and almost nothing is hooked up and it idles pretty good. at about 700rpm or so its fine anyways and i have no choke hooked up. When i got it half of the things were broken, choke didnt work etc. I did notice a few years ago i had some problems with breaking roll pins in the distributor. A few times it sheared half of the pin and the drive gear just shifted. It ran but didnt like load and wouldnt really want to idle very well. I went through two new distributors and it hasnt done it in about 3 years but i keep a pack of roll pins in the glove box because they are easy to swap out not that i know the problem.

 

Shift, maybe your oil pump is binding or the gear the dizzy mates to is shot also.
-Smoky

 

thats just it, the truck had a new oil pump a few years prior when this happened to my cousin. The shop though it was bottom end so they redid a bunch on it to figure out it was that 0.10c roll pin. After i put the second distributor in I went around the motor and cleaned up the vac lines and made sure the crank case vent stuff was working okay. Its been about 3 years and never had a problem since. Im guessing since the oil going through the dizzy shaft i may have had a problem with the crank pressure. Dont really know but if it ever happens again it will get a new oil pump thats for sure.