I have a mystery I can't solve despite my searching. I bought my Bronco in March. It has the 4.9 liter single bbl with T18 4 speed. All the emission equipment has been removed by the PO. I have since been tracking down all the issues with the truck between my college classes. I cannot get my truck to idle.
I have replaced: carb, plugs, wires, cap, rotor, brake booster, exhaust manifold, exhaust system (2.5" with steel pack muffler), intake/exhaust manifold gasket, carb to EGR gasket, EGR to intake gasket, PCV, fuel filter, fuel pump, and thermostat.

My friend tuned the engine be ear because the timing marks on the balancer do not correspond with either set of marks on the engine. (Boy that made a world of difference!)

The carburetor is a rebuilt Carter YFA. I have adjusted the float to factory specs because it came out of the box with a huge float drop. The float level change did not affect the idle or driveability of the truck. I have to keep the manual choke pulled out just a tad to keep it running at idle. the instant I put the choke in, it will stumble and die. It will cruise great at 60-65mph choke in or out. My gas mileage is 8.5 to 9mpg. I think this is awful even though I'm driving a brick on 32's. Currently It will not idle bellow 1200rpm. I have tried every adjustment on the carb that I can.

The vacuum at 1200rpm is 15 inches. I tried rotating the distributor to see where I got the most vacuum and it would not go above 15 in.

My compression is 115, 120 123, 125, 125, 120, 115

Some other thing I have noticed: The plugged ports on the vacuum tree fill up with gas and oil!?!? The Egr/carb spacer will fill up with a tablespoon of gas after driving 2 miles in town. When I come to a stop the brake booster seems to pull enough vacuum to kill the engine.

I have no idea what to try next. I really want to figure out what the issue is because I don't like broken things. If you have any idea whats wrong please chime in!

 

That is a mystery, for sure. I assume that all of that replacing didn't really change anything?

I would have said you have a vacuum leak, and while that is still possible it doesn't seem too likely given what all you've replaced. But, check the hoses for cracks anyway.

My guess is that you have burned valves. I realize that your compression test results don't look bad, but they are lower than they should be on a strong engine. However, a compression test will frequently miss bad valves. So, I suggest you do a leak-down test, which will show up burned valves. I know that because I had a 351W which ran poorly but the compression test said all was well. However, a leak-down test said that I had 5 burned valves. Sure enough, the machine shop agreed and rebuilt heads made a world of difference.

A leak-down test puts compressed air, typically at 100 psi, into a spark plug hole at TDC on the compression stroke. But it puts it through a small oriface, typically .060", with a pressure gauge either side of it. If there is no flow, meaning the rings, valves, and gaskets are tight, both gauges read the same. But if anything leaks the downstream gauge will show a lower pressure due to the oriface. And, you will be able to hear the air escaping in the intake, exhaust, or sump or see bubbles in the cooling system depending on what is leaking.

 

Gary, Thanks for the reply. You are correct, nothing really changed. I replaced all the vac lines that were in the truck. I will have to do a leak down test this week. How long does it take for the pressure to decrease?

 

It sounds like it may be too rich with excess gas in the vacuum tree. Have you adjusted the idle mixture screw down low, passenger front direction. Mine never seems to make a difference, but if youre overly rich at idle it's a possible culprit. Turn it in clockwise, then out like two turns. When it does run at 1200 rpm does it put out black smoke (rich)? The 300 will run fine with those compression ratios.

 

Get it running with the choke slightly pulled out like you do now. Take the aircleaner off and look down the carb with a flashlight. If you see fuel dripping inside the carb, your float level is too high. That's causing your bad fuel mileage, poor idle, and gas in those different places. You should see no liquid fuel inside the carb at idle(even your higher idle).

 

Dave and OO have a good point. If it is way rich it won't idle and will give very poor MPG, which are your symptoms. And, while I'm sure you understand this, the float drop and float level are two entirely different things. I think the 1-bbl is the same as other Carters, and You adjust the level, which is the critical adjustment, with the air horn (top of the carb) turned upside down. If you set it too high you'll get fuel pouring into the carb, as Dave said.

The drop is set with the air horn right side up and it is just to keep the float from dropping so far that the needle comes out of the seat. But other than that problem if set too wide, or running out of fuel on a hard pull if set too narrow, it isn't critical like the level is.

Do the carb check before doing the leak-down test as that may well be the problem. But, a leak-down test is instantaneous - it doesn't take time for the air to leak out, it happens immediately. There are percentages of air loss that are said to be acceptable, and most people say less than 10% is acceptable. But on top of that indicator you also get the sound of air rushing out through whatever is leaking, like rings or valves. Or you have bubbles in the coolant, seen in the radiator. Those are the clues that tell you where the air is going. The percentage loss just tells you something is amiss in that cylinder.

 

I have tried turning the idle mixture screw in to lean it out even with the idle a 1500rpm and it will die. Even a turn lean and it dies. I cannot get it to run in any other combination than what I have. I grab my flash light and check for raw fuel in the carb later today. OO It does not put out black smoke my my exhaust is already black (week old).

 

If your exhaust pipe is already black I'd say that is unburned fuel, aka rich mixture. We'll get there one clue at a time!!

 

OO, there is no doubt is running rich. I can smell it and see it on the exhaust pipe. I cannot get the truck to run on any other setting that is leaner than what I have. I was reading through my notes and I mentioned that the vacuum gauge was fluttering while I was trying to tune it. What does this mean?

Also, I noted when I did my compression test that cyl #1 and #5 dropped 5psi within 2 minutes of the initial reading. I had a system to ensure the tester was installed the same way each time. Does this do the same thing as a leak down test?

Just today I went and tighten up my intake/exhaust bolts thinking they may be causing an intermittent vacuum leak or something. That didn't make a difference either.

 

If fuel is pouring into the engine, you will never get it lean enough with the adjustments. You need to get back in the carb and see why it's pouring fuel down into the engine(if it is, you never did say if you looked yet).

If fuel is dripping into the engine this can be caused by dirt getting stuck under the needle valve, wrong adjustment, faulty float, etc.

 

Oh wow, another great clue!! The fluttering vacuum gauge needle. I dont have a chart handy with me, but there are charts showing every possibility, try google images. Or someone will volunteer help here. I'm not sure about that compression leakage, but I do know the purpose of squirting oil into the chamber is to help seal off the rings, so it's important I'd say.

 

Dave, I looked into the carb with the truck running and I can see fuel pouring onto the throttle plate from the venturis. There is nothing coming from the air horn. Addressing your suggestions, I have already tried the rebuilt old carb with the adjusted float, the new carb from Summit with the given float drop, low float, factory adjustment for the float, different the float needle from the rebuild kit. I have also checked AND cleaned the idle circuit twice. I also threw in the old float (and adjusted it to factory spec to see if there was any difference.

OO I will check the fluttering vac gauge on google tonight. I have a lot of hw to finish first.

 

Okay. You will notice one of the pictorials showing a steady needle being ideal. Any blips in the needle is the result of a loss in vacuum as a cylinder comes into play. Interesting stuff.

 

Yeah I noticed that. I found this helpful: Reading your vacuum gauge

"Drifts at Idle" and "Wavers Irregularity" are the symptoms I'm having. Gonna try gaping the plugs tomorrow morning.

 

You're not really in a true idle state, so some accomodation may be called for. If you change your "classification to needle wavers regularly, you could have a manifold leak at the intake port. I'm leaning that way....it matches that irregular compression possibly and the drop in compression. One thing I think is for sure to me is the fuel in the carb is a separate problem and may not be as serious. Maybe poor firing in those cylinders is leaving unburned fuel to be exhausted out the tailpipe. This is where my knowledge gets a little thin.