86 300/6 stock 1bbl
So i went to leave today and ran out of gas in my driveway but a strange thing happened. When i have a cold start my engine chugs and lopes for about 40 seconds til the idle speeds up and smoothes out. Well today it started and chugged but the last 3-4 seconds it idled like a purring kitten. This was only 10 seconds max into the warm up process. So i began wondering if maybe my air mix is too fat. Can anyone tell me where the mix screw is on a factory feedback 1bbl? Id like to play with it a bit.


Also this summer i put in a new fuel sending unit. My old one would go to past full but inside of 50 miles show empty even though i had 1/2-3/4 tank. The new unit worked well at the start but now it shows empty at 100 miles. I typically have 6gal left. What should i check over to try to get it to read properly? The unit wasnt damaged during the install. I ran out of gas today due to a pinhole leak. Ill have the tank out thursday to patch it. Id like to maybe fix the gage while im there. Is there something i can adjust in the dash?

Thanks in advance for your thoughts, ideas, and advice!

 

There is nothing to adjust anywhere, much less in the dash. If the gauge shows empty too early then you either have a bad sender or the wrong sender.

As for purring early as you run out of gas, I agree the mix is too fat when warming up. But the problem isn't with the idle mix, assuming that the idle mix is correct when warm. The idle mix screw is set for warm conditions and the choke is used to fatten the mix up. So back off of the choke a bit.

 

The choke is fully closed when i prime it. Guess that does make sense to me. Truck runs well when warm but i believe it idles a bit high. Need to knock it down 50rpm it sounds like. Was hopin to gain a bit of mileage from my 10-10.5mpg.

I will look at opening the choke a bit when i find the screw. Thanks for the advice!

 

The choke is adjusted by loosening three screws that secure the black cap and turning it a notch.

 

JayzDaddy,


I believe something is wrong... you should be able to get better mileage. Tell us more about your truck, engine, differential ratios, tire size, etc.

 

To start with: Thanks Gary, got it open to about 15% and it idles much better at first cold start.

Now 86 i have a 86 150 4x4 300/6 with about 202k. If the po's notes were correct i am about 70k on a rebuild. Dont know the extent of the rebuild but it was gone into. Tires are 31/10.50/15 at. Np435 granny 4. Axles are either 3.00/3.05 ratio, open diff 8.8/d44. I dont hot dog. Usually im in 4th by 25/30mph.

Plugs, wires, cap, rotor, icm, were changed last year and summer before. Less than 10k on all. Truck idles well but a bit high. The po removed the air pump. Temp runs low/mid never hot. Timing is spot on 10* but doesnt move. I have a ignition/carb swap planned for spring.

Im trying to source a 5spd/tcase. That should help alot.

Id welcome any thoughts you have.

 

Also there is a flat head screw low under the carb. Passenger side, about halfway between carb and manifold. Whats that for?

 

Glad it helped. But I agree that you should be getting better MPG. In fact, a lot better. Read this thread to see what you are missing. Granted you don't have his overdrive tranny, although you are looking into that, but you still should get much better MPG.

You say the timing doesn't change. Do you mean it doesn't advance when you rev the engine up? If so, there's a big part of your problem.

 

Gary beat me to adding the link. Thanks to him and others from our forum family, my mileage is good!


Your 1986 4.9L is computer controlled. It was a stop-gap measure until fuel injection was ready. When this "feed-back" system is working, it is great! But, when something goes wrong [bad/missing sensor, wiring, vacuum hose issues, computer, etc.], the computer locks the system in a "safe" or "limp home" mode, to protect the engine. When this happens, the carburetor is electronically adjusted to the richest mode and the distributor is locked at whatever the static timing has been set to [normally 10 degrees BTDC].


When in the "safe" mode, performance is diminished and gas mileage goes way d-o-w-n! When I purchased my 1986, the 1st tank gave 9 mpg and the 2nd was 11 mpg. Yuck!


As Gary pointed out, my truck has O.D., so where you have 1:1 direct when in 4th gear, mine has the advantage of 0.71:1 in O.D. [3rd gear is 1:1, on mine; 55 mph = ~1575 RPMs]


My rear gears are 3.08, so we are close.


Your truck weighs more [4 wheel drive] and with oversized tires [31X 10.5 X 15], most likely sits up higher and has more rolling resistance as well as aerodynamic drag.


By the way, unless your speedometer has been corrected for the larger than stock tires, it is reading slow, which means you are going faster and farther than shown. This means you are likely getting better mileage than you think.


Your mentioned the mileage [202K with rebuild at ~130K], mine has 180K with no internal engine work.


Read the link provided by Gary and ask any questions.


Speed kills mileage in these truck. I drive 60-62 mph and drive like an old man [getting easier all the time! ] and act like there is an egg between my foot and the throttle.


Short shift... get into the next higher gear just as soon as you can without lugging the engine. I normally drive between 1100 and 1700 RPMs.

 

Yall are correct. Timing is locked. Didnt know it made a fat mixture. Anyway i bought the wire plug from jims rebel. Buying new carb and dizzy. My max speed is 45/50. Very rarely do i see a interstate. 31s have been on since feb. Before i had 30/9.50. Got a great deal on these 31s and they "look better", gives a better stance. Mileage has not changed from 30/9.50 to 31s. I drive more by feel than speedo. Every few months i might do a full pedal pull just for fun but my guess is i stay idle to 16/1800rpm max.

I will delve into that thread. Thanks guys.


Any idea about that screw?

 

I don't know what screw it is. Can you post a pic?

 

Re: the screw, and I am going from memory, it should be the idle air/fuel mixture screw. The feedback carburetor originally had a plastic "anti tamper" covering over this screw, which was easily removed.


On the driver's side, there will be two screws. One is for the high speed idle adjustment [while on choke] and the other one is for normal idle speed.

 

I knock the idle down 100 or more (by ear). It idles smooth and sounds about perfect. Maybe a touch high still but when i get a tach ill get it set right.

This am i did cold start (42*) and no fast idle, but it was right in the previous idle speed and ran fine. Patched the tank (buy new one in spring with carb, dizzy, and maybe a ls for the 8.8), re-bled my brakes from my previous broken brake line adventure, and got it set up pretty good i think. I am going to try to attach a pic of the screw but im on my phone. Sometimes it dont work.




Between the bottom bolts

 

I agree with David, that's the idle mixture screw.

 

Thanks.

I ran it in fully then out 1-1/2. Gonna start there tomorrow cold and see what i can tinker out of it. I appreciate yalls help. Been quite a few years since ive owned carbs . Never had a 1bbl before. Used to be really good at tuning by ear.