86 150 stock 4.9 with some emissions pieces missing due to po removal. Went through drive thru today. While idling the rpms drop off and truck lopes like its running out of gas or something. Seems like rpms down in the 400 or so range. Then after 3 or 7 or 15 seconds (it varies) idle picks back up to normal. I do not have a tach yet so im guessing rpms. It occurs like im running ac (which i dont have) and the compressor kicks in and out. Truck runs good otherwise. I have saw posts where orhers adjust either carbs or timing by seasons but i forget which.

Any thoughts about possible causes. It has done this before but i just happened to think about posting it. Temp was near 90 today if that matters. Truck does not nor has not ran hot or overly warm in my ownership. Thinking back i do think it only happens on these hotter days.

 

Do you have a real temp gauge on it? How hot does it run? The fuel in the carb will start bubbling or boiling when the temp gets around 230F. That makes the carb start working like a coffee maker, where the water boils up and drops into the coffee. The same thing can happen in a carb, the fuel will start bubbling in the carb bowl and the bubbles will push fuel up and out of the main venturi and drip into the engine. No fuel comes out of the main venturi at idle, only when the rpms pick up.

 

No im on stock gages. They do work well. Temp stays around 1/4 gage. A year or so ago it went to 3/4 because my radiator was leaking and was low on coolant but no overheat. Oil pressure runs right around 1/2 gage. Voltage is always slightly right of center but goes to 3/4 with headlights.

Truck does run well when driving. A bit of power loss from when i first got it 2 years ago but i think thats just perceived as getting used to the truck. If i can get it to do it again ill look in the carb but even on a hot day like today, if i open the hood its not very hot in the compartment.

 

Anybody else have any ideas?

 

I'm thinking like Dave is - heat. Are you running the stock air cleaner? If so, is the blend door stuck or is the temp sensor broken and causing it to close all the time? The really hot air could cause the fuel to boil like he was talking about.

Next, do you have any temp-controlled vacuum switches in the vacuum advance line? Or, does it even have vacuum advance?

 

No, i have a open 3-1/2" or so cleaner. Similar to what the used to have on a 3 deuce set up. No vac advance. I still have the feedback system. Choke works fine, so does fast idle and it drops down to regular idle with a throttle blip. Underhood temps dont seem hot when i open the hood. I dont have a numbered gage though. I didnt go out today so i didnt get to see if it did it on a bit cooler day. Im not ruling out his coffee pot idea, im just looking for other insight too.

Was it clear that the idle drops and picks back up on its own? It bogged for a random number of seconds and then would smooth out for 3-5, then back to boggy stumble. When i let out on the clutch it pulls away on its own with no bucking or anything. Gonna try to get a look at it tomorrow.

 

A rich mix will cause the engine to bog down, but that pulls less fuel in so it allows the engine to run better and speed up. But speeding up pulls more fuel and it bogs down. I've seen them do that on and on. So, maybe the idle mix is just too rich?

 

Ill check that out.

 

Your opening thread stated that some emission pieces are missing and then later, you mentioned that the truck still has the feedback system. Unfortunately, that is not good. For the system to be properly controlled, all sensors, vacuum lines and components must be present and working properly.

There is an electrical port which can be tapped into so the computer can tell you if something is wrong. Our moderator, ctubutis, provided me with a link, previously. I will attempt to find it for you.

If anything is not connected, the computer locks the timing @ 10 degrees BTDC and adjusts the carburetor to its richest setting. This equals poor performance and bad fuel economy.

Let me try to find that link and I will get back with you. Here it is:

http://www.therangerstation.com/tech...ry/OBD_I.shtml

 

David - I'd rep you but can't. You deserve it for that post, though.

 

I got him Gary.
Yeah 86 i knew about pulling codes and the timing freezing. Im hoping to do a good bit to it in feb with taxes. Ds swap with carb. Maybe zf swap if i can get the pieces cheap enough. This is just something that recently occured and is different from its normal behavior.

 

I do have a old school timing light. Whats is base recommended timing on these 300s? Just got home and am gonna go tinker with it a bit in a sec. Ill look on the core support tags if i can read them and see if it tells me. Just wondering your recommendations as well.

 

I don't know about the six, but stock on the eights was about 6 BTDC. However, I usually run them closer to 12 - 14.

 

JayzDaddy,

On the six, it is 10 degrees BTDC. However, they seem to run well @ 12-14. You must disconnect the spout wire since yours is the feedback system.

Last night, I was thinking back over what has been said in this thread. You have indicated that it appears to be temperature related. Gary asked if there is a temperature controlled vacuum switch? Hmmm? What sensors the system originally has were O2, MAP and a temperature sensor which is mounted in the thermostat housing. This temperature sensor does not work with the temperature gauge, but tells the computer what temperature the engine is. I do not know which of these you still have ["86 150 stock 4.9 with some emissions pieces missing due to po removal."] On my son's 1984, which is equipped like your's, the O2 sensor went out and the truck ran rich. At another time, the MAP sensor went out and the truck pinged badly, but ran well otherwise. The temperature sensor went out and the computer thought the engine was cold at all times. The carburetor was very rich and power was down as well as gas mileage. I imagine that if the sensor failed another way, the computer might "think" the engine is hot at all times and the carburetor would again not be adjusted optimally?

One time with the engine idling at normal operating temperature, I pulled the electrical connector off the temperature sensor and almost immediately, the truck's idle increased significantly. When the sensor was reconnected, the idle dropped back to normal.

Check the wiring going to the sensor. Perhaps the sensor is failing or the wiring "jiggles" and a poor connection causes the computer to try and adjust to the varying conditions?

Random thoughts.

 

Very good "random thoughts".