yamaha
yamaha SuperDork
1/29/13 3:26 a.m.

Well, I know nothing about carburetors obviously, but after pulling someone out of the ditch tonight with the farm's 87 f150, it was spitting a fireball out the tailpipe at idle......is the a carb issue or maybe a misfire? It was hard to start beforehand as well if that matters. As usual, the I broke it, therefore I get stuck fixing it clause applies. Its an 87, 5.8L 4bbl.....and it sits a lot, prolly why it has 54k still. Thanks in advance.

fasted58
fasted58 UberDork
1/29/13 4:04 a.m.

ignition timing

Toyman01
Toyman01 PowerDork
1/29/13 6:42 a.m.

Step one for tuning a carb.

Ignition, plugs and wires. Check them at least and if it's been a while, throw a cap, rotor and a set of plugs at it. Then move on to the carb.

GameboyRMH
GameboyRMH PowerDork
1/29/13 6:47 a.m.

Yeah if it's at idle I highly doubt it's the carb so +1 for ignition timing.

Carbs are nice and simple to tune, you can generally get them running decently with a screwdriver. It's when an engine has more than one that things get complicated...

JohnInKansas
JohnInKansas Dork
1/29/13 7:14 a.m.

90+% of "carb problems" are really ignition problems. Another +1 for timing.

foxtrapper
foxtrapper PowerDork
1/29/13 7:17 a.m.

Don't discount a burned exhaust valve.

Streetwiseguy
Streetwiseguy UltraDork
1/29/13 7:39 a.m.

It not timing. Its never timing. People still come here and say, "I think the timing is off." In any vehicle built since the early 70's, if the timing is off, it means one of two things- either there has been some sort of mechanical failure in the cam drive or distributor advance mechanism, or somebody has been fooling with it.

I'm sorry. I should have out this in the "rants" thread.

Anyway... If the carb is flooding badly enough to send raw fuel out the tailpipe, you should be able to tell from the eye watering stench and the stalling and not restarting. Likely a dead plug or wire. If you have a dead hole, you can tell by the temp of the exhaust runners. There are 8 cylinders, you got 10 fingers, touch each one to a different runner. Whichever finger isn't blistered is the dead cylinder. I keed, I keed...a light spray of water will do it.

Zomby Woof
Zomby Woof UberDork
1/29/13 8:29 a.m.
Streetwiseguy wrote: I'm sorry. I should have out this in the "rants" thread.

I got you covered.

GameboyRMH
GameboyRMH PowerDork
1/29/13 9:03 a.m.
Streetwiseguy wrote: some sort of mechanical failure in the [...] distributor advance mechanism, or somebody has been fooling with it.

Both more common than carb problems in my experience.

iceracer
iceracer UltraDork
1/29/13 9:19 a.m.

Bet the choke is stuck.

iceracer
iceracer UltraDork
1/29/13 9:19 a.m.

Bet the choke is stuck.

yamaha
yamaha SuperDork
1/29/13 9:50 a.m.

The choke could be it, but ignition might make more sense........could condensation in the cap cause this havoc? I only wonder since cap, rotor, plugs, and wires were replaced last fall with factory motorcraft stuff.

If it makes any difference, the truck has been sitting in an unheated barn.....and we have had 30-40* temp swings in just the last few days.

yamaha
yamaha SuperDork
1/29/13 6:07 p.m.

Oddly enough I went to start it today and everything was fine. I assume that it was just the choke stuck?

curtis73
curtis73 UltraDork
1/30/13 8:32 p.m.

Could be a stuck choke, but if it was rich enough to shoot fire out the pipe it shouldn't have even been running.

Carbs are mechanically daunting, but they are just that... mechanical. They get dirty, or things stick from time to time, but they are pretty reliable pieces of engineering.

More often than not, something in the engine changes and therefore no longer suits the carb's mechanical settings. My neighbor refuses to accept my diagnosis of his old F150's tired 302. He keeps wanting me to adjust the carb to make it run right but his problem is the fact that it has 300k on it and the rings don't seal anymore.

mr2peak
mr2peak Reader
1/31/13 1:07 a.m.

Is it a carb that requires float oil? If so, it's probably all dried out.

yamaha
yamaha SuperDork
1/31/13 9:42 a.m.

I have no clue......I believe it is a holley.

iceracer
iceracer UltraDork
1/31/13 10:01 a.m.
curtis73 wrote: Could be a stuck choke, but if it was rich enough to shoot fire out the pipe it shouldn't have even been running. Carbs are mechanically daunting, but they are just that... mechanical. They get dirty, or things stick from time to time, but they are pretty reliable pieces of engineering. More often than not, something in the engine changes and therefore no longer suits the carb's mechanical settings. My neighbor refuses to accept my diagnosis of his old F150's tired 302. He keeps wanting me to adjust the carb to make it run right but his problem is the fact that it has 300k on it and the rings don't seal anymore.

In the old days with hand operated chokes, we would back off the throttle, pull the choke for a few seconds, get back on the throttle and would often get flames or a least a "backfire.\" then there was the spark plug in the tailpipe and a model T coil. Glorious flame.

Kenny_McCormic
Kenny_McCormic HalfDork
1/31/13 12:36 p.m.

Just remember, when they misbehave, they tend to enjoy spitting fire out the other way too. So keep a fire extinguisher handy and avoid looking down the carb with it running. I would first pull the air cleaner and check choke function. Does the truck sit a lot? A carb rebuild by a competent individual may not be a terrible idea, you'll want to find somebody 50+ years old for that.

I highly recommend marine grade stabil for anything with a mechanical fuel system that sits more than a day or two. Seems to keep the fuel from gumming up in the bowls, or in the case of ethanol gas, attracting water and corroding the bowl, a bowl full of fluffy zinc oxide will make it misbehave too.

yamaha
yamaha SuperDork
1/31/13 3:47 p.m.

It does sit a lot, and thanks to our local co-op, it drinks only 90plus no-ethanol fuel. We've had the carb rebuilt a few times over the years though.

dean1484
dean1484 UltraDork
1/31/13 4:47 p.m.

A friend of mine use to refer to carbs as "suck.. . . . . . " Ok .. . .if i finish this I will get baned from here. Never mind carry on.

iceracer
iceracer UltraDork
1/31/13 4:58 p.m.

Well they do work on suction.

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