carb or fuel pump problem? (1 Viewer)

Joined
Sep 2, 2005
Messages
84
 
I've been reading the forums for a while, but this is my first post. First off, a little background:

1977 FJ40 - I've had it for about 3 months in my buddy's garage

Since I bought this thing, it's been having trouble idling without choking and backfires or "front fires" in the carb (nice fireworks).

Repairs done:

Checked and replaced vacuum hoses.
Replaced distributor and coil/ignitor (w/ reman 1978 models)
Cleaned out Aisan carb and replaced gaskets/acc. plunger
re-adjusted timing

Okay - Now to the latest repair. This weekend, we got on a whole new exhaust (headers to tailpipe - bolt on from Man-a-Fre). With the carb and old manifold off, we once again cleaned out the carb after it looked like one of barrels wasn't getting any gas.

We put the headers, manifold, and carb on and started it w/o the muffler or tailpipe. Loud as hell, but it ran well. Idled smoothly at around 700 and accelerated and decelearted nicely w/o backfire. Excited, we put the rest of the exhaust on. Now it's back to it's old ways: Backfiring and not idling.

My buddy thinks it could be the fuel pump since the bowl doesn't seem to stay full and the only thing (he was told) that exhaust backpressure would affect would be fuel/air mixture. Or does it just need a carb rebuild? I also noticed there is no return fuel line, but the gas cap seems to hold pressure.

Any help would be appreciated?

Please help a newbie. Thanks - Matt
 
Joined
Feb 6, 2002
Messages
15,133
Location
OC, CA
 
 
 
 
Under what circumstances does it backfire through the carb? Idle, revving it up stoped or under load, like up hill in 4th gear pedal to the metal?

Not idling: Bad, non functional idle cut off solenoid, vacuum leak, plugged idle circuit.

Backfiring through carb. This is an indication of a lean condition. Causes would depend on the conditions at which it occurs.
 
Joined
Sep 2, 2005
Messages
84
 
It seems to only fire through carb when accelerating (worse under load) and usually at lower rpms. Once I get it over 2200 or so, there are no acceleration problems. Thanks for the response. How do I check a plugged idle circuit?
 
Joined
Feb 6, 2002
Messages
15,133
Location
OC, CA
 
 
 
 
If the bowl doesn't stay full, then I would check the fuel pump output first. If the pump is OK, then the carb has a problem. The fuel pump doesn't explain the idle problem. If you verify the idle solenoid works and there is no vaccum leak, then the carb is more likely to be the idle problem. Did you adjust the idle fuel mixture?
 
Joined
Sep 2, 2005
Messages
84
 
Yeah, I forgot to mention that adjusting the idle/fuel mixture seemed to do very little, but it is hard to get a read because I can't keep it idled very long w/o choking. I remember I one point turning the mixture screw all the way in w/o any change in engine speed. I don't remember if it was choked at this point.

It seems to me to check the idle solenoid, replace the fuel pump, and then if I still have the problem, get a rebuilt carb. How's this sound?

Thanks again for your responses.
 

cruiserbrett

Moderator
Joined
Feb 19, 2002
Messages
2,565
Location
San Diego
 
 
 
 
have you checked for vacuum leaks? I went through a similar issue with a new intake manifold/gasket on my 2F, and it wouldnt idle, backfired from time to time, etc. figured out there were some serious vacuum leaks where the manifold mates to the head, and fixed 'em. Runs great now.
 

65swb45

Elder Statesman
Supporting Vendor
Joined
Apr 14, 2004
Messages
28,620
Location
818-953-9230
 
 
 
Valve adjustment? Compression test?

A marginal intake valve will backfire more at idle, when the 'draw' is low, and will seem to disappear at high rpm when the 'draw' is high enough to reuse the air/fuel mixture returning to the intake manifold before it can reach the carb.
 
Joined
Sep 2, 2005
Messages
84
 
Thanks for the replies. I have not throughly checked for a vacuum leak, but replaced the majority of the hoses. It has been de-smogged, so there aren't many. A compression test was done. All cylinders looked good (135+) except no. 5 (around 125). Do these numbers sound okay? - Matt
 
Joined
Nov 14, 2004
Messages
3,399
Location
PNW
 
 
 
It DOES sound like a vacuum leak nevertheless. I'd get a acetyline torch, propane torch, hairspray, wd40, or any other flammable sprayable stuff and start carefully spraying it along every place where air could be entering the intake path. Under and around the carb and all along the intake manifold gasket top and bottom as best you can with the engine running at the best and lowest possible rpm you can get it to keep running.
When your engine smooths out, picks up a little rpm you've found your leak.
 
Joined
Sep 2, 2005
Messages
84
 
Rebuilt the carb and added an electric fuel pump. Running like a dream now. Just need to address some desieling. Thanks.
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top Bottom