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Old 11-13-2010, 09:41 AM
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Default 1975 fj40- Emission system help

Well, I have a question (or 10). I picked up a 1975 fj40, Colorado truck with 75k original miles 99.99% stock. I am the 3rd owner, it has set for years. I was told that the carb was rebuilt right before I bought it (not convinced of the quality of the build). Here is what I have done so far- drained the gas tank, installed new fuel filter, installed new oil pan gasket, new oil and filter, new air cleaner, new points, rotor, cap, plugs and wires, set the dwell, timing and adjusted the valves. Checked the compression, all cylinders around 110 while the oil pan was off, I looked at the cylinders and they look great. The plugs were quite black with carbon when I pulled them. The issue is that it is still idling quite rough. I checked the PCV valve and it seems to be working fine as there is strong vacuum and when I disconnect the hose the idle increases a bunch. I have checked the ports on the VSV and the ones that do have vacuum, have about 10inHg. I have the fsm on the 2f and Chassis /Body but do not have fsm for the emissions, just the Haynes manual which seems pretty vague on this system. Is there a good flow chart available for checking this system and it's components? Any help is greatly appreciated

Thanks, Brian
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Old 11-13-2010, 11:29 AM
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You might need to spring for the emissions manual -
http://www.sor.com/sor/cat223b.tam?xax=27398

-Phrog
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Old 11-13-2010, 12:56 PM
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Quote:
Originally Posted by Phrog View Post
You might need to spring for the emissions manual -
http://www.sor.com/sor/cat223b.tam?xax=27398

-Phrog
Yes I might... I could go here--
http://www.tlc4x4.com/2000/Parts/tlc_books.htm

Thanks
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Last edited by PhatFJ; 11-13-2010 at 01:01 PM. Reason: added link
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Old 11-13-2010, 06:52 PM
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Brian,

I recently (like this week) had a big idling problem that was due to an intermittent idle solenoid at the carb. I can't remember if the 75s have the solenoid or not; I know the 76+ does.

HTH--
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Old 11-13-2010, 07:27 PM
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Quote:
Originally Posted by PhatFJ View Post
...The issue is that it is still idling quite rough. I checked the PCV valve and it seems to be working fine as there is strong vacuum and when I disconnect the hose the idle increases a bunch...
Does the distributor have a vacuum hose connected to it? If it does, is the distributor a vacuum retard of vacuum advance? If the distributor is vacuum retard - remove the vacuum hose, plug the hose and see if that improves your idle issues.

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... Is there a good flow chart available for checking this system and it's components? ...
I don't have a good source of details for 1975 emissions. Is there a sticker in the hood detailing what should be there?
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Old 11-14-2010, 12:33 PM
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Hi Brian,

Lots of carbon normally means it is running too rich. A stock 1975 will generally have the vacuum retard distributor, and at our elevation 7 BTDC static timing is not advanced enough.

You might try advancing to around 11 or so to start - you'll likely hear the RPMs come up right off the bat. Before you do that make sure your points gap is around 0.018". It'll run like crap if it is too close.

Rough idle can also be caused by a vacuum leak. Try spraying carb cleaner around the base of the carb and around where the intake manifold mates with the head. If you hear a change in idle you have found a vacuum leak. You might also check the manifold nut/bolt torque - on a 75 they should be around 33 Ft/Lbs.

Don't forget to check your idle mixture adjustment. The screw is on the base of the carb on the valve cover side. To get to it you have to take the air cleaner housing off. I normally start at about 1.5 turns out, and then go from there. I usually end up somewhere around 2.5 to 3 turns out. This mainly has an effect on idle mixture, but has a slight affect above idle as well. Mainly the jets affect mixture when the throttle is opened up.

One last thing to check for is a crack in the intake manifold under the carb just above where it mates with the exhaust manifold. It is a pain to get to because you have to take the carb off for the visual inspection, but that is a fairly common place to crack. Cracking there is normally caused by the exhaust manifold butterfly either not being there or not closing after warm up. The hot exhaust gasses against the cold underside of the intake makes for quite the temperature difference.
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Old 11-14-2010, 09:05 PM
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Brian,

Would definitely recommend making sure your carb is tuned, and like Jeff said advance your timing a little bit more than it is. You should be getting more like 15" or 16" Hg vacuum I think at this altitude (someone who's done this more recently should chime in and verify that for me). It sounds to me like you have a vacuum leak somewhere. Emissions FSM is the shiz for emissions stuff. Your truck should have an idle solenoid too, I think they go back to '68 or even earlier. But having an erratic one is tough to diagnose, props to Carson for solving that one on his!
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Old 11-15-2010, 06:55 AM
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I probably should have expounded more on my hypothesis of the problem Brian is having. By disconnecting the PCV a large vacuum leak is introduced and with a vacuum retard distributor connected directly to manifold vacuum - the leak allows the timing to advance resulting in the increase in idle speed.

We experienced this a couple of weeks ago when we put the rebuilt carb on Pete's 72. The idle was not stable and the idle adjustment screw would actually kill the engine when we tried to increase the idle. The vacuum was also hovering around 10. When we removed the connection to manifold vacuum - everything improved to where we could dial in the adjustments.

Of course, I don't know if this is the situation with Brian's truck - but it should be checked.
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Old 11-15-2010, 07:03 AM
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Quote:
Originally Posted by subzali View Post
B...You should be getting more like 15" or 16" Hg vacuum I think at this altitude (someone who's done this more recently should chime in and verify that for me)...
I have read the "at least 15 at altitude" but have not hit that mark in my rig. The best I've been able to see is 14, which when adjusted for altitude is within an acceptable range. Here is the source I found that indicates 1 for each 1K of altitude for adjustment. The F Engine manual FSM indicates a healthy engine has at least 18 (non USA) and 16.5 (USA) of vacuum - so at 5K a desmogged F engine with at least 13 is within spec.

One of my back burner projects is to start a thread where others can post up the vacuum their rigs pull; along with engine information and altitude of where the vehicle was tested.

Quote:
Originally Posted by subzali View Post
...Your truck should have an idle solenoid too...
Yes, it will have an idle cutoff solenoid.
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Last edited by RicardoJM; 11-15-2010 at 08:26 AM.
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Old 11-15-2010, 09:11 AM
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Quote:
Originally Posted by RicardoJM View Post
I probably should have expounded more on my hypothesis of the problem Brian is having. By disconnecting the PCV a large vacuum leak is introduced and with a vacuum retard distributor connected directly to manifold vacuum - the leak allows the timing to advance resulting in the increase in idle speed.
Kinda sorta - with factory emissions connected properly, the distributor vacuum diaphragm is connected to the Vacuum Switching Valve (VSV), and the emissions "computer" decides when to apply vacuum to the distributor, which will depend the state of various inputs to the computer.

Instead of disconnecting the PCV valve, disconnecting the vacuum line to the distributor diaphragm will have a much more immediate effect of preventing any timing change due to vacuum, and if the line is pinched shut, then there won't be a vacuum leak through the VSV.
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