Thread: Desmog
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Old 12-13-2010, 02:27 PM
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subzali subzali is offline
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Join Date: Aug 2005
Location: Denver CO
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Originally Posted by Jacket View Post
I enjoyed reading the thread as well - just a good overall view of the emissions systems on these trucks. Other than a deleted air pump, mine is mostly original and I'm afraid to touch it....
That's where I've been for the past 5.5 years...

Originally Posted by Jacket View Post
I don't have anything to offer technically, but I too would question the reasoning if you stand to achieve little or no functional gain, but risk "destabilizing" the system to eliminate a few hoses? Or are you trying to solve another problem as well?
When I first got my 40 I followed the herd mentality I guess of "desmogging is cool." I was always of the opinion that these trucks ran great before smog equipment came along, so all the smog equipment does is add weight, rob power (smog pump, EGR), and "de-tune" the engine to control the emissions. But what I think I'm hearing is that while the smog stuff does all of that, it's also tied in with other things that actually make the engine run better by optimizing fuel and spark a little better. To give up the "disadvantages" of one you pretty much have to give up the "advantages" of the other. This is a new concept to me, so that's why I'm thinking of either taking it back to the original configuration or leaving it as-is, since it seems to work fine.

But the smog pump location is perfect for the on-board-air compressor I had in mind

Originally Posted by RicardoJM View Post
Yes and no; mostly no. You are correct, setting the timing is a "mechanical" action, i.e. you turn the dizzy. Each distributor has a "mechanical advance" system built in. As the engine RPMs increase the dizzy's built in advance system ( "mechanical advance") changes the timing - it advances it. The basic principle is that at high RPM, each cylinder can spark a little earlier than at low RPM - so that power and fuel usage are "maximized". In the evolution of things, a vacuum advance system was added to use ported vacuum to further maximize power and fuel usage under certain conditions, i.e. when ported vacuum is high.
I didn't know the distributor had a built-in mechanical advance system. That makes perfect sense and is what I would suspect, but I just didn't know that that was the case.

Originally Posted by RicardoJM View Post
Much of the above is mystery for me as well, relative to the late 70s Aisan carbs. I am hoping that your thread on MUD will result in some additional insight and understanding - someone out there has to know this stuff.
Jim C. does but he doesn't frequent the 40 series section much, and Charlie Glabe (Pinhead) usually knows what he's talking about, so I have respect for his comments in my thread.

Originally Posted by RicardoJM View Post
About all I know for sure is that automotive engineers have evolved things over time. In the 70s they started using engine vacuum, controlled by speed sensors (initially) and components/computers (next in the evolution) to advance or retard timing thereby altering the dizzy's built in advance system resulting in even better maximization of power and fuel. Like you, I'm not clear on the detailed specifics. We are now in the age where the dizzy is gone, and each cylinder has its own coil pack and the computer runs the show. Great for low smog, more power and improved fuel economy.

Not knowing how vacuum is routed through all the ports, it is not advisable to start changing connections on the dizzy. The results may likely not improve the performance of the engine. That said, I am pretty sure that if you have a ported vacuum port on the carb - it is ok to hook it to a vacuum advance port on a distributor.

Again, I'm not familiar with mid-70's and newer carbs - but on all the early carbs the ported vacuum port is just to the right of the idle mixture screw. This port does not exist on all early carbs but can be added. It is a real simple thing to do, but has to be very precise. I didn't have the confidence to do it on my own, so I sent my carb base out to get it done.

You are correct, when setting engine timing if you are pinging it has been advanced too far. Also, if it is difficult to start, it has been advanced too far.
I know I have ported vacuum, I'm just not exactly sure where it is. I think ported vacuum means it's located above the throttle plates, so it only sees vacuum when the accelerator is pressed and the throttle plates are opened.

My big problem with taking the system back to stock is with my ignition/dizzy/carb. The PO said that the vehicle backfired with the smog pump hooked up (I never experienced that but didn't have the smog pump on for very long), and I wonder if that's partly a function of the '78 ignition, '79-'80 dizzy, and '78 carb?

Sigh, I really like not having to adjust points...

Ricardo, doesn't the big cap dizzy have multiple vacuum cans on it as well? Do you just have one plugged off?
Matt Miller
TLCA# 13684
1977 FJ40 2F "Brahma" + Lockright, tach, Warn 8274, FJ60 Power Steering, no more Sanden OBA (factory emissions), so Puma OBA
1996 FZJ80 1FZ-FE factory lockers + Safari intercooled turbo, Warn M12000, OEM CDL switch, cup holder, and hand throttle, Metric TLC leather, heated seats, JDM switches, Puma OBA
2000 Tundra Limited TRD 2UZ-FE SOLD

Last edited by subzali; 12-16-2010 at 11:42 AM.
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