innertwist

iTrader: (3)

The cooling pipe just runs alongside the exhaust to the rear. If you still have the original muffler, the end of the pipe was welded to two standoffs on the lhs of the muffler. If the end of the pipe was removed, you should at least be able to see where it was ground off. It's not exhaust though, so it doesn't have to go all the way to the rear of the car. If it still does it after unblocking the pipe, The next most likely candidate is the gasket that Jeff mentioned, between the thermal reactor and midpipe.

Just so you have an idea of what you're messing with here, a brief explanation. From the air control valve (ACV) there are two pipes leading to the thermal reactor. The first one, which you can immediately see when looking under the hood from the rhs, goes directly from the ACV to the thermal reactor's air sleave. It's only purpose is to cool the thermal reactor (TR) under high load. This sleeve continues all the way into the midpipe, where it exits the system. The gasket between the TR and midpipe has two passages: the center for exhaust and then passages for the air sleeve. Given this, it's possible for the gasket to leek air in without burning through enough to allow exhaust to escape. Being so far upsteam in the exhaust, it also deals with a lot of heat.

The other "pipe" from the ACV goes through the intake manifold (though, fortunately for our debugging, it doesn't connect to any other system therein), out the rear of the mani, and to the outer sleeve of the heat exchanger. The heat exchanger is that exhaust lobe that looks like a catalytic convertor. It continues in this sleeve, through another sleeve in the midpipe where it exits into yet another pipe going to the TR. From there, funny thing, it actually shoots into the engine block before it joins up with the exhaust.

The ACV also has a solenoid controlled air bypass which leans the mixture under certain conditions, but for now focus on the exhaust plumbing.

If you did switch over to a header it would eliminate the backfire on all but the most extreme downshifts (eg, shifting to 2nd at 60mph and braking), but a header by itself would make the exhaust note obnoxious so you're better off with the whole system. However, the issue could be as simple as that pipe being capped and it would be so much cooler to restore that baby with the original system.

As for the idle, you want to adjust the speed first (air adjust - the big screw) before mixture. After you have the air set, just turn the mixture in until it starts hunting and then turn it out a half turn. Spec is 4/2 turns out.

780rx7

Thanks so much inner twist. I really appreciate you going to the effort to explain this. I knew the basics of the system but explaining the inner sleeves on the mid pipe helped a lot. Also, the idle mixture. Question though, stock setting is 4-1/2 turns out? or 4 half turns out (2 turns).

I have traced through the vacuum system and someone has really been mucking with this thing. The fitting on the altitude compensator is broken so a piece of heater hose was jammed around the outside of it and then really looselyattached to the carb fitting. The hose from the coasting valve was plugged into the idle compensator fitting on the bottom on the carb. There was nothing attached to the idle compensator on the air cleaner(first clue that something was wrong). The dashpot one way valve was on backwards and the dashpot was adjusted all the way up (probably because the valve would have been keeping the idle really, really high). Etc.

I'm no mechanic, but neither was who-ever was messing with it. Yeesh. Oh, well, that's why we got this car as a project. My son and I have been learning a lot. When I was a teenager I wanted this particular car sooooo bad. It is really cool to be restoring this thing.

Later

780rx7

finished testing everything tonight. Everything is hooked up properly and tested as per the fsm. STarted her up thinking this is going to be great! One block away, kaboom! Actually had to laugh out loud at that one. Too funny.

I'm pretty sure that there is a leak in the gasket before the down pipe. I think I'm going to go ahead with the Racing Beat Exhaust and hopefully pick up some more horsepower while I am at it.

Thanks again for all your help guys! I'll post pictures of our project.

Neil

innertwist

iTrader: (3)

Spec is 4 turns out for air and 2 turns out for mixture, but given mods, wear, minute vacuum leaks, ect, you're better off doing it to where it works best. Don't screw them in tight when you're adjusting them though.

I'm a little confused by the fact that you have a one-way valve on the decel dashpot, they switched to silicon type dashpots for 1980's (no hoses). Maybe you have a 79 carb? Otherwise, you might be confusting the delay valve with the choke delay valve. Oh well, the decel dashpot doesn't do very much anyway, many people leave it off completely.

780rx7

It's definitely the dashpot. My FSM is for a 1980 and the Haynes manual I have also details it the same as it is in my car. You sure you're not talking about '81?

Regarding the idle. The only way to get the idle down to around 1,000 (750 is spec's) is to have the air screw between 1/2 turn out and all the way in. The mixture screw is about 2 turns out.

The plugs look o.k., but I'm wondering about the other tune up parts. Probably time to get one anyway.

innertwist

iTrader: (3)

I have an 80' too, I know what I'm talking about. The 80 FSM makes no mention of a one-way valve for the dashpot, and the Haynes manual details a 79. Note, for instance, that the there is only one solenoid valve mounted on top of the carburator in the Haynes photos whereas your carb (should) have two mounted on top of it if your car isn't from CA. I realize that the Haynes says to adjust it (erroneously for an 80), but physically there's no place for a tube to connect. So, if your's does look like the Haynes manual photos, it's most likely that someone before you replaced the carb with a 79's.

If you can't get it to idle any lower than that with the air screw then either the throttle plates aren't closing enough or (more likely) you have a vacuum leak.

Tune up - that's up to you

chin428

I've had my 79RX7 since day1 in 11/78. Since new I had that normal muffled backfiring upon deceleration with an occasional bang when it was driven hard. Probably too much unburned gas fumes huh? Burned points like crazy and backfiring got worse as points got more worn. On third engine now, custom built by a little shop called Kearny's Racing w/ some mild porting done. They went electronic ignition w/ an 81 distributor and coils and got me out of the emmision testing drag by building on an old rx3? block. we removed all emmision stuff and the reactor and used a presilencer and muffler only. Its a bit loud now.

780rx7

Hmmm, interesting inner twist, I'll take a closer look. Regarding the idle, I actually thought of that. 1/2 turn out didn't make sense to me, so I set it to four turns out, mixture to 2 turns out as you mentioned, and then idled it down with the throttle stop set screw and then re-adjusted the mixture slightly. It idles nicer now, runs stronger and the backfires have calmed way down! They are way less frequent, and are more muffled - less violent when they do happen. Mostly on hard deceleration now. I'm sure it's not perfectly correct, but way better!

Still a work in progress though!

Thanks for all your help every-one! Without you guys I would be lost.

Neil

innertwist

iTrader: (3)

Word, glad to hear it (or not, in this case :-P). Wtg!

81gen1

i had this problem in my 80 SA too. i simply disconnected the hose from the smog pump to the carb hat and everything stopped.

the germ

I have an '85 rx7. When I re did the exhaust I had to remove the air pump hoses and when I drove it to see how it sounded, it back fired on the de exceleration. I would check on the air pump hoses. And remove on of the cats and see if its hallow.