1980 rx7 backfire
 

Help a new member! My son and I recently purchased a 1980 rx7 to restore. There is a serious backfiring issue on deceleration as well as when the rpm's get close to the 7000 red line. My dog won't even ride with me any-more! Poor thing just shivers. The exhaust has been redone, every-thing looks o.k. from the exterior including all of the air induction routing, except for the air cooling pipe coming out of the thermal reactor (sounds nuclear) which has been capped off. IS this supposed to route to the muffler? Will this cause the reactor to get too hot which will cause the backfiring?

Thanks in advance for your input!

i allso have a 1980 rx7 all the othere sa's 79-80 and fb's 81-85 can and seem to have this problem. dont flame me for this but you best bet to stop this is to replace the whole system. most people like raceing beat so you mite want to check them out. the carbs run rich stock which is part of the problem. if you have any leaks that will make it worse. my car shots fire balls :shocking: . . do you live in a state where you still have to pass emissions on the car? if not lose the smog pump

Could be a seized shutter valve, which is a little valve that is suppose to stop fuel from entering the rear rotor, when decelerating, there for reducing or stopping back firing. It's located in the intake manifold, and runs off vacuum. But back firing is a pretty common thing with rotaries.

Welcome the world of rotaries. Must us rotorheads actually welcome the flame ball that torches the Tailgater's bumper....

Oh and the honduh kids are all going to want to race you.

Tonight I unhooked the air pump from the system to see if that would reduce the backfires. Nope, same issue. These are loud like gunshots. I don't think this has a shutter valve as that seems to have been introduced in '81.

no way the removal of the air pump makes it worst from my exp lol. I love the back fires :)...FIRE BALLS FTW!!!

Looks like you answered yourself with that one. If an air cooling pipe is capped off don't you think something will get hot? hmmm... YES. Will it cause backfiring... possibly yet not likely.

******ANY car that comes from any manufacturer is designed to run smooth and quiet and perform as long as possible. Yanking stuff out does not fix problems; parts are put in a car for a reason. Problems usually occur when maintenance is neglected or things simply wear out.

With that approach:

Put everything back to stock, airpump, etc.
Check for vacuum leaks, lose or cracked hoses, crappy gaskets... fix or replace.
Clean off any gunk in and around the carb.
Adjust fuel mixture. Your car is probably running rich.
Adjust ignition timing. Tuneup time!
Not sure about the SA carbs but if there's an anti-afterburn solenoid next to the throttle make sure it works!!!

Yes we enjoy backfires :D

every engine ive ever had in my rx7 has back fired at one time or another even the fuel injected version. basically your carb needs to be adjusted go to blackdragonauto.com get there kit to adjust the carb turns blue when perfect. works pretty well should get rid of most back fires. but not all of them. most of us just live with a few every now and then but before i adjusted my carb i blew up 2 mufflers 1 stock one and 1 after market unit. so be careful it will happen but just not as much. and question why are you spending time at the red line as much as you are.

There is no shutter valve on the Sa models.

It's not a adjustment problem on the carb. Between you and 84-12a-GSL you are steering this guy in the wrong direction.:wallbash:

The Sa's were famous for the backfiring problem.
Most common problems but not limited to are.
Defective air control valve.
Defective or out of adjusted dashpot on the side closest to the firewall that controls the return of the throttle.
Also need to test the deaccel system per the FSM. Factory service manual can be linked per member trochoid profile.

Also do a search with the terms backfiring. I have posted and so have others with the relevent material that you need for the Sa models.

Ditto

My old SA loved to pop all the time.

Wow, you guys are awesome. Thanks for the input! I only experienced the red line backfire once as I don't normally bring it up that high. Every other time, it is on deceleration. I ran it in the garage last night and would rev it to 5000 and let it come down and it would backfire everytime. The exhaust stinks like unburnt fuel. Also I should mention that this thing sat for 2 years before we bought it.

I'll give the tips a try, especially the carb tuneup (unburnt fuel - running rich?) and the air pump switch and deceleration adjsustments.

Thanks again!

Thanks RX7 doctor. Should have searched this before. Tons of tips available. I guess stinky exhaust is normal!

Thanks again.

A big one that everyone seems to have left out: exhaust leaks, and not just the main exhaust, but the air sleave as well. Think about it like turning off an oxy torch. If you shut off the oxygen first the flame just gets dirty (in the exhaust, it would smother itself), but if you turn off the gas first you get a very loud pop. Does your car backfire when you turn it off? This is most likely the reason.

Edit: reread the top post. Don't cap off the air cooling pipe! You can run it short of the rear but capping it will cause all of the air to enter the thermal reactor, thus creating the overlean situation described with the torch.

Any car can backfire...I saw a dumb Honda with a flame kit the other day. I was laughing so hard when I saw a puny spark coming out from his tailpipe.

Learn more than one thing new everyday on this site! :)

And were just throwing out options, sorry if they're the wrong answer. No need to bash your head against the wall.

Options are great if they pertain to the vehicle in question. Remember this is the "Technical" section. Meaning that the information that we provide needs to be relevent or accurate to the case at hand. :)

The thermal reactor is a heavy cast iron piece with multiple shells surrounding it. Any crack in that casting will be hidden. The SA Nikki carbs are jetted rich so that the thermal reactor can burn any leftover HC's in that hellish environment created on purpose to meet emissions of that time. Problem is that when you have a crack or any kind of leak allowing even the slightest amount of fresh air to be sucked into the thermal reactor, your controlled burn of exhaust gases becomes explosive.

I had the problem too in my old '80 RX-7....changing nothing but the entire exhaust (header, cat, pre-muffler, then two mufflers...) to eliminate the thermal reactor stopped the backfire COMPLETELY at any revs or conditions. Air pump was still a part of the system with a 3-way cat, carb was untouched. You have a leak somewhere.

:cool:

It could be a leak between the thermal reactor and the down pipe. If you need a gasket, I have one.

Thanks for all the help. I'm going to try the dash pot and decel system tests first. Then maybe lean the carb slightly. Hopefully I don't need to replace the exhaust.

Thanks again, between every-body, looks like we have all the options/diagnostics covered.

I'll post again, to let you know if I got rid of it. It's driving me crazy!

Jack it up, fire up th engine and listen for exhaust leaks around the thermal reactor. If you see any carbon, it's a dead giveaway. Some heat shields might be in the way.

If the most noise is coming from the middle of the system, it could be the air pipe (about 1" diameter) sorry, I don't know the atual name for it. It is mild steel and can rust apart. That happened on my Cosmo and caused an exhaust leak sound near the heat exchanger.

Everything seems to be operating properly except the dashpot was way out of wack. ALso, the white plastic valve leading to the carb from the dashpot was in backwards? I assume so because it seems to be a one way valve and the suction was keeping the dashpot rod from going down at all. THe dashpot was also adjusted to the top, meaning that it didn't do anything whatso-ever. Would there be a reason for this? Anyway after adjusting it, I took it for a drive and it seemed slightly better but still got some lowd gunshots - though not as frequent. The main time that it backfires is not only on decel, but when you gear down and the engine braking is fairly strong. Even going down a hilll with no throttle applied, it pops away.

Two more carb. questions. First, does any-one know what the factory-stock setting is for the idle mixture screw, ie how many turns out is it supposed to be? Yes, I've been messing with it and now it seems quite down on power. I'd like to return it to stock and then re-adjust the idle. SEcond, the hose from the air cleaner at the idle compensator is capped off, would there be a reason for this?

THanks to RX7 doc for recommending the FSM and to trochoid for providing it. THis thing is really a life saver.

Edit: reread the top post. Don't cap off the air cooling pipe! You can run it short of the rear but capping it will cause all of the air to enter the thermal reactor, thus creating the overlean situation described with the torch.[/QUOTE]

It is capped right at the exit from the main pipe downstream of the reactor. I asked this question, but can't seem to find a drawing - where is this cooling pipe intended to end up? Does it route to the exhaust? This could be my problem, Although it doesn't backfire when I turn it off.

If I was to replace the exhaust with one from racing beat and eliminate the reactor, would this help with the backfiring issue? Would I then get rid of the air pump and valves etc.

Sorry for all the questions.

Yep we had one bad 79 out of two of them. Blew out the muffler after it was all said and done. We swapped out the ACV and it went away!

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.

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

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

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.

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.

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

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.

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

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

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.

About your back firing problem
 

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.