chinook/fj55

13Swords

John,
I have a '78 Longbed Pickup & have also been through the backfiring problem & now mostly have the problem solved. I think this is a problem with your throttle linkage being worn, sloppy or binding up. I will explain what I have done plus my results. There could be another problem (or two) causing your problem, I will mention those at the end.

I also went through a long list of repairs on mine: Full desmog done (no smog testing in my area): deleted/removed air pump, air piping to exhaust, all switching valves & EGR Valve, blockoff plates installed. New ignitor, coil, distributor, rotor, cap, wires, plugs, fuel pump, fuel pump relay, fuel pump resistor, fuel filter, all new fuel lines both steel & rubber, freshly rebuilt carb from National Carburetor. New exhaust manifold gasket & pipe gaskets. New timing chain, gears, guides & tensioner. New Battery & ground cables, new fusible link wire, fuse box removed/cleaned/restored. New ignition switch. New battery. Timing set to spec (8 degrees BTDC). Gap set to spec on pickup coil in distributor (.008"). New vacuum lines. Tested for vacuum leaks 3X, found none. Carb tuned to factory specs (Mixture 1.75 turns out, hot idle speed 800 RPM).

After all that, I had the exact same backfiring problem you describe; "pop" on letting off gas while in gear, "pop" when changing gears. The only difference was that it would happen half the time; the rest of the time everything would act normally.

Then I noticed that the ball & socket ends on the throttle rod under the hood, near the firewall, were very sloppy & "floppy". This part is no longer available from Toyota or aftermarket so I built a replacement using threaded rod & 2 new ball & socket joint rod ends from Dorman (Dorman Part # 115-001). That plus a little hardware from the hardware store & a little engineering/modding got it done. The backfiring didn't happen as often, it was still there, but it did improve. Next I removed the gas pedal rod & hinge assembly from the truck & had a look at it. It wasn't hard to take apart. I noticed that there was a groove worn into the rod where it contacted the hinge part. The holes in the hinge were still round. I filled the groove in the rod by welding it then grinding the weld back so the groove was gone. re-assembled the piece in re-installed it in truck. Again, this improved things a lot, but the backfire problem was still there, now just 5-10% of the time.

The last thing I need to fix is another hinge unit in the throttle linkage chain. There is a big black hinge on the firewall, behind the carb location. This hinge holds the end of the rod coming from the carb itself. It's there to allow for motion of the cab or engine but maintain the rod link from the gas pedal to the carb. This hinge also seems to have some slop & play in it, so my guess is that it's binding up. I just need to remove it & tighten up the hinge fingers, lube it up & put it back.

Slop or wear on any of the "pivot points" I described could be causing your throttle to bind & hang open a bit when you release the gas pedal. This keeps the butterfly valve in the carb open a bit too much, causing a lean condition (too much air). The mixture is too lean to be burned by the spark in the cylinder & passes out the exhaust valve into your exhaust system, where it can concentrate the gas vapor in your muffler. It then gets "rich" (more vapor than air) & is very hot. Once it gets near the end of the tailpipe, it finds the air it needs to burn & basically explodes. This is what probaly happened to your muffler during the shop testing it. Also you say it "calms down" after a big backfire; this could be the vibrations "unsticking" or unbinding the linkage. On mine, it would backfire & try to stall, & I'd slap the gas pedal with my foot. This sometimes brought everything back to "normal", sometimes not.

So have a look at the throttle rod system from gas pedal to firewall hinge plate & see if you have any wear or sloppiness in it. These trucks are now 40 years old & are bound to have some, somewhere. You can find some used replacement parts out there, hopefully from a 75-78 RV version of the truck, which have lots less miles on them & should be in good shape. I recommend Yoda Jims in Denver for this (yodajims.com). Or, you can just convert to a Toyota '79 & up throttle cable system & eliminate the linkages entirely. Toyota ditched the throttle rod stuff after '78 & went to throttle cable from '79 onwards. I've never done it before, but have seen write-ups on it on the web, so I know it's possible to do.

Other possible causes of your backfiring problem:

1. Throttle Positioner (Dashpot) not working due to being adjusted incorrectly. This is a mechanism on the front of your carb that allows the throttle to close gradually instead of slamming closed when you let off the gas pedal. It's working right when you totally let off the gas & the engine takes about 2 seconds (maximum) to come down to idle speed. If it's not working & the throttle slams closed, this creates an immediate rich condition (more gas vapor, not enough air). Again the mixture doesn't burn in the cylinders & goes unburned into your exhaust. It finds air near the tailpipe end (or even in the muffler) & goes "bang". There is a screw on this used to adjust it. I don't have a spec on it, but I did mine by ear & it works the way it should now.

2. Is the carb new, rebuilt, original? It could still have vacuum leaks in the carb if original, especially around the throttle shaft. You can check this while it's running by spraying some carb cleaner at the ends of the throttle shaft; if the engine speeds up, the throttle shaft bushing are worn & the carb needs rebuilt. Also, if the carb has ever "popped back" through the carb throat, that can damage the power valve inside, causing a rich condition. Again, the carb would need a rebuild if that's the case.

3. You didn't mention your ignition coil. Has it been tested to see if it's ok & in spec? A weak or faulty coil can cause backfiring, not putting out enough "juice", causing weak spark & incomplete burning of the gas/air mixture. You'd have to test the primary & secondary resistances with an OHM meter, or most parts stores will test it for you for free. The spec on ignition coil resistance for '78 is: 1.4 to 1.7 Ohms Primary Resistance, 6,500 to 10,500 Ohms Secondary Resistance. Anything out of these ranges, replace the coil with a quality unit.

4. EGR Valve could be clogged or stuck open. More likely clogged in your case, as a stuck open EGR will cause knock or ping, & you say the engine doesn't do that. It's not too hard to get the valve off the engine so you can clean it out. These things get carboned up bad over time. You just have to soak it with carb cleaner or gas & get the carbon deposits out of it. Unless the spring or metal diaphragm inside have broken, there is usually no need to buy a new one, just clean it out & put it back.

That's all I can think of (yeah, I know, that's quite enough!) Hopefully you can use some of my experience with this to solve your problem. Keep at it, you'll get there.

chinook/fj55

chinook/fj55

13Swords

You are most welcome, just hope it helps. I'm not too sure what you mean by "start coil". By Igniter I mean the box that sits on or near the coil. It is the "brain box" that reads the signal from the pickup coil in the distributor & fires off the coil. My Igniter gave up on me once I got the truck running again (naturally!) so I converted it to a GM H.E.I. Ignition Module. All of the early ignition modules pretty much work the same, & this is a reliable, cost effective solution. Whole conversion cost me $20 in parts, & the H.E.I. module works with the stock Toyota coil with no problems. Runs great.

Your starting routine does not sound common at all. If the carb is adjusted right & there is nothing wrong with it, you should be able to put it to the floor once, release, turn key & it should start right up & keep running, without any further gas pedal input. Once you do get the truck running using the method you describe, do you have to keep some pressure on the gas pedal to keep it going, or can you leave the pedal alone? Does it have high idle when cold? How is the idle when completely warmed up? When you say "That pool of gas is necessary or it dies and I have to wait a bit to try again", it just refuses to start? Does it act like it's flooded? Strong smell of gas underhood after that stall? Definitely sounds like an air/fuel mix or fuel metering problem. You do know where the the throttle shaft ends on the carb are don't you? If not, look for a round piece in the base that rotates when you work the throttle. There is one on the front & the back of the carb. Those are the specific spots to spray during the test. You might also want to try the spray test when holding the throttle open a bit. Sometimes bushing wear will only be in a certain spot.

The backfiring may seem better in neutral because there is almost no load on the engine. Engine load will affect vacuum levels, which effects how much gas the engine will pull from the carb. Exhaust back-pressure shouldn't be a problem, as you have a brand new muffler, exhaust manifold gasket & your pipes are clear (aren't they?) Might want to double check the torque on the exhaust manifold nuts (32 foot pounds) to make sure the manifold is seated. Also make sure the bolts/nuts on the fresh air tube to exhaust manifold are tight. Those joints have their own gaskets as well.

I do know some things about how the emissions are supposed to work on this truck, but need specific questions. As I said I deleted my emissions system almost entirely (kept the charcoal canister, thats all) but did a bunch of reading on it first before removing it.

"It will pop backfire on idle If I goose it and let off the gas pedal." Does the engine wind down back to idle fast or slow? Does it pop the instant you let off the pedal or is the pop delayed?

I'd be glad to offer other suggestions but hopefully the ones I've already given you will help solve the problem.Take your time on it, it's better to take a while & get it right. Anything you post I will get email notification on.

papawhellie

chinook/fj55

chinook/fj55

83

I did an almost complete de-smog of my 78, and it ran great that way. It included removing the smog pump altogether (didn't care for the extra weight), but yes, you can just take off the belt. You're in PA, right? Do you need to pass smog?

The vacuum systems on these trucks are so old. It's really hard to track down replacements, and it'll run better without most of that stuff anyway. Doing a de-smog is pretty cheap and easy. You can buy a pretty cheap kit, or do the really cheap way which just consists of vacuum caps, more or less.

chinook/fj55

13Swords

Just to be clear on 2 points.

1. As an actual test, did the mechanic or you take the belt off the smog pump & the problem went away? Or was the mechanic just suggesting that you do this? This isn't exactly clear to me.

2. The smog pump is a positive pressure pump, it has nothing to do with the vacuum system, emission vacuum or regular vacuum, at all. So taking the belt off the pump will have zero effect on the emission vacuum system. All that will do is stop pumping fresh air into the AIR system & your exhaust manifold (more on this below).

At least it's easy to take the belt off the pump & take the truck for a test drive to see if it makes any difference. No, taking the belt off the pump won't hurt anything.

If your thermo valve isn't working right, it's not regulating the emissions vacuum system the way it should. That thermo valve controls something called a "PAIR Valve" (Pulsed Secondary Air Injection). That valve regulates when the smog pump pushes fresh air into the exhaust system at the exhaust port, to cause ignition of unburned fuel vapor before it leaves the exhaust pipes. This is supposed to happen when the engine is below operating temperature (coolant temp less than 140 degrees F). The valve is supposed to shut off the airflow to the exhaust if coolant is above 140. That's what your thermo vacuum switch valve does, tell the PAIR valve what to do & when. If the PAIR Valve is stuck open, you should be getting fresh air into the exhaust all the time, & there should be no way for it to backfire in the exhaust, unless your carb is too rich or lean, or the engine isn't burning all or even most of the fuel/air mix.

BTW, you never got back to me on the condition of the carb; new, old, what?

If the PAIR Valve is stuck closed or clogged up with carbon, no fresh air gets to the exhaust. Again, the carb could be too rich or lean or the engine isn't burning all of the fuel. That could be an ignition problem. What's the status of your plugs, wires, cap, rotor, coil? Also, if the distributor is original it could have stuck advance weights, a stuck vacuum advance mechanism, or weak or broken springs. THAT will certainly cause the backfiring problem. I just went thru this with another guy with an 83 22R & it was the distributor being worn out. Yours probably isn't worn out, having only 27K miles, but it is 40 years old, & the innards of a distributor can collect dirt & get gummed up, or even rust inside. At the very least, take the cap & rotor off & shoot a bit of WD-40 on the pivot points of the weights. The weights are 2 flat pieces of metal below the plate that holds your pickup coil. Pic below.



At worst, ask your mechanic if he knows anyone who can clean & rebuild a distributor, or get a new distributor:
https://www.rockauto.com/en/catalog/...stributor,7108

I got that one & it works great.

PA rules on emission exemptions - if you have the truck registered as a classic, antique or collectible it is exempt from emissions testing:
https://www.dmv.org/pa-pennsylvania/smog-check.php

Given that info, & if you have the truck registered 1 of those ways, you have 2 choices:

1. Take the belt off the smog pump & if the problem goes away leave it off. You can then leave all the emissions equipment in place & it shouldn't hurt anything or cause other issues. One thing you absolutely should do if you do this, is cap off the vacuum port on the carb or intake that feeds the thermo vacuum switch. Also find & cap off any vacuum source from the carb or intake going to any part of the emissions system. This way you can make sure your carb & engine doesn't have any vacuum leaks.

2. What I did - Desomg the truck. Since it would be exempt from emission tests, you do not need to have or keep the stock emissions system. You also do not need any of that stuff for the engine to run well & actually run clean. This can be some work to do & there are always things that go wrong when you start trying to take out 40 years old bolts & such. Search Yotatech for "desmog" or "desmogging"; there are a couple of good write-ups on how to do it. I also have personal knowledge in doing this to my '78, so just ask specific questions & I'll answer them.

Melrose 4r

83

Yep, there's a good tutorial on this site. Once you get used to staring at it, you'll get a good feel for the bare minimum attachments the carb needs. It's...my memory is getting fuzzy but the distributer advance and...I mean that's pretty much it. Block off the EGR and all other ports. If you know the EGR isn't stuck open you can just plug the ports. But if it's stuck open you'd need to actually pull the valve and block off the opening.

These components are all in those desmog threads. It'll also mean more carb tuning since now everything is different. That's just part of it. This is the point where people start recommending buying a Weber carb (me included).

chinook/fj55

13Swords

That TVSV looks right; since it's out of a 20R vehicle the threads should fit in the manifold. Toyota calls the "anti-backfiring valve" am "Air Bypass Valve" but it does the same thing, cut off the fresh air flow to the exhaust once the coolant gets to 140 (controlled by the TVSV) so the thermo valve could still be the part causing this.

As far as I remember, the Air Bypass Valve is NOT in the smog pump, but mounted to the passenger side inner fender well. The air pump is just a pump; it has no valves in it.

Vacuum controlled secondary in the carburetor - Again, vacuum controlled stuff on the carb has NOTHING to do with the smog pump; it's just an air pump & provides pressurized air, not vacuum. It works off of ported engine vacuum. Nothing on the stock Aisan carb needs the emissions equipment in order to work ok.

I'd still check the advance weights & springs in the distributor just to know for sure, would only take a few minutes.

Melrose 4R is likely right, you have a lean condition due to the air flow not being shut off to the exhaust when engine is hot. Just IMO, but It's too expensive & frustrating to chase down all these emissions parts & try to make it work again as designed. Almost none of these parts are available new & you can never be too sure about used ones. If it were my truck I'd go ahead desmog it as 83 suggests. Since you can legally do it in your state there's nothing stopping you, & it sure eliminates all this headache. Up to you.

chinook/fj55

Melrose 4r

83

You could have good luck/skill. I've actually really enjoyed the process of rebuilding carburetors, but never had good results. In fact I haven't had good results with professionally rebuilt carbs from major online sources, either. It's why I vote for Weber, but I wouldn't try to stop you from rebuilding what you've got for cheap. It's worth a try.

13Swords

One of the backfires may have damaged the power valve in the carb; this can make the carb run very rich & is usually the thing that makes this happen.

If you want to replace the carb with a pro rebuilt stock unit for about half the price of the Weber, I recommend National Carburetor. They have been in business since the 50;s & do a first class job of it. They even live engine test them so all of the adjustments are set out of the box. I got a carb from them for me '78 & am super happy with it. Here the link:

http://www.nationalcarburetors.com/l...partnum=toy387

83

National is the place I tried, too, because they do have good reviews. I got a carb I couldn't tune, so I took it to a specialty shop and they couldn't tune it. The carb shop showed me how there was a huge vacuum leak at the...man I think throttle arm but it's been a while now. Sent it back to National and they did their fancy "live engine test" ( ) and told me nope, everything is perfect with this carb and sent it back to me. It ended up in the trash and a Weber ended up on my truck. This was during the countdown (days, literally) until I moved into my camper and hit the road. I couldn't mess with it any longer. Ordered a Weber shipped two-day, installed it with a friend the day before I moved out of my house and hit the road.

But that's just one persons experience.