84 T4WD Carb update

[Phatcat]

A few of you have probably helped me out over the past six months after I bought an old Tercel this spring. I’ve spent some time going through the carburetor system and emissions components. After rebuilding the carb and testing every component of the system several times over again per the FSM, the consensus was that these older carb feedback systems just aren’t capable of having a super smooth idle. At least, that’s the vibe I’d got here when describing my issues and possible remedies. Of course, this was all after going through all of the distributor, vacuum advance, timing, valve lash etc. to ensure all was working properly.

Recently, I was speaking to an old mechanic at my local Napa about my car and he proclaimed that he’d rebuilt many many hundreds of the Toyota and Honda carburetors from the 1980’s and 90’s, and that nearly every time he had to remove the factory plug on the idle mixture screw to make adjustments, more so in later years as fuel changes occurred. I’d been reserved about removing the factory plug and fooling with the idle mixture due to the literature and lack of documented cases here. Anyway, I finally did remove the Mass plug and upon opening up the mixture screw just slightly the idle smoothed right out. In addition to that, as the idle smoothed out and raised slightly, I was able to back off the idle speed adjustment, which has the car running way better overall.

I suspect that as fuel changed over the years, we’re talking about 40 years of engineering and changes through the life of this vehicle, such as ethanol etc etc etc, that the carb was adjusted via the idle speed screw to compensate and try for a smooth idle. In my case, every aspect of the system seems to be running much better with simply enriching the idle mixture slightly, which enabled me to place the idle speed adjustment at or very close to the designed location. Cold and hot starts are much better, and the car idles beautifully at stop lights. Now, there is a lot of science that goes into all of this regarding fuel types and mixtures, and I’m fortunate to have an ethanol free pump a mile from my home here in WA, and using ethanol free gasoline didn’t help my idle hiccup, which I suppose is counterintuitive to my findings here. However, and I don’t have all of the science here to list, but there has been many more changes to gasoline in addition to ethanol content.

To me, given the changes and time period since this car was manufactured in/around 1983, this all makes perfect sense and adjusting the mixture has completely fixed my idle and carb problems.

[Nordical25]

It is very true. There is a reason why a carburetor has a mixture screw and there is a reason why it is sealed in the first place.

You can do easily calculations scientifically for the ethanol fuel. 100% Ethanol has energy density of 27MJ/kg meanwhile petrol has 46,4MJ/kg.

It means that 5% ethanol fuel/kg has basically: 5%*27MJ + 95%*46,4MJ= 45,43MJ.

10% ethanol fuel has per kg: 10%*27+90%*46,4=44,46MJ.

You need to compensate your system for the 10% difference onwards based on Finnish government regulations.

In addition all gaskets will wear out faster when ethanol content will go over 5%.

So if you will find 100% petrol, no need to touch that screw.

[Phatcat]

Thanks, and yes, calculating the changes in ethanol content are possible. I’m not in Finland or anywhere close to that location though, I’m in WA state in the US. And interestingly, having ethanol free gasoline did not fix my idle hiccup issue. It wasn’t until I adjusted the idle mixture that the engine idled properly.

I wanted to post this finding because there seems to be a good number of people on here fighting similar idle issues, and it makes sense that some adjustments may be necessary given the age and time since these systems were originally engineered.

[Mattel]

Great result! glad its working well for you!

[dlb]

Interesting post. If you ever fill the car with gas that has ethanol in it, I'll be curious to hear if you notice it running or idling any differently then.

[Phatcat]

Oh I’ve had it filled with regular octane that has the usual ethanol in it, and the idle is still good after the mixture adjustment. I don’t run it all the time with ethanol free, but often, as I do have good access to it. I’d tried numerous different octane ratings and the ethanol free for a bit, and had no luck fixing the very slight idle misfire. This here was definitely a very slight lean misfire. It’ll run fine now no matter what I put in it. And I suspect that many of the people on here have the same lean misfire, at least, that’s the vibe I got while discussing here trying to fix my issue and watching numerous videos online. Keep in mind that the mixture plug, FSM, and recommendation to “not adjust the mixture screw unless there’s a known problem”, is 40 years old. The FSM we get our information from assumes (some) things. Most importantly, it assumes that the gasoline of the 1980’s is being used. Well, I’m not an expert on fuel types or the changes that have occurred in the last 40 years. However, there’s so many older vehicles having issues with vapor lock, pinging etc because of the modern fuel mixtures, I’d assume that enough changes have been made to fuel engineering to warrant an adjustment of our idle mixture screw on these old Tercels. In short, yes, there is a known problem… the gasoline I’m trying to run this 40 year old motor on wasn’t engineered to do so with the modern gasoline. All I’m saying, is that if you’ve been fighting a slight hiccup or misfire at idle, try removing the mixture screw plug and give it an adjustment. Worst case is that it runs worse and you put the screw right back where it was. Just count the turns in all the way when you initially get the plug out, write it down, and you’ll have a reference. This also assumes that you know there are no vacuum leaks or other sources to cause a misfire. In my case, this is where I was at and the only place that could possibly yield a result, and it did indeed fix my idle.

I believe the FSM said factory for the 3-AC was 3 1/4 turns, and others at 2 1/2 turns open. By the way, my mixture screw was EXACTLY at 3 3/4 turns out when I removed the factory mixture plug, go figure… And also, getting that mixture spot on also enables the idle speed screw to be backed off a bit, which is probably a more healthy position for the function of the fast idle cam, dash pot, and everything else, rather than compensate for a rougher idle by increasing idle speed.. Anyway, I hope this helps someone get their car running better.

[dlb]

How many turns out is your idle mixture screw now? My current Tercel came to me with the plug removed so I adjusted it and also found it idled a lot happier with it out more than FSM specs. I don't remember how many turns though because this was a little while ago. I've been meaning to look up how to set the idle mixture properly (the procedure in the FSM is absurdly complex, the instructions on the Weber carb I had were far easier and more clear) and redo, maybe I'll do that today.

[Phatcat]

Hey dlb,

I’ll be going through the process of setting my dashpot and fast idle cam here today sometime, and during that I’ll check exactly where that mixture screw is at. So yeah, I don’t know exactly where it is. I’d guess about 4-5 turns out. I adjusted it, saw the improvement, played with it to confirm what I was seeing, backed off the idle speed a bit, and have been driving it the past week set at that spot. But since the improvements, I want to go back through and set the dash pot and fast idle to FSM specs (1400 and 3000 RPM respectively), or about there, I think the 3000 fast idle is a touch high for my liking, but, you must have it up there a ways to function the fast idle cam and choke linkage properly.. I’ll get back to you with solid numbers.

[dlb]

When I got my current Tercel, the cover for the idle mixture screw was already drilled out. The idle wasn't great so I unscrewed the idle mixture screw a bit and the idle improved dramatically. So I just kept going until I was worried the screw was going to pop out. I just checked now and it's at 5 and 1/4 turns out. When I have time and the engine is warm, I will try to actually set it. The FSM instructions stink so I'll probably try using the Weber instructions:

"To set the idle mixture turn the idle mixture screw in slowly (clockwise) counting the amount of turns you make until the idle speed drops off. Turn the screw out (counter clockwise) counting the turns until the idle drops off again. Turn the screw now back in half the distance that you turned out. Now the mixture is set."

I had largely forgotten about it because the car generally runs great at this point, been getting 35 mpg with it.

[Mark]

I have a feedback carb on my '84 wagon. It idles rock steady as does my '81 Tercel, as did my '83 Tercel hatchback. The idle speed is as per the FSM for the automatic Tercels. The only time any of them idled a bit rough or occasionally stalled at idle was when they had vacuum leaks. Bumping up the fuel mixture with the idle mix screw compensates for these leaks. There are so many vacuum diaphragm components that can leak on these cars. The most common ones for me have been the AAP diaphragm, the distributor advance diaphragm(s) and the EGR vacuum modulator diaphragm. On my feedback carbed wagon, one of the vacuum switches (I forget if it was A or B) had a leaking diaphragm as well.

[Mark]

As for the ethanol blended fuel, the idea is supposed to be that it has less energy, but atomizes better for a quicker and more complete burn so power output is at least the same. I haven't noticed any difference between straight gasoline and ethanol-blended gas in my cars. Pretty much everything from the gas stations here has had ethanol in it now for years.

[Phatcat]

I counted the turns.. it’s sitting right about 4 1/2 turns out, maybe 4 3/4, it’s hard to tell and get a super accurate count while the carb is installed. But, I also tried my best to give it the lean best idle I could find. So just a hair under 5 turns would run smooth, but be maybe just a touch rich. Yes, the Weber tune directions can be a good place to start.

Hey Mark, yes, absolutely there are any number of opportunities for vacuum leaks on these cars. I had tested everything listed in the FSM procedures and replaced my dizzy vacuum advance, and even with every vacuum line capped off, my motor had a very very slight hiccup. I’d tested each diaphragm with a vacuum gauge and capped every line. I felt confident that I’d exhausted all other possibilities and or vacuum leaks. Now, I’m not taking about anything that most people would even notice. Your average driver wouldn’t catch it. It was there though, and just very slight. I’d describe my issue as a very subtle off beat shake coinciding with a very gentle random poof at the tailpipe, but you’d have to watch really close to catch it. So, when I speak of slightly adjusting the mixture screw to improve idle, that’s the context I was describing. And it’s entirely possible that the change I had to make is due to old fuel lines that I have yet to chase and blow out. Or some other culprit. All of the soft fuel lines under the hood I replaced. I suspect there are more soft lines entering the fuel tank at the rear as well, and I haven’t replaced those yet. That will happen when I drop the tank, clean it up, and shoot it with some quality undercoating. I suppose there could be other sources and I’ll continue to investigate here over the weeks and months. My bowl fuel level and float look spot on too.

You’re one of the few people I’ve chatted here that describes their idle as solid, and on multiple vehicles. I’d been learning this system on and off for a few months, and almost everyone I spoke with said that these cars just sorta run that way. However, Toyota didn’t sell cars with crap idles. And if they did, there’d be a TSB or whatever to remedy the situation. Anyway, I’ll continue to check through things on this car. For now, the slight mixture adjustment has it running very well.

[Mark]

Maybe the difference with my cars is that I never rebuilt any of the carbs. Many people seem to have some issues and decide to rebuild the whole carb and then it never runs right after that. On my '81 1st gen Tercel, it came stock with a 6-volt points-type distributor. I replaced it with a 2nd gen electronic distributor, which bolts right in and is 12v. The specs for spark plug gap are different (the old 6v, 1st gen gaps are smaller). Whenever I changed the spark plugs and accidentally gapped them to the original specs, there would be an occasional miss at idle. When I gapped them properly as per the 2nd gen specs, the miss went away so I assume even a slight difference in spark plug gap can also affect the quality of the idle.

[Phatcat]

Mark, yeah, I could see gap on plugs being an issue too. I didn’t look up the recommended plug for my 84, and just put in Denso W16 EXRU gapped at .032” or .8mm. What most 80’s and 90’s Toyotas recommend. I always check them while installing new plugs and put them at a liberal but fairly accurate measurement. My thoughts are that the slightly larger gap gives better ignition of the mixture, but that’s all biased information based from EFI fiddling.

[Mark]

According to the manual, the spark plug gap should be .043". I think the gap for my 6-volt ignition system 1st gen Tercel was .031".