weak acceleration, lacking power uphill, occasional backfire

[dlb]

reggie is generally doing pretty well but he does feel even more gutless than expected. at first i chocked it up to not being used to the stock carb/engine combo but i now think that something is wrong, and it is likely in the carb. looking at this thread,

https://tercel4wd.com/forums/viewtopic.php?f=2&t=7251&p=54078

and this post

https://tercel4wd.com/forums/viewtopic.php?f=2&t=7251&p=54078

i am going to check both of the accelerator pumps once it gets light out. any other suggestions would be appreciated though.

the other problem i've noticed that may or may not be related is that occasionally on gearing down, reggie will backfire out the exhaust. it's not very often and it's not very big but i have noticed it and am not sure why it would occur. the first time it happened i saw a puff of grey/black smoke out the rear window so i assumed it was just carbon buildup getting cleared out. it's happened a few more times though. i thought maybe the vac advance unit was sticking and not retarding as quickly as it should but it seems to have happened while slowing down immediately after lots of throttle, at which time the timing would not be advanced, so i don't think that's it. the timing is correctly set to 10* BTDC, i set the valve lash to the correct specs, and i have found no vacuum leaks so i'm not sure where to look next.

[Highlander]

David- Check your throttle positioner (dashpot closest to the radiator), It's supposed to slow the closing of the throttle plates when encountering sudden deceleration. The FSM has the setting procedure for it, I'm assuming that you checked that the timing belt hasn't slipped while you had the valve cover off, and that you checked that the vac lines to the dizzy are correct (not retarding when it should be advancing). Those are the only things that come immediately to mind.

[dlb]

timing belt and vac lines are correct but i have not checked the TP so i will do so. thanks HL.

[Petros]

Back firing means too rich, and it should not be any fuel going in on a down shift if the fuel cut-off solenoid is working properly. But that would not account for weak acceleration or lack of power, AAP diaphragm perhaps? Too much fuel would cause all of these symptoms, only tow things that occur to me that would cause that is bad float/needle valve or leaky AAP.

[Lil Beast]

A clogged or improperly seated jet would cause backfire! When my AAP went I didn't had any backfire, just black smoke at idle and incredibly expensive to drive and had a good performance still! How many turns out your mixture screw is? Should not be more than 2 1/2 turn! Bad gasket at the intake manifold or carb could cause a icing wich can cause backfires and bad performance at higher revs! If you have a timing light you can use it to test your advance by revving and pointing the light at the crank and look at the timing mark moving to about 13.6to22.8 degrees at 6000rpm(depending on models), start moving at 1000- 1200! If the timing works properly it's all around the carb! Bad emission or carb need adjusting or excessive wear! The mix of lack of power and back fire leads there!

[dlb]

done a few tests and everything checks out so far:

- accelerator pump and AAP both functioning properly
- choke, choke opener, and choke breaker all functioning properly
- TP and fast idle set to FSM specs
- blew fuel lines out with compressed air and changed fuel filter
- checked float level
- tested both fuel cut solenoids (both 'click' but i didn't remove them to check the o-rings since i don't have any replacement o-rings anyway)

but i'm still having the same problems. i've been able to pinpoint the problems a little better now so i'll be more specific here. the problem is worse when the engine is cold: it lacks a bit of power but runs ok at part throttle; if you give it full throttle, you can hear it sucking lots of air but at the same time the power drops off suddenly, causing the car to slow as if you're braking. when the engine is warm, it doesn't bog down like i just described but it still lacks power and i have to keep the revs up over 3k to get over any hills. mileage on the last tank wasn't great, something like 26 mpg.

further thoughts?

[Petros]

sounds like too lean during cold operation, there are three things that can cause this; Could be the temp vac switch at the water outlet (TVSV or whatever it is called). Could be the High Altitude compensator (HAC)-makes it run lean and advances the spark when above 3900 ft, but if it malfunctions it leans it out all the time (some cars do not come equipped with this device, it is on the right fender well next to the strut tower), or the other device on the right fender just aft of the coolant overflow tank with the vac lines (I forget what it is called) it also controls mixture. There are FSM procedures to test all of these, easy tests.

[dlb]

checked the TVSV a while back and it was within spec. no HAC on canadian T4's. but the MC valve was a part of this equation! it is not temperature-actuated but allows a metered amount of extra air into the intake manifold on sudden deceleration. i had disconnected the vacuum hose running from it to the port on the EGR body thinking that it was part of the EGR system. hooked it back up, did the FSM test, it passed, so i hooked it back up and now deceleration is much quieter. kudos, peter!

[dlb]

the problem turned out to primarily be a bad dizzy. after going through everything else and still having problems, i decided to re-check the timing which i had set to 10* BTDC. it was now at 0*. set it again, drove around, the next day i checked it and it had moved again. nothing i had done could have changed the timing so i figured the problem was either in the timing belt or the dizzy. took the cap off the dizzy and checked the rotor for play and it appears that the vacuum advance mechanism is partially frozen, probably due to the grease being gummed up with a dirt. usually the rotor will have a little bit of spring-loaded play in one direction (counter clockwise, IIRC) for advancing it but this one would turn the appropriate amount and then stick there. removed the dizzy, swapped a few parts with another one i had, put it back and set the timing and it's been great, like a whole new car. good acceleration and power uphill (for a tercel), no backfiring on down-shifting. i'll take the bad dizzy apart and re-lube it. should be a good spare then.

[celtglen]

the problem turned out to primarily be a bad dizzy. after going through everything else and still having problems, i decided to re-check the timing which i had set to 10* BTDC. it was now at 0*. set it again, drove around, the next day i checked it and it had moved again. nothing i had done could have changed the timing so i figured the problem was either in the timing belt or the dizzy. took the cap off the dizzy and checked the rotor for play and it appears that the vacuum advance mechanism is partially frozen, probably due to the grease being gummed up with a dirt. usually the rotor will have a little bit of spring-loaded play in one direction (counter clockwise, IIRC) for advancing it but this one would turn the appropriate amount and then stick there. removed the dizzy, swapped a few parts with another one i had, put it back and set the timing and it's been great, like a whole new car. good acceleration and power uphill (for a tercel), no backfiring on down-shifting. i'll take the bad dizzy apart and re-lube it. should be a good spare then.

What parts did you replace on the dizzy? The one I took out was fine except for the vac advance.

[dlb]

haha, i posted the link to this thread in the one about your current problem before i noticed you had posted here. good sleuthing, CG.

i didn't need to replace anything on the dizzy. all i did was disassemble it following the FSM, cleaned the old grease out, re-grease it with high temp grease, and put it back together. you can easily check for gummed up grease without doing all that though, you only have to do all that if the rotor sticks.