Trying to get the motor to stop backfiring. When it started we decided to rebuild the carb. Just rebuilt the carb, and still backfiring. Reset all the points on the distributor and found a wire that was going to the distributor terminal was broken. We fixed the wire, and I also bought a vacuum gauge and it is reading ~30. All emissions are still in place. It is backfiring at idle every so often but not through the carb, it is backfiring at the exhaust. It previously was backfiring through the carb prior to the carb rebuild. Any advice?
77 2f backfiring (1 Viewer)
- Thread starter KPruitt37
- Start date
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Start from cold and run for a few seconds. Are all 6 exhaust ports all warm?
Sounds like one isn't getting enough spark or has a tight valve perhaps?
Sounds like one isn't getting enough spark or has a tight valve perhaps?
It is running rich or intermittent bad spark. What are the units on the vac gauge 30 what?
backfire through the exhaust is often an ignition issue. check youre getting good spark. have you done anything to the distributor? check cap and points. wires plugs?
30 is cm hg vacuum and you want the other side of that which corresponds to ~12 which is pretty low
That's not a lot of vacuum, you could have way too much fuel for some reason - maybe your carb rebuild didn't go well and a gasket or something didn't seat correctly and fuel is leaking inside. Got an inductive timing light? Put it on each spark plug wire and watch it for while then move to the next and see if anything is different between all of them.
Check for vac leaks on the intake manifold and the carb sections. Start cold engine, let warm up enough to smooth idle. Lightly and quickly spray WD50 using the pee tube along the head intake gasket seam - when you hit a leak the rpm's will kick up. Do the same for the carb sections and base.
How much play is in the distributor shaft? Try rocking the shaft side to side, there should be very little play.
What kind of miles are on this engine?
Check for vac leaks on the intake manifold and the carb sections. Start cold engine, let warm up enough to smooth idle. Lightly and quickly spray WD50 using the pee tube along the head intake gasket seam - when you hit a leak the rpm's will kick up. Do the same for the carb sections and base.
How much play is in the distributor shaft? Try rocking the shaft side to side, there should be very little play.
What kind of miles are on this engine?
- Thread starter
- #8
All plugs have good spark, points were all just reset while the carb was being rebuilt
- Thread starter
- #9
What I don’t understand is prior to the carb rebuild, when I adjust the idle mixture screw, there is not any change in the motor rpm’s. After the rebuild it is the same where there is not much vacuum change when the screw is adjusted.
- Thread starter
- #10
There is about 82k original miles, I will do my best to check the other things. Where is the “pee tube” and when you say sections do you mean where there are vacuum hose connections?
- Thread starter
- #11
Right now the motor will run smooth at idle but every once in a while it almost is like the timing is off. We then tried to take it down the road and when the engine was under stress going up a hill it ran rough.
mattressking
10 digits to freedom
This truck is 42 years old and original (apparently). You need to download the 2F FSM & emissions manual from one of the many folks hosting it then.....
Go step by step in the tuneup section THEN emissions testing sections. There is no one answer. These trucks are managed by vacuum and are mechanical. You need to test each system and find your faults. People will guess and guess but you have to ultimately test it like any mechanic would.
In many cases you need to strip all the intake/exhaust manifold carb off and service manifold gasket and test each system. You can't pick and choose what you think here unless you truly understand these engines and their sub-systems....which is why de-smogging (removal of emissions equipment) is so popular. It eliminates 95% of the questions, but is only applicable if you don't have to emissions test or plan to sell to a place that does.
OR
Deliver the truck to someone who is well versed in diagnosing and repairing the faults.
Go step by step in the tuneup section THEN emissions testing sections. There is no one answer. These trucks are managed by vacuum and are mechanical. You need to test each system and find your faults. People will guess and guess but you have to ultimately test it like any mechanic would.
In many cases you need to strip all the intake/exhaust manifold carb off and service manifold gasket and test each system. You can't pick and choose what you think here unless you truly understand these engines and their sub-systems....which is why de-smogging (removal of emissions equipment) is so popular. It eliminates 95% of the questions, but is only applicable if you don't have to emissions test or plan to sell to a place that does.
OR
Deliver the truck to someone who is well versed in diagnosing and repairing the faults.
The can of wd40 has a pee tube. You need to spray the gasketed surface - that's where a leak can occur besides like crack in the casting. The head and intake manifold has a gasket that can leak - sucking air that isn't mixed with fuel vapor doesn't burn. Yep a rotted hose can crack and leak too - you can spray those connections too.
WD40 will burn, that's why it will find vac leaks - it also the reason you don't want spray it everywhere in large quantiles.
WD40 will burn, that's why it will find vac leaks - it also the reason you don't want spray it everywhere in large quantiles.
when I adjust the idle mixture screw, there is not any change in the motor rpms.
This usually means there is some fuel or air leak in the carb. You may want to take apart the carb again and study it closely inside. You wouldn't be the first or last (including me) to not get it right the first time. Be sure to have the FSM procedure 'lean drop method' to set up the idle mixture and speed correctly.
This usually means there is some fuel or air leak in the carb. You may want to take apart the carb again and study it closely inside. You wouldn't be the first or last (including me) to not get it right the first time. Be sure to have the FSM procedure 'lean drop method' to set up the idle mixture and speed correctly.
Why exactly do you want to use inches on a toyota again?
pb4ugo
SILVER Star
Typically reading the gauge you read the inside circumference #'s. So you say 30, but that would correlate to 12 on the inside #'s. Running properly the needle should be in the green. The higher the better. You say the idle mixture screws aren't working and the carb was rebuilt. As @SteveH said, fuel is probably leaking, bypassing the idle mixture screws. 1st make sure the throttle plates are closing. Sometimes carbon or a gasket can interfere with them completely closing. Then set timing to spec. I suspect throttle plates and vacuum leaks. Settings need to be baselined to spec.
It's just the units of measure that we use in the US. Inches of mercury vs. centimeters of mercury
Similar to lbs vs. kg or miles to km
It's just units.
Very easy to switch between, and much easier to use metric for all things mengineering.
My parents use inches. The kids now use millimeters. I use both as appropriate.
Just because it's quoted in cm doesn't make it wrong.
Living the international dream here
