Exhaust Popping On Overrun.

[rebuilder86]

I live on a very hilly island.

When i cross the island, going downhill in 3rd gear high vacuum, throttle fully closed, I get the old popping and gurgling out the exhaust.

I have i think, eliminated 2 exhaust leaks which i though were the problem, according to many internet armchari experts who say it is BECAUSE of an exhaust leak. My brain tells me if there is un-burnt fuel in the exhaust, then there is a problem with fuel getting into the exhaust, not air getting into the exhaust. The air leak is just doing me the favour of showing me that there is fuel getting in by helping it ignite nie n loudly.

This sound is not good, not like going to watch a subaru to a sick drift in the rally, more like a sad old motor.

 

A couple of points:

I have installed an idle fuel cut solenoid in place of the BOLT which was in its place originally and simply wired it up to the coil so that it opens when car is on. When I go down a hill and get these pops i turn the key off and the pops immediately stop. This can not be because of the fuel pump stopping (electric fuel pump in this car) because there is still fuel in the bowl. Its stopping because i am blocking the idle circuit, which is under tremendous vacuum from the high vacuum being creating with closed throttle high RPM. I am yet to isolate the effect of closing this solenoid but I'm almost certain it will stop the popping.

 

I also have done the paper up against the exhaust pipe test, and sure as hell every now and then at idle the paper sucks back to the pipe and leaves an imprint of the black soot ring on the paper. This indicates sticking valves or sad springs. The engine is pretty old, no idea of its kms, as the car is made from spare parts and the dash is from something random. I'm going to say the springs are old and worn. This would also be giving me shit power above 5k rpm, however I'm not getting any misfiring at any time except idle. I'm wondering if this shit springs issue could also be allowing the exhaust valves to get sucked open in this high overrun scenario. This would be a good source of fresh air to ignite this unburnt fuel coming from the idle circuit.

 

Question 1. Is that possible, that the valves could get sucked open against spring pressure??

 

Question 2. Has anyone installed any kind of high manifold vacuum operated switch to close the idle circuit on overrun? I want to do this. At the moment it looks like I'm going to need some sort of switch on the gearstick to momentarily open the solenoid circuit to close the solenoid while I'm going down hills haha.

[rebuilder86]

also, is there a tension difference between an exhaust spring and an inlet spring? its possible that I cocked up and mixed some around when I did a 'head-on' valve stem seal replacement.

In my defence, if i did cock up, its because i was in the middle of the jungle, with no tools, not even a spring compressor. I used zipties, my shoes, and a hammer and magnet. Ill let you imagine how that was performed.

[rebuilder86]

I guess I could just stick one of these in and set it to maximum vacuum, normally closed. Installed inline to the fuel solenoid.

http://www.ebay.com/itm/Adjustable-Universal-Vacuum-switch-6-24-inHG-lockup-700r4-2004r-350c-superior-/331667328971

[kickn5k]

also, is there a tension difference between an exhaust spring and an inlet spring? its possible that I cocked up and mixed some around when I did a 'head-on' valve stem seal replacement.

In my defence, if i did cock up, its because i was in the middle of the jungle, with no tools, not even a spring compressor. I used zipties, my shoes, and a hammer and magnet. Ill let you imagine how that was performed.

 

Nope all standard K valve springs are 40lbs at the installed height.

You should be fine as far as mixing springs.

[rebuilder86]

sweet. I wonder if I should bite the bullet and get new springs. don't really have much money, tempted to try devise some 'heath robinson' device to use as a shim to jam under them all and give them a bit more life haha.

[peterd]

I'm a backyard hack so take what I say with a grain of salt, but could it be a faulty coil?

[rebuilder86]

no not faulty, however I have been discussing in another thread how it is not the ideal coil for this setup. Its a 3.8 ohm coil designed more for a points type ignition, when I am actually using a 5k internal igniter reluctor distributor which requires a lower resistance coil for optimum spark. Perhaps this plus the fact I have dished pistons and cannot find the long reach plugs here in this stupid country, both adding up to poor combustion. You could have a point.

 

 

however... an update.

Believe it or not, I have just put in a momentary switch on the gear stick which allows me to close the idle fuel solenoid from within the car whenever I like. Interestingly, and kind of disappointingly, this has immediately made it clear that the idle circuit is injecting fuel all the way up to about 5000 RPM, as when I rev the shit out of it, and hit this button to close the idle circuit, there is a very noticeable drop in revs and some misfiring. This indicates the carb requires the idle circuit to run at high power up in the high RPM. that is shit house. Idle circuits are suppose to be for idle. :( My fuel is pouring in through the idle circuit all the time :(

I would love to know if this is normal operating behaviour for a stock 3k/4k carb, or if I have some strange issue whereby the carb requires more fuel due to starvation of fuel to the primary venturi, and its getting it from my 'wound up 4 turns' idle mixture screw.

 

ʞ©$ɟ sake, more problems.

[LittleRedSpirit]

Start with the basics like a hella good carby clean, adjust your idle mixtures and clean out the bowl.

 

Fuel cut solenoid is only to stop fuel coming through when the car is switched off (dieseling on). So cutting the solenoid while the car is on is just going to stop you filling the float bowl and make it run lean and pop as it dries up, backfire etc which are usually the sounds of lean running conditions. Its effecting high rpm running because it doesn't just work at idle, it works on the fuel supply as a whole.

 

You're looking around corners when you should be looking straight ahead. Crackling and popping like I said is more than likely a sign of lean conditions more than likely caused by incomplete combustion so if I was you Id be asking why is the car not burning all the fuel and passing some to exhaust. You would be experiencing bad mileage if this is the issue also.

 

If you want to see how it goes with the correct plugs and coil that would be best. You may just not be able to ignite the whole mixture with a weak spark and the short plugs. This would be exacerbated at times when the motor has little access to air like off throttle as the charge entering is coming comparatively slow and with little turbulence compared to when the throttle is open and able to let in enough air to get a better vaporised mixture. I'm kinda guessing based off your experiments but you cannot expect it to run great with the wrong coil and the wrong plugs.

Edited February 23, 2016 by LittleRedSpirit

[rebuilder86]

Little red

 

Start with the basics like a hella good carby clean, adjust your idle mixtures and clean out the bowl.

Thanks but I have the carb 100% clean. its been rebuilt, taken apart, completely clean.

 

So cutting the solenoid while the car is on is just going to stop you filling the float bowl

Fuel cut solenoid does not stop fuel coming through to the float bowl, it stops it going to the idle circuit from the fuel bowl. The fuel bowl is not affected by the solenoid.

 

and make it run lean and pop as it dries up, backfire etc which are usually the sounds of lean running conditions.

For yours and everyone's information, 'Lean popping' happens on acceleration, not deceleration high vacuum conditions and is the burning of fuel which is otherwise needed in the combustion chamber. Popping on deceleration is a unwanted and unneeded fuel vapour getting into the exhaust and meeting with fresh air and igniting. My post is about decelerating popping, also known as run on popping.

 

Its effecting high rpm running because it doesn't just work at idle, it works on the fuel supply as a whole.

I think you are mistaking fuel cut solenoid with the float valve.

 

Fuel cut solenoid is only to stop fuel coming through when the car is switched off (dieseling on).

yes, well i'm using it switched, to stop fuel coming through the idle circuit in high vacuum situations to test why I'm getting so much popping on deceleration, I know what its for :P

I think maybe you have missread or not read all my posts.

 

You're looking around corners when you should be looking straight ahead. Crackling and popping like I said is more than likely a sign of lean conditions more than likely caused by incomplete combustion so if I was you Id be asking why is the car not burning all the fuel and passing some to exhaust.

Nah I'm a very clever straight forward thinking guy. I do not have a combustion problem causing the cracking. I have too much fuel.

 

You would be experiencing bad mileage if this is the issue also.

Yepo, haven't tested it in a while, but last time I did the full tank to full tank test, i got close to 13 L/100 ks ( a corolla gets somewhere close to 6), but have since found the primary jet and secondary jet had been switched by some ʞ©$ɟtard, that is now better and it appears by eye looking at the fuel to have made the thing use us half as much fuel.

 

If you want to see how it goes with the correct plugs and coil that would be best. You may just not be able to ignite the whole mixture with a weak spark and the short plugs. This would be exacerbated at times when the motor has little access to air like off throttle as the charge entering is coming comparatively slow and with little turbulence compared to when the throttle is open and able to let in enough air to get a better vaporised mixture. I'm kinda guessing based off your experiments but you cannot expect it to run great with the wrong coil and the wrong plugs.

This is exactly what i needed to hear. You just made me think more clearly about the bigger issue, when its rich (acceleration) its burning fast and burning it all, when its lean, the problem is being amplified. Good point. However, on throttle closed high speed deceleration engine braking, you don't want combustion, you are trying to starve the engine of air. There shouldn't be much mixture to burn theoretically. Why would I want to burn fuel. If you look at any EFI car, the injectors stop injecting in this situation. Sure this is a carby and is ddifferent, but my point is the whole subject of making sure you have complete combustion is for load applied situations. Its not clever to simply replace ignition parts just to ignite fuel to stop it burning in the exhaust on deceleration. It would be clever to replace those parts if it was causing a power issue. If I replaced those ignition parts to solve this problem, I would be thinking to myself, "AHH THERE WE GO, I BURNT THAT WASTE OF FUEL INSIDE THE ENGINE INSTEAD OF LETTING IT BURN IN MY EXHAUST. " It wasn't being used in the first place,, it was a nuisance while trying to decelerate with the throttle closed. I demanded no fuel..

I hope I'm making sense.

Edited February 24, 2016 by rebuilder86

[Banjo]

Hi Jeremy,

Just a few thoughts. Have you done a health check on the engine itself ? If not, take it for a brisk run, until it is up to operating temperature.

 

Upon return home, immediately take out all four (4) spark plugs and lay them out on a piece of cardboard & number them 1,2.3 & 4 front to rear.

 

Then beg, borrow, or obtain a compression guage and do a full compress test, on each cylinder with the throttle wide open. You need an assistant for this.

 

Write down the results, for each cylinder. If all the readings are very low, then squirt a tablespoon of fresh engine oil down each cylinder & do compression test again.

 

The colour & state of the plugs & their insulators will tell you a lot about the current combustion process in your engine. The compression test will tell you a lot about the state of valve seats & valves & rings. All cylinders should be within say 5-10 PSI of each other.

 

Consistency between cylinder compression readings is more important than the actual compression number itself. A hack Corolla engine should be somewhere in the range of 120 PSI to 160 PSI, dependent on compression ratio & state of the rings & valve & valve seat conditions.

 

You'll be surprised how much info can be gained by this simple test, and point you in the direction to help you solve your backfiring issue.

 

Cheers Banjo

Edited February 24, 2016 by Banjo

[rebuilder86]

I've just had more of a think about it, it may simply be doing this because there is no load on the engine. No load on the engine means the manifold vacuum stays high, and will suck lots of fuel through idle circuit. So the carb is designed with the fact in mind that people don't usually just pointlessly rev the engine out of gear..... well, not with any regard for fuel economy anyway.

I think if I take it for a drive and give it some stick, and block the idle circuit it should be fine as the throttle plate would be open so much that the suction and venturi over the idle port and T-slot would be minimal.

I'll let you know my findings.

[LittleRedSpirit]

Little red

 

Start with the basics like a hella good carby clean, adjust your idle mixtures and clean out the bowl.

Thanks but I have the carb 100% clean. its been rebuilt, taken apart, completely clean.

 

So cutting the solenoid while the car is on is just going to stop you filling the float bowl

Fuel cut solenoid does not stop fuel coming through to the float bowl, it stops it going to the idle circuit from the fuel bowl. The fuel bowl is not affected by the solenoid.

 

and make it run lean and pop as it dries up, backfire etc which are usually the sounds of lean running conditions.

For yours and everyone's information, 'Lean popping' happens on acceleration, not deceleration high vacuum conditions and is the burning of fuel which is otherwise needed in the combustion chamber. Popping on deceleration is a unwanted and unneeded fuel vapour getting into the exhaust and meeting with fresh air and igniting. My post is about decelerating popping, also known as run on popping.

 

Its effecting high rpm running because it doesn't just work at idle, it works on the fuel supply as a whole.

I think you are mistaking fuel cut solenoid with the float valve.

 

Fuel cut solenoid is only to stop fuel coming through when the car is switched off (dieseling on).

yes, well i'm using it switched, to stop fuel coming through the idle circuit in high vacuum situations to test why I'm getting so much popping on deceleration, I know what its for :P

I think maybe you have missread or not read all my posts.

 

You're looking around corners when you should be looking straight ahead. Crackling and popping like I said is more than likely a sign of lean conditions more than likely caused by incomplete combustion so if I was you Id be asking why is the car not burning all the fuel and passing some to exhaust.

Nah I'm a very clever straight forward thinking guy. I do not have a combustion problem causing the cracking. I have too much fuel.

 

You would be experiencing bad mileage if this is the issue also.

Yepo, haven't tested it in a while, but last time I did the full tank to full tank test, i got close to 13 L/100 ks ( a corolla gets somewhere close to 6), but have since found the primary jet and secondary jet had been switched by some ʞ©$ɟtard, that is now better and it appears by eye looking at the fuel to have made the thing use us half as much fuel.

 

If you want to see how it goes with the correct plugs and coil that would be best. You may just not be able to ignite the whole mixture with a weak spark and the short plugs. This would be exacerbated at times when the motor has little access to air like off throttle as the charge entering is coming comparatively slow and with little turbulence compared to when the throttle is open and able to let in enough air to get a better vaporised mixture. I'm kinda guessing based off your experiments but you cannot expect it to run great with the wrong coil and the wrong plugs.

This is exactly what i needed to hear. You just made me think more clearly about the bigger issue, when its rich (acceleration) its burning fast and burning it all, when its lean, the problem is being amplified. Good point. However, on throttle closed high speed deceleration engine braking, you don't want combustion, you are trying to starve the engine of air. There shouldn't be much mixture to burn theoretically. Why would I want to burn fuel. If you look at any EFI car, the injectors stop injecting in this situation. Sure this is a carby and is ddifferent, but my point is the whole subject of making sure you have complete combustion is for load applied situations. Its not clever to simply replace ignition parts just to ignite fuel to stop it burning in the exhaust on deceleration. It would be clever to replace those parts if it was causing a power issue. If I replaced those ignition parts to solve this problem, I would be thinking to myself, "AHH THERE WE GO, I BURNT THAT WASTE OF FUEL INSIDE THE ENGINE INSTEAD OF LETTING IT BURN IN MY EXHAUST. " It wasn't being used in the first place,, it was a nuisance while trying to decelerate with the throttle closed. I demanded no fuel..

I hope I'm making sense.

 

My bad forgot how the solenoid works. Car will run without it but wont idle I forgot that. You seemd to enjoy harping on that one.

 

Please stop shouting.

 

Efi motors still inject some fuel off throttle, or the car wouldn't run, some form of combustion is still required to tick the motor over and protect it, even if its currently outputting less torque than the motor would need to sustain the speed hence deceling. Its got less air coming in thanks to the closed throttle on decel but it needs to return to idle conditions under driveline load so it must inject fuel or the motor would cut. Motors are air pumps and to make any rotation they need explosions. The fuel is not only for combustion but for protecting and cooling the combustion chamber, valves etc. Usually ignition advances to around 40 degrees btdc off throttle on most efi motors and minimal fuel is input to keep it ticking over, if you want cracks and pops you add more fuel and get the flames out the ass, but you're right its a waste of fuel, if you believe fuel has only one function, to make you go forwards, but therein lies your misconception, who cares about making power if your motor isn't protected.

 

If all you're really dealing with is decel combustion problems off throttle and you won't buy the parts to get everything on spec with your ignition what do you think will happen here?

[rebuilder86]

hey banjo, good to hear from you dood.

I never leave home without a compression tester. I bought one from Australia purely for this very reason.

Yeh ive done compression tests, before and after my engine rebuild. before was 110 even on all 4, now after rebuild (new rings and a good bush-style valve grind, ask me about that please) it 120 even across al 4. A bit of oil takes it up to 125 ish. The bores are a bit polished, I wasn't game to take to them with sandpaper, and there is no chance of getting a hone on this island. Seems to be sealing well.

 

I was getting black soot on all plugs before I discovered the stupid fuel jets where installed vice versa! haha. Its now hallway nice n dry and just a bit of carbon and some poo brown. Bit of browning of any of the insulator tips tho. I suspect this is due to the shit fuel in the Philippines. I even filled up here on the cheapest unleaded at a local shell, and found pure evidence of fuel additives, obviously used to bring an inadequate fuel up to RON requirements. Red Brick dust on all spark plugs after that refill.

The picture shows the conditions I have right now. Spark plugs are probably about 5,000 kms old at a guess. I think the spark plugs look good.

The picture of the compression also shows how as I am cranking the engine over, fuel is gradually sealing the rings and the compression is going up as I move from cylinder one to 4 ( i couldn't be assed disconnecting the idle solenoid for this test.). Also remember these are dished pistons so 120 is ʞ©$ɟing amazing.

 

Hey Little red, I wasnt shouting haha. I just saw you were a moderator, and worried that the advice had to be corrected as this is a forum which someone may stumble across and take the wrong information as gospel form your mouth.

Edited February 24, 2016 by rebuilder86

[rebuilder86]

so I just took a drive to the local parts store, to look for long reach plugs of course unsuccessfully...... "ONLY FOR SUZUKI MULTICAB SIR!!"

Anyway, on the way I was playing with the switch, and like I guessed, it makes no performance difference when under load, so a vacuum operated switch where the solenoid electrical circuit is only closed (fuel allowed through) during 19-20 inches of manifold vacuum is the answer. I will have to wait till I get back to Aus for something like this... ebay all the way. (postal service takes half a year in this developing country)

Ill probably need 2 vacuum switches, one to open the circuit when vacuum is very high (over 21 inches vacuum) and one to open the circuit when low (less than 19 inches). That will keep the valve closed for all operating conditions other than idle.

I should have been a carby engineer.

[altezzaclub]

Why not take the switch out and see if it diesels on after you turn it off. Its very unlikely with such crap compressions! If it doesn't just don't run the solenoid circuit.

 

The idle jet does work up past idle, as you found. Weber reckons it hands on to the main circuit around 2500-3000rpm. It makes a richer mixture at idle as there is so little fuel and air going in that at 'normal' jetting it wouldn't burn. Mine wanders up and down from 14 to 18 on the mixture display.

 

Of course you may have a hot cam grind you don't know about, and the extra overlap causes over-run popping and surging. ..or worn valves, but if your grind was good they shouldn't be doing it.

 

I get it on my 4K with twin SUs and cam. The uneven cammy idle conditions on over-run upset the inlet vacuum which upsets the SU floating piston, which upsets the idle and that causes it to pop...