Intake backfiring under compression braking?

CMF_Yom

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Hi all.

Since fitting the new intake I'm suffering a few problems.

Fuel economy isnt too crash hot anymore (this could be due to other reasons though hehe).

The next one is that under moderate to severe engine braking like going down a steepish hill in say 3rd at about 60/70kmhr, there is a popping noise from the front of the car.

its not pinging or anything like that, its like the mini backfires you get through the exhaust.

Could this be a backfire through the intake?

What would be causing this?

Have i broken something? lol
 

CMF_Yom

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The other option is that I'm just having the usual post-purchase worries and i'm worried about a perfectly normal sound, or something as simple as an exhaust leak!
 

CMF_Trev

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Yom,

I picked up on what you said.

a) Fuel economy isnt too crash hot anymore
b) its like the mini backfires you get through the exhaust

Could this be due to changes in fuel mixture? Richer mixture could account for both problems. Unburnt fuel in a hot exhaust manifold may cause post engine igniton and would account for the sound.

Do the spark plugs look sooty on the porcelin?

Trev
 

CMF_Yom

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It had the exhaust burbling before I modified the intake. I simply put that down to it being a straight through system (apart from the cat. converters).

But you're right in the mixtures being out. I have got 100km less from this tank compared to the last one.

THe popping noise under compression braking: well from what I've found out is most likely caused by the timing. As my timing light went bung today when I went to test (damn thing is older than me anyway :p) the timing I think I'll book it in at the mechanics so it can go up on a hoist so everything underneath can be re-torqued to factory spec. I suspect a loose rear engine mount and possibly one side of the steering rack are causing some quirky vibrations through the cabin.

I'll post up the results. Depending on what the spark plugs look like I might organise a quick dyno run to see what the a/f ratios look like. Not interested in power at the moment heh.

Cheers Trev.

Argh. Got to get a stone chip I picked up on the windscreen fixed as well.
 

CMF_Yom

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Okay thought I'd do a slight update on the matter at hand.

With the help of Alex_B, I have re-adjusted the timing to 17.5degrees from the 13.5degrees it was set to (looks like the mechanic who changed the distributor didnt know that you had to pull the TPS plug).

FInally I have that throttle response I have been longing for. It no longer hesitates in its power delivery.

Another thing I should mention is alot of the exhaust burble is now gone. I havent made it do the intake noise again and the air intake is unrestricted again as well.

I'm starting to think that I had a bad batch of BP Ultimate. I broke my rule and bought from a fairly small servo which is never very busy...

Oh plus I used a recommended doseage of Morey's Upper Cylinder LUbricant with this tank of fuel and I don't know if its my imagination or not but the engine seems to run smoother, blow less carbon, easier to start when warm and more importantly - the fuel economy seems to have improved dramatically... 300km to half a tank w00!
 

CMF_Yom

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correct me if i'm wrong but advancing the timing is relating to the spark sent from the distributor to the spark plugs. You can adjust this to perfect the tune of the motor (pretty much the only thing which needs to be tuned on modern motors). By advancing it I have been able to change when the spark plugs fire. This now means i get better combustion which should mean slightly more power and better fuel economy.

Very dodgey explaination. lol

Oh I also recommend setting the idle RPM at more sort of 700rpm. Much, much smooth idle.

But make sure you set the timing at 650rpm or you won't get a proper reading. You can put it back to 700rpm once you've adjusted the timing.

I should note that I definatley no longer backfire, there is still a slight burble though. It sounds too good to get rid of it. :p Doesnt sound like its pinging either. Yay.

What is more impressive is that I've clocked up 500km and the guage has only just hit the 1/4 mark! Thats with some pretty spirited driving too!!!

I love my micra!
 

CMF_Sean

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When you adjust the timing you are changing the distributor's position relative to the crank.

The distributor is run directly off the crankshaft and is set at a predetermined degree from the factory (13 degrees for instance refers to how many degrees it differs from the crankshaft when piston 1 is at TDC)

If you advance the timing (increase the degree say from 13-17) then you are in essence making the distributor fire the spark plug earlier. You can also retard the timing (decrease from 13-9 degrees for instance) This is the adverse and will send the spark to the plug later.

The main thing about timing is getting the spark plug to fire just a few moments before the piston reaches TDC allowing the piston head to absorb more of an impulse (impulse = force x time. The more force applied over a longer time = more power) to achieve maximum power.

However with timing you can run into problems such as 'knocking', 'pinging' or 'pinking' which is where the spark plug is firing way too early and is damaging the piston. To avoid this you need to retard the timing to a safe level.

Most cars come with pretty conservative timing. Running higher octane petrol or using water injection will also allow you to run more advanced timing than usual.

Hope this helped a bit :)

Sean
 

CMF_Yom

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If you're not sure of what you're doing, I don't suggest doing it yourself!

Find someone who does or get a workshop manual or a mechanic who can show you what to do.
 

CMF_1300supers

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YOM. I dont have enough knowledge to do that :( its true... but i'm really curious u know :))))

I think i understand, cause my mechanic had already change that plastic black top from the distributor, and then had to be careful adjusting the distributor.. cause at the time of puting back the dstributor i noticed that it can be rotated!!!! its that what u mean by changing the degree?

 

CMF_1300supers

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Thats it aaron!!!

After all i still know my micra :)

Now the question is:

- How do the mechanic know the right degree?is there any tool or meisure?
- The idle revs should be changed after the timing adjustment right?
 

CMF_Yom

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The correct procedure is as follows (i think it is anyway, its what i did with mine).

-Disconnect TPS (Throttle position sensor - it is the brown plug on the black thing which is screwed onto the throttlebody).

-Rev engine 3 times to 3000rpm (apparently puts ECU into some kind of mode)

-Adjust Idle speed to 650RPM using idle adjustment screw behind throttle body.

-Using a timing light, find out what the engine is currently running (use the sticker on the underside of the bonnet to figure this out).

-Loosen nuts on Distributor and rotate TOWARDS the rear of the engine bay (anticlockwise from where you'll be standing and watching the timing light flashing on the crank pulley).

-Adjust to where you want the timing to be adjusted to. Alex and I like 17.5degrees as it greatly improves highway throttle response. We use 98RON fuel.

-Very carefully you have to retighten the nuts on the distributor. Check the timing with the light again to make sure it hasnt slipped.

-Turn engine off. Reconnect TPS sensor and start engine up again. Adjust idle to either factory 650RPM or 750RPM if you don't like the standard idle harshness.

Take it for a spin with everything off and windows up and see if it pings (sounds kinda like a quiet ringing bell). If it does, retard the timing 1 degree using the above method.
 
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