backfire question

[Raggles]

I have been taking the van up and down a few steep places recently.

I noticed that when going down a steep hill in 2nd using the engine to aid braking, I can hear a gentle back firing when passing through an enclosed area. Is this normal? It isn't particularly loud or violent.

If not, any ideas as to the cause?

 

[ozziedog]

No , it shouldn`t. I`m guessing you got a manifold leak most likely. It will only get worse then it could get nasty if you don`t sort it.

Ozziedog,,,,,,,,,,,,, Are you sure it`s a backfire ? :mrgreen:

 

[Raggles]

No , it shouldn`t. I`m guessing you got a manifold leak most likely. It will only get worse then it could get nasty if you don`t sort it.

Ozziedog,,,,,,,,,,,,, Are you sure it`s a backfire ? :mrgreen:

no ... its more like a burble sound - I normally think of a backfire as being quite violent, but couldn't think how else to describe it. It only seems to occur when the hill is steep and the revs are getting quite high (over 20mph).

I found this on the net ... any thoughts?

When the throttle valve is in the idle position, fuel does not flow out of the main circuit, just the idle circuit. The combined effect of the closed throttle and elevated engine rpm is to create a high vacuum in the intake manifold. This vacuum causes a high air flow rate through the small gap formed by the throttle plate and carburetor throat. and the idle circuit will not deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there. When the fuel in the combustion chamber DOES fire, it's not fast, and the combustion still occurs as it is shoved out the pipe, where it sometimes ignites the raw mixture that has collected and the exhaust then pops or backfires. Mine does it, and it's normal to a degree, as long as it's not shotgun type fire that sends everyone around you hitting the deck and yelling "GET LOW! GET LOW!". Those types of backfires are accumulations after the collector that are caused by rich conditions and ignition misses, as well as leaky exhausts.

 

[ozziedog]

Sounds feasible and it does sound like something that everyone would know, so maybe that's it if it's more of a gentle burble.

Ozziedog,,,,,,,,, so how do we prove it ? :mrgreen:

 

[Raggles]

The exhaust is blowing a bit and I think needs replacing, which would provide the oxygen perhaps? I was not aware of the problem before the exhaust noise increased. Any tips on how to ensure that there are no leaks when fitting a new exhaust ... it would be the standard set up from VW heritage.

 

[lemur]

i just like the word 'burble'

 

[ozziedog]

i just like the word 'burble'

That is possibly because you are twisted, but I know where you`re coming from, I know exactly. If you had to describe burble,I think the very best description has to be the sound of a Morris 1000 panel van or pick up when changing gear. :mrgreen:

Ozziedog,,,,,,,,,,,,,, Thats gone and done it, :mrgreen:

 

[Happystamps]

The exhaust is blowing a bit and I think needs replacing, which would provide the oxygen perhaps? I was not aware of the problem before the exhaust noise increased. Any tips on how to ensure that there are no leaks when fitting a new exhaust ... it would be the standard set up from VW heritage.

Few things... Get a small mirror and some fine GOOD QUALITY(the cheap stuff will just fill the area with bits of sand) sand the mating faces on the exhaust flat, and if you're careful, the outlets on the cylinder head too.
Make sure that the studs are wound in properly- they should be all the same length coming from the heads, and they shouldn't wobble.

I don't use any exhaust paste, these things were designed not to need it.

Replace the nuts with brass ones so you can remove them next time- they should have more than enough strength in them to torque up properly.

Don't be afraid to fit, bash, fit, bash, refit. They never go on properly the first time.

Don't do them up tight until every nut/bolt is in, as otherwise you'll find one part (usually the inlet manifold heat riser) just won't go on. It's terrible engineering practise working like that but hey, I don't make exhausts.

The main problem you're going to face is the quality of new exhausts- I put a new one on my bus less than six months ago, and wouldn't be surprised if the tailpipe showed a couple of pinholes in the next few months. Paint just burns off, so if you can go stainless I'd say it's worthwhile.

Oh=- and the burble is because of the exhaust. Especially since you mentioned it happening in enclosed spaces, tunnels etc. I have no idea why a holed exhaust sounds different when you're in places like that, but it does.

 

[Bluesnailman]

Have. Check spark plugs lately, you might get some popping if the engine is running a little lean on fuel.... Possible

 

[Raggles]

I checked plugs and re set valves as part of my early checks.

My current thinking is that a mix of a leaking exhaust coupled to the steep hill leaning out the mix due to the high vacuum with no opening of the throttle is causing the issue - the leak supplying the necessary oxygen to ignite the un burnt fuel in the exhaust system. It may be that there is also a minor vacuum leak somewhere leaning out the mix(which is un-noticable except under these conditions) so I will check for this as well. I do seem to be getting a very slight flat spot at low revs which may back up this theory. I am running a 123vac distributor timed to 31 degrees max advance set to curve 6 (for a 34 pict 3) in line with the 123 specifications.

On another matter I have read somewhere recently that with the 10% ethanol content in modern fuel that the main jet should be increased in size. Does anyone have any thoughts/experience on this? My 34 pict 3 is a refurbished stock Solex by Volksbitz.

 

[Raggles]

Few things... Get a small mirror and some fine GOOD QUALITY(the cheap stuff will just fill the area with bits of sand) sand the mating faces on the exhaust flat, and if you're careful, the outlets on the cylinder head too.
Make sure that the studs are wound in properly- they should be all the same length coming from the heads, and they shouldn't wobble.

I don't use any exhaust paste, these things were designed not to need it.

Replace the nuts with brass ones so you can remove them next time- they should have more than enough strength in them to torque up properly.

Don't be afraid to fit, bash, fit, bash, refit. They never go on properly the first time.

Don't do them up tight until every nut/bolt is in, as otherwise you'll find one part (usually the inlet manifold heat riser) just won't go on. It's terrible engineering practise working like that but hey, I don't make exhausts.

The main problem you're going to face is the quality of new exhausts- I put a new one on my bus less than six months ago, and wouldn't be surprised if the tailpipe showed a couple of pinholes in the next few months. Paint just burns off, so if you can go stainless I'd say it's worthwhile.

Oh=- and the burble is because of the exhaust. Especially since you mentioned it happening in enclosed spaces, tunnels etc. I have no idea why a holed exhaust sounds different when you're in places like that, but it does.

Appreciate this Happy Stamps ... will try to follow on the instal.

I have just noticed a couple of pin holes in the manifold heat riser .... I filled a coupled of holes last time I had the engine out as well so fear the manifold is on its way out. Am I right to assume that the aftermarket offerings are pretty so so and I would be best off finding a half decent original manifold ... or is there a half decent new one to be had?

 

[slow-lane-Matt]

Raggles

Did you get to the bottom of this - my bus does exactly same thing (backfires only in second gear when using engine as brake.)

One test I read to check the manifold was to spray propane/bbq gas around the manifold, and check for increase in engine revs.
(Don't know if it is serious suggestion or a joke - but sounds a little dangerous and I don't plan to do it!)

 

[Raggles]

Hi Matt,

No not got round to it yet. I know I have to fit a new exhaust and I also have a replacement manifold I need to fit. Once this is all done I will test again. If it were me I wouldn't go anywhere near the heat risers with propane ... spray some carb cleaner or such like in the vicinity - should do the trick as far as detecting a vacuum leak.

I am inclined to think that a degree of 'burble' is par for the course in these situations but is accentuated and so more noticeable if there are leaks like mine on the exhaust. We will see!

 

[gas1man]

If you must spray use anything but LPG.

J & P

 

[sparkywig]

If you must spray use anything but LPG.

J & P

You'd only do it once.

 

[gas1man]

HAHAHA yeah I guess!!!

J & P

 

[TheHewsonFamily]

Have the same on my 1641 with twin 36 Dells and a big exhaust. Had it tuned on the rolling road and the tuner said it was normal to have some popping on the over-run, as long as it is not a 'back-fire'. The difference is quite noticeable.

I quite like it. It did get better when I replaced my dizzy cap and HT leads though.