back fire

M

macker1

0
I am after a bit of advice. When Dora is de-acelerating (going down hills)she back fires through the exhaust (on petrol and LPG) I have fitted a wider 50mm exhaust and it still does it. I have spoken to the prievios owner and she said it did it while she had the van. I have checked the timing and that seams spot on. I amnow scratching my head (and it is hard to get the splinters out of your fingers)
I am open to sugestions :D

Dora and Gloria
 
These highly tuned beasts do that, haven't you heard a rally car as it comes into a bend [}:)]. How loud are the back-fires, it could be a worn dizzy vacuum advance, you may think you are setting it up ok but something strange is happening as you decelerate. Thinking worse it could be a passing exhaust valve and unburned fuel is being forced through to the hot exhaust and igniting [:0] Or simply dodgy HT leads arcing between each other, try running the engine in the dark and see if you get a light show under the engine cover.

'Harvey the RV'
 
this sounds like the classic morris minor deceleration "fart" (which can become a backfire!) My morris used to get it until I changed the timing chain for a new duplex one.

I was guessing it was the slack in the timing chain was taken up when you do "engine braking" (i.e. the car is pushing the engine round) so the valve timing goes out i.e. the exhaust valves are open too early and the fuel/air mixture is let out of the exhaust before it is properly burnt.

could be wrong though (I'm a software engineer - not a proper engineer!)

Rupert

www.littleyellowcar.co.uk
 
That sounds very feasible, better than my ramblings :I I'm only a process manager not a software engineer [8)]

'Harvey the RV'
 
ht leads are. new silicon ones, The back fire can sound loud on steep hills, the back fire did sound like a moggy one with the small exhaust, but is now louder. I will do a compresion test to check out the valves next week, the timming chain also sounds like a posible culprit but no rattle from front of the engine

Dora and Gloria
 
Then my guess would be running rich enough to ignite in the manifold...are the plugs black?

Cheers
Jules

RatSport www.ratsport.co.uk
 
its either a classic symptom of overfuelling on the overun [xx(] or possibly a worn exhaust valve or seat.
also worth checking exhaust system for leaks :)

"you were only supposed to blow the bloody SLIDING doors off no.3"
 
Had a chance to do a bit more investigations. Checked the cylinder compretions three showing 160 lbs number one 150 lbs put a spanner on all the nuts on the manifold (got a little on each on) I also changed the exhaust dropped the dia back to standard.(mainly to stop frightening great danes and little old lady's :D )

question would the slight drop in pressure in number one indicate an exhaust valve pasing.

Dora and Gloria
 
Pressures sound fine to me, maybe start looking at electrical side -dizzy sounds favourite.

'Harvey the RV'
 
as you have a slightly lower pressure on 1 cyl it does indicate some sealing loss,either valve seats or piston rings/bore, but that amount is not serious so probably is not the problem.

"you were only supposed to blow the bloody SLIDING doors off no.3"
 
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