Backfiring on the over-run

Post by **Dominic** » Wed Jan 28, 2009 10:51 pm

Hi folks

I've been running on ordinary unleaded for the last 1000 miles, to try and get an accurate comparison on consumption against super-unleaded.
One thing I have noticed is that since the changeover, and retarding of the timing to suit the fuel, there is a marked backfiring when trundling down hills without accelerator. Even when changing gear there is a definite "crackle" instead of a mere "raspberry".
The timing has been retarded so that it has just stopped pinking when accelerating up a hill. The dwell angle for the points is currently within tolerance at 57 degrees (60 + or - 3) Plugs all clean and set correctly.
Has anyone any ideas as to the cause?
Cheers

Dom

Post by **bmcecosse** » Wed Jan 28, 2009 10:56 pm

It will be the retarded timing - you could reduce the effect by increasing the idling speed and mixture slightly - but then you won't get as much engine braking, or as good economy!
Interested to hear if you think you get better economy and/or running on the 'super' petrol ?

Post by **Dominic** » Wed Jan 28, 2009 11:22 pm

Thanks Roy. Can this be minimised by playing around with the mixture? I don't really want to change the idle, as that ticks over like a sewing machine at precisely 240 rpm - rock steady. Also, I don't want to advance any more, as pinking sets in. Maybe I should just go back to super, advance the timing to suit? It has to be said though that apart from the backfiring, the engine feels smoother and quieter with the timing retarded. (Great for starting on the handle too!)

Post by **mike.perry** » Wed Jan 28, 2009 11:23 pm

You must have money to spare. A series engines were only designed to run on 95 octane 3 star. If you get better economy I can't imagine that it will make up for the extra fuel cost.

Post by **bmcecosse** » Wed Jan 28, 2009 11:27 pm

Crikey -240 rpm (how do you know ??) is super slow idle. More usually 500 rpm ! Unless the backfiring is very intrusive I would leave alone. Wouldn't have theought the difference in timing would be more than a couple of degrees - should still start on the handle even in the advanced position. Can't remember if we covered it before - did you check/renew the vacuum advance module on your dizzy ?

Post by **Dominic** » Wed Jan 28, 2009 11:31 pm

I have one of those Gunson Testune meters, shows rpm quite accurately. (Also slow enough to time on a watch - 4rps!) 500 seems as if it's racing away!
Yes, timing was knocked back approx 2 degrees.
A new dizzy & vacuum was fitted 3 years ago.

Post by **toginthemog** » Wed Jan 28, 2009 11:33 pm

I have done a running test on high octane petrol and this high grade desie and we kept both vehicles at 60mph for the full tank and the only differance we see is the extra cost It went down on our record as a total waste of money. cant say this is the same for a modern high perfomance engine. we had one van doing over 100 thousand a year so we needed to be sure we wasn.t losing out on saving a coin

Post by **bmcecosse** » Wed Jan 28, 2009 11:35 pm

Could still be worth checking the vacuum unit - although surely should last more than 3 years!

Post by **Onne** » Wed Jan 28, 2009 11:35 pm

I did a test with the two different types on my DAF.
25 mpg on 98
30.8 on 95.

She is a bit of a drinker though seeing that she only has two cylinders and 746 cc (must get that timing right...)

Post by **bmcecosse** » Wed Jan 28, 2009 11:36 pm

Strange for it to be that way round - unless it went so very much better on the 'good stuff' that you drove it much harder!

Post by **Onne** » Wed Jan 28, 2009 11:40 pm

The car is still being run in though, she has done 300 miles in the last four weeks, and less than 500 in the ten years prior.

She should do about 40 mpg though, and 50 mpg on the motorway thanks to the Variomatic.

Back to Morris Minors now

My Minor doesn't do any miles on anything at the moment!

Post by **bmcecosse** » Wed Jan 28, 2009 11:41 pm

Well for goodness sake - buy an SU pump on ebay - there's lots to choose from, and a spare is always useful.

Post by **Onne** » Wed Jan 28, 2009 11:43 pm

I know

She'll be up and running at the weekend.

Post by **rayofleamington** » Wed Jan 28, 2009 11:44 pm

running my everyday car on the same fuel I get between 31 and 41mpg. The worst comes from short journeys in the cold and the best from constant 65mph on motorway. Tends to ~35mpg doing an indicated 80-85mph. When you're looking for a few % difference you'd need to do it over exactly the same journey at same speed with no difference in traffic etc..
The outside temperature will also affect economy so if doing a back to back test it's something to bear in mind.

Post by **Dominic** » Thu Jan 29, 2009 12:36 pm

When you're looking for a few % difference you'd need to do it over exactly the same journey at same speed with no difference in traffic etc

or (as I am) a test over several thousand miles - but then I suppose I should redo the test at the same time next year to take into account the temperatures! Mind you - I managed over 1000 miles in the last week - so maybe I can do another 3000 mile test in similar conditions!