Back firing and cutting out

I though she was running sweet on tickover since the weekend I re-timed skew gear and distributor (2.25 petrol), fitted new weber carb, I did valve clearances and strobe timed it to 6DBTDC once warmed up-it ran lovely round the block twice.
(it ran smoother at about 10DBTDC but I left it at 6 like book said.

had a nice 12 mile run to work this morning, overdrive unit engaged.

coming home tonight!!!!!!
it was backfiring like mad once up to temperature, still loads of power!
more noticeable when clutching in to slow down, it even cut out when approaching roundabouts (about 8miles into journey) another couple miles when I was going slow around the estate it was running really sweet again and no backfiring??!!


help?!!

Did you tighten up the dizzy pinch bolt tight? You may have a vac advance issue or a centrifugal advance issue. Put a new vac pipe on and test it by sucking with dizzy cap off. I had some running issues which turned our to a duff ht lead, check its firing on all cylinders by feeling exgaust manifold when started from cold to see if one stays cold. Good luck

yes pinch bolt tight

cheers for that, vac advance tube isn't very tight on the carb-fingers crossed its that!

my cylinder compression test kit arrived today so might give that a dose of
losing its pressure virginity ;0)

ps would losing vac advance pressure cause back firing??

joe27979 said:

ps would losing vac advance pressure cause back firing??

Click to expand...

I dont know but its potentially timing as im assuming your spark is igniting when exhaust valve open or maybe fuel getting into exhaust then igniting? Id recheck the valve lash to make sure thats ok maybe. Some others on here know far more than me on this stuff
It could be the carb causing a spit into the air intake so readjust but it sounds like yours is intermittent and maybe when hot so maybe check electrics which can do funny things when hot; coil etc

Do you mean a pop pop pop on the overrun - that's pretty normal for webers. Mine used to cut out when slowing down on long gardients, its the fuel sloshes forward in the float bowl and (deliberately) lowers the level in the main jet circuit. The opposite happens going up hill and you get a bit more power.

I'd try a quarter to a half turn out in the mixture screw (leaner) and then readjust the idle speed to 800 rpm (you can go lower but then you risk cutting out and also your alternator might not charge when idling which is ok for normal driving but not for farm work). Try that and if it improves you know where to look. If it gets worse try the opposite (i.e. go richer and re adjust idle). It's been a couple of years since I upgraded to a 32/34 weber and I can't remember how I ended up setting up my old Weber.

I can't find my Weber manual at the moment under all the ****e in my office but I'm fairly sure the vac advance pipe is from _above_ the throttle. This means if you have the idle mix rich and the throttle stop more closed then the advance doesn't do much when your foot is fully off the gas.

Dom

I would agree possibly a mixture issue or perhaps the condenser is knackered Ive had that a few times with my v8

I don't have a condenser chaps-electronic ignition, brand new top of range coil leads and plugs, I can guarantee electrics is running ok.
I'm keen to go down carb mixture route...
I don't have a tacho so how do I measure rpm to set up?

I'd buy a cheap multimeter plenty of them have RPM/dwell settings on. Worth having for when all the other electrics play silly buggers! I got one like this Uni-T UT107 Automotive Multi Purpose Meters Tester Voltage Temp Multimeter | eBay after I ran over my incredibly expensive one!

You can however do it by ear if you listen to another engine and set it up like that. Really you just don't want it to be idling too slow.

Once you're happy with it you can progressively lower the RPM until it just starts to play up then give it a bit more, this will give you a touch more mpg. I think I had mine idling around 650rpm in the end up, much lower than that and it would occasionally cut out when braking.

The other thing I've used is a wideband lambda sensor to help set up carbs, with one of those you can really start tuning, trouble is they're quite expensive. I got mine to set up twin-choke carbs and help my mate setup his race bikes (he's now lost all the leads). With that you can often get the carb set up better than new as things like the Weber can be tuned quite easily (by swapping idle, main and air jets). But unless you _really_ like reading a lot of books about carbs and fiddling about it might not be for you. I have managed to get about 5mpg more AND more power but for a fair outlay and a lot of playing...great fun though.