pop pop pop and backfire help please

i seem to have just developed a bit of a backfire and popping out of my exhaust, only when my foots slighltly on the gas just cruising along at any revs.. all the jetting is correct as ive had it dyno'd a few weeks back, and they they left the jetting as it was cause it pretty much spot on... all the nuts on my exhaust are tight, inlet manifold nuts are tight etc..

so where do i start???

thanks, fabo.....

What carbs are you running??
What sort of exhaust?

Backfire "usually" means 2 things

1 rich jetting
2 hole in the exhaust

The fact all the nuts are tight on the exhaust doesnt mean it is a tight fit.
Those clamps that join cylinders 1 and 3 to the exhaust are rubbish

My guess is you can move one of the pipes as it wont be grabbing the heater box

Backfire might also be caused by a vacuum leak at manifold or carb I think...
Could a backfire also be caused by very retarded ignition?

its a blocked idle/pilot jet.

remove and replace the ignition leads one at a time while the engine is running (from the cap). The motor should decrease revs everytime you remove and replace the lead, but you will find that one (or two) leads will make very little difference to the idle when you remove them. This will be the cyclinder with the blocked pilot jet. Remove and clean and it should fix the probs 😉

What HP did you get and where is your Dyno sheet ??

cheers for the replys guys..

carbs, 44idfs
exaust 1 5/8 headers...

i have been running these jets for a year,with no probs so i dont think itsthe jetting.

i thought inlet leek shows up with eratic idling? ill spray some wd40 around...

ill check the idles as well as of today i felt a bit of a engine splutter down low on occasion...

craig, no dyno sheet,sorry,it wasnt a dyno denamics dyno. dunno what kind of dyno it was but i dont think it prints out HP numbers like the shoot out mode ones..the dyno guy did say its propably 100bhp maybe alil more but thats it... i might take it to one with shoot out mode to see,but not in this 42 degree heat here in perth....lol...

ill have a play saterday and let u knows.. thanks...

As above with blocked or partial blockage of a jet, maybe a small manifold leak, I doubt exhaust leak. Sometimes if the jets are clear the idle mixture screw is holding a little bit of foriegn material, set mixtures again and the movement of the screw will let the rubbish out.

okay, i pulled all the jets out today and blew them out but the problem is still there...

ill pull the idles out again and give it a few revs when there out, maybe that will blow some crap out.if that fails ill try what matt said.if that fails , could it be a possible blockage in the fuel filter??

Any chance you have got water in there from the rain??

nope,. i was hoping this would be quickly solved as it does it while backing off when the revs are low and also when at a constant speed with my foot just on the throttle cruising along..

Might be worth checking the valve clearances

d d d d done that one already..... tomorrow ill check timing,points,fuel filter,exhaust and inlets again...

okay, i adjusted points and plugs today, all too tight...

pulled the carbs off to take them to a mates to blow some compressed air through them, and thought id pull the carb tops off to check the float levels. one was at 11-32mm the other 11-40mm, would this be causing my problems?.....

If it has been running well in the past - then it is something that is different

Float level wouldnt change by itself - so i would not think that is the problem

The backfire on light throttle AND backoff is definately a lean condition, mixture screws or jets, but valve adjust should also be done and definately NO exhaust leaks.

I was thinking points? cause when points burn out they back fire.
Well misses at least????
From Dylan

check your spark leads include the one from coil to dizzy
and check the coil to see the sparking distance, should be able to do at least 1cm

Do you have a fuel pressure regulator fitted ? Any chance the pressure has been altered recently . As I recall fuel presssure on IDF'S is fairly critical (3 1/2 - 4 PSI ??)

yeah i do have a fuel reg and the fuel pressure hasnt been touched..

have blown the carbs with compressed air, just yet to put them back on and see if problems solved,. should know by weekend...

PROBLEM SOLVED, MUST OF HAD SOME CRAP CLOGGED UP SOMEWHERE WHICH GOT BLEW OUT BY THE AIR COMPRESSOR...

THEY WILL BE UP FOR SALE NEXT FEW DAYS AS IVE GOT MY HANDS ON ON SOMETHING MAYBE BIGGER....HHMM..