As per heading, my Bay is backfiring shotgun backfires going downhill with my foot off the throttle pedal. If I slightly press the pedal I stops doing it, as it’s the last job I wanted to sort out before I am really happy with the bus, it is starting to drive me a bit mental. I have researched as many threads as I can, and done what I can but it’s frustrating. So far I have: 1. Checked valve clearances, they were tight, so have set them properly, no change 2. Changed inlet manifold gaskets from metal to card as per thread advice on here. No change 3. Bought new accuspark strobe light and set it to 8 degrees before TDC, it was way out, but this made it worse. 4. Checked vac advance and retard works in dizzy by sucking pipe, it does and checked all vac pipes. 5. Changed back box from bugpack to empi instarust silencer, No change. 6. Checked exhaust heat block off plates, tight and seem ok.... I am about to spit the dummy and put either the spare standard carb assembly on, or to be honest even the whole spare engine in as it’s driving me bonkers, and startling pensioners. The next step is a new set of ICT’s in case they are worn, or at least a refurb kit in each. Or perhaps a whole new stainless JK exhaust system. But this is starting to get expensive. It runs well and pulls well in every other way. any thoughts?
We had a similar issue years ago. One of the inlet manifolds wasn’t seated correctly, the tinware was pinching onto the manifold creating an air leak where it seated on the cylinder head. Some fettling soon sorted it out.
You don’t need a new engine. You (probably) don’t need new carbs. You’ve just got an air somewhere. I could make mine do it on demand for a while. Drove past 2 coppers in some crappy area of Birmingham and took my foot off; genuinely thought they were going to hit the deck.
Proper backfiring on overrun is usually attributed to an exhaust leak. I don't see that you checked the exhaust / head gaskets? These are sometimes quite poor quality and whatever it is sandwiched inside them crumbles.
I had a few hours lying under the bus, to change the driveshaft gaiters again. I used decent German ones so hopefully they will last more than 6 months. Anyway, while I was under the back I checked the exhaust over, and noticed the heat exchanger to exhaust joints were loose and the pipes were moving in and out. I replaced the bolts as they were solid, and really tightened it all up, there was soot on the pipes so maybe this was the air leak. I will give it a run tomorrow, fingers crossed, I also changed the nuts on the inlet manifolds to the small 11mm ones, and nipped them up tight so hopefully it isn’t that. I’m also not sure if the plug lead on the front right cylinder was on right, which may not have helped. Thanks all, I will report back.
My progressive setup has slightly worn throttle spindles, maybe. When I go down hill the AFR meter goes over to 20:1 (lean) and I start to get a gently crackling exhaust. Random pops every second or so.. There are cracks in my silencer too. I expect its a small exhaust leak, but I have come to accept it as a part of the character of using aftermarket parts. Just like its habit of idling on three cylinders from cold. Which is some characteristic of the carburettor and the manifold as three different engines all did/do the same thing.
Perfectly normal to be off the scale lean deccelerating. My stock exhaust did it without a pop. My big one does it and pops. 100% no leaks.
Sometimes I wonder if the lambda sensor sitting there is also hot enough to ignite stuff as it goes past. .. depending on ethanol content its 600F is smack bang in the range of auto ignition temperature.
Yes it does register lean. As lambda sensors are measuring O2 and implying the combustion from the percentage O2 remaining. You get the 18% or whatever O2 if you fail to ignite the fuel, or do not provide any fuel at all and try to ignite it. Idling with a misfiring plug causes the AFR to keep flicking lean.
Ok, I took it for a quick run at lunch time, and almost all of the way it was perfect, but then on the downhill approach to the house it gave one big pop, and made a lady walking her dog jump. Although it then seemed to run on three, so perhaps the plug lead has popped off again. I’m sulking a little bit, so will check tomorrow night after work and make sure, and maybe nip up the bolts again.
A second test run, seems to suggest it is fixed, no bangs. It appears to be the exhaust leak has been fixed and stopped the backfiring. Thanks for the help.
