I've put my engine in after a top end rebuild. 1600 type 1. I've done the tappets, points, timing and adjusted the Pict 34 carb. It's running pretty well and I think I can feel the improved compression. During the ring break in that I've been doing (accelerating to 50mph on 3/4 throttle and then coasting back to 20mph - to the annoyance of other road users) and while decelerating it consistently backfires as it decelerates from 40 down to 30mph. 3 or maybe 4 little backfires decreasing in power as it slows down. It looks like there's a leakage somewhere but I can't work out where. I have tightened the boots on the manifold, the hose going from the air filter to the top of the carb and the nuts on the exhaust to head studs just to see if there was a leak but no joy so far. Anyone got any ideas?
I was going to say Exhaust, but looks like you've already covered that Someone who understands these things will be along in a while
It is usually exhaust leaks that cause popping on slowing down. Did you use new gaskets? I would double check all joins, in particular the heat exchanger to head joints. If that doesn't show any probs, re check valve gaps and timing.
My type 4 rebuild was doing this too I had the mixture too lean after leaning it out for its MOT I have since turned mymixture screw out about 1/2 a turn and haven't had a backfire since drives better too
Is there any way I can tell whether it's exhaust or intake leak? I can't find it. Sent from my iPhone using Tapatalk
What about the gaskets that seal the manifold to the heads? I used cardboard ones here as I was advised that the metal ones are harder to get a good seal with.
If its an intake leak it will run poorly, get too hot, and pop and judder on accelerating usually. If you spray carb cleaner or plus gas around the inlet joints and carb base, you will hear the engine note change. If it runs fine bar the backfire on deceleration, its usually the exhaust side, or sometimes if the heads are not sealing properly to the barrels it can cause it. Stick your head under the engine with it running and listen for any slight blowing. Also shine a torch at the join between heads and barrels to see if there is any sign of blowing or oil leakage
I find every time my bus starts popping and crackling when I take my foot off there is an exhaust leak. Get a metre or so of hose, start the engine and while it is idling hold one end to your ear and point the other end at each exhaust joint and when you find the leak it will be a sharp cracking noise . Or if you have fireproof hands just after starting up and before things get hot you can often feel the pulse of an exhaust leak on your fingertips if you hold them near each exhaust joint. Then if no joy go above the engine and poke around the inlet and carburettor this time listening for a harsh hissing. That will be an intake air leak. The carburettor may make a more muted sound as air is flowing correctly inside it but if the throttle spindle bushings are worn the noise may vary and become harsh if you push it backwards and forwards in line with the throttle cable. I use annealed copper exhaust gaskets (Home made from £6 worth of copper sheet and using an Ikea hole punch intended for sink taps .. reusable after re annealing..) with a light smear of gun gum and do up the nuts fairly tightly with a 15cm ring spanner.
A common hole you get is one in the inlet manifold heater pipes that come off the exhaust, couple of inches from where they couple to the exhaust.
