Club 80-90 Forums

Morning all. I was driving to work this morning and noticed when braking that there were loud popping/cracking noises coming from the engine. This disappears when accelerating/cruising but comes back again when slowing. I didn't actually count how many time it did this but on my 25 mile commute it was at least 25-30 cracks or pops.

I thought I might have heard a couple yesterday too but wasn't sure, today was much much worse.

I'm not exactly tech-savvy but want to work through a kind of checklist to try and clear it up.

Change plugs
Change HT leads
Change dizzy cap
Change rotor (don't have 1 yet)
Change fuel filter? Was running on an almost empty tank yesterday but was fine doing so. Mis(or back)firing started bad today
Change 3.5mm vacuum hoses

I've also pretty sure I've an air leak near the left carb but there's a lot of hoses running around that are that I haven't got a clue about, but I can feel a blowing of air when I put my hand under and to the left of the left hand carb, it could be coming from the hose opposite the brake servo T-joint at the bottom but I've no idea what that hose is If the leak is from the carb itself I'm pretty stumped because I don't have the knowledge to even touch the thing. I've had the air leak since I've had the van though ( a few weeks) and not had a mis or backfire before.

Is there anything else I need to consider or any particular order I should tackle this job. I also need to get the 25 miles home, it's running fine apart from this and I don't have any loss of power so do I really run the risk of screwing the engine?

I've just had the same problem. Only ever happened on deceleration and often just as I touched the brakes. It was caused in my case by holes in the exhaust. Something to do with pulling in extra air causing unburnt fuel to ignite, or so I was told by a much wiser man than me.

Anyway the whole lot was shot anyway so I had the heat exchangers and back box replaced and hey presto, no more back fire.

Thanks! I do have a hole the size of a medium sized screwdriver in the silencer. What kind of temps do these things reach as I've got some filler which is ok up to 130°c and would rather just plug it up rather than replace.

81AirCooled wrote:Thanks! I do have a hole the size of a medium sized screwdriver in the silencer. What kind of temps do these things reach as I've got some filler which is ok up to 130°c and would rather just plug it up rather than replace.

No idea temperature wise but I used the proper "Gungum" exhaust filler on mine. It never stayed in for more than a couple of days but it may last long enough to find out whether it's the hole in the exhaust causing the backfire.

It'll be the hole in the silencer

I wouldn't worry too much about running back and forth to work with it. I ran mine for months with the backfire. Didn't do any harm to the van, plus it was hilarious watching people dive into hedgerows when I passed.

I finally decided to sort it out when it happened going through a tunnel. I had the window down and the ricochet scared the pooh out of me. I thought the Taliban had got me.

Hi, it might just be a hole in the exhaust or it could be more serious.

The "popping" on the over-run (when you come off the throttle) normally means your mixture is lean / weak. This could be caused an air-leak amongst other things. When you open the throttle you are letting proportionally more fuel into the engine so the mixture is less weak. Off the throttle the mixture has less fuel so is even weaker. Weak mixtures burn hot so not good for your engine and potentially catastrophic.

When you look for an air leak causing a weak mixture on your carb remember you are looking for where air is getting in, not coming out. If you spray WD40 around the carbs & manifold it will get sucked into any leak paths, causing the idle speed to change. With a weak mixture it should cause the idle to rise.

If you want to find the source off the air blowing under your left hand carb get some fairy liquid, water and a spray bottle with a mist nozzle. Squirt the soapy water around the joints and look for bubbles.

The hole was there since I've had the van too but I'll make sure I plug that tomorrow as well. I've been working so hard on the van since I've had it I've already lost a couple of kilos

My left heat exchanger was damaged in a shunt and ever scince it has been doing this, (it did have a whole new exhaust fitted )
Could it be this causing it even if its not directly the exhaust?

Its also developed a little bit of flat spot, could that be related ?

Same engine btw

Just noticed another hole in the exhaust along near the seam in the silencer. It's rusty down there so I think a stone might have gone through it which would explain why it came on suddenly even though the other issues were there before.

81AirCooled wrote:Just noticed another hole in the exhaust along near the seam in the silencer. It's rusty down there so I think a stone might have gone through it which would explain why it came on suddenly even though the other issues were there before.

Mine started with what looked like a small hole. However once I'd removed all the rust the silencer was like tissue paper. Probably time to invest in a new silencer. They're not expensive.

Just had a call from the garage about the Astra estate, cylinder head gasket, discs and pads need dpoing all round. Looking at the best part of €1500 so at the moment even the bloody gun-gum is looking expensive