South West Bikers | Motorcycle Forum

My bike sometimes gives off a massive BANG when starting it from cold. Once the engine is warmed up it starts 100% fine. Funny for scaring passers by, but not great for my engine/confidence in it.

The BANG happens exactly as the engine starts, it continues to run fine right after the bang. So it starts perfectly, except for the BANG sometimes, on the first revolution of the engine running by itself (ie after the starter motor). Hope you understand.

It happens rarely, but has more than usual these past few weeks. (could be weather related?)

Im not sure, but I thought it might happen when either i have too much choke or not enough choke. But today for example choke was not needed, and it still backfired.

Could it be something to do with the weather? Happens more in the colder months (did happen a few time in the Summer). I cant say for sure.

Bike is kept outside, covered. But sometimes it rains on the bike, and rain could get down the exhaust pipe, also condensation could build up.

One thing which may be useful for you is the fact that I'm pretty sure my fuel tap is faulty. When it's on the off postition it still flows fuel. So basically unless I get the position exactly right (near impossible to do), the fuel is on full flow most of the time. Not sure how this would affect the bike as there is a fuel pump between the tank and the carbs.

Thank you

Lo don't like fuel injected stuff but simple things first, when was the last time you cleaned the plugs or fitted new ones??? poor initial spark means the bike gets petrol into the exhaust, hense the bang when it starts as the petrol in the exhaust lights. Would advise changing the p-tap tho, as this can cause all sorts of problems. I'm not sure about the pump but fuel can still leak through and if a bike is left standing for a time the barrels can fill with petrol and that can cause bent crank and conrods when it is turned over. Thats why so many bikes were fitted with vacume ativated taps because people too stupid to turn taps off or faulty taps. Spray all electrics with wd40 or a slilicon spray to keep out the water. Try that and see what happens.

I would just add try not to twist the throttle before the engine is turning over.
It happened the first time I started my bike and I'm sure it was because I used the throttle too soon. It hasn't happened since.

I'm not trying to make you sound stupid with this, it's just an idea but I know alot of people who make this mistake....
Are you sure the fuel tap is broken? The bike will still run on the fuel in the carb with the fuel tap off and with bigger carbs it can run for quite a while on this petrol!

Maybe worth you knowing its cbr400 gullarm '92

Now I'll read the replies...

Plugs are good. Did the same problem on the old plugs as the new plugs. (changed them about 3000 miles ago).

Fair point techno bear. I'm sure I don’t add the gas before, kind of at the same time. But, I can try letting it turn over a wee bit before I add the gas. I will try. Thing is I start the bike the same way all the time, and it’s been fine for the most part. And the same starting procedure goes for when the bike is warm, it’s perfect when its warm and I start it in the same way. I only ever use a tiny bit of gas anyway. I test.

Dodgey, I’m pretty sure the tap is not right. When I was last working on it, (well the last few time actually) I would put the tap to off, pull hose off and loads of petrol still comes out. Then wiggel the tap left and right a bit to "find" the actual stopping point, which not only is very "exact" has to be 0.5 mm exact, but is not in the same markings as is on the bike. Kind of half way between off and reserve. Not pointing to the marker on the bike. I will test.

I can test some more, but it’s a fair point: the fuel in the bowls will last a while... But doesn’t explain when I put it on off, then take the fuel hoses off and fuel still flows... Something’s not 100% there. I will also test.

A mechanic I buy parts off suggested that the valve clearances could be to blame. Wrong cold clearances causing the valves to not be seated properly, causing the fuel to get past the valve/its seating. Going lean, backfiring. I suppose this would explain why when the engine is hot it starts 100%: metal parts expanding... But then if the shims were worn, then the valves would be seated "better", rather than worse, no?

I might get the valves checked, the bike has 28,000 miles and I doubt it’s been done in a while.

Cheers so far I'll try a few. I don’t have any tools, but I know someone who can help me out, so we will get cracking.

Funny you say that Ben, I had exactly the same problem with my ZXR when I first got it, it was a complete anchor except for the paint job and I had to re-build it. It wouldnt start and kept backfiring, I thought it was plugs, wiring, carbs, or CDI at first, wasnt till I had it running that I relised my fuel tap wasnt working properly and the "ON" was clogged up!

Claud 14.7 to 1";p="10635 wrote:mechanic I buy parts off suggested that the valve clearances could be to blame. Wrong cold clearances causing the valves to not be seated properly, causing the fuel to get past the valve/its seating. Going lean, backfiring. I suppose this would explain why when the engine is hot it starts 100%: metal parts expanding... But then if the shims were worn, then the valves would be seated "better", rather than worse, no?

I might get the valves checked, the bike has 28,000 miles and I doubt it’s been done in a while.

This would only be a problem if the valve clearances were too tight, and it would get worse as the bike gets hot because the metal expands and the gap gets tighter not loser as the metal expands.

Sure, exactly. I doubt they are too tight.

Someone else suggested that a low battery could be giving a low spark at turnover, hence BANG. Battery is/was abit low recently. I'll charge it right up and see how it goes. Try the cheap things first eh?

Didn't backfire today

I once had a VFR400R that had a faulty fuel tap, kept losing vacum making the diaghragm in the fuel tap cut the fuel whilst you where riding. So I removed the vacum diaphragm thingmy to solve it....

this would have the effect that if you forgot to turn the fuel off and left the bike standing for a week or so it would be hard to start and when you did get it going you would get a loud backfire from the unburnt fuel in the downpipes.

Its either shit spark(knackered battery,coils, plugs, leads,cdi/ingition unit or intervening wiring) or extremely rich mixture.

Its possible that your fuel injection is just putting too much fuel in on a cold start or its retarding the ingition too far or not enough.

or as has been said the taps leaking and flooding the engine.

It's ok, I found what it was long ago.

technobear";p="10631 wrote:I would just add try not to twist the throttle before the engine is turning over.
It happened the first time I started my bike and I'm sure it was because I used the throttle too soon. It hasn't happened since.

Exactly this!!! The slightest bit of throttle (too much) on the first revolution of the engine (turning over) and it would backfire. I guess it's a sensitive starter.

Now I let it turn over 1 or 2 times before adding the gas and it fires up no probs, 100%, 100% of the time.

Many many thanks Technobear. You are a genius.

*My thinks about how much money I saved*

My SV did it once, scared the crap outta me.
It turned over, went BOOM and then carried on ticking on idle perfectly. Just a little too much gas in the chamber I guess.
It's never done it since thankfully, changing my underwear after I already have my leathers on is a pain in the backside.

before i got mine on the road i picked up a set of carbs that i re-jetted but made the mistake of using 2.5mm needles in 3mm emulsion tubes. Trying to start it, every third revolution of the engine sounded like a shotgun going off!! Ran so rich that when it did eventually fire up there were regular two foot flames out the silencer!

Great isnt it? He,He