Gigantic whoomping backfire on startup
 

Why would I be getting that? Thanks to you guys I have the bike running REALLY good otherwise. Here is how I am set up right now:

Jardine race core hi-mounts.
Brand new stock filter w/ the Burnie Morgan mod.
Pilots 2.25 turns out.
One shim on the needles.
Idles at 1250 (by the bikes tach) when warmed up at around 170 degrees.
Have not done a carb sych yet, will be doing one tomorrow morning.

On cold starts with the choke on, as soon as I push the start button it almost sounds like a 4 X single cylinder vapor lock for a second and then it just goes "WHA-PAMM!!" out of the pipes. Then she fires right up and comes up to temp, runs fuggin' great, gets the best fuel range I've squeezed out of her yet, and makes my commute a joy...

Praps this was posted in the wrong section? Technical in nature, I guess...

Mine does the same thing. Really nice sound in the garage at 5AM. LOL..

sounds like it's flooded maybe?

I do not touch my throttle. I just pull out the choke, turn on the key and hit the start buttnn.

is there any smoke associated with the backfire?

No smoke with mine just a little fire. LOL


Mine does not do it everytime so I am not that worried about it. It seems to do it more often when it is really warm outside.

I wonder if you might have a seeping float valve in one of the carbs?

Just a thought. I have no idea if it's right. A high float level could cause it to flood on startup.

Sounds like the engine is cranking but not firing the plugs that first crank rotation. The next rotation fires and burns the unburnt charge in the exhaust system. Often I kil it with the kill switch at the bars and then when I go to restart, I often forget that the switch is still off. I crank the engine and when it doesn;t fire...Oh Yeah!. I switch in on and crank again and ...BAM!!!
HIgh mileage bike???
Leaky exhaust valves?

Hmmm...maybe the ignition is firing very advanced.....the ECU is having issues? Just a thought...THAT would do it alright....

Mine happens only on cold engine startup.


Since I have completely stolen this thread...



Sorry supermarto.

No that's all good Ghopki1! The more angles I hear about, the better!

Just turned 10k on the odo... no ECU probs I know of... Not started with throttle, just choke.

It did not do it today either time I started it. I am thinking it is the first condition REDMAN described.

Are you guys turning it off with the kill switch? or the key?
If it's the kill switch, that last revolution without ign may put gas/air in the pipes and when you restart it pops.

I've always used killswitch (old bad habit) and never had it pop unless I flooded it and knew about it. Like some A-hole in the parking lot messing with my throttle while we are standing around talking.

Popping in exhaust pipe means one of a few things:
gas is getting into exhaust pipe before ignition fires bike.
exhaust valve(s) is not closing completely
cam timing is off
ign timing is way advanced
You need to explore .....

Kill switch for me too... I feel stupid admitting I never knew anything was wrong with that method!

The kill switch does the exact same job as the ign key, so the issue would be the same.
I call using the kill switch a good habit, if something ever happens with the throttle you instinctively know where it is and you will have the ignition off before you'll think of it.
You never get ashamed by accidentally cutting it off, you look cooler cutting off the engine before complete stop and allows you to do my cool "get off the bike while turning off the key" motion, chicks fall for this.

I always use the kill switch. Same as dirt bikes. Old habit I guess.

If it was anything ignition related, wouldn't I see poor performance? I am getting better MPG than most on here so I don't think that is my problem.

Since mine does not do it every time, I think that something is allowing mixture to seep past somehow.:confused::confused:

most likely a misfire or serie of misfire that are lit by the next fire.
Can be caused by a whole bunch of stuff, probably lean or stoic since it's loud, rich farts smoother. May be out of synch, maybe a lot of things, voltage drop on ignition while cranking.
Maybe it greets you the same way I greet my woman in the morning :shock:

I just did a carb synchro using a home made meter and they were clearly out of synch... Test ride made me grin. Also bought an inductive tach at Harbor Freight for 32 dolla, looks like a Fluke unit but not nearly as accurate. I put it on the rear spark wire (easier access than the front) and it seemed to be pretty close. I set my idle and did the synch and it is running like a champ! I have not had one backfire since the initial post and I have started it a dozen times, although they were not all cold.

Glad you got it sorted out, you owe yourself a beer, and another for the saved money :D

I'll be buying those beers at a sports pub this evening sir, whilst watching the latest UFC fight on a big screen and staring at the barmaids' derriere as well!

After sitting for several days and during a cold start I can get some minor detonation before the bike actually starts. Never more than one but it can be a good report at 5AM. This is common for big bore singles and twins. Once mine fires it runs rich almost immediately so I have to get out of the choke to keep it from loading. Mine idles like yours. Runs strong. Gets shit for gas mileage :mrgreen:

Today after that carb sync and idle adjustment, it was running great till I was just idling at a light and -puhhhht.... she stalls. Just like that. No nothing, just dead. Starts right up and runs great again. WTF? It felt like something electrical, but ?????

This is normal, they all do this, they spit through the intake and die, nature of the beast.

As far as the backfire on start goes:boom:,mine only does it when battery is weak. Check your battery voltage before starting-- key on. I bet you find it on the low side.If it is low put a charger on it for 15 minutes or so(key off of course)then start it and see what happens. On my Gold Wing if voltage drops below 10.5 volts when cranking,it will not start till you let go of starter button. If the charger trick works,your battery may be on the way out or charging system needs checked. Do the obvious first--clean,tight connections.As long as I have the Battery Tender hooked up I never have any problems. As far as the stalling goes--mine will burp and stall at no particular time or for any reason. Just one of the things that give the Hawk character. Let us know what you find.

Redman called it...oops kill switch was on when cranking;)

KaBoom!