Odd backfiring issue

[afrozenfyre]

After a lowside last summer and being at school and away from my bike until now, I've finally got my Monster (almost) ready to ride.

I have a carbed 2000 M750 and my issue right now is that about 3 seconds after killing the engine I get an huge backfire (complete with flames shooting out the back). Before the crash I had some one-off "megaphone" style pipes with no brand as far as I can tell (and I have searched  here and the web for a while). I replaced them with stock pipes I got from a DMF'er so my initial thought was that is was running rich.

I'm not afraid of messing with the carbs (though it would be my first time) but I wanted some direction first. Two techs I talked to on the phone were confused about the whole thing, but one said it shouldn't be a problem as long as it doesn't backfire while running.

Other possible factors: Airbox is stock/unchanged with clean (enough) filter. I replaced the tank/fuel filter as part of the crash recovery. Runs/shuts off normally with open headers, which are stock (I ran it after changing the tank but before I put the pipes on). Choke is basically unnecessary and if it's on after the engine turns over it bogs it down, which was NOT the case before, I'd need to leave the choke one for a little while in 70F degree weather.

Should I worry? Is there a simple adjustment I can do or am I better off leaving it to the "pros"?

[koko64]

Does the bike have standard 38 Mikuni CV carbs?
How many miles on the bike?
Has the bike been previously re-jetted to suit the open pipes it used to have?
Would you clarify about the choke being required to start/warm up the bike?
What is the condition of your spark plugs?

Cheers

[afrozenfyre]

Does the bike have standard 38 Mikuni CV carbs?
How many miles on the bike?
Has the bike been previously re-jetted to suit the open pipes it used to have?
Would you clarify about the choke being required to start/warm up the bike?
What is the condition of your spark plugs?

Cheers

Haha I knew I couldn't include everything. 

The bike has 27,2xx miles, had the 24k service done at ~26k right after I bought it. So I don't know if it had been rejetted or about spark plugs (though I hope they would have checked them at the service?). Am I looking for soot buildup if I check the sparkplugs?. Carbs are stock.

Basically I usually start the bike with the choke open/on.  When it's cold out (<50) it takes a few minutes to warm up and then I take the choke off. When it's warm, I still need to use the choke to start, but can take off the choke after about 30s. Now, I have to have the choke off completely before I start it or else it feel like it is "flooded" after a couple cranks. Also once it starts it feels like the idle is really high--exactly like what it sounded like before when I would leave the choke on too long.

Hope that helps 

[koko64]

Haha I knew I couldn't include everything.  

The bike has 27,2xx miles, had the 24k service done at ~26k right after I bought it. So I don't know if it had been rejetted or about spark plugs (though I hope they would have checked them at the service?). Am I looking for soot buildup if I check the sparkplugs?. Carbs are stock.

Basically I usually start the bike with the choke open/on.  When it's cold out (<50) it takes a few minutes to warm up and then I take the choke off. When it's warm, I still need to use the choke to start, but can take off the choke after about 30s. Now, I have to have the choke off completely before I start it or else it feel like it is "flooded" after a couple cranks. Also once it starts it feels like the idle is really high--exactly like what it sounded like before when I would leave the choke on too long.

Hope that helps  

I would be interested to know if the carbs were touched during the service: if they were synchronized, and idle mixture and idle speed adjusted. If you have a high idle once the bike is warmed up and the choke is off that can be adjusted.

If your plugs are sooty (check after bike has been idling with no choke or you have been cruising around at lower revs), then the idle mixture screw may be adjusted out too far. However, if the needle jets are the original ones in the carbs then the following applies despite any other contributing causes:
With the mileage on your bike the standard brass needle jets/emulsion tubes will be worn oval by the needles. These needle jets wear oval and cause richness.They should be replaced and I would recommend Factory Pro stainless needle jets, cheaper and better than stock ones. The FP needle jets wear better.

Sometimes you can get crud build up in the carb float bowels and tiny bits of poo under the float needles making the bike flood and run rich and fart and backfire. Dissassembling, draining and cleaning the carbs with a carby cleaner, replacing the needle jets, checking and adjusting the float levels is worth it since your needle jets are shot. It has been gradually getting richer.

Then adjust the idle mixture screws and synch the carbs.
There have been a few threads on this and there is a good site that covers this with detailed pictures. A manual would be good too if you don't have one.

I'll find the site and be right back.

[koko64]

The site Ducati Suite has a section under Performance called 'jet kit/pod filter install' which among other things covers replacing the needle jets, adjusting the float level, etc. It has good pictures and step by step instructions.

Check the color of your plugs. They are probably a fluffy, sooty, black and not light brown or tan. This will help confirm the fuel build up problem.

Dont work on the carbs with a hot engine. If your fuel tap is a manual one turn it off. Remember to be careful with gasoline, no open flames, work in a ventilated area, etc, etc.

There may be other causes to your problem, but if your carbs have the original needle jets, then you need to do this anyway, be sure of that. A carb overhaul and adjustment will most likely solve the problem.

Let us know how you go.

Cheers

[koko64]

Don't forget to check the carb manifolds, exhaust connections and air box for air leaks. These things can get loose in a crash and let air in causing a backfire.  

Inlet manifold leaks make your bike run lean. But your plugs will tell you if this is the case. They will be white/cream.

Imagine if you have air leaks into the exhaust and a rich condition from worn needle jets!

Cheers

[Howie]

Anything short of a full exhaust makes little difference on carbie monster.  I would start with a carb synch and idle speed adjustment first.

[RAT900]

A stopped engine that blows out a backfire 3 seconds after ceasing to turn says carbon build-up and an open exhaust valve to me...just my thoughts on the subject as below....

detonation requires spark of some sort and if the engine is stopped completely that spark would be glowing carbon...if the engine stops on an open exhaust valve that would create the blow-out through the exhaust

the 3 second delay would be the residual gas in the cylinder vaporizing in the hot environment and achieving the right fuel-air mixture that will be touched-off by the glowing carbon

at 27K miles if the bike is running rich and/or has been running rich there will be carbon.

there was something called Seafoam that is like Draino for engines  

you may want to start there and work your way forward with some of the items mentioned above

here's a seafoam at work link       Seafoam in a WRX


probably should do it during mosquito season

[afrozenfyre]

Thanks for all the suggestions everyone. I think I figured it out!

I had some Seafoam lying around that I had forgotten about and added a good amount of it to my remaining gas. Unfortunately I didn't get cool smoke out the back, but after riding for 10 minutes I turned it off and no backfire! I puttered around another 20 minutes with occasional high revs (in neutral--I couldn't really get up to speed) until I ran out of gas (thankfully it died right in my driveway). Seems to run and shutoff much smoother now.

I guess the combination of old-ish gas and repeated cranking without really running for very long left a lot of carbon in the whole system.

I do still think the idle is too high though--that's next on my list of adjustments. I'm definitely going to look into the needle jets when I have some downtime because I'm pretty sure koko64 is right that they're shot--but while it's still running decently I'd like to ride a little first.  [moto]