Topic: Backfiring? Popping? What does it all mean?

[Greg]

hey all..
76 750f
rebuilt the carbs a while back and was having lots of problems figuring out the proper settings until i corrected the float heights -- finally got it right.  now the bike runs great, except for two minor things...

1) when i down shift (engine brake) the bike sounds like it backfires out the exhaust.  I don't know if backfiring is the proper word, maybe popping?  I recently took the header off, and also adjusted the valves for the first time ever.  Could these things create this affect?  Or is it a carb symptom?  It doesn't backfire any other time, and accelerates cleanly through the entire throttle...

2) My choke doesn't work.  the bike starts up fine with a little twist of the throttle while starting, but does not start up with the choke actuated.  also, if the bike is running and i open the choke, it sputters and dies.  am i running too rich, so that actuating the choke makes it TOO rich and it dies?  any ideas?

thanks guys!!

greg

[dusterdude]

greg,the popping from the exhaust is caused by the engine sucking in the colder outside air when you close the throttle,which usually happens when someone has changed pipes with litlle or no baffling.as fas the carb thing goes,sorry im no help there at all.

[SteveD CB500F]

If it starts without any choke, then I'd say it was rich.
They all die if you choke a warm engine (they should, anyway)

What's your fuel consumption like?

Try a plug chop - that'll tell you.

[Greg]

dusterdude --
thanks for the backfiring comments..

SteveD
fuel consumption is about 40mpg mostly highway.

i just got new plugs to try the plug chop....

[dusterdude]

your welcome

[TwoTired]

Have you done a compression check?  Poor valve seating make a bunch of strange noises.

Have you syncronized your carbs?  One or more slides open farther than the others, can leave a rich mixture in the exhaust collector, whereupon the exhuast of another, hotter cylinder ignites it.  Bank, pop, etc.

Tweaking carbs is the last part of a major tuneup.
Plugs, points, timing, tappets, cam chain, air filter cleaning, all get attention first.
You there yet?

Really should need some choke to start it when cold.  Are the idle air bleed screws set for too rich operation?

[dusterdude]

very astute observations lloyd

[Greg]

did compression check -- i get about 125 on all 4 cyclinders
carbs were synched about 3 weeks ago.
points, valves, cam chain were all adjusted, filter was cleaned and freshly oiled.

i set the idle air screws to 1 turn out as a base, then usually end up around 1.5.  if i go out to 2 turns out than the big seems to falter and surge when throttle is about 1/8 position.

i will try replacing the plugs, resynching the carbs, and rechecking the valves (that was my first valve job), and report back.

Thanks!!

[dpen]

Check where the muffler joins the headers. A lot of mufflers are a sloppy fit & suck air in. The comment about little or no baffles is also spot on. But...... why worry, doesn't it sound beautiful popping & gurgling on the overun?
What carbies have you got? If they're later ones with an accellerator pump you don't need a choke-two twists of the throttle & they're primed.
Spend a few dollars & buy a cheap remote tacho-gets the mixture screws spot on.
If the motor is hot, you pull the choke on & it DOESN'T splutter & die-then you worry.
I just noticed the time on the page-3.14 pm. Ha Ha it's 5.14 am here-time to finish work & go for a beer.

[dusterdude]

dpen,i agree with the open pipe popping,thats why i cut the nasty baffles off my pipes,now i can offend erveryone

[TwoTired]

One comment about oiling the air filter.  It can be over done.  If it gets over oiled, it can create a choking effect.
Something to think about, anyway.

Cheers,

[eldar]

many engines get a popping sound when you let off the gas, especially if you close it all the way and this is even more pronounced if you downshift to a lower gear to use your engine as a brake.

I have an accel pump on my carbs and it is on #2 and gee #2 runs way rich while the rest run fine. Your 76 I dont think has an accel pump but if it does it will be on #2.

If you are running rich to all cylinders, then you may want to check your float height AND the float valve seats.
As others say, leaky pipes can cause more popping but should not affect running rich or not having to use a choke.