1980 yamaha 850 backfiring

[exitandleave]

finally got the bike running- carbs rebuilt and new plugs/wires. Bike backfires a little bit at idle and backfires when you get around 7500 RPM, and again when you let the RPMs drop fast. If holding a steady RPM it won't backfire at all. Muffler chrome is starting to get rainbow-y.

Planned on getting a smaller jet for the carb, but after driving around checking stores- I was unable to get any. Also, I'm now unsure as to which direction to go with the jets. Should I increase or decrease the diameter? By how much should I deviate?

also not sure which jet to replace. Planned on replacing the center of the three but don't know.

Thanks a lot for any help.

[The_AirHawk]

"Blued pipes" and backfiring tends to point toward Lean. What do the plugs look like? Best way to judge is to put in a new set, fire it off and let it idle to warm up, then accelerate hard, upshifting to top gear as you wind it out. Shut it down and coast to a stop, pull the plugs and look at 'em.

But, that's typically not feasible for Joe Average.

Still you DO need to look at the plugs, if you have no other way of judging rich/lean (such as a wideband O2 sensor/datalogger).

[seruzawa]

Look carefully for leaks in the intake and exhaust. Even a slight leak at the head pipe/exhaust port junction can cause lean running. It'd check very carefully before trying re-jetting.

What altitude do you ride at?

[mscuddy]

Blue pipes can also mean very rich, as the flame front keeps going out the exhaust port, into the headpipe, discoloring the pipes.

Since it's not an "all the time" backfire (al la burnt valve) look at the electrical system, soulds like a short of some sort causing a misfire, like a bad plug wire.

It could also be a very large family of weasels moved into your airbox over the winter months, and globs of fur and rodent poop are fouling up the combusiton process. You can get a Weasel-0-Meter at your local Auto-Zone for about seventy five bucks. Detects weasels, warf rats and coatimundies.

[exitandleave]

I've looked at the plugs and two of the three come out fine. The third, the left cylinder, comes out black, wet, gassy. I misspoke in my original post, the bike has new plugs but not new wires. Will any spark plug wires work or are there a specific kind for bikes?

How would I go about checking for air leaks into the exhaust? I'll do a complete leak test on the engine later using WD40, but will this work to detect an exhaust leak?

Also, I am running only the right fuel tap to feed all three carbs, and have just plugged off the vacuum port on the engines left side where the left fuel tap used to call.

I replaced the airbox with three individual filters after finding the box and filter beyond use.

Thanks a lot for the replies everyone, I'll be back tonight or tomorrow and let you know what I find out.

[sarnali2]

Warf rats probably aren't an issue unless he lives near a seaport or river terminal. just to be on the safe side place some peanut butter as far up the pipe as you can with a stick or screw driver. Wait about 45 minutes or so then place the barrel of a shotgun loaded with bird shot against the pipe aperture as snugly as possible and let 'er rip.

Ensure the pistons are either on the intake or compression stroke to avoid valve seat damage from stray pellets, start the bike and run to maximum revs in neutral to expel any excess pellets or vermin carcass's and you should be fine

[seruzawa]

Pod filters often cause a lot of mixture problems. It can help to fabricate a tube about 2-3" long to stick between the pod and the carb opening.

Those bikes ran lean to begin with (smog requirement) and running pods only worsens the leanness. You might check Dynajet and see if there's a jet kit for the bike.

[exitandleave]

Didn't get a chance to run the bike and check for leaks today. Ended up having to JB weld the throttle holder back together after it broke on me. Waiting for it to set up before getting it all back together.

For proof of concept on the bike running lean due to the air filters could I put a rag or some plastic over the air filters to decrease the airflow through it and see how it affects it?

I checked Dynojet.com and I don't see a jet kit for the bike.

I'm 600 feet above sea level.

Thanks again for all the help and replies. Once I get the bike running I'll make sure I attempt to take care of the warf rats. I do live right between two rivers. I bet that's the problem.

[seruzawa]

Originally Posted by exitandleave

Didn't get a chance to run the bike and check for leaks today. Ended up having to JB weld the throttle holder back together after it broke on me. Waiting for it to set up before getting it all back together.

For proof of concept on the bike running lean due to the air filters could I put a rag or some plastic over the air filters to decrease the airflow through it and see how it affects it?

I checked Dynojet.com and I don't see a jet kit for the bike.

I'm 600 feet above sea level.

Thanks again for all the help and replies. Once I get the bike running I'll make sure I attempt to take care of the warf rats. I do live right between two rivers. I bet that's the problem.

The pods can cause turbulence in the carb throat that causes poor running. Installing a tube as I mentioned allows the airflow to smooth out before entering the carb.

Setting carbs is a bytch of a job. In your case I'd try the following:

1- Very carefully drill out the caps over the pilots screws. Then carefully turn the screws clockwise until they seat lightly. Then turn them out 2 1/2 turns.

2- Get main jets 1 and 2 sizes larger than stock. Try the one size over first.

Start the bike and warm it up. adjust the idle to about 1200 rpm. Turn the pilot screws 1/4 turn out.. keep the three screws the same. Let the idle settle. You are looking for the point where the idle just starts to get lower. Then turn the screws 1/4 turn in. You may have to play with it... like starting with the screws 1 1/2 turns out, but 2 1/2 is usually a good starting point.

Now that the idle is set you ride the bike. Using new plugs you ride it hard for a while then look at the electrodes. They should be a grey color. If while it's too lean. If black it's too rich. Once you have the main jets set for high rpm you need to set the needles. You can raise them with washers.

It's best to have a dyno to do this. Otherwise you have to do it by the seat of your pants.

[sarnali2]

You can't put the stock air box back on then? You'd be better off trying to patch it up and use it with a stock filter than fooling around with pods. JC Whitney or K&N probably carries a stock replacement filter.

A friend of mine had one of those back in the ancient 80's, it was a pretty good running bike, it would give my 750 Kawasaki a run for it's money...