Lean conditions? Spark Plug porn inside! NO ONE UNDER 18

[Incubus]

So my ride is giving me symptoms of a lean running condition. It runs hotter than expect, popping through exhaust on deceleration and occasional popping backfires. Other than that the bike runs very well, exvept it seems to ride just a touch rough what it goes for a lone high-speed jaunt onnthe highway. Not really running rough so you can hear it or feel it, just a real low, subtle "is that really a vibration" vibration.

The plugs are not as white as I expected:



These plugs are new. I put them in before a two ride of about 80 miles each yesterday and Friday, mixed highway/city but mostly highway, riding two-up, so it was working hard. Do these look lean to you? If so, I am running stock mains (137.5). Would one step up (140) be enough? Or should I plan on two steps?

The bike is a stock '79 standard with K&N in the original air box and a 4-1 Mac exhaust.

Patrick

[BA80]

Pardon my ignorance Patrick but I haven't been around in a while.

Did you rebuild the carbs and use after market kits, the ones that come with new pilots and mains?

[Incubus]

No BA80, I only rebuilt the carbs about 18 months ago using the parts that were already in they. The bike has run fine since then, but most of that riding has come during cooler months. This is the hottest spring in history, so far this year in Austin, and it is during the recent very hot weather that I first noticed a problem. I think I also have have an exhaust leak or two.

I first suspected a problem a couple days agi when I pulled the plugs I just replaced. They were not golden brown. They were a little lighter than golder brown. I always understood that the Japanese engineers who designed these bikes meant for them to run just a hair rich.

Patrick

[BA80]

Ok, the combination of 4 into 1, K&N filter and the exhaust leak ar probably your problem. I suggest repairing the exhaust leaks and seing how it is then.

If it is still lean, here are some guidelines for jetting.

Jetting Recommendations
by Denny Zander

Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.

1 jet size for custom 4 into 2 exhaust

2 jet sizes for 4 into 1 exhaust

1 jet size for K&N filter (single inside airbox)

1 jet size for drilling out the bottom of the airbox

3 jet sizes for individual filters

2 jet sizes for no muffler (open header)

1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size)

Under a mismatch condition, like individual filters and stock exhaust or 4 into 1 header with stock filter and air box, subtract an additional 1 jet size.

Check plug color often, sync carbs after each jet change, make sure the floats are set correctly, and seriously consider purchasing a Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).

Make sure your carbs are in perfect working order before making jet changes.

BTW, you might need to put a few more than 80 miles on a set of plugs to get a good read.

[TopCatGr58]

Hey Patrick,

Decel popping is common/normal with 4-1 pipes. Also can be aggravated with exhaust leaks. The plugs do look a little white, but are also very close to what mine look like as well..and I'm running high output coils.

As Greg said, fix the exhaust leak first. You can adjust the pilot screws out a 1/2 turn and see how the decel and low speed throttle response behaves.
Also if your idle is set very low...then your carb butterflies are probably very close to being totally closed when off the throttle....that also makes the Air/Fuel ratio even leaner during decel when you close the throttle. So setting a slightly higher idle speed could also help reduce the decel popping.

After doing these steps, if it's still not performing like you want, then you can invest in some new jets. Actually, the Yamaha engineers tuned these machines in the later years for lean conditions for EPA and mileage specs.

I ran 4-1 pipes with stock airbox for 9 years with stock jetting, no problems, but I've got the later 81SH model. SO...you have 4-1's, but OEM airbox but with freer flowing K&N filter...hard to say if 1 size up on the mains would be sufficient to fix your running/performance issuses, but if it was otherwise running okay before, I would think 1 size step would be enough!?

A cold engine needs more fuel than a warm one, and so if it ran okay lean cool, it should run okay or better when warm/hot.

T.C.