Yamaha Seca II
How to make it go fast!
The Seca II is jetted extremely
lean and the airbox is so restrictive, the engine can barely run.
Nearly everyone has already put in a DynoJet kit and maybe even a
K&N filter to try and improve things but this only makes a
modest change (now you only have to warm it up for 15 minutes
instead of 25). The K&N filter really doesn't do much at all
because the airbox won't let any air in anyway. The intake horn
on that airbox is smaller than a single carb throat and you have
four of those hungry carbs gasping for air.
So ... take the cover off the
airbox and you see a K&N filter with a very large center, all
open and friendly just inviting air to come pouring into the
airbox. Leave the cover off and let that air in - but you will
have to fabricate something to hold the filter in place so it
doesn't bounce out. I used two small angle brackets and bent them
slightly to use as hold down clips with two of the screws from
the airbox cover. The filter is now securely held in place.
Now the bike is REALLY lean with all that air coming in. I found I had to increase my dynojet #98 mainjets to #132 in order to get good performance above 6000 rpms (and I'm getting 55mpg like that).
I also found that my sealed gas tank with the carbon canister was not venting properly. It would develop so much vacuum that the bike would stumble until I pulled over and opened the gas filler to let some air pressure in. I pulled off the "roll over valve" that was stuck closed and left it off. Then I pulled off the canister and capped the two nipples on carbs #1 and #2 that the canister vented to. (about 5 pounds of worthless crap gone)
I pulled off the filthy, plugged up fuel filter and replaced it with a new hi flow filter. Still not much fuel flow. The petcock was full of rust and crap so I pulled that out and cleaned it up. Cleaned out all the crud in the gastank while I was at it. Still not much fuel flow - so I pulled the fuel pump apart and cleaned all the rust out of that. Hey, it pumps fuel now!
Ignition timing is fixed and
wasn't made to be adjusted. I elongated the screw holes so I
could get a range of about 10 degrees advance from the factory
setting. 10 is too much but I found that 4-5 degrees ahead of the
stock setting is just right. This picture shows it set too far
advanced so don't go by this for the setting. It's pretty easy to
make a degree wheel on a piece of paper and figure out where 4
degrees is. Oh yeah, you have to drill out or grind down the nub
in the plate that holds it at the stock setting.
At this point the bike was really running great and getting good gas mileage. Not content to leave well enough alone, I bought a Kerker 4-1 exhaust system. The stock mufflers are pretty heavy and I think I shaved off at least 30 pounds by going to the Kerker. The bad news (besides being LOUD) is that now there is a really bad stumble off idle. The bike runs great above 4000 rpms but really acts up in the low range. I determined that it is too lean (by noticing that the stumble is worse when cold and better when hot, also by noticing that the stumble is reduced with the choke pulled out halfway) and decided to richen up the pilot circuit. Pilot jets for these carbs are pretty hard to find at local bike shops and they require draining the carbs and pulling off the float bowls to install. I prefer to work on the pilot air jet because you only have to pull off the airbox to get to them. Nobody seems to stock those either but I have a trick for determining what size I want so I can order them.
First off, the pilot air jet works
opposite of the pilot jet. The pilot jet meters fuel and a larger
jet is richer. The pilot air jet meters air (air that is mixed in
with the fuel from the pilot jet) and a larger jet is leaner. So
an easy way to richen up the pilot circuit (low speed) is to
reduce the size of the pilot air jet. The pilot air jet is a #145
(all the Yamaha parts fische mistakenly label this jet as a
"Main Jet"). As a temporary measure, I use short pieces
of guitar strings to partially fill the air jet and reduce its
size. Guitar strings are great because you can buy them in
practically any diameter and they are very precise. I bend a hook
in the string so I can poke it in the air jet and the hook hanges
over the edge of the carb throat where the airbox boots can hold
them in place while I take a test run. I found a wound guitar
string that was about .5 mm in diameter works pretty good to beef
up the bottom end but I had a slight stumble at 4000 rpms where
it seemed to be just a bit rich transitioning from pilot to main
jet. I ended up lowering the needle half a notch (washer is half
the thickness of a notch so washer under the clip is half a notch
higher than the washer over the clip) and going to a smaller,
unwound guitar string (about a .018).
A .020 string is about .5mm (1mm = .0394"). If you figure out the area of the hole in the stock air jet (#145 turns out to be about 1.45mm in diameter by my measure) using pi times r² you find that the area is about 1.6513 mm². The area of a cross section of the .5mm guitar string is .1963 mm². Subtract .1963 from 1.651 you get 1.4549 and this "should be" the effective area of the air jet with the .5mm guitar string stuck in it. Divide 1.4549 by pi and you get .4631(radius squared). Take the square root of that and you get the radius of .6805. Multiply that by two and you get 1.361 as the air jet that should have the same area as the stock jet (1.45) with a .5mm guitar string stuck in it. So, once I settle on the size guitar string that runs best I know what size jet to order (in this case a #135).
As it sits right now (still with guitar strings in the air jets) it has so much more power than stock, I think I'll go down a couple of teeth on the rear sprocket and see how it handles that. I think I'll buy some "Silent Sport" packing for the muffler and toss the cheap fiberglass packing. Silent Sport sounds much mellower in my opinion.
***REQUEST FOR
INFORMATION ***
This bike came with the
dynojets installed already but the shop that did the work left
off the small plastic spacer that should be under the needle
e-clip. I'm finding that using the fifth notch from the top is
about right (instead of the third notch from the top as in the
dyno instructions) so I'm guessing the missing spacer is about
the thickness of two notches. If you have a stock Seca II carb
and can measure the thickness of the spacer I'd love to know what
it is.