Rich and lazy carbs

bdento59 · Post by **bdento59** » Thu Aug 21, 2008 9:13 am

Well, after several tanks of gasoline, I'm getting about 22 mpg, so I know
something is not right. I'm running 98 mains, needles shimmed 0.5mm, pilot screws 2 turns out, stock exhaust and a K&N air filter in a stock airbox. I pulled the plugs to take a look. Three were really black with soot (1, 4, and 5; #1 was the worst) and the others were also sooty although not as badly. I knew something had to be wrong, so I decided to pull the carb rack again. However, before doing so, I put the bike on the center stand and drew a plumb line on the #1 and #6 carb bowls as a reference for the rack angle. Turns out to be a ~22Âº forward tilt.

When I rechecked the float heights, all six floats were parallel to the carb
body when held vertical, which equates to 15-16mm when measured manually. However, when I propped the carbs at 22Âº and checked the fuel level externally with a tube, the gas settled out to a point just slightly above the top of the carb rack bracket bolt, not in the center of the bolt as I've been told it should.

So, I went back and reset the floats to 18mm above the carb body, then
rechecked the fuel level externally. To my surprise, it hadn't really
changed... still near the top of the rack bolts, or ~3-4mm above the inside lip of the carb throat. They're all pretty consistent, but still seemingly too high.

What the heck am I doing wrong?

alimey4u2 · Post by **alimey4u2** » Thu Aug 21, 2008 11:03 am

Are your float needle valves passing ?? The Viton tips may have become brittle ??

Also, weigh each float separately, a powder scale is excellent for this duty....

Choke butterflies full open ??

bdento59 · Post by **bdento59** » Thu Aug 21, 2008 11:35 am

alimey4u2 wrote:Are your float needle valves passing ?? The Viton tips may have become brittle ??

Also, weigh each float separately, a powder scale is excellent for this duty....

Choke butterflies full open ??

Float valves are all holding, no cracks in the brass overflow tubes, all floats are airtight and weigh the same, and the choke buterflies are working as designed. Thanks.

alimey4u2 · Post by **alimey4u2** » Thu Aug 21, 2008 12:53 pm

Float tab should just touch (plungers not compressed) the float needle valve when it is on it's seat. The measurement should be 15.5mm from the float chamber flange to the top of the float at the centerline of the main jet. If you are following this procedure the float height is correct. However, It is not unusual for old carbs to have been overzealously cleaned (or worn out) by previous owners increasing jet bore size.
With stock Mikuni & Keihin jets, the number 98 means that it will pass 98cc's ( milliliters) of fuel over a period of one minute. I don't know at what head that measurement is achieved. However, with a stop watch & a calibrated cylinder they can be compared. This is not the case for aftermarket "kit" jets...as the number is often drill size. More or less a useless guide.
Strangely float height on Mikuni's are measured with the carbs on their backs, therefore the plungers in the float needle valves are compressed. Which leads me to wonder if the manual has an error in print/translation ??

Correct temperature spark plugs being used ??

Ps. Not an expert on this subject by the way, others more knowledgeable may rightfully dispute...

bdento59 · Post by **bdento59** » Thu Aug 21, 2008 1:30 pm

alimey4u2 wrote:It is not unusual for old carbs to have been overzealously cleaned (or worn out) by previous owners increasing jet bore size.

Correct temperature spark plugs being used?

The main jets are new, as are the jet needles, o-rings, and everything else that comes in keyster carb kits. I have no idea if the pressed in idle jets are eff'd or OK. I kept the older float valves because the Keyster ones were shite and wouldn't seal. They seal well and pass the overnight leak test. Spark plugs are correct. Thanks.

alimey4u2 · Post by **alimey4u2** » Thu Aug 21, 2008 7:46 pm

Bill you really are pushing my envelope of knowledge here...

Keyster ? Educate me here guys is the jet notation the same as stock ??

I'm running 29 mm smoothbore Mikunis & my engine (1168cc) is also very thirsty but mixture is close to perfect.. OK a little on the rich side.

I run 1 & 6 one size smaller as they pull less air....

HELP......

EMS · Post by **EMS** » Thu Aug 21, 2008 8:42 pm

alimey4u2 wrote: I run 1 & 6 one size smaller as they pull less air....

HELP......

Did you flow the head with the intake manifolds to confirm that

I would think that with a straight bank of carbs, like the Mikunis are, the inner cylinders would have more flow loss due to the longer manifolds.

alimey4u2 · Post by **alimey4u2** » Fri Aug 22, 2008 5:20 am

EMS wrote:
alimey4u2 wrote: I run 1 & 6 one size smaller as they pull less air....

HELP......
Did you flow the head with the intake manifolds to confirm that I would think that with a straight bank of carbs, like the Mikunis are, the inner cylinders would have more flow loss due to the longer manifolds.

Plugs tell me Mike, The outer cylinder inlet tracts are more serpentine than the inner & are actually longer, they would "seem" to be more restrictive...I'm running shorter Ram Tubes on the outer carbs to compensate for the difference in length. I flowed the rubbers & the head to achieve the shortest path possible also...

cbx6ss · Post by **cbx6ss** » Fri Aug 22, 2008 6:37 pm

I had some similar problems after a rebuild. You may want to set the original and keister jet needles side by side to compare. I found the replacement needles significantly different in length and taper. I put the original needles and jets back in using just the "soft" parts from the rebuild kit to much improvement. This advice was from esteemed Carb Sensei, Mike Nixon.
/r
Brad

alimey4u2 · Post by **alimey4u2** » Fri Aug 22, 2008 7:13 pm

Thanks for that cbx6ss.....

cbx6ss · Post by **cbx6ss** » Sat Aug 23, 2008 8:00 am

"With stock Mikuni & Keihin jets, the number 98 means that it will pass 98cc's ( milliliters) of fuel over a period of one minute. I don't know at what head that measurement is achieved. However, with a stop watch & a calibrated cylinder they can be compared. This is not the case for aftermarket "kit" jets...as the number is often drill size. More or less a useless guide."

This was good info.
/r
Brad

bdento59 · Post by **bdento59** » Sun Aug 24, 2008 11:00 pm

cbx6ss wrote:I had some similar problems after a rebuild. You may want to set the original and keister jet needles side by side to compare.

That's a great point. Problem is that I'm not the first person to get into these carbs... what do the stock needles look like? The ones I found in there looked like aluminum and had the designation 55A stamped on them. Does that sound correct for the 1980 VB62A carb casting? The carbs also had had the #110 mains replaced with #98's previous to my purchasing the bike. TIA for any guidance you can give. I'm surprised at the paucity of information given in the Honda shop manual on the stock carb config, no jet needle designation and no pilot jet size given.

piit · Post by **piit** » Mon Sep 29, 2008 2:27 pm

alimeu4u2 wrote: "The measurement should be 15.5mm from the float chamber flange to the top of the float at the centerline of the main jet."

Q: If I understand this properly now, the measurement point in float is not at the end of the float? Where does this instruction based on? Please correct if I have understood wrong.

alimey4u2 · Post by **alimey4u2** » Tue Sep 30, 2008 5:36 am

piit wrote:alimeu4u2 wrote: "The measurement should be 15.5mm from the float chamber flange to the top of the float at the centerline of the main jet."

Q: If I understand this properly now, the measurement point in float is not at the end of the float? Where does this instruction based on? Please correct if I have understood wrong.

piit, It is in the Honda workshop manual, top of page 9, chapter 4. The picture ( "float level adjustment") shows the measurement being along/at the centerline of the main jets, that being said, no, it is not at the end of the float. I admit It's not that clear....

piit · Post by **piit** » Tue Sep 30, 2008 6:28 am

Good! I need to re-recheck it today...why I feel that quite many are measuring from the end of the floats?
Well, in my case now as I have an open dyno topic discussion ongoing this is the reason why I am running rich, not much rich but notable anyway. Interesting, thanks!