Any bike Tuners?

M-ATE · Sep 17, 2008

I added a custom built exhaust to my Nighthawk over the summer and am having trouble getting a little bog out at around 6-7000 rpm's. I thought my needles were set too lean so I moved the clip down a notch to richen things up but that worsened the problem. I went the other way with the clip and that was also worse as the bike had terrible acceleration. So now I'm thinking perhaps it is the main jet? The other thought I had was when I was reading the jetting instructions they said:

5. Locate the fuel mixture tab (Fig. B). Remove the float bowls and file
off the tab nearest to the screw on the float bowl. This will allow for easi-
er adjustment. Carefully turn the mixture screw clockwise until lightly
seated, then back out 2.5 turns.

This confused me because my tabs on my floats aren't adjustable and I don't know where the mixture screw is that they are referring to. So I didn't worry about it and assumed that my bike didn't have the screw.

Here are some pics of my carbs and one of the plugs after riding tonight between 6 and 7000rpms. The jet without anything in it on the far right is the main jet, directly to the left of that is the slow speed and then I was wondering if that pillar to the left of the slow jet was where the mixture screw was and they put a cap over it? A couple other things to consider is that my pipe is fairly black on the inside and I'm getting fairly poor fuel economy (34-36, when running it pretty hard) which would also lead me to believe that my jetting may be too rich. The side electrode on the plug is black/brown (indicating a rich mixture) but the plug is white around the center electrode (lean condition?) I guess I'm not sure which part of the plug I should read. Any help would be greatly appreciated as I'm a little bit stumped or maybe just confused right now.

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Motley Doo · Sep 18, 2008

I'm betting the exhaust itself is either too long or too short! I don't know personally, but my grandfather was a honda mechanic. He said that when they built race systems back in the 70's, they would have to make it too long, and keep cutting 1" incriments off the pipe until all the burbles were out of the powerband. Thumpers can be just as picky as a two-stroke about pipe length and size, especially with a collector, it affects the scavanging of the system! my $.02.

M-ATE · Sep 18, 2008

Motley Doo said:
I'm betting the exhaust itself is either too long or too short! I don't know personally, but my grandfather was a honda mechanic. He said that when they built race systems back in the 70's, they would have to make it too long, and keep cutting 1" incriments off the pipe until all the burbles were out of the powerband. Thumpers can be just as picky as a two-stroke about pipe length and size, especially with a collector, it affects the scavanging of the system! my $.02.

I suppose that is a possibility and was one of the things I was concerned about when building the pipes. Probably should have left well enough alone but I couldn't stand the crappy stock sound. I will probably go down a size on the main jets tomorrow and see if that helps at all. If it doesn't I will ride it like it is for the rest of the season and then maybe chop and tinker next spring. You don't happen to know which way the power band was affected by cutting or adding pipe do you?

Gimpster · Sep 18, 2008

First picture, to the left is a slotted adjustable jet. This is your fuel mixture screw. The stopper tab off to the side is so emission standards cannot be messed with by adjusting. Take a soldering iron and heat it up to remove it. Don't force it. Once removed, the fuel air screw can be removed and the circuit cleaned out (this is where your problem is at).
Although it is your idle fuel/air mixture screw, it affects your intermediate and top end and here is why. Your carbs have variable veturi diaphrams. Not all of vari venturi carbs have a intermediate passage but these do. the intermedite passage runs from one side of the vari diaphram to the idle fuel air passage. Thi is designed to sabilize the venturi height and to allow it to change to get rid of amid to high range accelleration flat spot. I think your pilot circuit is restricted and carrying one venturi at a different height than the other, making it drag down the next carb in the pair, that dragging the next carb etc... Throwing the whole deal out of sync causing the issues you are experiencing.

I know a little about Honda's as I am one of the top 20 world recognized Red level Honda technitions (Highest honors Honda has to offer)

Adding the pipes should not have required any more than a fuel air screw mixture change. If you would have put in like a a K&N air filter or such, then jetting changes would have been required. The issues you are experiencing are internal carb related and syncronization related

Any problematic help, call me
Shop phone number is (701) 663-8083
Cell Number is (701) 290-1600

I will help you farther

----- Gimpster -----

M-ATE · Sep 18, 2008

Gimpster said:
Adding the pipes should not have required any more than a fuel air screw mixture change. If you would have put in like a a K&N air filter or such, then jetting changes would have been required. The issues you are experiencing are internal carb related and syncronization related.

The jets with the limiter caps are set at two turns out so I will take them out and inspect them and then go to 2.5 turns out. The main jets were 115's and I know they made the bike run really lean. I have 122's in now, do you think it would be wise to go down to 118's? Thanks for all your help.

Killerk · Sep 18, 2008

Gimpster pretty much explained it.
The fuel screw caps are on a spline and held on by glue... Like he said, add some heat with soldering iron and wiggle lightly and lift cap up.
Don't force it or you will break it off.

I don't disagree gimpster, but I assume it idled/ran fine before the pipe install?
From my experience, opening up the exhaust (little to no restriction) causes a lean bog and can be hard to overcome.
Mainly because you can only change so much with the carbs on the bike.

While you are in there check to see if the float needles are steel tiped... If so, see if they are starting to groove. If so, you can replace with rubber tipped needles.
If they groove, they stick and leak.

Good luck.

Gimpster · Sep 18, 2008

Make sure when you pull them out that they are together in this order - screw, spring, washer, o-ring. Pull the pilot jets and veri venturis. Spray carb cleaner in the passages and make sure it cmes out the pilot jet hole and idle circuit on the cylinder side of the butterfly valve. Don't blow compressed air into these passages with the venturi covers on. You could pop a venturi daphram.
As far as jetting, Once the carbs are back on the bike, start the unit. turn the fuel air mixture screws in until the engine starts to stammer a little. Counting the turns, turn them out until the engine stammers. Turn back in half the counted out turns. If when riding the bike it backfires in the pipes when letting off, turn the screws 1/2 turn out. Any drivability issues thereafter will be carb syncronization related.I would worry about jtting after the carbs have been resynced because the placemnt of the variable venturi in all carbs will throw jetting readings off at the spark plugs if not opening evenly. for base tuning, drop in the 118 jets

----- Gimpster -----

M-ATE · Sep 18, 2008

Gimpster said:
As far as jetting, Once the carbs are back on the bike, start the unit. turn the fuel air mixture screws in until the engine starts to stammer a little. Counting the turns, turn them out until the engine stammers. Turn back in half the counted out turns. If when riding the bike it backfires in the pipes when letting off, turn the screws 1/2 turn out. Any drivability issues thereafter will be carb syncronization related.I would worry about jtting after the carbs have been resynced because the placemnt of the variable venturi in all carbs will throw jetting readings off at the spark plugs if not opening evenly. for base tuning, drop in the 118 jets

----- Gimpster -----

Should the screws be turned in individually or all at the same time when tuning? Again thanks for all the help.

Gimpster · Sep 18, 2008

Individual.
Each cylinder although cast near the same have different flow volume weather it be descrepencies in carbon build up, cam wear, valve clearance or what have you. That's why syncro is needed, to get the atmospheric pressure through the venturi bore the same on all cylinders. The flow will change pilot circuit screw setting.
Until then,set each one individually for ultimate tune as the carbs are set as they are now. Ifyou notice a big difference in turns per carb, the ones way out most likely are problematic carb/cylinders

hope this helps

----- Gimp -----

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