Again, carb tuning help

[SithLord]

Hello gents.
Yes, before asking, I've searched the threads, and followed suggestions therein. I have a 2017 Hawk 250. Here's my issue:


I'm trying to get my engine to run, full throttle, with choke all the way closed. I got it like that before I took it up in the mountains to altitude. there I had to do heavy mods with needle and still choke had to be all the way open for it to run.
Now that I have it in CA again, 95+ degree weather, I cannot get it to throttle correctly. It idles, warms, runs. I give it more than half throttle, about 6RPM, bogs down, loses power, sometimes shuts off.


Here's what I've done:
Adjusted valves to .003. Made sure it was on compression stroke by using finger over spark plug hole.

Airbox mod with K&N filter.
Stock exhaust with and without spark arrestor.

115 Main jet, stock pilot jet. Both were clear of obstructions, could see daylight through both.

Tried with Mikuni and Stock carb. Thorough cleaning of both carbs and components. Made sure float was level, and springs back to position.

ALL positions on needle for Mikuni carb.
Swapped CDI from brand spanking new Hawk that's never be out before.
Tried taking air filter out completely.
Examined my brand new NGK plug, regular fouling. That is not pitch black indicating too rich, nor power white indicating too lean. (Or vice versa not sure)
Running 91 octane fuel from Chevron/Shell, only thing i can get in this area.
Adjusted throttle cable tight and loose.
Dropped from 115 main jet to 105, all the way up to 125. Same thing.



So again, symptoms. In neutral, start the bike. Starts fine usually. Once I let it warm up, verified the RPMs are at 1.5. Rev engine past half turn (equal to about 6RPMs) RPM gauge drops, power loss, eventually shuts off if I keep gasing it. If I let off, it will return to normal eventually, but can never crank past half throttle.


Please help. I love tinkering on the Hawk, when things work! LOL. But I've tried every combination of idle air screw turns, main jets, different filters. Only thing I have not tried was bigger pilot jet. These were the extent of "fixes" I've come across, but none seem to bring my issue to a close.

[Digger]

Ate you getting flow to the carb? Maybe filter on peacock is plugged, engine uses the fuel in bowl and flow from tank can't refill.... Just a guess

[SithLord]

Good guess, I know peacock is pumping fuel to bowl, because every time I have to pull this darn carb to troubleshoot, its full of fuel.

Also thought it might be a gas tank breather issue. Ran bike with gas cap off (in neutral, idle) rev'd engine. Same issue. Bogs down, shuts off if I keep throttle pulled back.

[SithLord]

UPDATE:
Was told it could be clogged exhaust. Swapped exhaust from a never ran before Hawk, same thing.

Running out of options...Don't wanna take it in to the tune of 85 bucks an hour.

[Cravin01]

Here this may help, sounds like the main jet. this is a standard guide to which part responsible for problems with your throttle and where it happens.

[Cravin01]

I know that's a generic answer but I would go through and make sure your not pulling air somewhere like at the carb manifold and play with the main after checking all the seals.

[RedCrowRides]

I'd have to agree with Paul , you have been through so much other stuff that it seems like you must have a vacuum leak somewhere or a pinched hose .I get what you are saying about the fuel flow but i'd replace the fuel filter anyways, it's pretty common for the chinese fuel hoses to flake on the inside, as well as for the tank to have trash from machining /assy in it which can clog fuel filter and/or petcock itself. Actually I'd replace all the fuel line on it period if it was me with stuff i knew to be good, maybe some transparent silicone fuel line, can find it cheap on EBay in a variety of see thru colors.There has to be a cause but sometimes it can indeed be maddening running it down .

[SithLord]

Thanks for the input. Yes, maddening and frustrating.
My plan was to replace fuel lines with something..better as RedCrowRides suggested.


As Cravin01 suggested, I played with the main jets again. First time, I went up..a lot, to no avail. This time, I went way down to a 100 main jet. And what do you know, I'm riding!


Seems to stutter a little in 5th gear, like its starved for something, I'm assuming air. I'm not looking for this to be a speed demon, but even with my 250lb frame I'd expect a little more "ummph". I got up to about 50, 55ish.
I can play around with a 105 jet, maybe adjust the jet needle (currently in the middle)


I'm just so relieved I got some positive progress, thanks all!

[Sochin]

What's your elevation? I'm at 3000feet and I use a 105main and 25 pilot, mods in signature. It runs like a scolded ape. My needle is in the middle.........

[Cravin01]

Good to hear, the 5th gear shutter may be the fuel/ air mixture screw. too lean and it pops when releasing the throttle, too rich and it wont open up smooth and bogs a little.

Don't do like I did and back out past four full turns or you could lose your screw like I did.

Screw it in fully and then back off 2 1/2 full turns, start the bike(it will start with this setting), let warm up and hit the throttle and let go.
if it pops back out a 1/4 turn.
if it bogs at top throttle turn in a 1/4 turn.

if it doesn't clear up in two more 1/4 turns either way you need to upsize or downsize your main jet.
Bogging at top throttle then down size one jet size.
Popping at throttle release upsize one jet size.
Repeat process.

Take your time and listen to the motor, after you learn this you will be find yourself making play calls on everything that rides past you.

good luck with it!
'

[ChrisWNY]

I think running a #115 main jet on the restrictive stock Hawk exhaust pipe was your problem. The engine isn't breathing well enough to efficiently burn all the fuel the carb is allowing into the engine and the stock exhaust is your bottleneck. If you swap your OEM exhaust for the performance version (or remove the catalytic converter), your problems would probably go away at 1/2-full throttle when using a larger-than-stock jet. The #100 jet is what comes stock in the Mikuni VM26. The stock Hawk carb comes with a #98. Both are fine if you're using the stock exhaust but your engine will run lean (and therefore hotter) as your throttle opens up.



My understanding is that rejetting the carb to a larger jet without opening up airflow to the engine (that includes a less-restrictive exhaust) will cause performance problems...too much fuel, not enough air. Did you open up your air box and cut out the plastic that restricts the air intake from the front of the air box (aka air box mod)?

[SithLord]

Sochin,
Im in Sacramento, about 30' above sea level. But I do take the Hawk up north out of state to about 4000-8000 feet.
ChrisWNY,
Yes, I have done the airbox mod. I was debating the upgraded exhaust, but heard the bolt holes do not line up and the flang holes need to be drilled out. I don't have the equipment for that. As far as taking the CAT out the stock, the welders I know don't seem to want to do this type of work.

[ChrisWNY]

You can cut a slot on one side of the flange and bolt it right on. A standard power drill with a 1/2" bit is capable of enlarging the holes in the flange but it takes a lot of time and effort. After installing the aftermarket exhaust just yesterday, I'd recommend just cutting one side of the flange and not enlarging the holes at all - much easier. A dremel will cut through the stainless flange but you'll likely need a few cutting discs to do the job (they wear out fast going through stainless steel).

[blanc12]

I used a carbide burr bit in the drill to make the flange on the exhaust fit.
https://www.amazon.com/Carbide-KangT...n%3A3489079011


Then I took a heat gun to the inside of the air box and made room for the exhaust. Got the plastic pliable but not melted and moved it in with a putty knife.
Not that hard to do. just need a drill and a heat gun. Hair dryer may work but would take more time to heat up.

[SithLord]

That is an option indeed, thanks for the tip!