Breaking up highway speed

[shansel88]

Hello all,
Got 2004 big dog chopper 107 with mikuni carb. Ran great for years. Recently ran into issue where it coughs or sputter when cruising on highway 80mph or so - 1/2 throttle???
More gas I give it the more it coughs/stutters.

Originally thought dirty K&N - and it was - cleaned and treated - no good
Now have rebuilt Carb - nothing obvious - but just to be sure- can anyone confirm the plug (air screw) in bottom of carb should be a O - or blocked off- I am assuming this is correct as it ran fine before???
Also verified air fuel mix screw didnt back out its 1 3/4 turns from bottom

Symptoms-
Can ride around town - 55 and under - no issues
Can sit in neutral and rev engine to high RPM - no issues

But, when I give max throttle and get going to high speed - it breaks up??

I was positive it was fuel/air related - but now I am questioning that -

Any ideas appreciated>
Sam

 

[awg]

Happens only under load? Plugs?

 

[Sven]

The quick and easy is what awg said. Clear that and go to the next step like plug wires.

 

[Coolbreezin]

Petcock and/or filter may be plug. Sounds just like my ol' GMC did way back when.

 

[mleach72]

Yeah, I agree with these guys. It's something else. The way the mikuni is designed, if it breaks up at a certain throttle, then it always would, no matter how fast you are going.

 

[Sven]

Cool... rides around all day under 55mph, and WOT's the throttle shows no fuel/filter clog with that kind of delivery... sort of shows it's not a fuel problem if you walk the fuel vs spark. Under gear/tire/wind load resistances, that's where the spark vs load is a faster jump to the porcelain than the jump to the side electrode.

 

[FrankBDPS]

When you check you're fuel filter open the valve and check the fuel flow. If it is just a trickle you may have something blocking the opening from the tank such as a chunk of a failing tank liner covering the hole. It may also be a tank venting issue. Does you're bike have one or two tanks?

 

[shansel88]

Thanks for feedback guys. I am going to go after plugs next. Can anyone give me the champion or NGK plug number for 2004 107 motor??

Thanks,
Sam

 

[roadie1389]

Most folks will tell you to use the Champions.

Spark plugs

This has been asked many times on the forum. This link gives a list of all the different plugs you can use. Its the top list on the page (twin cam). I have used HD, Screamin Eagle, NGK and Champions and noticed no difference with any of them. Print it and put it in your manual. Spark...

www.bigdogbiker.com

 

[shansel88]

OK bought and modified road glide to be like I want - now I am back to trying to get the dog up and running

changed plugs- wires- even the coil

Cleaned carb - again

Still-
runs good cold - after warms up it will cough at steady state mid throttle????

I am looking at ignition, but the ultima ignition isnt showing anything on indicator lights - even when it runs great, like lights are burnt out?? smh

I am starting to the the magnetic piece behind the digital ignition pick up??

Any suggestions??????

 

[Sven]

Check plug color...
Cold/rich - runs good.
Warms/leans out - plug condition says?

 

[mleach72]

It definitely sounds like it isn't getting enough fuel. If it ran great before and nothing was changed, then it shouldn't be the carb. There are things can happen that can restrict fuel, though. Did you check your petcock screen? Tank liner clogging the screen is a fairly common problem. Did you check the vent on the mikuni when you had it apart? It's the copper colored fitting on the left side that points down. Did you check that the main jet isn't clogged up? Check that the emultion tube is clear too.

 

[shansel88]

plugs have always looked perfect on this bike - also in neutral there is no sign of the symptom - seems to be under load/moving only??
I was thinking of raising needle one clip - but im hesitant -even if that does work, because its more like im masking a symptom vs fixing - considering it has always run perfect where it is.

 

[shansel88]

very good points mleach,
I have torn carb apart 2-3 times, rebuilt with new orings and seals, verified all ports are open - did check petcock, but i will do that this weekend.
I wish i had another carb to throw on there and see what happened.

Im not professional mechanic, but to me these motors and carbs are very basic and some any true mechanic should be able to figure out quick. I am just trying to see if I can figure it out on my own, and learn along the way, before I give up and just take it to a local bike shop.

 

[mleach72]

plugs have always looked perfect on this bike - also in neutral there is no sign of the symptom - seems to be under load/moving only??
I was thinking of raising needle one clip - but im hesitant -even if that does work, because its more like im masking a symptom vs fixing - considering it has always run perfect where it is.

You can try raising the needle. You are right, it would be masking the real problem, but if it runs better, you would know that it is fuel related vs electrical.

 

[mleach72]

Also, check for any intake leaks. Spray WD-40 around the manifold and see if rpm changes. Make sure any unused carb vacuum ports are capped off. Intake leaks will become more intense under load.