Loss of power, open throttle, 93 Nighthawk

[Mick]

Hi Folks,
I have just acquired a '93 750 Nighthawk, with just 16,000 miles on the clock. Its spent years in a damp shed here in the UK, but I've cleaned and touched her up, changed the oil to fully sythetic, new oil and air filters as well as fixed brakes etc etc. (No the tacho doesnt work!)
She is now running fine, not totally happy starting when cold and damp, but running fine, except that when I'm on a long hill, on the motorway, she starts to run out of power, and opening the throttle further drops the power even more.
In first, second and third gear, all is well, but over 85 in 4th and fifth, she's a no go....(fine up to that speed!)
Anyone know a likely culprit?
No I havent changed the plugs, but are they likely, its unlikely to be blocked jets 'cos she snaps to life in the lower gears, but it rather feels like fuel starvation?...(tank is clean, spotless...no crud)
Is th9s a fault anyone recognises, would appreciate any advice....
Thanks
Mick

[LukeM]

Check the venting for the gas tank.  One way to check is pop the gas cap and see if 1) it's hard to remove due to vacuum in the tank) 2) the vent in the cap (if there is one) is blocked.

It may not explain why it doesn't happen in the lower gears, but normally acceleration in the upper gears means longer high-load engine speeds, which may use more gas.

Check the FSM here and see what it may say about loss of power at high speeds.

Might want to swap the plugs as well. Check the color first: should be a light tan color on the insulators, and no wetness.  Black shiny means too much fuel, oil = oil problems (valve guides or seals, or maybe rings, and blue smoke out the back).  

I hope this helps.
Luke M

[Mick]

Thanks Luke,
I had a Royal Enfield once that had a problem with the tank vent, very frustrating till I found it out!

I've just ordered a new set of Iridium plugs, guess they won't be here for a whle, but whether its the plugs or not (and I doubt its them), thats something else sorted.

I had a quick look at the tank filler cap last night, there's no obvious vent, its the hinge from the back sort, I guess it simply vents out through the lock mechanism, I cant see an overfull vent which my Ural Retro has (never a venting problem there!) so I guess my options include trying to flush out the lock, could all be full of gunged up wd40 or something I guess, or I could try using the scalpel and cutting a v shaped groove ito the rubber seal....

I'll have a look today..

Thanks again

[TOOLS1]

What kind of air filters do you have? It might be starving for air at speed. Some types of filters will do this. Especially pods.
TOOLS

[Mick]

Hi there, The filter is the single one that sits under the seat, accessed by 4 screws from the left of the bike. Its not a Honda one, but a pattern part, however, its a reputable japanese one and its brand new, and in far better condition than the clogged up one I took off. Lots of folks run their old brit bikes here without air filters, I sure did in the past, because the UK isnt very dusty, and up here in scotland its normally wet and damp!, so I could try a run or two without the filter in, and see if that  makes a difference, I'll try that over the next few days and report back....
Thanks for the suggestion..
Mick

[TOOLS1]

Actually what is happening is that a vacuum is being created past the air box at speed. Try using tape, and cardboard around the air box to create a dead air space, and see if that makes a difference.
TOOLS  

[Re-run]

What color are the current plugs? What brand is the filter? I used an emgo and it was too restrictive.
Another thing to try is to pull the choke some and see what happens. Try without the filter too, sounds like you will be doing that.
It does sound like a starvation issue though but that shouldn't be happening with the original jets in unless things are still dirty.

[Nighthawk409]

Hi
My 92 Nighthawk has developed a very similar problem ... poss of power at high speed  and yes opening the throttle makes it worse to the point of stopping all together. Sometimes Slowing down it chuggs along and feels like fuel starvation. However there is one notable difference .. found it hard to start again without the choke on full  even when hot  so further suspect fuel starvation .. on the last occasion I did this with full choke started fine and then ran fine without any choke. Seems to happen more at 95mph ish!! It was very hot weather - late 70's [which is hot for the Uk !!] and long distance - any advice on tank venting or fuel flow?  Never happened before - any advice appreciated over and above that already given ... cheers