miss at 3.8K

[billy192]

well i have done 600+ miles on mostly A roads. today I went on the Mway only to find at 3.8K i get a miss it keeps missing all the way up to 5K, not taken it passed 5K as it will not rev passed this under load.
when you slow done to 45 ish its fine.

I thought it was fuel starvation but fuel is flowing OK. checked the points, plug gaps all spot on.

a great pity as everything is great but for this miss.

Billy

[MarkCR750]

Hi Billy, I had very similar symptoms on a ZXR400, it turned out to be the float heights, i.e when the engine demanded more fuel at higher revs the level in the bowls was too low because the floats were cutting off the fuel supply before they should, trial and error re-setting the float heights fixed it, I couldn't use the factory settings because the engine/intake and exhaust were non standard.

[Bitsa (Ralph Wright - RIP)]

Billy
Pull yer plugs and see what the colour is like white weak black rich then you will know if its carb problem
Cheers
Bitsa

[billy192]

I pulled the plugs (burning hands) and they were white ish, i did try opening the choke but it did not make a difference. I am going to check the float level AGAIN. should i not lift the needle in each carb to rich top end?
Billy

[K2-K6]

Are you certain that the ignition advance is working correctly, if no advance then it would struggle to get much above 5000 at all.

Lifting the needles will richen mid range (throttle not fully open), top end is main jet size but I don't think you'd be finding that out if you are not getting the slides fully open anyway....I'm making the assumption that you have not got the throttles fully open as you've not exceeded 5thou, though if you are saying that it won't go higher AND the throttles are fully open then that would suggest something else assuming the carbs are near enough right.

White plugs do suggest lean running......I'd make sure of the ignition advance and timing first as it's easy to do, before taking the carbs down again.

Also are you sure of your method for setting float hieghts?

[billy192]

Yes I did them one , number 3 empties a bit on the floor from time to time so that needs adjusting.
The advance sounds as though it needs looking at

Will try Friday when I get back home
Thanks
Billy 


Sent from my iPhone using Tapatalk

[K2-K6]

Just to thought to add to my last post, it won't hurt to raise the needles if it's running a bit lean but one notch is considered a relatively small adjustment to trim an engine in to optimum so you shouldn't get a huge effect. As you probably know, the centre notch position is the starting point with room to trim it up or down depending on running conditions. if you get to either end notch and not get what you want, then it would indicate that the main jet was the wrong size.

Let us know how you get on with your checks on friday.

Nigel.

[billy192]

another question.
does it make a difference which way the petrol feed pipes route?
mine go under the float bowles (under the carbs) and up to the cross over feed pipes.
should they go above the carbs? like feed from the top?

Billy

[Bitsa (Ralph Wright - RIP)]

Billy
Mine are routed like yours.From the tap straight down in front of carbs up under the bowls through a filter then to the splitter.
Fuel flow is no problem.
Cheers
Bitsa

[billy192]

Bitsa,
I had an inline filter, but,,, the new tap and gauze in the tank look as if they would trap any unwanted debris.
is the tank strainer OEM part not up to it?
Niel

[Bitsa (Ralph Wright - RIP)]

Billy
All depends on how much rust and crap is floating around, my tank is rust free and yes the tap filter is also new but being over cautious wont hurt.I even have little filters on the float valves,
Cheers
Bitsa

[billy192]

went out today between showers and crap still missing. so I thought at 60-65 which is about 3.5 k it starts hesitating so i pulled up the choke halfway and boom no missing it flew along at 70-80 no problem.

so off with the carbs again an check the main jet and the needle clip position. clip is in the middle one (correct for std) the main is 105 and according to the book it should be 120. would this make it miss? i assume by pulling the choke its running lean.

the bike before i stripped had a four into one would they have had to re jet lean for that?

so next step 4 new main jets 120.
Billy 

[K2-K6]

Looks like you are getting there billy, as when you choke it you obviously richen it at all revs and the fact if took off ok then shows it needs more fuel. I'd go with the spec and return the mains to std.

I was slightly confused as with your second post you said the choke made no difference but it's always worth a play with at different revs to see what affect you get and can give a cheap and quick diagosis.

Just to cover a bit more of what is happening here, an engine will run at a steady state (continous revs and load conditions) with quite a lean mixture but it then needs quite a bit of a kick with extra fuel to get a nice clean acceleration (they do this on later F2 engines with an additional pump as you open the throttles) but older carbs like the K6 will be set as a compromise of steady running AND enough fuel to accelerate without going too lean and causing a missfire, yours appears to be jetted right at the bottom or even less than required to achieve that balance. If you jet as per spec then any small adjust should be possible with the needle clips if it needs any trimming at all, but it should be much better if you change the mains.

[hairygit]

Silly question Billy, but are they the correct carbs for your bike? maybe some previous owner swapped them over, as 105 is the main jet size for a 750F1 and possibly they have been replaced at some time with F1 carbs (probably thought F1 has 4 into 1 as standard, easier than changing jets, and cheaper which it was years ago!)

[billy192]

original carbs
billy