Thanks for taking a look at my post.
I am jumping in a bit early as I have only got the carbs off and not taken them apart yet but...

When fully warm the engine revs freely on the bench up as high as 10,000 rpm, sounds really crisp and clear, no excessive smoke...nothing out of the ordinary.
But when under load i.e. riding... it fluffs or gargles at about 4500 and will not pull through into the later rev range.There is no coaxing it through either, it refuses.
I initially thought it feels like a choke issue, maybe stuck partially closed, but altering the choke whilst riding did nothing to change the issue.

So before I rip the carbs down for a full inspection I thought I might put a question out there in case anyone has experienced this issue before and can point me in the right direction...?

Any ideas?

Cheers

Check that the needles haven’t been adjusted and that they are at the standard setting, also check that the choke plunger at the carb actually is moving when you pull the cable. I’ve had them feel like they are moving but they’re not!

Zeus wrote:Check that the needles haven’t been adjusted and that they are at the standard setting, also check that the choke plunger at the carb actually is moving when you pull the cable. I’ve had them feel like they are moving but they’re not!

I am being lazy but where will I find the needle height and float height info on here? Cheers

https://tech.nsr-world.com/nsr250/all-nsr250/nsr250-stock-carb-settings.php
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Andy wrote:https://tech.nsr-world.com/nsr250/all-nsr250/nsr250-stock-carb-settings.php

Thanks for that Andy, does the needle have a stock height setting? same q with the float height?

Standard needles are non-adjustable.
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OH, ok clear...
Does anyone have a knack for removing the alloy screw covering the power jet? mine is well stuck in and I can see the top deforming under load

Just a thought, how does the fuel tank breather system work? I see a hole in the well around the fuel tank filler which I imagine is a rain drain, but how can I check the tank breather system is working?
I wonder if it could be fuel starvation...

OK all sorted I think 🤞
I found a completely blocked power jet in one carb, years of crud inside....I guess enough power was generated from one carb to rev freely on the bench but obviously not enough to drive the bike under load.

The power jets are "progressive", so the wider the throttle opening, and the higher the RPM, the more the intake "draws" on them. That's how the standard setup can run such a small main jet (giving a crisp throttle response) and not go lean at the very top end. Basically, it just gets richer and richer as the revs rise.

Hopefully you've found the problem.
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Andy wrote:The power jets are "progressive", so the wider the throttle opening, and the higher the RPM, the more the intake "draws" on them. That's how the standard setup can run such a small main jet (giving a crisp throttle response) and not go lean at the very top end. Basically, it just gets richer and richer as the revs rise.

Hopefully you've found the problem.

First time of working on a power jet carb (or secondary main jet carb), and I wondered how it worked.
How does it draw then deliver fuel? I guess through some galleries in the carb body next to the slide...I am also intrigued as to how it can deliver a variable rate of fuel without the needle influencing it?

Max wrote:
How does it draw then deliver fuel? I guess through some galleries in the carb body next to the slide...I am also intrigued as to how it can deliver a variable rate of fuel without the needle influencing it?

The harder the motor "sucks", the more [supplemental] fuel drawn through the powerjets. Powerjet is probably not really the correct term, but it's what's been used as long as I've been into NSRs, and what the likes of Jha back in the day referred to them as, so who am I to question it?!
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The problem still persists, although it has changed a little... I have tried:

- cleaning out the carbs
- cleaning out the fuel tank and fresh fuel
- new plugs
- cleaned air filter
- cleaning out the fuel tap (full of crud)
- checking the coil primary and secondary for resistance (both are equal across both coils)
- checking the charging rate
- checking the new good quality and fully charged battery
- both RC valves rotate when blipping the throttle

The symptoms are that in the early gears the bike will pull through to about 7500 rpm and then refuse to pull any further, in the later gears 4,5,6 it will not pull past 4500. It sounds like it gargles, similar to when you hit a limiter.
Tickover and bench running sounds OK, there may be an issue but there is not enough load to show it up.
The plug chop shows as dry but maybe a little rich, not black sooty but definitely not brown, somewhere in-between.

Any advice gladly taken as I am running out of ideas!!

Cheers

Jonnymac had a similar problem to this at Mallory a couple of years ago, revved up fine on the stand, but wouldn't pull through the gears out on track, cant quite remember what the issue was, something to do with the air solenoids I think, was a schoolboy error if I remember, and easily fixed, he might be along shortly to comment.
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Here is an image of the air solenoid correction system, can anyone tell me if there is something missing?