A forum for owners of Yamaha TTR250 trail and enduro bikes!

zzrdd · Member

Stumble when throttle reduced from 3/4-WOT

TTRfan · Super Guru

Engine 'blips' when closing throttle from WOT

purdoy25 · Veteran Member

Hey there. So I noticed when I run my bike wide open/full throttle and then right after closing the throttle or even going half-throttle, my engine 'blips' and surges for about half a second before engine braking. I'm wondering if this is an issue with carb tuning. It's kinda annoying esp. when speeding uphill, otherwise the bike runs great. Also after a quick speed run checking top speed with phone gps, the bike idled really low and stalled when I went to the side of the road to check the GPS (It recorded 116kph btw, on a short stretch of badly maintained road).

It sounds like "BBRRRAAAAAAAAAAAAAAAAAAAAAAAAAAPPPP-PAPP!" The 'PAPP' being the blip after closing throttle.

My bike is a 96' or 97' model (I think) and using the stock carburettor but the engine seems to have higher compression than stock (I can't tell but all I know is that there's a thin spacer below the engine block and this bike has been thru several owner so god knows what they did to it). It's also been fitted a stainless short header with a powerbomb-like contraption where the heatshield is. It has the stock airbox but I just fit a scouring pad (scotch-brite) there for some filter-age lol.

Any answer would be greatly appreciated! biggrin

-- Edited by purdoy25 on Sunday 26th of November 2017 11:28:47 PM

-- Edited by purdoy25 on Sunday 26th of November 2017 11:31:31 PM

-- Edited by purdoy25 on Sunday 26th of November 2017 11:32:29 PM

ttboof · Super Guru

Mine did that

a couple of things I changed and it stopped

1 raised the clip on the carb needle one notch which leaned it out

2 adjusted the wide open stop as the slide was going well above the top of the carby bore so the needle was right out of the jet

Did both things in one go so not sure which one fixed it as yours also does it at part throttle maybe raising the clip a notch

JOHNYBOY · Super Guru

are you sure its not the front pipe gasket not sealing & drawing air to make it pop back

Dan-in-a-Van · Veteran Member

My recently purchased TTR was doing the same thing on its maiden run home from Bude. Coming into corners at high speed it would be like I'd blipped the throttle when I'd actually just slammed it shut. I haven't had chance to investigate this further as mine is now refusing to run above 1/4 throttle. The two issues could be related. I'll let you know what I find once I've had carb apart at the weekend.

ttr steve · Super Guru

Carb inlet stubs can be a problem, worth taking it off and inspecting, as sometimes it looks ok but has come unstuck from the metal part allowing uncontrolled air into the carb.

Ps I was out Bude way trail riding yesterday smile

purdoy25 · Veteran Member

ttboof wrote:
Mine did that
a couple of things I changed and it stopped
1 raised the clip on the carb needle one notch which leaned it out
2 adjusted the wide open stop as the slide was going well above the top of the carby bore so the needle was right out of the jet
Did both things in one go so not sure which one fixed it as yours also does it at part throttle maybe raising the clip a notch

THanks for the advice! I'll definitely try that when I have the chance. :D

purdoy25 · Veteran Member

JOHNYBOY wrote:
are you sure its not the front pipe gasket not sealing & drawing air to make it pop back

Not sure since it only happens after shutting the throttle at WOT but I'll also try to look into that.. cry

ttr steve wrote:
Carb inlet stubs can be a problem, worth taking it off and inspecting, as sometimes it looks ok but has come unstuck from the metal part allowing uncontrolled air into the carb.
Ps I was out Bude way trail riding yesterday

I did replace the inlet with a brand new genuine one about a few months ago. The engine also did this with the old, battered inlet. confuse

-- Edited by purdoy25 on Monday 27th of November 2017 07:23:39 PM

-- Edited by purdoy25 on Monday 27th of November 2017 07:27:17 PM

purdoy25 · Veteran Member

Dan-in-a-Van wrote:
My recently purchased TTR was doing the same thing on its maiden run home from Bude. Coming into corners at high speed it would be like I'd blipped the throttle when I'd actually just slammed it shut. I haven't had chance to investigate this further as mine is now refusing to run above 1/4 throttle. The two issues could be related. I'll let you know what I find once I've had carb apart at the weekend.

Ohhh. And here I was thinking I had a one-off problem biggrin

Hope to hear what happens after you do the carb job.

ttboof · Super Guru

purdoy25 wrote:
ttboof wrote:
Mine did that
a couple of things I changed and it stopped
1 raised the clip on the carb needle one notch which leaned it out
2 adjusted the wide open stop as the slide was going well above the top of the carby bore so the needle was right out of the jet
Did both things in one go so not sure which one fixed it as yours also does it at part throttle maybe raising the clip a notch
THanks for the advice! I'll definitely try that when I have the chance. :D

adjusting the throttle wide open stop is easy , once the air box /inlet is removed , maybe try that first .

the carb needle takes a bit longer to do but not difficult ( the trick is taking the two slide screws out of the top of the slide and tipping the carb so the needle falls out with the slide still inside , fiddly but not difficult )

TTRfan · Super Guru

Another thought is that the slide arm screw might be loose such that it looks like the quadrant spring has returned everything to the throttle closed position whereas the slide is still able to "float" and is not pushed tightly closed.

Not a common issue but it has been known and worth checking if and when you take the carb off wink

Lots of information to help here

Brian

purdoy25 · Veteran Member

Thanks for all the info! I went on a short ride today and also notice that the engine keeps stuttering at higher rpm's when i try to accelerate hard on a long uphill stretch (pavement). Might be related to this issue. Will hopefully find time to strip down my carb in the next few weeks. :D

-- Edited by purdoy25 on Thursday 30th of November 2017 10:02:39 AM

purdoy25 · Veteran Member

Hey guys. I had a long-ish mountain (street) ride last week and the bike just keeps on cutting off when going uphill in addition to the cutting off when on WOT but I mostly ride in the city so I only notice it on longer rides.

I haven't stripped down my carb yet since I couldn't find time but will probably just drop my bike off to my friend who knows this stuff alot more than I do. :)

katana · Senior Member

Keep us in the loop. Mine does the same.

Dan-in-a-Van · Veteran Member

Since I cleaned my carb and changed the air filter from an old sock to a foam twin-air my TTR is running a lot smoother. I haven't had chance to take it out for a long, high speed run yet, but I haven't noticed the engine blipping when I've been commuting this week. It might be worth looking at the air filter before you dive into the carb.

purdoy25 · Veteran Member

Hey guys. Just an update. I removed the scouring pad I placed as filter element and the engine breathes alot better now (only blipped once when I went on a long uphill ride). I'm leaving my TTR to a mechanic for the next two weeks to tune the carb properly, look for any leaks, and figure out a way to make the engine breathe nicely even with a filter element. I'll get back on this after getting my bike back and riding it uphill on WOT again. :)

purdoy25 · Veteran Member

Alright I had the mechanic sort out the problem and it was caused by a main jet that was just too big. He replaced it with a smaller one and now the bike runs a lot better even with a scotch brite filter! (but still under observation). :D

-- Edited by purdoy25 on Thursday 11th of January 2018 04:07:06 PM

TTRfan · Super Guru

Good news!

What sizes are the old and new main jets, just for the record, please?

purdoy25 · Veteran Member

The main jet was 142 and replaced with bored out 135 or 130 jet according to my trusted mechanic. Sry not so good with these things I just relayed info I got. :)

-- Edited by purdoy25 on Tuesday 16th of January 2018 10:33:06 PM

purdoy25 · Veteran Member

Wellp, the carb just kept on leaking, won't idle properly and I can't find a repair kit!

So I bit the bullet and put in a flatslide. Just took for a spin around town and damn! That snappy response is on another level!

I know. My bike looks like crap. Lol.

katana · Senior Member

Can you share some more photos of the carb and other specs or part numbers?

Kaos · Senior Member

katana wrote:
Can you share some more photos of the carb and other specs or part numbers?

X 2 please.

Thats got me interested.

ttr steve · Super Guru

Yes please to more info, no one is worried about your bike being dirty, etc. Just like to have the info on the carb, fitting, cables and all associated work. For the love of TTR smile

TTRfan · Super Guru

Hi Nick!

Is it a KOSO KSR? A web link to the site you bought it from would be good

Also what size did you buy and what were the issues with attaching it to the inlet manifold and airbox?

Any problems with the throttle cable - did you buy a special one as I see it is a single cable like on most of the after-market carbs.

More info on alternative carbs for the TTR here

Brian

purdoy25 · Veteran Member

Hello. Yes, the carb is a Koso KSR EVO 30mm flatslide (D-shaped slide actually). I bought it locally but this should be the item: oneteamstore.com/koso-ksr-evo-carburetor-assy-28mm-with-air-funnel or www.koso.com.tw/products-detail.php funny how the Koso site doesn't list a 30mm on their sizes.

For fitment, it was 30mm but my mechanic said it felt a bit loose on the stock engine manifold so we had to tighten it a bit harder to avoid vacuum leak. It also has that air scoop on the air intake side which would probably make it impossible to mount to the stock airbox connector. My bike was heavily modified from when I bought it and the old carb was connected to the airbox with some kind of rubber accordion tube so it just took some strength to pull it into the tube and it just kind of settled in the first groove. There are some air leaks though between the tube and airbox but the bike didn't seem to mind. My mechanic had to make another throttle cable from scratch (from parts he found in his garage) as the stock one was too short for the job. :)

Very happy with performance. We didn't replace any jets and stuff, we just slapped it in as-is, adjusted the air/fuel screw and go. Took it to the trails and had no problems (except from my lack of skill): www.youtube.com/watch :D

-- Edited by purdoy25 on Monday 19th of February 2018 09:13:30 AM

-- Edited by purdoy25 on Monday 19th of February 2018 09:21:12 AM

-- Edited by purdoy25 on Monday 19th of February 2018 09:21:38 AM