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PostPosted: Wed Apr 03, 2019 7:46 am 
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Still in the Driveway

Joined: Wed Apr 03, 2019 7:17 am
Posts: 4
Country: ne
Bikes owned: gt125
Hi

Have an Gt125 X4 that has been quite modded. Have just been putting it back together and had some test driving. I behaves quite strange, and I would be happy for some help how to proceed with the settings. Basic info, cylinders are ported, new bigger carbs and also other exhausts (moped chambers welded to stock first part). I have found out that it runs a little rich at low throttle low rpm, so I will buy smaller pilot jets. However, my biggest issue is with how it runs more up top, rpm-wise. If I for example hold 25-30% throttle steady in say third gear, the bike picks up normal speed through the rpm range between 2k - 7k. Then at 7k it´s like a second engine suddenly kicks in (note I´m still at same throttle), there is a very noticable power increase, and the bike pulls harder and harder (still without giving it any more throttle) up to 9,5 -10k where it hits a wall. I can in no way make it rev over 10k, regardless of throttle position.

What I don´t understand is why it pulls that hard at such low throttle (it pulls like a stock gt125 would at WOT). And I have no use of the remaining 70% of the throttle, the bike hardly runs any better at all there. So I guess something is really off here, but what? As said, it runs rich when cruising around town, but when it makes those pulls from 7k -10k (all by itself without me giving more throttle) I´m not sure that it´s rich. My worry is that somehow the chambers are so effective in that rpm range so it "runs by itself", maybe lean?

Haven´t done any plug chop yet, honestly I don´t dare to hold WOT any longer duration before I know what is going on with my carbs. They are 21mm, pilot 40 main 100. I´ve lowered needle one step, stock middle position was worse. Plugs are brown/black. Ignition set according to manual.

Any advice? I want the bike to behave normal, pulling hard at 80%-WOT, not 25%...

Thanks in advance!


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PostPosted: Sat Apr 06, 2019 6:21 am 
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Novice racer

Joined: Wed Jul 10, 2013 12:04 pm
Posts: 981
Country: England, UK
Bikes owned: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S
Brown black plugs are indicating a rich mixture. Have you balanced the carbs to ensure they are opening at the same rate and reaching WOT at exactly the same time? What air filter are you using? Is it clean and unclogged. My initial thoughts are that you may need to reduce the main jet size and possibly pilot jets. But first, have you made sure you have no air leaks? There must be no cracks in your rubbers and the must be sealed well. Are the needles in good condition? It will take you a lot of trial and error on a modified engine to get the carburation right.


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PostPosted: Tue Apr 09, 2019 4:23 am 
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Still in the Driveway

Joined: Wed Apr 03, 2019 7:17 am
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Country: ne
Bikes owned: gt125
sportston wrote:
Brown black plugs are indicating a rich mixture. Have you balanced the carbs to ensure they are opening at the same rate and reaching WOT at exactly the same time? What air filter are you using? Is it clean and unclogged. My initial thoughts are that you may need to reduce the main jet size and possibly pilot jets. But first, have you made sure you have no air leaks? There must be no cracks in your rubbers and the must be sealed well. Are the needles in good condition? It will take you a lot of trial and error on a modified engine to get the carburation right.


Thanks for your reply!

Carbs are brand new (and synced), so are the universal foam filters (lightly oiled) and that also goes for the rubbers. I have some very minor leaks on exhausts, I can tell because after some driving it shows some black sludge leaking out at 2-3 places, but we are talking like microscopic holes. I don´t know if that would be enough to deliver that much air into the engine, giving this symptom. I have ordered some new (smaller) pilot jets which will arrive this week, but up top I´m not sure at all that it´s rich.

Here's a very short clip, note that I'm holding about 1/3 throttle (fixed). Main jet shouldn't be in play there?

https://streamable.com/a4vy3" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;


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PostPosted: Tue Apr 09, 2019 6:55 am 
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Novice racer
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Joined: Wed Jan 18, 2012 8:01 pm
Posts: 946
Location: North of Albany, NY
Country: USA
Bikes owned: T500R (now), T500M (40 yrs ago)
Main Jet really is really only an influence starting around 3/4 throttle open. At 1/4 - 1/3 throttle, you are mostly controlled by the Needle Jet and Jet Needle, with a little of the idle circuit contributing.

Image

Source: Tuning and Jetting Mikuni Carbs


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PostPosted: Tue Apr 09, 2019 9:45 am 
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Still in the Driveway

Joined: Wed Apr 03, 2019 7:17 am
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Country: ne
Bikes owned: gt125
ConnerVT wrote:
Main Jet really is really only an influence starting around 3/4 throttle open. At 1/4 - 1/3 throttle, you are mostly controlled by the Needle Jet and Jet Needle, with a little of the idle circuit contributing.


Exactly, my point, main jets can't be the reason here. I've seen lots of those diagrams and they tend to differ slightly. Also read some people stating that pilot jet affects all way up top, however maginally. True?

Just an idea, can it be the throttle springs that are to weak, letting the throttles raise by themselves? Or is that a very uncommon issue? Carbs are Polini if that matters. I'm just quite confused over the fact that it pulls quite hard and rev that high at 1/3 throttle= slide not very open= limited amount of air can enter the engine.


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PostPosted: Tue Apr 09, 2019 10:06 am 
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Location: southwestern Pennsylvania
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Bikes owned: 69 T350 thru 75 GT750
wfs wrote:
...
Just an idea, can it be the throttle springs that are to weak, letting the throttles raise by themselves? Or is that a very uncommon issue?
...

Flange mounted carbs can be a problem. Over tightening the mounting bolts can distort the carbs to the point that the slides stick.

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PostPosted: Tue Apr 09, 2019 3:38 pm 
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Joined: Wed Jan 18, 2012 8:01 pm
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Location: North of Albany, NY
Country: USA
Bikes owned: T500R (now), T500M (40 yrs ago)
wfs wrote:
ConnerVT wrote:
Main Jet really is really only an influence starting around 3/4 throttle open. At 1/4 - 1/3 throttle, you are mostly controlled by the Needle Jet and Jet Needle, with a little of the idle circuit contributing.


Exactly, my point, main jets can't be the reason here. I've seen lots of those diagrams and they tend to differ slightly. Also read some people stating that pilot jet affects all way up top, however maginally. True?

Just an idea, can it be the throttle springs that are to weak, letting the throttles raise by themselves? Or is that a very uncommon issue? Carbs are Polini if that matters. I'm just quite confused over the fact that it pulls quite hard and rev that high at 1/3 throttle= slide not very open= limited amount of air can enter the engine.


The Pilot Jet has a low constant flow. So it always contributes. But is basically overwhelmed once you open the throttle, and allow much more air and fuel to rush it. Think it as your refrigerator humming quietly in the kitchen. Crank up the stereo, and the 'fridge is still contributing to the total noise in the room.

A thought on your situation - If air isn't going past the throttle slide, where is it coming from? I would be looking for vacuum leaks. Carb flanges, crank seals, head/cylinder gaskets, etc.


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PostPosted: Tue Apr 09, 2019 4:07 pm 
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Still in the Driveway

Joined: Wed Apr 03, 2019 7:17 am
Posts: 4
Country: ne
Bikes owned: gt125
jabcb wrote:
Flange mounted carbs can be a problem. Over tightening the mounting bolts can distort the carbs to the point that the slides stick.


Carbs are mounted with rubber adapters:https://www.scooter-attack.com/doppler-carburetor-adapter-d-25-25mm-cgn291582-39.html slides move freely

ConnerVT wrote:
A thought on your situation - If air isn't going past the throttle slide, where is it coming from? I would be looking for vacuum leaks. Carb flanges, crank seals, head/cylinder gaskets, etc.


I'm pretty sure there are no leaks at those spots but will try with some starter gas tomorrow. I guess a problem will be to isolate the filters, as it can easily get sucked in there giving false indications.

By the way do you agree with me that the bike acts weird in the videclip, or is it acting normal, given the fact that it's been ported, received chambers and what not? i mean maybe I think I have an issue but I don't? It's been well over 20 years since I last played around with tuned 2-stroke, so my memories are not that fresh..


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PostPosted: Sat Apr 27, 2019 4:16 pm 
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Novice racer

Joined: Wed Jul 10, 2013 12:04 pm
Posts: 981
Country: England, UK
Bikes owned: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S
Sorry it has been so long since I replied to this thread. I am wondering if the larger carbs
are poorly matched to your engine. How big are they? and what is the factory carb size in comparison?
From your video it is hard to tell if anything is amiss. It sounds great and pulls well when it hits the power-band.
It may be that the new carbs need a different profile needle to get them to match your engine. It can take a lot of trial and error to get jetting correct on a modified set-up. I would expect you will find it necessary to beg, borrow or steal several different size needles and jets to experiment with.
Does the bike idle ok?


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