Suzuki TR125 - gap at 5000rpm

[keith_vdw]

Hi guys,

Thanks for recieving me. I've read a lot of threads here and i would like your advise.
I bought a Suzuki TR125 from 1979 for my girlfriend, rare model in Europe but seems it was popular in Asia. It's very similar to a GP125 (rotary disc), only the position of the exhaust port has another angle.

I restaurated it, did a top end rebuild because piston was in bad shape and having a lot of blow-by. Fitted new oversize piston, new rings and cylinder is rebored.
Exhaust decarbonized, carb ultrasoon cleaned, ignition timed, new spark plug etc.
It runs nice in "race mode" going fast through the revs. But it's terrible at "cruise mode", it wont go past 5000rpm with a 1/4 throttle on and just drops dead completely.
I know how to set up a carb, i race a Rotax kart so i'm used to play with main jets, needle height and air mixing screw to find the correct setting on track.

The only difference compared to standard version is the airbox that was replaced by previous owner with a K&N filter, don't have the original air box to try.
I'm running syntechic Elf oil with standard oilpump setting.

I tried everything:
- All needle positions (for mid range rpms), even upped it with extra rings to see if it would have any effect.
- Tried all the positions (per 1/2 turns) of the air mixing screw (for pick up in low range rpms)
- Tried all kinds of float height positions, up to 5mm deviation from standard manual setting
- I even bought brand new NOS carb

Nothing...absolutely nothing. The best i can do is move the gap from 5000rpm to 6000rpm.
It doesn't have reeds, it has a rotary disc...and those are bulletproof right? I checked for air leaks on the carb inlet, checked the fuel tap, checked the carb for leaks, use fresh fuel, tried without air filter and or carbhousing cover...

Does anyone have any further suggestions? I'm fiddling for weeks now and can't cure it.

[keith_vdw]

Update:
I got a tip to try running castor oil instead of synthetic.
I hooked off the oil pump cable and mixed 2% of castor in the fuel tank.
The bike is a different machine, revs up to 8500rpm and is a lot more crispy.
I searched for the best carb setting and its a lot better now. There is still a drop at 5000rpm, but it doesnt completely drop dead like before. It s managable, but it s still there.
Does anyone have any other suggestions for me?
Also, because the of the big difference i m wondering if the oilpump is still working correct. I found this in the manual:
I dont have the special tool and i dont know where to connect it to. Is there a diy method to check the output of the pump?

[dollydog]

hi mate, one thing i would suggest is take the k&n filter off and try a foam filter instead. might not cure your problem, but worth a try. by the way, i always use foam air filters on my 2 strokes - NEVER k&n type,
cheers, dd.

[keith_vdw]

Dollydog, thanks for your reply.
Any particular reason why not to use the k&n type of filters? The original aribox is not an option, which would mean fitting something like this instead.

s-l200.jpg

You think this would have a positive impact on the motor behaviour? What's your theory behind it?

Concerning the oil pump check up i'm still stuck. I'm running premix now so i could easily run the test in the manual and see how much ml it spits.
But even if the reading is between 1.26 and 1.53ml..what is the percentage of the mixture? Does anyone know what was the default percentage Suzuki was setting it's pumps to at the time?

[dollydog]

using k&n filters is frowned on by a lot of people it seems, but it seems to be mostly on piston port engines like my gt250's. the theory behind it is with a piston port engine [don't quote me on this, it's just what i've read] it tends to spit fuel back through the carb, which in turn bounces off the back of the filter and back through the carb. thereby giving an over rich mixture and poor running. something like that with the foam filter it can't bounce back. that's the theory behind it. if you go on the .net 2 stroke forum they are always slagging k&n filters off and they all use foam pod type filters - like the photo above. and they mostly run reed valve engines which aren't supposed to spit fuel back up the carbs, so work that one out. yours is rotary valve, which is something i've never dealt with. anyway, foam filters are cheap, so worth a try surely?
as for your oil pump, i don't know the exact spec for oil flow, but i always set mine up as the manual says.line the 2 marks up at a certain throttle position. on mine it's when a little dot on the carb slide appears in a window in the side of the carb - then line the marks up. BUT, 2 stroke oil now is far superior to 2 stroke oil when the bike was new [1973-75 gt250], so i always turn the oil flow down as it doesn't need as much. good example is my 1966 t10 which runs on premix. in the original manual it should be 14:1 ratio, which is a lot of oil. i now run it at 33:1 on modern oils and it runs perfectly. perhaps you are over oiling?
cheers, dd.

[keith_vdw]

Hmmm, interesting theory indeed. I'm not a particular fan of k&n but it was on there when i bougth it so i stuck with it.
Unfortunately in my case it won't make any difference i'm afraid. The air filter is mounted on a connection tube that ends up in the housing where the carb is side mounted on the rotary inlet.

carb.jpeg

So spitting fuel will always splash against the cover and bounce back into the engine. Impossible to prevent this without removing the cover.

It's a pitty we don't have the info on what percentage the oilpump was set to back in the day. If it would 4% for exapmle i could simply adjust it to half of the outpour after 2mins running at 2000rpm and done.

[dollydog]

copied and pasted from another forum when i asked the question about foam air filters
cheers, dd.

quote: reversion of the intake process will basically 'bounce back off the back of the hard backed filter and will interrupt the signal creating a flatspot. you either need to run a smooth runner between the bellmouth and the filter, or run an all foam unit. any oem engineered airbox uses the same parameters for this reason

[dollydog]

hi mate, have a read of this thread: https://www.bikechatforums.com/viewtopic.php?t=153071
similar problem to yours and solved. gp125, but very similar bike
cheers, dd.

[keith_vdw]

quote: reversion of the intake process will basically 'bounce back off the back of the hard backed filter and will interrupt the signal creating a flatspot. you either need to run a smooth runner between the bellmouth and the filter, or run an all foam unit. any oem engineered airbox uses the same parameters for this reason

Makes sense.
With any other bike -with traditional carb mounting- i would immediately try it to see if it improves the situation!

But like i said, with this rotary inlet and side mounted carb, the air filter is in no direct contact with the carb inlet. There's a channel of 25cm between the two.
Further more, if above theory is correct, this would main Suzuki got the design of this bike TR125 and the GP125 completely wrong. The cover that seals off the carb housing is more or less closing off the carb inlet. There's only 5mm of free space between the inlet and the cover. Which means the blow back definitely bounches on the cover back in the inlet...

It's hard to imagine they got the design thàt wrong. This would actually mean every GP125 that's out there has the same issues...
And there's no elegant way to solve this issue.

[keith_vdw]

hi mate, have a read of this thread: https://www.bikechatforums.com/viewtopic.php?t=153071
similar problem to yours and solved. gp125, but very similar bike
cheers, dd.

Thanks! The guy's problem was caused by a leaking float chamber gasket.
Unfortunately i already solved this problem. I even have a second brand new NOS carb to test with and the behaviour is the same.

So the checklist:
- spark plug: i have two new ones
- check ignition: timed to 0.5mm, thats the closest i can get
- clean the carb: completely dismanteld and double ultrasooned, both of them
- check/adjust float level: done
- check inlet rubber for splits (if it's not between carb & engine, it'll be between airbox & carb): checked by spraying brake cleaner while running stationary.
- check fuel tap filter and sediment bowl and clean where necessary, rinse out tank (with petrol) if there's crap in it: did that before the tank was painted
- check cap for airflow: dont understand what he means here, which cap? airfilter?
- decoke exhaust: dismanteled it, pulled out muffler tube, scratched out a hand full of crap with a steelbrush but without any improved. afterwards filled it up with hot sodawater for a night and rinsed it.
- check inside the engine to see if you need new rings, a new piston or even a rebore: piston had a lot of blow by, so rebored, new oversize piston and rings are in it
- check fuel pump adjustment: ....and here we are

i tested 2% premix castor oil and had a very big improvement.
if the weather is ok this weekend i will do the same test with 2% premix synthetic oil.
if this would give a similair behaviour then this means it's not the type of oil but the mixture percentage thats to high.

but i don't want my girlfriend to ride around with a flacon of oil in here jacket
so i would need to be able to adjust the oilpump setting to 2% and thats where i'm stuck at the moment...don't have a clue how to check it.

[dollydog]

hi mate, he means the fuel tank cap. make sure there's a hole in it, so you don't cause a vacuum inside the fuel tank
cheers, dd.

[keith_vdw]

I checked the fuel cap, it's breathing, no risks for vacuum. Rinsed the tank again to double check for dirt, negative.
Did a second test with 2% premix synthetic oil. Same behaviour as with 2% castor oil. So we're sure now the cause is not the type of oil, bike runs nearly perfect with both oils.

I also tested the oil pump. Manual suggests a reading between 1.26ml - 1.53ml. My reading is 1.50ml so the oilpump is working and aligned correct according to the manual!

No one out there that can verify the mixing percentage? Of all the great mechanics on this forum no one has an idea ?
I could adjust the output to 1.00ml but without having any clue of what percentage i'm mixing...i don't want to risk a seizure
I thought maybe others would've had the same problem with old bikes since the oils of the seventies are very different to the ones we have now. So maybe a different oil pump setting is required nowadays?

[keith_vdw]

Don't know if anyone is actually reading this thread except for dollydog but i'll give the final update for those who care
Because there s no response concerning the oilpump mixture i thought i'd try and simulate the problem with premix. I upped the premix from 2% to 3%, then to 4% and finally to 5%. To my surprise that didn't alter the behaviour in a very bad way. It was still running ok, not as smooth as on 2% but definately no big drop at 5000rpm
So, emptied the tank, hooked on the oilpump again and filled her up with 5l of fresh fuel..and (this time less of a surprise) the behaviour stayed more or less the same (y).

The only thing i can think of is the alignment of the oilpump. Before i did the test to check the output i noticed she wasn't aligned 100%. So i fixed this first before i did the test.
I can't think this tiny difference would make such a significant change but it's the only thing i've changed since i went to premix.

So now it's just a little more fiddling with the carb setting to get her running 100% and she s ready to go!
Wont bother you guys any longer

[karl pa]

Don't know if anyone is actually reading this thread except for dollydog but i'll give the final update for those who care

Unfortunately when you have a rare model, you will have limited responses. I believe most, including myself never heard of that model and are reluctant to give a response because it would be just a guess.
Same said about oil pump rates, mine are all set to factory setting, I have no idea on rates, so if I were to say anything it would be incorrect.
I can tell you the GT models have oil guides on the crankshaft to pull oil away from main bearings so you will run the main bearings dry of oil on premix only, but again I have no idea if yours does the same or not.

[keith_vdw]

First of all thanks for replying karl pa, really appreciate it!
It's indeed a rare model but its actually nothing more than a special version of the well known GP125. So i thought i might've had some higher succes rate there
There's only one hose connected to the oilpump output and it's going directly to the carb. I don't mind premix to race, did it all my life. But for a roadbike with a girl on it it's just too much of a hustle.

I admit getting a clear answer to the rates was kind off a long shot i know
But I just thought of all these two strokers on here i would've at least get some opinions to be shared with me. It would be so interesting to do a poll and see how many ride with:
- standard oil pump setting
- premix with lower percentage
- deviating oilpump setting

I would guess or get the feeling most people don't bother and stay with the first option?
But then again, look at dollydogs reply, from 7% standard pump setting to 3% premix now!