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PostPosted: Sun Sep 29, 2019 2:53 am 
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On the main road

Joined: Thu Nov 07, 2013 1:59 pm
Posts: 172
Country: United Kingdom
Bikes owned: A100 GT250 & 4t Yams!
Hi all

after a year of on/off gentle fettling I took to the road on the GT250A a week ago. The machine idles fine but struggles under load..
..until the throttle is opened wide then it's off like a scalded cat. I imagine that's when the mains kick in. I had the carbs apart a while back
and gave them a fair clean but I'm wondering now abou the needle jet. I'm used to seeing a drilled 'emulsion tube' in this position
but this looks different. Position 11 here..

http://www.nichecycle.com/ncs/parts-fin ... -list.html

I have a cheap ultrasonic so might bung 'em in there for a bit..


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PostPosted: Sun Sep 29, 2019 6:52 am 
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On the main road
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Joined: Thu Oct 03, 2013 4:39 am
Posts: 161
Country: Germany
Bikes owned: GT250/380
Hi!
Needle Jet for GT250A/B/C, VM28SS:
Mikuni Series 172, size O-2, later size O-0

look HERE
in the table there klick on "172" to see a pic of the jet

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PostPosted: Tue Oct 01, 2019 4:46 pm 
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On the main road

Joined: Thu Nov 07, 2013 1:59 pm
Posts: 172
Country: United Kingdom
Bikes owned: A100 GT250 & 4t Yams!
Hi Thanks.

I had the carbs off again and they looked pretty clean inside - no blockages to see and float height looked about right too. I have some concerns
about airbox rubber air leaks but that would affect across the rev range - wouldn't it?

Cheers


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PostPosted: Wed Oct 02, 2019 4:52 am 
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On the main road
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Joined: Thu Oct 03, 2013 4:39 am
Posts: 161
Country: Germany
Bikes owned: GT250/380
Hallo...
It depends on how big the leaks are. In principle, additional openings in the intake channel can make the mixture lean. However, I do not see that as very problematic if the air filter intake manifold does not already have real holes.
Unnoticed small intake leaks between carburetor and cylinder are more likely to cause damages.

The GT250A engines have no usable midrange anyway. To accelerate from the stand halfway neat, you have to turn at least 4500rpm when engaging the clutch. Up to about 6500rpm, not much happens, only then comes a really noticeable boost. Quite different than the RamAir engines.

On the last GT250A engine I've set this year, I needed a pilot jet #35 and main jet #100 to get a good throttle response and not too bright spark plug face (B9ES). Fuel level in the float chamber at the level of the fixing screw heads. (like HERE)
The engine and channels were not machined, pistons, cylinders, carburettors and exhaust in original condition. Original intake manifold new, air filter foam inserts to fit the original air filter box were from the accessories. Maybe these have a slightly higher air flow.

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PostPosted: Wed Oct 02, 2019 2:01 pm 
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On the main road

Joined: Sat Sep 05, 2015 3:14 pm
Posts: 197
Location: southeast pa
Country: us
Bikes owned: GT
markush wrote:
The GT250A engines have no usable midrange anyway

I agree, GT250A & B don't like mid range.My GT250B likes wide open, and that isn't my style of riding.

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76 GT185
77 GT250
77 GT380
76 GT500
73 GT550
73 GT750
74 GT750
71 T250 scrambler
75 T500 cafe


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PostPosted: Wed Oct 02, 2019 5:46 pm 
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Moto GP
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Joined: Mon Dec 13, 2010 4:32 pm
Posts: 3477
Location: southwestern Pennsylvania
Country: USA
Bikes owned: 69 T350 thru 75 GT750
karl pa wrote:
markush wrote:
The GT250A engines have no usable midrange anyway

I agree, GT250A & B don't like mid range.My GT250B likes wide open, and that isn't my style of riding.

I had a 73 GT250K for a few years when I got a sweet 76 GT250A. I was surprised at how different they are & definitely prefer the early version.

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BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 project & 75
GT380 72
T500 73 project
T350 69, 71 & 72 project
T250 72
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE


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PostPosted: Thu Oct 03, 2019 6:16 am 
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To the on ramp

Joined: Fri Aug 11, 2017 7:09 am
Posts: 412
Country: england
Bikes owned: several gt250 ramairs
jab, you're a man after my own heart :D
dax, the way i've always jetted the 250a carbs i've done is [a] make sure you have the correct emulsion tubes in [the long brass bit] and make sure they match. they should say 100-0 on them, or sometimes just 0. [b] if it starts ok and ticks over fine, leave the #30 pilots in. [c] change the main jets to 112.5. [d] pilot airscrew out by 1&3/4 turns. [e] make sure your slides are opening at exactly the same time, and same amount of slack in each cable. [f] slides should be stamped 512.5 and make sure they are both the same, and obviously facing the right way. [g] needles on middle notch and both the same. also make sure the needles are being held where they should be with the butterfly and spring, the butterfly shaped ones have been known to pop off. apart from that, it should be perfect. can't remember what airfliters you have on, but DON'T use the k&n type with the solid ends. if you're going to use aftermarket, then get the all foam type :D doesn't sound like you've got air leak problems, if that's any consolation :up:
cheers, dd.

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GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.


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PostPosted: Thu Oct 03, 2019 7:15 am 
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On the main road

Joined: Thu Nov 07, 2013 1:59 pm
Posts: 172
Country: United Kingdom
Bikes owned: A100 GT250 & 4t Yams!
Thanks for all the tips guys..

I've put it all back now including the customary hour fiddling about trying to keep the (new) airbox rubber on :roll: ( the airbox seems like it needs to move forward at least 5mm to get a secure connection). If it still runs like before (bogging badly at under 50% throttle - not very neighbourly!) I'll check all the jet sizes (like fool I didn't note them down this time, although I think I check on teh first round of carb rinsing 12 months ago....) Airfilters are home-made filter foam on the original 'cage' sprayed with some ATF..


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PostPosted: Thu Oct 03, 2019 2:24 pm 
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To the on ramp

Joined: Fri Aug 11, 2017 7:09 am
Posts: 412
Country: england
Bikes owned: several gt250 ramairs
uh oh, spraying with atf might be part of your problem. i always leave mine dry, but mine are different to yours. reading up on the subject, apparently you should use air filter specific oil, either spray or loose in a can. air filter spray sticks to the foam and stays there, filtering particles out. the bike could be sucking the atf into the engine - just a thought :D
cheers, dd.
http://blog.pro-x.com/air-filter-maintenance

_________________
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.


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PostPosted: Thu Oct 03, 2019 4:08 pm 
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Moto GP
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Joined: Mon Dec 13, 2010 4:32 pm
Posts: 3477
Location: southwestern Pennsylvania
Country: USA
Bikes owned: 69 T350 thru 75 GT750
dollydog wrote:
jab, you're a man after my own heart :D
...

I sold the GT250A to a museum some years ago. A mechanic/friend now owns the GT250K. Its now a cafe racer: http://www.suzuki2strokes.com/forum/viewtopic.php?f=21&t=12998

The 72 T250 is 100% stock. My first bike was a red 72 T250. So of course, my collection has to have one.

I much prefer the T350, which is essentially the same bike except for a modestly larger engine.

_________________
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 project & 75
GT380 72
T500 73 project
T350 69, 71 & 72 project
T250 72
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE


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PostPosted: Thu Oct 03, 2019 5:19 pm 
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Expert racer

Joined: Tue May 18, 2010 5:38 pm
Posts: 1459
Location: New Hampshire
Country: USA
Bikes owned: Suz, Yam, Honda, Kaw.
I also own(ed) a gt250A and gt250L

The L was so boring ... why its a t350 now.

GT250A was/is awesome. It has no power at all under 4000 RPM. Period magazines that reviewed it stated how it behaves (go find article online and review how it should run).

And, as some have said here so far it needs to be above 6000 RPM to be enjoyable. However 4000 - 6000 should run OK. if you are say at 5000 rpm cruising, then you crack carbs WOT, it should not bog/slow down...however, it may not accelerate at all, that's how it came new from the mother land

If it bogs/slows down at WOT
Timing. ensure the ignition timing is per spec min... any retard makes big diff
Little leaner- one size for example on main jet can help for GT250 were jetted a tad rich IMO and that's what help keep the air cooled engine a bit cooler for those who stayed at high speed for a long time. Its is only air cooled and too much power for it to shed off (again IMO/experience). Oh, the A had two carb jetting specs, the later was safer against melt down

Now if you do run WOT all the time the be careful to ensure it is not going to melt down.

You need a fresh bore and rings. I mean low compression due to those worn items matter at lower rpms and way it is tuned

Once you sort out it out try putting chambers on it like DGs for RD. That bike will respond very well and give even more top end. It is a rocket. I went through cranks in about +2000 miles thrashing it most of the time... Running above 8000 RPM (maybe 10K with chambers). Those little twins in my opinion had too small big end Pin and bearing(different topic). This is now a T350 as well. I still have an early 250A engine with low mile new to me crank all waiting to drop in a bike again. A stock :ssh: t350 is probably just as fast but has bottom and mid range power torque much higher than 250A or 250L. (see Suz service manual for specs)

I would get another Gt250A/B but none show up. I must say with chambers it was the bike I would most likely would get hurt on... just can't drive it slow. Exciting, reckless, scary, foolish. When all is fresh and spot on, chambers it is very fast for what it is.

Curious how it compared to rd250. Both those bike like Chambers, added power, to be a fair comparison

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74 GT250 (T350 upgrade),
76 GT250 (T350 upgrade),
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77 Yamaha RD400 (Daytona Cyls),


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PostPosted: Thu Oct 03, 2019 6:51 pm 
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Moto GP
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Joined: Mon Dec 13, 2010 4:32 pm
Posts: 3477
Location: southwestern Pennsylvania
Country: USA
Bikes owned: 69 T350 thru 75 GT750
The GT250K currently has Higgspeeds. My friend is happy with it. I haven’t ridden it so I can’t comment on what the chambers did to performance.

From time to time I’ve thought about getting an RD400. So I’m a bit curious about how the RD400 & T350 motors compare.

_________________
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 project & 75
GT380 72
T500 73 project
T350 69, 71 & 72 project
T250 72
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE


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PostPosted: Fri Oct 04, 2019 8:27 am 
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On the main road

Joined: Thu Nov 07, 2013 1:59 pm
Posts: 172
Country: United Kingdom
Bikes owned: A100 GT250 & 4t Yams!
Thanks again guys - nice to have a bit of chat going on here

Vintageman wrote:
I... Oh, the A had two carb jetting specs, the later was safer against melt down


Do you know what those are/were so I can compare with what I have (eventually!)

Cheers


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PostPosted: Fri Oct 04, 2019 10:11 am 
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On the main road

Joined: Sat Sep 05, 2015 3:14 pm
Posts: 197
Location: southeast pa
Country: us
Bikes owned: GT
The start of 250A was 92.5,after some holed pistons they went to 95 mains,I changed my B to 92.5 to make it more rideable, but you don't want to hold wide open to long.

_________________
76 GT185
77 GT250
77 GT380
76 GT500
73 GT550
73 GT750
74 GT750
71 T250 scrambler
75 T500 cafe


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PostPosted: Fri Oct 04, 2019 12:02 pm 
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Moto GP
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Joined: Mon Dec 13, 2010 4:32 pm
Posts: 3477
Location: southwestern Pennsylvania
Country: USA
Bikes owned: 69 T350 thru 75 GT750
Service Bulletin GT-32 lists the changes going from the early GT250s to the GT250A http://www.ozebook.com/compendium/techbuls/SerBulletin%20GT%20027-33.pdf

Service Bulletin GT-35 lists the mid-model-year changes for the GT250A: http://www.ozebook.com/compendium/techbuls/SerBulletin%20GT%20034-39.pdf

They also had a timing marks snafu with the early GT250A. See Service Bulletin GT-30: http://www.ozebook.com/compendium/techbuls/SerBulletin%20GT%20027-33.pdf

_________________
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 project & 75
GT380 72
T500 73 project
T350 69, 71 & 72 project
T250 72
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE


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