GT250 left cylinder issues

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Alan H
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Re: GT250 left cylinder issues

Post by Alan H »

The primary (low voltage) side needs earthing.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

Well I have installed new aftermarket coils, new plug caps and one condenser (other I bought was wrong).

Unfortunately I m back to same problem, left cylinder firing but not getting very hot at all, and overall anemic power - I truly don’t understand as I thought the new coils would solve it, given that the problem moved to right cylinder when I switched the old coils.

So I rechecked a few things:
1) I have good strong spark both sides
2) Timing is correct, I had set it carefully statically and dynamically, and rechecked dynamically now
3) Compression good at 140 psi both sides with a tester that has a 12” or so hose (so it’s probably even higher due to hose volume
4) I spent much time on the carbs, floats are set right, jets clean, no air leaks
5) Spark plugs read pretty good on right, very clean almost like not firing on left…but I know it’s sparking and at least firing sometimes or partially….

I’m really getting frustrated with this, I have other bikes I need to work on
74 CL200 scrambled
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Re: GT250 left cylinder issues

Post by dollydog »

this is more common than you think. the ones i've fixed have been 90% fuel or exhaust.
[1] do the cranks have drain plugs? some gt250's do and some don't, even the same year. if your's does, then undo the left drain plug [14mm socket] and let the fuel drain out of that side of the crankcase. replace drain plug with new copper washer, and do it up TIGHT. if it doesn't have drain plugs it's slightly more difficult to empty, but it can be done. the bike will run on one with fuel in the crankcase, usually a pig to start, but once there's about an eggcupful in there it won't run on both and the crankcase won't empty on its own.
[2] i did one of my own with the same problem and that turned out to be the left spanny loaded with black goo. took the spanny off, stood it on its end to drain it. after that the bike ran pefectly. after that i sorted my jetting out and got no more black goo.
[3] the only other time i've had this problem was with an electronic ignition system breaking down - but you don't have that. that was when the battery voltage dropped below 11.5v. it ran perfectly on the right side and after charging the battery overnight it ran perfectly on both.
if i think of any more i'll post them up. once these little 250 engines are set up right they should run virtually forever. i've seen them in karts, which obviously run nearly always at wide open and the engines have lasted a full season without a problem :D
cheers, dd.
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

dollydog wrote: Tue Mar 01, 2022 4:32 am this is more common than you think. the ones i've fixed have been 90% fuel or exhaust.
[1] do the cranks have drain plugs? some gt250's do and some don't, even the same year. if your's does, then undo the left drain plug [14mm socket] and let the fuel drain out of that side of the crankcase. replace drain plug with new copper washer, and do it up TIGHT. if it doesn't have drain plugs it's slightly more difficult to empty, but it can be done. the bike will run on one with fuel in the crankcase, usually a pig to start, but once there's about an eggcupful in there it won't run on both and the crankcase won't empty on its own.
[2] i did one of my own with the same problem and that turned out to be the left spanny loaded with black goo. took the spanny off, stood it on its end to drain it. after that the bike ran pefectly. after that i sorted my jetting out and got no more black goo.
[3] the only other time i've had this problem was with an electronic ignition system breaking down - but you don't have that. that was when the battery voltage dropped below 11.5v. it ran perfectly on the right side and after charging the battery overnight it ran perfectly on both.
if i think of any more i'll post them up. once these little 250 engines are set up right they should run virtually forever. i've seen them in karts, which obviously run nearly always at wide open and the engines have lasted a full season without a problem :D
cheers, dd.
Thks for response…
re 3) I have a new Lithium battery, fully charged, actually had a smaller one and got a bigger one…it shows 12.5-13V
re 2) I did take the pipe off the offending side, there was a little black goo, but I drained it, blew it out with air, same problem continued
re 1) I will ck on this, thks I have had gas get in cranks in 4 strokes, it can be super dangerous, due to combination of bad float and bad vacuum petcock, but I caught it…
74 CL200 scrambled
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Re: GT250 left cylinder issues

Post by jabcb »

You could have a wiring problem. Checking voltage drops could be useful.

With the left points open & right closed, check voltages for both coils at: battery + , coil + , coil - and points (on part that pivots).
Repeat with right points open & left closed.

Results should be similar, but with left - right flipped.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
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GT550 72 & 75
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

Yellow02Z06 wrote: Tue Mar 01, 2022 6:43 am
dollydog wrote: Tue Mar 01, 2022 4:32 am this is more common than you think. the ones i've fixed have been 90% fuel or exhaust.
[1] do the cranks have drain plugs? some gt250's do and some don't, even the same year. if your's does, then undo the left drain plug [14mm socket] and let the fuel drain out of that side of the crankcase. replace drain plug with new copper washer, and do it up TIGHT. if it doesn't have drain plugs it's slightly more difficult to empty, but it can be done. the bike will run on one with fuel in the crankcase, usually a pig to start, but once there's about an eggcupful in there it won't run on both and the crankcase won't drain on its own
Thks for response…
re 1) I will ck on this, thks I have had gas get in cranks in 4 strokes, it can be super dangerous, due to combination of bad float and bad vacuum petcock, but I caught it…
Did have the 14mm drain plugs, but no fuel came out when removed….
74 CL200 scrambled
75 GT250 cafe
71 A7 Avenger 350
85 RZ 350
77 XS400 “No XS”
73 H1 500 Aubergine
87 GSX-R750
85 GPz900R Ninja GWB
83 CB1100F
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

jabcb wrote: Tue Mar 01, 2022 7:53 am You could have a wiring problem. Checking voltage drops could be useful.

With the left points open & right closed, check voltages for both coils at: battery + , coil + , coil - and points (on part that pivots).
Repeat with right points open & left closed.

Results should be similar, but with left - right flipped.
Actually had done this before - good voltage at battery, at points and at terminal pins for wires feeding primary side of coils across back to kill switch terminal pins and if touched to battery…

A buddy was over (he is not a 2 stroker but is very good with vintage bikes) looking things over hearing it run, etc. only thing he didn’t like was that I had twisted and shrink wrapped wire connection to new coils and same on new condensor (old condensor was that way)….I am going to solder the connections just to make sure…

I also switched the coils (or at least the inputs and plugs wires) and this time unlike with old coils when I had done it the issue did not switch cylinders (this is why I thought I had found problem and got new coils)….I guess could indicate a problem in the points to condensor to coil circuit wiring side since this was left in place even when I switched the coils but if there is I am not seeing it in voltage drops and will eliminate any connection problems with the soldering….
74 CL200 scrambled
75 GT250 cafe
71 A7 Avenger 350
85 RZ 350
77 XS400 “No XS”
73 H1 500 Aubergine
87 GSX-R750
85 GPz900R Ninja GWB
83 CB1100F
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Re: GT250 left cylinder issues

Post by jabcb »

What voltage are you getting at the coil +?

Check the wiring harness electrical connectors for the motor. They will be near the battery box.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

jabcb wrote: Tue Mar 01, 2022 5:48 pm What voltage are you getting at the coil +?

Check the wiring harness electrical connectors for the motor. They will be near the battery box.
At the connector to the coil wires (the aftermarket coils had the wires pre-installed and I had to splice in between the connector and these wires) I get 13.2V.

I have checked connections in general including those you mention. I soldered the connections that were previously well twisted and shrink covered…

Still cold left cylinder firing but somehow not good, spark plug still wet….

Both pipes produce a good amount of smoke, but the right gets way hotter….I was thinking maybe oil pump putting too much oil in but but smoke seems fairly equal although kind of heavy but have not messed with pump at all, check valves, crank seals etc. but when I boroughs the bike last year it was running well, did a lot of cosmetic/cafe racer setup mods, took it out for first ride, was running good, then lost power and have had these left cylinder issues ever since, and none of the carb and ignition work has solved it…
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85 RZ 350
77 XS400 “No XS”
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Re: GT250 left cylinder issues

Post by jabcb »

It's normal to have around a 1 ~ 1.5 V drop from the battery + to coil +. (The drop would be zero when both points are open.)

These bikes tend to run on only one cylinder when the battery is weak or not charged up, or when the alternator &/or rectifier is problematic. So check voltage at various rpms.

If it were my bike, I'd install new points & condensers. And possibly a new air filter. Make sure there is no leak at petcock vacuum tap.
And balance the carbs using the old-school technique: (1) warm the motor so it's OK without the choke, (2) with one cylinder running set idle to something like 2k, (3) with other cylinder running set idle to same #, (4) with both cylinders running set idle. If it runs better, use the bike & then set idle again after the motor has cleared out.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

jabcb wrote: Tue Mar 01, 2022 8:21 pm It's normal to have around a 1 ~ 1.5 V drop from the battery + to coil +. (The drop would be zero when both points are open.)

These bikes tend to run on only one cylinder when the battery is weak or not charged up, or when the alternator &/or rectifier is problematic. So check voltage at various rpms.

If it were my bike, I'd install new points & condensers. And possibly a new air filter. Make sure there is no leak at petcock vacuum tap.
And balance the carbs using the old-school technique: (1) warm the motor so it's OK without the choke, (2) with one cylinder running set idle to something like 2k, (3) with other cylinder running set idle to same #, (4) with both cylinders running set idle. If it runs better, use the bike & then set idle again after the motor has cleared out.
Thks so much for your continued support.

How and where would I check voltage while running and at various RPM?

I have only replaced one condensor as the other one I ordered was wrong I guess I can get another and points and maybe even a rectifier and stator….can’t really see a rotor being bad, but I hate throwing parts at it rather than diagnosing it better somehow….
74 CL200 scrambled
75 GT250 cafe
71 A7 Avenger 350
85 RZ 350
77 XS400 “No XS”
73 H1 500 Aubergine
87 GSX-R750
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83 CB1100F
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Re: GT250 left cylinder issues

Post by jabcb »

Checking the voltage at the battery is fine. The battery is a PITA to get to on the cafe builds we done, so I installed a convenient battery tender connector. That also provides a convenient location to check the voltage.

Took a quick look at this topic.
The alternator is likely OK. The voltage vs rpm test will confirm this.

Stock, the bike does not have a voltage regulator — it uses the combo of a switchable alternator lighting coil & the battery. At high rpm the voltage will be too high for your lithium battery.
If you are using any type of modern sealed battery, then you should also use a modern rectifier/regulator.
But high voltages are not causing your weak left cylinder. It is a separate issue.

The flange mounts on these old carbs can be problematic. Over tightening the flange nut can warp the carbs to the point the slides don’t move smoothly or stick.
I’ve had this problem. I’ve found the torque required to collapse new modern lock washers to be too much.
Trying to get a bike to run correctly when the slides are misbehaving is only useful if you want a really bad headache.
Visually check to see that the slides fully close & fully open in unison.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
Yellow02Z06
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

jabcb wrote: Wed Mar 02, 2022 5:26 am Checking the voltage at the battery is fine. The battery is a PITA to get to on the cafe builds we done, so I installed a convenient battery tender connector. That also provides a convenient location to check the voltage.

Took a quick look at this topic.
The alternator is likely OK. The voltage vs rpm test will confirm this.

Stock, the bike does not have a voltage regulator — it uses the combo of a switchable alternator lighting coil & the battery. At high rpm the voltage will be too high for your lithium battery.
If you are using any type of modern sealed battery, then you should also use a modern rectifier/regulator.
But high voltages are not causing your weak left cylinder. It is a separate issue.

The flange mounts on these old carbs can be problematic. Over tightening the flange nut can warp the carbs to the point the slides don’t move smoothly or stick.
I’ve had this problem. I’ve found the torque required to collapse new modern lock washers to be too much.
Trying to get a bike to run correctly when the slides are misbehaving is only useful if you want a really bad headache.
Visually check to see that the slides fully close & fully open in unison.
I have been studying the wiring diagram a lot….so would the + side of the coils primary be the orange wires that are ganged before the coil that come from kill switch and the - side be the white and black that go to the points and condenser? If so then the continuous flow of correct current at correct voltage would indeed be critical to getting coils to fire well…

Yes my Lithium battery on this cafe is a PITA to get to, and I made custom bracket to hold it in which relays on the tank bolt so it’s a hassle now every time I take the tank off too….but I have gotten pretty good at it…I am worried about the whole rectifier thing…but I will check voltage at battery when running today…

Carb slides are working smoothly and in unison….
74 CL200 scrambled
75 GT250 cafe
71 A7 Avenger 350
85 RZ 350
77 XS400 “No XS”
73 H1 500 Aubergine
87 GSX-R750
85 GPz900R Ninja GWB
83 CB1100F
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Re: GT250 left cylinder issues

Post by jabcb »

What battery are you using? (link, or brand + model #)
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
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Re: GT250 left cylinder issues

Post by Yellow02Z06 »

jabcb wrote: Wed Mar 02, 2022 1:01 pm What battery are you using? (link, or brand + model #)
Anti Gravity, AG801. I tested voltage at battery, it’s giving 13.2 at all RPMs…

I pulled one spark plug lead off at a time, ie ran one one cylinder, then added other, and it is absolutely firing the left cylinder and the right, and both at once, it was just still smoking, left not getting nearly as hot, and smoking a lot…so that was good to re-confirm…

I then started looking at oil pump adjustment and I found out a few things:
1) spring was not strong enough to return it all the way, it was basically near full open even with throttle closed
2) opening window (by removing the “small plug style” bolt in right carb to see mark that is supposed to sync with mark on pump per owners manual) revealed I had the sliders switched….now I don’t think this makes a difference in running as they are not backwards like is possible on other bikes which causes shut off of air when throttle closed, (trust me I know I had done this on my CL200) but maybe it does
3) I added a compression style spring to fuel pump cable after seeing if I could wind the stock spring (I could not) and adjusted oil pump opening after switching slides, too…

Anyway I am hopeful this will help!
74 CL200 scrambled
75 GT250 cafe
71 A7 Avenger 350
85 RZ 350
77 XS400 “No XS”
73 H1 500 Aubergine
87 GSX-R750
85 GPz900R Ninja GWB
83 CB1100F
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