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PostPosted: Thu Sep 27, 2018 8:00 am 
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Moto GP
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Location: Illinois
Nice looking bike. Good to hear that you have it sorted now. Do you know what make of pipes you have? They look interesting and modern.


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PostPosted: Thu Sep 27, 2018 12:01 pm 
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Country: Portugal
Bikes owned: GP125, GT380
Thanks.
Not quite sorted yet. Still chokes in 6th at WOT yet drop a gear and it pulls nicely again.
Also still spits black oil out the pipes yet the plugs are not oil fouled and have the correct colour.
The pipes are my own design locally fabricated.


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PostPosted: Thu Sep 27, 2018 12:40 pm 
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Joined: Wed Jul 10, 2013 12:04 pm
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Country: England, UK
Bikes owned: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S
CFS wrote:
Thanks.
Not quite sorted yet. Still chokes in 6th at WOT yet drop a gear and it pulls nicely again.
Also still spits black oil out the pipes yet the plugs are not oil fouled and have the correct colour.
The pipes are my own design locally fabricated.

Only on WOT?
Would make me think main jet is wrong size.


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PostPosted: Thu Sep 27, 2018 2:12 pm 
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Bikes owned: GP125, GT380
Yes, only on WOT. What is strange though is that I didn't have any of these symptoms with the engine that broke.
The only difference being the Newtronic ignition as opposed to points. Carbs are the same ones including all jets.
The other motor was mildly ported and the head was skimmed but don't think this to be the problem.
Will continue experimenting until I get it right.


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PostPosted: Tue Oct 02, 2018 9:56 am 
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Country: England, UK
Bikes owned: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S
CFS wrote:
Yes, only on WOT. What is strange though is that I didn't have any of these symptoms with the engine that broke.
The other motor was mildly ported and the head was skimmed but don't think this to be the problem.

It could be the problem. If the other motor was modified it would have required different jetting.


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PostPosted: Tue Oct 02, 2018 11:18 am 
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Country: Portugal
Bikes owned: GP125, GT380
The modifications were minimal but that may affect it as you suggest.
What is confusing though is that the plugs don’t show it to be running rich. I will post some pictures tomorrow.

From previous posts you have mentioned that you have run a 380 with 550 carbs - are these set up with the original jets and settings?
What difference do these make?


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PostPosted: Tue Oct 02, 2018 6:48 pm 
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Location: Illinois
Just a thought that with those pipes, it may need more or less fuel and possibly less ignition advance at the top end - assuming that they are generating more back pressure ie more pressure in the exhaust.

If those same pipes worked fine with the other ported motor, the pulse timing may now be wrong.


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PostPosted: Wed Oct 03, 2018 3:04 am 
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Bikes owned: GP125, GT380
Yes, those are the same pipes used in the other motor. The porting was very mild, exhaust and inlet raised/lower about 1mm each only and the head skimmed 1mm as well. The carb settings used were generally only to increase the main jet size due to pod filters and pipes. Timing was maintained as per the manual and the bike with the previous motor ran fine. As mentioned before this is an Italian motor where the capacity is larger at 384cc and not 371cc so I don't know if this will affect it as well.
Unfortunately I don't have access to any specialized equipment (Dyno, etc), so have to rely on what I can see/hear to get it right.

Herewith photos of the plugs after the ride. I think the colour looks fine.


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PostPosted: Wed Oct 03, 2018 9:00 pm 
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Was that after a plug chop or just riding around? If it was flat open, then the plugs are running too cold. There's no sign of self cleaning on the side electrode, so that tells us the plugs did not get hot enough. That could be the way it was ridden or the timing is retarded, plugs are too high number heat range (too cool).

If it was general riding then plugs don't tell us much of anything unless things are waaay out of the ballpark.


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PostPosted: Thu Oct 04, 2018 2:14 pm 
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Country: Portugal
Bikes owned: GP125, GT380
Once again thanks to this forum and all who have assisted. Keep the advice coming be it negative or good.
No, definitely not after a plug chop.
This was after a ride of approximately 50km mainly at WOT but of course you need to maintain speed limits when off the motorway.
Just to clarify the timing was set with a dial gauge as per the manual, the carbs are set as per the manual just the main jet has been made larger (110 I think) mainly due to pod filters and pipes.
Next time when home if time allows I will replace the plugs and attempt to do a plug chop but any advice you may have is greatly appreciated.

Thanks


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PostPosted: Sun Oct 07, 2018 1:34 pm 
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Country: England, UK
Bikes owned: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S
CFS wrote:
From previous posts you have mentioned that you have run a 380 with 550 carbs - are these set up with the original jets and settings?
What difference do these make?

I have yet to find out if these are jetted correctly. I'm still not finished restoring my bike yet. I have had it running, but with no fluid in the front brake and the paintwork needing sorting, I have not ridden it yet. I made an educated guess as to the jetting after consulting with some of the other guys on here. I put the same pilot and needle jetting as on a GT250A in the GT550 carbs, but went one size up on the main jets to compliment the Higgspeed pipes. It was good enough to get it running and it sounded sweet, but I am sure it will need a little fine-tuning once it is rideable.

Judging by the colour of your plugs they are possibly running ever so slightly too rich. As TZ said they don't appear to be self-cleaning, especially on the right and centre plugs.
Question is...what is different on those two? Could be different float levels in the carbs. Or clip on needles in wrong grooves. Or poorly synchronised carbs (Perhaps the left carb is opening slightly more than the other two. Or the pilot screw is not set equally on all three (hard to get this bang-on perfect without a gas analyser)).
Once you have eliminated those options I would try going down one size on the main jets and see if that makes a difference.
As for your plug choice, I would keep it standard unless you ride it very hard a lot or do lots of high speed motorway mileage. Correct heat rating is BR8ES NGK plugs. (BR9ES is cooler and BR7ES is hotter).


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PostPosted: Fri Mar 15, 2019 10:03 am 
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Joined: Tue Nov 07, 2017 3:25 pm
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Country: Portugal
Bikes owned: GP125, GT380
Hi to all,

Just a quick update and of course some queries where I would appreciate your valued input.

Since my last post I have changed all the wheel and neck bearings which were not in great shape. I have also stripped the GT550 carbs, ultrasonically cleaned them and installed new jet kits.
The spare oil pump has also been stripped, checked and re-assembled.

I took the bike out for a test ride last weekend and it pulls very well in all gears up to max 8500 RPM from where it will not accelerate any further.
I took some photos of the new plugs I installed prior to the ride and would welcome any feedback. Please note that the photos are not after a plug chop as I still haven’t found a road where I can undertake this safely. The photos were taken after a ride of approximately 20km of which most was done at WOT on the motorway. Photos show left plug on left, centre & right plug.
The bike starts first kick and doesn’t spit out black oil as before.
I did notice that after the ride the motor was hot to the touch but the exhausts were cool from the headers all the way to the baffles (could hold my hands on them without burning). This is baffling me as they should be extremely hot. Can anyone shed some light as to why this would happen.
The timing is set as per the manual. I think that I have read somewhere that timing should be altered due to the lead free fuel now available from the pump - is this true and if so what should it be set at?

Thanks to all once again
Carlos


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PostPosted: Wed Mar 20, 2019 8:26 am 
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Location: Manchester, UK
Those plugs look pretty good, the small amounts of dark brown carbon deposits on the ground electrode and on the metal 'ring' of the plug are a good sign that the carb jetting is correct. Also there are no signs of detonation.

On a more basic level, the fact that you can hold the bike at WOT for several km without it seizing / holing a piston shows that the jetting and timing settings are pretty safe :up:

About ignition timing - there are lots of arguments about modern fuel with ethanol etc causing problems with older 2-stroke engines. Suzi GTs don't have squish heads or high compression so my personal opinion is that modern fuel does not cause problems in these engines.

However, because you are using chambers / mild porting / skimmed head, I would be tempted to retard the timing just a little (maybe 0.1 or 0.2mm) from the factory setting. Just to be as safe as possible.

The chambers on my old 380 used to cool down really quickly after a ride, too - it is because the metal in the headers and the 'belly' of the pipe is thin - it is WAY thinner than the headers of the factory exhausts, for example. I used to be able to hold my hand on the headers of my chambers within a minute or two of stopping the engine after a fast ride.

_________________
1976 GT380 - wounded by me, and sold on
2006 SV650S - killed by a patch of diesel and a kerb in Feb 2019
2017 SV650 AL7 - naked and unashamed


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PostPosted: Wed Mar 20, 2019 8:28 pm 
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Moto GP
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Location: Illinois
Plugs are still looking cold ie no sign of heat on the center or side electrodes. That's usually a sign of plugs being too cold heat range or retarded ignition or mixture too rich.

If the exhaust ports were only raised by 1mm, the motor will peak around 800 to 8500 and maybe the pipes are designed to resonate at a higher peak. With a 54x54 (or so) motor, it should be OK to more than that but needs more exhaust time area. There may be a mismatch between pipes and port timing.

But why are the plugs looking so cold. What is the ignition timing set to in mm BTDC? Are you using points or electronic ignition? What plug caps - resistor or non and are the plugs resistor type?

What is the cold cranking pressure ie when you kick it over with plugs out and with a pressure gauge in the plug hole, what does the pressure rise to?


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PostPosted: Thu Mar 21, 2019 5:10 am 
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Joined: Tue Dec 16, 2008 11:37 am
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Location: Trowbridge UK
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Bikes owned: T500R, SV1000S, TS125, Seeley T500
Hi Carlos,

Just to add my thoughts on your plug reading. One thing I have been advised, by competent engine tuners, is that modern day unleaded fuel makes it quite tricky to read a plug in the traditional manner. The cleaner, and faster burn works against you. If you could get your bike to a good 2-st Dyno operator, with the sensors to read Air/Fuel ratios, you would get a much better indication of what you have. Also I time my road 2-strokes, and race engines, with more retard than that stated in the workshop manual (T500 - I use Max advance 2.9mm BTDC) to take into account faster burning modern unleaded. Sorry to add to the confusion :oops:

Cheers Geoff


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