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PostPosted: Thu Mar 21, 2019 8:54 am 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 6087
Location: Illinois
Geoff is correct that unleaded burns differently. Well that's not a good way to describe it, but when Unleaded was introduced, Shell issued a document that basically said that it leaves more soot on the plug body and needed hotter plugs to burn that off. I don't know if that is still the case. The ethanol content makes a difference to octane and burn rate and combustion speed.

All we can see on the plugs is that they are running cold. At normal mild street cruising that's normal. At normal street riding, we rarely use the main jet. Most of our riding is on the pilot jet and needle/needle jet. Different main jets make little difference at say half throttle unless the needle jet is far too large.

In the pictures, we cannot see down to the bottom of the insulator to see the mixture ring, which is where we read mixture at wide open throttle. That's why a plug chop at full load is useful to determine main jet size. Have you tried to drop the needles (raise the clip) to see if that cleans up midrange? It is possible that the needle and needle jet combo in those carbs is not the best for your motor. You may have to change them to get mid range clean.

Suzuki typically run 22-24 degrees of advance, but at higher revs that needs to come back to 19-20 degrees depending on the compression. With mild porting, you can probably increase the compression a little to say 6.9:1, and if you raise the exhaust ports again, compression can also be raised. Once you know exactly what the timing is set on each cylinder, you could try to increase it by 1 degree or retard it by 1 degree or 2 degrees and see what difference it makes. Use a dial gauge and ignore the timing marks.

Comparing that motor to say an RD250, they are the same bore and stroke but the RD (or DS7) has much more Time-Area. In fact almost twice as much time area on all ports at say 10,000 rpm on a mildly ported DS7, so that makes a difference to power at higher revs. Stock bikes peaked at 8,000 and with an almost stock ports, that may be the limiting factor for speed.


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PostPosted: Fri Mar 22, 2019 3:22 pm 
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Around the block

Joined: Tue Nov 07, 2017 3:25 pm
Posts: 51
Country: Portugal
Suzuki 2-Strokes: GP125, GT380
Hi to all and thank you for your valued input.

Geoff,
Unfortunately I don't know of any dyno let alone a 2 stroke specialist anywhere near me - I'd gladly pay the fee.
All the work on the bike is carried out by me with my limited knowledge and the experience of the good people on this forum which I hope will continue. Together I'm sure we can get it right eventually.

TZ
The motor is a substitute I purchased from a breaker when mine popped some time back. It is an Italian import and when I had it stripped never measured the port dimensions to see if these had been altered. From memory they looked like they hadn't been touched (normal casting imperfections within) so I therefore assume that it is stock standard. The previous motor had the ports slightly touched and the head skimmed.
The plugs are as recommended by the manufacturer and what I have always used NGK B8ES non-resistor type. The plug caps are still the originals which I assume also to be the non-resistor type.
The timing is set as per the manual 2.3mm BTDC for the right and left with 2.25mm BTDC for the centre cylinder all as per manual. These were set with a dial gauge and the bike runs on a Newtronic electronic ignition.
I know I did check the compression after re-assembling the motor and it seemed fine. Unfortunately I don't remember the figures but will re-check when next home and post these together with some photos of a run on hotter new plugs (B7ES). This will not be a plug chop as explained previously but will attempt to get better photos this time round.
The carbs are standard GT550 28mm bores running mainly standard jetts and settings. The only difference on the carbs from standard is that the mains are all 95 (should have 97.5 left and right) and the clip is set on the 3rd slot and not the 4th.
You mention that I should drop the needle to get the mid range clean which it may need, but it pulls very well through the full rev range. I will also attempt this and post my findings.
I may not have the time to do all the recommended changes but will post my findings as they occur.

Once again thanks for all the help.
Carlos


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PostPosted: Fri Mar 22, 2019 10:22 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 6087
Location: Illinois
I could not find dyno operators in Portugal presumably because I need to translate that into Portuguese... So I googled Dynojet distibutors and the closest appears to be in Spain. https://www.dream-machine.net/ They have a few distributors or dealers in Portugal https://www.dream-machine.net/distribuidores and it's possible that one of them may know of a dyno shop near you.

I had forgotten that the last motor was slightly ported and this one was a replacement. That would explain why it will not rev above 8500.

The oil and black plugs may be a symptom of riding the bike responsibly and it may need to be ridden harder. It's also possible that stock 550 jetting is too rich for a 380. I will try to find time to compare the needle and jets from a 380 to 550 to see if that reveals anything. I think the 380 uses O-2 needle jets and a 550 has O-2 on the outers and O-5 in the center and the needles have the same taper angles (DH). I don't appear to have any measurements of the 5DH21 GT550 needle to know how it compares to the 4DH7


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PostPosted: Sat Mar 23, 2019 3:51 am 
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Expert racer

Joined: Tue Dec 16, 2008 11:37 am
Posts: 1025
Location: Trowbridge UK
Country: UK
Suzuki 2-Strokes: T500R, SV1000S, TS125, Seeley T500
Hello Carlos,

I understand that access to a Dyno, and an operator sympathetic to old 2-strokes, is not available to everyone. A Dyno will enable you to establish good basic carb settings but they are not a magic solution all the time. When I left the Dyno with one of my race engines we were on 290 main jets, when we subsequently raced at Brands Hatch we ended up using 360 left and 350 right. You still have to be able to asses and adjust to the conditions. The pain on a road bike is having to strip the carbs each time to make adjustments with all the air box gubbins to complicate matters :) .

Cheers Geoff


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PostPosted: Sat Mar 23, 2019 7:51 am 
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Around the block

Joined: Tue Nov 07, 2017 3:25 pm
Posts: 51
Country: Portugal
Suzuki 2-Strokes: GP125, GT380
Hi to all,
The bike doesn't smoke or spit black oil out of the exhausts since I replaced the oil pump. The plugs also seem cleaner.
The bike pulls well from low RPM to 8000 where the red line starts from where it stagnates.
Since replacing the 380 with the 550 carbs the bike accelerates a lot better and quicker with the revs climbing pretty quickly. This of course is no race spec bike. When on the motorway I will always try to extract the maximum out of the bike when possible.

TZ - I found this link which seems to show the differences between the 4DH7 and 5DH21 needles. I hope this is what you were after.
http://kawtriple.com/mraxl/carb/mikunineedle.html
I did a search for dyno tuners in Portuguese but to no avail. Will need to do this old school style.

Geoff - I have pod filters which makes the exercise a little simpler but it still remains a pain.

I will be trying out all the advice given and post my findings as and when I can try them out.

Once again Thanks
Carlos


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PostPosted: Sat Mar 23, 2019 12:53 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 6087
Location: Illinois
Thanks Carlos. That gave me the missing data. Odd that I had all the needles (and a few more) except that 5DH21. That filled the gap. Odd thing is that I had a set of GT550 carbs in the shop last year and didn't measure the needles because I was sure I had the numbers.

Sounds like the bike is now working well with maybe just a little fine tuning left. When you have some spare time - maybe next winter - might be time to port the barrels and get more top end for the following riding season. :wink:


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PostPosted: Sun Mar 24, 2019 4:40 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
Suzuki 2-Strokes: GT250/380
Hi!
Several months ago, I collected all the data about jet needles and nozzles that I could find.
I brought the data together in a spreadsheet, similar to findings on the internet. However, I have completely rebuilt the sheets and sketches to match the data and their names.

The file contains three sheets:
Needle data
Nozzles data
Evaluation
If interested, it can be downloaded here:
http://suzuki-gt250.de/sonstiges/DuesenNadelnTabelle.html

site translated by google:
https://translate.google.de/translate?sl=de&tl=en&u=http%3A%2F%2Fsuzuki-gt250.de%2Fsonstiges%2FDuesenNadelnTabelle.html

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PostPosted: Mon Apr 22, 2019 12:24 pm 
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Around the block

Joined: Tue Nov 07, 2017 3:25 pm
Posts: 51
Country: Portugal
Suzuki 2-Strokes: GP125, GT380
Hi to all,
I recently tried out a set of hotter plugs (B7ES). Photos 6467 & 6471 are of these after a 14km run at mainly WOT.
Photo 6474 is of the next run on std plugs (B8ES). I done these two runs one after the other to get a comparable reading of both sets of plugs, same settings, same day, etc.
Unfortunately on my return during the second run the bike started to loose power as if running on 2 cylinders, it managed to get it home. When home I removed the plugs and was shocked at what I saw - the centre cylinder plug was burnt.
I proceeded to strip the motor and photos 6476, 6477 & 6478 are of the pistons. The right piston (photo 6476) and left piston (photo 6477) are salvageable but the centre can't even be used as an ashtray!
No settings were altered from when the bike was parked up last or prior to these two runs.
I am confused as to why this would have occurred and of course why only to the centre cylinder.

I assume that the correct procedure would be a crank rebuild as well as a re-bore correct? I inspected the crank bearings, seals and big ends and these don't seem to show any signs of damage.

I would appreciate any feedback as to what may have caused this.

Whilst doing the test runs the bike would rev to 7500/8000 RPM at which time the clutch seems to slip and take again reving to 8500 - what could be causing this to happen. This happens in 2nd to 5th years. It also seems to loose power in 6th gear.

Thanks
Carlos


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PostPosted: Mon Apr 22, 2019 12:27 pm 
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Around the block

Joined: Tue Nov 07, 2017 3:25 pm
Posts: 51
Country: Portugal
Suzuki 2-Strokes: GP125, GT380
Seems that I missed photo 6477


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PostPosted: Mon Apr 22, 2019 2:12 pm 
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Yeah Man, the Interstate

Joined: Tue Mar 10, 2015 12:35 pm
Posts: 531
Location: Wales
Country: WALES
Suzuki 2-Strokes: suzuki gt 550j
Sorry to hear of your problems
Here are my thoughts
Its quite obvious that the middle cylinder has massively overheated and melted the piston crown. All the plugs 7's and 8's have been overheated hence the white central electrodes. Now there are several reasons that this overheating may have occurred. 1. Excess air in the cylinder (leaking gasket\carb rubber.badly adjusted airscrew) 2. Poor quality fuel low octane ethanol (alright if ridden normaly but not at sustained WOT) 3. Poor ignition timing(allowing mixture to self detonate before the spark)
Provided the bore of the cylinder is not damaged no need to rebore. Provided the piston fragments have been recovered and the seals are undamaged no need to replace.
As i say these are my opinions only.
Good luck

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98 carb Blackbird and GT550j


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PostPosted: Tue Apr 23, 2019 10:34 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 6087
Location: Illinois
Sorry to see that hole Carlos. Typical two stroke.

Electrodes really should be white or close to it after a plug chop. The plugs do look like they are a little too hot though. Side electrodes are not too bad but appear to be burned clean for too far down the side strap.

That could be a plug that's too hot for the type of riding, too much advance or mixtures too lean at the top end.

It could also be from fuel with too low an Octane rating combined with any or all of the above. It's possible that the jetting was slightly different even if the jet numbers were the same. We had an RD350 on the dyno a couple of weeks back and at idle, one side was much leaner than the other and that with all new OEm Mikuni jets. Swapped the lean one for one size larger and like magic, both sides were now OK.

On a triple it could also be timing differences between cylinders, or differences in compression ratios. Heads are not always identical and barrels can be different heights.


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PostPosted: Wed Apr 24, 2019 1:33 am 
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Joined: Mon Dec 01, 2008 2:52 am
Posts: 1228
Location: Manchester, UK
+1 to the above, sorry to hear about the melted piston, I did the same on my GT380 at wide-open throttle a few years back. In my case, it was the left-side piston that got a hole, because the engine's original left-side outer crank seal was weeping slightly (and had been for years, although the bike started and ran great). While it was fine for general (and quite hard) riding, unfortunately, it was just a little bit too lean for more than a few seconds at WOT.

The melted piston could also be caused by a little bit of dirt getting stuck in the centre carb's main jet - it really doesn't take much to melt a piston at high load / high rpm.

From my own bike (a 1976 380M with stock heads & porting), I found that running the stock timing (2.3mm / 2.25mm) was a little too far advanced, the plugs showed very slight signs of detonation, so I used to set the timing at the spec for the A/B models (2.05mm BTDC). This made no difference to performance and mid-range running, and completely eliminated signs of detonation.

As TZ mentioned above, Suzuki's factory timing data is a compromise between good mid-range running and top-end performance. More advance = more lively midrange, but also risks overheating at high rpm. Every engine is a little different, but I would suggest retarding the timing by 0.2mm when you rebuild.

Good luck!

_________________
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: Thu Apr 25, 2019 1:29 pm 
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Around the block

Joined: Tue Nov 07, 2017 3:25 pm
Posts: 51
Country: Portugal
Suzuki 2-Strokes: GP125, GT380
Hi to all and thanks for the replies.
I also had the inclination that the bike was running way too lean when I looked at the plugs.

If you look at the photos of the plugs on the previous post (page before) they seem to be a bit on the rich side and this is what is confusing me - that fact that no changes were made, no new fuel, no carb adjustments, no timing adjustments, etc.

I simply replaced the plugs and went for a ride. The riding style is mainly always the same.

Carlos


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PostPosted: Thu Apr 25, 2019 2:08 pm 
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Yeah Man, the Interstate

Joined: Tue Mar 10, 2015 12:35 pm
Posts: 531
Location: Wales
Country: WALES
Suzuki 2-Strokes: suzuki gt 550j
Its worth remembering that a proper plug chop can only be done with new plugs and the engine switched off at WOT. Then the plugs are removed and cut down. Examining the plugs just gives us an indication as to whats going on inside the cylinders. Also with modern fuels it is actually harder to read the results. It takes much longer for the burn to alter the plug colour. I can only suggest that perhaps you ran the bike for longer at WOT and the heat built up gradually.
You didn't add any 2t oil to the tank did you? Because this alters the mixture in the cylinder and can cause overheating as petrol is a cooling element too.

_________________
98 carb Blackbird and GT550j


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PostPosted: Sat Apr 27, 2019 9:57 am 
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Joined: Wed Jul 10, 2013 12:04 pm
Posts: 1087
Country: England, UK
Suzuki 2-Strokes: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S
Craig380 wrote:
From my own bike (a 1976 380M with stock heads & porting), I found that running the stock timing (2.3mm / 2.25mm) was a little too far advanced, the plugs showed very slight signs of detonation, so I used to set the timing at the spec for the A/B models (2.05mm BTDC). This made no difference to performance and mid-range running, and completely eliminated signs of detonation.

+1 on that. The only time I have ever put a hole in the middle of a piston was when I had experimented with advancing ignition timing. Knock it back a teeny bit and use a cooler plug is good advice.
You could also get your new piston crowns ceramic coated to help prevent overheating.


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