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Re: GT380 not revving out under load
Posted: Wed Dec 29, 2021 4:55 pm
by karl pa
The leak down results look good to me. Did you figure out why you holed the center piston before?
You say you are running pods, did you increase carb jetting?
Re: GT380 not revving out under load
Posted: Thu Dec 30, 2021 7:58 am
by slosher
CFS wrote: ↑Mon Dec 20, 2021 12:33 pm
Prior to the rebuild it had no problem reving to around 10K in most gears.
I hope that someone out there has a possible solution and hopefully it will be a simple fix.
Thanks
Carlos
Hi Carlos after reading this post mate IMO that's your initial problem they are not really built to sustain that sort of rpm range.
Having said that it sounds like a exhaust blockage of some kind sounds like it's been restricted check your 2 lowers
Has it got joiner pipes on the headers
Just noticed it's an "A" so assuming no crossover so as above check your 2 lowers
Cheers Dave
Re: GT380 not revving out under load
Posted: Thu Dec 30, 2021 9:32 am
by tz375
Compression and leakdown are fine. That's a start and it eliminates a couple of possibilities.
If the pipes are still off, pull the baffles and shine a flashlight from each end and see if there's anything in there that shouldn't be there. You could also try to pass a long length of fence wire (reasonably stiff) from the outlet end and see if it will pass through or if it gets stuck on a nest of some sort.
You could also try to use a garden hose for that trick and it it gets stuck, turn it on and see if the muffle fills up or pours out. Repeat on the center header pipe.
I wouldn't remove the head or barrels because they should be OK with those Compression and leakdown results.
If that's all clear, test the sparks on each cylinder to see that all 3 are similar fat sparks. It is possible that the Newtronics unit is breaking down under load or a a plug or plug cap.
Then I would clean out the carbs again. This time use a can of WD40 (safer) or carb cleaner and blow through each carb circuit - not just the jets. Do one jet or drilling and repeat on the other carbs for comparison. Then move on to the next one.
It sounds like a main jet is blocked in one cylinder or a choke plunger leaking. How do the plugs look after a blast?
Re: GT380 not revving out under load
Posted: Mon Jan 03, 2022 1:40 pm
by CFS
karl pa wrote: ↑Wed Dec 29, 2021 4:55 pm
The leak down results look good to me. Did you figure out why you holed the center piston before?
You say you are running pods, did you increase carb jetting?
Hi Karl,
No, never figured out why. It's a strange one. I went out a few times to try alternate plugs as per a recommendation. No settings were changed from when the bike was parked up or between rides. The ride prior to the bike being parked up showed the plug colour in my opinion ok (tan). For the first run I installed new plugs of a different heat range, and these showed the bike running lean (white plugs). For the seciond run I again installed new plugs with a different heat range and this run unfortunately produced a holed piston but the plug colour was also lean (white). Some photos at this link
viewtopic.php?f=5&t=13497
Have always run pods and expansion chambers. The mains on the 380 carbs were increased from 80 to 100 and the plug colours always seemed ok (tan).
Presently re-cleaned and set the 550 carbs which have now been installed on the bike and are all on the standard jets except the mains which are at 100 on all 3 carbs.
I'll post more once I'm able to take it out for a ride.
Re: GT380 not revving out under load
Posted: Mon Jan 03, 2022 1:48 pm
by CFS
slosher wrote: ↑Thu Dec 30, 2021 7:58 am
CFS wrote: ↑Mon Dec 20, 2021 12:33 pm
Prior to the rebuild it had no problem reving to around 10K in most gears.
I hope that someone out there has a possible solution and hopefully it will be a simple fix.
Thanks
Carlos
Hi Carlos after reading this post mate IMO that's your initial problem they are not really built to sustain that sort of rpm range.
Having said that it sounds like a exhaust blockage of some kind sounds like it's been restricted check your 2 lowers
Has it got joiner pipes on the headers
Just noticed it's an "A" so assuming no crossover so as above check your 2 lowers
Cheers Dave
Hi Dave,
The motor has got mild porting, skimmed head and runs on expansion chambers and these are not blocked/restricted.
Before it ran a big end on the centre pot a good few years ago it could rev to 10K in all gears without any problem. I understand that these are not designed to sustain this sort of RPM but for a short burst it should be ok.
Cheers Carlos
Re: GT380 not revving out under load
Posted: Mon Jan 03, 2022 2:01 pm
by CFS
tz375 wrote: ↑Thu Dec 30, 2021 9:32 am
Compression and leakdown are fine. That's a start and it eliminates a couple of possibilities.
If the pipes are still off, pull the baffles and shine a flashlight from each end and see if there's anything in there that shouldn't be there. You could also try to pass a long length of fence wire (reasonably stiff) from the outlet end and see if it will pass through or if it gets stuck on a nest of some sort.
You could also try to use a garden hose for that trick and it it gets stuck, turn it on and see if the muffle fills up or pours out. Repeat on the center header pipe.
I wouldn't remove the head or barrels because they should be OK with those Compression and leakdown results.
If that's all clear, test the sparks on each cylinder to see that all 3 are similar fat sparks. It is possible that the Newtronics unit is breaking down under load or a a plug or plug cap.
Then I would clean out the carbs again. This time use a can of WD40 (safer) or carb cleaner and blow through each carb circuit - not just the jets. Do one jet or drilling and repeat on the other carbs for comparison. Then move on to the next one.
It sounds like a main jet is blocked in one cylinder or a choke plunger leaking. How do the plugs look after a blast?
Hi TZ,
The pipes are clean and no restrictions either in pipe or baffle. I have replaced the waddle in all 3 baffles.
It sparks on all 3 cylinders and they look the same. The plug caps have recently been replaced and I have installed new plugs.
When I stripped the carbs I didn't notice any blockages. Choke plungers seem to be working properly.
I have stripped and cleaned the 550 carbs which are now fitted on the bike. I have also replaced the carb to motor rubbers with new units.
Pod filters gave been re-cleaned and oiled.
When possible will take it out for a good burn and post my findings.
Cheers
Carlos
Re: GT380 not revving out under load
Posted: Sat Jun 11, 2022 10:46 am
by CFS
Hi all,
I know its been a while since I wast last here but only recently have I been able to take that test ride.
Firstly I would just like to say that the carbs were cleaned, pod filters cleaned & oiled, timing checked, new fuel (95 unleaded) and no blockages in exhausts.
The bike pulls well from 1st to 4th up to 8000 RPM (it doesn't really pull much beyond 8K).
When 5th is engaged it starts to loose power & the revs drop to around 5000 where they will maintain. The throttle at this time is around the 3/4 position. Opening the throttle further has no effect and closing it has also no noticeable effect (except losing more revs).
Engaging 6th gear worsens the situation further as it starts losing more revs.
Am at a loss as to what may be causing this and am hoping that someone on the forum can identify the possible problem and recommend a proposal for repair.
Many Thanks
Carlos
Re: GT380 not revving out under load
Posted: Sat Jun 11, 2022 11:22 pm
by Alan H
The only thing I can suggest, is to remove the baffle wadding and try without it. I have a 550 on expansion chambers without any wadding and it revs well in all gears. I realise it's going to be loud, but just to try it. In the lower gears, restrictions in the exhausts will have less effect, but will restrict power as you get to higher gears (and speed). Are you on genuine NGK B8ES plugs?
Re: GT380 not revving out under load
Posted: Sat Jun 25, 2022 3:46 am
by CFS
Thanks Alan.
Surely this cannot be a permanent fix! They are already so loud can only imagine the noise without the wadding.
I'll remove the wadding and see how loud it is before taking it out on the road. The last thing I need is a fine & the bike taken off the road.
I assume you don't ride yours without the wadding. What would a permanent fix be?
The plugs are new done approximately 150km and were purchased from a reputable supplier so assume they are genuine.
I'll post more should it be possible to take it out on the road.
Carlos
Re: GT380 not revving out under load
Posted: Sun Jun 26, 2022 6:24 am
by Craig380
Hi, have you tried running without the K&N filters to check if they could be causing the problem? If the filters have the metal end cap (or a solid plastic end cap), they can cause weird problems with the carburation because the cap can reflect the pressure wave which travels back through the carbs and affect the fuelling.
Re: GT380 not revving out under load
Posted: Sun Jun 26, 2022 2:55 pm
by CFS
Thanks Craig.
No, I haven't tried running without the K&N filters.
The weird thing is that I've always run K&N filters ever since I've had the bike (1982) and never had any issues.
Have run K&N's and expansion chambers for approximately 20 years, again no issues.
These K&N's have the metal end cap. I'll definitely give it a bash when possible and let you all know the outcome.
Carlos
Re: GT380 not revving out under load
Posted: Sun Jun 26, 2022 5:30 pm
by Evans Ward
Single layer UNI filters are what most 2 stroke guys use. They are the clamp on type but are are filter including where your K&N’s have the back chrome covering.
Re: GT380 not revving out under load
Posted: Mon Aug 29, 2022 3:13 am
by CFS
Well, finally got the chance to try a run without the K&N's and it doesn't bog down like before in the upper gears and will pull to around 7K in 5th & 6th. Still not going as it should but hopefully will get there soon. Now need to budget and get some foam filters.
Didn't try running without the wadding as it's way too loud.
Any further comments/help will be greatly appreciated and thanksl for all the help.
Cheers
Carlos