GT380 spluttering at 3000RPM

[CFS]

Hi All,
I've had some time to install the refurbished crank in the 380.
As mentioned in previous posts in the repair forum section, the crank bearings were incorrect (no locating pins) and the conrods had small ends for 16mm gudgeon pins.
The crank bearings are held in place with glue similar to bearing loctite and IMD supplied me with pistons to suit a GT380 but with 16mm gudgeon pins (I assume these to be for a GT250).
The bike starts easily and revs under NO LOAD throughout the rev range. On the road it will not rev beyond 300RPM and will start spluttering. The symptoms are the same in all gears.
Prior to the crank/piston change the bike pulled fairly well in all gears (no real issues).
The carbs and jetting are the same as before. Timing is set as per the manual and runs on a Newtronic ignition.
I have removed the head and everything seems ok except that the pistons and bores are bronze in colour.

Can anyone help with what the possible issue may be please.

Many thanks
Carlos

[Alan H]

Timing wrong, or carbs blocked?
Make sure your timing electronics have good voltage to them - if it drops below 12v, it will run, but weak sparks will cause problems as you describe, so check battery voltage to start with during starting & running as revs rise. Bad/dirty wiring contacts between battery & Newtronics plus battery & earth can cause all sorts of issues.

[CFS]

Hi Alan,
Thanks for the reply.
I have just checked the battery and it's over 12V with the ignition on. I can't check whilst working as the head, carbs and exhausts are off but the regulator/rectifier is new so I think all's ok.
The wiring and plugs were recently redone (contacts cleaned, plugs changed where required) so again all looks good.
The timing was set with a dial gauge and I've just re-checked and this is spot on.
The carbs were ultrasonically cleaned, some jets changed for new Mikuni jets and set as per manual.
I have attached photos of the pistons/bores. Never had any piston/bore with this colour before. Almost looks like excess oil.

Anything else that comes to mind?

Thanks
Carlos

[CFS]

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[Alan H]

Just curious as to what settings on the dial gauge were used.
Firing should be 24 degrees btdc, or 3.37mm btdc.

https://www.oldjapanesebikes.com/mraxl_ ... /page-A501

[CFS]

Hi Alan,

Thanks for the link.
That particular link is for the 550 but through it I found the specs for the 380 (attached).
Below i have listed the values BTDC for a 76 (A) model which is what mine is
L&R = 2.09mm
C=2.05mm
This is the same information I have and is the current setting of the timing and don't think it's the problem.

380A_Timing Spec.jpg

Anything else i can check?
My next move will be to re-strip and install the old crank and pistons to eliminate them as the problem.

Thanks
Carlos

[karl pa]

What jetting are you running? I would try different size jets before going deep in the engine, If I remember correctly that engine has holed a couple pistons, I wonder if the color you are seeing might be from heat, from a lean mixture.

[CFS]

Hi Karl,
Yes, you are correct I holed the centre piston a few years back but I don't believe it was due to jetting.
I'm running the same jetting for over 20 years. It's all standard except for the mains which are 100 instead of 80 due to pods and chambers.
I have gone ahead and opened the motor. It definitely wasn't running lean.
I will reinstall my old crank and pistons and hopefully it will be back to running ok.
I'll post some photos and queries once done.

Thanks
Carlos

[dollydog]

i've had a very similar problem. you can rev the ar$e off it standing still, but under load it dies very quickly. turned out to be the coils - or one of them - i changed the coils and all was good
cheers, dd.

[CFS]

Thanks DD,
I'll check that out.

Cheers
Carlos