GT380 not revving out under load

[CFS]

Hi all,

I'm hoping someone on the forum will be able to shed some light on the problem I'm having.
The bike revs to red line easily in neutral and will also rev to red line in first, second and third but, will only rev to 4000/5000 in fourth. Fifth and sixth react the same as fourth.
I have tried the 380 carbs (VM24) and also tried a set of 550 carbs (VM28) but no difference.

The bike is a GT380 A model(1976) running on a Newtronic ignition.
Carbs have been ultrasonically cleaned, set to spec and synced (both).
Ignition is set as per manual using a dial gauge and there is good spark.

I have tried retarding/advancing timing to see if this improves but to no avail. I trust someone will have a possible solution.

Thanks
Carlos

[karl pa]

Is the tank vent and exhausts open, unrestricted ?

[CFS]

Hi Karl,
Yes, the exhausts are unrestricted. As for the tank vent I'm not too sure but have a new cap to try over the Xmas break.
Will post a reply once I've tried it.

[CFS]

Curiosity has got the better of me.
Although I have purchased a new cap due to a completely different reason, why would venting issues only affect upper gears and not the lower gears in the upper RPM range?

[karl pa]

It takes a little till the vacuum in the tank builds enough to hold fuel from flowing, you don't stay in the lower gears long enough to see. A quick way to check for restricted vent is to open cap while it is acting up.

[CFS]

Hi All,
Took the bike out today with the new tank cap and it faired no better than before.
It will rev fairly easily in 1st, 2nd, 3rd and 4th to around 7000RPM. Between 7 & 9000RPM in the lower gears is a struggle and it hits a wall at around 5000RPM in 5th, with 6th being even worse. This is with the throttle fully open.

Presently it has the 380 carbs on it with the same jets and settings it has always had.
The motor has had a rebore, new pistons & rings fitted and done around 350km.

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

[jabcb]

With the fuel lines disconnected, check to see how quickly the fuel flows on the prime setting. If that is good, then try running the bike on prime. Just make sure to set the petcock on run after doing the test.

[CFS]

Hi Jacb,
Thanks for your input. The tank and tap on the bike are not standard.
The tap has an on and off setting and no prime like the standard tap.
I have checked the fuel flow and I dont think it's restricted there is a good strong flow.

As previously mentioned the only change was the top end repair.

Thanks
Carlos

[tz375]

What changed between the time it ran fine and now? That might help us to point you in the right direction.

It could be as simple as crank seals failing from age or rings not sealing if it was rebored or rodent/insect nests in the pipes or air box, or timing being way off, or jets still partially blocked.

I would start with a compression test and then if that is OK, a leakdown test. Then I would run it hard through the gears and do a plug chop to see what the plugs look like.

[Alan H]

Clean the baffles and replace air filters. New genuine plugs of the correct grade and make sure timing is spot on, on all 3 pots.

[CFS]

What changed between the time it ran fine and now? That might help us to point you in the right direction.

It could be as simple as crank seals failing from age or rings not sealing if it was rebored or rodent/insect nests in the pipes or air box, or timing being way off, or jets still partially blocked.

I would start with a compression test and then if that is OK, a leakdown test. Then I would run it hard through the gears and do a plug chop to see what the plugs look like.

Hi Tz,
In my last post a couple of years ago, I mentioned that I holed the centre piston. Since then I rebored, fitted new pistons, rings and tidied/matched the ports so that the sizes/dimensions were similar/closer.
Prior to reinstalling the motor I done a compression check and leak down test (all good).
The bike hasn't been ridden much, done around 350km since the re-bore.
Not much done/modified to warrant what I'm experiencing.

There are no rodents residing in the pipes and there is no air box (pod filters). The bike lives in a garage that is in constant use.
As previously stated in a previous post I don't have any roads nearby where I could ride the bike safely to undertake a plug chop.

I can redo the leak down and compression test, ultrasonically clean and reset the carbs, redo/check timing, remove the head and check clearance between rings and bore, etc, etc, etc but, was hoping of a quick & simple fix.
Wishful thinking on my part!

[CFS]

Clean the baffles and replace air filters. New genuine plugs of the correct grade and make sure timing is spot on, on all 3 pots.

Hi Alan,

Pipes were cleaned and insulation in the baffles replaced around 400km ago so I don't think that is the problem.
Air filters are K&N pod filters recently cleaned and oiled.
Spark plugs are new, correct grade (B8ES) and I assume genuine as these were purchased from a reputable supplier.
Timing was set with dial gauge and I think is pretty spot on on all 3 pots. Neutronic electronic ignition is in use and I assume this to be ok - good spark on all 3 pots.

[tz375]

After the meltdown, you bored it and reassembled it, but how did you flush out the cases? I would re-do compression and leakdown tests to see if a bit of debris tore a crank seal.

[CFS]

The motor was practically completely stripped down and cleaned/washed out and checked/rechecked, but as you say rather safe than sorry.
I'll do a compression test & a leak down test to remove any doubt.
Will post findings once done.

[CFS]

Done a compression test and all 3 cylinders - 10 Bar/140Psi.
The leakdown test gave the following results:
Right & Left cylinders show no drop after 1 hour at 6 Psi.
Centre cylinder dropped from 6 Psi to 5 Psi after 10 minutes and dropped further to 4 Psi after 1 hour.
I sprayed soapy water around the exhaust and inlet bungs - no leaks. No leaks also noted at base gasket or head gasket so assume the leak to be at the crank seals.

As I've had to remove the exhausts & carbs for the breakdown test, I will replace the waddling in the baffles, re-clean & set the carbs and re-check/set the timing

Would the loss in pressure on the centre cylinder only cause the symptoms I'm experiencing?

Thanks
Carlos