Lacking Power

[Glyn.G]

Hi again,
I have recently rebuilt a GT380 replacing many major components.
That is :- New pistons, rings, pins, and small end bearings.
Rebuilt Carbs with replacement kits bought online, jets, screws, needles, gaskets, etc. Also new intake rubbers as old ones were cracked.
New points and condensers & plugs.
The big end bearings are fine with no up and down play at all.
The engine runs ok at idle and will rev into the redline with ease with no audible rattles or thumps.
I had a few problems with the timing at first but that seems ok now. I set the before TDC to 2.1mm using a dial gauge ( as suggested on this site ) and set the points to just opening with bulb connected to the corresponding points. The timing propeller marks also line up perfectly.
My problem is that after reaching a speed of 65mph the engine seems to be strangled of power and remains at that speed. Up to 65mph the engine seems to pull fine ( yet will not red line ). I know its not recommended to red line the engine when in gear, but it would be nice to know it will.
Has anyone got any suggestions.
Ps, What should the top speed of a GT380 be.

[Craig380]

Hi, are you running the standard airbox and filters too?

With an average rider (i.e. about 13 stones in weight), a healthy, standard 380 should pull round to 8,000rpm in 5th gear easily on a level road in neutral conditions, with the rider sitting normally. On standard gearing, this is about 90mph.

6th gear is an overdrive on a standard bike and used only for more relaxed cruising. If you want to accelerate at all, knock it down to 5th, unless you're on a slight downhill or with a tailwind

Sounds like something's not right. What colour are the spark plugs - not the porcelain, but the metal ring that the earth electrode is welded to? If that's a dry, sooty black, or a wet black, the carburation is too rich. You're looking for a medium brown colour like strong coffee with a splash of milk.

Also, which pistons were used? Pistons for Suzuki X7s will fit, but they have a different gudgeon pin to piston crown height, which lowers compression quite a bit. The bike will run with these, but will be gutless. Can you borrow a compression gauge? With the engine hot, throttle held wide open (ignition OFF) and after 6 or 7 kicks in quick succession, you should see at least 125 - 130psi with no more than 10% difference between each cylinder.

Oh, one final thing to check - make sure the inlet snorkel to the airbox isn't being blocked by anything. A while back I'd put an old polishing rag by the battery, then forgot about it and closed the seat. Next ride, the bike started and ran fine, but went 'wuuuuuuuurrrrrrggghhh' every time I tried to accelerate over 50mph. Took me a while to realise the forgotten rag was being sucked up the inlet snorkel when I opened the throttle

[Cragdog]

I am having a similar issue with my 550.
It only seems to occur while riding freeway speeds, usually 5th gear around 5k-6k rpm or 70-80mph the bike hits a wall with power, there's plenty of throttle play available, but if I pull for more the engine sounds starved, almost the same as if it's running out of gas like when you need to switch the petcock over to reserved. It doesn't always happen because 70-75mph is usually my cruising speed so I haven't investigated much, but when it does I have to back off the throttle slightly until it is comfortable once again. In my mind it was a fuel delivery issue, I think there was a service bulletin for the problem and some of the pre 74 petcocks had to be modified for better fuel delivery at high speeds. I will search for it and post a link when I find it. That's where I was going to start with mine and go from there, maybe yours is the same issue?

[Cragdog]

http://www.oldjapanesebikes.com/mraxl_G ... etins.html

Found it, Service bulletin GT-21


Not sure exactly if it's the problem we are both experiencing, but worth a look.
What year is your 380? My 550 is a 74, but I'm using a petcock I bought off eBay so I'm not sure what year it is from and might need the modifications mentioned in the bulletin. I'm going to try and take a look this weekend and report back if it changes anything. The bulletin says that if your petcock has yellow or blue dye on the vacuum side then it should already have been corrected.

[Coyote]

It sounds to me like your carb slides are not topping out like they should. With the engine off and airbox removed, look through the throat of the carb and pull the throttle wide open. No part of the slide should be below the full circumference of the hole of the bore. When properly adjusted, at wide open throttle the slide should be 1mm above the carb throat.

[Cragdog]

Coyote - that's also a good point and definitely worth checking out.
But if there is plenty more throttle to be had when the problem arises (approx 1/4 - 1/2 more available in my case) that means the slide hasn't fully opened but the issue still occurs. I could see how if the slides are topping out prematurely it could cause this problem, but since there is more slide movement available I don't think this is where the problem lies, at least in my case. Still going to check this though. Maybe this is the case for Glyn.G?

Glyn.G - When you experience this problem are you at wide open throttle or do you still have more throttle to play with when it happens?

[Vintageman]

Use factory timing (but if you propeller marks line up you may be?).

Verify gas tank cap venting not plugged.

run iridium plugs in case ignition weak. I now run expenasion chambers an d forget how stock exhuast was ( Regardless could not get much above 8000 with standard tip. I now get +9500
Those are just some other things, but seams like more wrong if you only can only go 65. I can go ~100 mph which probally more like 95mph due to speedo error

Whose carb kit (keyster or cruzinimage?) Did you verify Jet needle the same shape as OEM? Keyster's may be OK since 72-77 GT380 only use done jet needle style (what year is yours) so far cruzinimages carbs kits are way off (but I haveenot tried Gt380 kit)

Main jet and Pilot I always use genuine Mikuni. I would not change needle jet unless OEM show signs of wear

+1 on Carig380 challenging what piston you used.

You say top end quite? Maybe pop the head and take a look see how things are breaking in and looking into exhuast / intake port for scuffing.

[tz375]

I would throw out the Jet Kit jets and needles and order new genuine Mikuni jets and refit the original needles and try it again.

[Glyn.G]

Hi,
Thanks for all your advice, here's a few answers to your questions.
The bike is a 1977 model, and when I hit the power wall there is still plenty of throttle movement left. The throttle slides are also fully opening as shown in the photo.
The plugs are a nice coffee brown but I am running it a bit on the rich side ( 1 1/8 turns out on the air pilot screw ).
I got the piston kits off e-bay from JTS motorcycles Swansea, the advert stated they are for a GT380 but will also fit a GT250 & X7.
The carb kits were from AW motorcycles Doncaster which I am going to trash and try replacing the old ones I took out as they did not look worn.
I have a set of Higgspeed expansion chambers arriving soon as the originals may have fallen to bits inside.

[Glyn.G]

Well I've just changed back to original jets and needles to double check. The carb slides are opening fully and floats seem to be at the correct height, yet made no difference at all.
I have just noticed though that the right hand exhaust seems to be blocked in some way, as I rev the engine I can feel no emissions on the palm of my hand. Think I'll wait till my new ones arrive, but those after market pistons do sound as if they could be the problem.

[tz375]

Sounds like it's time to remove the blocked pipe and see what rodents or wasps nests are hiding in there.

[Coyote]

A blocked or badly clogged exhaust will cause the exact symptoms you are experiencing.

[GT750Battleship]

++with Coyote,I'm betting those pipes & baffles just need a good clean,one of the most overlooked problems with 2 strokes,I try to look at my baffles every 5,000km,this also ensures that they don't get stuck in there if ignored for too long
Cheers,
Roger

[Glyn.G]

Cheers again for the advice, I cleaned all the baffles before I put the exhausts back on the bike, but there are a few distinct rattles inside the silencers. If I remove the exhaust and take the baffle out then look down it I cannot see any daylight through it. I think it has fallen to bits inside. I have also ordered a compression tester to check all 3 cylinders.

[GT750Battleship]

Yes,a common problem with the internal baffle guide plates collapsing with age ! I'm surprised you're not having difficulty getting the baffles back in the pipe !!
I had to get the top righthand pipe on my 750 opened to sort out an internal problem,
"Expensive" but I had no choice ! Delkevic Replica pipes not being available at the time,unfortunately I hope the compression check helps you fix the problem
Cheers,
Roger
Suzuki GT750A