rattling sound under accleration

66ccmbjr · Sep 15, 2011

hey guys just got a new motor Grubee SkyHawk GT5 66cc for my bike.
I have ran two tanks at 16:1 now I am on my third tank of gas now mixed half 16:1 and half 32:1. The 32:1 was left over from my old motor.
My motor is making a little rattling noise under acceleration. Is this normal or is something about to break?

The_Aleman · Sep 15, 2011

Welcome to the forum!

I've had 2 of my motors do that, a Dax 66 and a GT5, both times was the upper conrod bearing going bad. Someone else might have more insight to offer.

66ccmbjr · Sep 15, 2011

Ok thanks for the input

mapbike · Sep 15, 2011

66ccmbjr said:
hey guys just got a new motor Grubee SkyHawk GT5 66cc for my bike.
I have ran two tanks at 16:1 now I am on my third tank of gas now mixed half 16:1 and half 32:1. The 32:1 was left over from my old motor.
My motor is making a little rattling noise under acceleration. Is this normal or is something about to break?

Yep Aleman hit the nail on the head here I do believe, sounds like the wrist pin bearing/upper bearing.

Don't run that engine anymore until you replace it because if it fails you will be rebuilding the entire top of that engine at the very least.

Here is a link to motion industries were you can at a very small price get a replacement bearing that is much higher quality than the ones that come in the engine and the ones most MB site sell as replacements because they are just the same stock soft weak bearings that may hold up a long time or may not.

here is the link:

Motion Industries - Product Search --

if it doesn't show the bearing info. just type KBK 10X14X13 in the (SEARCH BOX) and the choose (Manufacturer Part Number) in the next box and then leave the last one at (All Manufacturers) and just move over and click the GO button, you will now see the bearing details but NO Price will be listed you will need to either call or just go to a local Motion Industries and they will order the bearing for you, I ordered 6 of them back in june of this year and they were $3.69 each, there is a K 10X14X13 also but the KBK is the best of the two, get one of these in that engine and you wouldn't have to worry about that issue any more, and while you're at it you may as well pick up a couple of higher quality crank seals for when you need them for only $2.74

Seal # and description is...

SKF Sealing Solutions
17X29X5 HMS5 RG GREY NITRILE SEAL
17x29x5 RG Oil Seal - Metric SKF# 563933

Item No. 02508698

the picture that is shown of the seal on the motion industries site is the wrong picture so dont be concerned about that, but the seal is the correct seal.

Again.........!!!! DON"T.........!!! run that engine with that rattle in the top...... just go to one of the local vendors on the forum like Thatsdax.com and order a new (correct) either 6mm or 8mm stud size jug to case gasket if you dont have the tools to make your own and order the bearing from M. Industries and you can have that thing going again in no time.

Best wishes.

Peace, map

66ccmbjr · Sep 16, 2011

Hey mapbike thank you for all of the info.
I called motion industries and they do not carry kbk 10x14x13 they do have the k 10x14x13 so I went ahead and ordered that one.
Is there a guide some where on how to install these bearings? Any help would be great thanks.

bairdco · Sep 16, 2011

before you get too worried about it being bearings, check everything else, not only on the motor, but on the bike.

it could be your motor, but it could be as simple as a loose chainguard, or the muffler end cap, and as you speed up, it hits the right resonance to vibrate and rattle.

check the basics before you panic.

biknut · Sep 16, 2011

66ccmbjr said:
hey guys just got a new motor Grubee SkyHawk GT5 66cc for my bike.
I have ran two tanks at 16:1 now I am on my third tank of gas now mixed half 16:1 and half 32:1. The 32:1 was left over from my old motor.
My motor is making a little rattling noise under acceleration. Is this normal or is something about to break?

The_Aleman said:
Welcome to the forum!

I've had 2 of my motors do that, a Dax 66 and a GT5, both times was the upper conrod bearing going bad. Someone else might have more insight to offer.

I hate to say this but I just had a new motor fail with the same sympton. From day one, when it got up to about 5500 rpm it would start squawking something awful. Seemed to vibrate a bit too much too, but I'm not sure about that. At about 550 miles something gave out in there. I haven't got apart yet but I'm thinking it's a main bearing on the magneto side.

bairdco · Sep 17, 2011

lemme say it again, and start by saying all of the other responses are from experienced people...

but before you expect the worst, check the obvious,

you gave us one tiny paragraph of your symptoms, and everyone's assuming faulty bearings.

as speed increases, things vibrate. you might have a loose fender, or you might have a major engine problem.

what i'm trying to tell you, is start with the basics first. something's making noise? well, what kind of noise is it? could it be caused by a loose bolt, a rattling pedal, a fender strut?

or is it serious, like a main bearing, or a missing piston clip?

take your time and inspect your bike, and check the simple things first. that's rule number one in diagnosis class...

66ccmbjr · Sep 18, 2011

I looked over the bike and tightened a bolt on the muffler which seems to be the only thing loose. Took it out again and I still hear the noise which is some short of internal rattle. It gets worse with rpm so I believe it to be a bearing going bad as others have suggested.

I have ordered a new needle bearing so I suppose I will tear the motor apart and replace it when the new one gets in hopefully that will be it. Will keep you guys updated thanks for the advice and info.

66ccmbjr · Sep 18, 2011

O and I really only hear the noise when I am about half or more throttle.

thegnu · Sep 19, 2011

i think those guys are correct in their diagnosis , but sometimes a centrifugal clutch will do that when they are badly worn ,I experienced that on a mini bike an couple go carts in the past . I just thought I would throw that out there , centrifugals dont seem to be the way you guys roll tho .

mapbike · Sep 21, 2011

66ccmbjr said:
I looked over the bike and tightened a bolt on the muffler which seems to be the only thing loose. Took it out again and I still hear the noise which is some short of internal rattle. It gets worse with rpm so I believe it to be a bearing going bad as others have suggested.

I have ordered a new needle bearing so I suppose I will tear the motor apart and replace it when the new one gets in hopefully that will be it. Will keep you guys updated thanks for the advice and info.

your welcome and yes keep us posted on the results you find.

My last bearing change ended up that the bearing itself was in good shape but the wrist pin is what was wore real bad on one side which allowed the upper bearing to have enough clearance to knock on the top end, of coarse I would never reuse the bearing but it looked to be fine, I just replacd both bearing and wrist pin and now she purs like a kitten again.

Peace, map

PerryP180 · Sep 21, 2011

I had problems with a rattle like that. Searched all over the motor looking for it. I only heard it when the motor was under load going up a hill. Drove me crazy. Had to remove my saddlebags one day for some work. Amazingly to me, the rattle disappeared. Turns out it was a few tools in the saddlebags making the rattling. Live and learn.

66ccmbjr · Oct 6, 2011

Ok so I pulled the cylinder off and took the piston off. The bearing appeared to be fine bit I replaced it with the stage 6 extra wide 15mm bearing and I still have the noise.
I was told by a mechanic that it could be the piston slapping which had happened on a 2 stroke motorcycle he had. So he suggested a new piston and rings which worked for him. Gonna try that and see what happens.

mapbike · Oct 6, 2011

66ccmbjr said:
Ok so I pulled the cylinder off and took the piston off. The bearing appeared to be fine bit I replaced it with the stage 6 extra wide 15mm bearing and I still have the noise.
I was told by a mechanic that it could be the piston slapping which had happened on a 2 stroke motorcycle he had. So he suggested a new piston and rings which worked for him. Gonna try that and see what happens.

Good luck with the piston change, hope that cures the problem for ya...

Peace, map

66ccmbjr · Oct 26, 2011

So a new piston kit did the trick! My bike runs amazing now! I did the easy does it break in for the new piston staying under 20mph with a 36tooth sprocket for 4 tanks.

thanks for all the help suggestions and help.

mapbike · Oct 27, 2011

Glad to hear that you got that bug worked out, now just happy safe ridng ahead if all goes well.....

Best wishes

Peace, map

66ccmbjr said:
So a new piston kit did the trick! My bike runs amazing now! I did the easy does it break in for the new piston staying under 20mph with a 36tooth sprocket for 4 tanks.

thanks for all the help suggestions and help.

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rattling sound under accleration

66ccmbjr

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