Engine bounce

[Pretzelguy]

Hi everyone,

Maybe some of you have seen this before. When i hit the throttle on the go kart it seems to bounce the engine, I'm a total noob at this and was wondering if the chain tensioners may be a little tight? not sure, thoughts?

Im running a 41 chain, with a jack shaft and 8 HP engine.

Thanks for the help.

 

[USAMAC]

The video show's us what the motor is doing but can you take some pics of what's around the motor and how it's mounted? Sides, front and maybe even under neither? I don't know that kart, but I am sure plenty others will be able to name it quickly and how it's configured. Is it mounted with any kind of motor mounts or hard bolted to the frame?

Either way, I would certainly be checking to make sure that those bolts/nuts are properly secured. Looks pretty new and clean. Has it been ridden much?

 

[madprofessor]

That motor looks loose, either by the bolts, or by some squishy rubber motor mounts. It's not torque pulling on it (one direction), it bounces up and down. Acts like it's mounted on springs like a cast iron HVAC compressor. Looking at the valve cover compared to the edge of the dash panel it's clear that the motor's doing the moving, not the whole machine.
I had a chain skipping problem once from a motor mounting plate that was flexing down under load, although the motor was rigid. What's your mounting area doing? Bet that's not it. Try rigid bolting it metal-to-metal.

 

[Functional Artist]

Yup, I agree, that's NOT normal
...& a go kart engine MUST be bolted down solid

IIRC A lot of folks have gotten the bright idea to insert rubber washers/spacers, when mounting their engines (thinkin' it'll "smooth" things out)
...but, it just causes problems

 

[Pretzelguy]

I built the kart myself, there's 4 bolts with lock nuts going through to a plate that's welded to the frame. @madprofessor From what you're saying, maybe my mounting plate might be too thin? @USAMAC, its my first test run. The mounting plate is a little less than 3/16"

 

[Pretzelguy]

Yup, I agree, that's NOT normal
...& a go kart engine MUST be bolted down solid

IIRC A lot of folks have gotten the bright idea to insert rubber washers/spacers, when mounting their engines (thinkin' it'll "smooth" things out)
...but, it just causes problems

nope just plain steel washers. the bottom ones look funny because i got them off of an elliptical. they just were to added for thickness so it wouldnt bend under strain.

 

[Karttekk]

The driver pulley is wobbling as you rev the engine. What's up with that? Re-check for the correct belt, correct driver pulley bolt torque, sticking driver sheaves. Something out of balance or incorrectly installed possibly.

 

[Karttekk]

Bent crankshaft?

 

[madprofessor]

Looks like the sprocket side of the motor's bolted down tight right over some structural components, wouldn't be flexing down there. If that mounting plate is indeed flexing, it's only flexing upward, letting the motor rock on the rigid side (the fulcrum) with the force of the chain pulling down on the sprocket.
Not really important anyway, as I now believe (after a video rerun) the jerking motion (not bouncing) is only the symptom of the real problem, and I always try to treat the cause instead of just treating the symptom.
You can watch the lettering on the belt moving as the kart starts creeping along (shouldn't be moving). The lettering is moving in fits and jerks, not smoothly, and the motor flexes in perfect time with the jerking of the belt. You need to stop the TC from grabbing the belt that way, that is, first run it hard a little bit to wear in the belt some. If that doesn't do it, then you need to take apart the driver/clutch and lube everything EXCEPT the sheaves' shiny surfaces with dry lube ONLY, preferably a dry moly lube.
Get all of that correct, and then see if a little flexing of the mounting plate still concerns you or not. Fix the cause, vanish the symptom.

 

[Denny]

I agree with the above and would also like to add you did use a Genuine Comet belt didn’t you? Secondly that mounting plate seems a little flimsy for the engine and should be beefed up.

 

[Pretzelguy]

The driver pulley is wobbling as you rev the engine. What's up with that? Re-check for the correct belt, correct driver pulley bolt torque, sticking driver sheaves. Something out of balance or incorrectly installed possibly.

thats the bolt loosening up again. had to torque it a bit more and use some Loctite.

 

[Pretzelguy]

Looks like the sprocket side of the motor's bolted down tight right over some structural components, wouldn't be flexing down there. If that mounting plate is indeed flexing, it's only flexing upward, letting the motor rock on the rigid side (the fulcrum) with the force of the chain pulling down on the sprocket.
Not really important anyway, as I now believe (after a video rerun) the jerking motion (not bouncing) is only the symptom of the real problem, and I always try to treat the cause instead of just treating the symptom.
You can watch the lettering on the belt moving as the kart starts creeping along (shouldn't be moving). The lettering is moving in fits and jerks, not smoothly, and the motor flexes in perfect time with the jerking of the belt. You need to stop the TC from grabbing the belt that way, that is, first run it hard a little bit to wear in the belt some. If that doesn't do it, then you need to take apart the driver/clutch and lube everything EXCEPT the sheaves' shiny surfaces with dry lube ONLY, preferably a dry moly lube.
Get all of that correct, and then see if a little flexing of the mounting plate still concerns you or not. Fix the cause, vanish the symptom.

i will check it out this week. thank you

 

[Karttekk]

Use an impact wrench on the torque converter pulleys.

 

[Pretzelguy]

yup, done.

 

[anderkart]

I agree with the others but i also suggest that your chain tensioner should be located on the bottom/slack side, not the top/working side of your chain.

 

[madprofessor]

chain tensioner should be located on the bottom/slack side

I concur with that, mainly because of (1): nylon roller instead of idler sprocket, and (2): bracket's not welded down yet.
You could cut a new slot in the bracket down lower to tension the underside that goes slack under load, and replace that nylon roller with an actual sprocket that won't groove out or just wear out. Here's one I'm using..............
HEAVY DUTY ROLLER CHAIN SPROCKET IDLER 22 TOOTH #35 CHAIN 1/2" BORE. | eBay

 

[Pretzelguy]

I agree with the above and would also like to add you did use a Genuine Comet belt didn’t you? Secondly that mounting plate seems a little flimsy for the engine and should be beefed up.

i used whatever belt came with the torque converter (a 669 variable drive for motorcycle). I will stiffen up the plate. thanks for the advice

 

[Denny]

When you shread that belt, and you will. Replace it with a Genuine Comet. It will save you from so many problems in the future and will outlast the cheap ones 10-1.

 

[Pretzelguy]

are all belts the same size for torque converters?

 

[madprofessor]

Many, many different belts, chosen by measurement................
Comet Drive Belts | Comet Torque Converter Belts | Go Kart Belts (gokartsupply.com)