Go Back   DIY Go Kart Forum > Building Plans And Advice > Mini Bikes, Trikes, and Choppers

Notices

Reply
 
Thread Tools Display Modes
  #1  
Old 02-15-2020, 01:00 PM
ghost civic ghost civic is offline
Senior Member
 
Join Date: Mar 2015
Location: maine
Posts: 254
Thanks: 0
Thanked 14 Times in 12 Posts
Default 70CC engine problems

I got my trike built from a 70cc kids atv
It was running good
I had to remove the carb to adjust the float, and at the same time I changed the oil. The engine had a 1/8'' layer of caked stuff at the bottom after draining the oil. Put new oil in and figured it would be fine.

very shortly after that, it started to die any time you gave it throttle. It would idle and always start back up fine but got so that it couldn't even move.

The carb on it was old, and I figured a new one would solve the issue. Put a new carb on today, it started right up but then died when I gave it throttle. I checked the valve lash...both were tight but since it was able to start and idle I figure they couldn't have been too tight. I gave them both a bit of play but the bike still would not move. I adjusted the mixture screw on the carb and off it went...but only for 100' or so, then started acting up again. No adjustments to the carb made it any better, it actually got so it would start, idle for 10 seconds or so and then die.

I went to check the valves again and noticed a hole in the engine which would lead to the timing chain cavity.

I am wondering, would a hole there prevent oil from being pushed up to the valve train? I don't honestly know what moves the oil to the valves/rockers...and I'm wondering if either the hole is preventing oil from flowing there, or if that caked up stuff I found when changing the oil ended up blocking an oil passage.

The picture shows where the hole was, I just put high heat stick weld over it.
Attached Thumbnails
0215201437.jpg  
Reply With Quote
  #2  
Old 02-15-2020, 01:10 PM
JTSpeedDemon's Avatar
JTSpeedDemon JTSpeedDemon is offline
2019 Build Off Winner!
 
Join Date: Jan 2019
Location: Texas
Posts: 3,401
Thanks: 774
Thanked 523 Times in 476 Posts
Default

That hole was certainly a problem.
But I don't think that was the cause of your erratic running.

I have weird erratic running issues with my kart, too. I now know that it's because of a worn throttle shaft causing a vacuum leak.

Obviously your throttle shaft is fine as it's a brand new carburetor, but check the intake manifold and all gaskets. That's probably where your problem lies.

Oh, and try to find the ACTUAL spec for the valve lash instead of guessing.
__________________
Currently Flathead-Crazed
Reply With Quote
  #3  
Old 02-15-2020, 01:34 PM
Texan's Avatar
Texan Texan is offline
Senor Member
 
Join Date: Oct 2016
Location: Deep In The Heart of Texas
Posts: 453
Thanks: 196
Thanked 224 Times in 176 Posts
Default

A small hole on the Cam cover would not cause erratic behavior.

These engines have an oil pump, so if it was really dirty as you say, the pump may be clogged and causing your valves to overheat from lack of oil.
__________________
Whatcha says no good.?
Reply With Quote
  #4  
Old 02-15-2020, 03:49 PM
ghost civic ghost civic is offline
Senior Member
 
Join Date: Mar 2015
Location: maine
Posts: 254
Thanks: 0
Thanked 14 Times in 12 Posts
Default

Quote:
Originally Posted by JTSpeedDemon View Post
That hole was certainly a problem.
But I don't think that was the cause of your erratic running.

I have weird erratic running issues with my kart, too. I now know that it's because of a worn throttle shaft causing a vacuum leak.

Obviously your throttle shaft is fine as it's a brand new carburetor, but check the intake manifold and all gaskets. That's probably where your problem lies.

Oh, and try to find the ACTUAL spec for the valve lash instead of guessing.
You wanna guess what part of the old carb I reused? throttle shaft/slide. Put the new one on and she ran great...for about 5 minutes...then she died at idle. Had trouble restarting. Tried messing with the carb a bit and hit the starter button and suddenly the engine sound changed...starter motor was spinning faster.
Put a compression tester on it, reading only 25psi!
Checked the valve lash, both seemed good.
Looked at the timing chain, looked good.
BTW, both the exhaust rocker cover and the timing chain cover showed fresh oil...so the top end looks to be getting oil.

Was about to give up, thinking a ring must have gone...put the spark plug back in, hit the button and she fired back up. My son drove it around for 10 minutes with no problems.

Now this engine has smoked some since I got it, and while the problem I first posted was almost certainly the damaged throttle cable causing a vacume leak (which at times could go away based on the strain on the cable shroud) the engine must have worn valve guides...I'm betting that is why it burns oil and when the compression dropped that the exhaust valve stuck slightly opened and it freed itself up.

I will have my son give it a good thrashing tomorrow...more than 10 minutes to see if it holds up. Higher temps could cause the valve to stick again. Not sure I can do anything for it without pulling the motor out.
Reply With Quote
The Following User Says Thank You to ghost civic For This Useful Post:
JTSpeedDemon (02-15-2020)
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 03:25 AM.