800 Mile Engine Running Very Slow Then Fast (??)

[Shyzah]

Unimportant background story: Engine is from 2015 and my bike once had a shift kit, nitrous bottle, "performance" carb, sbp exhaust, and other goodies on it. After it hit 760 miles the shift kit failed and I decided to take it easy on the engine and convert it back to stock to have a more reliable set up. Bike sat for over 1 year while I was enjoying my second bike. Bike has been on 25 miles cruises to the beach and back many times. After I got it running again, it sounded like someone got a hammer and would hit the engine every 10 seconds. Took of the head and nothing inside. After idling for 30 minutes the banging cleared up and it was idling like a new engine again.

My Problem: Engine is VERY sluggish wont go above 15mph sometimes. Then other times it will run very strong and hit 32 mph. When its running strong it has a hard time revving down as if there is an air leak. If its running strong and I turn it off and leave it for 1 minute and turn it on it will be running slow again. When its running weak it runs like there is no air leak but it bogs when given gas and then slowly revs up. I've adjusted the float bowl, bought two new carbs, resealed the aftermarket intake, clean fuel 91 octane w/ royal purple synthetic oil 1:24 ratio. I've checked for air leaks by waving propane around the engine and it made no difference in revs (also tried water). I'm very lost. I've been trying to fix this issue for 5 months now. Compression is 110 PSI.

Could it be one of the gaskets inside the engine went bad?

Also, I deleted the nitrous bottle and I have the tube for it that goes into the intake plugged with a screw. There is no air leak from that from what I can tell with the propane test.

 

[Shyzah]

bump

 

[GTEFGHA]

Maybe a cdi? Even tho if it was a cdi it would probably just die. Other than that I have no idea.

 

[GTEFGHA]

bump

Maybe ask dragonfly75, he knows a lot more about these engines than you and I.

 

[Shyzah]

Maybe a cdi? Even tho if it was a cdi it would probably just die. Other than that I have no idea.

Nice 86. I have replaced the CDI as well. I'll ask dragonfly75. Thanks.

 

[Jug]

Check your magneto for oil in it, you could have a blown crank seal.

 

[Shyzah]

Below are photos and videos of it running as requested.

Ok so I believe I have found what is causing the engine to run slow sometimes then as if it's on steroids other times. Worked on it with patience today and found some stuff. Below are photos and with captions labeling what they are.

What do I believe the problem was? Ignition timing. I had a pull start kit on the bike before and ditched it around 300 miles ago when it failed. I didn't put the nut back on for the magneto magnet. After popping the cover off I decided to see if the magnet was still being held in properly by the key. Turns out it had some wobble in it! I decided to put the magnet as advanced as I could with the few degrees of play that it had developed over time and put the bolt back on (found it in the parts bin). The engine was always running like it was on steroids. It would NOT run slow after restarting. I even took the head off and inspected the spark plug a few times then restarted and it was still being powered by buff horses.

I ran the bicycle for 25 miles today and the spark plug looked a bit lean. I'm going to drop the adjustment on the needle by one or two. I kept my RPM in the middle high -> low range. I did see how far it would rev out and it hit 30.1 mph on stock 44 tooth. When the engine was new top was 22mph on 44tooth. I'm going to get a 38 or 40tooth sproket to run at lower RPM to save my engine.

My set up:
20200515_092952 by Shyzah, on Flickr
20200515_093014 by Shyzah, on Flickr
20200515_093004 by Shyzah, on Flickr
2020-05-15_06-50-47 by Shyzah, on Flickr

Engine Inspection (@805 miles)
(Carbon cleaned out at 720 miles 2 years ago)
What I fixed today: Head bolts were going loose and caused head gasket leak. Re tightened to 16 ft pounds.
Weird cylinder damage from a long time ago is still there.
20200515_104023 by Shyzah, on Flickr
20200515_104009 by Shyzah, on Flickr
20200515_103749 by Shyzah, on Flickr
20200515_103745 by Shyzah, on Flickr

Popping the magneto cover off
I found some gasoline/liquids inside the cover.
Brake cleaner sprayed on to clean everything.
Magnet had play in it. Put it as far as it will advance (a few degrees) and put the original bolt on. (It was not on since ~300 miles ago).
20200515_093317 by Shyzah, on Flickr
20200515_092943 by Shyzah, on Flickr

Spark plug inspection
When bike is running like trash:
20200515_102705 by Shyzah, on Flickr

When bike is running fast:
20200515_115128 by Shyzah, on Flickr

Videos: (Click on them and it will go to flickr video)
When bike is running slow (16 mph max speed):
20200515_101056 by Shyzah, on Flickr

When bike is running fast (31 mph top speed):
20200515_124736 by Shyzah, on Flickr

Sounds like a vacuum leak. It's definitely NOT the intake or carb. I have ruled those out after testing. I hear is slight hissing sound from the engine. Can't tell if its an air leak or a tired internal part.

 

[hazmat]

with the engine off turn the back wheel in a quite area you should hear it if there is a airleak

 

[Shyzah]

Update: Solved the issue a month after this thread was posted. The problem turned in fact turned out to be a worn magneto half moon thingy that stops it from rotating out of place as I stated above. It wore out because the bolt was removed some time earlier. Sometimes the magneto would rotate forward or backward retarding/advancing the timing.

To fix the issue, I left the timing slightly advanced (for some more power) and put the bolt back on to stop the magneto from rotating out of its timing setting.

Engine lasted until 980 miles. I crashed the bike on gravel and the throttle got stuck on full. Engine developed a knock shorty after. Sold the bike frame (Schwinn Moab S) and will rebuild the engine later on.

 

[Member 30571]

Did the bald tires have anything to do with your crash ? ... https://prnt.sc/1sjxhn9