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Post by wiscootsin on Jun 30, 2021 11:55:31 GMT -5
I still have a cobra sitting around as parts with a bad engine (top end at least), so when a clean title sorta running cobra came for sale I bought it for my friend.
I bought it from the dad of the 22 year old who bought it from his neighbor. I guess the story is he got it last fall and did a complete engine rebuild on it (crank, cylinder, piston, everything). His dad said he took the motor into a local shop to get the crank pulled and such. But then the 22 year old got a job and moved away before he really finished it. Over text the 22 year old said he started it once and drove two blocks around his neighborhood but the spark plug boot didnt stay on.
He rebuilt it with stock parts, so it has a new kymco crankshaft and new kymco 49cc cylinder/piston kit. He put different things in the CVT from some generic china type kit. Im' not exactly sure what, but the scooter does go to 8k rpm and stay there all the way up through to 35mph (or however fast it goes). I've left the RPM limiter wire on (my friend wants to be able to use this as reliable/fun transportation).
Anyway.
I brought it home, took of the autolite spark plug and put the NGK BR8HSA stock plug in it and also put the stock airbox on it instead of the malossi pod filter and it drove around fine.
But eventually at WOT it starts to backfire and stall out. It doesn't completely die. If you start it back up it will idle fine.
To me it seems like it runs out of gas but i can't be certain.
Things i've tried (one thing at a time): - swapped autobystarter from the parts scooter on it. - swapped carburetor between the parts scooter. - cleaned jets.
- put an 80 mainjet in it (stock is 85) to see if that changed anything (it didn't). - swapped exhausts
- the stator/magneto gap seemed quite large so i filed holes down to get the gap to about .017" This seemed to make it run smoother but still backfires. - replaced fuel filter. - emptied gas tank, cleaned fuel petcock, put everything back together. - removed engine shrouds, pulled off head to inspect. Tightened back together.
I'm not sure what to try next. Could this just be tuning? should I try a larger mainjet? Is there possibly an air leak somewhere from the engine rebuild? Even though the vacuum petcock works and is perfectly clean could it somehow have a slower/restricting fuel flow?
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Post by wiscootsin on Jun 30, 2021 14:27:33 GMT -5
i opened the airbox and the air filter looked like it potentially was allowing a leak on the side. I cleaned filter and oiled it. I think it's running without backfiring and stalling now. It will have to be taken on a longer ride to test it out more fully.
Maybe just the simple things.
This does have 31km on it, so there may be a chance there's some inconsistent electrical thing? We will see.
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Post by wiscootsin on Jun 30, 2021 20:42:12 GMT -5
After I did the airbox thing I had it running for 20 minutes and I set the carb idle and air mix screw.
It sat for 6 hours then my friend tried to drive it. He got a block and it started to backfire and died.
I got on it and drove it at 1/4 to 1/2 throttle only and it died once. I started it back up and kept it lower on throttle until I thought it was fully warm.
Now my friend took it in a drive and it worked fine.
So it backfires after a block at WOT when cold but once warmed up is fine.
Is this just because of breaking in the rebuilt engine or do I need to look into the auto bystarter situation?
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Post by wiscootsin on Jun 30, 2021 21:31:05 GMT -5
Just tried it again and was entirely inconsistent. Couldn't take it safely without it backfiring and dying.
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Post by wiscootsin on Jun 30, 2021 22:31:26 GMT -5
I swapped the voltage regulator/rectifier, CDI box, and made sure both fuses were tight. I put dielectric grease on all of the connectors.
After that I started it up and it drove around great. I only drove for 3 minutes so no telling if the inconsistent thing is still around.
Feel free to throw out any ideas or confirm what I'm doing as the right track.
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Post by oldgeek on Jun 30, 2021 22:36:23 GMT -5
I had one backfiring once and the reeds were jacked up. Not sure if the backfire damaged the reeds or the reeds caused the backfire.
I would start there and definitely check to see when the iggy is actually firing if you can. You could also swap out the CDI with a spare if you have one.
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Post by wiscootsin on Jul 1, 2021 10:41:27 GMT -5
Good idea I'll check the reeds.
The person who rebuilt it had no experience messing with scooter engines before. Too many options of what could have been done in a non-ideal way.
It's just very inconsistent so hard to figure out. I told him not to, but my friend drove it to work this morning and he said it maybe backfired once but made it fine.
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Post by wiscootsin on Jul 27, 2021 10:42:04 GMT -5
My friend has been riding it daily for 3 weeks. I think it's had a backfire issue once or twice since then. I'm thinking the reeds were the issue, maybe some build up or the guy who built it got some gasket maker squished into the inside.
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