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Engine running weird

3K views 13 replies 4 participants last post by  The Ghost 
#1 ·
Hi guys
I have recently upgraded my 5sc from 26cc to 29cc and all was running fine for the first 4 or so tanks but now it doesn't want to start unless I trim the throttle along way open but then it idles very high if I adjust the trim to get it to idle right it just stalls out, if I apply the brake it will stall ( the idle screw is ALL the way in) iv got to trim the throttl so far open to start it that the idle scew doesn't even touch the throttl shaft, iv got a brand new 990 carby and all new gaskets and new plug, everything is tight and no leaks as far as I can tell. This is doing my head in hope someone can help!!! Cheers
 
#4 ·
Those settings are stock for a 668 walbro carb

The 990 settings are
Low speed 1 1/8
High Speed 1 5/8

Try the new settings and see how you go
 
#6 ·
Let us know how you get on, hoping it is just tune settings that are stopping it from running
 
#7 ·
Well, I tried it at those settings, it did run a little better but still not right, so I pulled the motor down to check it all out again and what I found was, in the cylinder bore around the exhaust port intake port and around the top of the bore, it seems to be delaminating or kind of a little rough in those areas it's hard to explain but it doesn't look right at all. Not sure how to post any photos of it, if I could I would. Have you ever heard or seen anything like this.
 
#8 ·
Sounds like the chrome lining coming away from the bore or being stripped away...yes I have heard of it.

My very first Baja motor had a similar problem, one tank and I couldn't get it to fire up, spark was good, coil was set correctly, carb setting all were fine, but no life in it. Pulled the head off and the chrome lining had been stripped away from the rear near the exhaust port.

New barrel, piston, ring, roller bearing for the rod and reassembled and the thing went like a steam train, motor has about 50 litres on it now and still goes hard
 
#10 ·
Sounds like the chrome lining coming away from the bore or being stripped away...yes I have heard of it.

My very first Baja motor had a similar problem, one tank and I couldn't get it to fire up, spark was good, coil was set correctly, carb setting all were fine, but no life in it. Pulled the head off and the chrome lining had been stripped away from the rear near the exhaust port.

New barrel, piston, ring, roller bearing for the rod and reassembled and the thing went like a steam train, motor has about 50 litres on it now and still goes hard
If it is above the exhaust port, it is because of a lean condition.
See photo, this is what happens when it is too lean on 50% nitro, this is at the exhaust port

Cheers
 

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