New "racing" carb bogging down

[bluenosegoat]

I'm building a bike with a gt5 engine and racing carb- the one that looks like the cns. It starts up fine and will idle but has no power! Once you get it going and give it more than a quarter throttle it bogs. I've tried every needle setting and the problem wont go away. Its not as bad the richer I make it (clip down on needle) but this thing is killing me!! I've spent all day with it and it just wont run correctly! I've messed with the air screw but that doesnt seem to help much either. Any help would be much appreciated

 

[bbamboo]

I had the same problem with a Superrat engine. I discovered that if I would open and close the choke that it would run good,teling me that it needed more fuel, so I drilled the main jet out to a 72 size and raised the needle as for as it would go.It would then run much better but still acted to lean.But doing a plug check showed a very wet plug and the top of the piston was a black mess also. I tried lowering the needle and it wouldnt run right. So next I checked the timing and discovered it was 43 degrees advanced! That explained the popping sounds I heard at start up. I set the timing to 22 degrees and its a new engine.It starts right up and pulls good to top rpms. So I would check the timing first. I moved the needle back to the middle and the plug and piston top look good now. This might not be your problem but worth checking.

 

[bluenosegoat]

Thats very interesting. So are you saying that the magnet on your crank shaft was put on wrong? I know that they are keyed and the only real way, that I know of, to change timing is to modify the shaft and keyway to change its posistion. I dont think its a timing problem though. It starts up perfectly, will rev up well with no load but under load on acceleration anything more than 1/4 throttle it just bogs down really bad.
This bike is for a customer and its really upsetting that it wont run right! To much time wasted on it as it is!

 

[bbamboo]

I used a timing light and degree wheel to verify where the timing was set.I removed the key and rotated the flywheel back to get 22degrees.I just used the star washer and nut without the key to hold the flywheel on for testing.I tried different positions but it ran the best at 22 degrees. It turns out that I moved the flywheel about 1/2 the width of the key so I have ordered a offset key to use.The flywheel did not move during testing however.From now on I am going to check the timing on these engines first because you will never tune one if the timing is way off.Once the timing is right make sure the carb is sealed to the manifold so there are no leaks. Another thing my cns carb came without a gasket under the top cover so I made one for it. Also check the head gasket for signs of leaking. Once all this is right then it is a matter of jetting the carb right.

 

[cory151]

I never got mine to work, utter and complete junk. Swapped back to the standard and its been fine ever since.

 

[bluenosegoat]

Thats very interesting on the timing- I'll have to start checking that on future bikes. One thing on this bike is I had to make an offset intake for the carb clearance but I checked for vacuum leaks and have none. I also noticed (after being in there a 100 times!) that there is no gasket on the top of the carb but still leak testing showed its fine. I give up on this carb and will refuse to ever work on one again! Give me an old nt or a real Cns and things will be good. The customer got the bike this morning as is taking it to another person to see about a standard carb, I dont have one on hand that I can let go. I'm just upset because I've never turned a bike over that didnt run to its best ability. But at least she got a quality install and custom motor mounts and exhaust for super cheap! Glad to see that carb go bye bye!

 

[Potsos1]

I think those carb are defected, 3 of my buddies tried them none of them work they bog flood ect. I went back to the speed carb, and its running it's ass off. we have built at least 20 bikes always trying to improve, do not use this carb...

 

[bluenosegoat]

I have to agree it must be a defect. The bike runs perfectly now on a standard nt carb. Kind of a bummer though. I was expecting this carb would work well and I like having the air screw to make fine adjustments. But if it dont work, well it just dont work! I'm just glad she found a carb is all is well. That doesnt get me my day back wasted on it though!