GN400 Carburetor..Desperately Need Help!

StryfeS13

New Member
The model of the carburetor is a BS36SS CV Carb.

Hi guys, I'm building a GN400 and I'm desperately trying to figure out the jetting/problems I'm having with my carburetor.

I tried running a VM36 but I could never get it to not be rich (went all the way down to a 15 pilot and still smokes black), so I put the stock carburetor back on. The bike has an emgo cone filter, and a pretty much straight through exhaust. The problem I am having is that it will pop through the exhaust and shoot out flames.

I completely disassembled the carburetor, soaked it for 24 hours in the solvent tank at my work, blew EVERYTHING out with compressed air and reassembled it. When I pulled the needle out, I found 3 O-RINGS under the clip that the previous owners placed there, which obviously were making it super rich. I removed them and put the needle back in how it's supposed to be, which I found out by following the exploded diagram on our parts fiche at work.

I started the bike up like this (completely stock carb, totally clean, everything perfect), and it will only run on choke or half choke, with no response to throttle except popping and banging. When I try to take it off choke it will die out immediately. The pilot is not dirty, neither are the passages. I also checked the slide diaphragm for rips/holes and there are none at all. I have also checked and set the float level, etc.

It still has the stock jets in it, 42.5 pilot 132.5 main. I have tried a 140 main and it made no difference in the way that it runs either.

I am SO confused haha...can anyone help? Oh and none of this happened when I was using the VM36 carburetor. It ran O.K. but just super super rich - black smoke. So it IS the carburetor causing this issue.
 

NBraun

Member
Sounds like its running lean being as it only runs with the choke on, and its popping. Try upping the pilot and main jet again. Should check for air leaks around the carb boots too.
 

StryfeS13

New Member
See this is why I'm so confused. If it were lean, why would it be shooting flames out the exhaust? Wouldn't that be unburnt fuel igniting in the pipe and flaming out the back? Also, the plug is black..but it isnt a new plug so I might be getting a skewed reading.

It also gets very hot very fast which is a condition of being lean as well...What's confusing is that even if I swap from the stock 132.5 main to a 140 main it makes absolutely no difference in the way it runs.
 

Davem8499

Member
Hi StryfeS13,
Im having the exact same issue that you had.
Did you over come this issue? What did you do to resolve the problem?
 

cxman

Active Member
DTT SUPPORTER
idle circuit is plugged that is why it only runs with the choke
that carb has to be ultrasonically cleaned and all the brass removed

the 36 is bigger than is optimal for the gn400 a vm34 will give much better performance
 

teazer

Well-Known Member
DTT BOTM WINNER
DaveM, you managed to resurrect a very old post, so please tells us what your issues are? Is the bike stock? Air filter? exhaust? symptoms?

CXman has pointed out that the symptoms the OP described with his stock carb were caused by a blocked pilot system. Do you have teh stock carb on and if so, the carb sounds like it needs to be deep cleaned.
 

Davem8499

Member
Hey guys.
So...i bought the bike ages ago from a guy who had the bike sat in his garden (not covered) for 8 years. So i bought it really cheep to do up.
It looks like a complete bike except the last owner took the air filter and housing out and took the exhaust off for some reason. I think his idea was to turn the bike into a bobber.
I went with the idea and read loads about other people bobbing the bike (seems to be the done thing now).
The bike is stock ( 400cc 4 stroke single with a decompression valve, 6v electrics, kick start only) apart from a pod air filter, and a slash cut exhaust. When i try to start it (which takes a million kicks) it sounds like it wants to start, occasionally it bangs really loud and the odd flame comes out but nothing. Once it does start the choke has to be kept open fully or the throttle has to be open, once closed or released it cuts out instantly.
Ive read that there is a mixture screw under an aluminium cap that can be drilled out to adjust the screw to help this but before i do this i was wondering if anyone knew what the issue could be? Thanks
 

teazer

Well-Known Member
DTT BOTM WINNER
Did you rebuild the carb or replace the spark plug with a fresh one? If the bike sat for years outside, there's a high probability that water got into places that are not designed to get wet inside. It's also possible that the exhaust and other openings might be full of mud from ant and wasp nests or full of nuts and poop from rodents. Give it an end to end through cleaning.

That would be a good place to start. I'd replace the spark plug, change oil, strip and clean the carb and check all the jets and tiny drillings, replace the air filter and clean every contact/connection in the electrical system. Fit a new fresh battery and then try to start it.
 

Davem8499

Member
Oh yes sorry, forgot to add that i stripped the carb down completly, cleaned it and rebuilt.
Put a new plug in and its getting a good spark. Also im not using the fuel tank at the mo instead im using a fuel bottle with a tap on it (mimics a tank) so its getting fresh clean fuel

I havent taken the engine apart completly but have given it a proper clean up and checked all gaps/openings and cant see any blockages. Ive replaced the oil with new stuff to.
The battery is new and appears to be working fine too
 

Maritime

Well-Known Member
is the timing off? I crossed the plug wires on my GL bu mistake once andnit acted like
sthat as it was firing 2 cylinders at the wrong time. I know thisnis a single but if timing is out it wi be hard to start and not run well if it does.
 

ex119x

Member
Sounds like the pilot jet/circuit is still clogged. I believe it is time to strip the carb again and double check the pilot stuff.
 

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