[82SU]

So after months of not riding and fixing little things its time to get my 82 GN400 on the road with the good weather. What I am experiencing is a lot of kicking before it actually starts, When it does start then it may back fire once or twice. I get it on the road and almost everytime I decelerate I get pops of what seem to be out of my exhaust. A lot of research Ive done says that this is caused by a lean mixture. But when I pull my plug, it is fouled to death with soot. So I'm under the impression it is a really rich mixture. Another thing is that in first gear, it seems to be going faster than should, still propelling under its own power at about 10-15mph with out throttle, this does not make it fun to brake. wondering if these two may be related. A lot say it has a CV carb, but here are the specs on the carb according to the manual:

MIKUNI BS36SS:

MAIN JET #132.5
MAIN AIR JET 0.6
JET NEEDLE 5C33
NEEDLE JET Y-6
PILOT JET #42.5
PILOT OUTLET 1.2
STARTER JET 35
PILOT SCREW ****PRE-SET***** This has an aluminum plug over it that some say you need to drill out to adjust.

The person who I bought it from had put after market pipes on it, and a cone filter. He claims he rejetted it the same as a "friend" because it sounded the best. I personally have not tore into the single carb yet, so to answer any future questions, I dont know the jet size of what is in there.
Not sure what to do, if i should tear into the Carbs and figure out the jets, drill out the aluminum and adjust the pilot screw, or what. I dont have a tachometer, so i can only do the rpms by sound

Thanks in advance!

 

[TrialsRider]

Step one would be to replace the spark plug if you haven't already, I have seen plugs develop cracks in the electrode insulator and the plug will periodically spark through that crack and not at the electrode gap. This can result in a sooty plug even if there is a lean carburetor situation which indeed does relate to some of the symptoms you have described. Add to that yes, your bike is equipped with a CV (constant velocity) carburetor and those were typically installed to make the bike run as lean as possible thus reducing emissions. Messing with both your bikes intake and exhaust was a serious bad idea and it has left you with no base line to determine what is needed in the way of a carburetor change :/ in fact an entire change of the carburetor is what you might need to actually match the ideal setup for what you now have. If your "cone" filter is one of those cheap crap units that block some of the intake jets, take the filter off and see if your carburetor starts working slightly better. Naturally when you do this you will not want any dirt and debris getting into the engine or it will destroy it. That should get you started in the right direction, but nobody can tell you what you need in the way of jet #'s and changes, there are too many variations including altitude and operating temperature that would be native to Utah.
... cool place by the way, love that Moab.

"This has an aluminum plug over it that some say you need to drill out to adjust." yes indeed that screw is set and capped to prevent you from tampering with the pollution control characteristics of that motorcycle :/ which is illegal under federal law and which your previous owner has already messed with by altering the exhaust and probably the airbox.
No big deal because you are turning it into a competition race bike for use in closed course competition events only, right?

 

[hillsy]

With the changes to the intake and exhaust you DEFINITELY need to adjust the idle mixture screw regardless of what jets are in it.

 

[generalyen]

Sounds like a rich condition to me, first step on my opinion is the check your valve lash, if your out of spec then you will never get your carbs dialed in. Sounds like you need to back down your idle screw and figure out what your idle rpm is. Does it bog down anywhere in conjunction to throttle position? Throw new plugs at it, take it for a spin then pull them, make your adjustments and repeat (you can clean the new plugs after each run). I had bst36ss on my cb750 and I loved them once I got them dialed in.

 

[TrialsRider]

Plug singular it's a single cylinder engine.

 

[knappyfeet]

Let's start by taking the carb off....inspecting the jet sizes and do a complete, thorough carb clean with all new items. Gaskets, jets, float needle, etc.

Cone on the exuast......do you mean like an actual cone or like a supertrapp exuast with discs.

 

[generalyen]

Ah gotcha, well than that should make it easy!

 

[TrialsRider]

Once you get it running reasonable, put it on a dyno tester and figure out if you have gained or actually lost performance. Naturally to do this now; you will need a 1982 GN400 that has not been messed with.

I read it as being a cone air filter aka pod :/ probably emgo cheapo

 

[collector000]

I had one that was stock. Mine would pop and not run right if you left off the airbox, you are probably running lean or the guy never changed the jets at all. Also check the intake boot for leaks.

 

[TrialsRider]

I sure hope the people modifying their street bikes emission control equipment realize where they stand with respect to law. If you are doing all these things to build a competition only motorcycle you are probably good to go. Street motorcycles :/ do your research on "motorcycle emission control tampering" just because they aren't going after owners now, doesn't mean they won't in the future. When you buy a motorcycle and it's got questionable or removed parts, make darn sure you get all the original parts the dude took off, you might need those parts some day to set it right!

 

[TrialsRider]

... that being said:

Carburetor replacement with a Mikuni VM36 is apparently the hot setup

 

[hillsy]

I'd imagine a VM36 would be a tad too big for a 400 - might be good at full throttle on the track but a 34 would probably be a more user friendly carb for the street.

 

[TrialsRider]

I kind of assumed that his stock carb being a MIKUNI BS36SS that it was a 36mm carb, but I must admit I did not investigate that any further. If it's not a 36mm what is it? VM's are available in plenty of sizes and the price is right even when you include a tuning kit, which includes jets to bracket the most likely range of adjustment required.
... probably can even find some used ones, they put them on snowmobiles as well as a kazillion dirt bikes.

 

[82SU]

Step one would be to replace the spark plug if you haven't already. Add to that yes, your bike is equipped with a CV (constant velocity) carburetor and those were typically installed to make the bike run as lean as possible thus reducing emissions. Messing with both your bikes intake and exhaust was a serious bad idea and it has left you with no base line to determine what is needed in the way of a carburetor change :/ in fact an entire change of the carburetor is what you might need to actually match the ideal setup for what you now have. If your "cone" filter is one of those cheap crap units that block some of the intake jets, take the filter off and see if your carburetor starts working slightly better. ?[/COLOR]

Thanks for the advice Trial, my spark plug is new. I was really confused when I just pulled the plug after a 20 minute ride and noticed I had a semi clean/normal plug, but I guess what is or was happening to make me think it was rich was the plug wasn't getting up to its self cleaning temperature because the times the engine ran up to this point wasn't very long. So I am posting pictures of what it currently looks like, and what it looked like before to make me think it was rich.

I still have not taken apart the carbs to see the jets, but I did figure out that it is a UNI pod filter, sorry for the confusion everyone. So does this look lean now? Because I haven't made any adjustments to fix the popping.

Remember, the fouled sooty black plug was before my 20 minute ride

 

[kenessex]

Just dicking around with it is going to be a hit and miss proposition. So, approach this with a plan and a series of steps. Give it a tune up, adjusting the valves and the cam chain. It probably doesn't need it, but you don't know for sure so go ahead and do it. Replace the plug, I know you said it is new, but if the 2.98 for a new plug is a deal breaker then you should probably reconsider owning a bike. Check the intake manifold for checking or cracking. If it is suspect, replace it.

The real problem is your air filter, carb and pipe. Who knows what they are doing together. Your plug looks oK to me, but was that a plug chop at wide open throttle, or cruising around on midrange or what. I suggest you pull the carb abd clean it entirely, including removing the aluminum plug and removing the mixture screw so you can bast carb cleaner through that passage along with everything else. Once the carb is clean, begin the jetting process. I am not going to get into that here because everybody seems to have their own pet way of jetting carbs and I only do what I know works for me. I suspect that you will end up with a 1 size larger pilot and may have raised the needle either via clip if it has one or via a shim. With a CV carb, an air filter and a more open pipe it is likely that you will never be able to get the carburation perfect, but it should get pretty decent with enough tweaks and adjustments.
Once I got the carb cleaned and pretty well adjusted, I would just run it and live with a pop on decel.

For Trialsrider, in most of the USA, we can get away with whatever we want as far as carbs and pipes on street bikes. Most of us don't have to deal with inspections much less emission testing.

 

[TrialsRider]

...
For Trialsrider, in most of the USA, we can get away with whatever we want as far as carbs and pipes on street bikes. Most of us don't have to deal with inspections much less emission testing.

What I am hearing is that so far you are not seeing the already existing federal EPA laws being enforced at the individual level, Yet! All they need to do is as they did here, make emission testing a part of the process to renew your license plate sticker and what you can get away with changes real quick. My provence is lazy, they just adopt California emissions standards and then added a few other items like noise restriction laws at the municipal level. Age of his bike Might also put him outside the realm of federal EPA law, but I would not know about that until it hits the news.


Plug looks to me like it is plenty white enough on the electrode now, certainly no indication that it is drowning in fuel. Length of that plug electrode into the combustion chamber makes me think it is a very Hot range of plug! Reading the plug only goes so far towards determining carburetor tuning, the bike has to perform well throughout it's operating range before you can read the plug. CV carbs typically have more fuel circuits (as many as 5) then a more simple carb like the one I pointed to which has only 2 fueling circuits and lots of ways to easy adjust both.

Photos are very good! It helps us spot things like: Is that foam air filter suppose to be oiled? It looks bone dry from here :| Huge difference in the way it will effect your fuel/air mixture.

Additionally; venting of the crankcase is fairly critical on a one lunger, what did you do with the crankcase ventilation which would normally terminate inside the airbox, some place after air filtration and where it can spew oily gasses and condensation etc.

As others have already indicated; if you haven't already drilled out the plug that was intended to prevent you from tampering with the low speed fuel or air screw, you are almost certain to need to do that the moment you made the intake and exhaust less restrictive. Screwing with all of the other jets is not addressing your low speed issues.

 

[TrialsRider]

By the way, how big is the carburetor? Did it end up being a 36mm carb or something different?

 

[kenessex]

The Bss36 is a 36mm carb, but since it is a CV I always figure it flows about the same as a VM32.

I believe in the US the states have more freedom from federal regulation than the provinces have in Canada. Our federal government is slightly less oppressive than yours, especially when it comes to fun stuff like bikes, cars and guns.

 

[82SU]

So, approach this with a plan and a series of steps. Give it a tune up, adjusting the valves and the cam chain. It probably doesn't need it, but you don't know for sure so go ahead and do it. Replace the plug, I know you said it is new, but if the 2.98 for a new plug is a deal breaker then you should probably reconsider owning a bike.

The real problem is your air filter, carb and pipe. Who knows what they are doing together. Your plug looks oK to me, but was that a plug chop at wide open throttle, or cruising around on midrange or what. I suggest you pull the carb abd clean it entirely, including removing the aluminum plug and removing the mixture screw so you can bast carb cleaner through that passage along with everything else. Once the carb is clean, begin the jetting process.

Thank you Kenessex, Saturday I will be checking the valves to see if they are in spec, and yeah the plug has only seen the engine run twice, so like i said, it is new. The plug was at wide open, cruising, and obviously decel. After I check the valves for spec I plan on tearing into the carbs and seeing what is going on in there. But I am suspecting that the pilot jet is a size or 2 to small like you said as well.

As Trials said, yes the filter is bone dry. I knew K/N Filters needed oil but becasue i bought it dry it didnt even cross my mind to oil it. But just looked as Uni's website and it says to oil the interior and exterior filters. Thanks for the heads up! On a side note, that is to bad that your province just adopts California's requirements because they are the most liberal on anything environmental in our whole country! Where I live They dont care about emissions even on cars, just a safety inspection.

The previous owner had the NGK R DCPR7E plug in it with a gap 3 times the width it was supposed to be, I took it out and put what was supposed to be in there that the manual calls for which is the NGK D8EA. The crank case vent you can only see the hose in this photo, starts down by the oil cap and comes up to the top rail with that black zip tie around it.

 

[Cyorg]

Haven't read the whole thread, so maybe I missed it...but does the MIKUNI BS36SS have an air cut valve? Could be wrong, but me thinks they do. If it's stuck or leaking, that could also account for popping on decel.