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6k wall while under load

  • Thread starter Thread starter System9
  • Start date Start date
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System9

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My bike runs good. It starts right up, runs smoothly, has very snappy acceleration. I have thoroughly cleaned the carbs, replaced all seals, it has a new petcock, etc.
I live in the city and don't get too many chances to really get the rpms up. But when I do get a large stretch of road, when i hit 6000 rpm, it sputters out, like it isn't getting enough gas. I have Larger main jets in this bike, since I made my own exhaust for it (i bought it with a damaged exhaust)
But, if I'm riding along, and pull in the clutch just to rev it up and see what happens, It will rev right up to redline in a heartbeat. No sputtering at all.
So what is preventing it from reaching 6k while under load as opposed to just revving it?
 
Is the airbox installed? Could be carb jetting.
 
No, this bike came with a damaged exhaust pipe, and the previous owner drilled holes in the airbox. I have individual air filters on it.
I have run different sets of jets to tune it in, and it has a custom exhaust with custom exhaust baffles for back pressure. When I do plug chops, there is no indication of lean behavior, and it otherwise runs like a dream. I was under the impression that the Jets come into play at different throttle openings, in which case my bike is very responsive at all throttle positions. Its just when I hit 6000 rpm while under load. Again, if I rev the bike with the clutch in, it will rev to infinity without a hiccup.
 
With vacuum slide carbs the mains don't come in until you have some rpm's and high load. Sounds like your main jets are too small, but it could be other issues as well.
 
Wow, I always thought the mains came into play more often. I think they may actually be too big, as I tried a few when tuning it, but maybe I was only tuning low and mid range.
 
Fixed. Main was a little too big. I made some custom baffles to restore back pressure, and it was still running great up to 6000 rpm, but I never really tried max rpm range with the new baffles.
I just put in a smaller set of mains and on my first run it went to 7000 no problem. I opened the screws up some, tried it again, and it went screaming all the way to 9000! I actually went through all the gears from 6-8 k, and if a cop was around I would have been in trouble. :D
 
Wow I'm having the same exact problem. Maybe my main are too big... When you say you "opened the screws up some" what exactly do you mean? Sorry for the newbie question. Thanks
 
biketrials25 said:
Wow I'm having the same exact problem. Maybe my main are too big... When you say you "opened the screws up some" what exactly do you mean? Sorry for the newbie question. Thanks
Click to expand...

exact same? do you have a gs450? you should add your bike model / make / year/ to your signature so people can give specific advise.
 
I have a '82 GS750 with the same mods as yours and the same symptom. In second gear around 6k it just falls on it's face. In first gear and sitting still it will rev the full range.
 
FYI for all you guys running modded bikes with messed up carburetor jetting: even if the engine is reving though the range that doesn't mean the mixture is correct, or optimum. If you are cheaping out and trying to tune the engine via main jets alone, the midrange mixture is sure to be lean, and this is where the bike runs most of the time. Lean means HOT, and hot is not good for air cooled motorcycles. One of the main benifits of a Dynojet kit is the new needles, with more taper than the stock needles. You can raise the stock needles to improve the mid range mixture for pods/exhaust use, but you can never get the proper mixture this way since the stock needle is too fat.

I strongly advise you guys to go though with a full and proper plug chop procedure to verify the mixture is correct for your application. Failing to do this can result in engine damage.
 
biketrials25 said:
Wow I'm having the same exact problem. Maybe my main are too big... When you say you "opened the screws up some" what exactly do you mean? Sorry for the newbie question. Thanks
Click to expand...

On the 450s there are air/fuel mixtures screws on the side of the carbs, they come factory sealed, you have to remove the metal plugs covering the screws. You will probably want to remove these adjustment screws while you dip your carbs for cleaning because they have tiny rubber o-rings on them that usually need to be replaced. On the 4 cylinder models these screws may be on top of the carbs. But! These screws are not the main way to tune a bike with a modded exhaust / air intake. My bike has altered needles, needle jets, pilot jets and main jets. You have to tune different parts of the carb to match. Running lean is bad for the bike.
 
While tuning my Flatslide carbs I have learnt a couple of important things - testing while stationary is nothing like testing under load. And the main jets really don't come into play until you're really pouring it on.

From what I have read here and on various sites that sell carbs it would seem that the main jet is where all the adjustment and "correction" happens. In my experience with flatslide carbs the mains are just part of the whole picture, and if you ride conservatively they are not even the most important part. The pilot jets have been more critical for me tuning my carbs. For me it has been getting the pilots right, then the valve needle and only finally the mains. That is of course leaving out the screw adjustments, synching, etc.

I guess my point is that it is misleading to think that changing the main jets is all there is to setting up carbs with different filters. The key is to stick the bit of tape on the throttle sleeve like Mikuni suggest and that will tell you where to adjust the carbs.
 
System9 said:
On the 450s there are air/fuel mixtures screws on the side of the carbs, they come factory sealed, you have to remove the metal plugs covering the screws. You will probably want to remove these adjustment screws while you dip your carbs for cleaning because they have tiny rubber o-rings on them that usually need to be replaced. On the 4 cylinder models these screws may be on top of the carbs. But! These screws are not the main way to tune a bike with a modded exhaust / air intake. My bike has altered needles, needle jets, pilot jets and main jets. You have to tune different parts of the carb to match. Running lean is bad for the bike.
Click to expand...
Just learning the proper name for these screws will help to explain that. :-k

These are the "IDLE MIXTURE adjustment screws". Because they are for the idle mixture, they won't do much good at full throttle.

As mentioned in other posts, there are different jets for different operating parameters, they all need to be checked. When you change your air filter or exhaust, the amount of air flow at wide open throttle will be increased. When running at part-throttle, the throttle blades (or slides) are controlling the air flow, the filter and pipe don't really make that much difference. Because of less restriction with the pods/pipe combination, more air can flow, so more fuel needs to be added via the mains.

.
 
Also, I understand that CV carbs tune from the main jet down because the main jet plays a role in each circuit. VM carbs tune as you move up the circuits - so pilot, then needle, then main.
 
Steve said:
Just learning the proper name for these screws will help to explain that. :-k

These are the "IDLE MIXTURE adjustment screws". Because they are for the idle mixture, they won't do much good at full throttle.
.
Click to expand...

They are also known simply as "mixture screws", maybe so they aren't confused with the idle adjust screw? Naming aside, this screw does play an important role at wide open throttle. A full turn on my mixture screws will give me an extra 3 or 4000 rpm, where it would otherwise be starved of fuel.
 
Aha!! That's why guys are always on about the mains. I was wondering because with the RS carbs they don't make so much difference until you hammer it. Well at least that's how it seems to me.
 
the radial slides and GS vacuum carbs have zero in common as far as tuning.
the vacuum carbs need a st. 3 jet kit to get 100% correct....period...on the 4 cyl. anyways.
like comparing apples to watermelons.
 
JimmyR said:
Aha!! That's why guys are always on about the mains. I was wondering because with the RS carbs they don't make so much difference until you hammer it. Well at least that's how it seems to me.
Click to expand...

You still need to get the mains right upfront. If you are too lean at WoT, it will burn up your valves pretty quick. That's why there is so much concern about main jets and pods. Doing the pilot wrong makes for crappy idling. Getting the bigger circuits wrong will destroy expensive things.
 
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