Trumpets

[hirsty]

Hi I have a h1 with a mishaped airbox were it sits on the carbs. I can't find a h1 air box so was thinking can I put trumpets on like you would with a car on carbs.

[gavinfdavies]

apparently it's tricky to get these carbs well set up on open trumpets. your best bet is to ask around the racers to see if any one has an original air box left over - they nearly all fit ram air boxes!

[hirsty]

I would put filters on them if that makes a difference.

[gavinfdavies]

i think it's all down the the resonance behavior. trumpets have very well defined resonance characteristics, boosting power at particular point buts sapping it at others. Hence why modern bikes often feature variable length trumpets, to smooth out the power. It's all down to the pressure waves, when the engine is reving at the right speed for a given length trumpet, the high pressure wave will hit the cylinder just before the intake valves close, boosting the pressure, and hence packing for air/fuel mix in, and thus more power. At the wrong revs, the low pressure waves hits, lowering the intake pressure, and reducing power. Expansion chambers on two strokes work a similar way, leeding to the notorious small but potent power band. Get the length right, and you'll get a nice boost at the top end. Get it wrong and it'll die a death. The trumpets on my gpz are tuned to give massive pull of the bottom, but they die just short of the redline. I loose top end power, but by then i've already pulled ahead.

the other problem is getting the fuelling correct. Trumpets i expect would make it difficult to get the mix right over the whole rev range and over a range of throttle openings considering the relative lack of adjustability with carbs. Fuel injection gives the right levels to work around this, but we don't have FI.

All in all, you have 3 options... 1) just use the original box and original settings, they're pretty good to start with, cheapest and easiest option. 2) you could junk the whole air box, and try trumpets, but you'll need to re-jet the carbs to allow them flow enough fuel, best to get some proper dyno time in once you've done this not too pricey, but not easy to do well from what i've heard. Took me days to get the carbs working well with my gpz. 3) get a full ram air system installed, and then get your carbs re-jetted to cope, and make a good deal more power. This is done alot, so it's easy to get right (alot of advice around), but not cheap.

I'd recommend 2) for now, 3) if you have a few hundred quid spare.

[hirsty]

Will just try find a airbox then. Will a l model airbox fit? Its just mine looks like it has melted wear it should sit in the rings that are cut out in the carbs so completely wrong shape wich would answer why she wont idle very well.

[gavinfdavies]

i think i recall some one else having a similar problem. try using the search for something like melted bell mouths or the like. I think they put them in boiling water to soften and re-shape them.