Main & Needle Jet Sizes (Bing 32 CV)

gog

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Afternoon chaps. I am just getting started rebuilding my bike (1987 R80RT mono) a year after taking it apart, and have been raking through stuff I bought to do the refurb but have of course forgotten why I bought some of the parts I did. The new main jets I have are 135 and needle jets 2.68, though they dont match the ones they will be replacing (145 and 2.70) - I know I bought the different sizes for a reason, but cant remember or find where I read that these would be a good idea.

The R80 barrels and pistons are being replaced with R100 ones, and cylinder heads machined to match.

Anyone know off hand if the 135/2.68 combo is a good one for the 32mm carbs on 1000cc heads, or should I just spend the few quid on the standard sizes?

Cheers.
 
135 main jet and 2.66 needle jets instead of the 2.68 you have. It will be really lumpy on the midrange and loose power compared to the 2.66. Needle on third groove from the top - this is standard R100RT/RS mono Jetting
 
The 32mm carbs are so much better than the 40's. I'd have them every time if I could.
 
Funnily enough, I had considered buying 40mm carbs. I had to get replacements as mine were 'removed without consent' from my garage along with half of my (dismantled) bike. Got another set of 32s instead though which have cleaned up nicely.
 
I think because the air velocity through the carb is greater which atomises the fuel better, I am just about to change the 40mm Bings on my R100GSPD for 32mm.
 
I had an American 100gs with 32mm carbs. The mid range was superb compared to the 40mm equipped bikes.
 
timeandtide said:
Will you explain why Rob?
Had a 79 R100RS back in the 90s and have recently acquired a R100GS and am eager to learn.
Click to expand...

I had a 78 R100RS. It was probably the best airhead engine I've ever had, very quick at the top end.

the airflow through the 32mm carbs means they tend not to stumble at tickover and have better midrange. I was looking for some old dyno printouts, without success, from a Mono R80 G/s I had a few years back. the smaller carbs have less depression on the airflow when the throttles whacked open so tend to sit very well on the optimum air/fuel ratio where the 40mm carbs have a nasty dip that tends to take them too rich.
The clamshell airboxes on the heavy flywheel bikes worked much better with the 40s where the later, black, airbox seems to work much better with the 32mm carbs. I've heard that the later airbox volume isn't high enough for the 40's. Certainly if you look at dyno printouts for the smaller carbs they work beautifully. Pity I couldn't find the printouts otherwise I could have shown you the difference.

I think it was Beemerboff who said the 32mm was optimal on an engine that produces less than 80Bhp.
 
Vincent's ran fine on 1 1/16" / 27 mm carbs and even the Black Lightning only had 1 1/8" / 28.6mm jobs.
If you do the maths for 400 ft/min intake velocity on a 1000cc airhead you come up with 28.6 mm too, but CV carbs are by nature slightly more forgiving on over sizing, 32 mm will comfortably flow enough for a genuine 80 hp so they should be enough for most road bikes.
Rob, a long time ago when you were playing with some fancy gauge you came to the conclusion that the stock flat air box didnt flow enough for a warmish 1000 cc bike, but that when you freed the intake up a little a 150 main and 2.68 needle worked well.

A few weeks back the float bowl on my high compression 1000cc R80 G/S started leaking and I borrowed the newish floats and float valves from the untaxed GS while the parts arrived from 'Bins.
Borrowed the 150 main jet to replace the existing 145 too, and left the 2.68 needle which was already there. Airbox and exhaust are both modified for better flow.
To my surprise the 150 main livened things up right across the range, and consumption improved from 17 to 18 km/l too.

Flies in the face of conventional wisdom that main jet size only effects the last third or so of throttle opening.

While that might be true on a slide carb it doesn't take into account the fact that on a CV carb the throttle cable connects to the butterfly, not the slide, and the slide bobs around merrily, influenced by but not directly connected to the butterfly position.
Most others seem to find the 135 jet works well, but it gets pretty hot around here and cool running is just as big a concern to me as extracting a little more power.
Someone in a position to know once said that the easiest way to find out if you were running a air cooled motor too lean / hot, was on a dyno - if you had the jetting which gave the most power you were running too lean!
 
I swapped to the Clamshell for a while. Cant remember what was happening with the jetting at the time but with the same jetting the mixture ran so weak on the clamshell it was as if the kill switch was being hit. With the black airbox it was running rich. if you look at the heavy flywheel jetting for equivilent models theyre 10 sizes bigger on the main jet.

It was the innovate LM2 and MTX-L I still have them - http://www.innovatemotorsports.com/products/MTXL.php

it basically gives the same info as a dyno (uses the same Bosch wideband sensor) the problem always is getting the jetting at the top end right before messing with everything else. Its easier and safer to get the main jet right on a dyno.

The LM2 logs the data to a memory card so you can run it back later

lm2_zpsiwg7fgij.jpg
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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