cb500 649a carburetor questions

[paul_D]

Hello everyone. I am new to this forum. I am a member of sohc4.net

Seeing as my bike is European I was hoping you could tell me your experiences with the 649A carburetors on the cb500. I understand England receive 627B carburetors but being so close to Germany etc. I would guess there was more crossover of bikes than here in Canada and the USA.

I wanted to know how the bike ran in stock compared to without the air duct assembly and if any of you used 100 main jets in 649a carburetors instead of the stock 78's.

Lastly what settings were used on the carburetor and bike for these various setups to run nicely. (Main jet, slow jet, air screw turns, needle clip position from top, float height, air filter type(foam or paper), exhaust type(Original or aftermarket), spark plug)

[deltarider]

The 500 with 649A carbs (78 main jets) and airbox cover performed equally well as models with the 627B carbs (100 main jets). Top speed in Dutch and German tests prooved 179 km/h. BTW, I never managed more than 170 km/h @ 8500 rpm, but I weigh 90 kg, am 196 cm tall and was not dressed like a test pilot. On the dyno my bike reached the theoretical top speed of 185 km/h @ 9250 rpm.
Many years ago, out of curiosity I tested with 100 main jets and the airbox cover removed on the road. Germans did the same. With standard 4 in 4 exhausts it didn't bring anything but extra intake noise, but feel free to experiment. Needle standard is in 3rd (middle) groove, air screw is one turn out +/- 1/8. Slow jet is 40. Stock paper filter element is best. Float level is 22 mm (but is always right unless somebody messed with them).

[paul_D]

Thanks delta. I am asking because I have been trying 100's and no cover on my cb500 with needle in 3rd and air screws out 1 turn with no luck and fouling until it stalls while riding.

I am looking to put the 78's in on my carbs but I don't have the duct assembly. As you know from sohc4.net I am repeatedly told these carbs simply cannot work without the duct. Seems ridiculous to me.  I am also told I should give up and use 022A carburetors. I know these 649a's will work. Just want to return to stock as much as possible to eliminate any variables.

By the way I am located in Canada.
And I've even read your posts on Frances sohc board in French about cb500 and 649a's. Was nice to see you consistently use deltarider as your username.

[paul_D]

The 500 with 649A carbs (78 main jets) and airbox cover performed equally well as models with the 627B carbs (100 main jets). Top speed in Dutch and German tests prooved 179 km/h. BTW, I never managed more than 170 km/h @ 8500 rpm, but I weigh 90 kg, am 196 cm tall and was not dressed like a test pilot. On the dyno my bike reached the theoretical top speed of 185 km/h @ 9250 rpm.
Many years ago, out of curiosity I tested with 100 main jets and the airbox cover removed on the road. Germans did the same. With standard 4 in 4 exhausts it didn't bring anything but extra intake noise, but feel free to experiment. Needle standard is in 3rd (middle) groove, air screw is one turn out +/- 1/8. Slow jet is 40. Stock paper filter element is best. Float level is 22 mm (but is always right unless somebody messed with them).

But delta
I was wondering how your bike runs without the air duct and the 649a carburetors. I am consistently told over on sohc4 that its impossible to run them without it, which seems ridiculous.
And what are the measurements of the airduct as a temporary fix ? I am looking for one to fit and try out but cant find one.

[matthewmosse]

For many years I had my cb500/4  running a K & N filter and a one off exhaust. Work was done by previous owner - carbs looked standard (so assume stock british markett item for 1974,) phlenem chamber was stock but filter box was replaced with a K & N item. I immagine the jeting may have been modded. I could look but might have to wait until next week to have time to loft dig. Best thing about that setup was it looked totally stock, even had the tool tray in place, but went like stink. Exhaust was a bsm system with a triumph rockett 3 (70's vintage, very very rare) raygun silencer. It was loud and antisocial  and gave the ability for my 500/4 to spin it's rear wheel if putting the power on mid corner with a bit of front brake ( sidecar outfit so that meant power slides, much fun)

[LesterPiglet]

I don't have any experience on the road with or without stock airbox but in the garage it seems to be fine.
I do know, no matter what configuration exhaust I have had on my bike I have felt no difference.

It's had stock headers with a Micron(two stroke) end can, just stock headers, stock headers with a gsx750 can and now a Motad.

[matthewmosse]

Oh, and on my 550k3 the thing was a pig to start with individual k&n style filters, but actually running it was ok. I think with cb500.4 carbs I've found them fine for bench testing with just the k&n style (think mine are s&b brand in point of fact) fiters, start up and run fine, even with multiple exhaust studs missing, I tested each of my project bike engines on the bench using a not very restrictive slash cut 4 into 1 chop exhaust and those 4 cone filters and not even a frame fitted but they were still a doddle to get running, even with carbs full of furry corrosion. The 500/4 carbs in my expirience have been nice reliable and easy to get the bike running. the 550k3 ones that I found a swine to get right.