Hey there I have a 1982 cb 750 F with 150 compression over all 4 cylinders. I just bought 2 sets of used carbs at a great price one is 33mm RC specials which I see on bikes with pods more commonly. And a set of 31 mm FCR which is a weird size I know. What 's better for a really fun riding street bike.
33mm RC & 31 FCR/ for street what's better
- Thread starter Frit
- Start date
amc49
CB750 Guru
If rest of bike is stock the 31s would be better.
How are the fcr carbs built? Many of them are built for more of a downdraft configuration and wouldnt work well on engines that the cylinders are more upright.
I'm putting a cbr 4f front forks and rear sway arm on it. If guy I know that thinks he has them finds it lol. He just has to measure the sway arm and make sure it's 10 inch at the pin. And double ohlin's on the rear. Because of the upgrades I want to gusset the frame. I'm not sure if that's overkill with the frame but all the parts are designed in different years with different materials I'm thinking it would be a good idea. Also upgrading the electrical and charging system. Keeping the ignition.
amc49
CB750 Guru
Downdraft ones actually won't work at all, the angle change is too great, the float bowls won't work. They need to be sidedraft ones.
Have seen the down draft carbs on sohc, they did get them to run just not well.
If you look at the down draft carbs you will see the float bowl is at at an angle compared to the rest of the carb body because they are meant to be mounted at an angle. You will notice an obvious difference when you compare them to side draft carbs.
More modern sport type bikes used down draft carbs, they have very steep cylinder angle. I cant say for sure the carbs you have wont work because I have not seen the ones you have. If you post a picture I could better tell. I use nothing but CR carbs, easy to tune but the choke system sucks.
amc49
CB750 Guru
'Pick up the draft' means nothing literally, the carbs mount according to how the fuel bowl is kept level and that depends on the carb design. The float action direction changes up inside the bowls to be at the standard right angle to gravity to let the floats work right. There can easily be more than one degree of downdraft as the angles have been steadily changing to make the ports straighter over the years.
As well, if the carb bank was off another engine you may well have issues with the carb spacing too, not all are the same distance between carbs, and more fit issues.
You'd best show some pics there. Still wondering what 'RC' carbs are unless referring to RC Engineering.
As well, if the carb bank was off another engine you may well have issues with the carb spacing too, not all are the same distance between carbs, and more fit issues.
You'd best show some pics there. Still wondering what 'RC' carbs are unless referring to RC Engineering.
I believe he is referring to Keihin CR carbs. RC eng didnt make their own carbs, bored out stock and webers were their go to carbs. Hopefully we get some pictures.
The FCR carbs will not work for your application, they are made for a newer motor with very slanted cylinders. The others will work if you can get the correct boots and if the carbs have the correct spacing. They are also CR not RC. The CRs are adjustable for width but it takes some time.
amc49
CB750 Guru
X2 all of that last post, the CRs will be too big for a stock 750 DOHC regardless if 33 or 34. At 750 they will need a pretty well modded engine to work. The FCRs are 45 degree from the verticle ones, the floats will be hanging up.
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