Running CB450 carbs on a CB350

[woody1981]

Okay, I know I kinda have this wrapped up in the carb cleaning post, but I thought maybe I would get some more info if I made it it's own post.

So, I have an extra set of CB450 carbs in good shape. I am currently cleaning them thoroughly and installing new seals and o-rings.

The stock jetting in the 450 carbs are a 145 main jet and a 38 slow jet. I don't know anything about the 350 carbs, I've never done anything to them, its always run pretty well. If I stuck with the 350 carbs I'm pretty sure that shimming the needles and adjusting the A/F mix would be enough to get it running right... probably.

Its running pretty freakin' lean right now I'm pretty sure. I can get it running pretty easy... I let it warm up for a few minutes with the choke on full until the idle picks up to about 1800 RPMs. Once I open the choke it settles in to a really nice idle about 1100RPM. If I rev it slowly it's pretty okay, but it pops etc... on decel (leading me to believe its really lean). And if I try to rev it pretty fast, not overly fast, it totally bogs with lack of fuel.

Okay... you carb geniuses. Let me know what your thinking. I am thinking that I try the 450 carbs first... if I can't make them work right I know the 350 carbs are at a good starting point to just need richening up. But I really love the idea of the 450 carbs. Some of you 350 roadracers have to know something about this.

Ask me whatever you need to know and I will do my best to answer.

Thanks,
Woody

 

[johnson_steve]

I am far from a 'carb genius' (in fact I'm about to give up on mine all together.) but, I know on cars they put bigger carbs on all the time so as long as the 450 carbs physically fit, I don't think this would be a problem.

 

[woody1981]

I'm pretty sure I can make them fit. I am still in the process of rebuilding them and waiting for a smattering of new jets and new seals and o-rings to arrive. I have a pretty good thread going with some pretty knowledgeable dudes over on the hondatwins.com forum. I have read instances of people (mostly vintage racers) using the 450 carbs on their 350s, so I'm sure it can be done. I will post my results.

- Woody

I am far from a 'carb genius' (in fact I'm about to give up on mine all together.) but, I know on cars they put bigger carbs on all the time so as long as the 450 carbs physically fit, I don't think this would be a problem.

 

[rockcitycafe]

I'm pretty sure I can make them fit. I am still in the process of rebuilding them and waiting for a smattering of new jets and new seals and o-rings to arrive. I have a pretty good thread going with some pretty knowledgeable dudes over on the hondatwins.com forum. I have read instances of people (mostly vintage racers) using the 450 carbs on their 350s, so I'm sure it can be done. I will post my results.

- Woody

technically, a bigger bore carburetor will allow the engine to breathe better, but you might run into issues at lower throttle with bigger carbs because airflow can be less restricted and atomization is less effective... overall i doubt you'll gain much power unless you've overbored the engine and ported the head, ultimately carbs and engines are designed for one another and perform better that way, my guess is the racing guys seeking these carbs are either after the CV qualities of the carb, or their engines are overbored, or both

 

[woody1981]

* This is the post I made over at Hondatwins.net, so there is some talk about carb boot reconditioning etc... over there. Here is the original post link.

http://www.hondatwins.net/forum/viewtopic.php?f=17&t=21

- Woody

-------
Awww crap.

Well, this project might have just come to a screeching hault. Maybe even the end.

Last night I took my newly softened carb manifolds and drilled out the mounting holes slightly to allow them to be mounted on the 350 head... worked no great. Went to install the carbs and boom, problem. The 450 Manifolds don't have any angle to them... they just point straight back... not outward at all like the 350 manifolds. The pivot shaft of the choke butterfly that protrudes from the side of the carb hits the spine tube of the frame and stops the carb from fully seating in the boot. It goes in far enough to hold without a clamp, but not all the way. And the drag from the pivot on the spine makes the choke lever not operate smoothly... that and the velocity stacks were resting against the battery box. As much as the twin vibes I don't really want ANY hard parts touching the frame. The only solution I could see is if there were other manifolds that cant outward like the stock 350 boots and were big enough for the larger carbs... I briefly tried to fit the 450 carbs in the 350 boots, but it was pretty obvious that was a stupid idea.

How big are CB360 carbs? Do the boots cant outward? Boots off an XS650 maybe? Suggestions?

In the mean time I went ahead and shimmed the needles in my 350 carbs and re-installed them. I also adjusted the A/F screw to richen the mixture. It starts and idles perfect... but I haven't ridden it yet to see if I am okay or if I am going to go lean at WOT.

I might just keep my fresh and clean 450 carbs, put the 145 jets back in and use them on my 450 when it gets back together. Or I might have a nice set of rebuilt carbs for sale soon. I was planning to put mikunis on the 450... but maybe I'll just save the cash.

Who knows. Kinda disappointing after all the hard work and $$ I've spent on rebuild kits, jets, etc...

At least I have learned A LOT in this whole process. I just want to get this bike to the point that I am happy with it for the time being and I can concentrate on my 350F and 450 builds.

- Woody