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CB900 carb tuning on Pod filters

36K views 26 replies 10 participants last post by  Ravivos 
#1 ·
Hey All.
I need some advice please.
I've just put a CB900F engine in a CB750F frame not that makes a difference.
I've got pod filters on the 900 with 900 carbs.
Can anyone tell me what size jets I would need to use please?
Any other advise you may have with regards to cafeing a CB900f would be much appreciated.
Many thanks all
Ride safe[emoji111]
 
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#8 ·
says the person who has never worked on these carbs and doesn't know that the needles are non-adjustable. Also 165? really? it's not a top fuel dragster. stock is somewhere around 95-100 for the mains.

Here are a couple of things the OP needs to know about these carbs (and why I am not just going to give you a jet size).

- The airbox is not restrictive. It is consistent and given the sensitive nature of these cv carbs why the bike runs as well as it is ever going to be with an airbox in place.

- the internal vacuum passages are actually slightly too large. One of the bigger complaints people have about the DOHC cb750s is the throttle response is terrible, and part of that is the carbs just not drawing enough vacuum. the old school fix was to put 750 carbs on a 900 engine to choke down the engine in hopes of getting the pressure up - you give up top end but gain increased throttle response and better mid range part throttle fueling. There has been some modern stupidity to replicate this fix, one of my favorite stupid fixes is taping up 75% of the pod filter to help get the vacuum pressure up. You could also ride around with the choke half in too. Here is a perfect example of carb tuning where all involved have as much comprehension of carbs and how they work as the monkeys had of the obelisk in the film 2001 (seriously if you read this and it makes sense to you stop and go find another hobby that doesn't require carburetors): Honda CB750'S - I GOT PODS TO WORK!!!!!!!!!!

- Not all pod filters are created equal If you buy K&N filters, they are usually well made and actually filter stuff. If you bought EMGO, Dixie, or any of those other cheap brands that sell metal mesh filters chances are the filters are blocking the cv vacuum passages in the carb anyway and causing all sorts of havoc.
 
#7 ·
CV carbs with pods....you can have good cruising performance, or good WOT performance. Pick one, you can't get both. Even with serious modifications, I've not seen anyone get rid of the transition issues.
 
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#9 ·
just because you cant, doesnt mean no one else can
You must be from the "you can do anything you put your mind to and everyone is winner" generation. Minds much smarter than any of ours who know these bikes TOP to BOTTOM have proven beyond doubt that these bikes can NOT be correctly tuned with pod filters.

They can be bandaged sure...but unlike some other CV carbs that can get close to full performance, these DOHC Honda Carbs require an air box for anyhting that could be considered acceptable by anyone who has a clue.

There are some bad ass things you can do to these bikes. But pod filters is not one. If you must have them then you need to find other carbs that will bolt in.
 
#10 ·
I know the general consensus is that you won't be able to get your carbs running with pod filters, I think you can, at least reasonably well.
I've spent a week or so messing about with the jets on my 750kz. The standard jet sizes are similar to your 900.
This is what I've done so far..... Pilot jet standard
Primary jet 118
Secondary jet 155
Needle height left standard (larger pimary jet has the same effect as raising the needle)
Air screw 1 3/4 turns out.
Hope this helps.
 

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#11 ·
You've successfully jetted a dohc 750/900/1100to work with pods? And by successful I mean at least 90% of stock performance including low/mid/top and total throttle response. Yeah...I didn't think so.

As I said...some cv carbs can be jetted to be pretty good. These can't. Don't generalize my statement and miss the point.
 
#20 ·
Just got in from another test ride, there was me thinking I was close to having the carbs sorted, had I bollocks. Had it cruising nicely at 85 for 5 miles then gave it full throttle, massive flat spot. So back to the drawing board. To make it worse even the weather pissed on my cornflakes, got an absolute soaking, and had to spend an hour cleaning the bike. So all in all not a great day.
I'll brace myself for all the I told you so's:(
 
#14 ·
Thanks... your right most of what I've read on various forums said it was going to be a waste of time. I watched a video on youtube the other day which claimed the guy had set up the carbs to run perfectly on pod filters, it sounded like shite, and didn't rev over 3000. So fingers crossed:)
 
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#18 ·
Youre right...I'm not getting anything done and dont know shit. I guess the fact that I am actually riding my bike instead of building incomplete shit to sell to other people is the problem. I suppose the TD3 replica I'm working on to actually race doesn't count either bc I'm not a "builder". Fucking moron. Get a clue and then come back.
 
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#19 ·
You can say you are successful all you want. Show me a dyno sheet that proves what you are saying. Without that you have no idea if you were successful or not. Butt dyno's are the most inaccurate devised known to man. I'm a racer as are many other members here who have taken the time to build and tune race machines and spend time on the dyno to make sure the changes are positive. If you want us to believe you put up some dyno results or shut up because you will never convince those of us who have proven pods don't work that they can make the same or more power than an air box.
 
#22 ·
Hi steveo,
have to admire your persistence with these carbs.
in the time you spent try to tune them, you could have converted to ... wait for it... that's right, EFI...
i know i have been preaching about it, but the more you try (and sad to say, fail), the more i think this is the right way to go.
once it is set up, you can ran whatever pods you like... , hell, you can even put your wife's panties on it, and it would still run like a champ.

i am planning on converting my XJ550 seca, after i rebuild the engine (damn cyl-2 have a compression of 30...).
if you are gamed for a nice project/challenge, let me know, we can make a list of needed parts.
i already got most of them for my conv.
 
#24 ·
Cheers Ravivos
I hope the wife can't see me typing this, but you obviously haven't seen the size of her underwear, I could probably use them to cover the whole bike.
 
#25 ·
Steveo....no I told you so's here...trying and providing hard data as to why something did or didn't work is the way to go about things. I don't believe you will ever remove the flat spots but I applaud the attempt. Gucci throwing out jet sizes and mods like he knows what he is talking about just makes me laugh.

Keep on testing....though I think you will eventually come to same conclusion that those before you have. :)
 
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