Author Topic: 750 carb body orifice (Read 1517 times)

Beergineer · « **on:** July 13, 2011, 07:44:29 PM »

Hey all, I've been lurking here for a while and thanks to you guys my CB is almost on the road! Only problem is that my #2 carb isn't working properly, upon taking the carbs apart for a third time I noticed what might be the issue. On the underside of the carb body there are two small brass orifices, I am missing one of them. Can this part be purchased or even replaced?

Hondell · « **Reply #1 on:** July 14, 2011, 11:57:08 AM »

Those are vents to the outside and should not affect running. Should be fine to run without the orifice.

Spanner 1 · « **Reply #2 on:** July 14, 2011, 12:22:57 PM »

Have you put one carb in the wrong place on the rack ??
The #1 carb and the #4 carb have only 1 vent/balance oriface, while #2 and 3 have 2 orifaces.
Sorry I don't have a pic. handy, but do a search for a pic. of the carbs assembled.... you'll see the balance/vent hose setup

Beergineer · « **Reply #3 on:** July 14, 2011, 03:17:53 PM »

Thanks for the advice. I swapped the carbs from my buddy's K3 and the engine ran great, so I've at least isolated the problem. I have kept the carb order the same but the PO had done more foolish things, like ran the bike until the drag pipes were black I was actually astonished when some dripping engine degreaser revealed chrome underneath. I will check the carb assembly order later tonight, sucks not having your own garage. I'll keep you posted!

Beergineer · « **Reply #4 on:** July 14, 2011, 08:21:37 PM »

I checked to make sure the carbs were in the right order, which they were. #2, which had never fired, was bone dry. So I checked and reset the float heights on all four (each off by a few mm). Put the carbs back on, and it wouldn't start at all. For some reason the bowls didn't take on any fuel, so I filled the bowls with gas and it ran on all four! Now to figure out why the carbs didn't fill like before...

Hondell · « **Reply #5 on:** July 14, 2011, 08:48:58 PM »

Float installed upside down ?

Beergineer · « **Reply #6 on:** July 15, 2011, 06:08:32 AM »

Nope, I already made the upside down float mistake. I think it might have something to do with the needle clip position, as that's the only other thing I changed. I had them at one position closer to the stubby end, I'll put them back there tomorrow afternoon; Tonight is devoted to finishing a lab report.

WarwickE36 · « **Reply #7 on:** July 15, 2011, 06:32:37 AM »

My bet is on you set float height with the plunger in the float needle depressed. The spring loaded plunger must be all the way out with the tang just touching it. If you have the plunger depressed you get the outcome you are describing of no flow to the bowls. Re-do the float height with thr carbs on their sides making sure the tang on the float is just barely making contact with the plunger inside the float needle. You're almost there, good luck!

Beergineer · « **Reply #8 on:** July 19, 2011, 04:59:02 AM »

So the other day I reset all my float heights (properly this time, thanks Warwick). The bike now idles nicely at about 1.3k rpm, but doesn't want to run without the choke. I have the needle clips in the center position, and the air screws set at 1.5 turns out. I'm thinking that because adding air kills the engine I need to enrich my fuel mixture, right?

jahmic · « **Reply #9 on:** July 19, 2011, 05:26:49 AM »

I had that same issue when I first brought my bike home. Have you pulled the pilot jets out to clean them? If not, that could definitely be your issue...partially clogged jets could be contributing to your lean conditions at idle.

WarwickE36 · « **Reply #10 on:** July 19, 2011, 05:27:12 AM »

did you take the time to install the O.E. airbox? ... no air leaks around the boots? ... box firmly in place? Filter installed???

Will run like #$%* without the airbox + filter

Beergineer · « **Reply #11 on:** July 19, 2011, 06:24:54 AM »

Jahmic, I've done a full rebuild on the carbs-they were gross. The bike has velocity stacks, although I can pull the airbox from the parts bike I just picked up. I haven't tightened the boots down on the carbs yet, seemed kind of pointless since I was pulling the carbs pretty much daily, but that will be my first step if I get a chance to work on the bike tonight.

WarwickE36 · « **Reply #12 on:** July 19, 2011, 10:23:02 AM »

Unless you have jetted for the stacks they will cause your bike to run super lean and cause a myriad of running issues.

Put the stock box back on, run it. You'll be surprised how well it will run. Running stacks just destroys the stoichiometric ratio of fuel:air that honda designed for this bike. You can tune for stacks, but if this is your first rodeo I recommend saving that treat for when you are experienced with tuning stock first.

Good luck, keep us posted.

KingCustomCycles.com · « **Reply #13 on:** July 19, 2011, 10:41:07 AM »

Dude...........you have to tighten the clamps on the rubber boots or you are asking for a lean condition and possibly no run condition as air rushes into the engine that the carb did not know about.

Beergineer · « **Reply #14 on:** July 19, 2011, 07:48:08 PM »

So I socked down the clamps on the boots, no difference. Even since removing/installing the carbs so many time those boots are friggin tight. looked at the airbox on the parts bike, its filthy and will definitely need a new filter. Although the excess air theory seems to fit: when I start to close the choke the engine will rev up to 4-5k (more air=more bang) but after about 1/4 choke the engine rapidly dies. So I tried adjusting the air screws in a 1/4 turn: engine didn't like that at all; out a 1/4 turn: didn't like that either. So the air screws are set at the stock 1.5 turns out. Does anyone know if there is a place that stocks the air filters in the Rochester, NY area? Or is it possible to clean an old filter somehow? I really don't want to be stuck waiting on parts again. Thanks for the advice.

WarwickE36 · « **Reply #15 on:** July 20, 2011, 05:52:49 AM »

Depending on the integrity of the old filter you can give it some bursts of shop air backwards through the filter to blow out the big debris. As long as it doesn't have any holes in it, install it in the airbox. Run the bike and see if it at least runs better. In the meantime order yourself a new filter.

Beergineer · « **Reply #16 on:** July 20, 2011, 07:36:49 PM »

For #$%*s and giggles I took the screens out of the velocity stacks and put a piece of paper towel over them, then put them back on the bike. It ran and I had the choke nearly closed when the 3 or 4 back fired and coughed fuel out of the intake and then the bike died. I consulted my friends dad (been chopping CBs since they came out) and he said to lower the needle clips, so I lowered them by one position. The bike idled at 3k, but the choke issue was back.

So, I pulled the airbox from the parts bike. What a chore! The airbox said cb750F, even though the bike was a K3. Is this the airbox off a later SuperSport model, or do all the airboxes say cb750F? The filter inside didn't look too bad, so I took the shop vac to it. Either way it was near impossible to fit that thing into the frame and even harder to get those boot thingies onto the carbs. When it was on there wasn't any noticeable improvement in the way it was running-maybe because the needles had been raised. If this is the wrong airbox I'm just going to stick it out with the velocity stacks.

I'm really at a loss here. At the rate I'm progressing I think I'm going to have to buy snow tires.

WarwickE36 · « **Reply #17 on:** July 21, 2011, 05:43:07 AM »

The airbox is a chore because its the right way to do it

Shortcuts are like masturbation in the end you're only screwing yourself

Just my opinion, other are entitled to theirs.

Beergineer · « **Reply #18 on:** July 21, 2011, 05:48:19 AM »

Kind of what I figured, but is this the right box? I can't imagine the box designs would be very different between the K and early F models. We'll see how hot it is when I get home from work, if its not too bad I'll reset the needle clips, wrangle the airbox on, and pray it runs right.

KingCustomCycles.com · « **Reply #19 on:** July 21, 2011, 06:34:11 AM »

This is about the time the computer controlled, fuel injection fantasies start to become unbearable....................

WarwickE36 · « **Reply #20 on:** July 21, 2011, 09:09:09 AM »

I keep toying with a turbo F.I. setup for my 550... not for performance just for the sheer look how much money I spent on this 35 year old bike effect.

Kickstart · « **Reply #21 on:** July 21, 2011, 05:11:22 PM »

I'm not sure if all the air boxes have cb750f on them (all of my bikes are 75 CB750F's - and their air-boxes have "HM CB750F-F" on them).

I believe all the air-boxes should fit, but with slightly different air-flow restrictions. I would think any of them should get you in the ball-bark though. That said, I do believe the F and later K model air-boxes were less restrictive than the earlier K models (but not the K0 which didn't have noise reduction baffles).

When you cleaned the carbs, did you make sure all the passages and jet orifices were clear? Also, did you check to see if they had the proper jet sizes? (the PO may have changed them)

Beergineer · « **Reply #22 on:** July 22, 2011, 05:45:42 AM »

The PO did some strange things, like 4" over forks and drag pipes. Since it had velocity stacks on it I'd imagine the jets were changed, I will confirm this the next time the carbs are off, probably tomorrow morning. If the jet sizes have been increased and I want to go with the stock airbox I'll need to compensate for this by lowering the needle, correct?

Kickstart · « **Reply #23 on:** July 22, 2011, 04:31:55 PM »

I don't believe it works that way... lowering the needle to compensate for the jet size increase. It may would work at certain throttle positions but I don't think it's going to solve your problems.

Since it ran fine when you swapped the carbs out with your friends, I suspect your issue is with the carbs. I would leave the stock air-box on there and try to get those carbs as close to stock as possible and start from there.

Beergineer · « **Reply #24 on:** July 22, 2011, 05:57:45 PM »

It can't be simple, can it Kickstart? As a correction, when I said it ran fine I meant it ran on all four cylinders (1 and 2 did not want to work for a long while). I can't remember if it ran without the choke. I can't repeat the experiment since he's in Indiana until Sunday night. However the next 50 hours will be dedicated to some very careful and mindful carb adjustments/note taking/and hopefully tuning. Would have worked on it tonight but there's a lab report due at midnight. =(

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Author Topic: 750 carb body orifice (Read 1517 times)

KingCustomCycles.com

KingCustomCycles.com