Author Topic: 400F carb rebuild questions (Read 7363 times)

.RJ · « **on:** January 16, 2007, 08:03:44 AM »

I picked up a new project this past weekend - '75 400F. Bike is in great shape, needed a battery, brake caliper rebuild and a carb cleaning. Initially I threw a battery in it and tried to start it but it spewed fuel out of the float overflows. Carbs came off, cleaned all the gunk out of the floats, got the floats and slides moving freely. Then pulled the main and slow jets out, and the float valves, soaked them and put new o-rings in. I bench balanced the carbs, set all the air mixture screws to 2 turns out, put it all back together and.... it runs! But, it dies with the choke all the way off, wont idle under 2000 rpm, and if you give it a lot of gas it will die as well. I'm thinking its not getting enough fuel and the emulsion tubes under the main jets are clogged up, and possibly the needle valve.

My plan is to get a float bowl gauge (I bought one, waiting for it to show up) to set the float height, pull the jets back out and soak the carb bodies (I just sprayed them before) in simple green, and reinstall. I'm also going to put a set of pod filters on it, the airbox is a bit of a pain - will I need to change the main jets for that? Am I on the correct path with this?

Thanks all!

The bike in question:

supersport_CB400F · « **Reply #1 on:** January 16, 2007, 08:20:18 AM »

Get it running good before you put the pods on and then make any changes that need to be done would be my advice…..nice bike!

.RJ · « **Reply #2 on:** January 16, 2007, 08:26:33 AM »

I would usually agree, except the airbox isnt in the best shape

supersport_CB400F · « **Reply #3 on:** January 16, 2007, 02:08:32 PM »

Quote from: .RJ on January 16, 2007, 08:26:33 AM

I would usually agree, except the airbox isnt in the best shape

I think your going to have some head scratching time, I have no simple solution but I think your carbs need to be cleaned like you said you will do and then see how the bike, runs if it idles less than 2K and around the norm then add the pods. Have you adjusted all the other variables?

.RJ · « **Reply #4 on:** January 16, 2007, 04:10:33 PM »

When I got it running, I didnt even put the airbox back on - i only had it running for a minute or so. I havent adjusted anything else, I figured I should get them cleaned out properly and the the float heights set before I really go any further.

750goes · « **Reply #5 on:** January 16, 2007, 05:17:44 PM »

From what I have read on this forum and references from other 400F owners - this model must have the airbox fitted to run properly...having said that...put the airbox back on....get it running properly then change to the pods..but get it running right first....

flatblack · « **Reply #6 on:** January 16, 2007, 07:12:56 PM »

Sounds like your slow jets are still plugged. Not only must they be clean through their entire length, but the holes in the bottom must also be clear.

Other posters have said the 400F should have the airbox on it to run right. Dunno if that's true, but it'll surely run better when everything's sorted out and back together. Then get a manometer and synch them dynamically (I *like* the Morgan...).

And FWIW, my 400F runs fine at about 1-1/4 to 1-1/2 turns out.

HTH...

fb

aptech77 · « **Reply #7 on:** January 16, 2007, 07:14:25 PM »

CLEAN,CLEAN,CLEAN.........

.RJ · « **Reply #8 on:** January 16, 2007, 08:54:41 PM »

Quote from: aptech77 on January 16, 2007, 07:14:25 PM

CLEAN,CLEAN,CLEAN.........

Done, Done, Done

I pulled all the jets out again this evening and soaked everything in simple green - looking much better. I'm going to throw a set of larger mains in it as soon as I get those and the pod filters in the mail. I thought about getting it sorted out with the stock airbox, but I'd hate to then change everything when I put the new filters on.

Is it worth picking up some larger slow speed jets as well? Or will the stock ones be fine?

MadDogMcq · « **Reply #9 on:** January 17, 2007, 04:39:51 AM »

That's interesting RJ, cos I'm pretty much in the same position as you. I'm doing up a 400/4 and I've stripped the carbs right down to clean the things out. I've replaced the slow-jets with NOS ones because I couldn't get one of them 100% clean. When I rebuild the bike, I'm going to fit power-filters instead of the airbox so I hope I don't have similar problems to you.

www.mcqart.com

Tom

flatblack · « **Reply #10 on:** January 17, 2007, 04:48:44 PM »

Quote from: .RJ on January 16, 2007, 08:54:41 PM

I pulled all the jets out again this evening and soaked everything in simple green

Why Simple Green? The basic stuff is corrosive to aluminum, although they have another product that isn't. There are any number of products much better for carb cleaning than Simple Green. Although if that's what you have, I guess that's what you use...

fb

.RJ · « **Reply #11 on:** January 17, 2007, 05:23:39 PM »

Quote from: flatblack on January 17, 2007, 04:48:44 PM

Why Simple Green?

Because it works... not sure why, but it just gets all the crud out of everything. I dilute it 2:1 or 3:1 usually.

After about 30 minutes of soaking:

.RJ · « **Reply #12 on:** January 17, 2007, 05:40:25 PM »

Also - what kind of Keihin carbs are on these bikes? I found some suppliers for pilot jets, but I'm not sure which carb I need them for.... just want to be sure

http://www.sudco.com/keihinjets.html

http://www.rageperformance.com/carburetorparts.asp

.RJ · « **Reply #13 on:** January 19, 2007, 08:02:25 PM »

Still having a problem with fuel pissing out of the float bowl overflow tubes........

Float heights set, checked, double checked and triple checked. I can hook up the tank on my workbench, elevated, and the carbs in a pan with the fuel turned on and I can push the floats up and the fuel stops flowing out of that carb. The floats all float in little tub of gas.

Are the float valves just worn out and not sealing properly with the weight of the float in the gas?

750goes · « **Reply #14 on:** January 19, 2007, 09:27:15 PM »

Are the floats in upside down

If they leak fuel out of the overflow then something is not sealing properly....

when you turn the fuel on does it overflow on each carb ??

can you check the fuel height with a clear hose attached to the overflow or another from a drain screw ??

Sounds more like float height and non sealing needle jet...

.RJ · « **Reply #15 on:** January 20, 2007, 05:23:01 AM »

Quote from: 750goes on January 19, 2007, 09:27:15 PM

Are the floats in upside down

That was my first guess, but i checked another set of carbs that I got with the bike... the round side is 'up', flat side 'down'

Carbs upside down in the pic:

supersport_CB400F · « **Reply #16 on:** January 20, 2007, 05:51:38 AM »

Your floats look much higher than mine in that upside down photo, take a photo from the side so I can compare the height with my carbs.

750goes · « **Reply #17 on:** January 20, 2007, 02:19:53 PM »

Floats look to be the right way up - but if they are sitting on the bench upside down I would have expected to see them sitting further into the carb body -

when you fully compress the float and jet does it stop the fuel ??

.RJ · « **Reply #18 on:** January 20, 2007, 08:51:19 PM »

Quote from: 750goes on January 20, 2007, 02:19:53 PM

when you fully compress the float and jet does it stop the fuel ??

Yes - I tried this on the bench, propped hte carbs up with the fuel tank above the carbs. Turned the petcock on and the fuel would drip out the floats - one by one, I lifted the floats and shut the valve - no more fuel out. But I was thinking that the float valves arent sealing with only the pressure of the floats, but will seal with me pressing on them. I have no idea how old they are. I just took the carbs apart again, since I'm waiting on more the parts anyways.

750goes · « **Reply #19 on:** January 20, 2007, 09:13:12 PM »

take the floats off and then use your finger or something else to hold the needle in place to make sure it actually seals and how much pressure is needed to stop the flow - it should not take very much at all

the needle end should also have a small spring pressure to close it... do they all work ??

.RJ · « **Reply #20 on:** January 20, 2007, 09:17:33 PM »

Yeah, all the spring things work on the float valves - I'll try sealing them without the floats, thats a good idea.

Although thats just stuff to keep me occupied while I wait for parts

Too bad there's nothing else to fix on the bike!

.RJ · « **Reply #21 on:** January 22, 2007, 07:50:17 PM »

Keyster rebuild kits showed up today.... assemble the carbs and one of the f'ing pilot jets breaks off while screwing it in to the carb body - I hadnt even bottomed out the threads!!!! I broke a drill bit off in the jet trying to extract it too. That one's my fault, but now the carb body is trashed.

Ugh. Cheap ass aftermarket junk. I'm going to get the dealer to order me the parts I need. I just tore them down again, and I tore down the nasty spare carbs I have as well - I'll use those bodies instead. Guess I'll get some carb cleaner tomorrow and soak them for a few days.

Ugh.

.RJ · « **Reply #22 on:** February 02, 2007, 06:43:37 PM »

Another set of carbs in, bike starts, runs... I need to change the plugs and balance the carbs so I havent gone much further with it. Stock main/pilot jets, pod filters, needle clip lowered one notch to start with.

However - I Still have the problem of the fuel flowing out of the overflow tubes. The float valves are new. Float heights are set. If the bike is running, they dont leak at all. Bone dry. When I stop the bike and let it sit, fuel starts pissing out of the overflows, pretty evenly on all 4.

What gives? This is starting to piss me off.

750goes · « **Reply #23 on:** February 02, 2007, 08:08:32 PM »

I have run out of ideas...but check your float height again..............its the only thing I can think of .........

.RJ · « **Reply #24 on:** February 03, 2007, 03:39:54 PM »

Took the carbs apart again, soaked/cleaned them, lowered the floats a little - seems to have stopped puking fuel. All the floats float, the float valves are sealing.

But, new problem. Argh... as soon as I start the bike, the engine revs shoot up, with the idle screw backed all the way out it idles around ~3000, sometimes will surge, sometimes will die, choke on/off makes no difference - so, I've got a massive air leak from... somewhere. But, where? The carb boots are tight. This is totally new and I've no idea where it came from..... dammit. Ideas now?

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Author Topic: 400F carb rebuild questions (Read 7363 times)

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