Author Topic: '78 CB750F2 Carbs (Read 5919 times)

SurfinBird · « **on:** June 02, 2011, 07:58:39 PM »

So i'm 99.9% sure the carbs are the issue on getting this thing running.

I adjusted the floats to the specified height of 14-15mm (using Clymers and a downloaded shop manual for reference). When I turned on the tank fuel poured out of the overflow hoses. Does anyone know the height I should be using? The actual floats are flat on the fuel side (bottom) all the floats I see are rounded. I don't think that's the problem since that's the illustration in the clymers manual shows for the F2/3. So at the stated height the bottom would be parallel to the carb lip (hope that makes sense)

Also, it seems like the fuel isn't getting past the carbs. When I pulled the plugs, after I flooded the carb, they came out dry and had no fuel smell (brand new plugs). These carbs are different from the other 750 carbs, there's a accelerator pump on the #2 carb and no diaphragms on 1,3 or 4.

Any ideas? Here are some photos for reference

Spanner 1 · « **Reply #1 on:** June 02, 2011, 09:04:02 PM »

O.K. well......your carbs need cleaning to get the bike running. Starting with the slow jets. Hmmp?
You have to pull-out the slow jets to clean them..... also a try of the accelerator pump with the air box removed...... best thing would be lots of reading here first, do a search.... loooooads of info, good luck

bollingball · « **Reply #2 on:** June 03, 2011, 02:48:57 AM »

Surf forget about measuring use the clear tube method.if you look at one of the bowls at the top is a little bit of a ridge it measures 3mm below the gasget that is where I set mine. The flat bottom float is correct for these carbs.
Clean the carbs then clean them again then one more time should do it

Ken

jtb · « **Reply #3 on:** June 03, 2011, 05:41:28 AM »

Surf, I set mine the same way you did. Silly question, but did you close the drain screws at the bottom of the float chambers? I had one that leaked even when closed, I chucked the screw in a drill, and smoothed the tapered end with some 400 grit.

Spanner's probably right about the slow jets, they're a b!tch.

SurfinBird · « **Reply #4 on:** June 03, 2011, 06:03:43 AM »

Thanks guys, after doing some research it looks like I have a F3, not that it matters that much, the carbs are pretty much the same.

I'll tear this thing down and clean it within an inch of it's life. I read the slow jets are press fitted and don't come out, do I just clean them in the body? Or is there a trick to getting them out?

It appears the floats are set correctly (downloaded an original shop book for the F3, illustrations show the floats as I have them), with that said, would anything else cause fuel to pour out if the overflow tubes?

Bankerdanny · « **Reply #5 on:** June 03, 2011, 06:11:20 AM »

Just because the floats are set doesn't mean that the float valve is closing.

You also should check the overflow tubes for cracks. I had a persistent leak on one of mine from a cracked tube.

Finally, pull one of the float bowls and GENTLY push the float up and see if it stops the flow of fuel. If it doesn't then the problem is a bad valve or something obstructing the hole to keep the tip from seating and sealing out the fuel.

It does seem odd to me that ALL the carbs are leaking heavily out of the overflows. To follow up on JTB's thought, take out the float drain screws and make sure that they have their o-rings on them.

SurfinBird · « **Reply #6 on:** June 03, 2011, 06:32:24 AM »

...and this is why I come here for help. I should have taken a minute and see if it was all the carbs leaking or just one. When I saw a pool of gas forming I shut off the tank and pulled the carbs without checking to see which one(s) was overflowing.

SurfinBird · « **Reply #7 on:** June 04, 2011, 09:33:43 AM »

Would the pilot screws being cranked down tight keep it from starting. These things were cranked down to the point that I may have to tap one out.

Also, do I need to completely separate the carbs from the stay to give it a decent cleaning. I have #1 and 4 off, but don't want to take the throttle linkage apart if I don't have to.

TwoTired · « **Reply #8 on:** June 04, 2011, 10:31:20 AM »

Quote from: SurfinBird on June 04, 2011, 09:33:43 AM

Would the pilot screws being cranked down tight keep it from starting. These things were cranked down to the point that I may have to tap one out.

The pilot screws block the entire pilot circuit from delivering fuel. If the acell pump is working and the throttle held open the engine will start and race, but dies at/near idle settings.

The pilot screw tips have a very shallow taper. And must only be gently seated. Forced seating damages the seat in the carb body and even the screw itself. At worst, the tip seizes (even breaks off) in the carb body rendering the pilot circuit worthless, (and the carb body as well). At best it simply distorts the adjuster screw seat, making predictable settings impossible.

Quote from: SurfinBird on June 04, 2011, 09:33:43 AM

Also, do I need to completely separate the carbs from the stay to give it a decent cleaning. I have #1 and 4 off, but don't want to take the throttle linkage apart if I don't have to.

No, they can be cleaned as a bank. All the bits that get fuel deposit residue are accessible with the carb bowls off.

The pilot jets are pressed in, and really can't be reliably, predictably cleaned while installed. But, if you are confident they aren't blocked/restricted, they don't need to be removed.
They can be grabbed and pulled straight out. I use a soft jawed parallel pliers for this. But, they can be hard to find. Regular pliers can be used if you use a leather pads on the jaws.

Carefully check the stand pipes in the carb bowls for cracks.

Cheers,

BobbyR · « **Reply #9 on:** June 04, 2011, 12:21:28 PM »

Quote from: SurfinBird on June 03, 2011, 06:03:43 AM

Thanks guys, after doing some research it looks like I have a F3, not that it matters that much, the carbs are pretty much the same.

I'll tear this thing down and clean it within an inch of it's life. I read the slow jets are press fitted and don't come out, do I just clean them in the body? Or is there a trick to getting them out?

It appears the floats are set correctly (downloaded an original shop book for the F3, illustrations show the floats as I have them), with that said, would anything else cause fuel to pour out if the overflow tubes?

The slow jets are easy to remove and should be cleaned out with a strand of copper wire or high E guitar string. You really only need to be careful about deforming them, if you get a scratch on them it means nothing, using pads can cause you to squeeze to hard. Once they are clean you can tap them back into place. I promise you they are dirty.

The 78 carbs do have a pump and there is such a small amount of fuel in there it goes bad real fast. Remove the screws and clean the crap out. Be careful of a small ball in there.

There is is problem with the tubes in the float bowls developing cracks which causes them to leak as you described. The cracks can be soldered

Spanner 1 · « **Reply #10 on:** June 04, 2011, 06:58:07 PM »

Are 'we' sure were not confusing the bowl drain screws with the 'pilot screws.'... just sayin' ......?
( Pilot screws being vertical between the carb and intake manifold and accesed from below with a small stulbby straightedge screwdriver ).

jtb · « **Reply #11 on:** June 04, 2011, 07:16:32 PM »

Pilot screws should be turned out 1 3/4 turns after seated. If the one that is stuck is a 2 or 3, I probably have an extra body if you need it. You won't need to pull the slow jets to try getting a high e string through.

BobbyR · « **Reply #12 on:** June 04, 2011, 08:39:35 PM »

Quote from: jtb on June 04, 2011, 07:16:32 PM

Pilot screws should be turned out 1 3/4 turns after seated. If the one that is stuck is a 2 or 3, I probably have an extra body if you need it. You won't need to pull the slow jets to try getting a high e string through.

You will need to pull the emulsion tubes to do a proper job.

bollingball · « **Reply #13 on:** June 06, 2011, 10:46:45 AM »

jtb There is no way you can get the wire through the cross drilled holes of the slow jet without pulling them also no way to tell if they are clear without pulling them.

SurfinBird · « **Reply #14 on:** June 06, 2011, 11:08:45 AM »

Spanner 1:
Yup, i'm sure that its the pilot screws, they are vertical, outside of the bowl (into the body)and have a spring and looks kind of like a needle. The stuck screw is on #4.

I'm getting a e string today or tomorrow. Hope thing starts after this I want be riding this summer

jtb · « **Reply #15 on:** June 06, 2011, 06:15:31 PM »

Quote from: bollingball on June 06, 2011, 10:46:45 AM

jtb There is no way you can get the wire through the cross drilled holes of the slow jet without pulling them also no way to tell if they are clear without pulling them.

Bollingball, I can see all the way through the slows on pd carbs with them still in the body.

Surfinbird, if the pilot screw seat is trashed, I've got a #4 body.

John

TwoTired · « **Reply #16 on:** June 06, 2011, 07:12:17 PM »

Quote from: jtb on June 06, 2011, 06:15:31 PM

I can see all the way through the slows on pd carbs with them still in the body.

How can you offer credible advise if you have never seen what a pilot jet looks like?

The first pic shows you the small emulsion tube buried in the body when installed. Still think you clean the emulsion tube holes with a wire with them installed?
The next pic shows the pressed in pilot type that the OP has. They look the same, apart for no threads on the body.

You simply can not clean them positively while still in the carb body.

SurfinBird · « **Reply #17 on:** June 06, 2011, 10:43:37 PM »

Thanks jtb. I'll let you know once I get the screw out.

Spanner 1 · « **Reply #18 on:** June 06, 2011, 11:03:58 PM »

With respect

... with a PD carb off the bike and the bowl off I think it is possible to see a speck of 'daylight' thru' the slow jet if you care to peer up thru' it......... if it's clear that is. Agreed it doesn't prove the side emulsion holes are clear........

BobbyR · « **Reply #19 on:** June 07, 2011, 12:55:02 PM »

True you can peer up and see daylight. The holes in the emulsion tubes do serve a purpose and there are posts in which Hondaman has discussed the merits of adding additional holes to improve performance. In bikes that have been run for any amount of time in the last decade you will invariably find white deposits in those hole from the now banned MBTE additive. I have found nothing to dissolve it, so it must be mechanically removed.

I rebuilt my carbs 3 times and the reason I had to go through the removal and installion process 3 times is I cut corners, I finally took my time and went through them item by item.

Now I have a nice even 1200 rpm idle, and good response through the entire power band. TT is correct, nothing beats attention to detail when it come to carbs.

jtb · « **Reply #20 on:** June 07, 2011, 05:08:36 PM »

Well, you're right. It's been so long since I had one of the slows out, I forgot they had the emulsion tube holes. Last night I just grabbed a body from my carb junk box, and could see light on the other end when I looked through it. All of which proves two things:
1. Senior moments are real... and
2. There is light at the other end.

Blitzburgh207 · « **Reply #21 on:** June 07, 2011, 06:18:29 PM »

I was missing one of the tiny washers that goes on the end of the pilot jet spring and it made it a bi$% to turn when it was close to tight. A lot of pressure let me back it out - but I'm not sure how trashed your seat is..

SurfinBird · « **Reply #22 on:** June 08, 2011, 12:57:32 PM »

I noticed that the washer and gasket didn't come out of the pilot screw holes, I think I see them in there, looks like they might be jammed in.

Any ideas to get them out so I don't ruin the seat? I only found it on #1, i'm cleaning/rebuilding them 1 at a time and think they might all be jammed considering how tight they were to begin with.

SurfinBird · « **Reply #23 on:** June 09, 2011, 11:07:41 AM »

So i'm going to get a screw extractor to get the pilot screw out of #4. The screw is pretty small, is there anything in need to know before I tap it? Will liquid wrench help at all?

SurfinBird · « **Reply #24 on:** June 14, 2011, 07:20:10 PM »

Well, the screw extractor did nothing but make it even tougher to get the pilot screw out. Thankfully jtb is sending me a #4 body to swap out this one. I was tempted to put the carbs back on and see if the cleaning helped, but i figured I might as well wait for the #4.

Mew problem. Before I disassembled the carbs the choke and throttle worked perfect. I have since found out everything moves fine until i screw in the holding plate facing the engine. When I put it back on both the choke and the throttle bind, pretty much non functional. Am I missing something?

Here are pics of the #4 with the pilot screw stuck and the plate that's causing the problems.

thanks again jtb, haven't got the body yet, but i'm sure it'll be real soon.

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Author Topic: '78 CB750F2 Carbs (Read 5919 times)

bollingball

bollingball