Topic: '78 CB750k conversion to cafe racer (yes, more issues)

[SOHC4 Cafe Racer Fan]

Well, I am back.

This past winter was real rough on me. I was closing in on buying a 550 to add to the 750 I already had, and I found a real good deal on CL and was all set to close on the sale the next day when I got a pink slip. 

Fastforward to now.. things are stabilizing and I'm slowly getting into the swing of things- just got my 750 out of winter storage, giving her the once over. She fired up good, right out of the gate but a weird problem developed:

When I accelerate, the rev stays stuck for a while before it starts to come down. Problem doesn't seem to be with the throttle handle.. it always springs back after I let it go. I took the gas tank off to look at the other end of the throttle cable and twisted the throttle a few times and it always snaps back every time on the carbs end. This is a problem that seems to occur only when the engine is warmed up.

When the engine was running, I twisted the throttle and the rev stayed high so I reached inside and tried to close it down by pressing on top of the throttle valve lever that goes across the 4 carbs but that didn't help.

That sounds like it was a nasty patch.  Glad to hear that things are more stable now. 

[flybox1]

re-secure all the boots attached to the carbs.
air leaks can cause hanging rpm's

[deafcaferacer]

Thanks CFR, it'll always be the 550 that got away. Suspect the guy selling it was an intermediary or got a hand me down.. it was vastly undervalued.

Flybox, I did go over the carb boots, they are secure.

But I went over the throttle pull cable again, carefully, and it appears there is a split in the cable where it meets the knob. Probably best to play it safe and order a new pull cable.

[SOHC4 Cafe Racer Fan]

Thanks CFR, it'll always be the 550 that got away. Suspect the guy selling it was an intermediary or got a hand me down.. it was vastly undervalued.

Flybox, I did go over the carb boots, they are secure.

But I went over the throttle pull cable again, carefully, and it appears there is a split in the cable where it meets the knob. Probably best to play it safe and order a new pull cable.

We all have a few of those, both with two wheels and two legs.

[onetruepunk87]

I just dealt with the same problem of hanging RPM's and it ended up being my spark advancer. Try taking the advancer off and cleaning/lubing it. It's behind the points plate so you'll have to reset your timing afterward, gave me an excuse to replace my points/condensers while i was at it. After that, no more hanging RPM's.

I was chasing my tail checking for airleaks, checking my throttle cables and making sure the grip wasn't hanging up until I found the fix. Here's the page I found the info from: http://forums.sohc4.net/index.php?topic=89464.0

[deafcaferacer]

Thanks for the heads up punk.. the hanging revs continue for me, but I am working things out, progressing from the more simple remedies to the most complex. Right now I'm trying a seafoam mix in the tank to see if it's a matter of gunk in the carbs being responsible for the hanging revs in not allowing the slide to seat properly.

Failing that and other diagnoses I have in mind, I'll be moving on up to trying the more complex solutions including examining spark advancer, and worse case, cleaning the carbs.

[flybox1]

Thanks for the heads up punk.. the hanging revs continue for me, but I am working things out, progressing from the more simple remedies to the most complex. Right now I'm trying a seafoam mix in the tank to see if it's a matter of gunk in the carbs being responsible for the hanging revs in not allowing the slide to seat properly. 

Failing that and other diagnoses I have in mind, I'll be moving on up to trying the more complex solutions including examining spark advancer, and worse case, cleaning the carbs.

  are you saying you haven't cleaned the carbs yet, and you're trying to 'fix' it with seafoam?

[deafcaferacer]

  are you saying you haven't cleaned the carbs yet, and you're trying to 'fix' it with seafoam?

Yes, why not?

If the seafoam doesn't do the job, wouldn't that indicate serious crud or some sort of impediment or blockage that would point to hands-on cleaning of the carbs being required?

If the seafoam does the job, or if the new throttle pull cable does it, then I would deem it unnecessary to take the carbs off.

To be more clear, the revs don't hang that long, just enough to be annoying, but they eventually come back down. May just be a matter of the bike just coming out of storage.

[flybox1]

seafoam is not going to clean it as well as your hands, some brushes, wire and carb cleaner, and its really not that hard.
get in there.  you've had this bike for over a year now, and never cracked them open to see if the tiny holes in the emulsion tubes or pilot jets are blocked beyond what seafoam could potentially liquefy.  how lone before you purchased the bike were the carbs adequately cleaned? 
how positive are you that your accel pump is working and your check valves are not blocked? no doubt you've done the valves and timing, and a few other things required to get the bike running, but why leave out carbs? in storage all winter, too. 
i'm just a little puzzled

[deafcaferacer]

seafoam is not going to clean it as well as your hands, some brushes, wire and carb cleaner, and its really not that hard.
get in there.  you've had this bike for over a year now, and never cracked them open to see if the tiny holes in the emulsion tubes or pilot jets are blocked beyond what seafoam could potentially liquefy.  how lone before you purchased the bike were the carbs adequately cleaned? 
how positive are you that your accel pump is working and your check valves are not blocked? no doubt you've done the valves and timing, and a few other things required to get the bike running, but why leave out carbs? in storage all winter, too. 
i'm just a little puzzled

I understand what you're getting at and yes I'm eventually going to take the carbs off at some point, but not before I get my soda-blasting setup. In the meantime I'm sure it won't hurt to see what seafoam can get done.

I got the new pull throttle cable and installed it this past weekend, and that seemed to help a lot in getting the slides to reseat. Still working on my first tank of seafoam infused gas, so we'll see how that goes.

But another problem has emerged- my taillight bulb has suddenly burned out, but the strange thing is that the plate illumination LEDs are fine, only the running and brake LED's refuse to work, and there doesn't seem to be anything wrong with how the wiring is set up, and it was fine last riding season. Two bulbs have burned out quickly.

This is the bulb I am using:

http://www.superbrightleds.com/moreinfo/tail-brake-turn/1157-led-bulb-dual-intensity-25-led-motorcycle-bulb/170/

[deafcaferacer]

Had a talk with my landlord and asked him if I could soda-blast the carbs on the premises and he nixed that idea before I could even finish my sentence.

What I'd give to have my own garage.

I'll still take the carbs off for cleaning. I also bought a volt meter to try to figure out what is going on with my taillight.

[deafcaferacer]

Testing, one, two, three, four. Am I talking to myself?

Pulled my carbs. Ready to dive in for the cleaning.

[deafcaferacer]

Using this excellent reference:

http://www.salocal.com/sohc/tech/carb/carb.htm

I had the carbs disassembled in just a little over an hour without too much trouble.

After reading various threads on the philosophy of cleaning carbs, I'm going to go with the simple green APC approach and a scrub brush. Off to the store to get those plus #60-80 drill bits and soft wires for the emulsifier air bleed hole.

Nothing looks suspect except for the gaskets which look and feel like they need replacing.

[deafcaferacer]

Found these at CVS after reading up about it on a thread in this forum. Good tip!

Used them to scrub out the bigger holes and passageways in the carbs.

[deafcaferacer]

Before and after results of soaking in simple green 1:1 for a few hours and then scrubbing, rinsing, and blowing off with compressed air.

[harisuluv]

Hanging revs are often a sign of being too rich.  Another cause could be an out of sync issue. 

Looking at some of the pictures it looks like you have some kind of custom airbox? 

Unfortunately there is quite a laundry list of possible carb issues that it could be.  What are your air screws set at?

By the way, it looks like someone at some point swapped your float bowls for the earliest style k0-k1 bowls with the smaller drain screw and no oring, instead aluminum washer.  This type of thing is pretty common, but it is just evidence of some parts swapping, so I would check the other parts inside for the right part. 

I just saw that you have a '78 bike/engine, but using round top carbs?  The engines are different in some specs so I don't know how this affects your bike.  The "PD" style carbs that come with a 78 run leaner, which would make sense with what you're experiencing.

Also, are those the boots that came with your bike?  They look like they might be 77-78 style boots, which are different than 69-76 boots. 

If you are interested in getting the right core for your bike I might have the correct PD set in stock, you can PM me.

[deafcaferacer]

Hanging revs are often a sign of being too rich.  Another cause could be an out of sync issue. 
Looking at some of the pictures it looks like you have some kind of custom airbox? 

No custom airbox, just pod filters.

Unfortunately there is quite a laundry list of possible carb issues that it could be.  What are your air screws set at?

one full turn backed out from closed.

By the way, it looks like someone at some point swapped your float bowls for the earliest style k0-k1 bowls with the smaller drain screw and no oring, instead aluminum washer.  This type of thing is pretty common, but it is just evidence of some parts swapping, so I would check the other parts inside for the right part. 

I just saw that you have a '78 bike/engine, but using round top carbs?  The engines are different in some specs so I don't know how this affects your bike.  The "PD" style carbs that come with a 78 run leaner, which would make sense with what you're experiencing.

I have a '78 frame, but the motor and carbs are '73. I don't think this is strictly a mechanical problem as the bike was running fine last season. The revving issues only came up after I retrieved the bike out of storage. I now wish I had stored it with the carbs dry.

Also, are those the boots that came with your bike?  They look like they might be 77-78 style boots, which are different than 69-76 boots. 

Yes, those boots came with the bike, and I don't suspect them of being '78 as they've never been removed from the bike as far as I can judge.

If you are interested in getting the right core for your bike I might have the correct PD set in stock, you can PM me.

Will do if the need arises, thank you.

[deafcaferacer]

Just noticed that one of the pinhole washers for the bowl vent is missing (bottom hole). Is this cause for concern?

My assumption is no because those vents simply supply air into the bowl for the fuel level balance, and push to feed the jets?

[harisuluv]

that's normal, there's no washer in there actually.  you should find the same thing but opposite configuration in the #1 carb.  on that side there is no vent, #2 and #3 have actual ducts on both sides.  1 and 4 has no venting on the outer side.

So nothing changed with your bike.  Are you using the same gas?  It could be bad.

[calj737]

Found these at CVS after reading up about it on a thread in this forum. Good tip!

Used them to scrub out the bigger holes and passageways in the carbs.

Thankfully, your carbs will not suffer gingivitis in the near future... Good hygiene is important on more than just your teeth 

[deafcaferacer]

that's normal, there's no washer in there actually.  you should find the same thing but opposite configuration in the #1 carb.  on that side there is no vent, #2 and #3 have actual ducts on both sides.  1 and 4 has no venting on the outer side.

So nothing changed with your bike.  Are you using the same gas?  It could be bad.

It's been through two tanks of gas, one had the seafoam additive.

Nothing changed, physically- except for winter storage. I'm thinking I didn't properly winterize in terms of stabilizing the gas and leaving the carbs dry.

We'll see what happens when I fire it up again after I get and install the new float bowl gaskets. Will report back here then.

[deafcaferacer]

Found these at CVS after reading up about it on a thread in this forum. Good tip!

Used them to scrub out the bigger holes and passageways in the carbs.

Thankfully, your carbs will not suffer gingivitis in the near future... Good hygiene is important on more than just your teeth 

The pilot jets on #3 and #4 had plaque buildup!

[deafcaferacer]

Went back together without too much trouble. Managed not to miss a step except for one of the washers on the choke yoke for #2 which I found under the carbs when I lifted it to go put it back on the bike. That made for some choice words.

Back on the bike now and waiting for the float bowl gaskets.

[deafcaferacer]

Well this is very frustrating.

Got the  gaskets and the new pull throttle cable.

Gaskets went in good.

Pull throttle cable doesn't fit at all. Even though the motor is a '73 (and so I got a '73 throttle cable) the throttle case on the handlebar is a '78, so the cable doesn't fit in on this end.

But if I get a '78 pull throttle cable, it's certain to be too long because of the different position of the carburetor linkage.

WTF?

[calj737]

Get a generic brand like MotionPro and order it by length, not model.