1982 CB750F...Better Devil

[Jimbonaut]

My guess is this carb vacuum synching game is like a lot of things motorcycle...that is it'll get easier the more you do it. Took me a decent hour, these carbs are really sensitive. Just touching the nut/adjuster bolt would set them off, let alone moving anything. Still, got there in the end. Here's the before -

and here's the after -

Setting up the Carbtune tool was really straightforward, it's a very good piece of kit. You have to install some dampeners (supplied) into the hosing which stops the levels from vibrating too much and makes it easy to read. Glad I finally bought the thing. Next up, Colortune.

 

[cbrianroll]

Sweet, how erratic are those?

 

[Jimbonaut]

Sweet, how erratic are those?

Not very - I mean I don't really know (I've never done it before or used any other manometer) but from what I can tell it's a solid piece of kit

 

[cbrianroll]

Looks like a good one. Looking forward to your colortune review.

 

[doc_rot]

both solid investments. A good tool worth the money.

 

[Jimbonaut]

I like the fact that it’s pretty much idiot proof. Doesn’t require much setting up (once you’ve installed the dampeners) or calibration, and because the mercury has been replaced by steel tubes there’s no danger of spilling the stuff all over me or sucking it back into the engine. I’m sure there are plenty of other models out there but I can certainly vouch for this one.


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[Jimbonaut]

The Colortune was money well spent too. Really easy to use, works best in a darkened garage so you can see the colour of the ignition. Essentially it’s a windowed spark plug that allows you to see inside the combustion chamber while the air and gas is igniting. You’re looking for a “Bunsen blue” flame to indicate correct air/fuel mixture - blue/white is too lean and yellow is too rich. My mixture was dead on, but you can adjust the colour of burn if necessary by adjusting the mixture screw accordingly.

This is at idle. There are other tests you can do at different throttle positions but I’m yet to do them -

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[CarbsAndCylinders]

What throttle positions do the instructions say that you can use. From what I have read, you can't use it for wide open throttle positions to read the main jet.

 

[Jimbonaut]

What throttle positions do the instructions say that you can use. From what I have read, you can't use it for wide open throttle positions to read the main jet.

Good question, not sure the answer yet. I spent most of last week up a ladder or wrapped around a baseboard as we're painting our place but I'm hoping to duck out to the garage later today. From what I understand there are other tests you can do with the the colour tune but I'm not sure the extent of them. I thought the main jet only really came into play when the engine was under load anyway? Could be wrong about that. I'm not sure how much load you can put on this tool, not much I don't think. Maybe a gentle trot around the block.

 

[Jimbonaut]

Never mind the colossal fail I had trying to reverse bleed the new brake lines on my 750F build. When I reverted to the usual method of filling up the reservoir and pumping the brake lever, I could hear a wheezing sound coming from the mc. Sure enough it was was leaking fluid from the rubber boot on the bottom. Removing the mc and taking it apart didn't reveal much, but was I doomed to fail anyway with the internals I've got in the thing? I rebuilt the mc with new spring and gaskets etc, but the piston is the original one and looks a bit beaten up -

The mc itself looks a little roached but I think the bore is ok. I measured it and it's within spec. The outside is in poor shape but from what I can it's because it's had a bunch of battery acid dumped on it back in the day.

Need a new piston?

 

[chickenStripCharlie]

That is a strange looking piston.
Is that the rear MC?

The surface where the seals sit looks better than the rest of it. What does the bore inside the MC look like? Same as outside or smoother?

 

[cbrianroll]

There are a few videos on youtube regarding idle color, 1/2, and 3/4 or something like that. If I followed it correctly it gets less bunsen blue as it opens...I could be wrong though lol...only watched it once.

 

[cbrianroll]

You can download the instructions from their website also.

 

[Jimbonaut]

It's all good - I have the instructions and all is well. I've now tuned all 4 carbs with the ColourTune and get a nice blue bunsen flame across all 4, going more white with an open throttle.

So that master cylinder piston - think that could be the cause of the leaking mc?

Edited - my bad, I mean push rod not piston.

 

[pidjones]

Yeah, that is the push rod. The piston is up inside. If you haven't had the piston out, or if you put it in dry, it might be your problem. There are two cups inside - one on the piston rod near the back and one against the spring with the piston pushing against it. Both cups open towards the outlet. There are two holes to the reservoir from the cylinder. One is easy to see, and one is tiny. Both need to be open. The tiny one on my build was behind a hex bolt tgat seemed to do nothing but cover the holes. I use a high "E" guitar string to open the smaller hole.

 

[Jimbonaut]

Jesus Murphy. Brakes.

Rear brake, no drama. Took about 20 minute, tops. Front brake...different story. Figured out what I was doing wrong with the whole reverse bleed process, and after getting fluid appearing in the reservoir after injecting it into both calipers I started congratulating myself on almost bleeding the lines in about 5 minutes. Big, big mistake.

Started bleeding the lines the usual way to finish the job - 4 hours later and still squishy brake feel. Fuck me running. Kept my zen on pretty much the whole time which I was very proud of (it was a lovely day after all) and then after 4 hours and 1 minute I lost my shit, chucked my toys out of the crib, steamed home and bought a vacuum brake bleeder tool. 5 star ratings, here in a couple of days. Bollocks to it.

 

[chickenStripCharlie]

"Fluid appearing in the reservoir"

Hmm .... Were you trying to pump ALL of the brake fluid in the reverse manner?

I poured brake fluid in, did conventional bleed until I got fluid at the calipers. Stopped. Did reverse bleed (slowly!) until bubbles stopped coming up at the master and the fluid level came up to the top.

Then did one more conventional bleed to make sure. Worked well.

It took me two days, because I took a break in between to research reverse bleed a little bit. But in total maybe 2hrs. Worked well for my setup (dual calipers, both lines running direct off master cylinder)

 

[Maritime]

I reverse bleed until the master is to the full line and then try them, they usually don't need to be re-bled in the conventional way as there should be 0 air at all in the system.

 

[Jimbonaut]

I'm thinking maybe I didn't reverse bleed enough. I did it (on both calipers) until I saw the first signs of fluid in the reservoir. Maybe I'll drain most of the fluid in the reservoir and have another go at the reverse bleed again. Thanks chaps.

 

[Maritime]

Do you still have the splitter in the lines? id so you need to bleed to the splitter, then to the MC.