85 GPz 750 Cafe Project

I've got 135s in it now which seem silly big. I think I have 142.5's, why not. I have not run it without the K&N's on it. I was thinking I would do that just to see the sliders operate dynamically. Although I expect them to behave with no load.
I had a local buddy ask me about slider springs. As I don't know a lot about the carbs history, I pulled out the ones I have for the BS34's. I found two different sizes, see pic below. The larger springs where in it and I thought AHA! No, behaves slightly better but if I was to characterize the high rpms performance, I'd say it runs like $hit. Tomorrow is shot with chasing down the CSR. I'll be back at it Saturday.

I'll watch the sliders with the filters off running. I'll put boots on it and see how it performs. If there isn't a breakthrough I'm going to break down and try the other set of carbs I've been threatening to put on.

I'm struggling to connect the ignition issues to running lean, and it is running lean. See pics below.

No change in symptoms or performance with the lighter slider springs. Today I tried removing the K&N filters, installing the stock airbox boots and running with those uncovered like velocity stacks. Same, maybe even worse performance. Short vid of the bike running in neutral while watching the sliders.

For some reason I thought you were running a stock airbox. With pods you might have to make some pretty large changes in jetting. I would search for a Dynojet kit for your bike, known to work with pods. They usually have radically different metering needles.

Have you changed needle positions? (Sorry if this is stuff you already mentioned. I didn't go back through all of the pages.)

I've messed with the needles a bit. With 120 mains I raised the needles all the way and got another 1K rpms out of it. I put it 135s with the needles on the 3rd notch from the top. Almost no difference. (Much bigger jet, dropped the needle two slots)

You have your work cut out for you trying to make the CV carbs work as well with pods as they do with the stock airbox since a lot of engineering went into the design of the airbox to tune it to make the carbs work well. Take a look at the video in the link below; it shows how CV carbs work. Notice at time 1:20 when the throttle is opened the throttle vacuum passage opens and the throttle vacuum passage retracts. This causes the throttle slide and jet needle to rise. The amount of air that is drawn through the main intake is critical in that the more air drawn through it the less vacuum is available at the throttle vacuum passage which in turn affects the opening of the throttle vacuum passage. At time 1:56 with the throttle wide open the vacuum at the vacuum passage pulls the diaphragm and jet needle all the way up.

As I see it the difficulty in getting pods to work well on the CV carbs is dealing with the fact that the resistance to air flow through the main intake is reduced which in turn reduces the vacuum available to pull the diaphragm up. Consequently, tuning the carbs so the engine runs great at wide open throttle will normally be detrimental to the way the engine runs at other throttle openings and vise versa. This may not be of any importance to a person using the bike on the track since he will not necessarily care about how the engine performs at moderate throttle openings, but for street use this can be an issue.

In any case, you may find the video interesting and informative. Ed

I'm following Ed and it is very interesting, expands my understanding. Are there modern carbs which are not CV carbs? Buying new carbs may be preferred versus a complete redesign of the bike. The real estate that used to be occupied by the air box now houses the electrical components and battery. All would have to find new homes and there is no room at the inn.

You could always just do the old cardboard flap test.

Take all four filters off.
Cut a rectangle of stiff cardboard so that it's wide enough to cover all four intakes.
Start the bike and let it warm to where it will idle.
Then slide the cardboard over the intakes, but leave it held open at an angle, sort of like a trap door, so you don't cover up any breather ports etc.
Then try revving.
There will be a position for the cardboard such that you will get full RPMs available to redline.
If this does not work there may be a more problems than just the jetting. Like clogged mains, or ignition, etc.
This has always worked. By restricting the opening, you restore vacuum to the main throat.

In your video, I see the slide blip open, but I don't really see any fuel spray out of the needle jet. When the slide first opens there should be a pretty good spray visible (but it might not show up on video very well).

NEVER LOOK DIRECTLY INTO THE THROAT OPENING WHILE RUNNING. Flames can shoot out 3 to 4 feet, and douse you with flaming gasoline.

I'll see if there is a video online.

Somewhere after 3:30 ish, you can see the fuel spray.

I'll do that today Loudhvx. Yesterday I took a step back to verify some basic items. I checked cam timing. Some have suggested it may be off one tooth. Once assembled I'm going to put another bank of carbs on it off of an 82 GPz750 I have to see if that changes anything. I'll do the cardboard test then. Cam timing shown below:

I thought this Q&A might possibly hold a clue for your problem. If not, its still interesting. This is a great project that you are doing. It's inspiring and you are very creative and talented. You will fix this, I'm quite sure.

Here's the link: www.motorcycleclassics.com/tech-corner/h...roblems-zb0z1203zbea

New set of carbs, 42.5 pilots, 125 mains, runs fantastic all the way to 9K. ( I stopped there) I pulled a plug and I know with certainty it isn't rich. They look just like they looked before. Since the white powder of lean was on them when I started with this set of carbs, I suspect I won't be able to read these unless the AFR is extreme one way or the other. Tomorrow I'll color tune the idle circuits and vacuum sync the carbs. Over joyed may describe my state of mind well. Between the tank BS and the carbs, I lost about 6 months of my life. I think that just means I'm slow.

Nice, that's good to hear. I'm slow too so I can't say anything.

Are they a different type of carb, or can you see any difference between the good carbs and bad carbs?