Topic: Tuning some Mikunis

[MisterKnife]

Alright, couple years back I bought a set of mikunis(VM34) for my cb550 2->4 which was a stupid buy (saving up for some Cr26's)
I'm now tuning dialing in the tune but I've ran into an issue where I got a hanging throttle at around 1/4 sometimes if I blib it it drops straight to idle then.

I have an A/F gauge on my exhaust and I'm not even close to lean on that point (around 11 VERY rich, just to get rid of this spot)
My throttle cable I made myself and is routed ideally and smooth as butter.
There is a set of UFOs in the carbs that could increase the vacuum pressure in a certain way maybe.
I even polished the carb slide hole to make it even smoother but that didn't help.

Maybe the venturi is so large that the fuel doesn't get properly atomized thus creating a lot of unburned fuel in the exhaust while the burning of fuel is actually lean?

I'm a bit lost on what I could do to remedy this. Any insights would help

Thanks

[MisterKnife]

Tried al sorts of different needles and needle jet combinations. Now running a H3 which has a early taper, thinking it will get richer on the 1/4 and get rid of that sticking. It did get richer but it didn't help

[BomberMann650]

Provided the A/F meter is calibrated.  That much fuel down low makes a certain sound, possibly some smoke, from the exhaust

I recall the VM's having an air jet in the bellmouth.  And they offer a variety of needle jets (the brass tube holding the main) which can fine tune the mid opening.

Another suspect for poor return to idle can be the advancer mechanism.  If the timing unit is equipped with the oem part.

Good luck, the twin carb setups do work once adjusted

[MisterKnife]

Yeah at 11 af it smells quite a bit so I think the af gauge is good.  Also my fuel economy validates this. Ha

Yeah I have all sorts of needle jets and jet needles to bring it to a 12.8just had it this rich to validate its not lean somewhere

I’m going to be checking the advancer mechanism; maybe that gets stuck in the advanced position.

[MisterKnife]

Just took the advancer assembly apart; did notice that one of my springs was 'loose' removed a loop, lubed it up and seems to advance correctly at around 2600-2800 rpm. Hoping this was it.

But alas. Problem still persists
I did calibrate my AF gauge.

here is a video (the first number on the gauge is not readable in the video but it's a 1)

https://youtube.com/shorts/1V37hyBsBBE

So it 'hangs' just after the advancer kicks in.

[MisterKnife]

Maybe the springs of the advancer are still out of sync in terms of NM force. Will measure the sizes needed for those and order new ones just in case.

[CB750R]

Used carbs, maybe make sure there’s not a slide hanging on you? Pop the filters off and inspected in the shop? Air intakes leaking?

You’ve synced the carbs with gauges?

If you’re saving money for carbs I’m beyond happy with the RS34 s on my build. Been extremely easy to dial in, very close to ideal jetting right off the bat, and each change made has been informative. I’ve read that the CRs can be finicky.

[MisterKnife]

I'm the first owner of those carbs. I did had once some grains of sand inside one of the carb. cleaned them out perfectly and polished out any imperfections. No hanging of the slides what so ever if I try them dry. I need to apply a decent amount of force pushing it from the filter side to simulate vacuum to get them to stick. But I dont think that it's the amount that will be applied by the engine

Yeah the carbs are synced with a digi sync close to perfection as I could get them.

[CB750R]

There must be a difference in AF from normal idle to where it’s hanging. What are the differences?

Probably want to confirm that at operating temps.

If you can get it to hang in the shop I’d be wiggling cables and having a look in the mouth of the carbs to see if one or all are hanging.

[BomberMann650]

Watched the video.
Sounded like it can idle at 1400 without throttle input.
What happened at 3500rpm before the blip though?  Was your hand on the throttle

Seemed to return well enough after the blip.

If you can find someone with a dyno and patience for rapid carb rejetting.  Would recommend doing that.

[MisterKnife]

Before the blibi forced the throttle back to idle so it’s just hanging in the advance.

TheAF on the hang is 11 I also had this around a good 12.8 with a different set of needles but same outcome. Oil temp was around 85c

I do have an electric ignition and going to validate that the hall sensors are at the right distance. Might be off in spacing

Also when it hangs I see both slides just above idle hang. Yes there is enough slack in the cable

[CB750R]

Is this one of those dual carb setups like Murray’s or speed moto? If so maybe looking into others who’ve got that setup would be some help.  Those VM carbs have individual cables I’ve hot had a bike with that setup so not super familiar.

Also it’s sounds like cam chain rattle at idle, is the tensioner set? Hate to ask, but if not , cam timing could be off and as that has a major effect on how the engine ‘breathes’ it can appear to be a carb tuning issue… hate to admit but I speak from experience on a ZRX I owned.. was one tooth off and spent weeks jetting the carbs…

[MisterKnife]

The cam chain tensioner is a tad worn but what you are hearing is the primary chain. Still within limits but will be replaced once I do the engine overhaul.

Cam timing is correct; Installed the chain myself after a good head tune up. (clean valves en remove carbon)

It's a set from murray indeed. But had to tune it after. From what I remember before I converted my fork and all the electronics I didn't had a holding at full advance.
I'm going to verify that the ignition is correct and report back.

[MisterKnife]

So I moved the hall sensors closer to the shaft. Jetting turned way lean after that. So happy me thinking that was it. Adjusted the jetting annddddd bam still there.  :'(

 

[BomberMann650]

before I converted my fork

Might be worth double checking the cable in the throttle tube. 

Glad you got it to spark a bit stronger.

What number did you determine as "way lean"

[MisterKnife]

15 going into 18.

I also rebuilt the advance mechanism today. made the two springs equal in length and strength. Advance is now decent slowly increasing from 2100-2700 rpm range. So that's a (check)

I got a feeling I didn't clean or synced float levels of my carbs well enough. Cyl 1 = dark plug Cyl 4 = beige. Then I can have all the AF gauges I want but that will throw off readings. I also might be left over residu from the ignition issue.

Will take them both apart tomorrow (again) just to be sure.

What do you mean by throttle tube?

[BomberMann650]

15 going into 18.

15 okay - 18 not so much

What do you mean by throttle tube?

The business end of the handlebar!

Its easy for a cable to twist or kink inside that housing.  Drag on stuff, get stuck against a ledge or burr.  Sometimes the last quarter of the rotation doesn't happen cause the carb spring has used most of its sprung..

[MisterKnife]

O yeah no the throttle is super smooth and also new. Using 1.25mm wire so sticking g is no existent

[MisterKnife]

So i've tried multiple stuff again.

- Rebuild the carbs totally with even more slack in the cable and they are super clean now. Idle is set correct at around 1200-1300 rpm with an AF of 13 (1.75 turn out of air)
- Replaced the spark plugs and noticed that only Cylinder 1 is dark. the orthers are in the good color range.

This is weird as Cyl 1 and 2 share the same carb so how would this be possible? I have adjusted the valves to the loose side of the spec.
Spark cap on that cylinder is also new

Any suggestions?

[MisterKnife]

I did check for air leaks but couldn't find any. and 1 and 2 share the same inlet so if that would be leaking 2 should be dark also right?

[MisterKnife]

Tried stepping up one pilot yet and get more air in but that was too rich so back to the current one.
Waiting not for the gasket sheet and will make new intake gaskets, also do a 3k tune up once more, might even remove the head to check the valves and cam position

[CB750R]

Tried stepping up one pilot yet and get more air in but that was too rich so back to the current one.
Waiting not for the gasket sheet and will make new intake gaskets, also do a 3k tune up once more, might even remove the head to check the valves and cam position

Going up a pilot jet in the bottom of the carb will of Richen the mixture, where as bigger air jet in the mouth of the carb will lean the idle circuit.

It kinda sounds like you’re throwing a lot of adjustments at things at once. Carb tuning should be a one thing at a time and asses process. You’ll end up chasing your tail if you can’t rule out what works and what doesn’t.

[MisterKnife]

I didn't do multiple adjustments at the same time for tuning the carbs. I just enlarged the pilot yet and was too rich at 2-2.5 turns out. Was seeing if I can get the bigger pilot to be in the higher range of turns out instead of the lower range on a smaller yet.

What I suspect what is going on is that because the needle jet and jet needle are set too rich on the 1/8-1/4 and are adding to the problem. Read some conversations that it could happen if the needle system is too rich it could suck fuel while returning down and sustaining the engine in that RPM range.
This and/or float height that makes it too easy to get fuel from the needle jet while in the off idle position.

I'll be testing both systems tomorrow after i've done the 3k tune and replaced the gasket on the intake manifold (just to be sure)

[BomberMann650]

Any progress?

The swapping needle jet isn't a bad idea.

[MisterKnife]

Yes progress, at least in ruling out stuff.

Tried all of the above and nothing helped getting rid of the issue and the sometimes handing throttle.
The smaller needle jet leaned it out to a nice 13-15 but then lacked in power. Float height I adjusted only 1mm and it leaned it out a tad but no help either.
Also made a 3d printed fixture for the throttle cable splitter so it's in the center line of the bike in the most advantageous position. As I'm starting to think this would be a mechanical issue.
The cable movement is yet again super smooth.

I've also ordered a set of VM32's to see if my carbs have just #$%*ed slide throats. Wanted to go smaller in venturi so I can drop the UFO's which pull in more vacuum from the needle jet.
They are coming in somewhere this week so I'll report back once I have tested those out.