Gave up on the Mikunis

[MustangSix]

The Mikunis had a lot of potential and a lot of good qualities. Immediate starts, instant revs, potential for high power. But they were proving to be a bear to tune. I couldn't get the idle to my liking and the mixture needs to be fine tuned. The idle air bleeds, jetting, etc. all need the attention of a motorcycle mechanic, and I am not the guy. I think that this is a system that could really wake up a 200, but I am not in for all the effort.

So this past weekend I cobbled up something more easily tuned. I added a "log" to the stubs.

Actually, it looks like a log, but it is internally divided with each CD175 feeding three cylinders. There is a balance hole in the two dividers giving me a common chamber to pick up manifold vacuum for the PCV. I'll use ported vacuum from the front carb for the distributor until I get the EDIS working.

This should idle better and be very responsive. And I know how to tune these things. 8)

 

[Anonymous]

Did you synch the carbs?, I know it makes a big difference on my bike. It'd be like the old Honda six cyl bike.

 

[Asa]

it's a Jag!
hey.... Jag is now owned by Ford, maybe we could get 'em to bring back the six through them....
hrmmm...

 

[Anonymous]

how did u internally divide it? are there holes in the divider plates to allow vacuum for the balance hole thing? the way i picture it is two plates welded up on either side of the balance hole...but that would mean that its sealed and vacuum wouldnt get to the balance hole...i feel sorta dumb but i would really like to know, thanks...It looks like a really sweet setup. when are u gonna fire it up?

 

[Anonymous]

Did that free up enough room to solve your distributor clearance problem??

Still looks cool enough!

Doug

 

[Ronbo]

Aha! There is a reason to split the log! Its ironic that I just started inquiring about splitting the log on the 170-250 forum. Anyone care to explain why you would do this so I can be edumacated?

 

[Asa]

better distribution of fuel, wouldn't have the leaning out of the end cylinders
and it looks cool 8)

 

[MustangSix]

I cut two slots in the log about 2" apart at the center. I inserted two divider plates with a 1/2" hole in the middle and welded the plates into the slots. You can see where I have a hose barb in the top. That's picking up the vacuum between the two plates.

The idea is to allow the divider to allow some balance between the carbs at idle. At higher rpms, each set of three cylinders only effectively sees one carb, so carb velocity is higher, giving me better atomization and, hopefully, improved torque. The manifold is functioning like a dual plane manifold.

I'm using a modified Volvo linkage that is pulled by the motorcycle cable that was previously operating the Mikunis. The distributor is still tight, but I can get the cap off now. With the Mikunis I couldn't even see the distributor.

The air filters will be hanging in the breeze outside the bonnet. I think I'll carve some out of aluminum. I have some 6" filters that should work.

I still need to make spacers between the carbs and the manifold, and then I'll start it up. I have a bunch of these Strombergs. Maybe next I'll try three. 8) Actually, the same design would allow me to mount one of these 2bbl Autolites I have on top of that log.

Aussie head, US block, Swedish linkage, British carbs, Jaguar gauges, MG wipers, Triumph wiring, Mustang Brakes, Granada rear......All I need now is a few parts from a Nissan....

 

[addo]

Funny you should mention the Swedish bit. It's reminding me of an old Volly and also a Triumph 2500S... Dare I add, if you use HS-6s, you can index the fuel bowls to hang more vertically!

 

[Anonymous]

Thats pretty awesome...thanks for the explanation on the dividers. i just picked up a new hood so i can customize it and give me more clearance...i will then be trying a 2x1 setup later in the year, alhough it wont be as fancy as yours. Im also curious as to what that engine is in...is it a hot rod or a dragster type car?

 

[Ronbo]

I'm going to guess and say that two plates were used in order to create a chamber for the manifold vacuum otherwise one would have been fine?

 

[MustangSix]

Exactly right Ron. I wanted a chamber for the PCV connection. Also I added a second tapped hole which I can use for diagnostics.

 

[SuperMag]

Looks slick! But... (here he goes again)

With that location, I'd be concerned that the incoming PCV vapor would dilute the mixture going to the #3 and #4 cylinders, thus causing a leaning or starving effect at those locations. I'd be inclined to "Y" the PCV line making sure that each leg was equal length, and mount a PCV bung in each of the carb spacers.

 

[MustangSix]

Ideally, you could have a separate line tied from each runner to a common line to the PCV and an individual throttle blade and venturi for each cylinder to make sure everything is equal.....wait, that's what I had!

SM, you could do that. But in the real world, a little incoming crankcase fumes is not going to have a big effect, especially since it's really being fed into a plenum, not the cylinder runner.

On every intake stroke, even at just 80% VE, theres almost 27 cubic inches of air/fuel mixure being sucked in. That effectively evacuates the plenum along with crankcase fumes on each stroke. In reality, every cylinder ends up getting pretty much the same thing.

 

[SuperMag]

On every intake stroke, even at just 80% VE, theres almost 27 cubic inches of air/fuel mixure being sucked in. That effectively evacuates the plenum along with crankcase fumes on each stroke. In reality, every cylinder ends up getting pretty much the same thing.

True. But keep in mind that the PCV line is effectively open to the atmosphere on account of your vented oil cap. I have no way to support this supposition, of course, but my thought would be that your "effective plenum" for each bank of cylinders is the plenum itself, the PCV line (and to a certain extent, like you say, the adjacent plenum). Hence the holes closest to the PCV source would draw the greater percentage of PCV supplied air.

I still remember the set-up on the dual-carb Datsun roadster I rebuilt years ago. Nissan ran the crankcase vent line from the valve cover to the backside of the oval shaped air cleaner, which served both carbs. Inside the air breather the line split into a "T", with each split dumping its "charge" of vent gas directly into each air horn. Pretty slick in that this arrangement assured that whatever came in through the air horn got mixed in the carb. It certainly made carb tuning easier and more precise, as you weren't trying to jet (needle, actually-- they were SU carbs) for the incoming air through the carb plus some unknown volume of air being sucked directly into the manifold.

Having said all that, with your arrangement I'd nix the vented oil cap. That way, the only thing that came through the PCV line would be a little inert gas, instead of a little inert gas + some amount of "dry" air that varied with the level of vacuum. *gets off his soapbox*

 

[MustangSix]

My turn on the soapbox!

It doesn't quite work that way from what I have seen. I think you are completely overestimating the effect the PCV has over a space of mere inches thru a 1/2" hole, and that you are incorrectly envisioning the way air flows in a plenum chamber.

#1, #2 and #3 runners are less than 3" apart from each other. None of those three cylinders ever inhale at the same time in this dual plane manifold. Each cylinder, in turn, will completely ingest everything in the small plenum, plus whatever the PCV feeds it, plus whatever the carb feeds it. The mere proximity of the PCV line makes no difference. If the PCV were feeding a non-homogeneous wet mixture like fuel/air, maybe, but this is all gaseous. Just because it is closer to #3, the PCV gasses will not get "lazy" and travel no further to #2 or #1. Those gasses have no choice. They'll get sucked up by whatever cylinder is on intake stroke. To take your lead, I offer up a dual carb Datsun manifold as an example.

On the left side of the plenum is the PCV connection. If it worked the way you envisioned, only #4,5, and 6 woud be fed by the PCV because they are closer to the line. But it's a constant vacuum for all intents and purposes and both ends of the manifold are serviced equally thru that balance passage.

As another example, on the crossflow the PCV is connected to the plenum directly opposite #6 runner. No noticeable effect, zero, none, on mixture on that cylinder. That plug looks just like all the rest when you pull it. IMO, trying to equalize the flow of a PCV in a plenum is an engineering exercise in vaulting over anthills. Equal is good, but some things ain't worth equalizing. Taken to an extreme, I guess I could equalize the vacuum to the distributor, too. Just kidding.

Also, without a vented cap, you would be sucking in unfiltered air thru the oil seals. Not good. In the stock crossflow arrangement, as in most cars, that opening goes to the air cleaner, not to dump vented gases, but to a small filter as a clean air source.

 

[SuperMag]

Boy, does that pic bring back memories! Although mine was a four-banger, and the heat shield was water cooled. And I suspect the above pictured manifold is 1970 vintage? (Mine was a 1966, and had no PCV provision in the manifold, just the vent to the airbreather as I described above.)

It doesn't quite work that way from what I have seen. I think you are completely overestimating the effect the PCV has over a space of mere inches thru a 1/2" hole, and that you are incorrectly envisioning the way air flows in a plenum chamber.

That's entirely possible, and the way you explain it does make sense. But my thoughts on the matter are not mere suppositions. In the past I've had idling problems and other carb related tuning difficulties in a number of vehicles that either went away or became much easier to dope as soon as I put the PCV inlet in a central location. Just below the base of the carb seems to be the best location, in the absence of a provision in the baseplate of the carb itself. Bent Eights with single plane manifolds seem to be particularly sensitive in this respect, just as much as dual plane eights.

Given the discussion to this point, would it be safe to say that location of the PCV inlet is irrelevant, unless the plenum volume exceeds that of the cylinder displacement?

 

[MustangSix]

The Master wrote? Boy, that is giving me WAY tooo much credit, in spite of my self proclaimed title....(maybe I could just change it to El Presidente for Life)

Hmmm..... Plenum volume would probably make a big difference. In a huge plenum, the smal volume of air entering from the PCV might dilute the nearest runner.

 

[SuperMag]

 

[Anonymous]

Oh, I'm so sad..
Your work is so beautiful!
I wish I lived closer to you, so I could have helped you with the tuning. I've worked with the bike carbs for so many years that they're second nature to me....

One of the things that made this particular setup harder to tune is the CV type carbs. They are not as intuitive as the slide type, especially in the off-idle regions. The mix goes rich until the pistons start to rise and lean if they rise too far, too fast. The passage that controls this, sometimes referred to as the "vacuum jet", must be resized for the larger bores of the 200 CID engine. The ones that are in those carbs are for a proportionately smaller bore, I imagine, so the passages are too big for the 200.

A small 'balancer' vacuum line is often connected between the carbs in larger bike bore setups to help reduce the roughness in the idle-to-1/8-throttle region. This causes some crosstalk between the cylinders to even out the piston movement between cylinders in the larger bores. This is also the ideal place to pick up some vacuum, via a check valve, for those power brakes. The brake booster itself is enough of a reservoir, in most cases.

If you ever try it again, PLEASE contact me, I can help you understand it better.