Mikuni manifolds and intake tract length

[JimF]

As you who have been following my Mikuni jetting thread and my "ankle buster" thread know, I have been trying to resolve a 6000 RPM artificial rev limit and a over-rich mixture which may or may not be related to each other.

While I play with jetting to cure the too rich condition and while I also try a HV coil swap to see if that impacts the rev limit as part of a comprehensive process of elimination, I just want to ping the experts/racers here regarding the possibility that the (and I am not sure I have the terminology correct) intake tract length, that is to say the distance from the carb outlet to the cylinder head manifold could be creating the artificial rev limit and/or the over rich condition.

The carb is connected to the manifold by means of a rubber hose that is about 2" (50mm) long.

intake tract.jpg

I have not as yet looked at the manifold to understand why the carb outlet is not directly attached to the manifold but I expect that it would have been if not for some mechanical impediment between the manifold and the carb body .

Would increasing the intake tract length:

a)adversely affect the quantity of fuel mixture entering the cylinder head at higher RPMs causing the engine to sputter (not sure if it's starved for fuel or beginning to foul the plug at this point.)

or

b) create an overly rich condition? I know there is a whole science of fuel/air mixture associated with the tract length, the cylinder's volume, the exhaust stroke and valve timing etc.

Jim

[DewCatTea-Bob]

" While I play with jetting to cure the too rich condition and while I also try a HV coil swap to see if that impacts the rev limit as part of a comprehensive process of elimination, I just want to ping the experts/racers here regarding the possibility that the
intake tract length,
could be creating the artificial rev limit and/or the over rich condition. "

____ I doubt that the ign.coil is responsible for the issue, but is the correct/stock 'green-label' ign.coil the particular HT.coil you're using ?
I'd try swapping the carb for one off another (good running) Duke, before trying same with the ign.coil.
__ The inlet-tract length should not directly effect the air/fuel-mix,
but it could effect power @ RPM if the cam-timing is off...
Does the engine produce good/normal power at all the RPMs that it will run at below 6-grand ?


" Would increasing the intake tract length:
a)adversely affect the quantity of fuel mixture entering the cylinder head at higher RPMs causing the engine to sputter (not sure if it's starved for fuel or beginning to foul the plug at this point.) "

____ No, not even UNadversely, or even at less than "higher RPMs" .
And certainly it's not beginning to become "starved for fuel" OR fouling the plug at that point in the RPM-range.


" b) create an overly rich condition? "

____ If duct/tract length could directly reduce the amount of AIR or increase the amount of FUEL that the carb has already metered, then that sure would over-complicate life for tuners !
__ Where did you get that Mikuni-carb from ? _ You seem to be experiencing the same issues as others who try to install a carb intended for a 2-stroker onto a 4-stroke engine.
__ Did you determine it's 'size' yet ?
Can you list all the meter-numbers of the internal air/fuel metering-parts ?
____ I must say that trouble-shooting most everything-else while trying to use a STRANGE carb, can quite likely be a real time-waster !
You should've tried another (known to be good-to-go) carb, from the onset of any such trouble (as you've ran-into).


Hopeful-Cheers,
-Bob

[wcorey]

Would increasing the intake tract length:...

While the length can have some effect, it would be very small/subtle and nothing remotely on the scale of what you're experiencing.

I have not as yet looked at the manifold to understand why the carb outlet is not directly attached to the manifold but I expect that it would have been if not for some mechanical impediment between the manifold and the carb body .

Since the mikuni's use a spigot as an attachment method, the type of manifold adaptation on your motor is pretty typical. It can be an advantage over a straight bolt up arrangement (like with the amal's) as it provides isolation for the carb from heat transfer and vibration. It also allows for some latitude on location and placement angle.
If you wanted a shorter/cleaner looking arrangement, while still retaining the isolation benefits, there are molded rubber adapters available that have a two bolt flange and spigot all-in-one but not sure if there are any with the right size combination for your setup. I used one for the TM mikuni on my 450 but had to make an adapter to alter the mounting angle a bit.



These guy's have some:
http://www.shadetreepowersports.com/cgi ... anges.html

Bill

[Eldert]

a friend of mine had a built in rev limiter all of a sudden last year .
it turned out to be 1 broken hairpin valvespring .
it took him a long time to find out . changed ignitions a couple of times

Eldert

[JimF]

When I take the tank off to change the coil I will take a look at the valve springs. Thanks for the tip on the valbe springs. I had a report of similar symptoms that were coil-related.

The coil is likely the correct one as evidenced by a visible green-colored label.

I could take the SSI29 off the Mach 1 and bolt it on.

I have not had time to take the carb apart. The side business suddenly picked up and I am prepping another (non-Ducati) motorcycle for sale this weekend.

I like the idea of using the Mikuni only because the PO affixed a Dell'Orto air filter from a Scrambler to it so to the untrained eye the whole thing has a stock look to it.

I have had great success with SSI29s, but never have I got one to work right with an air filter of any kind mounted up.

[DewCatTea-Bob]

" I had a report of similar symptoms that were coil-related. "

____ The battery-type ign.coils which depend on 'satuation' (and lose time to become sufficiently satuated as RPMs climb), may possibly become too weak to produce a fucnctional spark at extra high RPM,, but if that issue became a problem at only 6-grand, then at even half that RPM the ign.spark would be marginal, (since satuation-time is relatively short).
__ BUT your 'green' type ign.coil doesn't depend on becoming saturated ! _ It's the type which rather depends on a (rather strong) 'pulse' of DC (from the supplied AC-waveform) in order to produce IT'S ign.spark. _ And as the revs build-up, it gets STRONGER (rather than weaker), which has been the known benefit of magneto-type ign.systems,, (and may be the reason why the Mk3-model out-tops the otherwise virtually identical Mach-I).
So (with an ohm-meter) you may wish to check for any power-leakage perhaps found somewhre in the ign.circuit, from the alt.stator to the points location... A leaky condenser could possibly bleed-off some of the juice needed by the ign.coil.
However, I much doubt this unlikeky possibility so long as that non-stock carb is employed.


" I could take the SSI29 off the Mach 1 and bolt it on. "

____ Since Bill is willing to allow you to borrow his 18-degree AAU, perhaps he'd then also let you try-out his R/T's 29mm-VHB carb.


" I have not had time to take the carb apart. "

____ So you didn't make any note before of what the meter-numbers are of the Mikuni's metering-parts ?


" I like the idea of using the Mikuni only because the PO affixed a Dell'Orto air filter from a Scrambler to it so to the untrained eye the whole thing has a stock look to it. "

____ I do agree that that's a fair reason to stick with it (if there's no cost, of course).


Hopeful-Cheers,
-Bob

[vintagetour]

Jim,
I have experimented with both 2 stroke and four stroke (yes a Mikuni is designed for both) and changing the intake distance a little doesn't seem to make any difference. Mine is butted up against the intake and connected with a rubber hose exactly like yours. I don't have a tach so I can't say what it runs like at 6000 rpm's but it runs and sounds good all the way to max rpm's. You might want to check the low speed jet. I know that if I change the size of the low speed jet I will have to make needle adjustments. If you haven't already cleaned the carb good you might want to try that before swapping carbs. The difference between a modern Mikuni and the old Dellorto or Amal or any other factory carb is evident the first time you crank the motor. Now I know all the purists will tear me apart for that statement but all I know is they work on my Montesa, my Vincent, my Yamaha and soon on my Goldstar. Experience is my only expertise

[DewCatTea-Bob]

" (yes a Mikuni is designed for both) "

____ Not any-O Mikuni is intended for "both", so you must've actually meant that Mikuni makes Mikuni-carbs for both (2-cycle and 4-cycle engine-types).


" I don't have a tach so I can't say what it runs like at 6000 rpm's but it runs and sounds good all the way to max rpm's.
The difference between a modern Mikuni and the old Dellorto or Amal or any other factory carb is evident the first time you crank the motor. all I know is they work on my Montesa, my Vincent, my Yamaha and soon on my Goldstar. Experience is my only expertise "

____ So how about telling of what the specific meter-numbers are of the metering-parts within your Mik.carb (on your Mikuni-carbed Duke) ?


Hopeful-Cheers,
-Bob

[graeme]

Jim, I have read your posts re the various problems with your bike. I may have missed some bits but, do you definately know that this bike ran well previously? In this same configuration? Has it been poorly built? Bad porting? Poor compression? Is there a smooth transition from carb to head port? May be worth checking to be sure. Good luck. Graeme

[vintagetour]

190 main, 22.5 pilot, 00 needle, 5E75 jet needle, 1.50 cutaway on the slide (previously posted in Where to buy Mikuni Jets). And yes Bob I meant they make carburetors for both. Mine is for a four stroke and the main jet/atomizer is smooth with inlet bore (not protruding). Something to check though Jim if you take it apart.