MikuniCarb

MikuniCarb

Leonard discovered during pre-flight inspection at Oshkosh 2002 that as he puts it, "The Mikuni gas inlet pipe is pressed into the carb base and a gas line clamp can be selected that is too wide and further the clamp can be over tightened"... In the next three pictures you see how the pipe was forced out of the carb base and then safety wire being used as a clamp to resolve a future failure... A one cylinder flight would be one to remember...

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Ultralight Aircraft Alert

Aircraft:

All ultralight aircraft using the Bing or Mikuni carburetor.

Incident report:

Ultralight pilots using engines equipped with the Bing or Mikuni

carburetor, report a sudden loss of power, although the engine will still

run at an idle, this has happened both in flight and on the ground.

Upon closer examination it was found the retaining clip which holds the

needle located inside the throttle slide of the carburetor, controlling the

fuel air mixture to the engine, had slipped or vibrated off allowing the

needle to fall into the brass insert thus cutting off the fuel supply to

the engine.

Suggestions:

On the Mikuni carburetor it is recommended the customer update his slide

and needle to the new style which allows for a plastic insert underneath

the clip so that there is no metal to metal contact, and provides a

vibration dampening effect.

On the Bing carburetor, I recommend that you update to the new clip and to

replace the clip on a regular basis (say on a yearly basis) as they seem to

wear with time, also I recommend that the pilot do a preflight check on

this part on a regular basis.

Incident Report:

AAIB Bulletin No: 12/98 Ref: EW/G98/07/25 Category: 1.4

Aircraft Type and Registration:

Rans S6-ESD XL, G-MZBD

No & Type of Engines:

1 Rotax 503 piston engine

Year of Manufacture:

1996

Date & Time (UTC):

19 July 1998 at 1700 hrs

Location:

Sittles Field, Nr Lichfield, Staffordshire

Type of Flight:

Private

Persons on Board:

Crew - 1 - Passengers - 1

Injuries:

Crew - None - Passengers - None

Nature of Damage:

Significant damage to the landing gear and cockpit floor

Commander's License:

Private Pilot's License

Commander's Age:

42 years

Commander's Flying Experience:

226 hours (of which 25 were on type)

Last 90 days - 32 hours

Last 28 days - 20 hours

Information Source:

Aircraft Accident Report Form submitted by the pilot telephone inquiries

with owner, aircraft repairer and other agencies

The pilot reported that during the final leg of a cross-country flight,

whilst flying at approximately 1000 feet amsl, the engine RPM decreased

from 5,800 to approximately 3,800 and the unit began to run very roughly.

Various throttle positions were tried, none of which improved the

situation. After some 30 seconds the engine stopped completely, the height

by then being 750 feet agl or less.

In view of this low height, no attempt was made to re-start the engine.

The pilot judged that the only safe landing field available was directly

beneath the aircraft and accordingly he commenced a steep right-hand turn

through 360 degrees to position the aircraft into wind and facing up the

local slope. Unfortunately, after straightening out the aircraft,

insufficient airspeed and height remained to enable the aircraft to

round-out into the uphill flight-path necessary for a landing. The

aircraft stalled whilst about 10 feet agl.

Examination of the engine subsequently revealed that a needle in the jet of

one of the carburetors had detached and fallen into the jet area. It

detached as a result of breaking in the plane of the locating groove after

severe machining type wear had been inflicted as a result of continuing

rotation of the needle relative to its mounting circlip. This problem has

been encountered in the past; it occurs under certain conditions of

vibration and resonance. It led to the fatal accident of the similarly

powered Colb Twin-Star, Kolb, in 1994, see AAIB Bulletin 11/94. As a

result of that accident, the AAIB made the following comments and

recommendation to the CAA:-

"The cause of the partial engine failure was that the needle on the rear

carburetor had worn through at the circlip and had dropped into the

carburetor jet, restricting the flow of fuel to the rear cylinder. The

needle was retained in position by a circlip which was spilt at the apex

of the hole holding the needle; the edges of the split had 'machined' away

the groove in the needle as it rotated under the forces generated by

vibration. This problem is well known and both the UK distributor and the

manufacturer (Rotax) have issued safety bulletins calling for a 50 hour

inspection of the needle and an ultimate life of 150 hours.

Rotax are developing a modification for new production which will

introduce an 'O' ring to each side of the circlip to prevent the needle

rotating. The modification is due on the production line at the end of the

year. It is therefore recommended that:-

94-35 The CAA require that the modification to the carburetor needle

fixture on Rotax engines be made retrospective and mandatory, and that in

the meantime the 50 hour check be also made mandatory."

The CAA did not accept this recommendation. Their reasons were stated in

their document 'Follow-up Action On Occurrence Report ' No F30/94.

The modification ultimately engineered by the manufacturer is slightly

different from that described above. It has now been available (at minimal

cost) for retrospective installation for approximately 18 months. It is

installed on current production engines, although the fact that suppliers

and builders of kits frequently store engines for extensive periods results

in new aircraft continuing to come into use with engines which may have

been built before the modification became part of the production standard.

The aircraft involved in this accident is understood to have completed 49

hours total flying at the time of the accident.

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Leonard's head modifying video does not say how long the intake tube should be... The tube needs to be as short as possible... We mounted the flanges with just enough room between the head and the flange to put the carb mounting bolts thru with the bolt head on the cylinder side...

There are numerous Internet and after market manual information sources for the Mikuni... Some of these sources have misleading information in them... Click here for the Adobe acrobat .pdf format manual an authorized Mikuni VM carburetor Super Tuning Manual... Study the carb function drawing on the last page carefully for important understanding of the parts interaction within... In this manual you will see references to aluminum or zinc carbs - you want aluminum carbs as the zinc carbs weigh 1/2 pound more!... Further note that the choke circuit only works with the throttle slide closed!...

We have tried VM-32 rebuilt carbs and strongly recommend builders use NEW carbs... If you go with rebuilts be sure to totally examine them... Our two carbs had 4 exceptions that should ground an airplane and included one worn jet needle at the circlip!... You could rebuild the carbs yourself and install the correct wear parts - we will supply a list of SUDCO part numbers for the necessary items at a later time...

Leonard calls out a carb that has a choke lever mounted right on the carb... We were able to get VM32-282 carbs (we have changed many parts in these carbs and they may not be the best choice for you) which require the choke mechanism be added to them... The Universal choke cable will not reach back to the instrument panel so we bought the lever type choke conversion kits... Don't worry about forgetting to close these chokes before you get in the cab because they are really aggressive and have one position... We hate the crude look of the lever chokes and now are searching for the plunger type choke controls used on motorcycles...

You will need to understand the workings of this carb and test your way to the correct power jets in your engine... The #60 drill Leonard mentions in the engine plans is the equivalent of a #140 Hex power jet opening - much smaller than we are using... Be sure the Dual throttle cable is adjusted to open both carb throttle slides at the same time...Tuning dual carbs is very tricky if you don't have EGT's... If you are using leaded Av gas tuning is done by reading spark plug color - light brown... When the engine is broken in enough to take full power, run it at 3100 RPM for three minutes and then check the color of a plug... Adjust the main jet sizes if required... Then run the engine at 1800 RPM for 3 minutes and adjust the main jet needle heights if necessary... Finally adjust the idle needles for best idle - 2 turns on our engine... The initial idle speed setting is 1/4 to 1/2 turn... Unleaded auto gas always leaves a black spark plug - you must have an EGT system to tune!... The main power jet size vary from one engine/cylinder to another... Our engine is using #260 (#2 cyl) & #280 (#4 cyl) jets, Joe Maynard has two engines that use #320's... Mike Ward is getting 1100 degree EGT readings on 34mm carbs (VM34-???) with a #140 jet in #2 cyl and a #120 jet in #4 cyl - engine is 69x92 stock cam 7.5-1 compression approx. at 4000 FT ASL turning a 52x22 prop 3220 RPM static...

(Service note: Be sure to replace the clamps provided on the rubber flange carb mounts with the wormdrive screw type commonly found in plumbing and hardware stores. The provided clamp will buckle and release the carb if a side load is put on the aircleaner which is easy to do accidentally.) We are running a safety wire from the carb throttle slide barrel across the rubber mounting flange and around the intake pipe as an additional precaution to having a carb come loose in the air... Examine the rubber flange carb mounts regularily for wear and secure attachment to the carbs and the aircleaner attachments too...

Be sure to dig a hole for the tail wheel to sit in that establishes the climb out angle of attack and do a power run getting at least 3200 RPM for 4 minutes before first flight...