Author Topic: NGC mikuni carb question  (Read 435 times)

Online Canuck750

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 1042
  • Location: Edmonton, Canada
NGC mikuni carb question
« on: June 20, 2021, 05:36:44 PM »
I need some advice for tuning a pair of Mikuni carbs I have on my Morini 500. Previous owner fitted them and they work much better than the Dellortoís that were on the bike. I have watched some videos on tuning the VM Mikuni and I have the idle jets were I want them. Video says get the main jet sorted for WOT before playing with needle and clip. Ignition is electronic Sasche and itís set and forget. The carbs were set for sea level but I am at 2000+ above sea level, I knocked the idle jets down from 30 to 27.5 and the plugs now have a nice light brown colour.  Bike idles and runs well but at WOT it just bogs and runs quite rough. How do I know if itís trying to gulp more fuel or drowning from too much?



48 Guzzi Airone, 57 Guzzi Cardellino, 65 Benelli Barracuda, 66 Aermacchi Sprint, 68 Gilera 106SS, 72 Eldorado, 72 Benelli 180, 74 Guzzi 750S, 73 Laverda SF1, 74  Benelli 650S, 75 Ducati 860GT, 75 Moto Morini 3-1/2, 78 Moto Morinii 500

Offline SED

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • Posts: 1549
Re: NGC mikuni carb question
« Reply #1 on: June 20, 2021, 07:55:22 PM »
Hit the kill switch, pull the clutch, stop and pull a plug?  Have you tried flipping the chokes on when it starts to run rough?  I did this when playing with a VM on the GTV and if the mix was too rich it would stumble worse.  This was at part throttle.

Often it is said that if the engine picks up when backing off from WOT it indicates a lean main.

What size main have you got in it?
1983 LeMans III
1981 Monza
1947 Ariel Red Hunter
1939 Ariel Red Hunter
1937 Guzzi GTV

Online SIR REAL ED

  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 1190
  • uh.... it's personal....
  • Location: Forest, VA
Re: NGC mikuni carb question
« Reply #2 on: June 20, 2021, 07:57:25 PM »
I need some advice for tuning a pair of Mikuni carbs I have on my Morini 500. Previous owner fitted them and they work much better than the Dellortoís that were on the bike. I have watched some videos on tuning the VM Mikuni and I have the idle jets were I want them. Video says get the main jet sorted for WOT before playing with needle and clip. Ignition is electronic Sasche and itís set and forget. The carbs were set for sea level but I am at 2000+ above sea level, I knocked the idle jets down from 30 to 27.5 and the plugs now have a nice light brown colour.  Bike idles and runs well but at WOT it just bogs and runs quite rough. How do I know if itís trying to gulp more fuel or drowning from too much?

Does it run better when the bike is hot or cold?  If it runs better cold it is rich, if it runs better hot it is lean.

Also when you have the throttle wide open and it starts to bog, rapidly close the throttle (which leans it out), if it runs better for a second, it is lean.

You can also play with the choke, if it runs better at a set throttle with a bit of choke, it is lean.

Hope that helps.
« Last Edit: June 21, 2021, 04:40:11 AM by SIR REAL ED »
"There's two kinds of people in the world; those for whom no explanation is necessary, and those for whom no explanation is possible."

2016 KTM Duke 690
2019 Beta EVO 250
1982 Yamaha Seca 650
1999 Suzuki DR 650 w/790cc kit
1986 Honda XL600R

Online Canuck750

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 1042
  • Location: Edmonton, Canada
Re: NGC mikuni carb question
« Reply #3 on: June 20, 2021, 09:59:24 PM »
Main is a 130

I donít think it runs much better hot or cold, bike is warmed up by the time I reach the freeway and get a chance to open it up.

If I back off slightly from WOT it runs better.

I will try a bit of choke at WOT tomorrow

I had the bike out three times today trying different things and double checking the electronic ignition settings.

Tried lowering the needle, ran very poorly so went back to 2nd notch from bottom.

Thanks for the suggestions, will try again tomorrow

Jim

48 Guzzi Airone, 57 Guzzi Cardellino, 65 Benelli Barracuda, 66 Aermacchi Sprint, 68 Gilera 106SS, 72 Eldorado, 72 Benelli 180, 74 Guzzi 750S, 73 Laverda SF1, 74  Benelli 650S, 75 Ducati 860GT, 75 Moto Morini 3-1/2, 78 Moto Morinii 500

Wildgoose Chase Moto Guzzi

Re: NGC mikuni carb question
« Reply #3 on: June 20, 2021, 09:59:24 PM »

Offline SED

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • Posts: 1549
Re: NGC mikuni carb question
« Reply #4 on: June 20, 2021, 11:42:53 PM »
From memory Honda 160 twin with VM22s is running 115s.  GTV VM28 ran a 140 and 500 Bullet VM28 ran 130.

What is the original Dellorto size?  When I get home I could measure the diameter of several Mikuni jets in mm to convert to Dellorto numbers.

The Honda and GTV Mikunis from Sudco had very small atomizers (needle jets) and drilling them out made a big improvement.
1983 LeMans III
1981 Monza
1947 Ariel Red Hunter
1939 Ariel Red Hunter
1937 Guzzi GTV

Offline huub

  • Guzzi Mentor
  • ****
  • Posts: 638
Re: NGC mikuni carb question
« Reply #5 on: June 21, 2021, 03:46:08 AM »
at WOT , slightly close the throttle , if it accelerates ,  it is too lean
130 sounds small ,
i run my V65 lario with 34 mm mikuni VM , i run 210 main jets.

Online Canuck750

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 1042
  • Location: Edmonton, Canada
Re: NGC mikuni carb question
« Reply #6 on: June 21, 2021, 11:35:55 AM »
at WOT , slightly close the throttle , if it accelerates ,  it is too lean
130 sounds small ,
i run my V65 lario with 34 mm mikuni VM , i run 210 main jets.

Thanks!  More to check today. One of the carb to head rubber boots ( a new one I bought from NLM ) has torn, need to fit an d hard rubber one in its place.
48 Guzzi Airone, 57 Guzzi Cardellino, 65 Benelli Barracuda, 66 Aermacchi Sprint, 68 Gilera 106SS, 72 Eldorado, 72 Benelli 180, 74 Guzzi 750S, 73 Laverda SF1, 74  Benelli 650S, 75 Ducati 860GT, 75 Moto Morini 3-1/2, 78 Moto Morinii 500

Online John A

  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 3710
  • No way to slow down...
  • Location: Hager city ,western WI
Re: NGC mikuni carb question
« Reply #7 on: June 21, 2021, 12:12:33 PM »
I chased a wot running problem and learned that you need 2.5 times (theoretically)the throat diameter of clearance between the carb inlet and the end of the filter or it disrupts the air flow and because the intake air is not just flowing into the carb but is puffing back and forth at wot. On a Guzzi it was easy to feel if I put my left hand near the inlet with the filter off, while the right held wot running down the road. The air puffing back and forth was warm, I could feel that because ambient temp was about 50F. Maybe thatís something to consider , the symptoms you described are exactly what I was having. I took the foam off the ends of the velocity stacks and it was cured
« Last Edit: June 21, 2021, 12:19:29 PM by John A »
John
MGNOC L-471
It is easier to fool people than it is to convince them that they have been fooled-Mark Twain

Offline huub

  • Guzzi Mentor
  • ****
  • Posts: 638
Re: NGC mikuni carb question
« Reply #8 on: June 21, 2021, 02:12:30 PM »
Thanks!  More to check today. One of the carb to head rubber boots ( a new one I bought from NLM ) has torn, need to fit an d hard rubber one in its place.

by the way, on aliexpress you will find cheap jet kits for mikuni's,
just buy 100  to 150 in small steps, they are stupidly cheap.
saves you going to a shop to get the next set of jets,
i bought the complete set of dellorto main and pilot jets, and a complete set of mikuni main and pilots.
trying other jets is just grabbing them from the cabinet.
makes setting up carbs so much easier.
i wish i had those 20 years ago.
« Last Edit: June 22, 2021, 01:54:34 AM by huub »

Offline wirespokes

  • Guzzi Hero
  • *****
  • Posts: 1915
Re: NGC mikuni carb question
« Reply #9 on: June 21, 2021, 03:17:42 PM »
at WOT , slightly close the throttle , if it accelerates ,  it is too lean
130 sounds small ,
i run my V65 lario with 34 mm mikuni VM , i run 210 main jets.
A 210 main is GIGANTIC! For example, the T3 is spec'd for a 130. My LeMans 3 and 4 are somewhere between 130 and 140. Even my BMWs are in that range.

From all the evidence so far, it sounds like dropping the main size down a few notches should help.

Online Antietam Classic Cycle

  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 12289
  • Happily stuck in the past.
    • Antietam Classic Cycle
  • Location: Rohrersville, Maryland
Re: NGC mikuni carb question
« Reply #10 on: June 21, 2021, 03:31:02 PM »
A 210 main is GIGANTIC! For example, the T3 is spec'd for a 130. My LeMans 3 and 4 are somewhere between 130 and 140. Even my BMWs are in that range.

From all the evidence so far, it sounds like dropping the main size down a few notches should help.

Different designations for the orifice size. Dellorto designates by millimeters (a 130 has a 1.30 mm orifice), most other manufacturers do they same. Mikuni is an exception. Instead of measuring theirs like all others, they decided to measure them according to the amount of fluid that will flow through the jet during a given time 
Charlie

Offline Rough Edge racing

  • Guzzi Hero
  • *****
  • Posts: 4476
  • Location: North of Finger Lakes
Re: NGC mikuni carb question
« Reply #11 on: June 21, 2021, 03:47:41 PM »
:gotpics: :gotpics: :gotpics: :gotpics:
by the way, on aliexpress you will find cheap jet kits for mikuni's,
just buy 100  to 150 in small steps, they are stupidly cheap.
saves you going to a shop to get the next set of jets,
i bought the complete set of dellorto main and pilot jets, and a complete set of mikuni main and pilots.
trying other jets is just grabbing them from the cabinet.
makes setting up carbs so much easier.
i wish i had those 20 years ago.
I run Mikuni TM flatslides on five Brit bike engines, Three racers, two street bikes, 650 and 750 twins.My advice is never buy any jet unless it's genuine Mikuni. The sizing can vary too much...
89 Honda 650 NT , 79 Triumph 750,61 BSA A10, 650 Triumph land speed racer , dual 650 Triumph engine land speed racer.
" I  don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"

Online Canuck750

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 1042
  • Location: Edmonton, Canada
Re: NGC mikuni carb question
« Reply #12 on: June 21, 2021, 07:10:52 PM »
I replaced both carburator to head rubbers putting on the old rock hard originals back on, one of the new boots had split wide open and the other one was starting to split. The originals seem to have a deeper flange on the head end and they fit snugger.

I noticed a wee bit of exhaust leak out of the muffler joints so I pulled the mufflers off and ran a bead of copper high heat RTV around the header and the reinstalled the mufflers and snugged up the clamps.

Installed new plugs and then hit the road. Stopped pulled the plugs, front sooty, turned in the front carb air screw 1/2 turn.

After I got to the back roads on WOT it still runs quite rough, backing off the throttle a wee bit and it cleans out a bit but not great.

I tried several electronic ignition settings, it ran poorly on position 1, a bit better on position 2 and ran best on curve number 3, all higher curve positions ran worse and worse until it barley ran at all.

I was able to get some larger Mikuni hex head jets from a local bike shop, unfortunatley they did not have smaller than 130, installed 150's fresh plugs ran worse, way down on power could hardly get past 60 mph. I ran it up to WOT and then turned off the key and pulled in the clutch, plugs all sooty.

Put in new plugs and put back the 130 mains, ran much better, when I got back to the shop the plugs looked much better, but on the dark side. As per the Elektronic Sasche instructions I am running resistor plugs.

Bike seems to be clattering a bit, will check valve gap tomorrow morning.

Will look for 120, 125 main jets tomorrow and see how that feels.

At WOT it actually feels like its stumbling as if the spark is not conistent but I don't think it can be the ignition module, maybe the HT leads have a week connection that starts to fail at WOT, is that even possible???
48 Guzzi Airone, 57 Guzzi Cardellino, 65 Benelli Barracuda, 66 Aermacchi Sprint, 68 Gilera 106SS, 72 Eldorado, 72 Benelli 180, 74 Guzzi 750S, 73 Laverda SF1, 74  Benelli 650S, 75 Ducati 860GT, 75 Moto Morini 3-1/2, 78 Moto Morinii 500

Offline Rough Edge racing

  • Guzzi Hero
  • *****
  • Posts: 4476
  • Location: North of Finger Lakes
Re: NGC mikuni carb question
« Reply #13 on: June 21, 2021, 07:38:18 PM »
over rich is often called 8 stroking. it has a rhythm ,like dah, dah, dah ...lIn reality every plug check should have new or cleaned plugs..I find it takes up to 5 miles to get a plug color..
 Many here might call this lean but it's just a touch rich.Notice the carbon on the end of the screw shell...About 10 miles ion the backroads running between 60 and 80 mph on my 750 Triumph using 90 octane non ethanol pump gas. E10 might need to show a bit of color..



89 Honda 650 NT , 79 Triumph 750,61 BSA A10, 650 Triumph land speed racer , dual 650 Triumph engine land speed racer.
" I  don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"

Online Canuck750

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 1042
  • Location: Edmonton, Canada
Re: NGC mikuni carb question
« Reply #14 on: June 21, 2021, 07:45:35 PM »
My plugs are much, much darker than yours, I used three pairs of new NGK plugs for each cop, ran for about ten minutes before killing the motor. Called another bike shop and they have the smaller jets I need, back to plug chops tomorrow.
48 Guzzi Airone, 57 Guzzi Cardellino, 65 Benelli Barracuda, 66 Aermacchi Sprint, 68 Gilera 106SS, 72 Eldorado, 72 Benelli 180, 74 Guzzi 750S, 73 Laverda SF1, 74  Benelli 650S, 75 Ducati 860GT, 75 Moto Morini 3-1/2, 78 Moto Morinii 500

Offline SED

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • Posts: 1549
Re: NGC mikuni carb question
« Reply #15 on: June 21, 2021, 11:15:27 PM »
I've got a couple of wire gauges to measure Mikuni mains in mm.  A 130 Mikuni is about .9mm, so a Dellorto 90 main.  A 150 Mikuni is a tight 100 Dellorto.  These seem small.  What is the stock Dellorto main?  115?

Dellorto, like Charlie says uses the diameter in mm.

Mikunis were based on Amals.  Amal used a flow rating like a 100 jet would flow 100 ccs/ minute (don't know if this is the correct conversion but you can find it on the internet). Mikuni hex jets use the same measurement system as Amal - they are interchangeable.  My 1939 and '47 remote float Amals are running Mikuni hex mains.

In my experience Sudco supplies too small an atomizer (AKA needle jet is too small).  The taper on the Mikuni VM needles also start lower than Dellorto needles and the combination is lean just as the slide lifts (which leads you to compensate by enrichening the idle) and stays lean which may lead you to run too rich a main to compensate. 

The Mikuni tuning guide has a table of atomizer sizes in mm so easily compared to Dellorto sizes.  Let me know if you need it and I can post a picture.

BTW my LMIII had lots of running issues related to badly worn intake guides and it gave sooty looking plugs when the mixture was weak and burning too slowly.  Richening the mix (bigger idle jets and raising the needles) gave the engine more power, it got noticeably hotter and the plugs looked leaner.

Hope this helps!



1983 LeMans III
1981 Monza
1947 Ariel Red Hunter
1939 Ariel Red Hunter
1937 Guzzi GTV

Online Canuck750

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 1042
  • Location: Edmonton, Canada
Re: NGC mikuni carb question
« Reply #16 on: June 22, 2021, 09:45:49 AM »
Thanks Shaun great information as always.

Jim
48 Guzzi Airone, 57 Guzzi Cardellino, 65 Benelli Barracuda, 66 Aermacchi Sprint, 68 Gilera 106SS, 72 Eldorado, 72 Benelli 180, 74 Guzzi 750S, 73 Laverda SF1, 74  Benelli 650S, 75 Ducati 860GT, 75 Moto Morini 3-1/2, 78 Moto Morinii 500

Offline moto_marco

  • New Goose
  • *
  • Posts: 23
  • '73 Eldorado, '73 Eldorado, '08 Triumph Tiger
  • Location: Los Angeles
Re: NGC mikuni carb question
« Reply #17 on: June 22, 2021, 09:53:47 AM »
This is a good resource to have on tuning Mikuni carbs. I used it to help me setup my VM32s. Very detailed. Good luck!

http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html


Online Canuck750

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 1042
  • Location: Edmonton, Canada
Re: NGC mikuni carb question
« Reply #18 on: June 22, 2021, 06:36:41 PM »
This is a good resource to have on tuning Mikuni carbs. I used it to help me setup my VM32s. Very detailed. Good luck!

http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

That is a great resource, explains the systems and how to trouble shoot and tune, Thanks!

This morining checked the valve lash and did a minor adjustment on 3 out 4 of the valves. I found a pair of 120 and 125 Mikuni main jets, nothing elses available in stock where I checked. Took the Morini out into the country and after half hour pulled over, changed to fresh plugs and swapped out the 130 main jets for 125's ....... then the bike would not start on the electric starter, dead battery, WTF!!

The eyelet crimp connection of the regulator wire to battery and come undone, no charging. I was able to kick start it easily but decided to not do a plug chop and risk not being able to start it again.

With the 125 mains the WOT no longer bogs / skips, it now has much more power, touched 90 mph previoulsy it would top out just over 80 mph.

Rode back home without stopping, about a half hours ride and less than half a block from the shop the battery gave up the ghost and the bike just died, pushed it back into the shop and pulled both plugs.

These pictures are of the two plugs running on the 125 main jets, the last 15 minutes was riding through town with stop and go traffic



The bottom plug is the front cylinder, the top plug the rear.

Sooty outside of the center, I should probably try another clean plug chop for WOT and take another look at the plugs

I need to check the idle / pilot mixture scews and resync the carbs, the bike felt a bit rough at speed.
48 Guzzi Airone, 57 Guzzi Cardellino, 65 Benelli Barracuda, 66 Aermacchi Sprint, 68 Gilera 106SS, 72 Eldorado, 72 Benelli 180, 74 Guzzi 750S, 73 Laverda SF1, 74  Benelli 650S, 75 Ducati 860GT, 75 Moto Morini 3-1/2, 78 Moto Morinii 500

***Wildguzzi Official Logo High Quality 5 Color Window Decals Back In Stock***
Shipping in USA Only. Awesome quality. Back by popular demand. All proceeds go back into the forum.
http://www.wildguzzi.com/Products/products.htm
Advertise Here
 

20 Ounce Stainless Steel Double Insulated Tumbler with donation credit
Buy a quality tumbler and support the forum at the same time!
Better than a YETI! BPA and Lead free.
Advertise Here