Mikuni HSR 45 and DTT ignition

[dynaglide]

thanks.
It looks like the shroud might cause the main to starve even though there's still fuel left in the rest of the bowl, sort of like raising a drain pipe?  Or do those little slots allow fuel from the main bowl into the main jet's extended bowl?  But if they do, doesn't that still allow for the fuel to slosh around?  Or does it reduce the sloshing enough to help?

[Nowhereman]

A long time ago, someone used to make clear float bowls for Miks.
That would show if sloshing and foaming are going on.
I'm not buying into that theory and I say that based on my almost 45 years of Mikuni usage.
All those motos over some of the roughest terrain there was.
My only concern for HD applications is that for non pressurized applications there are times you can actually starve the bowl.
With a 48 on a large displacement motor it happens.
The way around it is to use the small float valve and run a  pump.
Works nicely too if you can hide the pump cleanly.
A clear bowl would show a lot.

[Admiral Akbar]

A long time ago, someone used to make clear float bowls for Miks.
That would show if sloshing and foaming are going on.
I'm not buying into that theory and I say that based on my almost 45 years of Mikuni usage.
All those motos over some of the roughest terrain there was.
My only concern for HD applications is that for non pressurized applications there are times you can actually starve the bowl.
With a 48 on a large displacement motor it happens.
The way around it is to use the small float valve and run a  pump.
Works nicely too if you can hide the pump cleanly.
A clear bowl would show a lot.

Didn't know the HSR carbs have been around for that long.. 

[Nowhereman]

A long time ago, someone used to make clear float bowls for Miks.
That would show if sloshing and foaming are going on.
I'm not buying into that theory and I say that based on my almost 45 years of Mikuni usage.
All those motos over some of the roughest terrain there was.
My only concern for HD applications is that for non pressurized applications there are times you can actually starve the bowl.
With a 48 on a large displacement motor it happens.
The way around it is to use the small float valve and run a  pump.
Works nicely too if you can hide the pump cleanly.
A clear bowl would show a lot.

Didn't know the HSR carbs have been around for that long.. 

They haven't Max but their basic design has and I have run Miks on my bikes since I was 16.
If I understand correctly also, all fuel companies do formulate their products to fight foaming issues.

[Admiral Akbar]

A long time ago, someone used to make clear float bowls for Miks.
That would show if sloshing and foaming are going on.
I'm not buying into that theory and I say that based on my almost 45 years of Mikuni usage.
All those motos over some of the roughest terrain there was.
My only concern for HD applications is that for non pressurized applications there are times you can actually starve the bowl.
With a 48 on a large displacement motor it happens.
The way around it is to use the small float valve and run a  pump.
Works nicely too if you can hide the pump cleanly.
A clear bowl would show a lot.

Didn't know the HSR carbs have been around for that long.. 

They haven't Max but their basic design has and I have run Miks on my bikes since I was 16.
If I understand correctly also, all fuel companies do formulate their products to fight foaming issues.

We'll talk about differences later.. Ever put a 44mm round slide on a panhead?

Add this is not an EGO bragging thing. It more "have you done it? did it work? have any issues"   To anyone..

[Admiral Akbar]

thanks.
It looks like the shroud might cause the main to starve even though there's still fuel left in the rest of the bowl, sort of like raising a drain pipe?  Or do those little slots allow fuel from the main bowl into the main jet's extended bowl?  But if they do, doesn't that still allow for the fuel to slosh around?  Or does it reduce the sloshing enough to help?

Slots are on the bottom not the top.. The you idea of a stand pipe is correct but since the slots are on the bottom, it's to keep air out.. Each slot is 4-6 times the diameter of the mainjet.. Do you think it will effect fuel flow? Fuel get to the jet just less air bubbles.. Its not the best design.. What whould be better would be a fine nylon mesh that could trap the bubbles and keep them out..Some Honda dirt bikes had them...

[dynaglide]

thanks.
I'm still having a hard time picturing what's going on inside the float bowl.
The fuel gets aerated/foams because its sloshing around inside the float bowl, causing it to mix with the air that's above the fuel level while the carb is vibrating, right?
Your "stand pipe" fits tightly enough around the main jet extender so that air from above the fuel level doesn't get into the fuel well under the main, right?
But the fuel getting into the well through the slots is already aerated from the air that's in the bowl, isn't it?
And why is this problem unique to the Mik's and not a problem with S&S or CV carbs?  Don't they have air in the bowl, too?  And don't those carbs vibrate, too?
Not questioning, just trying to understand. 

[Admiral Akbar]

thanks.
I'm still having a hard time picturing what's going on inside the float bowl.
The fuel gets aerated/foams because its sloshing around inside the float bowl, causing it to mix with the air that's above the fuel level while the carb is vibrating, right?
Your "stand pipe" fits tightly enough around the main jet extender so that air from above the fuel level doesn't get into the fuel well under the main, right?
But the fuel getting into the well through the slots is already aerated from the air that's in the bowl, isn't it?
And why is this problem unique to the Mik's and not a problem with S&S or CV carbs?  Don't they have air in the bowl, too?  And don't those carbs vibrate, too?
Not questioning, just trying to understand. 

The idea of the cover is that that the only way fuel can enter the main jet well is through the holes.. The holes are essentially right at the bottom of the float bowel so the air had to travel down to the bottom of the bowel and get through a hole at the bottom before it can enter the main jet cavity..

As far as why miks have the problem and the SnS / Keihin  CV carbs don't is the basic design of the carb..  All carbs can suck air through the main and probably do but it's what happens after the bubbles get through the main that makes the difference.. All carbs will go lean but it what the bubble can do after the main.. Mikunis don't have emulsion tubes.. Once the bubble goes through the main, there is only one place it can go.. Out the main jet. That bubbles places a big negative fuel spike in the AFR.. So why doesn't a CV or SnS carb do it? Because both carbs have emulsion / mixing tubes.. If you get a bubble in the main, there is already a bunch of fuel in the emulsion tube being mixed from the air bleed. It simply leans out the mixture a little over a few power intake stokes. Fuel level in the emulsion drops a little but recovers.. The emulsion tube acts as a damper for the bubble..

So miks suck when sucking bubbles.

Now go talk to the guys doing LSR stuff.. They'll tell you fuel aeration is a big issue.. Most have gone to fuel injection as it doesn't have the issue.. Fuel is always under pressure once it passed the fuel pump. Air cannot get in..

Kinda wish I knew this when I built my 131.. Would have gone EFI.. That bike had an SnS G carb on it for a while.. Did not have the same issue of the Mik..  For me it was running along at cruise and a lean pop through the carb.. Going to a G that problem went away..

[Admiral Akbar]

A long time ago, someone used to make clear float bowls for Miks.
That would show if sloshing and foaming are going on.
I'm not buying into that theory and I say that based on my almost 45 years of Mikuni usage.
All those motos over some of the roughest terrain there was.
My only concern for HD applications is that for non pressurized applications there are times you can actually starve the bowl.
With a 48 on a large displacement motor it happens.
The way around it is to use the small float valve and run a  pump.
Works nicely too if you can hide the pump cleanly.
A clear bowl would show a lot.

Didn't know the HSR carbs have been around for that long.. 

They haven't Max but their basic design has and I have run Miks on my bikes since I was 16.
If I understand correctly also, all fuel companies do formulate their products to fight foaming issues.

We'll talk about differences later.. Ever put a 44mm round slide on a panhead?

Add this is not an EGO bragging thing. It more "have you done it? did it work? have any issues"   To anyone..

BTW,, Got you beat by about 4 years.  How many round slides did you put on 4 strokes?

[Nowhereman]

4 to 6 Max.
And, I fabricated everything to mate a side draft Weber to a pan before I got my first car....
It worked.... most of the time.

[Admiral Akbar]

4 to 6 Max.
And, I fabricated everything to mate a side draft Weber to a pan before I got my first car....
It worked.... most of the time.

So what were the bikes?

Add: and how well did they run? Seems like you might have had issues..

[Admiral Akbar]

FWIW,

I've installed to helped other guys install the 44mm Mikuni round slides on 5 bikes (IIRC).   We used to take the sock manifold wack off the linkert / Tillotsen mound and braze on a 1/4 inch plate that matched the rubber spigot.. Made up 3 manifolds for sportsters and 1 for a pan and one for a knuckle. Of the 3 manifolds for sportsters, did the assembly and tune on 2. They ran great.. The Pan and Knuck were buddies and they weren't ever able to get them to run right.. They ran OK everywhere except WOT at high revs they would miss.   Up or down on the main-jet made no difference as far as the miss goes..It would feel rich or lean but would still miss.. IIRC they went to SUs and those didn't have the issue.  the guy with the pan tried to put a 44 in a VW bug.. That was a disaster..

Used to think the fuel wasn't emulsified (mixed) enough and the spark not hot enough but I wouldn't be surprised if they had issues with pulling air through the main..

Taiwan Ted used to sell a 38 mm kit for shovels and I've seen a couple bike with the kit on them but don't know if they ever had any tuning issues..

One shop I worked in sold CZs and Maicos.. We'd install a 38 on the 400s up and 34s on the 250s, most of them before they even went out the door.. Never have an issue with that carb on a 2 stroke..

[Nowhereman]

FWIW,

I've installed to helped other guys install the 44mm Mikuni round slides on 5 bikes (IIRC).   We used to take the sock manifold wack off the linkert / Tillotsen mound and braze on a 1/4 inch plate that matched the rubber spigot.. Made up 3 manifolds for sportsters and 1 for a pan and one for a knuckle. Of the 3 manifolds for sportsters, did the assembly and tune on 2. They ran great.. The Pan and Knuck were buddies and they weren't ever able to get them to run right.. They ran OK everywhere except WOT at high revs they would miss.   Up or down on the main-jet made no difference as far as the miss goes..It would feel rich or lean but would still miss.. IIRC they went to SUs and those didn't have the issue.  the guy with the pan tried to put a 44 in a VW bug.. That was a disaster..

Used to think the fuel wasn't emulsified (mixed) enough and the spark not hot enough but I wouldn't be surprised if they had issues with pulling air through the main..

Taiwan Ted used to sell a 38 mm kit for shovels and I've seen a couple bike with the kit on them but don't know if they ever had any tuning issues..

One shop I worked in sold CZs and Maicos.. We'd install a 38 on the 400s up and 34s on the 250s, most of them before they even went out the door.. Never have an issue with that carb on a 2 stroke..

Nope Maicos and Miks were made for each other.
All the top MXers had Miks on their Maicos back in the 70s.
Lets see, put Miks on a 350 Honda, a BSA, a Yamaha 400 twin and Norton Comando.

[Admiral Akbar]

FWIW,

I've installed to helped other guys install the 44mm Mikuni round slides on 5 bikes (IIRC).   We used to take the sock manifold wack off the linkert / Tillotsen mound and braze on a 1/4 inch plate that matched the rubber spigot.. Made up 3 manifolds for sportsters and 1 for a pan and one for a knuckle. Of the 3 manifolds for sportsters, did the assembly and tune on 2. They ran great.. The Pan and Knuck were buddies and they weren't ever able to get them to run right.. They ran OK everywhere except WOT at high revs they would miss.   Up or down on the main-jet made no difference as far as the miss goes..It would feel rich or lean but would still miss.. IIRC they went to SUs and those didn't have the issue.  the guy with the pan tried to put a 44 in a VW bug.. That was a disaster..

Used to think the fuel wasn't emulsified (mixed) enough and the spark not hot enough but I wouldn't be surprised if they had issues with pulling air through the main..

Taiwan Ted used to sell a 38 mm kit for shovels and I've seen a couple bike with the kit on them but don't know if they ever had any tuning issues..

One shop I worked in sold CZs and Maicos.. We'd install a 38 on the 400s up and 34s on the 250s, most of them before they even went out the door.. Never have an issue with that carb on a 2 stroke..

Nope Maicos and Miks were made for each other.
All the top MXers had Miks on their Maicos back in the 70s.
Lets see, put Miks on a 350 Honda, a BSA, a Yamaha 400 twin and Norton Comando.

Hot sure what the "nope" is for. Sounds like you are agreeing..
All of them except for the Commando would not have issues with fuel foaming in the bowls..   Roommate installed mics on his CB350, It had a bore kit that got the motor over 400cc..

[Diesel Pro]

Thanks for the pics of the shroud.

Latest run with 96 needle in center clip, Y6 jet and 20 pilot and Yost extender 175 main using timing table posted earlier was 41.3 mpg 2 up and getting on it from time to time.  One key variable is that I aired up my rear tire.  I've always ran it a bit low to try to keep from wearing the center so bad.

At any rate, it still had a bit of ping.  I have the Y8 needle jet installed to test next as it was simplest install.

[Admiral Akbar]

Thanks for the pics of the shroud.

Latest run with 96 needle in center clip, Y6 jet and 20 pilot and Yost extender 175 main using timing table posted earlier was 41.3 mpg 2 up and getting on it from time to time.  One key variable is that I aired up my rear tire.  I've always ran it a bit low to try to keep from wearing the center so bad.

At any rate, it still had a bit of ping.  I have the Y8 needle jet installed to test next as it was simplest install.

The Yost extender is probably a good idea..  I asked earlier whet effect a drilled emulsifier tube did if since there is no air bleed there was nothing to emulsify.. Well if the main happens to suck air, the tube could spread the air bubble out in the flow through the needle jet and make the AFR mixture a little more stable.. 

[dynaglide]

snip:  "... What whould be better would be a fine nylon mesh that could trap the bubbles and keep them out ..."

What about using a very fine screen, like from a petcock or inline filter (or a bowl  ), either in place of the shroud, or maybe even screwed in between the main jet and the extender?  That would break up the air bubbles as the fuel is drawn into the carb throat, sort of like the CV/S&S emulsifiers, wouldn't it?
Or maybe even use both a shroud and a screen together?
Or do you think the screen would be too much of a restriction to the fuel in the main circuit?

[Nowhereman]

First and foremost, you need to prove that your being bubbled to death.
I would not be adding "Potty mouth" just for piece of mind.
Call Sudoco up or call Mikuni and ask them their opinion.
Maybe they can throw some light on it.

[dynaglide]

I'm not necessarily thinking about doing anything - I don't think I have a bubbling issue, and even if I may, I wouldn't know how to test for it.  Is there a certain range that the AFR swings wide of if there's fuel foaming goin on?  Something else?

No, to me this is just an interesting theoretical discussion about what's going on in the fuel bowl and what can/should be done about it.  Armchair analysis, nothing more.  Kinda like guys debating PCSA-to-valve size ratio...

[Nowhereman]

And that's fine because that's how great development and innovation start all the time.
It sounded to me like someone was going to go through these gyrations without actually identifying an actual cause.
Some pretty sharp minds live here on this site and lots of it backed up by actual testing theories.
I would like to get Mikuni's input to things like this as  they have spent millions plus on their product development and application.
It's a great carb and my first choice always around building a V twin carb application and the only solution for two stroke off road apps.
I would like see Mikuni come out with some new upgrades to their design but everything is going FI an their sales are still very good.

[Admiral Akbar]

And that's fine because that's how great development and innovation start all the time.
It sounded to me like someone was going to go through these gyrations without actually identifying an actual cause.
Some pretty sharp minds live here on this site and lots of it backed up by actual testing theories.
I would like to get Mikuni's input to things like this as  they have spent millions plus on their product development and application.
It's a great carb and my first choice always around building a V twin carb application and the only solution for two stroke off road apps.
I would like see Mikuni come out with some new upgrades to their design but everything is going FI an their sales are still very good.

So you don't think that every time I was able to reduce vibrations to the carb made the jetting richer as a valid indicator? 


Add..

http://harleytechtalk.com/htt/index.php/topic,5476.msg53554.html#msg53554

[Nowhereman]

And that's fine because that's how great development and innovation start all the time.
It sounded to me like someone was going to go through these gyrations without actually identifying an actual cause.
Some pretty sharp minds live here on this site and lots of it backed up by actual testing theories.
I would like to get Mikuni's input to things like this as  they have spent millions plus on their product development and application.
It's a great carb and my first choice always around building a V twin carb application and the only solution for two stroke off road apps.
I would like see Mikuni come out with some new upgrades to their design but everything is going FI an their sales are still very good.

So you don't think that every time I was able to reduce vibrations to the carb made the jetting richer as a valid indicator? 


Add..

http://harleytechtalk.com/htt/index.php/topic,5476.msg53554.html#msg53554

Not sure Max, Don't have enough info.
Did you measure fuel temp before and after?
I'm not saying it did or didn't.
Something you did apparently had an effect on what you perceived as a richer fuel mixture.
Have not seen or heard of anyone installing anything like you did to a Mik to achieve that outcome thats all.
Years of MX racing showed me how much abuse a Mikuni could take and still deliver excellent performance.
That's why my comment on getting Mikuni to chime in on their experience with high, (whatever that can be) vibratory applications.

[Admiral Akbar]

And that's fine because that's how great development and innovation start all the time.
It sounded to me like someone was going to go through these gyrations without actually identifying an actual cause.
Some pretty sharp minds live here on this site and lots of it backed up by actual testing theories.
I would like to get Mikuni's input to things like this as  they have spent millions plus on their product development and application.
It's a great carb and my first choice always around building a V twin carb application and the only solution for two stroke off road apps.
I would like see Mikuni come out with some new upgrades to their design but everything is going FI an their sales are still very good.

So you don't think that every time I was able to reduce vibrations to the carb made the jetting richer as a valid indicator? 


Add..

http://harleytechtalk.com/htt/index.php/topic,5476.msg53554.html#msg53554

Not sure Max, Don't have enough info.
Did you measure fuel temp before and after?
I'm not saying it did or didn't.
Something you did apparently had an effect on what you perceived as a richer fuel mixture.
Have not seen or heard of anyone installing anything like you did to a Mik to achieve that outcome thats all.
Years of MX racing showed me how much abuse a Mikuni could take and still deliver excellent performance.
That's why my comment on getting Mikuni to chime in on their experience with high, (whatever that can be) vibratory applications.

Didn't know I needed to measure fuel temp.. What is that supposed to do? 

This bike always ran fairly well at WOT. The main problem for me was that it at steady speed it would hiccup and cough.. I thought it was electrical for a while. Even swapped a DTT from another bike to check.. I rewired the power to the DTT so that it got power through a relay directly from the battery.. None of this helped.  The bike is a 131 ci RevPerf/SnS motor in an FXR frame..  With the 48 it made 144 hp on a DJ250  dyno..   Still.. Cruise down the road and maybe 2-4 times in  100 miles, the bike would cough through the intake when I first go the jetting close.. Hot / cold ambient temps made no difference.. Jetting was easy. Run the motor to 2K-3k no load and if it starts hacking and coughing go up on the needle / needle jet..  Went all the way up to a Z-0 and 96 needle.. Adding the black chingas got me a needle step or 2 smaller before the cough and before the jet change to smaller the mileage would tank.. 28-30 mpg.  Remounting, lightening and shortening the pipes, mileage tanked but main felt good. Step down needle jets / needles and mileage comes back,  coughing drops to maybe 1-2 times in a 100 miles and mileage jumps back up to 38-40 (not getting on it too much).. Same thing happens when I rubber mount the carb.. Now it might cough once in 100, maybe not at all.. This bike has about 12000 miles on it with this issue.. There was even a time when I pulled the 48 off and installed a G for some time early on.. G ran great tho it's about 3-4 MPG less than the 48 (did find a crappy tank vent) .. I got a 3.3 tank so I need the mileage..

I can see on mild builds that this would be a good carb..  I was at one time thinking about making a float bowel extender and an even longer jet holder.. The only thing I'm not doing that Mikuni recommends is adding a 2 psi fuel pump.. That might help.. Fuel it 5/16 tubing with a straight shot from a Pingle to the carb.. I did also find that the tank vent sucked (or didn't) and it now has an MX style hose running off the cap.. It might have helped a little but it did help the G..

Anyway the last change, rubber mounting the AC made the bike run acceptable.

The old round slides are good carbs.. Put em on a lot of stuff.. Heck we use to put the 24s on built up ATC 90s / 110s and they ran pretty good but unless you MXed a rubber mounted big twin, I doubt you'll have issue.. You might think that bouncing around is going to cause fuel foaming / aeration but the frequency of the vibe ain't there..

If you got a phone number Mik support, please post it..

BTW,, Eric Bennett of Bennet's Racing mentioned fuel issues when I got a stack of carb needles as did Ron Dickey (Axtell) when I talked to him.. Ron felt air was getting into the pilot system. He was seeing it on the dyno..    Again these guys were having issue on high performance builds 4 stroke HD builds..

[1FSTRK]

When Vic was running the HS 40 carbs and testing with the HSR 42,45,48, and 52 carbs he found the exact problems that Max is reporting. At the time Lee Chapin was the guy over at  Mikuni American Corporation and the dyno and tuning information went from Vic's shop to Lee and then to Japan. The Mikuni manifolds were designed to use the rubber spigot adapters and the air cleaner elements were made with a rubber backing plate that had mounting studs built in for exactly this reason. I have witnessed several different build combinations that when the carb is mounted in too rigid a manor it will cause the bowl to foam at certain rpm range to the point that gas will spray out of the bowl vent and on to the bike and dyno. The cure has always been to modify the carb mounts to help cancel the frequency in that range. The worst offender was a 88ci race motor that had a 7600 rpm shift point on the track and it took several tries to finally come up with a working mount and manifold but in the end the problem was solved with mounting and welding up a larger volume float bowl.

Not all builds will have this problem but the ones that do can not be fixed with jetting alone, It did seem to be much more prevalent with the FXR/FLH rubber mounted bikes and as Max stated you could effect it with the mounting as well as the length and weight of the exhaust system. It is not a violent shaking of the bowl as in a MX bike that does it. The problem stems from reaching a specific vibration frequency that certain HD's seem to easily achieve.

 

[Nowhereman]

As much as one degree in fuel temperature changes the specific gravity of the fuel.
The specific gravity of a volatile fluid can react to vibration in your term of bubbles if it is too light.
Colder fluids as you of course know tend to gas less when agitated.