Mik 48 tuning

[hrdtail78]

Does anybody know what would cause a mik 48 to go lean at 5500 and piss out the overflow.  This is roll on runs up to redline.  Assuming I am lean because of the overflow.  But why does it flow out at that rpm.

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[jabarr]

Vibration.  Try holding the carb with your hand during the dyno pull.  May be touching cylinder fins.

[hrdtail78]

Thanks for the reply.  Carb is solid mounted to a G manifold with a raceback.  Great clearances around everything.

[FLTRI]

Thanks for the reply.  Carb is solid mounted to a G manifold with a raceback.  Great clearances around everything.

What is a raceback?
The carb must have positive support to the engine. The a/c backing plate must be directly bolted to the heads with a bracket, assuming you have evo or tc.
This, as jabarr is indicative of vibration. Seen it more than once, especially with dual mikuni setups.

[fuzznut5197]

A raceback is a machined aluminum adaptor that bolts to both the Mik and the manifold. It doesn't get any more solid than that. My 48 did exactly what happened to you on the dyno, although your curve just died whereas my hp peak just flattened out.

[FLTRI]

A raceback is a machined aluminum adaptor that bolts to both the Mik and the manifold. It doesn't get any more solid than that. My 48 did exactly what happened to you on the dyno, although your curve just died whereas my hp peak just flattened out.

The carb must have positive support to the engine. The a/c backing plate must be directly bolted to the heads with a bracket. Maybe this is what you are saying but carb to manifold is not carb/manifold to engine.

[hrdtail78]

I believe my carb is mounted solidly.  I have changed cams and carb on this bike.  Came from a RR62T and S&S G to the S&S585V and Mik.  Same fall off at same RPM.  I took out the reed valve and put a S&S cut out screen one in.  Pipe is a RB 124 challenge pipe and doubt that is my problem.  Thinking about this today I am leaning toward ignition.  Have a crane HI4E.  Going to throw another ignition in it to check it out.  Here is the other sheet from a few years ago with the G and RR cam in it.  Done on an older dyno with no A/F meter, but you can see it in the tq and power.

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[fuzznut5197]

Going to throw another ignition in it to check it out.

Tried that, and just about everything else. Still a mystery until spring arrives...

[Admiral Akbar]

From what I've seen the 48 don't handle vibes too well. If it's rigid mount and it overflows might want to rubber mount it..  Here is your dyno run with AF.. Looks like something is definitely amiss up top. IIRC Dougly and Mike R also had problems.. Max..

[Admiral Akbar]

BTW,

One thing you might try and do is add a baffle or fine screen between the main part of the float bowl area and the main-jet well. Some dirt bike carbs do this..  You could try a baffle between the vent and the bowl but I'm not sure this would be effective. If you've ever pulled the bowl off a SnS super carb you'll find that bowl gasket acts partly as a baffle for the air vent.. It only lets air past in the middle of the carb where the emulsion tube is.. Making a float bowl extension like Bobby does for the CV51 might also help. Daughter might get a CNC machining class spring semester. If so I'll try and get her to do a a bowl extension. I might try and make some main-jet well baffles on the lathe out of plastic but aluminum would be good for experimenting.  Max.

[FLTRI]

"From what I've seen the 48 don't handle vibes too well."

The fact fuel is coming out of the float bowl vent is the telltale for excessive vibration. As I stated before, seen this a few times. Everytime was due to the carb mounted to the manifold without a rigid bracket mounted to the heads.
Rubber mounting would be nice if you could do it successfully on a harley (ie: hold the carb still while the engine shakes), but if you were to do that the carb would shake and flop around like a fish out of water.
Most other engines run like sewing machines compared to HD V-twins so rubber mounting works great.

Mikunis work well on HD until they get big ci, then the shaking @ high rpms needs to be put in check. As mentioned the dual miks are famous for this anomaly. Gas just pours out of the vent tube @ high rpms and it is almost impossible to put all that weight hanging out there in check.

I believe hrdtail78 fells his carb mounted solidly to the intake manifold is good enough, so we're done.

That said, the test is simple: Hold the carb with your hand while doing a run on the dyno as jabarr suggested. That will prove it one way or another. Or if the dyno is not available or expensive. Just get the braketry for mounting the backing plate to the heads.

HTH,
Bob

[Admiral Akbar]

"I believe hrdtail78 fells his carb mounted solidly to the intake manifold is good enough, so we're done. "

What do you do if the motor is rubber mounted?

Max

[FLTRI]

Max,
Fortunately the carb's problem shows up at high rpms not idle, so there is really very little difference at high rpm as far as "shake" is concerned. However I see the problem more prevalent on the solid mounted engines for whatever reason(s).
Bob

[hrdtail78]

Thanks for the replies.  My Mik is solidly mounted to the manifold by the race back.  My manifold has a brackets from the bottom bolt to the top center through bolt of the case.  I will try and hold the carb next.  I have a A/C backing plate with ears I could mount to the head for testing.  The fuel out the overflow isn't a constant thing.  I will have somebody watch for it as I run the bike.  Maybe it is hitting a certain vibe and the carb dont like it.  Seems with the other cam and carb the harmonics would be different and do this at another RPM.  I have made 20 pulls changing main jet from a 200 to a 185 tying to get it to move.  Did the same with the G.  You can feel when the bike gets to 5400.  Dont know if it would be possible to feel on the street, but the dyno is smoother.  It's not really a stumble or fall on the face type thing.  The noise and vibes are there.  Just seems to quit pulling for a second.  Hard to descibe.  Got some things to try.  Thanks

[Admiral Akbar]

Another thing to think about.. If you have a miss the AF ratio will show up as lean if the sensor is sensing O2. (eh bob?) I know the Bosch sensors sense O2 not sure about the dynojet.. What ignition are you running? This is a 127 Utlima?  You could try closing down the gap on the plugs to say 0.030... If the same problem occurred with the G, it may be some weird vibration problem but it may also be somewhere else. Max

[FLTRI]

Could be the ignition system falsely rev-limiting. I've seen it before, however it is the AFR and fuel that comes out of the bowl vent that points toward vibration.
Since there is another backing plate with capability to mount solid to the heads, that would be my first try. The present mounting cannot keep the carb in check albeit keeps the carb from sagging down.
Suppose I would change the mounting and take another ignition unit with me to the dyno. Then, if the anomaly still rears its ugly head I would swap out the ignition and retest.
Hope this helps and as always, JMHO, Bob
PS - The sensor in the Dynojet AFR pump is indeed a Bosch wide band O2 sensor.

[fuzznut5197]

OK, what happened to the old HTT, looks like they closed it down a month early. I had a post there with a million AFR graphs showing the same type of problem. If you guys can remember, the front cyl AFR on my bike was doing the exact same thing as hrdtail78's, but the rear cyl was FINE. Sticking the sniffer up the front primary is the norm on a dyno, the first thing to do when revisiting the dyno is to check the rear AFR. If you find that the rear AFR looks good, then we have the same type of problem, and this is what I checked:

Carb- not the problem

Remove single bore tuned induction- no change

New plugs and several gap changes- nope

Different plug wires -no

Swapped Ign module, DTT --> HI4E - no change

Single fire coil, swapped primaries and secondaries - nope

Hooked up a digital recorder to the front cyl coil primary, to record
every ignition pulse. No pulses were missing.


I was told by this "munkey" guy that it was the pipe. 

[FLTRI]

Fuzz,
So "this "munkey" guy" told you it was the pipe that completely starved the front cylinder of fuel @ a given rpm or rpm range?

Well, don't keep us in suspense here! What happened when you changed the pipe??

I have found in life, anything's possible, however the pipe (RB124 challenge) hardtail78 is using has been proven to work just fine on big inch engines @ high rpms.

hartail,
What is this all about? "I took out the reed valve (what and where) and put a S&S cut out screen one in"

Bob

[hrdtail78]

Bob,
S&S cam out with a nonrotating breather gear for the evo.  Had three reed vavles on it.  My thoughts were maybe this thing isnt capable of dealing with the air and oil of a 127, going to vavle flutter or something.  I put a traditional rotary breather in it, that I cut the screen out just to eliminate thinking about the reed valve.  My thoughts are inline with you on the pipe.  High reving, turbos on the salt flats has proven this pipe.

Bruce,
S&S cases and fly wheels, 7.69 carrillo rods, Hyperformance cylinders, gearotor pro-flo oil pump, JE pistons and R&R billet heads.  I haven't messed with gap.  I have moved my ignition around with the pot.  Changed from single to multi spark and have moved it some with the whole ignition unit.

Fuzz,
I usually read back cylinder, and tune there.  Then I move to the front to check things out.  Have swapped the probe around couple of times.  About the same readings.

This is being done on my dyno.  So cost isnt a big deal.  This morning I had to tune a fatboy, now I need to start on putting some apes on a bagger.  Hopefully tomorrow morning I will get a chance to make some runs.

[fuzznut5197]

Well, don't keep us in suspense here! What happened when you changed the pipe??

The pipe it was dynoed with was the old supertrapp 2-1, I believe the the front primary is 29", rear 23", 2" collector and core, 30 disks closed. Thinking that the lean top end was from this pipe being too small for a 124, I acquired a boarzilla headpipe (40" front, 38" rear, 1 7/8"dia, 2.5" coll), welded O2 bungs 4" from the flanges, and got a WEGOIII. The muffler is 2.5" tubing welded to the ST meg, with a 2.5" core now. Tried 20 disks, no disks, no core, lean problem still there. The only time the AFR didn't skyrocket out of sight, but was still lean, was when I ran the headpipe alone.

This is being done on my dyno.

I wish I could say that!   

[Admiral Akbar]

"So "this "munkey" guy" told you it was the pipe that completely starved the front cylinder of fuel @ a given rpm or rpm range?"

Sometimes you got to watch that munkey guy cuz he's FOS.. 

Jason,

See you tried to ignitions DTT and Hi4. How about powering the ignition system separate from the bike?

Maybe it's something like weak springs and the values aren't staying on the seats?

Are you running it with a filter installed? If not see it that helps. I've seen this on my 116. AF would hook up slightly without a filter.

I'd try adding 2 inches to the intake track length just to see if there is something funky going on in the intake.

Bruce

[fuzznut5197]

Ok guys, I am going to attempt to analyze the Mik 48 and it's problems, also looking at the G carb.
Mik48s go on big engines that suck big fuel, require a big float needle and, regardless of vibration, has big design issues regarding the quality of fuel delivery to the float bowl. Beware of many pics!!
But first, let's look at the S&S G:




Here is the G float bowl. It's obvious that the float/needle/seat is part of the bowl and not the main body. Looking at the float-needle relationship, it would appear that there is a lot of leverage to keep the needle closed:





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Here is the G needle and seat. Notice there is a pretty good amount of space between the needle and seat bore for fuel to flow past:





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Here is the G seat. Wow, 2 giant slots for the fuel to exit, heck, I'd be surprised if much fuel needs go go up through the needle bore:





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While we're in there, let's look at bowl venting. We have a big gasket that isolates the bowl from the main body, except for a small notch in the gasket. The other hole is for the accel nozzle and while the gasket is loose around it as well as the main jet casting, I would consider the notch as the official bowl-to-main-body vent.





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Look, they put the vent gasket notch on the opposite side of the actual vent! Maybe they were thinking that when you're pulling Gs and the gas in the bowl want to ride up the back of the carb and out the vent, that they should block that with a full gasket and near-center vent location. What were they thinking! 




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OK, on to the Mikuni. Here is the needle seat. 4 bypass holes instead of 2 big slots. Do these holes exit directly into the bowl? You wish!





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Needle and seat in main body. Notice how much less room there is around the needle/bore than the G for fuel to flow. And the fuel exiting the 4 holes in the seat exits through the 2 side channels, after making a couple of turns and then hitting the seat flange. And look at those unobstucted bowl vents. Perfect for getting gas on your pants when you pull some Gs!!





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OK, what's wrong with this picture? Apart from the space around the needle tab, the fuel flow is obstructed entirely by the float bracket. I bet fuel is splashing every which way and, with the right vibration, foaming or launching right up out of the vent. And it looks like there's less float/needle leverage than the G:





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The float needle tab. This is how much the frickin' thing has to be bent to get the proper float height. I would imagine that this will side-load the needle and, with the right vibration, the needle could get "stuck". The G is dead perpendicular.
UPDATE  Idiot alert!
When I adjusted this last year, either the needle pin spring was stuck, or I was retarded. Probably the latter. Anyway, after readjusting, it's only slightly bent.





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These last 2 pics are the G and Mik needles. Which one will be easier to close with the force of fuel flowing around it?

[Admiral Akbar]

The float needle tab. This is how much the frickin' thing has to be bent to get the proper float height. I would imagine that this will side-load the needle and, with the right vibration, the needle could get "stuck". The G is dead perpendicular."

I'd figure out how to bend the that so that it's perpendicular.. Side load got to be a problem may even be with vibes. Are you sure the plunger on the end of the needle ain't stuck? Mine don't over flow .. You got me to wondering if the G needle could be made to work on the HSR48.. Looks like it'd flow more.

Some say the G is outdated but I still think they are built well.

Max.

[fuzznut5197]

Yeah I'm going to bend the bracket behind the tab to get it as level as possible.
The G needle didn't fit, and there's not enough meat in the bore to open it up. I guess you could modify the G needle, but I'd be more inclined to do mods to get the bulk of the gas to exit the rear slot, so it has a direct shot down the back side of the bowl.

Hmm, something just came to me....   The G's float pivot is perpendicular to harley's vibration plane; the Mikuni's is PARALLEL. And there's an awful lot of slide slop on mine, looks to be dependent on the casting. I'm getting mental images of the float pontoons moving side to side in the gas, displacing it...   essentially digging a hole in the gas...   meanwhile more gas is flowing in...
pushing the floats deeper with the bracket...   runaway situation...   gas out the vent!

Or not.   

[FLTRI]

Fuzz,
That's some great pics and analysis. I have a little story to share that goes right along with your findings:
Back in 1992-4, IIFC, I sold a CDS data acquisition system to Zippers for their drag racing effort (Big inch Sportster). I worked closely with a fellow, "Mac" Mcnally who headed their engine development of that team.

They had noticed the engine "lay down", or get weak at about 60ft. Then, as the run continued the fuel richened back up and ran fine. Drove them nuts! This is why they looked for instrumentation, in an attempt to find what was causing this lay down, then run fine again. They had a good rider who had a great feel for how the power was being delivered, and felt it lay down right after launch, but down the road a bit. (about 60ft)

Well they instrumented the bike O2 and EGT sensors, and saw clearly the engine was laying down due to it leaning out causing the lay down. But couldn't, for the life of them, figure why this was happening. A real stumper.

They were using a highly modified (T-jetted) Mikuni (for its superior air flow). I suggested, just because I was aware of problems I've seen on the syno with Mikunis, that for just sh$ts and giggles they toss on an old modded up G (also w/T-jets) they had laying around just to see if it did the same thing. Low and behold the enleanment stopped happening @ about 60ft.

Wanna guess why the G cured that problem?
Hint1: This anomaly was not due to inlet fuel restriction, as there was plenty fuel available @ high rpms, down the road apiece.
Hint2: It is pointed out in your pics.
Thought provoking, but kinda simple when you think about it for a bit.

Bob