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Author Topic: Draw thru supercharger carb weirdness  (Read 14868 times)
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generatorshovel
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« on: August 31, 2012, 06:58:40 PM »

I'm having trouble setting the carb jetting, (250cc/4, aisin AMR300 / alcohol)
Carb #1 HSR42 Mikuni, alcohol supplied from a reputable supplier whom I've dealt with previously.
Filthy rich, fuel spraying from exhaust while warming up.
Ive re-jetted the pilot jet which cured the fuel spitting, the bike answered to gobs of throttle in the shed ok, but a test ride showed a problem with engine cutout over 1/4 throttle.
Needle jet and needle changed, problem persisted
Main jet changed from way too rich, to a tad lean,,with no noticeable change in the engine cutout problem.
In an attempt to eliminate the carb from the equation, I fitted the smaller carb from my LSR 175cc single (Iv'e calculated the 42mm flows 190 CFM, and the engine,with 10 psi boost, would require 110 CFM, the 32mm carb is close to this number)
Straight away, with the smaller carb (which an fine on the other engine) the ugly fuel spray from the exhaust @ a 3,000 rpm idle problem returned, and the engine drowns before warming up is achieved.
Why ?
What effect does a supercharger have upon a carb that will make it run so rich at idle, when it's jetted "right" for a non blown engine
I can reduce the jetting, but other than the idle mixture screw (air) this involves a smaller pilot jet, which is fine for small throttle openings, but the transition from pilot, to needle jet seems to be where the problem is ?
I've tried rolling the throttle past the cutout point, to see if the main jet can pick up the slack,,but the engine doesn't recover untill the throttle is returned to under 1/4 throttle.
I have plenty of fuel flow (pump) about 2 ltr/min




Tiny
« Last Edit: August 31, 2012, 07:16:42 PM by generatorshovel » Logged

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« Reply #1 on: August 31, 2012, 07:31:54 PM »

Limited to my experience with a supercharged MGB, but -

First thought always turns to ignition.  If it's spitting fuel out the exhaust, something is not burning.  Weak spark/too much spark gap is the first thing I'd check.  According to electromotive, a little less gap when pressure goes up is advisable.

You mentioned good flow - but too much pressure, perhaps?

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generatorshovel
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« Reply #2 on: August 31, 2012, 08:07:46 PM »

First thought always turns to ignition.  If it's spitting fuel out the exhaust, something is not burning.  Weak spark/too much spark gap is the first thing I'd check.  According to electromotive, a little less gap when pressure goes up is advisable.

You mentioned good flow - but too much pressure, perhaps?


Thanks chris ?
I'm aware of the potential ign energy problem (have a fist full or iridiums gapped @ .020" ready) and found a place in the UK who do a mean coil to suit the lil hondas
The pump is regulated to 4psi, with a suitable needle/seat fitted.
Maybe I'll have to live with the spitting of unburnt fuel problem, just to hav e it rideable at full noise ?
Maybe the fuel is falling out of suspension when it gets to the rather large, cold plenum (I forget what capacity I worked out the intake volume was,but did the research before fab)
I haven't spoken  with Doc G since his return, I hope your blokes didn't abuse his tired 'ol body too much during his visit?
Tiny
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Bob Drury
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« Reply #3 on: August 31, 2012, 10:00:22 PM »

  Shovel, the fastest pass so far in my 496 BBC on straight methanol it left the line spewing raw fuel out the exhaust (according to my engine builder) and it was so fat that the plugs had absolutley no sign of use, all the cad was still on not only the threads but still on the ground straps.
  If I had not pulled the plugs myself, I would not have believed it.
  What I am saying is don't freak out unless it won't clean out under power or just plain feels like it has nosed over.
  One thing to remember with alcohol (or any other non petroleum based fuel such as Nitro, is that when the motor is cold or idling, the fuel is running right past the rings and diluting the oil (and filling the crank case).
  I try to preheat the oil and water and start it, clean it out, and go like hell.        Bob
  One more thought, if the blower has tight clearances (roots type) maybee it is pulling a vacuum above the rotors and actually literaly pulling fuel through the carb.
  Perhaps you are running too much fuel pressure to the carb.
  I really have no idea what I am talking about cause its a Blue Moon here on the top of the World...........................
  p.s. Don't forget to run a alcohol lube such as Red Line as the methanol will wash the oil off of the cylinder walls which is not a good thing at all.
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Bob Drury
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« Reply #4 on: August 31, 2012, 10:32:49 PM »

No abuse of the Doctor on my part - I was only in contact via phone - stuck in Beerhaven, again.

One other thought - did you just convert this over to alcohol?  Are there floats?  Alcohol is heavier than petrol, and I'm wondering if that might be causing the floats to keep the bowl jet open and flood it out.

I'm running about 2.5-3 psi on my SU and on my Weber to avoid overfilling the bowl.

I'm taking stabs in the dark, but I am very interested in the solution you come up with.

Chris
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« Reply #5 on: August 31, 2012, 10:37:35 PM »

If it's spitting fuel out the exhaust, something is not burning.  Weak spark/too
I haven't spoken  with Doc G since his return, I hope your blokes didn't abuse his tired 'ol body too much during his visit?
Tiny

Just ran a diagnostic, got a pulse here, O2 sensor reading normal zone. Thanks to Carmen's I was spitting a bit of unburnt alcohol out the exhaust early in the week at Wind-Over, Having proper work to do in LA kept my nose clean.....as did the impending start to my new job three days after disembarkation at Tullamarine....I'm positively healthy right now, off for a creek-walk/bird-watch this arv'.
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« Reply #6 on: August 31, 2012, 10:39:40 PM »

You might want to put a vacuum gauge on the intake below the carburetor to see how much vacuum signal the carb is seeing. They are only designed to work within the typical vacuum range seen in a NA engine. I as mentioned above the blower is pulling a vacuum in the intake, it could be sucking fuel right past the butterfly, transfer slots etc.

Larry
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generatorshovel
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« Reply #7 on: August 31, 2012, 11:52:29 PM »

Thanks Guys (blokes)
Bob & Larry,I think your on the money with the carb/blower vacuum , The thought has entered my mind  huh What to to to solve it, I dunno, yet.
The engine has done about 1 hour, 3 miles, and 3 oil changes, I'm also onto the oil pollution thing. )I'm a tad worried about hydrailic lock at the moment.

Chris,
I bench tested the carb / pump / regulator combo @ 4PSI with no sign of overflowing.
The carb(s) were set up by "pure power" (Jim Payne) for use on Jr Dragsters
Have I mentioned its a pain in the freckle to start (cold) when its set up as it is now ? (Aerostart needed)

Doc,
     I'm pleased to read you left a good collection of your grey matter in Bendover mate,
You still crack me up when I need a smile fix.
Tiny
EDIT
I finally got some heat into it, and monitored the dribbles, bugg'rall smell , and settles once the heat gets in, there's a fair amount of condensation going on in the collector I think (ambient temp 65 degrees here)
I had time to play with the idle mixture screw this time (on 'tuther side from throttle) starting with 1 1/2 turns out, poor throttle response, then 2 turns,,same result,,then 2 3/4 out,,it stopped.
Restarted with 3/4 out, it started to hunt (It's always done that) and throttle response improved, so I went with 1/4 turn,,,vast improvement,,but still sounds lean just above idle, but manages to hurt my ears when blipped from past that.
That "fuel" might be mostly water ? although it made Mrs "T" gag when I poked a wet finger in her nose, for a second opinion  evil
I'm gonna up the pilot jet and try again.
EDIT # 2
Manifold vacuum is 18" @ idle, raising to 20" just above idle.The fuel dribbles have stopped ?
#65 pilot jet is in, it seems close enough for another test ride, get some uniform heat into it, and play with needle position and main jet if needed.
« Last Edit: September 01, 2012, 03:34:12 AM by generatorshovel » Logged

Tiny (in OZ)
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« Reply #8 on: September 01, 2012, 02:22:45 AM »

A float bowl spacerfor the HSR42 is available from Mr. Turbo in Houston, a bit spendy, maybe you can make one.
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generatorshovel
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« Reply #9 on: September 01, 2012, 03:38:22 AM »

A float bowl spacerfor the HSR42 is available from Mr. Turbo in Houston, a bit spendy, maybe you can make one.
Thanks Moxnix, Iv'e looked @ those, but if another few ounces of fuel won't make much difference, if the supply & demand ain't right.
Plus,,its awkward enough to swap main jets already, without not being able to see 'em too.
Hey,,,how's your voice after Bubs ?
Tiny
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« Reply #10 on: September 01, 2012, 06:47:21 AM »

Tiny

Something that may help with warm-up is to get yoiurself a sure-shot spary can, fill it with gas, pressurise and spray gas down the carb throat with the methanol turned off. (old top fuel bike trick)
This will reduce ring wash whilst warming the engine, and probably make tuning the engine easier.

HTH
Neil
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« Reply #11 on: September 01, 2012, 08:23:58 AM »

Oh, my delusional self-importance made me post that.  I have a face for radio and a voice for print.

If you were running 20 lbs. of boost, it might be a valid suggestion.

My voice never came across the webwaves, I kept running notes over to the radio tent for the 4 announcers to share.  Delvene, Denis Manning's daughter, runs the meet; she is an Ozzie, smiled when I told her someone in Broken Hill, NSW was tuning in.
« Last Edit: September 01, 2012, 08:28:46 AM by Moxnix » Logged

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« Reply #12 on: September 01, 2012, 11:59:41 AM »

What I use is a Propane bottle with a regulator and about 8' yes feet of copper line with one-way flashback protectors ---from oxy-acclene--in the line.  put the line over the intake opening--- much safer and much more controlable when one shuts off the gas there is virtually no danger from lingering fuel.
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« Reply #13 on: September 01, 2012, 02:30:19 PM »

Sounds to me like you are probably fighting low speed rotor pulse here mate. Its especially a problem with two segment rotors & the closer the carb is to the blower, the worse it gets. The carb is responding to a huge signal & then nothing & the engine's on the receiving end going WTF.
This is why SU's work so well in this type of situation, they have a variable main plus they are a variable air flow carb so too big or too small never happens.
You might want to check & see if you are having a problem with manifold icing during warm up too.
Once you get into hot run mode, keep checking all the plugs to make sure you're not having a distribution issue.
  Sid.
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« Reply #14 on: September 01, 2012, 04:20:41 PM »

One other thought is vibration might be causing the cutout at high speed. Clamp some lead around the intake bell. Solder works.
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