Leaning the carbs for altitude - on the cheap


19 posts in this topic

Posted

Someone mentioned the HACman system for leaning the carbs for altitude.  It seemed like a cool idea.  Digging deeper I learn that there once was an automatic leaning setup, but it is no longer offered.

Digging even deeper, the explanations seem to be a little obscure, perhaps so we'll want to buy something we could easily cobble together on our own.  I hate to ruin the Hacman's day, but c'mon!  My understanding is that one takes the low pressure from the (initially plugged) tap into the Bing carb's throat, mixes it with another pressure source for some reason taken behind the air cleaner screen, and uses this "mixed" pressure to influence the flow through the jets of the carb via the float bowl.  Basically, you're messing with the float-bowl vent pressure.  Lower float-bowl pressure means less fuel flow through the jets.   Elegant and brilliant, no?  Well it seems to me that the float-bowl by default vents to the local pressure inside the cowling, whatever that is, so any pressure drop thru the air filter element is irrelevant.  Plus, if we only want a leaner mix, couldn't we just admit a teensy amount of air into the carburetor throat?  Take out the throat plugs, install hose barbed fittings, connect the two tygon tubes into a Y-fitting, then route the one tube to your panel, and install a brass needle valve from the hardware store, the other side exposed to the cockpit pressure.  Closed would be full rich, and then at altitude open the valve to admit some air directly into the carb throats to lean the mix to get the correct EGT.  Here, you're not even messing with float-bowl pressure.

Now I've done no calculations, so it's possible that you wouldn't be able to lean the mix enough with this simple scheme.  In that case, we just use two needle valves, one of which connects to the carb throats as before,  and a second, which connects to engine compartment pressure.  We connect the two pressure sources together with another Y-fitting, the third leg of which then goes back to connect to the float-bowl vents (another Y required).  Now we're doing the HACman thing on the cheap.  We set the needle valve connected to engine compartment pressure to some nominal opening.  We will immobilize it after initial tweaking, maybe mounting it behind the panel where we don't normally even think about it.  We leave the valve connected to the carb throats mounted on the panel.  The panel-mounted needle valve is closed for full rich, and again is opened a bit if we want leaning, making the pressure applied to the float bowls something between engine compartment pressure and carb-throat pressure, depending on by how much we open the needle valve.  We would only have to play with the cabin-pressure needle valve in the beginning, in order to set an appropriate sensitivity for the panel-mounted valve.  Clearly it's not set right if full leaning at 10kft requires only 1/8 of a turn, or 5 turns.  In this case we reach behind the panel and adjust the other valve.  Again, EGT is our guide.

Comments?

Share this post


Link to post
Share on other sites

Posted (edited)

Ski doo snowmobiles used their HAC and DPM system on sleds for about 10 years.

I had a bunch of them and even added hac to a sled that didn't have it (just looked up the hac parts on the microfiche).

Float bowl pressure regulation...  worked really well but I did have some failures over the years.  Default fail is rich.. which is good, but the sled would only pump about 40% power when it failed.  Reasonable failure mode in a sled but not so much in a plane!

Man, I dont know... an attempt to re-create a alt compensation unit in a 2 stroke would require tons of testing, fiddling, reworking, and possibly engine rebuilding if you mess it up and overtemp in the R&D process.

Think I'd rather pick up a 600 etec and try to build a psru adapter plate for it! (Direct injection, "FADEC", and 120 HP)

Edited by Yamma-Fox

Share this post


Link to post
Share on other sites

Posted

Fails?  This would be a fully manual system.  Clearly one would be monitoring egt, rpm, and coolant temp, as in normal operation.  Shutting the needle valve brings you back to the basic configuration.  At least with leaning we have some control over egt.  Perhaps I am missing your point here?

Share this post


Link to post
Share on other sites

Posted

The Artic Sparrow mixture control works great. One turn of the control knob is equal to moving the clip one notch. 

Share this post


Link to post
Share on other sites

Posted (edited)

I have a HACMAN on my 582, and it works well. It has enough authority to lean at least 1.5 GPH at altitude (from almost 6 GPM to about 4 or 4.5), and would raise the EGT from 800 to about 950 or 1000 degrees. Using the HACMAN always seems to prevent any rich plugs. I flew across the US from Stockton CA to New Haven CT, across Wyoming, so I got to see all types of altitudes from SL to about 11K', and I could always lean to keep the plugs clean.

The HACMAN principle is simple - it has a screw valve in the cockpit that drops the carb bowl pressure by mixing some of the very low pressure area at the carb throat (the air at the throat has low pressure because the air in the throat is flowing so fast.)  The low pressure from the carb throat reduces the bowl pressure so less gas is sucked up into the throat, the HACMAN lets you  lower the pressure differential between the bowl and the throat.   It basically reduces the suction so the needle valve lets less gas into the carb, thus leaning it out. (This really just restating what Turbo wrote above, his sense of the physics is spot on).

Frankly, the HACMAN is just an air needle valve and some cheap hoses, my guess is we could make one fairly easily, but since I was flying over the Rockies and didn't want to walk home, I paid the money.

The altitude compensating HAC that Rotax sold at one time appears just like the HACMAN except it used a simple diaphragm to adjust the mixing of the two air sources. 

Edited by nlappos
Fixed some confusing language

Share this post


Link to post
Share on other sites

Posted

I have the oil injection on my 582. Recently bought the HACman mixture control. Can I install it using the oil injection or will I have to prefix. If I install it, using the oil injection. Does leaning the carbs mean the oil/fuel ratio changes?

Rick

Share this post


Link to post
Share on other sites

Posted

My understanding is that the OI system responds to two things only: throttle slide position and engine rpm.  How rich or lean your air/fuel mixture is should not affect engine oiling.  It seems to me that you're good to go.  Of course, you'll be watching EGT and coolant temp. - Turbo

2 people like this

Share this post


Link to post
Share on other sites

Posted

I'd be cautious of directly admitting air as a means to lean out the engine.   It might be possible to dial it in for one moment, but changes in engine RPM or throttle position could change the vacuum at the air-injection point and dramatically change the mixture.   

There's been a lot of success with carb bowl vacuum systems.   I'd stick with that. 

If you want simple, I wired up a very cheap 12VDC vacuum pump to a 100 Ohm rheostat, and plumbed it to a T in the normal vent tubing.    The vent tubing continued on to its normal atmospheric opening, which I adjusted in size until the pump running full-out pulled enough vacuum on the vent line to increase EGT by about 80 degF.   It increased my ceiling from about 9,000' to 13,999' and made the engine much happier.      

I didn't like pulling gas vapor through a crappy electric pump, but the pump head seemed pretty well isolated from the sparky parts.   Also, if the vacuum pump was on, and the downstream vent tube got completely or partially plugged, it would excessively lean out the engine and could be catastrophic.    It seems unlikely, but it's worth thinking about all the failure cases. 

Here's a pic of me holding 1000degF EGTs at (just under) 14,000'.   

IMG_20170705_091326.jpg

1 person likes this

Share this post


Link to post
Share on other sites

Posted

The system isn't on the plane anymore, but here's the rough layout.  

The "calibrated orifice" was pretty large, maybe almost 1/8".  It easily passed liquid gasoline that got into the vent tubes. 

 

IMG_20180414_200445.jpg

Share this post


Link to post
Share on other sites

Posted

Cowlove, I like your solution!   BTW, we were t alking about a float bowl vacuum leaning system; that's what the hacman system is, but based on the carb throat as the vacuum source.  Where did you find the vacuum pump?

Share this post


Link to post
Share on other sites

Posted

My mistake!  I did originally mention a direct-leaning approach.  Cowlove's point regarding same is taken.  

Share this post


Link to post
Share on other sites

Posted

Rfphtmfxr, I agree with you in that there is a difference between OI and premix wrt leaning. With premix, less fuel means less lubrication when leaned out at altitude despite essentially the same cruise rpm, whereas with the oil injection you should be getting the same oil flow at all altitudes.  I think this is an advantage for the OI system over reverting to premix.

1 person likes this

Share this post


Link to post
Share on other sites

Posted

Seems to me it is an advantage. But then the next question is. Since there is more oil to fuel, will the plugs foul easier. Or will the optimum GETs keep the plugs clean

Looks like I'm going to have to drill one of my carbs to install the HACman system. The casting is there just need to drill and tap. More research on the drilling before attempting. No spare carbs, if I screw it up y

Share this post


Link to post
Share on other sites

Posted

Good question.  Since the oil is ostensibly less atomized with the OI system, maybe it won't be any worse.  Same relative motion, same friction, same temperatures, same lubrication. Sounds right to me.  Beats hell out of potentially under-lubricating the engine.  What say y'all with leaned, high-altitude time running premix?

 

1 person likes this

Share this post


Link to post
Share on other sites

Posted

LOW EGT's lead to plug fouling.  Keeping the EGT's in the upper range will keep the plugs nice and happy.  I am in the same camp of OI keeps the same amount of oil going into the engine when less fuel is going in is a good thing for 2 strokes. 

Basically what you are doing in "rejetting" the carbs at the higher altitudes such as we have done with snowmachines for eons.  The only difference is, with the mixture control you don't have to stop on the side of a mountain, fumble around with half frozen gas soaked fingers dropping the jets in the bottom of the belly pan cursing up a storm just to keep your engine from blubbering and farting as your trying to go higher up in elevation.  There have been attempts at the "automatic" systems in sleds but I never had much luck with them really working right.  With the hac man system or for those of us lucky enough to have the Arctic Sparrow mixture control, it is a simple twist of the knob or knobs to keep the engine humming happily along and not popping snorting and belching as we soar along on the winds.

Yes we ran premix on the sleds and didn't have an issue as long as you remembered to stop on the way down and swap to the fatter jets before you ripped across the low lands. 

:BC:

 

Share this post


Link to post
Share on other sites

Posted (edited)

The HACMAN leans the fuel mixture using the same method as the Rotax approved HAC system (which is illustrated in the Rotax parts manual in the accessories section), except the manual mixture valve. It basically tricks the carb bowl pressure to a lower pressure, making the carb deliver less gasoline to the metering valve (functionally the same as having a smaller main jet). I used a HACMAN on my 582 for 3 years, and it always produced clean plugs and high egts, as I flew from 11000 foot legs in Wyoming to sea level long cruises.

I do think the simple mixing valve and tubing set is not worth $200 and I applaud the guys here who make it cheaper and simpler!

Edited by nlappos

Share this post


Link to post
Share on other sites

Posted (edited)

Mixture control comes up here from time to time. I did a lot of experimenting with this about 15 years ago with great success. I thought I'd posted this stuff here before, but couldn't find it when I looked. I'm posting some misc sketches, photos and notes about mixture control. It's not organized but maybe someone will find it useful. "Chris Diagram" is the thought experiment that led to 'manual mixture" which was one of my first implementations of the manual mixture control. it continued to evolve but I did not document that. "Carb balance" is just a thing i came up with to balance the carbs. Worked great for me and was free to make from junk tubing.

Chris Diagram.pdf

Manual mixture.PDF

_Carb balance.pdf

Edited by Chris Bolkan

Share this post


Link to post
Share on other sites

Posted

newer carbs come with a low pressure port. older ones do not. this is how i modified my carb for a low pressure port. 

Carb mod.pdf

Carb Mods2.zip

Share this post


Link to post
Share on other sites

Posted

Here are some pics of the mixture control in my mkiv

IMGP0063.JPG

IMGP0064.JPG

IMGP0065.JPG

IMGP0066.JPG

IMGP0067.JPG

IMGP2122.JPG

IMGP2123.JPG

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now