Author Topic: Prop Wash interfering with 1/2 VW dual Mikuni Intake??  (Read 610 times)

Offline Aaron

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Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« on: August 01, 2021, 07:22:18 PM »
I am running a 54x24 prop on a 1/2 VW on my LE with dual mikuni carbs. Has anyone ever experienced the prop wash causing a low pressure  region on the intake itself, interfering with the intake vacuum on the # 2 cylinder (left cylinder from pilots view). I know it sounds far fetched, however let me explain how I got to this strange conclusion.

It started with rough running at high power settings, probably anything over 3/4 throttle. This appeared upon bolting the engine for the first time onto the airframe from the test stand, so there were many variables (note this problem does not occur on the test stand, also note that I just finished my LE and am in the initial taxi testing stage). I have good fuel flow as well as good gravity feed, and leveling the engine (not on tail) didn't fix, thats the first things I ruled out. While trouble shooting I found that holding just the left carb while running made the engine run normal again (right carb seemed to function fine). My next thought was carb spitting due to excessive vibration. My though being that the airframe may flex more than my test stand, making the carbs floats incapable of doing their job effectively from the vibrations as the carbs are bolted directly to the head, and yes I have read all the other threads about the carb spitting and vibrations etc. I went as far as mounting the carbs to the airframe and connecting the intake with rubber hose to eliminate the vibration from the carbs, this did not fix the problem either even though the carbs were completely still, the engine still had the same symptoms. It was then my I realized that when holding the carb that maybe my hand was covering the air filter partially rather than dampening the vibrations like I thought I was doing. I ran it up again and held my hand over the left air filter and once again it ran great. I then started making mixture adjustments, which was unsuccessful, because as I got it richer it would run better, however as I would go back to idle it would start puffing black smoke out of the stack. This got me thinking that whatever the problem is, there is something in this setup that is different then the test stand that is affecting the left carb. It seemed no matter how rich I made it the engine would just not run right at high rpms. I put everything back to original settings, and ran it up again. What I found next caught my surprise. I advanced the throttle to full which of course resulted in rough running. What caught my attention was as I reached out toward the carb it almost  seemed as if the engine knew my hand was coming and came to life before I even touched the filter. Confused at what I just thought happened I pulled my hand back, waited for it to act up again (about 2 seconds), and reached forward again. Sure enough, this time I didn't even make it to the carb, all of a sudden the engine was running perfect with my hand a few inches from the carb, like magic! I found that just holding my hand out just forward of the carb the engine ran perfect. Having read the threads about the bowl overflow vents being effected, I tried plugging, rerouting to the cabin (a static pressure area), ram air, etc, all failed. My conclusion is that the prop wash on the left side is causing a low pressure region in the area of the left intake which is causing starvation of the left cylinder. I was still stumped to why my engine was not effected by the prop wash on the test stand. However I realized that due to the nature of my test stand, the air filters pass thru the frame work of it, so I think the 2x4's that are in proximity to the left filter disturbed the prop wash, effectively dissolving the low pressure region that I am now experiencing on the airframe as the carbs are now out in open space.

Alright, now that I explained myself, has anyone ever experienced this? I figured I can't be the only one. Does someone think it is really something else? I am open to other theories or anything that could help me out. I am going to try and reroute my intake next weekend. My other idea was making a baffle, however I decided that making a baffle that would protect the air filter is not the most airworthy choice (losing that baffle would mean losing 25% power, not a good feeling). Let me know your thoughts!

Offline Tom H

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #1 on: August 02, 2021, 07:26:20 AM »
Aaron, it has been a really long time, but we went through similar issues with mikuni carbs on half VW.  We ended up putting some plastic bowls over the intake of the carbs to help with air delivery.  It seemed to run well.  Again, though, it was so long ago that I don't remember the details.  I've attached a poor quality picture - hope it is clear enough to be helpful.
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Offline Dan_

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #2 on: August 02, 2021, 09:12:47 AM »
I am running a 54x24 prop on a 1/2 VW on my LE with dual mikuni carbs. 

I am open to other theories or anything that could help me out.

Is it a cut case?  Have you switched carbs side for side to see if the problem follows?  Have you ran it without the air filter?  Does it still do it with the plane moving at near take off speed?

Just thinking out loud here...


“A superior pilot uses his superior judgment to avoid situations which require the use of his superior skills.”

– Frank Borman, Apollo 8 Commander

Offline Aaron

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #3 on: August 02, 2021, 01:34:52 PM »
Thanks for the pic Tom!I will have to try something similar maybe. I tried something similar by bending up a quick and dirty baffle, the goal being to disturb the prop wash around the air filter. It helped enough to fly but I still am lacking a good 150-200rpm of top end, which makes the climb pretty sad :/

@Dan, it is full case. I have switched carbs, cleaned both of them twice, tried adjusting, ran without filters, ran with choke, and probably more that I can't think of at the moment. Yes it still has the same symptoms even when flying 10 ft off ground over runway.

My current plan of attack was to put an elbow in and route the air filter either 90 degrees upward or to the outside, in which those orientations in space seemed to not be affected by the prop wash. Would you advocate those air delivery bowls? I am tempted to try that, but then again I am worried about the vibration causing something like the baffle or a bowl to fail and if it departed the airframe it would guarantee at least 25% power loss, whereas a rubber boot would take that vibration for quite a long time. Could be paranoid but that's just my thoughts :)

Offline Aaron

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #4 on: August 02, 2021, 01:36:52 PM »
Tom, do you remember if you had a direct flange to flange bolt on to the intake of your head with your mikuni carbs (rather than a rubber boot to isolate some vibrations)?

Offline Dan_

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #5 on: August 02, 2021, 02:15:39 PM »
Would you advocate those air delivery bowls? I am tempted to try that, but then again I am worried about the vibration causing something like the baffle or a bowl to fail and if it departed the airframe it would guarantee at least 25% power loss, whereas a rubber boot would take that vibration for quite a long time. Could be paranoid but that's just my thoughts :)
Paranoid is good especially on initial flights...

Mounting a funnel on the firewall with the small end toward the carb jumped out at me as an experiment but you prolly don't have room.  Whatever you try --if you can mount it to the firewall, it would likely tend to stay put and not stress the intake mounting.

Tom is good help, Les Homan is good help as well as many others who don't check in very often.


“A superior pilot uses his superior judgment to avoid situations which require the use of his superior skills.”

– Frank Borman, Apollo 8 Commander

Offline Tom H

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #6 on: August 03, 2021, 09:25:34 AM »
Aaron, we had rubber boots connecting the carb to the head, using hose clamps.  The hose clamp screws were wired to prevent them from loosening.  Also, we had used safety wire connecting the head to the carbs to keep the carbs from squirming free from the boots.  We did not use add'l supports to keep the carbs from shaking, but some others have done that.
Tom H
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Offline Aaron

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #7 on: August 03, 2021, 01:22:43 PM »
Thanks for the info, I bought some stuff to reroute the intake 90 degrees upward and will see how that goes this weekend, will report.

Offline Aaron

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #8 on: August 09, 2021, 02:39:55 PM »
Update: Engine is running great now, just like it used to on the test stand. There were two culprits, both victims of prop wash. The first was the left carb's intake, I fixed this by putting a 90 degree rubber elbow on the carb, pointing the filter upward. The other culprit was the left carb's float bowl vent. Strangely the engine ran terrible when routing the vents to the cockpit (static atmospheric pressure source), instead, the vent demanded some positive pressure, so I bent up a piece of copper tube to give the vent some ram air as opposed to vacuum or static pressures. Also just an interesting note, the right carb was completely unaffected by the prop wash. Anyway thanks everyone for all the input!

Offline Kamcoman77

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Re: Prop Wash interfering with 1/2 VW dual Mikuni Intake??
« Reply #9 on: August 09, 2021, 05:08:57 PM »
Please try to take a couple of photos so other builders can try your solution. It sounds great! Here is another idea that a builder used on his Mosler 1/2 VW. The Mikunis are mounted to the fuselage tubing so they do not move with the engine.

 

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