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MA-4-5 Mixture Throw on RV-8

Sam_B

Sam_B

Well Known Member
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Hey everyone, I’m hoping to take advantage of the collective brain power and experience of this group. I’m just getting back to building after several months of life getting in the way and man does it feel good! But I digress, I ran into a snag this evening while working on getting the mixture hooked up. It appears that I don’t have enough throw in the quadrant to get the mixture from stop to stop. When I investigated drilling a new hole in the quadrant, it appears that the standard Van’s green cable is running out of travel, so a new hole in the quadrant might not gain me much, if anything.

See the image below of the cable in the quadrant at full rich:
072998B2-0FB6-4E7E-9930-9DD2EB18E0D8.jpg

Then see the image below at the carburetor at idle cutoff:
A8061B6A-CC25-4BD3-B141-B0A594EA6F4F.jpg

When I look at the mixture lever, it appears I need about 1/4” more of travel from the cable to actuate the mixture stop to stop (not even including the standard ‘cushion’ in most engine control setups).

My next thought was to drill a hole in the mixture arm on the carburetor closer to the pivot point but the shape of it doesn’t really allow for that:
D3CB6371-F2E2-4F12-97D2-BA6B0F578B07.jpg

I don’t have an image on me, but the OP instructions from Van’s show a mixture arm that looks to be the same general shape as what I have, so it’s not like I have an oddball carburetor (well maybe I do, but it doesn’t seem like it). Furthermore, I found this Kitplanes article that specifically calls out the RV-8 as frequently having this problem in the ‘Controls’ section:

https://www.kitplanes.com/carburetor-maintenance/

So I’m kind of thinking this might be somewhat of a design flaw with the RV-8 quadrant.

Does anyone have any suggestions as to how to overcome this? Is there an aftermarket mixture control arm that I could buy with similar, but slightly different geometry that might work better in my application? The current mixture control arm appears to be made of brass or bronze or something similar, so without knowing the specific alloy, I’m apprehensive about any sort of bending/welding/modification that may weaken or break the arm.

Surely I’m not the only person to have taken on a carbureted RV-8 so I’m hoping this group can offer some advice.

Thanks!
 
Is it being limited by the available throw of the *cable*, or the control?

If it's the control lever itself, which was what i had on my 7's throttle quadrant for one of the controls, you can drill a new hole higher up on the lever to get more of the full extent of the cable's available throw.
 
Quadrant hole/Outer sleeve position..

I can't tell by your pictures, but I have an RV-4 I built with an MA4SPA and standard Vans Green Teleflex. I believe you need to adjust the jamb nuts where your outer green sheath is "centered" between the quadrant and carb so there is equal travel at both ends. I built my own quadrant and cable attachment at the carb (which may be your limiting factor), so I was able to do custom geometry, but I get full travel easily. My full forward stops at throttle and mixture hit (carb) before my quadrant hits its stop as desired .I have attached some pics of mine. I do not recommend altering the mixture lever.
 

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RV7A Flyer said:
Is it being limited by the available throw of the *cable*, or the control?

If it's the control lever itself, which was what i had on my 7's throttle quadrant for one of the controls, you can drill a new hole higher up on the lever to get more of the full extent of the cable's available throw.
Click to expand...


The cable seems to have less travel than is required by the carburetor, so drilling a new hole higher up on the quadrant wouldn’t gain me anything.
 
fixnflyguy said:
I can't tell by your pictures, but I have an RV-4 I built with an MA4SPA and standard Vans Green Teleflex. I believe you need to adjust the jamb nuts where your outer green sheath is "centered" between the quadrant and carb so there is equal travel at both ends. I built my own quadrant and cable attachment at the carb (which may be your limiting factor), so I was able to do custom geometry, but I get full travel easily. My full forward stops at throttle and mixture hit (carb) before my quadrant hits its stop as desired .I have attached some pics of mine. I do not recommend altering the mixture lever.
Click to expand...

It is a little tough to tell from your images, but your mixture lever looks quite different than mine. Perhaps it requires less throw than mine does. I don’t think centering the cable will help since the cable itself seems to not have enough travel in it as compared to what the carburetor requires.
 
haven't had my carb for many years. However, your arm appears longer than I remember mine being. Might be worth a call to Marvel to see if they sell different length arms.
 
Thanks!

Thanks for that. One question: Hiw do I find such information for myself? Is there a ‘carburetor parts catalog’ out there somewhere for me to reference? I haven’t had much luck finding one for download or for purchase.
 
I had the same problem with my RV8 mixture and prop control cable. The mixture cable was replaced once with a Van's cable. After the second failure, I decided to go with a custom McFarlane cable. I have also replaced the prop control cable with a McFarlane cable. They are expensive, but they perform flawlessly. Mine have been trouble free for over three years. I highly recommend them. Also, you should consider putting fire sleeve on all the control cables forward of the firewall.
 
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Mark Cigal said:
I had the same problem with my RV8 mixture and prop control cable. The mixture cable was replaced once with a Van's cable. After the second failure, I decided to go with a custom McFarlane cable. I have also replaced the prop control cable with a McFarlane cable. They are expensive, but they perform flawlessly. Mine have been trouble free for over three years. I highly recommend them. Also, you should consider putting fire sleeve on all the control cables forward of the firewall.
Click to expand...

Will do. Thanks for the tip!
 
pauldan181 said:
I have the exact same problem with my mixture arm. I found this site.

https://www.mcfarlaneaviation.com/products/product/155-357/

I've never done business with them but thinking about giving it a shot.
Click to expand...

The only problem I see here is this - from the photos I have been able to obtain of part# 155-357 show a flat mixture lever. The one on the “brand new” 10-3878 Ma4-5 I just got from Spruce is goose-neck shaped - i.e. the hole for the rod end attachment is ~(wild guess) 1.5” above the shaft base. The flat one won’t work. I eagerly welcome any other ideas, as I am at my wit’s end.
 
Machine a spacer?

Could you fabricate a spacer similar to what is shown in the plans for the throttle to go on top of the flat mixture arm but below the heim joint and then use a longer bolt of appropriate size to attach the cable to the lever?
 
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rocketbob said:
You need part # 155-357 which is the shorter arm.
Click to expand...
I ordered this from McFarlane, was on back order for several months and just came in the other day. Unfortunately, this isn’t a viable solution, at least without some other modifications. It places the cable attachment point to the rear of the carburetor where the lever would be actuated side to side instead of front to back. So it really can’t be actuated without a longer cable, modifications to the Vans brackets, etc., see picture of the two arms below. One of the stock carburetor arm (155-367), and the other of the #155-357. The levers that attach to the cables don’t point the same way in relation to the slot in the hole that would clamp to the mixture control of the carburetor. At this point, I’m wondering if the fastest method is just to reverse engineer this thing (should have the metrology equipment at work to make a blueprint), and then just machine my own. That said, I’m open to ideas
 

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I‘m leaning towards getting the McFarlane cable, though I’m having trouble finding it. My build is an -8, so it would just be a cable threaded with 10-32 on each end. Can anyone point me to a link?
 
Sam_B said:
Perfect, thanks!
Click to expand...
I have been using the ACS cables from Spruce on my 7 for the past 7 yrs and 750 hrs. Work fine. They are rated for 500*F and are much less expensive than the McFarlane's. The cables come in different travel length's. Do you know what the original one is? Is the quadrant lever travelling fully end to end?

I had a similar issue with my quadrant limiting the travel. Couldn't find an appropriate arm so I cut the mixture arm shorter and used JB Weld and 3/32 rivets to add a new shorter .060" aluminum arm on to the cut off arm. Still solid after 750 hrs. FWIW.
 
RV7 To Go said:
I have been using the ACS cables from Spruce on my 7 for the past 7 yrs and 750 hrs. Work fine. They are rated for 500*F and are much less expensive than the McFarlane's. The cables come in different travel length's. Do you know what the original one is? Is the quadrant lever travelling fully end to end?

I had a similar issue with my quadrant limiting the travel. Couldn't find an appropriate arm so I cut the mixture arm shorter and used JB Weld and 3/32 rivets to add a new shorter .060" aluminum arm on to the cut off arm. Still solid after 750 hrs. FWIW.
Click to expand...
It’s definitely the cable travel being the issue. The quadrant is close, but the cable is the issue. I don’t recall at the moment what the cable travel is, but the cable is end stop to end stop.

Do you have a link for the ACS cables? I searched for those as well with no luck.
 
Sam_B said:
It’s definitely the cable travel being the issue. The quadrant is close, but the cable is the issue. I don’t recall at the moment what the cable travel is, but the cable is end stop to end stop.

Do you have a link for the ACS cables? I searched for those as well with no luck.
Click to expand...
Aircraft Spruce has the cables. They are special order. You may be able to buy direct but for me in Canada it was easier to deal through Spruce. I paid an extra $75 for the expedited 5 day manufacturing service. The cables were less than half the price of the McFarlane's and have worked fine for me. I couldn't justify $500 US per cable from McFarlane when I could get all 3 cables for my 8 for $700 Cdn. The ACS cables don't have the sheath over the spiral casing but they are teflon lined and 500*F rated. If you Google ACS control cables you will get their website. Sorry, don't know how to post a link.
 
RV7 To Go said:
Aircraft Spruce has the cables. They are special order. You may be able to buy direct but for me in Canada it was easier to deal through Spruce. I paid an extra $75 for the expedited 5 day manufacturing service. The cables were less than half the price of the McFarlane's and have worked fine for me. I couldn't justify $500 US per cable from McFarlane when I could get all 3 cables for my 8 for $700 Cdn. The ACS cables don't have the sheath over the spiral casing but they are teflon lined and 500*F rated. If you Google ACS control cables you will get their website. Sorry, don't know how to post a link.
Click to expand...
Thanks!
 
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