MA-4-5 Mixture Throw on RV-8

[Sam_B]

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:


Then see the image below at the carburetor at idle cutoff:


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:


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!

 

[RV7A Flyer]

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.

 

[fixnflyguy]

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.

 

[Sam_B]

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.

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.

 

[Sam_B]

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.

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.

 

[lr172]

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.

 

[rocketbob]

You need part # 155-357 which is the shorter arm.

 

[Sam_B]

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.

 

[pauldan181]

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.

 

[Mark Cigal]

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.

 

[Sam_B]

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.

Will do. Thanks for the tip!

 

[Sam Staton]

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.

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.

 

[Sam_B]

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?