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chazdo2 05-09-2011 02:47 PM

Throttle linkage
Any of you experianced guys had this problem? I have a 38MM Mikuni flatslide on my Yamaha cart. I have the 16HP Duromax motor in it. After creating a manifold out of 90 degree radiator hose to alleviate the clearance problem another problem arose. The throw on the gas pedal does not allow the throttle to fully open even with no slack at all! I moved the gas pedal to almost completely straight up and down and it almost does the trick. Any of you guys have a better solution?

sho305 05-09-2011 02:55 PM

Re: Throttle linkage
Make a lever like this, see this one 'gears down' the throttle rod to the carb. The pedal actually travels more than the rod. Just put your carb cable outside the pedal cable, from the pivot. I used the factory governor arm for this one, I drilled a set of holes in it so I could pick the right ratio to make it come out right. There is a return spring on them on other side.

The choke on the right I used the other side to change direction of it. There is more pics on page before that but not good ones of linkage.


You don't really want the carb to go all the way open anyway do ya? LOL! Let us know how the carb works I want to put one on my clone I think this carb sucks. I'm trying to make a curved intake.

chazdo2 05-10-2011 07:20 AM

Re: Throttle linkage
I will take some pics. I used a tight 90 degree bend off a radiator hose cut as small as I could and mounted the flatslide on it. I supported it with a couple of washers welded on some threaded rod from the top of the carb to a bolt on the engine. Works well and doesnt shake to much. I love the 38 MM flatslide carb. I have a junk box full of old Mikunis mostly round slide. I got them off e-bay over last year. I bet I have 10 carbs and have not invested 100 bucks in the whole lot with shipping. Just gotta shop a little. I still cant figure out why I could not make the motor run well and start right with any of the other smaller roundslides. I played with the jetting and the needles a ton but with limited success. I put the 38 mm flat slide on with a 320 main on it. Started right up and ran a little rich. I knocked it down to a 140 main and it runs and starts great!!
I had a small problem with the size of the intake I welded up and the much bigger inlet on the flat slide. I slid a piece of thinwall conduit in the intake of the carb that fit perfect. Now the inlet matched perfectly with the radiator hose. It slid over it tight and appears to have no air leaks. Kind of hokey but it works great. I guess maybe its a 34 mm flat slide now. Well anyway it runs great and with your throttle idea should be done.........FINALLY!!!

chazdo2 05-10-2011 07:36 AM

Re: Throttle linkage
I looked at the pics you suggested. The problem I have is being a throttle cable and not the rod moving the throttle this is confusing me a little. I can envision some sort of mechanicle advantage such as a bigger cam on the pedal rod or something like that. I can see your arm deal working with a rod but I cant get it through my head to work with a cable. Maybe a better picture? I am sure you well know that the travel of a Mikuni flat slde is about four times the travel of the stock carb on these engines. Seems that the flatslide is a lot more than the round slide. I bet I need about 4 inches to open all the way. Can you make it a little clearer to a hack like myself?

sho305 05-10-2011 11:48 AM

Re: Throttle linkage
Its all about the diameter of a circle, if you put a small and large circle together. Lets say you have a saucer centered on a big plate, its 12" around (circumference) the saucer a 30 around (the edge) of the plate. If you turn the plate 1/4 revolution (90 degrees like my lever approximately) then the saucer (hooked to your pedal) turned 1/4 that or 3" and the plate (hooked to your carb) turned 1/4 so 7.5", the ratio is 12:30, or 6:15, or 3:5 to simplify. Sure you don't get that exactly because your cable or rod is not turning with the circle (unless you use a cam/pulley to hold the cable), but in about 90 degrees it can be close, just like your tie rod ends work on the spindle that swings in part of a circle.

What you would be doing here is cutting a pizza in four pieces (quarters) and then measuring the width of the crust, that is how far it moves in quarter turn (with a rod or no cam). Then you have to make the circle (pizza) the size you need, and then use a lever because that would be a waste of good pizza lol.

The easy way to do it is with paper and a rule. If you know how far your throttle cable travels and how far your carb needs to go, you know the ratio you need to match them up. Don't forget a little pedal play in there. Cut out a lever out of paper and hold the pivot point on the paper, then swing it and measure where you need to get your pedal travel mounted on it. Then go out more until you get the travel you need for the carb, at the same rotation as the pedal. Its better to mount it up so the lever is more angled to the cable at idle and closer to 90 at WOT. That gives you throttle tip in you want the WOT part to move faster. It will move slower the closer the lever gets to pointing up the cable, where it stops moving.

You can make a cam, but I don't recall there being much room at the pedal. You could use a garage door or patio sliding door pulley(s) for that just off the top of my head. If you make it on/around the engine then yeah you need two cables or a cable and a rod like I did it. The rod of course has to be inline with the carb mine is in same place the governor rod was before I removed it. If you found the two right sizes you could just shoot a bolt through the pulleys to make them turn as one. Still have to make the cable hook to them of course, and that is why levers are easier.

chazdo2 05-10-2011 02:48 PM

Re: Throttle linkage
Thanks that makes it easy. I greatly appreciate the time you take to help us backyard guys out!! Remember if in Central New York look me up and ill buy you a beer. That pizza comment was pretty funny!

sho305 05-10-2011 03:41 PM

Re: Throttle linkage
You can do it with geometry, but not as easy lol. You would look up the formula to give you the long side of a right triangle (90 degrees) with equal short legs, put that into an excel formula, then you can just punch in how far you need it to move and it would solve it for you....tell you the distance from the fulcrum/pivot you need to be for X travel of your cable. But its easy to just use paper and measure it quick, you can design it in a minute just have to figure out where to put it and how to hook the cables up. Then drill some extra holes in it close together so when its not perfect you can adjust it up/down a little. When I did that one I held the lever there with my fingers and flipped it back and forth holding a rule next to it, then tried it with the cable and carb to verify, marked it and drilled the holes in it. The bolt for the throttle lever is on the back side of the plate, so maybe 1.5" behind the front of that plate. I think thats 1.5" x 1/8 plate or maybe 2. Lever is .5" x 1/8".

I had a job where I had to stand on the ground and tell you how tall something like a building was, and I didn't have to move to do it with the help of a couple tools. Its all about angles and geometry, but if you don't use it you don't have much reason to remember it. A carpenter could figure it out I bet, they use angles a lot, more than I did. Glad I can help we can't have a carb not opening all the way lol.

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