No more broken throttle shafts
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Newb
Joined: Jun 2009
Posts: 15
Total Cats: 1
From: San Jose, CA
No more broken throttle shafts
When I began driving CSP Miatas, I heard of throttle shafts breaking at the butterfly screws, the engine ingesting the screw and the next thing to do is rebuild the damaged engine. The guys I knew would weld, braze, or even JB weld the screws to the throttle shaft. Pretty insane considering you are spending good money on a pro built engine then protecting it with JB weld.
So, I personally have had two throttle body's shafts break on me in the past two years. We usually would carry a spare. The latest one broke during the El Toro pro. Lucky for us, Tom had welded the screws to the shaft. The engine was saved but runs were lost. After this event I had enough of the same problem. I took pictures and was just going to copy Bill's throttle setup and run the Skunk2 throttle body. I called Bill and he told me to call Skunk2 directly and see if the throttle body that they had in the works was finally going to be built.
I am thrilled to say they are making exactly what we all have needed for years. A Miata specific throttle body with a throttle shaft that won't break! It is 64mm, instead of the stock 55mm. When we would use our factory stuff and would clean up the inner lip, it would then show hp gains on the dyno, so I can't wait to see what this increased diameter will prove on the dyno. They use all factory components/sensors for 1.8L NA and NB cars.
Tom just got the prototype for us to test. I will post up improvements after putting it on the dyno. As far as I'm concerned if it makes the same power AND will stay together it's a win!
Hopefully this will save someone else's engine,
Scott Fraser
So, I personally have had two throttle body's shafts break on me in the past two years. We usually would carry a spare. The latest one broke during the El Toro pro. Lucky for us, Tom had welded the screws to the shaft. The engine was saved but runs were lost. After this event I had enough of the same problem. I took pictures and was just going to copy Bill's throttle setup and run the Skunk2 throttle body. I called Bill and he told me to call Skunk2 directly and see if the throttle body that they had in the works was finally going to be built.
I am thrilled to say they are making exactly what we all have needed for years. A Miata specific throttle body with a throttle shaft that won't break! It is 64mm, instead of the stock 55mm. When we would use our factory stuff and would clean up the inner lip, it would then show hp gains on the dyno, so I can't wait to see what this increased diameter will prove on the dyno. They use all factory components/sensors for 1.8L NA and NB cars.
Tom just got the prototype for us to test. I will post up improvements after putting it on the dyno. As far as I'm concerned if it makes the same power AND will stay together it's a win!
Hopefully this will save someone else's engine,
Scott Fraser
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I'm not a pro racer at all. But, not once have I ever had a butterfly screw come out. Not on Subaru sandrails, not on Porsche's, bikes, or anything. I am willing to be proven wrong, please do. However, not once in my years has a perfectly good throttle just come apart. Bad assymbly maybe? Dumb people? I pulled screws out of race car oil pans before, never even once thought buttterfly screw. I was another shop dropping carb screws in a open motor.
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I'm not a pro racer at all. But, not once have I ever had a butterfly screw come out. Not on Subaru sandrails, not on Porsche's, bikes, or anything. I am willing to be proven wrong, please do. However, not once in my years has a perfectly good throttle just come apart. Bad assymbly maybe? Dumb people? I pulled screws out of race car oil pans before, never even once thought buttterfly screw. I was another shop dropping carb screws in a open motor.
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I've seen a DP Miata ingest a throttle screw and the results was not pretty. I've talked to a guy that builds Spec Miata and other race cars about what to do on my car. His opinion on why the throttle breaks is the gas pedal and cable are not properly adjusted. After checking it on my car, I'd have to agree he has a point.
It seems the stop on the gas pedal is not very strong. You can actually flex the pedal and bracket when you really mash it. Once you get the throttle to fully open any extra pull from the cable is still getting applied to the shaft. It is this twist that fatigues and eventually breaks the shaft. His solution is to either put a better stop on the pedal once you get to fully open, or adjust the cable to have enough slack.
I had a friend push the pedal as I watched the throttle and shaft. It did look like there was more pull after the throttle fully opened. I adjusted the cable so that there is a little more slack and less tug after the floored position.
After pulling the pedals during the off season upgrades last year, the whole side of that thing for the gas pedal is pretty flimsy. I think I'll be reinforcing the stop on the bracket the next time I have some down time.
It seems the stop on the gas pedal is not very strong. You can actually flex the pedal and bracket when you really mash it. Once you get the throttle to fully open any extra pull from the cable is still getting applied to the shaft. It is this twist that fatigues and eventually breaks the shaft. His solution is to either put a better stop on the pedal once you get to fully open, or adjust the cable to have enough slack.
I had a friend push the pedal as I watched the throttle and shaft. It did look like there was more pull after the throttle fully opened. I adjusted the cable so that there is a little more slack and less tug after the floored position.
After pulling the pedals during the off season upgrades last year, the whole side of that thing for the gas pedal is pretty flimsy. I think I'll be reinforcing the stop on the bracket the next time I have some down time.
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I was always under the impression the screws/shaft failed because the throttle cable tension was not set properly. So at full throttle the shaft was actually being twisted/loaded instead of the throttle cam hitting the stop.
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Junior Member
Joined: Jun 2011
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From: Houston
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Junior Member
Joined: Oct 2010
Posts: 245
Total Cats: 4
From: las vegas
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A broken throttle shaft nearly destroyed the $4000 motor in Rover a few weeks ago. I would pay a lot more than $325 to ensure that I never broke another throttle shaft ever again.
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