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Wednesday, May 1, 2013

Convert a Trail 70 carb bowl to Trail 90 carb bowl

So the Trail 90 sat too long with gas in the carb again and won't idle(again). I tore down the carb to clean everything and one on the 3mm threaded holes on the petcock gave up the ghost. I tried to oversize it to 4mm threads. The metal was just way too soft and the tap just reamed the hole larger and didn't cut threads. I had a spare Trail 70 bowl and I notice it was the same casting, but the drain nipple on the Trail 90 goes back while the drain nipple on the Trail 70 goes straight down. So I mocked up the 70 bowl onto the 90 with carb and intake, and you guessed it the drain nipple needs to go back and won't fit pointing down.

This is how you make a 70 bowl into a 90 bowl and more or less vise versa. These photo's are from my second attempt at the this. My first attempt successfully relocated the drain nipple, but heat or time or both loosened the overflow tube's press fit. It may have been from the heat Columbus had already applied to the get the stuck broke drain screw out of that bowl. Anyhow I worked for hours to set the float height with that bowl and it keep leaking and then I discovered that it was leaking bottom of the overflow pipe not the top.

Back to the task at hand. Trail 90 bowl on the left. Trail 70 bowl on the right.

Just a touch of heat and a tug to get the nipple we want out. I had to do this to both bowls, because I decided to use the 90's smaller diameter nipple on the finished product. I would think that the just swapping the 70's nipple's position would have worked fine, too.

Find the drill bit that will make a decent press fit.

This is where you need some special tools or friends with special tools. Columbus lent me this angle vise. I just eyeballed the boss square and center to the drill bit.

I carefully drilled just far enough to get some daylight through the new and checked the nipple fit. Then squeezed it all together nice and tight.

Next I installed the drain screw with its o-ring.

Then I covered the old nipple hole with my finger and filled the bowl with rubbing alcohol. The base of the overflow tube is NOT leaking. YEAH!!

I kept my finger on the old hole and checked for drain function and for leaks around the nipple's new location. That looks good too.

Mix up some JB Weld to patch the old hole and let that sit up.

Be careful and check that you don't get too much JB Weld in the hole and block the drain that you've worked so hard to move. Still clear!


  1. Hey man, Im doin the lash adjusters on my Vulcan 1500 and have checked out your blog about the Vulcan. In it you said you were not going to put back the KACR- did you put it back in or did you leave it out? If you left it out, does it start ok? I couldnt find any other way of contacting you.

    My email is

  2. I didn’t use them and it has never been an issue. I learned that from a Guy Mobbley who built a lot of high compression hot rod Vulcans. He left them out and never had a problem.