How to Use JB Weld Epoxy on a Motorcycle

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How to Use JB Weld Epoxy on a Motorcycle

When is it okay to go with a bodge repair? Learn how to use JB weld epoxy on a motorcycle engine case.

One of my goals when recommissioning my 1988 Paso was to put it back stock running the original-spec Weber 44DCNF 2-barrel carburetor, the selfsame carburetor largely responsible for the Paso's overblown reputation as a wretchedly poor performing, unreliable machine.

By the time it got to me, my Paso was wearing a pair of Mikuni flat slide carbs, a common replacement for the Weber. Yet a few stubborn, diehard Pasophiles worked out the Weber's issues, solved almost entirely with jetting and auxiliary venturi changes. A notable plus in retaining the Weber is the benefit of the factory airbox and its huge panel air filter; the twin Mikuni or alternative DelOrto swap leaves little option beyond small, stubby pod filters.

I spent hours diligently prepping the Weber, stripping it bare and rebuilding it following an ultrasonic bath with suggested new auxiliary venturis, main jets, idle jets and emulsion tubes.

Unfortunately, the changes didn't appear to be effective, and the Paso ran terribly. Back went the Mikunis and I tore into the Weber again to see if I could suss out what was — or wasn't — happening, and I found two glaring issues — one an easy fix, the other more involved — I can't believe I missed.

The easy fix was so stupid it's embarrassing, but worth admitting because it underscores how quickly assumptions get us in trouble. I had assumed a threaded hole in the intake manifold where the accelerator cable bracket attaches is a blind hole. It isn't, and without the bracket installed I had a massive vacuum leak; it's no wonder I couldn't get the bike running properly. Weak defense on my part: Said hole is on the back/opposite side of the carb from where running adjustments are made and not visible while tuning, and I simply didn't hear the leak over the noise of the engine.

The more involved issue was a bent throttle shaft I somehow failed to identify, replaced along with new sealed throttle shaft bearings. The original open bearings are packed with grease, with external washers aiding sealing; best practice is to replace them with sealed units to stave off vacuum leaks at the throttle shaft.

And that was it. With the Weber back on, the throttle bracket secured and the fuel on, I hit the go button and boom, it ran. Some fine tuning of the fuel/air jets and the air bypass screws and it settled into a steady idle. Wow. It not only works, it works well. It's great on the road, with crisp throttle response and none of the mid-rpm jerking the Weber's famous for. Is it better than the Mikunis? Probably not, but it's what I wanted, and it was worth the effort getting there.

How to Use JB Weld Epoxy

No sooner had I declared victory than I noticed oil dripping from the right side of the engine. Removing the timing covers revealed oil leaking from behind the timing belt jackshaft gear. Assuming a leaking jackshaft seal, I removed the belts and lower timing gears. Seal failure would have been welcome. What I found was oil leaking out underneath a wad of J-B Weld putty smeared on the case below the jackshaft. What the hell?

I knew when I bought the Paso that the original owner had blown up second gear. What I didn't know was that the engine case apparently suffered a crack at the same time, "repaired" by simply epoxying over the crack. Removing said epoxy revealed a crack just below the jackshaft, curving up half an inch or so in either direction like a sick smile. Ha ha. Funny. Not.

I decided I had three options: gut the case and properly repair it; replace the case; bodge repair. The first two options being the most advisable, I went with the bodge. Why? My logic was simple: The first "repair" had lasted 25-plus years and 5,000 miles, and near as I could tell it hadn't been executed with great care. As for how to use JB Weld on a motorcycle properly, I removed the putty — surprisingly easy as it hadn't adhered well to begin with — and using a Dremel carefully ground off any paint in the affected area and roughed up the metal. Using a small, pointed chisel, I gently vee-d the crack, then thoroughly cleaned the area before wiping it down with acetone. I mixed up two-part JB-Weld epoxy (not the putty-type), carefully applied it, and then let it sit undisturbed for two days before covering it with black paint and putting everything back together.

Result? So far, my bodge is holding just fine. How long it will last is anyone's guess, but if it does leak again I have an excellent set of used cases I picked up from Ed Milich at Duc Power.

Ride safe.

— Richard Backus/Founding Editor

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