Classic Bike Guide

A problem where you least expect it...


The letter from Paul in January was very familiar to me as I started to read, but his problem ended with another cause. My friend acquired two Honda four-cylinder bikes, a 400F and a 550. After rebuilding both bikes, the fun began!

The 400 started easily but as it warmed up it began to pop and bang in the exhaust on one cylinder. Off with the carbs, check for blocked jet, but nothing. Check the plugs, all seemed to be okay. Try new plugs but the problem still there.

Think a bit, perhaps a bent valve, so pull off exhaust pipes to look at valve. Seems okay. Wonder if there is a cam/rocker problem? This means a strip down. Exhaust pipe was examined and seemed that a carbon build-up was in the offending pipe, so tried to remove it. But the ‘carbon’ was very hard, and scraping the surface showed shiny metal!

The pipe was slit with an angle grinder to display a blister-like bulge on the inner pipe that was closing off the exhaust; it has been reduced to half its size! On examinatio­n of the pipe, the manufactur­er appeared to have assembled the double skin pipe by pushing the inner pipe into the outer while they were both straight, then welding the inner to the outer at the muffler end only. The pipe must then have been mandrel bent to its final shape.

The blister on the inner pipe was in direct line with the exhaust flame and this area is subject to high temperatur­e that expands the inner pipe, which cannot move due to the initially cold outer pipe.

Due to this, I think the inner pipe deformed inward, due to high temperatur­e annealing the area of least resistance and forming the blister. After this, with the pipe partially blocked and velocity of the gas increased, the expansion movement was greater and the blister increased in size. A similar problem was found on one of the newly-chromed 550 pipes.


Thanks Charles, goes to show the issue could be where you least expect it – Matt

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