The law of unintended consequences: replacing exhaust headers on a 31-year-old motorbike - Honda CBR250RR MC22

Side profile: New headers and muffler on the Honda CBR250RR MC22 (1991) - yes, that is a temporary zip tie on the exhaust hanger..

One of the things that drew me to the CBR250RR MC22, was the glorious sound it made.

However, after a while riding the bike I noticed that the clamp holding the end pipe on, between the header and muffler, was coming loose. I also noticed one of the flanges on the aluminum headers had cracked, ouch.  At some point, someone had modified the exhaust to accept a 2-inch slip on muffler, rather than the three-bolt flange style of the original muffler.

I considered repairing the headers, but I’d been told by several welders that they could not guarantee welding the flange would work. I looked high and low, but it seems no one sells an off the shelf set of headers for the bike anymore and I didn’t want to spend an exorbitant amount getting a custom set made.

Aside from being loose and causing some amount of exhaust leak, which in itself isn’t ideal, I thought the fixing the leak might remedy another issue I was having. Once the bike had been tuned, it spluttered and struggled, at any speed (under load), as soon as the engine dropped below 4,000RPM – above that, the bike was perfectly okay.

New headers

I decided to buy a pair of used headers, being the only option I had left. I got a set that looked just like the stock/original headers and were said to be off a CBR250RR MC22 – so far so good, or so I thought.

Me being me, I decided to get them vapour blasted and ceramic coated, given they were old, had some surface rust and what looked like some very old paint.

Install

Once I’d gotten the headers back, I promptly removed the old set of headers and installed the ‘new’ ones. I installed the headers on without too much trouble, although it was very tricky trying to keep each of the four header gaskets between the headers so that they could sit between the engine and the pipes, by myself. 

Then came time to install the muffler. Ahh…While the muffler sat on the headers fine, the muffler didn’t line up correctly with the muffler bracket which hangs off the rear subframe. Instead, the muffler sat too high above the hole in the bracket, in other words, the stock bracket was now too long.

I then noticed that the rear section of the headers comes up quite high behind the right footpeg. This has meant that I cant have my foot flat too far back on the peg, otherwise it’ll hit the pipe.

Clearly the headers aren’t, at least not entirely, original for the MC22. Having looked at numerous images of MC19 headers (which anecdotally fit the MC22) these headers don’t appear to be original MC19 headers either. Hmmm.

It is possible someone modified or replaced the headers from the rear section, where the pipe begins to curve upward to the muffler.

 For now, I’ve got a couple of zip ties holding the muffler in place of where the bracket should be – yes I know, certainly not ideal and far from pretty. However, that bracket serves to maintain tension and reduce the extent to which the muffler goes down when the bike hits bumps and so on – it does not hold the weight of the muffler and so it is relatively safe as is. Not that I intend on keeping it like that!

Luckily, I have a friend who is designing a new bracket which is shorter for me, which I’ll get laser cut. 

Rear view: new headers and muffler on the Honda CBR250RR MC22 (1991)

Drivability changes

Once I installed the headers the terrible stuttering at under load at 4,000rpm disappeared, but the bike became more lethargic until 6,000RPM, where it suddenly comes to life.

The new headers are slightly smaller in diameter compared to the old ones. Exhaust leaks also tend to make engines run slightly lean. The bike had previously been tuned with the exhaust leak present and with the slightly larger diameter headers. So now, the bike is likely running too rich in the lower RPM range, causing that ‘doughiness’ below 6,000rpm.

I’m going to have the bike re-tuned, and it’ll be interesting to see what the A/F ratio is prior to the actual tuning occurring. Watch this space!

Lessons learnt

Ah yes. We learn from everything we do, regardless of whether what we engaged in resulted in the outcome we intended – even if you aren’t actively thinking about what you might have learnt from a situation. In this case, I learnt that when buying second hand parts for a 30-year-old bike, don’t just assume they fit before you sink money into restoring them, check first! This was also a reminder that you don’t know what you don’t know. Without having had an experience like this, I hadn’t thought to check that the headers fitted well before going ahead and getting them ceramic coated. I also didn’t anticipate that fixing the exhaust leak (which wasn’t too bad) and having slightly smaller diameter exhaust pipes, would change the engine characteristics as much as it did.