This week, I have been messing about with the Bonnie, trying to squeeze an extra couple of horsepower out of that unsophisticated engine. I know that when I got the Bonnie I said I wasn't interested in outright power, and that's still true. I'm having more fun with 61 bhp on the Bonnie than I did with 125 bhp on the Pan. But a couple of extra ponies wouldn't go amiss. Especially if they were virtually free of charge.
I read a bit of information that mentioned a restrictor plate inside the airbox, with the additional information that removing this would give a noticeable increase in performance for no penalty in fuel consumption or driveability. This seemed too good to be true, but for the sake of half an hour's work it had to be worth a try.
The inlet tract of the standard Bonnie is a pretty convoluted affair. There's a rubber intake snorkel, which has a 90° bend in it, and then the air goes through the paper air filter, through another 90° bend, then through 90° in two directions, through a plate with a small oblong hole in it, through another 45°, twice, and into a chamber which feeds the carbs. Removing the snorkel would straighten the path of the air considerably (straight path = less resistance to flow = more air) and removing the plate would allow the full measure of oxygen to pass directly to the carbs. The bike already has the Truimph "Off-Road" (TOR) silencers fitted by the previous owner, which are far less restrictive than standard, so this should match up inlet and exhaust and let the bike breathe better.
So I set to and removed the snorkel and then partially dismantled the airbox, and took the plate out. What I had failed to take into account was that more air needs more fuel to retain the proper air/fuel ratio, so when I tested the bike it ran like a dog, and not a very fast dog at that. In fact, truth be told, I had ruined a perfectly good motorbike. Not permanently - the plate slid out and could slide right back in if I so chose - but disappointing nonetheless. I needed a pair of bigger main jets.
My local dealer had some of the right size so, at a pound each, I took them. Of course, they were the wrong fitment and had to go back - Sod's Law. The ones he did have that would fit were slightly bigger than I needed, but at that price I thought I would give them a try anyway. (One point to watch - the standard jets are 110s, and when the TORs were fitted the jets should have been replaced with 115s, but what was in there were the standard jets. Take nothing for granted, even a dealer's word that the bike had been correctly jetted by his own mechanics.) I ended up with 140s. I had to take the carbs off to fit them, as getting to the jets was impossible any other way, but the effort was worth it. At first, the bike idled very badly but I took it for a test ride anyway. The bike went well, confirming that the problem was with the idle jets, not the mains. I must have got some dirt in one when I had the bowls off. Carbs back off, blow through the jets, and back together. And a test ride ...
Well, it's like someone had taken the engine out and given me a bigger one. It rides slowly just as well as before, but open it up and there is a huge surge forward that wasn't there before. I haven't tried it for maximum speed yet, but for the part of the power curve that makes riding enjoyable and easy (the mid-range, where you do most of your accelerating and is vital for easy overtakes) it is massively better.
I need to fine-tune it, as I suspect that the jets are too big and the bike is running a bit rich now. A plug chop will tell me all I need to know. And I will need to check the fuel consumption as well. No point in having 10% more power if it's going to use 20% more fuel. As the modification is geared towards gaining more airflow and therefore greater efficiency, there should be no increase in fuel consumption, and people I have spoken to who have done this confirm that, but I need to see the numbers for myself.
So far, though, a big success.
The last thing was to make a little mesh screen to cover the air filter, now that the restrictive snorkel is no more. I made it out of a stainless steel Tesco kitchen sieve, and I think it looks quite good. Perhaps not 'factory', but tidy enough for me.
I'll report some results when I have them. I am doing a bit of a charity ride on Friday (see this post), so that will be at least 240 miles to check it out. I'm looking forward to that.
Next - new rear tyre. I'm down to the limit.
Plug readings normal, fuel consumption unaffected. Thunderbirds are GO.