Despair Shop 23 – Lean on me

As we had  a few hours without rain I thought I would drag the Vigor outside and have a look at the hot exhaust scenario I talked about last time. I took the seat and side panels off so I could have a better look and noticed that somebody had rather inexplicably drilled some holes in the airbox where there shouldn’t have been any!! I’m pretty sure if they needed to be there Honda would have put them there but some guy in a shed obviously knew better. There was about 8 or 10 of them, I covered them up with tape for now, I will sort something more permanent when I have sorted out the running issues – it’s too lean to be caused by a few holes in the air box.

I thought cleaning the carb couldn’t do any harm so I did that first – it’s a wee bit of a sod to get off as the air pipe to the airbox is a bit tricky. With the carb removed I took it to the workshop for a damn good seeing too – it was filthy inside with a lot of rust flakes in it, I need to fit a filter between the tank and carb I reckon. Cleaning out the tank might be a good idea too.

Before I go too far off subject – the carb was cleaned and put back on the bike, I thought that the jets being grotty was part of the problem but not the whole story, I strongly suspect the big issue is the choke cable retainer, which is plastic and way below Honda’s usual standard of quality.

It had snapped off and I hadn’t noticed as when the choke was on it held the broken bit flush with the carb body – I had only ever looked at it when the choke was on.

To check my theory out I put it all back together as best as I could and  started the bike. She was a bit reluctant and required a snort of easy start before showing any signs of life. I run her at high tickover until she seemed to want to run but she was still popping and farting as though the mixture was very weak. I was able to confirm that by restricting the air flow to the airbox inlet. Next job was to squirt around some easy start and listen for changes in the engine note – sure enough if  I sprayed any near the broken piece of plastic the engine responded, showing that air was indeed getting where I didn’t want it.

I double checked it and am happy that is where the problem is – fixing it is a whole different ball game as the part seems unobtainable. I thought it would come with a new cable but it doesn’t, which is a mare, I will have t try and find a part number for it and see what I can find, otherwise I will have to machine one up out of brass.

I am just hoping now that the valves haven’t been burnt out by running it too long like this, I have no way of knowing how long it has been like that. The compression is immense so I can’t imagine anything too bad has happened.

Looking at the way the choke works there is a plunger that the cable operates. The plunger has a needle that goes in to a jet – just like the needle on a CV carb. To the side of the chamber that the choke plunger lives in is a drilling that goes in to the normal fuel circuit, when the choke is open the jet can supply fuel to the normal running circuit. If air is allowed in to the chamber there is nothing to stop the air entering the circuit instead of fuel – I think that is what is happening. The cable seems to have rubber boots at each end to prevent air leaks, which seems to back up my theory.

With a quick repair done to the plastic cable end I tried again and found it was still running way too hot so I went back to the internet for further advice. I found a few articles that suggested Honda had made the bike run lean to pass emissions and several references suggested hot running as a result, which sounded plausible. I took the spark plug out and sure enough the electrode was white indicating very lean running. I tried taking the idle mixture screw out to about 5 turns which made a pretty big difference but it was still not right.

I have now ordered larger idle and main jets – the idle was a number 50, I have ordered a 55, the main was a 165 I have ordered a 175. People that have already done this report much better running and easier starting so that seems like a good starting point. It will take a few days for them to arrive so I will report back next time. Just a word of warning here: there are several subtly different styles of jets – particularly the idle jet. The one in the carb fitted to my bike is 23.5mm long overall, many that I saw on line were longer at about 27mm – I saw numerous versions so it’s important to take the jets out and measure them to be sure of ordering the right ones. One other word of warning – it is an absolute mare to get to the mixture screw when the carb is in the bike – you need a screwdriver of about 1 ½ inches long to get at it, it’s worth sorting that before you start if you have one of theses bikes.

Article provided by David Powell of Boston Bike Bits.

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