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[zonggong]

I have recently installed Hitchcock's EFI carburettor conversion on my 2015 B5. Ever since new the bike had been running rich and the spark plug was always fouled with carbon. It didn't matter what spark plug was used, hotter, cooler or iridium and I wanted the ability to adjust the tuning - and the carby option is less expensive than a Power Commander.
The retrofit was reasonably straightforward and I added an inline fuel filter to the Hitchcock's kit. The bike started first turn and has been running well since and most of the time seems better than with the EFI. I am however, experiencing a couple of problems . Problem 1: most of the time the idling is stable and regular and much more consistent than with the EFI, however, occasionally, and usually at the most inconvenient time, the engine hesitates and stops. It will start again but takes a little time and then seems to run OK again. Problem 2 : There is some banging (backfiring) in the exhaust, especially when changing down when coming to a bend or a stop i.e. when the throttle is closed or nearly closed. There was some of this with the EFI but not quite as much. AMAL's literature says this can be due to either a lean pilot mixture, or, a rich pilot mixture and an exhaust air leak.

The retrofit carby is an AMAL Mk 1 concentric 932/300 with a main jet of 220, needle jet of 106 and a throttle with a # 3 cutaway.

I'd be grateful for any suggestions.

[Rattlebattle]

I did the same on my 2015 C5 about a year ago, largely for the same reasons. I have had absolutely no bother with it since. At the time I had the original bazooka exhaust. I believe that the carb comes jetted for the after-market pipe and silencer. The needle is supplied in the middle groove as a starting point. For me it has worked fine after initial tickover and throttle stop adjustment. Assuming there are no leaks in the carb to manifold and manifold to head joints, or in the exhaust system, it must either be a weak mixture or possibly a partially blocked jet. Perhaps the tickover is set too low? Mine very occasionally stops when idling. The supplied settings are the same as mine.

[Rattlebattle]

BTW with a carb fitted the fuel you use makes a difference. If you buy the lower octane stuff it could have ethanol in it, up to 10%. This is equivalent to a 7% weaker mixture. You can't mix and match and expect the carb to fuel consistently. For this reason and because ethanol dissolves carb bits I always use super unleaded, which by law in the U.K. cannot yet have more than 5% ethanol in it.

[Adrian]

You can buy ethanol-resistant floats for the Mk1 and Mk2 Concentric carbs. A.

[Rattlebattle]

Yes, the "stay up" floats. The fuel system can indeed be ethanol proofed but as far as I know there's little that can be done about the varying carburation resulting from using different fuel mixes. Best to stick to the same type as far as possible. An advantage of EFI that is lost on converting to a carb is the loss of self-adjustment to fuelling via the O2 sensor input. Then again many people remove that anyway when fitting a Power Commander.......

[zonggong]

Thanks for the comments. What I think I will do in the coming week to try and address the exhaust banging, and eliminate potential sources, is to obtain a new exhaust gasket and re fix the exhaust header pipe into the head to make sure there is no air leak. I just read Deanobat's thread about the missing gaskets and wonder if there has always been an air leak at this joint. I wonder why copper crush washers are not used at this joint as I thought they would be more effective. Secondly, I will dismantle the carby and clean it again. I have read in other threads about the troublesome pilot bush - perhaps it is partially blocked. If there is a method for cleaning it I'd be pleased to hear about it. I used guitar string to push through the various holes in the past. For info the needle clip has been on the middle groove, as supplied. When reassembling, I'll re-check the gaskets to ensure no manifold leaks and may even put a thin coating of Loctite MR 5923 aviation gasket sealant (this stuff resists fuel and oil and remains flexible) and the joints. I'll report back in due course.

I'm not using any fuel with ethanol content. Where I live in Australia fuels with ethanol content are marked as such. Where it is available it is limited to 10%. In the past we found that ethanol content in fuel could also affect some engine oils and cause them to go gel like, although I suspect this is more likely with a high ethanol content. There were no tests completed to confirm this but quite a bit of anecdotal evidence. Another reason to stay away from ethanol fuels.

[Rattlebattle]

Is it possible that the fuel supply is restricted? An American guy who fitted the Hitchcock conversion to his EFI had an issue with the fuel tap supplied in that it had a restricted flow on account of a displaced rubber washer or seal in the tap. Could the fuel tank vent be partially restricted? I chose to leave the MIL connected on mine so that I can use it to confirm that the ignition is on. If you did the same could it be that the ignition occasionally misses? I.e. Does the light flicker or are you certain it's a fuelling issue. Just a thought. It probably is a bunged up pilot jet. Sadly Amal no longer fit detachable pilot jets to the Concentric. Personally I wish our host would offer the kit with either a Monobloc or, better still, the pre-Monobloc. Certainly that's what an early to mid-fifties Brit single would have had. I'd have paid the extra.

[Adrian]

Our hosts do of course stock a range of better carbs from the Monobloc to the Mikuni. I'm sure if you asked them nicely they'd sort something out. The first EFI carb conversion kits had the Mk2 Concentric. A.

[zonggong]

Thanks again. So far I have stripped the carby and cleaned it, especially in the area of the pilot bush in a similar manner to "Bullet Whisperer's suggestions (quite helpful). I used a piece of 'B' string from a guitar for this. It seems to be idling quite sweetly now at about 900 rpm. I didn't use the fuel tap supplied by Hitchcocks because it was loose in the pentagonal plate that replaces the fuel pump. I obtained a more simple on/off version locally that wasn't so loose. The fuel runs very well from this tap. I've left the MPI connected for the same reason as Rattlebattle and whilst the lamp is loose in the fitting it doesn't seem to flicker. I'm waiting on an exhaust gasket, but in the interim have checked to see if I can locate any exhaust leaks - which I can't. There are only 2 places - at the head/header interface and the header/silencer interface. The exhaust banging is a nuisance. It is more than a crackle and I get a really loud bang every now and then when changing down with the throttle closed. I did get a bit of this with the EFI but not as bad. AMAL's notes say exhaust banging is the result of a lean mixture or a rich mixture with an exhaust leak. For a rich mixture to ignite in the exhaust I assume that air must be able to get into the exhaust, but, if there is a leak I find that the exhaust gases are exiting, not entering, or does this change when slowing down through the gears at zero throttle? is air sucked in these circumstances perhaps? The idle seems to be slightly on the rich side (about the best setting) and on my rather ancient exhaust analyser it shows a mix of about 12:1 - would this be rich enough with an exhaust air leak to create banging. I'm wondering if it is being caused by the valve timing not being correct since I had a bit of it with the EFI.
Interestingly, I pulled the spark plug and found that for the first time in the bikes's life it wasn't fouled with carbon. So one of my objectives in making the change back to a carby has paid off, but I really would like to solve the exhaust banging too.

[ric]

I've been contemplating the carb kit but at the moment am running efi without an 02 sensor. I exceeded 4,500' above sea level today and the bike fitted with a £10 eliminator plug is running really well. My only observation is that on light cruise the resistor does seem to let the engine run quite weak. For the last 1400 miles I've been averaging 103mpg since leaving UK last week.

Anyway the point of this post is to ask how the carb kit compares economy wise with what I have as I've had to dispense several measured litres to an accompanying bike which has a small capacity teamed with bad economy ps riding in 34' heat is rubbish. Also although it's cold riding at cloud level at least it warms up when you ride above them