Greg's 73 Commando

Yep, those little PWKs are a good economical solution. FCRs are the high end of the spectrum.

"Ya' gets what ya' pays fer."

That old adage seems to guide my purchases more the older I get. I'm sixty-four, but still get suckered once in awhile. If those knock-off Keihins work well, then you got yourself a hell'va deal. If not, refer to the top line of this post.

They work well.

...although they aren't all that much cheaper than new replacement Amals, which I prefer.

When I got home from work yesterday there was a little surprise sitting on the step.


A couple of nice new Keihin carbys.



I took them out of the box for a quick look. The Keihin PWK carbs are normally a spigot mount that has a rubber hose go over the end and is held with a hose clamp. Because the Norton is a flange mount, rather than replace the manifold with a spigot mount type one like the Mikuni's use the company that supplies these carbs has modified them to be a flange mount like the normal Amal is.










It appears that what they have done with them is turned down spigot and fitted a machined flange then fixed it with grub screws top and bottom and epoxied it to prevent air leaks.

One thing that I did notice and am not too happy about is that both carbs are (edited after proper look) left handed, unlike the Amal's which were left and right.
This means that the right side one is going to be a little difficult to tune because the mixture and idle speed screws are going to be on the inside up against the other carb.

You can see in the photo below the epoxy that is sealing the new flange to the carby body. You can also just see one of the grub screws that hold the flange to the carb.




I guess I am going to have to try and make some time over the weekend to fit these to the bike and see how she runs.

The idle and mixture appear to be OK for the left carb, it's the right carb that has the mixture buried between them. The idle appears to be on the right side of that carb.

I'm real interested in how they work.

Veefore said:

One thing that I did notice and am not too happy about is that both carbs are right handed, unlike the Amal's which were left and right.
This means that the left side one is going to be a little difficult to tune because the mixture and idle speed screws are going to be on the inside up against the other carb.

Click to expand...

In your second photo, I can see what looks like a blanking screw in the same position on the left side that would appear to match the idle screw position on the right, so can they be swapped over for the L/H carb?

I had a little time today so I had a go at fitting the Keihin's. I was able to swap the idle speed screw over so that they are both on the outside and it looks like I might be able to make up a little tool that will get to the mixture screw if I need it. I may not need it though. After fitting the carbs and putting the tank back on I decided to give it a kick, just to see what happened. To my great surprise the bike started straight up and ran at a brisk idle. I will need to adjust the cables as they are a bit tight at the moment causing it to idle too fast. Once that is done I will get the mercury balancer out and get the idle speeds matching and take it for a ride and see how it is through the rest of the rpm range. I am pleasantly surprised so far.

Nice project it's looking good...my Interstate was laid up for a similar amount of time inside a garage covered up so the amount of work I had to do was limited. But and there is always a but, the tyres were absolutely dangerous although the were inflated ok as soon as you pushed them a bit hard you started to slide ( a bit like bayliss on a Ducati) I found out after talking to an Avon rep at a vintage club meeting that all tyres have a date code on them the normal safe life span is Six years the TT100's on mine were at least 10 years old as far as I new....the other thing was delaminating insides of hydraulic brake pipes, great pressure but all of a sudden they don't let off, fortunately mine was on the rear! You may well have changed all these but there again you may not have.

P.S like the Viper I have a MAC and I am desperately try to finish my Venom clubman but the money keeps disappearing into my son's trials bike...On the Interstate I have also fitted a 36mm Mikuni with a K&N and mine is running a PW3 cam from Norman White the Mikuni really makes a difference to the all round running.

Very interesting on the carbies. I ordered a set which may be quite the same that I hope will come in this week. I have a few sets of carbs around but I thought these might be the best fit for what I want to do with this bike.
I have a pair of 32 mm Mikunis, and I do love how they run on bikes I've converted, but I suspect they are too much carb for this stock 750.

Had all kinds of trouble trying to get the bike to idle on both cylinders. It ran OK (just) at higher RPM but not idle.
I messed with mixture of the left cylinder (luckily it was the side that had the mixture screw exposed), the idle, and everything else I could think of. I even put my colortune in today and couldn't see a spark. This made me start to suspect that it wasn't the carb at all. So, first thing is first I swapped over the spark plugs to see if it was one of them that caused the problem. BINGO! The dead cylinder moved to the other side. So I took the spark plug out and just out of curiosity tested it with an Ohm meter. I thought it should be OK since it was less than 100 miles old.
About 10 Ohms resistance along the centre electrode and about 340~ Ohms resistance from the top to the threads. This leads me to believe that there is a breakdown in the porcelain and the current was going straight to earth at idle under cylinder pressure. I will have to pick up some new plugs tomorrow on the way home from work and see how it goes then.

Reminds me of the days 20 years ago when I would buy a dozen spark plugs for my racing Kart and throw away at least three of them because the Kart wouldn't even start with them. Another 3 or 4 wouldn't last a single race meeting. I guess plugs haven't improved much over the years. :lol:
Below are a couple of pix of the carbs fitted to the bike.





Oh yeah, I finally finished hand rubbing the petrol tank. I haven't put any glaze or swirl remover on it yet so it is just how it was after the rubdown.
I will let it sit for a month or so more before I put anything on it.

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Are you running resistor plug wires and resistor plugs?
The Norton electrics are weak at low RPM.
You will get beter idle by switching one or both to non-resistor.


BTW, nice job you've done there.

MichaelB said:

Are you running resistor plug wires and resistor plugs?
The Norton electrics are weak at low RPM.
You will get beter idle by switching one or both to non-resistor.


BTW, nice job you've done there.

Click to expand...

Regular BP7ES plugs. I don't know about the wires though. I am going to replace them after christmas and might also do the coils.

A new spark plug to replace the dud one and I fired her up and took her for a ride. I got about 1km down the road before the bike stopped. I did a little fiddling and got it going again then took it for about a 20 mile ride. It started to run a bit rough toward the end then promptly died at the bottom of the hill at the end of my street. Would you believe I ran out of petrol? I must have checked the thing at least 3 times and each time I thought I saw plenty of fuel in there. I leant the bike right over to get the fuel onto the reserve side of the tank and managed to get it going again, but only enough to get it halfway up the hill. I had to push it the rest of the way. It took about half an hour to cover 100m with a stop to catch my breath every 10m or so. I never realised the hill was that steep. No wonder my car has to go back to second gear to get up it.
Once I got home I put some fuel in and messed with the mixtures some more. Idling on the left side only it seems that the idle speed increases most when I have the mixture screw turned all the way in which leads me to believe that maybe it needs a larger pilot jet in it.
Overall though I am quite impressed with these carbs. The idle is rock solid and I can turn it down until it is barely turning over and it will just keep chugging away smoothly, not lumpily like it did with the Amals.

I'll update more when I get some time to sort it out some more.

You DO NOT want the slow chugging idle. You want your engine to spin freely at about 1,000 RPM tickover.

I have a set of those carbs in the Post Office today. When I pick them up I'll get to see if they are the same as yours.
I hope I have a bit less trouble fitting them, your struggles have been illuminating.

My carbs came today and they do look identical to yours. I may have to do a bit of re plumbing to install them, and will look them over a bit for flaws before assembly.

I picked up some new Pilot jets at the local Honda dealer. I went to a number of dealers and they all wanted to know what bike the jets were for. Just telling them what kind of carb they were for wasn't enough information. Eventually I found a dealer who let me go through his jet boxes. I found a couple that matched what I had seen when I pulled the carbs down for a look before fitting them. The part numbers were 99103-437040 for a #40 jet. I spent Friday afternoon pulling the carbies off, changing the jets, putting them back on, then pulling them off again to adjust the mixture (of the right one that is blocked). It still isn't perfect but it is pretty damn good. The bike starts brilliantly now and idles crisply. There is still a little popping in the right exhaust on the overrun but it isn't too bad and I must admit I kinda like it. I haven't had time to yet to look into the other ranges because I have just been too busy. Maybe after Christmas I might get a day to spend on it.

This is proving to be quite interesting. I fitted them to my bike yesterday but I'm having some clearance problems. My bike is a hot rod and does not have as much room as a Commando under the tank. It also only has one petcock.
I'm hoping not to have to rejet on my stock Commando engine, one would think the pilots should have been chosen for a 650-750 engine when the development was done. I may also have problems using the enriching valves with limited clearance. Did you have to drill anything out to fit the slide stop to the other side?

Cookie said:

This is proving to be quite interesting. I fitted them to my bike yesterday but I'm having some clearance problems. My bike is a hot rod and does not have as much room as a Commando under the tank. It also only has one petcock.
I'm hoping not to have to rejet on my stock Commando engine, one would think the pilots should have been chosen for a 650-750 engine when the development was done.

Click to expand...

From what I can gather, this size carb was actually spec'd for about an 80cc two stroke and is jetted as such. The retailers don't seem to alter the jetting from that which comes standard.

Cookie said:

I may also have problems using the enriching valves with limited clearance.

Click to expand...

I find the right side one very difficult to get to requiring some kind of digital origami to get my fingers around the front of the carb, beside the balance tube and up to get to the knob. The left side is easy though and is often enough to get the bike to fire straight up and idle happily away.
When I get some time (yeah right!) I might look into joining the two together so that I can operate them both from the left side or maybe attach a cable and run it to the handlebars.

Cookie said:

Did you have to drill anything out to fit the slide stop to the other side?

Click to expand...

No. There is a small brass screw on the opposite side that just screwed out and swapped sides with the slide stop screw. VERY easy.

I rode the bike to work this morning and it was the best it has ever run. It was an easy second kick start (must turn on the ignition first if I want a first kick start) and the bike idled happily away at every stop light like it never did with the worn out Amal's.

Xmas shopping seems to be getting in my way today. I got a bit of fuel line and I have a couple of t fittings that are not quite what I want, but will do for testing.
I'll look at swapping that adjustment as usually the pilots on real Kehins are not all that sensitive. On my Hondas if you have no air leaks you set them to factory specs and don't touch them.
If the jets are set for an 80 cc odds are I'll have to rejet too, now where did I put my drill.....