Starts well from cold.....

Thanks Robert. If anything it is showing signs of being a bit lean.

The carb is a 1 1/8" 389 Monobloc, 25 pilot jet, 105 needle jet, 320 main, 3 slide, needle on 2nd notch. This is all as per the Norton manual from 1962. That advises to up the main jet for continuous large throttle openings, as they put it. I was going to put a 340 main jet in.

It needs full choke and flooding from cold, and fires up straight away. It needs choke for 1/2 mile or so, when it can be taken off, and it will settle down to a steady tick-over.

When it has been run, even after standing for an hour or so as it did on Sunday when I went to a VMCC breakfast meet, that is when it would not start. The odd thing is the dry plugs, despite flooding, full choke, no choke, you name it. Fortunately the cafe was on a hill, so sitting astride and rolling down the hill brought the engine to life after three or four revolutions.

Fuel vaporisation has been mentioned, but the engine was cool to the touch by then, so unlikely, one would have thought. It has an insulating washer between the carb & manifold.

Frustrating!

Ian

Gents. Thanks for all your comments & suggestions. btw a correction; I said the slide is a 3 1/2. I was typing from memory, which can fail me these days! It is a 3.

Thanks to Phil for posting up the carb spec. Problem is it depends which one you read. My 1962 Norton manual says 1 1/8" 389, 25 pilot, 105 needle jet, #3 slide, needle in position 2, 320 main.

My Amal book says the same, apart from a 106 needle jet. Phil's list says 105 needle jet, needle position 3. Thus, all different, but not massively so.

Float level is correct. I checked it 2 weeks ago using a banjo off a twin carburettor link pipe, and a length of plastic tube.

Valve clearances are dead right at 0.006" & 0.008". They would take some closing up! Also, if the valves stayed open once the engine got hot, it wouldn't run too well, would it?

Good fuel causing the problem? They were designed for 100 octane leaded. Avgas is 100 octane leaded, mixed 50/50 with Esso Synergy 97 super-unleaded. I doubt that is an issue. If I used crap supermarket 95ron 5% ethanol fuel, I may think that was the problem!

The engine was not hot, barely warm, so unlikely to be vaporisation.

Fuel flow is fine, tickler flooding happily. Over-flooding doesn't seem to be the problem as the plugs aren't getting wet!

All mag parts checked or changed by the mag man 3 weeks ago, including pick-ups, carbon brushes etc.

I suspect it is a Monobloc made perhaps 15 years ago, and I know some had issues, particularly with the pilot drillings. I sonically cleaned the carb recently and blew it all through with an airline, and, as I mentioned, it ticks over ok. I will check the pilot drillings in a minute, also put a 106 needle jet in, as it appears a bit lean, but should not affect this issue. 

Out for a breakfast meet in the morning. I will report back!

Cheers. Ian

Reason for edit - missed a few words out!

Robert. I just said that! That is why I mentioned it. It does tick over ok though, so the pilot system can't be far out, surely?

Yes David, I swapped plugs over. I keep a few spares with me.....!

To test out John's theory, when the engine is hot I could whip the rocker covers off and check the valves for clearance. I had a thought about this, because when I rebuilt the engine about 10 years ago, the camshaft, along with everything else, was worn out. I got it into my head I wanted a PW3 cam for it, but at the time no-one had the Dommie version in stock, so I bought a 4S from Norvil. It took a long time to realise that something was not right with the performance; extended running in period, other bikes taking my attention away etc. Finally it ended up having to be the valve timing. I re-checked it and it was correct to the timing marks. Not satisfied, I clocked it with a dial gauge and timing disc. It was a mile out. Opening waaay late, and nothing tallying up. I re-timed it, ignoring the timing marks, starting where the 4S should open. I then measured what was going on at various points in the cycle, and it just did not tally with 4S data. I had conversations with Les Emery, and he was happy for me to send the cam back to him so they could clock it to see what was going on. I had to remove the engine, completely strip it etc in order to do so, at my time and expense, of course. He agreed it was possible they had sent me the wrong cam. At that point I decided not to do that.

Studying the data from the new settings, it appeared to be pretty close to that of the standard SS cam, or at least not a million miles away. The conclusion I came to was that they sent me an SS instead of a 4S, compounded by the keyway being machined incorrectly, hence not following the timing marks. Apparently some have been known to have been produced with such mistakes in manufacture.

The performance is ok now I have done that, although it still will not rev as high as I think it should. This may be due to the single carb, but I don't want to thrash it anyway, just for it to be reliable.

So, back to valve clearances. Assuming it was an SS cam, I set the clearances to 0.006" & 0.008" respectively. If it is a 4S cam, they should be set to 0.016". It wouldn't do an SS valve train any good to run at those clearances, but likewise a 4S cam at SS clearances? who knows?

My brain hurts! I'm going to bed.....

A compression test shows 130 psi in each cylinder, which isn't that great.  I am aware that, when the engine was rebuilt, I suspect the rebore could have been tighter; not a good job. As a result, I struggled to get good ring gaps. It is on + 0.040", so will need a resleeve back to standard. I know this is on the cards, but a fairly expensive job I was hoping to put off for a while. Apart from that, the decision is, having taken the top-end down, that is the obvious time to pull the engine out to find out exactly what the cam is. Having done it once, I haven't the stomach for doing it at the moment, with other restoration projects I would rather be getting on with. Only so much space, time and pocket depth!

Ian

I changed to a 106 needle jet, also fitted a new in-line fuel filter, although the old one was ok, fuel flowing happily through it, carb flooding when tickled.

Started 3rd kick this morning, and with several stop/starts today, started 1st kick every time!

Whenever I stopped I left the petrol on. When starting I did nothing, no choke, no tickling, just a touch of throttle and away it went each time. It is not happy with the 106 needle jet though, as it is very snatchy/lumpy on a constant throttle at 2500-3000rpm, typically in 4th, or 3rd, on country roads with traffic, in 50mph limits. As soon as the throttle is opened, however lightly, it picks up smoothly and away it goes. Plug inspection shows it to have gone from a bit lean to a bit rich. I may try going back to a 105 needle jet, and raise the needle from 2nd to 3rd position, to see how that goes. I will then have tried every setting from every book!

I will look out those timings, Robert, It was about 4 years ago I did that, but I made copious notes, as one would have to. They will be somewhere safe....!

Thanks for your interest.

Ian

The snatchiness on constant throttle, having gone from 105 to 106 needle jet indicates running rich. The difference there Anna is your #4 throttle slide. All the lists, despite any small differences they have, agree on a #3 slide. In the past I have found a big difference going from #3 to 3 1/2, let alone 4.

I will leave the needle height as it is, but may try a 3 1/2 slide with the 106 needle jet, before going back to 105.

As you say Robert, curious.

Referring back to my previous post in this thread about the 4S cam I thought I had bought from Norvil, which I had to time with dial gauge & degree wheel, here are the timings as they are now.

Bear in mind I was setting it up as a 4S, as I thought, my assumption being that the problem with the cam as supplied was confined to an incorrectly machined keyway, meaning I could not use the timing marks. When I originally set it up using the timing marks; as I had no reason to suspect anything was wrong with my shiny new camshaft; it eventually transpired, after a process of elimination, that the valve timing was the cause of the odd performance. When I clocked it, the inlet valve was opening at 40 deg btdc. Hmmm, time to set it up manually.

The 4S inlet opens 65deg btdc, closes 83deg abdc. Exhaust opens 88deg bbdc, closes 60deg atdc.

Set using 0.016" valve clearances, and setting the inlet to open as per the 4S; as close as; mine are as follows:-

Inlet opens 64deg btdc, closes 52deg abdc.. Exhaust opens 94deg bbdc (86deg atdc), closes 27deg atdc.

Not good; miles out.

However, the standard SS cam (which I am now thinking is what they sent me; although still of course with the keyway incorrectly machined) the settings are as follows:-

Inlet opens 50deg btdc, closes 74deg abdc. Exhaust opens 82deg bbdc, closes 42deg atdc.

Thus, if I reset mine for the inlet to open at 50deg btdc as per the SS cam, the theoretical settings would be as follows:-

Inlet opens 50deg btdc, closes 66deg abdc. Exhaust opens 77deg bbdc, closes 42deg atdc.

The inlet would be closing 8deg early, the exhaust would be opening 5deg late, but it is not a million miles away. Kind of as good as one can expect from aftermarket parts these days. 

Since you asked, Robert!

Cheers. Ian

That is interesting, John. Just shows how critical these settings can be. After the success with starting on Saturday with a 106 jet, which unfortunately made it unpleasant to ride, I re-fitted the 105 jet. Started from cold first kick; once warm, nothing. A bump start had it going in no time.

Makes no sense. I have re-fitted the 106 needle jet along with a 3 1/2 slide in place of the #3, to see if that will tame the richness. No time to try it today, or possibly even tomorrow. Off to France on it on Friday. Fortunately it is quite hilly in Brittany I believe!

Certainly no time to re-set the valve timing before I go, so priority job on return.

Ian