MkI Square Four fine tuning and performance troubles

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Gui.dorey
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MkI Square Four fine tuning and performance troubles

Postby Gui.dorey » Mon Aug 03, 2020 12:10 am

I’ve been having some issues with my square four during the run in stages, which I haven’t been able to quite figure out yet. I started discussing this with Paul Jameson on another thread, but decided it was best to start a new one.

My bike starts with one kick, settles to a very nice and steady idle, pulls very well from low speeds even in top gear, and runs very smoothly up to 80-90kph. Above that the engine loses its smoothness, becomes harsh and a bit rough, I feel vibrations coming through and she seems to struggle to accelerate beyond that. To reach 100kph, feels like it’s being forced, although I haven’t really tried full open throttle. I’m still reluctant to push her as I’ve only done 500 miles since the rebuild, and so until now I’ve only tried higher rpms and speeds very shortly and very few times.

I have the ignition set now where I get the best performance, having tried more advanced and more retarded from my starting point. Same with the Solex, which now seems to be on the sweet spot.

To answer some of Paul’s questions and suggestions:
- valve timing: I followed Bruce Longman’s method and my peak opening on the inlet on Nr 1 was just about at the recommended 105deg. Rechecking my notes perhaps a couple of degrees more advanced, but not less than 102-103 deg. I could of course have made a mistake and the timing is indeed off, but feeling very lazy about taking the tank and everything off To get in there again. So I’ll leave that for last if things really don’t improve. However, if valve timing is the cause, which direction should I go considering the symptoms? More advanced or more retarded?
- rockers getting caught in the housing somehow: I just took the covers off today to readjust the valve gaps, which needed to be buttoned down after the head has settled in a a number of retorques. I didn’t see any signs of anything catching, not scuffs or scratch marks anywhere and everything was moving smoothly and freely.
- coil or HT leads shorting: both coil and HT leads are new and as far as I can tell, nothing seems to be wrong there.
- distributor: I have an electronic ignition from Thorspark, so no points. It’s a Hall sensor type thing with the magnet turning on the cam and the sensor fixed. Distributor cap and finger are new. As far as I can tell the automatic advance mechanism is assembled and working properly. I have a slight play on the cam - when I turn it with my fingers I do feel a slight play, but I wouldn’t think it is so much as to cause a problem. The problem is I have no reference so I don’t really know what is acceptable or not. This could be the contributing cause, but I’ve no idea how to really check it. And I do believe the hall sensor ignition should be more forgiving of play between the cam and shaft, or am I wrong in assuming this?

Otherwise I have good fuel flow, carb has recently been cleaned and all the jets are free and the proper size. I have a washer under the float valve. Spark plugs are new and gaps adjusted according to the manual. Fuel here has 5% ethanol and I add a lead additive. Not sure what else I could tell you, so start with the questions and your always helpful ideas.

Cheers
Gui

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simon.holyfield
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Re: MkI Square Four fine tuning and performance troubles

Postby simon.holyfield » Mon Aug 03, 2020 6:37 am

I’ll defer to others with more experience than me, but I didn’t reach 100 kph, or about 60 mph until I had 1000 miles on the clock. My Mk1 seemed to loosen up a lot at that stage. Earlier, it seemed more reluctant and as you got to 55-60 mph it was a little tingly in the fingers; now, with 2500 on the clock it will whip up to 60 very easily, and beyond.

Personally I think I would use it and stretch its legs over the next few hundred miles and see how it goes.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
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'30 Model A
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Gui.dorey
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Re: MkI Square Four fine tuning and performance troubles

Postby Gui.dorey » Mon Aug 03, 2020 7:54 am

Hullo Simes, that could be. She does run happily all day on country lanes between 70-80 kph and doesn’t get excessively hot or anything. That’s a good sign and I probably should not worry too much at this stage. We’ll see.

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Re: MkI Square Four fine tuning and performance troubles

Postby klaus gerhard » Mon Aug 03, 2020 3:39 pm

Hello Gui,
do you think the the engine runs rough because you rev it beyond a certain range or could it be that other influence e.g. wear from the wheelbearings cause the rough running above 60 mph. You could try to run in 3rd gear above the equivalent speed. Just an idea. My Mk1 runs very nicely up to 120 km/h.
What size engine sproket is fitted to your bike?
Regards Klaus

Gui.dorey
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Re: MkI Square Four fine tuning and performance troubles

Postby Gui.dorey » Mon Aug 03, 2020 4:22 pm

Hello Klaus,

It’s definitely the engine. Happens in all gears above a certain rpm. I can’t remember exactly which sprocket combination I have, but I do believe it’s the solo gearing. Stupidly enough I never wrote it down. I remember I checked the front springs during the rebuild and they are the solo version.

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Re: MkI Square Four fine tuning and performance troubles

Postby paul.jameson » Mon Aug 03, 2020 9:04 pm

Is the camshaft new or is it an old one? If old, uneven wear could lead to incorrect valve timing, as could badly worn cam followers.
Paul Jameson
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
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Now Archivist once more - but not Machine Registrar.

Gui.dorey
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Re: MkI Square Four fine tuning and performance troubles

Postby Gui.dorey » Tue Aug 04, 2020 4:30 pm

The cam followers are new, but the camshaft is not. When I set up the valve timing, I did find that the total lift was slightly less than the 0.3125” by about 0.0027”, and when plotting the graphs my line going through the mid points was slightly leaning to the right. It didn’t seem like a sign of excessive wear to me, but do correct me if I’m wrong. I must say I am more suspicious of my measuring skills and instruments, though - I never measured exactly the same values every time I clocked the same cam.

Reading through Longman’s instructions on valve timing as well as other sources, if my timing is off then it would seem to be too advanced by looking at the symptoms - good power at lower revs and reduced power at higher revs.

I’ll keep observing how she goes over the next 500 miles and see if there is any improvement like Simes predicts, but everyone’s thoughts are highly appreciated as this is a very interesting discussion.

Cheers,
Gui

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Re: MkI Square Four fine tuning and performance troubles

Postby Gui.dorey » Tue Aug 04, 2020 4:35 pm

Another thought is of course ignition timing.
Could it be that the advance mechanism on the distributor is not working properly and not advancing as it should?
How does one check for this?
Without the engine running it all seems all right to me, but how does one really know?
I’ve really no idea how to check it...

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Re: MkI Square Four fine tuning and performance troubles

Postby Paul_Linden » Wed Aug 05, 2020 4:08 pm

Slacken the lock nut on the distributor so that you can just move the distributor by hand. Take the bike for a run and as you are going along move the distributor round to see if there is an improvement. This was what I used to do on my C15 back in the 70s.
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Gui.dorey
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Re: MkI Square Four fine tuning and performance troubles

Postby Gui.dorey » Wed Aug 05, 2020 5:05 pm

Paul_Linden wrote:Slacken the lock nut on the distributor so that you can just move the distributor by hand. Take the bike for a run and as you are going along move the distributor round to see if there is an improvement. This was what I used to do on my C15 back in the 70s.


Paul, that sounds like a quick way to crash. Don’t think I’m that flexible :lol:

Basically it’s what I did, but stopping every time to do a small adjustment until I got the best performance.


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