SQ4 Misfire

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simon.holyfield
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SQ4 Misfire

Postby simon.holyfield » Sun Jul 22, 2018 10:27 pm

I've just returned from a 60 mile round trip this evening - the temperature here is about 18 C.

Driving back, the bike was flying along at 55-60 most of the way, feeling great. On the way down, a misfire appeared after 10 miles or so, missing and holding back, though it would get up to 50-60 eventually. The problem seemed worst around 30 in top. I stopped for an hour before coming back.

The other day, I made a 10 mile round trip - on the way there it was fine, on the way back the same misfire appeared after a couple of miles.

Any ideas? I can hold my finger on the carburetter float bowl after a fast run - the carburetter overheating seems to be behind me. I'm wondering about oiling up or some intermittent ignition thing?
Last edited by simon.holyfield on Mon Sep 03, 2018 10:53 pm, edited 1 time in total.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
http://ariel-square-four.blogspot.com

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Re: SQ4 Mis-Fire

Postby MARK.WALSH » Sat Jul 28, 2018 2:25 pm

Simes
difficult to say. Could either be ignition or carburation. Mine has a tendency for the pilot to partially block causing problems at the lower end but usually I can nurse it through to the main jet where the problem will disappear. I also have a volt meter fitted and if travelling at low speeds for prolonged periods the voltage will drop to about 5.85 where misfiring can occur until you get the speeds up and dynamo charging. - All this heat doesn't help matters though, as the engine soon gets hot and bothered anyway unless you are doing 50+.
Mark

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simon.holyfield
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Re: SQ4 Mis-Fire

Postby simon.holyfield » Fri Aug 31, 2018 10:36 pm

I'm still chasing this misfire around. I straightened the bent emulsion tube and repaired a longitudinal crack in it, but whilst the bike is very smooth and rapid when its happy, I still get this misfire.

Today, I drove for about 16 miles - then, coming into a village and slowing down it appeared and stayed off and on for the next 4 miles. I pulled over and the engine died, but restarted easily and was then fine. Carb is very cool, and I'm starting to look at the ignition again. A hint from the Austin 7 forum (same distributor you see) suggested that the auto-advance some times sticks. Mine is very free, but do others have these odd springs?

IMG_20180831_192624.jpg


I can see that odd springs would alter the advance curve and I think that Lucas did use this technique, but I would like to know if others have springs like these. You will note that the loop in the larger spring is arranged so that the large spring only becomes effective when the small spring has stretched somewhat. I see the 419109 spring on Drags website looks more like the smaller one than the larger one.

By the way, I also found a bad crimp on the coil wire to the distributor - just the sort of thing that would give you an intermittent fault like this!
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
http://ariel-square-four.blogspot.com

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Re: SQ4 Mis-Fire

Postby Simon.Gardiner » Fri Aug 31, 2018 10:55 pm

My BM suffers intermittent misfiring when the coils start to play up - I only nailed this when I was glaring at it in the dark and I happened to spot the arcing from an HT connection.

So - I'd suspect the HT side. Maybe even a duff/dirty plug??

SG
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)

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Re: SQ4 Mis-Fire

Postby nevhunter » Fri Aug 31, 2018 11:32 pm

The small and big spring is fairly normal, but you would have to put it on a machine and plot the advance of you are serious about it. Many simple ones just retard it at slow (idle) speeds and don't do much otherwise. I doubt it would make it misfire. Mostly it will make it sluggish if the weights don't move free enough to go to full advance at revs as they should. Nev

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simon.holyfield
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Re: SQ4 Mis-Fire

Postby simon.holyfield » Fri Aug 31, 2018 11:45 pm

Thanks for the tip Simon, I'll try that tomorrow night.

I'm working through the HT system at the minute - a rocker gasket leak has covered most of the leads with oil which won't help, and I've got some very large heat shrink to make something to protect the leads - but that, and fixing the leak, will have to wait until the winter.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
http://ariel-square-four.blogspot.com

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Re: SQ4 Mis-Fire

Postby roger.fellows » Sat Sep 01, 2018 6:22 am

They're known for going better uphill when there's greater depth of fuel at the taps - do you keep the level well up? I expect you know this.

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paul.jameson
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Re: SQ4 Mis-Fire

Postby paul.jameson » Sat Sep 01, 2018 8:00 pm

In my experience with Square Fours, it doesn't matter what the problem is - the symptom is a misfire. You will need to check out the whole system till you find it but I would start on the ignition side by swapping parts one at a time. So condenser, coil, distributor cap, distributor base plate, rotor arm, etc. The most difficult one to find I have had was when the stud on the side of the distributor had become unsoldered from the strap which runs round the inside of the distributor. The bike was fine till it had done about 30 miles when the misfire set in.

My own Square has had the habit recently of being perfect but then suddenly stopping at low revs and being awkward to restart. I finally found a small drop of water in the carb below the main jet. How it got there I have no idea at all, but they bike runs much better without it. The carb is the early type though, without the bi-starter so different from yours.

Good luck, and let us know what the problem is when you get to it.
Paul Jameson
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.

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simon.holyfield
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Re: SQ4 Misfire

Postby simon.holyfield » Mon Sep 03, 2018 10:53 pm

Thanks Paul.

So another day, another road test. After the weekend, where I checked out the B5HS plugs (pale grey centres, all the same - as expected) and fixing a very dodgy crimp in the distributor circuit and filling the tank, I took an early evening bimble along the lovely A149 North Norfolk coast road, into the setting sun. As usual, the SQ4 was fine for the first few miles, though I thought a little more reluctant to rev from a closed throttle than normal. We stopped for a break and a picture at Blakeney:

IMG_20180903_182009.jpg


There's a narrow lane down to the beach here, maybe 300 yards down a hill which you do at 20 mph or less - idling, essentially. At the T junction at the bottom, there was a reluctance to rev and I coaxed the bike to a parking place where I enriched the idle mixture half a turn thinking that it had been reluctant to come off the idle jet since leaving the house.

A few more miles, and once out on the open road again she was flying. A little stumble now and then but generally great, until I got to a little village called West Runton, which has a mile or so of descent down a narrow lane into the village centre - again, 20 mph max, throttle closed, followed by a short hop over a railway bridge. By the time I was at the bottom, opening the throttle would kill the engine and there was no way I was getting over that bridge until the engine had cleared its throat. Once on the open road - more or less OK again. I'm convinced it gets very rich on the overrun, or descending hills on closed throttle.

So, back in the shop I am thinking that I have the idle mixture screw set way too rich - but it's only two turns out. My second thought is that there is an internal leak - possible in the bi-starter, or possibly the bi-starter is not closing fully.

We'll have to pull the carburetter off again.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
http://ariel-square-four.blogspot.com

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paul.jameson
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Re: SQ4 Misfire

Postby paul.jameson » Tue Sep 04, 2018 8:49 pm

"Most carburation problems can be solved by attending to the ignition."

A mild paraphrase of the words of the late Len Ore. He actually said "magneto."
Paul Jameson
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.


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