1961 350 Is 50 psi compression too low

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
Member
Posts: 25
Joined: Tue Apr 16, 2019 8:36 pm
Location: UK

1961 350 Is 50 psi compression too low

Postby Oldcarguy » Fri Apr 26, 2019 8:59 am

Hi new member here recently bought a 1961 Ajs Model 8 when I went to see the bike it started 2 kick and idled a little lumpy I did not get a test ride because it was on a busy main road, I noticed there was oil at the head/barrel joint. Anyway picked it up Monday this week started it up ran for 15 minutes the an hour later would not start. Since then I discovered no oil in the gearbox and engine oil very dirty, so I changed both oils and removed filter and cleaned it, then changed wrong very worn spark plug managed to get it start but not happy running to I removed the carburettor striped it and ultrasonically cleaned it put it back together and back on, it now starts but does not rev and pops back though the carb. It was then I decided to do a compression test on a hot engine and could not get more than 50psi (Now in my experience of 30 years motor mechanic 50psi is not enough to run a car engine)


I wait in anticipation


Iain

Member
Posts: 6449
Joined: Wed Sep 01, 2004 12:00 am
Location: WEST SUSSEX UK

Re: 1961 350 Is 50 psi compression too low

Postby SPRIDDLER » Fri Apr 26, 2019 1:37 pm

Welcome to the Forums Iain.
A couple of quick thoughts..........
I assume with your experience you did the compression test with the throttle wide open..........?
Apart from carburation perhaps the auto A/R is sticking at the retarded 'start' position? If it doesn't advance it won't rev (much) although spitting through the carb suggests a weak mixture/air leak at the carb/manifold.
I poke badgers with spoons.

Member
Posts: 25
Joined: Tue Apr 16, 2019 8:36 pm
Location: UK

Re: 1961 350 Is 50 psi compression too low

Postby Oldcarguy » Fri Apr 26, 2019 4:18 pm

Yes and it has Pazon ignition my main concern is the lack of compression

Member
Posts: 6449
Joined: Wed Sep 01, 2004 12:00 am
Location: WEST SUSSEX UK

Re: 1961 350 Is 50 psi compression too low

Postby SPRIDDLER » Fri Apr 26, 2019 10:36 pm

I didn't refer to the low comp specifically as with your experience I didn't want to bore you ;) Possible causes: Lost rocker to valve stem clearance, sticking valve (esp. inlet valve, producing spitting back from carb), pitted valve seats, worn bore/rings or a leaky head gasket.
I've not had a L/W or owned a comp tester, merely relying on gut instinct and being able to stand on the kickstart of my H/W 350 single for about two seconds, but you're right, 50 is too low and I believe you should be seeing far greater; in the region of 100 p.s.i.
Last edited by SPRIDDLER on Sat Apr 27, 2019 3:41 am, edited 1 time in total.
I poke badgers with spoons.

Online
User avatar
Posts: 2642
Joined: Wed Dec 14, 2011 1:02 am
Location: ESSEX UK

Re: 1961 350 Is 50 psi compression too low

Postby Ozmadman » Fri Apr 26, 2019 10:54 pm

Does it kick over easy without using the decompressor ? no gearbox oil is a real worry as they are prone to rapid box wear if the are not kept topped up right up to the inspection plate (3 pints SAE 50) good chance that it will then leak somewhere(probably why it was dry) usually out of the output shaft seal, kick start seal or selector shaft seal. Spitting back through the carb sounds like timing to me should be 1/4" BTDC (compression stroke)with the ADV/RTD wedged fully open. Leaky head gasket (they are prone to that as well) could cause a weak mixture depending where the leak is and mine ran very lumpy until I realised that the throttle slide flopped around like a c**k in a sock and I got the carb renovated by Martyn Bratby and now it runs like a swiss watch. Just a thought, choke is off when the lever is pulled toward you when sitting on the bike, many make that mistake and think the opposite is true.

https://www.youtube.com/watch?v=DFgHSyX8dGI
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed

Member
Posts: 25
Joined: Tue Apr 16, 2019 8:36 pm
Location: UK

Re: 1961 350 Is 50 psi compression too low

Postby Oldcarguy » Sat Apr 27, 2019 8:25 am

Yes it kicks over without valve lifter you can even turn it over my hand by just putting pressure on it.

Member
Posts: 6449
Joined: Wed Sep 01, 2004 12:00 am
Location: WEST SUSSEX UK

Re: 1961 350 Is 50 psi compression too low

Postby SPRIDDLER » Sat Apr 27, 2019 8:45 am

Oldcarguy wrote:......it started 2 kick and idled a little lumpy ...... I noticed there was oil at the head/barrel joint. Anyway picked it up Monday this week started it up ran for 15 minutes the an hour later would not start. .......I decided to do a compression test on a hot engine and could not get more than 50psi

It may be that the head gasket was leaking initially and has now failed further.
If you depress the k/start can you not hear any hissing noise of escaping air from the head, or past the piston, or from the valves, or out of the carb mouth?

Maybe first check the pushrod to rocker clearances and the valve lifter (decompressor) adjustment. Next put a dessertspoon of oil into the plug 'ole and do the comp test again. If there's a significant improvement chances are it's a worn bore or rings (black smoke from exhaust, oily plug?) but that won't have occurred between purchase and today.

From your other findings (dry 'box, knackered plug....) there'll be more to check. It won't solve the low comp issue but is the (Monobloc?) carb correctly jetted?
Whereabouts are you in 'U.K'. ?

If you don't have Workshop, Owners and Spares manuals they can be found here under 'Books':
http://archives.jampot.dk/
I poke badgers with spoons.

Online
User avatar
Posts: 2642
Joined: Wed Dec 14, 2011 1:02 am
Location: ESSEX UK

Re: 1961 350 Is 50 psi compression too low

Postby Ozmadman » Sat Apr 27, 2019 11:26 am

Oldcarguy wrote:Yes it kicks over without valve lifter you can even turn it over my hand by just putting pressure on it.

Mine developed a slight head gasket leak after it sitting outside in the cold all winter when I started it up after a few months. I took off the rocker cover and re-torqued (after slightly loosening) the four main head bolts back to 35 lbs ft, also just nip up that smaller 5th 5/16" bolt a bit (don't do that to the same torque of it will strip the thread in the barrel!) and that seems to have stopped my leak. Worth a try before going the whole hog and taking the head/carb / exhaust off. Remember that to remove the rocker cover there is a bolt hidden behind the tappet cover plate!!

Tappets should be set when the engine is just warm so you have no free play but enough to be able to spin the push rods easily by hand. Yes and as Spriddler has mentioned, loosen off the decompressor cable so that is one thing out of the equation.
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed

Member
Posts: 25
Joined: Tue Apr 16, 2019 8:36 pm
Location: UK

Re: 1961 350 Is 50 psi compression too low

Postby Oldcarguy » Sat Apr 27, 2019 12:23 pm

Well a slight change for the better,took off seat & tank to get to the decompressor and found the cable was not completely free so wound the adjuster out a little and now I have close to 100 psi on a cold engine this may have been the reason it would not start again when hot as everything would have expanded, I am now going to remove the rocker box and try a re torque on the head bolts to see if this will stop the leak.Fingers crossed

Member
User avatar
Posts: 3706
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: 1961 350 Is 50 psi compression too low

Postby clive » Sat Apr 27, 2019 2:47 pm

Make sure you dont catch the cable with the tank when replacing it.
clive
if it ain't broke don't fix

Next

Return to G5 / Model 8

Who is online

Users browsing this forum: No registered users and 3 guests