Model 50 Compression problem

Previously terry_robinson wrote:

Took charge of a very original 1956 Model 50 about 18 months ago. Started fine from cold and went well. Started OK within about 5 mins of stopping. Joined the VMCC recently and started going on runs with them. Getting the bike started again after 15 or more minutes of a stop took forever. Some of the guys who took over kickstarting commented on the serious lack of compression. Suggestions such as tight tappets and a tight valve compressor were made. I have investigated both of these and they are not the problem. With a compression tester the compression got to 70psi, but on the kickstart there is very little resistance.

As I say the bike starts first kick from cold and there doesn't seem any lack of power (to me). Any help will be gratefully received. I'm not completely useless but am not a mechanic. Anyone know of someone in the NW who would do a top-end or engine rebuild if needed? I live near Blackpool.

70lb isnotalot, doesitsmoke? if no measureyourtappetclearances, Justcheckyourdecompressorisclearingeverytime when youcheckthetappets,ifok pour a drop (capfull) of oil in the boreandsee whatpressurechangeyouget when you take areading and doesitdeteriorate themore you kick it??

Anysign ofseepagearound thecylinderheadjoint? If themotor isbasically sound Iwouldgivethe boreabit of asoak inlight oil fora fewdaysthenkick it overwith theplug out to expel whatsleft; getit out on a runfor agooddistanceandkeep it hot. ifyou gotsomehillsrun it up thehill in a pullinggearandkeep acheck on theexhaust, theextrapressure will generate smoke ifthereis a ringproblem, conversly rundown thehill atspeed with throttleshut thencheckyourmirrorwhen you open up... therewill bea puff of blue smoke ifitsvalvetrainrelated.

My guess, you mighthave abit of a stickingring problem. theydolike theirthroatscleared now andthen....

Let usknow howitgoes....

Cheers andgoodluck

Jon

Previously terry_robinson wrote:

Took charge of a very original 1956 Model 50 about 18 months ago. Started fine from cold and went well. Started OK within about 5 mins of stopping. Joined the VMCC recently and started going on runs with them. Getting the bike started again after 15 or more minutes of a stop took forever. Some of the guys who took over kickstarting commented on the serious lack of compression. Suggestions such as tight tappets and a tight valve compressor were made. I have investigated both of these and they are not the problem. With a compression tester the compression got to 70psi, but on the kickstart there is very little resistance.

As I say the bike starts first kick from cold and there doesn't seem any lack of power (to me). Any help will be gratefully received. I'm not completely useless but am not a mechanic. Anyone know of someone in the NW who would do a top-end or engine rebuild if needed? I live near Blackpool.

hi i had the same problem with a mod19 1st kick when cold , would not start when warm, turned out to be rich mixture , wrong cutaway on slide, check the slide & jets , hope this helps, pete

The compression does look low but compression testers don't work well on singles. When you're trying compression with the kickstart, are you doing it with the throttle wide open as otherwise it can feel low.

I would also check that you have an insulator on the carb flange as you can get vapourisation in the carb due to heat soak when stopped for a while. My old 16H of fond memory was prone to this. I remember kicking and sweating in central Brum once when an old bloke (well he looked old to me at the time) suggested flooding the carb - which worked! He told me he'd been a Don R in the war and it was a common problem.

Do you not have a slow speed adjusetr screw for the throttle slide, Terry?.

As the bike is more usually hot rather than cold there is a good argument for setting the tickover hot and holding the throttle a bit open whilst it's warming up. If it has a high tickover when hot it could be that the mixture is thus incorrect when you try to start it in the normal way with the throttle fairly shut.

George

Hi Terry,

Re.- lack of compression on your very original (meaning very old) Model 50, it's possible the exhaust valve may have hit the piston and slightly bent the valve head so it doesn't seal properly. The valve spring could have become weakened with age and allowed the valve to float, i.e. lose contact with the cam profile. The only way you will be sure of the problem is a top end strip-down to find the cause.

Re.- the poor starting, there is a knack to starting cold and starting hot. When cold you need to retard the ignition about half way, close the air choke and just flood the carb slightly. When hot all you need to do is retard the ignition. After 15 plus minutes you will need something between these.The bike should start within a few kicks and if not it could be a blocked pilot jet/airway or just general dirt in the carb. If the spark plug oils up this is, as Jonathan says, most likely to be piston rings and/or cylinder bore wear. A hotter running spark plug might help with this.

Mike Pemberton (see Roadholder) is not too far from you, but he does this as a business so it may be best to get an estimate first before you commit. A top end strip down is fairly straight forward if you have an original maintenance book and check and measure everything as you dismantle.

Regards, Richard.

The carb insulator would be a Tufnol or similar spacer between the inlet flange and the carburettor flange, usually about 3/8" thick.

As said, it's normal to set slow running when warm as this is how the bike is running most of the time.

Each bike will have its own starting routine - for example my ES2 doesn't need any flooding at all when cold (at least in summer temperatures) and starts first kick on half retard, air slide closed (when cold) and a tiny amount of throttle.

Hello, heres a photo of the carb insulator

Always check the easiest thing first. When it's warm is there still valve lash? I've been bitten by this in the past and it's the easiest thing to check and remedy.

Mike