Starting problems '72 Commando

You may already know all this but you need to do some diagnostics:

You must have 3 basic things: a spark, fuel and compression. Make sure the spark is happening close to where the manual says on the compression stroke. You can find compression by holding your finger over the plug hole. The compression will be obvious as you kick the engine over.

I don't know how you test for a spark with the Pazon (probably by holding a spark plug against the head and kicking the engine over like any other system) but you must do that.

If you don't trust the fuel taps or carburettors then get a little fuel into the cylinders by some ingenious method.

I have never had any luck playing with the timing. Just get it spot on according to the manual.

Go through your entire (I mean all of it) tune-up procedure. While the valve covers are off, check that the sequence of valves opening and closing makes sense. No valve stuck open or stuck shut or fallen in.

It could be that nothing you do gets the bike running but what you want is to find something not quite right. The manual lays out the minimal settings for a running engine so we must start there.

After you have done all of this, report back with your findings and I am sure we will get to the bottom of this.

First check that fuel flows freely when you turn the taps on. If it doen't then find where the blockage is.

Before you do anything check the batt voltage is at least 12.6v with nothing switched on. Then check it again with ign on just in case there is an unwanted discharge somewhere. 12.5v would be good. Pazon will as low as 8v they claim but its worth checking......low voltage is not conducive to good starting.

Maybe the clue is the backfire before it died.That does point to an electrical fault, probably not timing as such. Low on fuel or lack of tends to show up as a slow to quick 'die' and maybe spitting back depending on throttle applied and engine speed. Pazon check is easy....plugs out laid on head (or a spare pair) connected to HT etc. Remove points cover, remove both trigger wires, ignition on and touch trigger wires together intermittently. Plugs should spark at rate of touching wires....... Ignition kill switch needs to be checked and best to bypass for test taking a wire from neg terminal (if still + earth) to Pazon 12v supply wire.

I would be looking at one of three possible causes. Firstly I would suspect that the Pazon trigger plate has moved on its taper. This is not an uncommon fault due to a poorly machined batch. Unless your battery is really duff then the Pazon will run on just 8 volts but prefers more.

My next suspect would be the coils or the connections to them. Their exposed environment does allow the connections to go manky and hence increase the resistance through the set. Headlights, rectifiers and earth wires have the same issue and all work far better after a clean-up of the terminals.

Lastly the carbs......I can not see the carbs as the problem. Possibly one but not both, unless there is a fuel supply problem.....but turning both taps on would answer this. How good is the fuel?

Hi all

Bike was running OK & then backfired & died (thankfully as I pulled up at my garage. Plugs badly carboned, replaced same thing. With great difficulty got it started but not running well & then would not start at all.

Having replaced both carbs & coils, new throttle cable & fitted Pazon (the Boyer was shot) my bike will not start or might start on full choke & then die once choke taken off.

Compression is 150 on one side & 100 on the other but also an issue with fuel not pulling into plugs though no probs when carbs tickled. Fuel is the same as used on my Bonneville with no problems so don't think it is that.

Am now thinking of retarding the timing & advancing slowly.

But is there anything else I may have missed? Am now getting pretty desperate & low on ideas.

All advice welcome.

Thanks in advance.

Christine

Well how did you check your compression ratio , was the engine warm or cold, it dose make a difference maybe you have a valve not seating right this is the most common cause of compression loss on one side the other is a broken piston ring or rings, are you getting any oil on that one side on the plugs , your engine will not run right with the compression down on one side, so this needs attention first before anything else, yours anna j

Hello Everyone, Anna J has a good point. If everything else checks out I would suspect that it could well be oil fouling the plugs and thedifference incompression reported on one pot is a strong indication that all is not well with the engine. I have experienced this myself and once the plugs are contaminated with oil, that's it! they will not run again. New plugs will run, but only until they to are fouled and then they will cut out. The difference between over rich sooty plugs and oiled plugs is quite obvious so if this is evident on either of the plugs than this is most likely the problem. Oil can be drawn into the cylinder between the head and the valve guides if they have been ragged out without care and also down the valve stem if valves are worn or have missing oil seals. A bad rebore or damaged/broken rings are just as good at fouling plugs so these must be suspect too. I hope this helps. Cheers and good luck! Peter Austin

Previously christine_boulton wrote:

Hi all

Bike was running OK & then backfired & died (thankfully as I pulled up at my garage. Plugs badly carboned, replaced same thing. With great difficulty got it started but not running well & then would not start at all.

Having replaced both carbs & coils, new throttle cable & fitted Pazon (the Boyer was shot) my bike will not start or might start on full choke & then die once choke taken off.

Compression is 150 on one side & 100 on the other but also an issue with fuel not pulling into plugs though no probs when carbs tickled. Fuel is the same as used on my Bonneville with no problems so don't think it is that.

Am now thinking of retarding the timing & advancing slowly.

But is there anything else I may have missed? Am now getting pretty desperate & low on ideas.

All advice welcome.

Thanks in advance.

Christine

Well how did you check your compression ratio , was the engine warm or cold, it dose make a difference maybe you have a valve not seating right this is the most common cause of compression loss on one side the other is a broken piston ring or rings, are you getting any oil on that one side on the plugs , your engine will not run right with the compression down on one side, so this needs attention first before anything else, yours anna j

As it was running and then stopped it won't be wet sumping. Wet sumping can be a starting issue whe a bike has been left a while and oil drag makes it somewhat challenging to kick over. I'm a bit concerned at your plugs being fouled and by the engine stalling when you take the choke off. Firstly ensure you have a good spark at the plug and then be quite sure your fuel flow to the carbs is not restricted - check that the petrol tank cap breather is clear.If that is blocked it can cause mysterious engine stoppages. And finally, get someone else to have a look and think. A fresh pair of eyes may just spot thecause of your problems.

Previously christine_boulton wrote:

Hi all

Bike was running OK & then backfired & died (thankfully as I pulled up at my garage. Plugs badly carboned, replaced same thing. With great difficulty got it started but not running well & then would not start at all.

Having replaced both carbs & coils, new throttle cable & fitted Pazon (the Boyer was shot) my bike will not start or might start on full choke & then die once choke taken off.

Compression is 150 on one side & 100 on the other but also an issue with fuel not pulling into plugs though no probs when carbs tickled. Fuel is the same as used on my Bonneville with no problems so don't think it is that.

Am now thinking of retarding the timing & advancing slowly.

But is there anything else I may have missed? Am now getting pretty desperate & low on ideas.

All advice welcome.

Thanks in advance.

Christine

Funny enough I have the exact same problem with my 1972 Commando. It also died when arriving home and I fitted a Pazon system, since the old mechanical ignition was worn out. I tested and re-fitted the Pazon several times, until I realized the problem was probably fuel related. And it was. After cleaning the carb (SU) it started immediately. I still have problems keeping it running below 2000rpm, but I will continue to look at the carb, which has a few other issues. I will follow your endeavours with interest.

How old is the wiring loom, in particular the cable that runs down the frame to the ignition pick-up ? I replace mine every year or two in order to pre-empt precisely this sort of problem.

Although you're making a link to the choke, this could simply be that the cable is pulling on the wiring loom as it's tightened. Engine revs too, by making the motor bounce more or less on the isos can also cause odd misfires by tugging at the cables.

In my experience, 90% of Commando electrical problems relate to wiring, connectors or switches.

My first steps would be to run a live feed from battery to coils and replace the pick up wiring. If that helps then you can get down to finding out exactly where the problem lies.

OK there's a lot of electrical checks still to be done but forgive me if I ask, how long have you had the bike and if a while has it always run ok? Has yr Bonny got the same choke arrangement with bar lever and cables? Classic or modern Bonny? So, tight for off etc. Just a thought as its not been mentioned..........

Thanks Tom - new carbs still not starting - put old carbs back on - same problem. New replaced after thorough cleaning - no joy. Fuel same as I used on my Bonneville & is flowing OK when carbs tickled. Retarding ignition to 20 & it started on full choke cut out when choke taken off & obviously can't run it at 20 as pipes very hot. The issue remains fuel is not being pulled into plugs & it will only run on full choke & cuts out if choke taken off. Have checked there is no oil on plugs, no air leaks round flanges or elsewhere. It won't bump start either. Someone said it's a Combat engine issue but not what & whilst bike falls within Combat engine number range VMCC said there is no mention of Combat on original factory docs relating to bike. Have taken it to experienced mechanic used to British bikes but not so much Nortons. He is also much perplexed & thinks it likely to be a timing issue but what exactly is a mystery. So if anyone has any bright ideas I am all ears (well eyes at least). If it were valves or valve guides surely they would have to be pretty bad to stop bike starting altogether & why will it start on full choke but not run without also it was running OK until it sulked. I think it's got it in for me!

Well you answered your own Question !! will only run on full choke, so this is fuel starvation, if it then cut's out when you take off the choke, and you need to set the timing to 29 degrees before Top Dead center Drive side first, then Check the right hand side,, and use a timing disc on the drive side, and use some Bosch W7DTC spark plugs , and clean the pet-cock and the fuel lines and carburetors And too do a compression test the engine should warmed up ,then you may get somewhere, Yours Anna J

Blocked idle jets give the same symptoms as you are experiencing i.e. very poor running below half throttle and no start. Very hard to clean on a concentric. I think a 12 thou piece of wire will go through them and clear them. You get to them by taking the idle adjustment screws out.

Also a little water at the bottom of the float bowl gives a similar symptom. All more or less likely but you have to check everything and it only takes a few seconds.

Hi i had simlar symptoms, what it was i found i had used the wrong type of charger on the battery and without me knowing it boiled it dry.the battery still functioned then one day it packed up i came to a stop but on kicking it over it would start all though only for a short time.it was back fireing through exhausts and carbs whilst running.this was fitted with a pazon ignition.take the battery off and give it a good looking over to see if its bone dry. if so it will need replaceing.

Try a separate, fully charged battery to power the ignition. ie completely divorced from the bikes electrics.

For sure blocked idle jets will not let the engine run under 2000-2500 rpm. But why are the plugs sooty when the bike stopped? If you fitted new carbs are the bowl plugs still clean? No sign of any debris that could block the jets. New carbs can block up very easily as I know only too well. Filters on my feed pipes now. I only asked about the bike vintage re. the chokes and how long you'd had it since old Triumphs and old Nortons chokes work the same way with Concentric and Monoblocs. You say they are different which is odd. Anyway good luck with Emery and hope you get it sorted. Love to know what the problem is.

Can you confirm that the slides are high enough in the body of the carbs to allow a flow of air through when on the throttle stop? In otherwords will it run with the throttle stops screwed in a couple of turns? This is the $50,000 question, did your new carbs come from Amal direct or from somewhere else (in the west midlands)?