Myths

I have come across a lot expert opinions in my time. Now take into account I am self trained in motorcycle repair. I am an Electrician by way of a 4 yr apprenticeship. Everything I know is either from the school of hard knocks , ie. trial and error, usually only one error is enough to learn, or some reading .  I read  Phil Irvings tome in my 30’s, this helped me some ,but understanding engineering principles is difficult with only grade 10 level education.

Anyway, I was on youtube recently and saw a bloke explaining why his Norton was smoking. He tried to explain to any viewers who’d listen,  that the 2 compression rings were lined up at their ends. He then stated they should be 180 degree apart. Well those of you who have experience in 2 strokes will know that they usually have 2 compression rings only and that these rings are pinned in the ring groove to prevent them turning. Why ,because 2 strokes have ports in the bore and when left to turn they can catch the port at the ring end causing disastrous results. Now given that 2 stroke rings are known to turn in the groove during operation it follows that 4 stroke rings will do the same. When setting up the rings and pistons people are told to place the 2 compression rings 180 apart. Fair enough , this may prevent a minute loss in compression on start up but should have no bearing on oil use as below those rings is the oil scraper and that’s it’s job. As an engine is running, a certain amount of harmonics is produced and this  is what causes the rings to rotate, as I understand. Like  orbiting planets these rings will line up occasionally. It causes no problems , it’s normal and may last a very small time. So go ahead and set them up at 180 but be aware they will line up at some stage but won’t cause smoking or any other problems.

Now as for the cause of smoking in a 4 stroke. If an engine is smoking constantly as confirmed by a mate following you , then it is highly likely to  be a bore problem . This can be worn rings, out of round or tapered bore caused by ring wear. Usually this means high mileage. Or something  else like a gudgeon pin rubbing a groove in the bore,  due to a missing circlip or even the rings unable to do their job due to a previous nip up, which can grab the rings or deposit piston material into the ring groove rendering them useless.  A nip up can also deposit aluminium on the cylinder wall and  stop the rings doing their job. These conditions cause continuous smoking.

Smoking on over run or at idle is caused usually by worn valve stems or valve  guides or both , giving excessive  valve guide to valve stem clearance or the guide itself coming loose in the head can be another cause. Not unknown in Triumph twin 650-750 models. Always use a .001 instead of a STD guide  unless of course it needs to be bigger on the OD.  Coming loose  is also known to happen with cast iron guides in alloy heads as the coefficient of expansion is quite different between the 2 materials. With bronze guides in allay heads the expansion rates are similar and should not be a problem unless fitted  with not enough interference. This type of smoking clears for the most part when the throttle is opened . When the throttle is shut the engine hasn’t changed its volume so tries to pull in air  from wherever it can. With worn or loose guides the extra air also pulls oil with it.

No 2

Another I hear a lot is the main jet will fix it. “But I changed the main jet and it still runs like crap”. Well as a lot of you know carbs , you might want to find something else to do. For those of you who don’t understand carb tuning then read on. I may be able to help. First off ,the main jet takes care of fuel requirement from about 3/4  throttle to wide open but has to be of a size big enough to pass enough fuel for all requirements. The long needle usually looks after the mid range, the bit you use most. The pilot jet then central air bleed hole then the slide cutaway with some needle, look after idling and low speed running just off  the bottom.   Providing the carb is not completely worn out it should be able to be made to run. Never go too far outside manufacturers specs. If they say a 200 main jet, then a 190 , 210 or 220 will be ok. In OZ it doesn’t hurt to lift the main jet  1 or 2 sizes as it is a hot country with wide open riding conditions. Take into account your exhaust , typically lots of engines can be over exhausted , too big a pipe or wrong  or no muffler. Some back pressure is desirable. Recently after firing my Goldie I realised the muffler was too open, I changed to a smaller ID  mini mega  and away it went. This was because I had detuned with touring cams and smaller carb  on a small valve head but the header was still the same. A header pipe should never be bigger than the exhaust valve for best results. It will run fine but will never be optimum on a stock engine.  It only needs to exhaust about 80% of what went in . So on more modern engines ( late 50’s on)the exhaust valve is only 80% as big as the inlet valve. Once the main jet is checked , go to the pilot jet , again this should not vary from spec by more than 1 size either way. To counter a slightly wrong pilot jet there is a mixture air screw to adjust and a wrong jet can be accounted for slightly either way. 1 size too big , give it more air and one size too small , less air, but this might cause spit back. Usually  it is wind out for more air, on most carbs. To prove the pilot jet is blocked or not, when winding in the low speed air screw the idling engine will want to slow and stall ( rich) and winding out ( adding more air ) the engine will want to race . If blocked not much happens in or out. Next up the mixture needle , the long one down the slide. If there are no markings or if the clip grooves are worn badly then it would be best to replace. If it seems ok you might check to see if it is in a  useful  parameter. Take the clip off the needle , take the idle adjustment screw out and take the cable and spring off.  Leave the needle jet block with main jet  in place. Drop the needle in the hole after putting the slide back in, if it disappears below the level of the top of the slide then it will be too short and will be difficult to stop an over rich mixture. If it sits up past the first groove or more then it may be too long to prevent a lean mixture but if it sits at about flush or between the first groove and the needle top  then it may be good to use. Check that the needle jet is 106. This can go 107 or 105 and sometimes I have been able to get rid of too rich or lean a mixture by changing the needle jet.  If on the leanest  clip setting  and it stills 8 strokes on over run( holding a steady throttle, not  on or  off) then a change to the next leaner jet, say a 105 is as good as one more clip on the needle. This is where all the running is done so is important to get as close as possible.  1968  was the first year of Amal concentrics  and they had a rubbish needle and  needle jet , the 106 had a small bore and no side bleed hole. Change to the later 99-1036 and larger bore 106 and later longer jet block . Badly worn carbs can be a pain on twin or triple applications and sometimes is best to just get new ones. But with a single carb you can usually get something happening. Although I threw away 2 worn bodies trying to get my M20 to run . Total junk.  If it is running rubbish around town but seems to clear a bit on open running then check the slide cutaway opening. If it has 2-2.5 or 3 it may be too rich ,ie. Not enough air at the lower opening so try a 3.5 or 4 cutaway. I discovered this when as a young and dumb one , I made a 2 into 1 inlet manifold for my 750 Norton. All Commandos were twin carbs but on checking a 750 Tiger triumph it had the same size for one or two carbs. So I reasoned,  as one does, it should work on my 750 Norton. Eventually I also figured out that the slide needed to be more like a 4 than a 2.5. With the help of a file , end of problems. Unless you ride flat out everywhere then it ain’t your main jet  that’s the problem. I also figured out that if setting up a single carb where there was 2,  the main jet size to start with should be half of one added to the other , ie, if a 200 with twins then add 100 and use a 300 main as a starting point. Triumph mostly used main jets a small as 230 on say a 750 single carb Tiger.  UK Green lanes against hot open OZ roads. Too big a pilot don’t help nor does too little or too much cutaway on the slide.  Usually the main offender will be the needle and needle jet . Amazing  how much difference a new needle and needle jet makes . As they rub away at each other and together with that  up- down action and fuel passing through them they wear larger in the jet bore and smaller in the needle diameter, causing excessive richness. The next time someone tells you about his main jet changing escapades you can put him on the right track.

No 3. I don’t need an oil pressure indicator , the oil is returning to the tank. Well on a Triumph twin the pump is dual piston and with all engines except total loss there is a feed and return side to the system. Ariels up until the MK2 SQ 4 use a similar plunger pump. If oil is in the sump and the oil is returning to the tank then the return side of the pump is indeed working but is the feed (pressure) side? Is the pump just returning oil you put there before a startup or left from the last run. This is a moment in time.  A  pressure relief valve indicator as used on pre-unit and early unit Triumphs or a gauge or a light will warn you if it isn’t. The factory fitted it for a reason, not just to leak oil. For a few years in the middle ‘60’s there was no indicator at all fitted to Triumph twins then a light was fitted. I used to see myself or hear from customers  that the light wouldn’t go out when started. Usually after an oil change when a bit of crud gets loose and lodges under the ball and it’s seat. Don’t assume the gauge has stopped working as they don’t usually. Anyway, a friend, instead of pulling over , continued on down the road after reasoning the gauge was broken and  ended up with an expensive repair bill. While I am dribbling on I might as well tell you about a T110 Jim at Blacktop fired up recently. It was brought to him with a knock to investigate. Had over 100lb oil pressure. As you all should know by now I am an oil pressure gauge nut on anything with plain bearings. Well as this pre-unit Jim was working on , was rebuilt by another shop, one can never know want went on. The pressure stayed high for 10mins or more after turning off. It should normally leak down through the rod bearings in 30 seconds or so to a minute if the bearings are in good condition when cold. Because this motor also had a unit twin type seal conversion on the crank instead of the factory bush it will hold pressure for a while. Unlike gear pumps where the built up pressure will just push back through the gears when the engine is stopped and drop to zero quickly. The Triumph pump as I said , is a ball and spring plunger type and will hold the back feed pressure showing a slow drop  via the rod bearings when stopped. As yet Jim has not gone inside for a look but  his suspicion that the rebuilder drove the sludge trap nut of the crankshaft in too far and blocked the oil feed into the crank will almost certainly be correct. This will be the second time he has found an engine done this way. The other was also a pre-unit and had been rebuilt but never fired over 15 years ago. When the owner decided to put it on the road finally, he was unsure of what had happened and decided to have Jim have a look inside. Had this one been fired it too would have  had a big knock, until the conrod decided to step outside for a look. So as you can see an oil pressure gauge  or light can work in many, not so mysterious ways, all good.

©2018 by British Motorcycle Classics. Proudly created with Wix.com