ronwright Posted September 6, 2016 Report Share Posted September 6, 2016 Recently acquired an Ossa TR77 - well used but sound. Been standing for about 5 years. Initially it had no spark but I tweaked the points and they started to show life. The bike is not a good starter so I have some new points and a new condensor to fit tomorrow. The spark is good but it is not bright blue - hoping the new points will fix it. Got it running yesterday but it would not rev and kept bogging and backfiring in the exhaust. After a while it was running of a fashion but with cloud s of blue smoke which I hope was just unburnt fuel. What is the best plug for this engine. Is it possible to fit the mar engine into a gripper frame. On the carb there is a large air screw - what is the general setting - 2.5 turns out ?? To the left of the airscrew is a another smaller screw which can be turned clockwise and anti clockwise - what is this and what is the geneal setting. I am hoiping to use the bike in the Beamish trial in less than 12 days time so I need to get it sorted. Quote Link to comment Share on other sites More sharing options...
bullylover Posted September 6, 2016 Report Share Posted September 6, 2016 I don`t really know much about the Ossa`s but I think it would have an Amal carb on it so I think the large screw you are refering too is the idle screw and the small one is the air screw. It also sounds as if the timing is wrong. That would cause the backfiring and the not reving so put the new points and condenser in it and time it as well. I thought all the Ossa trials bikes had a CDI ignition in them. I know all the MAR`s did. Graham. Quote Link to comment Share on other sites More sharing options...
woody Posted September 6, 2016 Report Share Posted September 6, 2016 (edited) The TR77 came with different carbs, Bing for the 350, Amal for the 250. Either will have roughly the same position for the pilot (airscrew) which is around 2 turns out for a starting point but there is no 'fixed' setting as all bikes run differently. On an Amal the idle screw is at 45 degrees and the pilot is 90 degrees to vertical. On a Bing they are both 90 degrees. From your description of the airscrew and idle screw you have a Bing. The larger is the idle which just controls the height of the slide so if you screw it in it raises the slide and lets fuel through which increases engine speed. The smaller one is the air mixture or pilot screw which controls the amount of air through the pilot circuit at up to 1/4 throttle opening. In shuts air off, out allows more air through Most if not all TR77 (green MAR) had points but some may have had CDI. The CDI ignition from an earlier MAR or a Gripper will fit straight on should you want to. The backfiring and missing is generally the result of a weak spark. On the MAR, the wires from the stator up to the coil exit through the bottom of the casing and they can often get trapped between the bashplate and become frayed or broken so check there. They also become brittle with age which increases the chances of them breaking as they have to ben 90 degrees. If this has happened it can cause a weak spark or kill it altogether. It may not be just a points issue. A mod is to run the wires behind the stator plate and out through a hole you will have to drill in the top front of the casing and straight up the front down tube (as standard they go up the rear of the frame) You also have to grind back the ribs behind the stator to give room for the wires. There are topics on this mod in this forum if you search. The MAR engine will go straight into a Gripper frame as they are the same basic engine but the exhaust will prove an issue as will the carb as the Gripper frame won't take the MAR exhaust and the carb may well foul one of the rear frame tubes due to it sitting higher on a Gripper barrel. The Gripper exhaust won't fit a MAR barrel without modding. Probably more trouble than it's worth when the TR77 frame will handle nicely with good shocks and the forks pushed up through the yokes by up to 2" as they are too long. Edited September 6, 2016 by woody 4 Quote Link to comment Share on other sites More sharing options...
goudrons Posted September 7, 2016 Report Share Posted September 7, 2016 I wouldn't ignore Woody's advise and check the wiring from the stator out first, I'm sure he's forgotten more than I know, but difficult to start and when it does run it won't rev cleanly, backfires and bogs down, all sound like air leak to me. With it being stood for so long and the blue smoke, perhaps consider the crank seals are perished. Quote Link to comment Share on other sites More sharing options...
lotus54 Posted September 7, 2016 Report Share Posted September 7, 2016 Two of my vintage OSSAs had bad crank seals. Easy to replace once you get the primary drive apart" Quote Link to comment Share on other sites More sharing options...
ronwright Posted September 7, 2016 Author Report Share Posted September 7, 2016 Thanks everyone for their input; now got a new problem which is of my making and ignorance. See new thread Ossa TR77 clutch problems Quote Link to comment Share on other sites More sharing options...
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