Jcarteradams Posted April 10, 2020 Report Share Posted April 10, 2020 Good afternoon all I have a 76 Sherpa 350 and have been riding it on and off for a few months now. It’s my first bike but have spent many of hours getting to know it and from a non starter it works and runs well, apart from when I’m riding it will all of a sudden erratically just max out on the revs, similar to when she runs out of fuel! I then pull the clutch in and pull the kill switch, immediately. It tends to do this through the gears at any given time but usually when it’s in higher revs, and certainly keeps me on edge the whole ride. It’s like she has a mind of her own - the Bultaco is exactly the same! Is this anything to do with a blockage, how the carb is set up or the throttle? Many thanks in advance. Regards James Quote Link to comment Share on other sites More sharing options...
b40rt Posted April 10, 2020 Report Share Posted April 10, 2020 Could be an air leak, check everything. Also look for split or cracked hoses. 2 Quote Link to comment Share on other sites More sharing options...
htrdoug Posted April 10, 2020 Report Share Posted April 10, 2020 Magic combination of engine temp,exhaust temp,combustion chamber temp,tiny bit of combustion able fuel causes rpm to skyrocket. You can usually yank the plug wire off and they will still scream. Your bike is barely doing it compared to what it can do. My Sherco had a slow cooling fan and I was just hoping between sections last fall at a trial I was trialmaster so bike was getting hot soaked every time I shut it off. It tried doing it at one section so I decided to get it back to the truck. When I pulled up it went to max revs ? I calmly( to the shock of those around me) just jammed the front wheel against the front tire of my van and dropped the clutch and started up shifting. Had to get all the way to 4th gear before it started to pull down and the stalled. Waited several minutes,made sure throttle wasn’t sticking, started it and it immediately did it again. Allowed to cool completely and it was fine. Replaced cooling fan and has never done it since. I have seen rumors that if you open the throttle wide open when they go into free rev it will cool the combustion chamber enough to get it to stop revving, I have never had the wherewithal to try that cure though. Honda experimented with a 400cc race bike way long ago that put itself into free rev at part throttle,emmisions go way down I guess. I know this sounds like bunk but it really isn’t your throttle slide going wide open. 1 Quote Link to comment Share on other sites More sharing options...
htrdoug Posted April 10, 2020 Report Share Posted April 10, 2020 (edited) http://dwolsten.tripod.com/articles/jan97 https://www.greencarcongress.com/2005/10/honda_making_si.html Edited April 10, 2020 by htrdoug 1 Quote Link to comment Share on other sites More sharing options...
misscrabstick Posted April 10, 2020 Report Share Posted April 10, 2020 First thing I would do is inspect the needle valve and float, sounds like it's flooding. 1 Quote Link to comment Share on other sites More sharing options...
greychapel Posted April 10, 2020 Report Share Posted April 10, 2020 Check the throttle slide they are soft metal on the Bing carb and wear quickly and can then jam, especially on full/ high throttle openings....I know from experience ? 1 Quote Link to comment Share on other sites More sharing options...
feetupfun Posted April 10, 2020 Report Share Posted April 10, 2020 From your description it sounds like it is free revving without much power meaning an air leak. On that bike the two likely places for this would be the rubber connector between the carby and the cylinder and the igniton side crank seal. 1 Quote Link to comment Share on other sites More sharing options...
lineaway Posted April 10, 2020 Report Share Posted April 10, 2020 Could also be missing the clip to hold the needle in place inside the slide body. The needle being loose with a mind of it`s own. 1 Quote Link to comment Share on other sites More sharing options...
Jcarteradams Posted April 11, 2020 Author Report Share Posted April 11, 2020 17 hours ago, feetupfun said: From your description it sounds like it is free revving without much power meaning an air leak. On that bike the two likely places for this would be the rubber connector between the carby and the cylinder and the igniton side crank seal. It does seem to increase in power when I’m going along. It gets hairy! 19 hours ago, greychapel said: Check the throttle slide they are soft metal on the Bing carb and wear quickly and can then jam, especially on full/ high throttle openings....I know from experience ? That sounds like a good shout. It’s an Amal carb but I’m sure that won’t matter too much!? Ive had a good inspection and it does seem as though there is a slight oil leak coming out from the spark plug. Would that cause anything such as air intake etc? The plug is in pretty tight so could there be an additional washer or something to help seal it? I will certainly take a good look at the float and pin, is there anything in particular that I need to look out for? Could the head gasket need replacing too? Thanks in advance Quote Link to comment Share on other sites More sharing options...
the yukon sizzler Posted April 11, 2020 Report Share Posted April 11, 2020 I had a 325 Beamish that used to do this pretty regular. I got ride of the bike before it killed me. I used to be able to stop it by putting a gloved hand over the end of the exhaust and pushing very hard. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted April 11, 2020 Report Share Posted April 11, 2020 7 hours ago, Jcarteradams said: It does seem to increase in power when I’m going along. It gets hairy! That sounds like a good shout. It’s an Amal carb but I’m sure that won’t matter too much!? Ive had a good inspection and it does seem as though there is a slight oil leak coming out from the spark plug. Would that cause anything such as air intake etc? The plug is in pretty tight so could there be an additional washer or something to help seal it? I will certainly take a good look at the float and pin, is there anything in particular that I need to look out for? Could the head gasket need replacing too? Thanks in advance OK now that you have said that there is an increase in power when it happens it's more likely to be the throttle slide sticking. This is a common problem with AMAL carbies mainly because they suffer from rapid slide and body wear but also because the slide and body are made of metals that seems to become sticky where it wears. Water inside the slide bore can make any stickiness a lot worse. 1 1 Quote Link to comment Share on other sites More sharing options...
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