ET: Engine (Twin) Gunson colortune

charles d cannon jr

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My 48 HRD Rapide, built up from parts, has 67 miles on the rebuild and idles and runs a bit lumpy, but accelerates well. On the advice of another, I purchased a Gunson color tune to check the mixture at idle. My current set up is 32 mm Amal Mark 2 carbs, K and N conical filters, 106 needle jet, main jet 240, pilot 25, needle in third position and 3 1/2 slide. My pipes are a little blue. I warmed the bike up with a 12 mile ride and then inserted the colortune on the front cylinder running a fan on the engine. My mixture screw was about one and 1/2 turns out. The color flame was orange/yellow at idle, not unexpected because of the somewhat sooty plugs and lumpy running, but to my amazement, no amount of turning, in or out produced the desired blue color, supposedly representative of ideal air/fuel ratio using this tool. Any thoughts or comments will be appreciated. Thanks, Charles Cannon
 

charles d cannon jr

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I disassembled them before use to check the size of jets, etc, They were very clean and nothing, slides included, looked worn. Magneto was rebuilt by Doug Wood stateside who does good work. ATD rebuilt by me with new gear and springs and ears bent mildly to cut advance a bit for "modern fuels" I feel fairly confident on the timing--I use the piston stop and degree wheel method. Thanks for your thoughts.
 

Albervin

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If adjusting the air screw makes no difference then one or more of needle position, slide or pilot jet is wrong. Chae the needle position first. If that does not help then return needle to original position and change pilot jet. As soon as you notice an improvement on one of the changes move on then to the other possible culprit(s). Time consuming and annoying. Are your air filters the correct size for the air flow of your carbs? Good luck.
 

BigEd

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My 48 HRD Rapide, built up from parts, has 67 miles on the rebuild and idles and runs a bit lumpy, but accelerates well .........
My mixture screw was about one and 1/2 turns out. The color flame was orange/yellow at idle, not unexpected because of the somewhat sooty plugs and lumpy running, but to my amazement, no amount of turning, in or out produced the desired blue color, supposedly representative of ideal air/fuel ratio using this tool. Any thoughts or comments will be appreciated. Thanks, Charles Cannon
Assuming all jets and passages are clear:- If you are checking the colour at idle and can't get an air screw position orange to just turning blue first take the air filters off and check again. If colour improved with adjustment then your filters are wrong or blocked.
If it runs well off idle then your other settings are not far off. You could try a smaller pilot jet.
Also, check that your carbs are opening equally ideally with some vacuum type gauges or failing that by sound, listening for the slides clicking into the bottom together or, use two drills of the same size trapped under the slides when the drills should be released together as you open the throttle.
 

charles d cannon jr

Active Forum User
VOC Member
If adjusting the air screw makes no difference then one or more of needle position, slide or pilot jet is wrong. Chae the needle position first. If that does not help then return needle to original position and change pilot jet. As soon as you notice an improvement on one of the changes move on then to the other possible culprit(s). Time consuming and annoying. Are your air filters the correct size for the air flow of your carbs? Good luck.

Thanks so much. I found an old service bulletin from Triumph on "rich running" Mark 2's and their first suggestion was to make sure the cable to the choke was not tight, followed by checking float levels. Sure enough the front choke cable was tight and once loosened, the Gunson light was blue. Rear cylinder didn't respond to loosening the choke cable and the float level was ok, so needle position fiddling tomorrow! Thanks again!
 

charles d cannon jr

Active Forum User
VOC Member
Assuming all jets and passages are clear:- If you are checking the colour at idle and can't get an air screw position orange to just turning blue first take the air filters off and check again. If colour improved with adjustment then your filters are wrong or blocked.
If it runs well off idle then your other settings are not far off. You could try a smaller pilot jet.
Also, check that your carbs are opening equally ideally with some vacuum type gauges or failing that by sound, listening for the slides clicking into the bottom together or, use two drills of the same size trapped under the slides when the drills should be released together as you open the throttle.
Thanks so much for your input!
 
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