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Mk1 concentric carb

derek

Well Known and Active Forum Website User
VOC Member
Having fitted a Mk1 concentric to my Comet,I find that I have problems. Difficult to start and to adjust.
Two Questions.
1. Does the cutaway affect the pilot jet settings?
2. Turning the pilot air screw in or out has no effect on running, the plug would appear to be over rich. What is the problem?
Thanks Derek.
 

Albervin

Well Known and Active Forum Website User
VOC Member
Having fitted a Mk1 concentric to my Comet,I find that I have problems. Difficult to start and to adjust.
Two Questions.
1. Does the cutaway affect the pilot jet settings?
2. Turning the pilot air screw in or out has no effect on running, the plug would appear to be over rich. What is the problem?
Thanks Derek.
What is the set up? Slide, jets, needle position etc. What brand & heat range is your plug? It is easier to make suggestions when we know what you have.
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Derek, It sounds like the holes are blocked in the carb, Some concentrics have pilot jets, some not, if the engine is soft I think a 25 jet, If it is a quick bike maybe a 30. The slide affects it as soon as you twist the grip, which you are doing when you are starting !!. As we have said before, The petrol now is not good, I only use the best, not supermarket stuff. I have just fitted a pair to my twin, Which is fast, And I found them very rich, I had to hand cut the slides to make them 3 1/2 to 4 cutaway, But first make sure your floats are at the right setting, If they are too high, It will upset all settings. Good Luck , Bill.
 

derek

Well Known and Active Forum Website User
VOC Member
Tanks, I have as supplyed, slide 3 cutaway, needlw in middle notch(Ihave just tried lowering it, 180 main jet, and I belive that a pilot jet is fitted(but do the come in different sizes) I have Mk 2 cams fitted, and hve just ordered electronic ignition to replace the magnito.! Derek.
 

mercurycrest

Well Known and Active Forum Website User
VOC Member
Hi Derek,
I agree with Bill, it sounds like you have plugged airways. Most Mk 1's have a fixed pilot jet (#25) so you maybe stuck with that size. Try your best to clean out the airways with compressed air, brake/electrical cleaner and fine wire.. anything you can think of to unplug it. If you can't open the airways back up, all is not lost. You can always place the offending carb in a 20 ton press, flatten it and use it for skipping across ponds.:D
Cheers, John
 

Peter. C

Active Forum Website User
Non-VOC Member
Hi Derek - I had similar problems with Mk 1s and gave up and went to Mikunis, there appears to be various types of float bowls for these carbs I have three that are different and someone told me there are ones about that were made for two strokes. I found they would not stay in tune and gave very unreliable tick over but were excellent when wide open.

Peter.c
 

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
The answer to your original question, does the slide influence the jet, is in practical terms, no. The tuning sequence is pilot (yours sounds blocked), slide, needle, THEN jet. There is rarely a need to go through the procedure again to find settings for the final jet choice. Needle, however, does influence slide, and to a limited degree can be used to "richen" or "weaken" the slide mixture. IME it is unusual to need a different pilot jet. The screw varies the amount of air being mixed, so there is a considerable range available.
Sid Biberman has posted several pieces on Mk I's on jtan, which having 289's I've forgotten - except that a common problem is getting the float level right, by bending the shut-off tab.
Burlen Fuel systems run a technical helpline which on the one occasion I've used it (to find out what the float level for a 289 should be) they came up with the goods.
 
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