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Adjusting stock carb with modified engine (Read 417 times)
Gary_in_NJ
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Re: Adjusting stock carb with modified engine
Reply #15 -
06/13/17 at 12:08:24
kojones,
Do you have the same model carb as we use in the US? Even in the US there are two versions of the Mikuni BS40SS that use different needle profiles.
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kojones
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Re: Adjusting stock carb with modified engine
Reply #16 -
06/13/17 at 21:00:34
Gary_in_NJ wrote
on 06/13/17 at 12:08:24:
kojones,
Do you have the same model carb as we use in the US? Even in the US there are two versions of the Mikuni BS40SS that use different needle profiles.
It's a BS40 for sure, but I'm not sure if it's a SS. Stock jetting was 47,5/127,5.
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kojones
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Re: Adjusting stock carb with modified engine
Reply #17 -
06/14/17 at 22:54:24
It's definately running rich now after lifting the needle, have to move the lock ring back to the center groove.
Threads on the idle mixture screw hole are busted, think I'll have to buy and rebuild another carb, the Mikuni VM36 can wait for winter
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justin_o_guy2
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Re: Adjusting stock carb with modified engine
Reply #18 -
06/15/17 at 01:14:58
Try a tap and die.
Hmm, didn't sound so hateful in my head, but it sure looks bad in print.
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kojones
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Re: Adjusting stock carb with modified engine
Reply #19 -
06/15/17 at 01:26:58
justin_o_guy2 wrote
on 06/15/17 at 01:14:58:
Try a tap and die.
Hmm, didn't sound so hateful in my head, but it sure looks bad in print.
I have a tap, but no die. The screw is one turn out and glued in place. Got another carb for cheap already.
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kojones
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Re: Adjusting stock carb with modified engine
Reply #20 -
06/16/17 at 00:52:16
Strange behavior after carb rebuild, the bike starts fine without a choke when cold but pulling the choke one notch on idle raises the rpm's significantly. Twisting the throttle from idle and releasing it gives a black puff. After engine braking the idle gets low for a few seconds when warm, then raises to normal. Mid range feels fine, needle in the center slot with the original spacer.
New needle and needle jet, otherwise same jets than before.
Feels rich, but really isn't?
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batman
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Re: Adjusting stock carb with modified engine
Reply #21 -
06/16/17 at 06:28:59
Sounds like the idle jet may be to large ,and the mix screw is in to far.when you chop the throttle the rpm drops ,and when you pull out the choke the idle speed goes up(mix screw in to far).Twisting the throttle from idle your getting black smoke (pilot jet to large).You have to remember that there are two paths for fuel coming from the idle jet and the mix screw sets in one the other is unblocked,when you blip the throttle vacuum increases pulling more fuel into the carb ,and there is a delay before the side opens ,if the idle jet is to large you'll be very rich ,(your puff of black smoke).Ithink you said your running a 55 pilot ,standard here is 52.5 ,and stock setting for the mix screw is three turns out, but I think 1 1/2 to 2 1/2 would be good for you.I think you said you were at 1 turn that sounds to lean and is why your rpm is sagging when you close the throttle.
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verslagen1
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Re: Adjusting stock carb with modified engine
Reply #22 -
06/16/17 at 07:18:15
batman wrote
on 06/16/17 at 06:28:59:
and stock setting for the mix screw is three turns out, but I think 1 1/2 to 2 1/2 would be good for you.I think you said you were at 1 turn that sounds to lean and is why your rpm is sagging when you close the throttle.
The initial set point for the idle mix is 1 1/2 turns out, at 3 turns you wanna go up a jet, 0, down.
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LANCER
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Re: Adjusting stock carb with modified engine
Reply #23 -
06/16/17 at 07:33:51
kojones wrote
on 06/16/17 at 00:52:16:
Strange behavior after carb rebuild, the
bike starts fine without a choke when cold but pulling the choke one notch on idle raises the rpm's significantly. Twisting the throttle from idle and releasing it gives a black puff.
After engine braking the idle gets low for a few seconds when warm, then raises to normal. Mid range feels fine, needle in the center slot with the original spacer.
New needle and needle jet, otherwise same jets than before.
Feels rich, but really isn't?
No choke required when cold ... pull choke rpm increases...black smoke...TOO RICH
So what are the pilot/main jets, pilot screw setting, and needle setting you have now ?
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batman
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Re: Adjusting stock carb with modified engine
Reply #24 -
06/16/17 at 19:55:43
The fact that we don't know what jet needle /needle jet you used ,or if we are even familiar with that type, and the DR cam which we don't use because we think it might be a bit to radical for the S-40 makes it hard to advise what jets you need to use . The fact that you jumped from a 47.5 to a 55 pilot (3-steps) and a 127.5 main to a 155 (11-steps) seems to me a bit drastic,even with the larger piston and cam ,it may have been better to move up jet sizes in stags. You are going to get a lot of fuel just by the fact of the piston size ,if the cam has to high a lift ,(opening the valves more than 25% of their diameter) and you've "closed the centers " to much (a to long a valve overlap ) you may have a hard time sorting out your jetting .good luck!
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LANCER
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Re: Adjusting stock carb with modified engine
Reply #25 -
06/17/17 at 06:00:36
Look up the stock jetting on the carb used on the old DR650's.
I just don't remember off hand, but it is a Mikuni CV type carb very similar to ours and is also a 40mm. I cannot remember for sure but it seems like it uses the same type of jets as well.
Anyway, that will give you a place to start.
Remember though, that the DR head has very nice
straight exhaust ports
, so with our
crooked exhaust ports
I would expect that the jetting requirements would be slightly less than the stock DR...or at least somewhere between the 2 stock carbs.
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kojones
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Re: Adjusting stock carb with modified engine
Reply #26 -
06/18/17 at 06:35:22
LANCER wrote
on 06/16/17 at 07:33:51:
No choke required when cold ... pull choke rpm increases...black smoke...TOO RICH
So what are the pilot/main jets, pilot screw setting, and needle setting you have now ?
160/55, needle clip in the center groove, pilot screw
one turn out. Slips off the threads when 1'5 turns out. Went on a 200 km trip, got some problems at first 80 km's that felt like the needle was sticking, after that ran like it should. 50 mpg.
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batman
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Re: Adjusting stock carb with modified engine
Reply #27 -
06/18/17 at 18:14:16
Kojones, you never told us what air filter your running ,if your running a K&N or similar an you oiled it, it's likely some of that oil ended up on your carb slide at first ,making it stick slightly ,before moving on.(normal)
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kojones
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Re: Adjusting stock carb with modified engine
Reply #28 -
06/20/17 at 08:13:51
batman wrote
on 06/18/17 at 18:14:16:
Kojones, you never told us what air filter your running ,if your running a K&N or similar an you oiled it, it's likely some of that oil ended up on your carb slide at first ,making it stick slightly ,before moving on.(normal)
It is a K&N indeed, in the stock airbox.
I don't think it's that, just found my bike in a middle of a pond of gasoline, I drove it an hour ago and thought that the needle is sticking again. Didn't remember to turn the petcock to off position...
Time to drain the oil, luckily it's already been there for 1000 km's and a friend bought a van
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batman
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Re: Adjusting stock carb with modified engine
Reply #29 -
06/20/17 at 09:35:00
Do you have a stock petcock ?or is the float valve sticking ,or the level in the float bowl set to high? If you have gas in your oil,you may want to check your air filter and air box drain too.
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