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Post by nokturnal on Apr 16, 2019 15:43:13 GMT 1
There are a lot of threads on the 31k carbs being used on the LC engine, but none the other way around.
Any differences besides the lack of vacuum?
I am swapping a 1UA engine into my LC and was just going to use the LC carbs. Just dropped a mint on new needle/seats...
I was just going to use standard 31k, with maybe a couple extra on the main for the chambers. (still running LC airbox)
Thoughts?
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Post by Tobyjugs on Apr 16, 2019 19:30:33 GMT 1
Ive never noticed any performance differences as long as they are set up correct. Once you get up to higher than 450 main jets they don't seem to respond very well after that. Strange thing is i have two very different engines, one engine makes 65 PK and the standard 31K and 4LO 01 carbs cant hack it. The other engine makes 70 pk and uses 4LO 01 carbs with 480 main jets.
I never understood why people like the 31K carbs more. I also don't like the spring loaded mechanism for holding the end of the throttle cable in place on the later type carbs. I much prefer the loose locking plate type for the end of the throttle cable
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Post by nokturnal on Apr 17, 2019 10:42:04 GMT 1
cheers. thanks for that toby
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Post by 4l04ever on Apr 17, 2019 20:14:02 GMT 1
As standard, 31K main jets are 240 whereas 4L0-01 carbs have 220 mains, so you need to go up two sizes if putting on a YPVS, and extra sizes for your pipes.
The other difference is the drilling for fuel going to the needle/float valve is smaller, as is the hole in the brass seat, so may need reviewing if tuned to make sure you get enough fuel through.
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Post by nokturnal on Apr 18, 2019 2:20:12 GMT 1
The other difference is the drilling for fuel going to the needle/float valve is smaller, as is the hole in the brass seat, so may need reviewing if tuned to make sure you get enough fuel through. Good info. Thanks! |
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Post by Tobyjugs on Apr 18, 2019 6:58:23 GMT 1
As standard, 31K main jets are 240 whereas 4L0-01 carbs have 220 mains, so you need to go up two sizes if putting on a YPVS, and extra sizes for your pipes. The other difference is the drilling for fuel going to the needle/float valve is smaller, as is the hole in the brass seat, so may need reviewing if tuned to make sure you get enough fuel through. I got in touch with Topham Mikuni about getting some larger needle valves as they come in different sizes for VM carbs. Topham told me there was only one size available for the 31K and 4LO carbs. Rob are there two different part numbers and do you know the size difference. |
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Post by 4l04ever on Apr 18, 2019 10:03:24 GMT 1
I do not have measurements to hand, but noticed when I put the carbs side by side. The cross drilling from the fuel inlet to the valve is bigger on 31K carbs, as can be seen by the size of the blanking ball. Also, I replaced LC valve seats with YPVS ones, as I like the rubber tip, and noticed the drilling was a little larger.
When I get time I will try to measure the hole size of both types of valve seat.
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Post by Tobyjugs on Apr 18, 2019 19:35:17 GMT 1
I do not have measurements to hand, but noticed when I put the carbs side by side. The cross drilling from the fuel inlet to the valve is bigger on 31K carbs, as can be seen by the size of the blanking ball. Also, I replaced LC valve seats with YPVS ones, as I like the rubber tip, and noticed the drilling was a little larger. When I get time I will try to measure the hole size of both types of valve seat. Thanks Rob, I have replaced all my carbs with new needle valves from Topham. |
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Post by nokturnal on Apr 19, 2019 13:35:38 GMT 1
presumably when running smaller jets, as in with the airbox, the bowl should fill well enough to cope. I hope... since I just dropped $100 on 4L0 needle valves...
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Post by Tobyjugs on Apr 19, 2019 14:30:17 GMT 1
presumably when running smaller jets, as in with the airbox, the bowl should fill well enough to cope. I hope... since I just dropped $100 on 4L0 needle valves... You will be ok. Rob and I are talking about extreme situations. |
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