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Post by bikeman on Oct 24, 2018 15:29:10 GMT -6
hi guys. I know I'm usually answering questions not asking them but thought I would run this pass you all.
I'm experimenting with a pair of OKO carbs on the spare bike. I've got it running pretty well except for the transition between pilot and main circuits. it ticks over spot on and is the right mixture. it also revs well in fact much better than the CVK's due to the different power arc. but I have an issue with low revs before it swaps to the needle jet. there is a flat spot at this point hunting and hesitation. once the throttle is opened past 1/4 throttle it takes off like a scared rabbit.
I have tried loads of different needles, pilot jets, and pod configurations together raised and lowered fuel levels but the problems persist any ideas in case I'm missing something. to be honest it feels like a rich hump at this point [like pulling on the choke] but making the system weaker makes no difference. I have an idea it is to do with the carbs not metering the fuel correctly as it comes off the pilots but not sure how to change it.
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Post by bikeman on Oct 26, 2018 16:44:46 GMT -6
yeah I'll take that as a no idea then. not surprised though I have just about tried everything to cure it. it is obviously a issue when the carbs just come of the pilots and before the needle jet start. will just have to keep at it in hope of some inspiration before it gets too cold to test it. shame really it's 95% there just that little bit that it seems no one has a answer for. coincidentally it seems I'm not the only one with this issue a few threads I have read from other bike forums speak of the same issue when using sliders on bikes that were meant to run CVK's but as usual the thread ends before a solution is posted.
bit like this one then.
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Post by bikeman on Nov 15, 2018 15:52:26 GMT -6
hi all. Well for the ones interested "that will probably just be me then from the response of the enquiry I made" just thought I would communicate the fact that I have found the answer to the problem. in short [as this has taken months to find] and after 6 sets of needles and 10 sets of jets without any seemingly improvement in the situation. I decided to go back to basics and look at the slide carb design.
it became apparent that these carbs are designed to work with large bore 2 stroke engines and so have in there design a power jet. this injects fuel as the slides open directly into the inlet tract to stabilise the mixture on a 2 stroke engine that has virtually constant vacuum while it is running.
a 4 stoke works differently having a lull in vacuum once the inlet valves close. the upshot of this is the power jet over riches the mixture just off idle to 1/2 throttle [the point of the most vacuum] and where the issue was. changing to a weaker set up doesn't work as it leaves the idle far too weak .
the answer was to disable the power jet by blocking it with solder. and then re-jet and re-needle the carb to suit the 4 stroke set up. [coincidentally these were very close to the stock set up] FYI. there is a similar carb on sale without the power jet. so I would suggest if anyone thought of doing this conversion use those instead.
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Post by yaatri on Nov 15, 2018 22:52:52 GMT -6
hi all. Well for the ones interested "that will probably just be me then from the response of the enquiry I made" just thought I would communicate the fact that I have found the answer to the problem. in short [as this has taken months to find] and after 6 sets of needles and 10 sets of jets without any seemingly improvement in the situation. I decided to go back to basics and look at the slide carb design. it became apparent that these carbs are designed to work with large bore 2 stroke engines and so have in there design a power jet. this injects fuel as the slides open directly into the inlet tract to stabilise the mixture on a 2 stroke engine that has virtually constant vacuum while it is running. a 4 stoke works differently having a lull in vacuum once the inlet valves close. the upshot of this is the power jet over riches the mixture just off idle to 1/2 throttle [the point of the most vacuum] and where the issue was. changing to a weaker set up doesn't work as it leaves the idle far too weak . the answer was to disable the power jet by blocking it with solder. and then re-jet and re-needle the carb to suit the 4 stroke set up. [coincidentally these were very close to the stock set up] FYI. there is a similar carb on sale without the power jet. so I would suggest if anyone thought of doing this conversion use those instead. Brilliant bikeman. Quite unrelated to your problem, I was just thinking about the difference between carburettors for two stroke vs four stroke engines. Very nice work bikeman. Thanks for your posts.
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Post by knoizy on Nov 17, 2018 13:59:10 GMT -6
Respect to you Bikeman for putting the work in here, somebody at some point will be grateful for the info...
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Post by bikeman on Nov 18, 2018 18:53:05 GMT -6
yeah thanks for that. when you are the first to do something. [or appear to be the first] there are no reference points to work from it's all down to experimentation and hard work not to mention the cost. if in some way this episode helps someone save a load of time and expense it is worth it.
BTW I can say these carbs really do transform the bike, it's not so much a power increase but the way that power is used.
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