06-17-2022, 07:34 PM
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#1 |
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Join Date: Apr 2021
Posts: 2
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172ffm ohc engine carb jets
Hi I've been lurking the forum for a while this is first time I'm posting.I just thought I'd throw a question out there about my BMS CRP 250 ( basically a bashan storm). I found a 172 FFM overhead cam engine and replaced the pushrod CG motor with it. I immediately noticed the power improvement as the displacement is 249.9 CC versus 229. My question is should I throw bigger Jets on the carburetor with no mods on the exhaust or the air box? I took the original carburetor and jetted it at 34 and 120 up from the original 105 main and one washer on the needle. It seems to do well. I guess my overall question is should the larger displacement motor require larger jets with an otherwise stock air box and exhaust. I just don't want to run this thing too lean once it gets good and broken in. Thanks for any advice.
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06-17-2022, 10:26 PM
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#2 | |
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Join Date: Dec 2018
Location: PNW
Posts: 983
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Quote:
Tuning carburetors is a "contact sport," meaning hands on with adjustments and then evaluations. Your question is a theoretical one for which it seems you have already found a fair answer. If we think of an engine as an air pump then more CC equals more air movement and it should require larger jets. How much larger is the question and given all the variables with tuning the answer is achieved by applying good tuning techniques. Carburetors do not always act how you expect them too though and that is why it is necessary to assess and adjust and over break in that may change. If it runs well from idle to whatever your RPM limit is for break in I would evaluate the spark plug for running condition good, lean or rich. |
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06-18-2022, 03:52 AM
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#3 |
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Join Date: Apr 2021
Posts: 2
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That's kind of what I figured to be the case. As it is right now the plug color is good. We'll just have to keep an eye on it once it's fully broken in. Very much appreciate the feedback
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