I have a mb45 mag that I added a mikuni hsr 42 to. On A small boat 15x44 with hunt deck. Highest rpm's are around 4200. From 3/4 to full throttle motor goes flat and rpms drop. Let off from full throttle to 3/4 or less throttle rpms increase and motor seems to gain power. Had 172.5 main jet in. Put 175 jet in carb and ran it. Seemed to get more rpms and power now closer to the full throttle, but with little bit of throttle left lose rpms and power until backing off of throttle. Should I continue to go up on the main jet size. seems that going up on the jet size helped getting towards a true wide open throttle. Is there something else I'm missing. What are the usual main jet size for these motors. Also got the needle clip in the middle position
Any suggestions appreciated

You need to read some new plugs after a long run or install an o2 sensor to get an idea where your fuel is. Search for info here. You still have stock heads sounds like.. possible the motor can't eat all 42mm

That thing should need closer to a 165 than 175. I'm running same internals (different cam, but similar overall compression), gutted exhaust, port and valve work, needle clip dropped one groove, and below 50 degrees with a 167.5 is pretty well close.

Thanks for info. Yes I still have original heads and dual bps exhaust. When I bought the carb I was told that the 175 main jet was what I needed to run. I will get smaller main jet and try it with new plugs. Where should needle clip be usually. Also what valve clearance should I have. My rockers have eagle printed on them.

Umm, who told you to run a 175?

Needle clip position drives throttle response through the midrange rpms (less than 3500 or so, not always the same with every engine, but above 1500 or so). I've seen them at bottom position (needle as high as it would go) and I've seen them in the the middle, and the one in between. Don't typically see an engine that wants the clip any higher than the middle. If the engine reacts well above idle through the midrange, it's fine. If you think it should be a little snappier, drop it one and see what happens.
Someone else can answer the roller rocker question. I haven't run them or ever adjusted an engine with them.

Thanks for your time and info Russ. I bought the carb from mudbuddy and they told me that was the jet I needed so that's what I have been running. Didn't think I was getting all the performance out of it so that's why I started tinkering with it.

What's actually happening is you're way too rich when the needle is past 3/4 as you're solely on the main jet. By "backing it down" you're actually leaning it out and it gets happy. The 160-165 sizes are the common jets for it. You may find 160 is good in the summer but 162.5 or 165 for winter.

Try a 167.5 and see if it helps. but with stock head ports, you may need even less. Its a game to play with every engine build.

What elevation are you at? Mb suggesting a 175 doesn't seem right.

I m in central Louisiana. When I bought carb they sent the 175 jet and told me to take out the 165 it came with. I was always worried about going to smaller jet size in fear of it running to lean. I ordered 170 and 167.5 jets to give it a try. I still have the 165 also.

I'm near monroe. Don't worry bout that 175. Put it away. If you need help with it, I'll be down in Alexandria this weekend and will have some time tomorrow evening.

I'd put that 165 in

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