Would like some input from the pro's on Carb jetting for 489 with AFR's
06-20-2013, 02:36 PM
#14
List# is 9022, which is the same carb that Merc ran in the carbed HP500's. Merc ran a 6.5 PV and staggered the primary and secondary jetting to try to get more even fuel distribution. Not sure if I will need to follow suit, since I will be running a similar Dart single plane manifold. Of course, the different flow characteristics of the cam and AFR heads might throw that right out the window!
06-22-2013, 12:08 AM
#16
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For whats its worth, I did a set of carbs for a buddy of mine.
468CI
GM Rect port heads
236/245 cam
Single plane intake
Holley 750
stellings headers.
Carbs had 82 in primary with 6.5 PV. 92 in secondary no power valve. Cruise was 12.0-12.2 AFR, WOT was 11.0-11.3. Obviously a bit fat at wot for a N/A engine. Prob go 88 in secondaries with it and see how that does. He's not looking for max speed, he wants a safe tune. These carbs came with 75P, and 80 secondary. Im pretty sure if I left them stock he'd be pretty lean.
468CI
GM Rect port heads
236/245 cam
Single plane intake
Holley 750
stellings headers.
Carbs had 82 in primary with 6.5 PV. 92 in secondary no power valve. Cruise was 12.0-12.2 AFR, WOT was 11.0-11.3. Obviously a bit fat at wot for a N/A engine. Prob go 88 in secondaries with it and see how that does. He's not looking for max speed, he wants a safe tune. These carbs came with 75P, and 80 secondary. Im pretty sure if I left them stock he'd be pretty lean.
06-25-2013, 03:10 PM
#17
Found a couple of threads on some other boating related boards asking about jetting, with responses from some of the usual suspects around here:
http://www.speedwake.com/forums/showthread.php?t=19455
http://www.seriousoffshore.com/forum...b-jetting.html
Some general consensus I seem to be gathering is that the out-of-the-box jetting for Holley's carbs is usually on the conservative side and unlikely to result in an engine being burned up. Of course, if I ran WOT all weekend without a proper tune, I'm sure I would find out otherwise.
Also, it seems that Merc had some specific reasons for their unusual staggered jetting in the HP 500 carb motors, but no one seems to be duplicating that staggered jet arrangement in their builds with custom cams. I guess you would need to be running on a dyno with sensors in each exhaust port to tune to this level - otherwise you are just guessing.
Also, I still haven't totally decided on the Dart single plane over the Performer RPM Air Gap. With the AFR 265's, the AG will make huge numbers up until about 4600 RPM, at which point the Dart will pass it (but not by much). Although I do most of my running at RPM's lower than that, wouldn't this mean that I would have to step up to a much larger prop?
http://www.speedwake.com/forums/showthread.php?t=19455
http://www.seriousoffshore.com/forum...b-jetting.html
Some general consensus I seem to be gathering is that the out-of-the-box jetting for Holley's carbs is usually on the conservative side and unlikely to result in an engine being burned up. Of course, if I ran WOT all weekend without a proper tune, I'm sure I would find out otherwise.
Also, it seems that Merc had some specific reasons for their unusual staggered jetting in the HP 500 carb motors, but no one seems to be duplicating that staggered jet arrangement in their builds with custom cams. I guess you would need to be running on a dyno with sensors in each exhaust port to tune to this level - otherwise you are just guessing.
Also, I still haven't totally decided on the Dart single plane over the Performer RPM Air Gap. With the AFR 265's, the AG will make huge numbers up until about 4600 RPM, at which point the Dart will pass it (but not by much). Although I do most of my running at RPM's lower than that, wouldn't this mean that I would have to step up to a much larger prop?
Last edited by Budman II; 06-25-2013 at 03:59 PM.
