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Carb tuning

Old 09-04-2013, 09:47 AM
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Also, Dan, I wouldn't plug the PV because to compensate for the huge PCVR's, youd probably have to run a monster primary main jet, to keep WOT AFR where it is. Then your primary light cruise at 3000-3500 will be pig rich.
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Old 09-04-2013, 09:55 AM
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Originally Posted by MILD THUNDER
Same thing I told him mike. His power valve channel restrictions are .0935. that is HUGE. At 3500RPM, he was pulling 8" of vacuum under the carb. At 4000, 0'' of vacuum. Somewhere in between there his PV opens, and his 02 reading goes from low 12's to 10's. I believe when the Power valve opens, that is causing his rich spot, rather than the secondaries starting to open. Rich afr means more fuel than air. In order to go from 12.2 AFR, to 10's, within a hundred rpm or so, its getting a large amount of fuel from somewhere. That somewhere I believe is the powervalve. Typical power valve channel passages on a 4150 style carb, are much smaller than his. The only carbs holley used those large restrictions is on a dominator. Which we all know were pretty much meant for one thing, WOT racing.

We all know when you plug a power valve, you have to jet up to compensate for fuel flow. What determines how much you need to jet up, is based on the size of the power valve restrictions. In otherwords, when the power valve opens, it can add 6 sizes worth of jet, 8 sizes, 10 sizes, 12 sizes, etc. If the power valve restrictions have been enlarged, then you will get a LARGE amount of fuel when the valve opens.

Your typical carb setups are usually 6-10 jet sizes larger in the secondary when the valve is plugged. Following this traditional setup on his carbs, netted a very rich AFR at wot. So it was recommended to Dan to put smaller secondary jets in to lean the wot afr. So now he is 2-3 sizes larger on the secondary, but has the rich midrange.

What I would do, is drill and tap the restrictions, and go back to what a standard 850 holley uses for size. Then, install larger secondary jets. This should clean the midrange issue, and still have a good wot AFR, and also much better fuel distribution. We don't want 75% of fuel coming from primaries, and 25% coming from secondarys on sideways mounted carbs. You then get into one bank running leaner than the other. Decreasing secondary jet is just band aiding the problem. Secondary fuel flow is proportional to the airflow thru the venturies. As airflow increases, so should fuel flow. Hence why as rpm goes up, AFR should go down (richer). Not leaner like his is doing. And ultimately, have a even amount of fuel coming from the primaries and secondaries at WOT. Just my opinion.

This is getting into a area most average weekend guys don't get into or talk about much. 10 Years ago this was unheard of here on oso. Nobody had/used widebands. The amount of fuel that can be changed thru small changes in the pcvr size is huge. Mike, you and I learned that when tuning those blow thru aerosol carbs. Next set of carbs hands down will be quick fuels, if you are into tuning them to work like EFI. Theres no need to drill and tap things, its already done for you. Air bleeds, PCVR's, emulsion jets, idle feed jets, etc. A properly set up carb should have no "lean' spots, and no "fat'' spots. It should be linear from idle to wot. And to get there, in a boat, you just cant do it without a wideband meter, on the water. Not a bench, and not a dyno. Theres much more to a carb then main jets, especially if you want it to have a EFI like fuel curve.
You think like I do.... I was never a huge fan of tuning on a dyno and forgetting about it. Conditions change. I'd routinely be at the track just doing tuning passes on the car, get in 4th early and just run the whole rpm range while datalogging. You can't particularly see max power from timing that way, that was where the dyno did come in, but as for fuel, you can't get any better then real world conditions.

If you don't mind me asking because alot of it is still over my head, what's a general guideline you go by when setting up the carb. i.e. idle adjustment for x range, primary for y range, secondary for z range and how to dial in the power valve and throttle enrichment? I know every setup likes something different, and try as I may I've never found a really good breakdown on setting it all up.
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Old 09-04-2013, 06:24 PM
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joe,c&s sells the jets dan needs,6 sizes,4 of each size for 24 bucks,i agree with you on the sizing,go with whatever holley puts in the 850 metering block and it can be as even as efi,it is just a matter of drilling&tapping the metering block.i like the quickfuel carbs if starting new,they are boost referenced and are quality parts,and are fairly priced not to mention they look great.if you dont feel like drilling&tapping,make dan spring for 4 new metering blocks from quickfuel.
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Old 09-04-2013, 08:26 PM
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2 circuit and non boost referenced carbs? no wonder you beating your heads off the walls.....wait until you chop your props,,gonna have to do some more jetting....have you guys tryed 3.5 and 4.0 power valves?
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Old 09-04-2013, 10:18 PM
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Yes, the engine needs to be at operating temp, with no choke it will be a little lean and may not want to idle with first startup, but after it warm it will run like a champ. You may have to push the the accelerator arm by hand while warming up. Pumping the throttle with add more air and lean the engine even more
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Old 09-04-2013, 10:31 PM
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SKATERMAC,a blown engine with a air-fuel ratio of 14.7 at 6 lbs boost at full throttle will burn down in seconds,take that to the bank!
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Old 09-04-2013, 10:36 PM
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14.7 afr is the manufacture setting for the best possible emissions . A roots blown marine engine will more than likely puke when shifted into gear with a afr of 14.7 at idle.

And 14.7 while underway will for sure smoke pistons and or valves.
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Old 09-04-2013, 10:41 PM
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back when i was in high school[about 100 years ago]they thought and taught that 14.7 was the perfect afr,we sure have changed our way of thinking in the new world.

Last edited by mike tkach; 09-04-2013 at 10:46 PM.
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Old 09-04-2013, 10:44 PM
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Originally Posted by Skatermac
Yes, the engine needs to be at operating temp, with no choke it will be a little lean and may not want to idle with first startup, but after it warm it will run like a champ. You may have to push the the accelerator arm by hand while warming up. Pumping the throttle with add more air and lean the engine even more
why on earth would you tune an engine so lean at idle that you need to pump the acc pump lever to keep it running,,just curious.
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Old 09-04-2013, 10:44 PM
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Originally Posted by mike tkach
SKATERMAC,a blown engine with a air-fuel ratio of 14.7 at 6 lbs boost at full throttle will burn down in seconds,take that to the bank!
I said 14.7 at an idle his wot AFR will be 12.5. The info he posted to start with confirms this. Most stock carbs lean out under power someone spent a lot of time to correct that problem on his setup so why send him down the wrong road????
..
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