Cujo

I have dual 525 SC engines..I boost referenced both carbs, used original 6.5 power valve,91 jets in front and 92 in back..
My plugs are white..Any body else have the same set up??

MILD THUNDER

iTrader: (3)

Stock jets are 98 Front, 98 Rear, with 6.5 PV front and rear.

Boost referencing was really not necessary on those, they worked fine stock. My guess is with the cool air you're probably running in right now, 91/92 jets are not enough. If you were in Florida, you'd prob be ok. Just not in Canada.

Cujo

In all my research the boost referencing was recommended.. Yes they worked fine stock but i was tooo rich..
Always carbon buildup on the transom..tested engines prior to installation and was running 12.5 to 1 on idle and off idle..couldnt test open throttle, so just assumed would have been just as rich,now the power valve is working now..

Young Performance

An AFR or 12.5 at idle will not soot the transom. Sure, you could get it a little leaner and go into the 13's, but 12.5 is not bad at all. Are you burning any oil? That could be the source of your soot. Also, it could certainly be richer off idle in the mid and upper rpm that is causing the soot.
Eddie

MILD THUNDER

iTrader: (3)

The power valve worked on the stock setup too. The power valve being NON boost referenced, typically would be opening later in the RPM range, than the boost referenced valve. Main reason for boost referencing the power valve, is guys want to avoid a situation where the the power valve might see a vacuum at WOT, and close shut, leaning out the engine. This would really not be a problem for a 525sc.

Eddie is right. 12.5 AFR at idle wont make the transom black. The power valve really has nothing to do with that anyhow, unless it was ruptured.

My theory is, at part throttle now, say 3000RPM cruise, or planing off, , your power valve is opening being boost referenced. The Dominator carb has very large power valve channel restrictions(holes behind the power valve fuel flows thru). A typical holley 750/800/850 carb might like 6-8 jet sizes more IF you were to block off the power valve, where a dominator will want probably 8-12 jet sizes. What Im saying is, by changing the power valve opening point, you are simply introducing more fuel earlier, but no more fuel at WOT.

Lets look at the stock setup. The stock 6.5 Non referenced power valve, would probably open up around 4000RPM, when the vacuum above the blower dropped. So below 4000RPM, you were running soley off the #98 main jets. But now, below 4000, you're running on Main jets, plus the power valve's fuel. At full throttle however, you've reduced the total amount of fuel, by installing smaller main jets. So, you probably have a really rich low rpm area, and a lean high rpm area.

So now, what you'd need to do, is get back the proper amount of fuel needed for WOT, but somehow reduce the amount of fuel at low rpm/cruise. You may end up needing a 2.5 power valve(power valve opens later), with larger main jets, and or reducing primary main jet, and increasing secondary main jet to ensure you have enough fuel at wot.

Basically what you did, was simply change the PV opening time, and reduce overall fuel flow (smaller main jets). This mod would change the entire curve of the carburetor. On a heavy 38 Scarab, my guess would be that around 3000RPM, you arent seeing intake manifold "Vacuum", and by 3500, you're probably seeing a slight positive pressure (boost), which would mean your PV is open.

Are you running a PV in the secondary? If not, I would start by upping those secondary jets by quite a bit.

mike tkach

iTrader: (3)

i sugest putting the carb back to the way it was .assuming things instead of verifying can cause big problems.imo your decision to boost reference the carb was a mistake.merc would have done that if it was necessary.you really need a air/fuel meter to properly tune the engine.good luck .

MILD THUNDER

iTrader: (3)

I agree with Mike. The 525sc stock lives a long life and very reliable with normal maintenance. Leaning it down a bit simply via #93 or #94 smaller main jets was a common mod for those who boat in hot humid temps.

For a Canadian I think you went to small on jets and if you plugged the rear power valve like everyone likes to do, with your jetting , I pretty much guarantee its gonna melt down at wot if you hold it long enough

Young Performance

Brother, that is one hell of an excellent explanation.

Biggus

iTrader: (1)

For all the mindless bullschitt we wade through on this site, I'm glad to see some still are willing to put forth the effort to share their wisdom.

MILD THUNDER

iTrader: (3)

Thanks guys! I should prob switch to decaf . I can be kinda long winded in the mornings !