Carb tuning...........................

[76Tahiti460]

Ok...I also run the exact set-up you do, 660 center squirters, but what is "Squareing up?"

[LVjetboy]

Squaring means running same jets in primary and secondary. Squaring up means using all jets matched to the higher secondary (or larger) size. Usually done after blocking off the power valve. The power valve does the equivalent of increasing jets 6-10 sizes depending on vacuum signal.
My guess is blocking the power valve and rejetting may work good for full throttle race applications, but for most lake jet use, may just use more gas and hurt part throttle performance.
jer

[Rat Raft]

I keep reading over & over that Vacumn secondary carbs don't work well on tunnel rams. That is a misconsecption brought on by the "bigger is better syndrome" that has affected hot rodders ever since some one put 2 stromburg 97's on a flathead and soon they were running 6 or 8 of the little three bolted critters. Remember that a vacumn secondary carb works exactly the same as a double pumper does once the throttle plates are wide open. The vacumn secondary carb can respond better than the dp if its set up right. We want to be able to stand on the throttle from a idle and have the engine reach full rpm without missing or stumbleing. The double pumper does this by having accelarator pumps in all 4 venturies to deliver fuel until the boosters start pulling fuel through the main system. For this to work right the air velosity through the venturis & boosters must be high to create a vacumn. When this happens the air pressure drops below the ambient air pressure on the float bowls and fuel flows through the main jets & into the venturi via the boosters. These carbs need to be very close to the ideal size for everything to work right. That is why Holley makes them in 50 cfm incrediments. On the vacumn secondary carbs only the primaries have accelarator pumps. These cover for the main system until the main system picks up fuel. As the velosity increases a passage leaks some of the vacumn to below the secondary diaphram and this pulls the secondarys open. This can be done either very quickly if the carb is the correct size and you use a weaker spring or very slowly if the carb is too big or a stiff spring is used. These carbs need to be close to the right size too but its not as critical. Hense Holley doesnt make as many sizes. One problem with these and also some of the DP carbs is Holley sets them up for single carb use. They have to be tweaked for 2 carb settups. Since you have twice as much throttle plate area the main system comes in later. You have to cover for it by using bigger squirters and sometimes larger accelarator pumps. You also have to tie the secondary diaphrams together so they will each receive equal vacumn. Then you have to play with the springs. If everything is right you should have smooth power from a idle to full rpm and the same rpm as a DP. If these carbs are much too big the air velosity through the venturis will be too low and the signal to the secondarys may not be enough to open them. This makes the carb somewhat self adjust for being too big. If you force the secondarys to open anyway there may not be enough of a vacumn drop to pull fuel into the main system and the engine will go lean. This is also what happens when you use too large a dp but you have no vacumn control on the secondarys so you just hold the petal down and wonder why it doesnt run right.

[LVjetboy]

I think you're right RR about dp's and the "Bigger is Better Syndrome" or may a better fit in the "Urban Legend" category.
Was going to mention it but already beat it to death on the other site Maybe another one of those urban legends like bigger oil pans keep your oil cool type dealies. From experience I do know my secondaries aren't opening much on my TR with twin 715's at 5k, and figured this was because it didn't NEED more flow for the displacement and rpm I'm running. But, always willing to offer my boat as the floating test lab, next time out I'll force the secondaries open (against their better judgement) and see if I gain in top speed thrill factor...will let everyone know the results as always.
I'd also truely love for GS to do a lake test of different size carbs (600-850) and vs versus dp secondaries (complete with mph and rpm data) on a BBC and report the results. Could be the ultimate "Is Bigger Really Better?" jet boat article. So far no bites on this tech article...Greg?
jer

[skeepwerkzaz]

The best thing to use to check for vac leaks is a propane torch. You don't light it, you turn it on a little, and wave it around all of the suspect places, and when a leak is found, the engine speeds up.
Skeep

[cheyenneNY]

here s the math on carb cfm =[cubicinchs x rpms divided by3456] x volummetric effciency% =cfm