have any of you removed the rear power valve and jetted the secondaries up and found any et by doing this? and if so what differences did it make? also, is it about 8 jet sizes differences? thanks Dave

Dave,

I did this on my car and it made no difference whatsoever. IMO your time will be more wisely spent tuning the accelerator pumps. If you decide to block off the rear power valve be sure and add rear jet extensions. The rear power valve isn't uncovered during accleration, therefore the carb can still get fuel from the secondary bowl. If you block off the rear power valve you may go lean under hard acceleration ;) As far as jet sizes, go 6 sizes up in the rear with a plug as a good starting point.

BTW, you need to update your signature.

can you tell me a little more about the pump stroke settings, as it sits right now the car jumps right out from under me. theres no hesitation. the pump stroke settings are box stock at this point. ive already installed the extensions. do i need a 50cc rear pump? is there et to be found optimising the pump circuit? thanks, Dave

If the car is leaving as it should then there is nothing to gain going to larger pumps. There is no advantage to a PV in the secondaries in a race car. The front PV should stay in or you'll be extremely rich moving around in the pits and staging lanes. The PV should be called the economizer valve. It allows leaner mixtures at part throttle. In a car that only idles about then goes to WOT, the secondary PV serves no useful purpose. Also, since the secondary PV is higher in the metering block than the jets, it is more likely to be uncoverd during hard acceration. If you are jetted correctly, and the PV is not uncovering, you may not see any ET gains by removing it. On the other hand, it's not difficult to plug it to see what happens. Looking in the Holley catalog, they put 9-10 jet sizes larger in the carbs without a rear PV.

Originally posted by kjett:
Dave,

The rear power valve isn't uncovered during accleration, therefore the carb can still get fuel from the secondary bowl. If you block off the rear power valve you may go lean under hard acceleration I am having trouble visualizing that? Ken, please enlighten me, I thought it was the other way around (front and rear PV's)

I picked up 2mph .20 et blocking the rear PV and jetting up. I had a problem at 900ft the car would lay over, not any more.

As with Jim, Ken, I don't think that's accurate. 'Splain.

Block it, and move up 6-8 sizes. Jet extensions not required unless you cannot set your rear float correctly. K-I-S-S rules here.

Only problem I ever had with a rear PV was with a 750, think it was a stock 065, and at around ?? rpm ( maybe 6500), the darn thing would start to pull a vaccum. Now, that carb never took a trip to 7400 rpm, but if it had, the rear PV would have shut down. Nice. :eek: Blocked it, stuck a set of 78s back there and ripped around town until I got a big 850. Fool. Now 7400 was too easy.

Originally posted by Epistuff:
</font><blockquote>quote:</font><hr />Originally posted by kjett:
Dave,

The rear power valve isn't uncovered during accleration, therefore the carb can still get fuel from the secondary bowl. If you block off the rear power valve you may go lean under hard acceleration I am having trouble visualizing that? Ken, please enlighten me, I thought it was the other way around (front and rear PV's) </font>[/QUOTE]Jim, my mistake. I had it backwards. For some reason I was thinking that the power valve fuel circuit was slightly lower than that of the main jets. However, I still stand by my test results. I gained exactly ZERO performance improvement making this swap. Here's a pretty interesting article on plugging the rear PV and how to calculate increased jetting requirements after blocking the PV:

http://www.netwelding.com/double_pumper.htm

Carry on :D

Thanks Ken, I feel better now. You know your stuff and I was a bit confused there. smile.gif

Yessir! graemlins/waving.gif

IMO, Blocking the rear power valve not only ensures that it will not come uncovered, it also makes for a more consistent ET since you no longer rely on vacuum and you have a constant air/fuel ratio.

With the power valve you are keeping it lean until the valve opens to bring up the A/F mixture to optimum. (Not what you want on a race car).

Ken, I would say you did not do enough testing or did not jet up high enough to see a gain. There is not only power but consistency on the table. The link you provided for reference is discussing a vacuumed secondary carb. You need 8-10 sizes for a double pumper!

For a race car block both the front and back PV's and jet for optimum ET. Also remove the vacuum advance and lock the mechanical. Set timing at 36*, this will aid in the burning of the rich idle mixture, make the car run better, add power as well as consistency.

For a street car just do the rear, and BTW rear jet extensions are a MUST!

Originally posted by gatewayracer:
IMO, Blocking the rear power valve not only ensures that it will not come uncovered, it also makes for a more consistent ET since you no longer rely on vacuum and you have a constant air/fuel ratio.

With the power valve you are keeping it lean until the valve opens to bring up the A/F mixture to optimum. (Not what you want on a race car).

Ken, I would say you did not do enough testing or did not jet up high enough to see a gain. There is not only power but consistency on the table. The link you provided for reference is discussing a vacuumed secondary carb. You need 8-10 sizes for a double pumper!

For a race car block both the front and back PV's and jet for optimum ET. Also remove the vacuum advance and lock the mechanical. Set timing at 36*, this will aid in the burning of the rich idle mixture, make the car run better, add power as well as consistency.

For a street car just do the rear, and BTW rear jet extensions are a MUST! Ken,

Every car is different. I jetted my car up until the MPH started to fall off. I ended up with 79p/86s jetting with the rear PV blocked off. As for the PV requiring vacuum to ensure proper mixture I agree to a point. However, with a properly sized induction system the engine shouldn't be making more than 1-1.5" of vacuum under WOT. Therefore the PV shouldn't be an issue. As for your comment about timing, again every car is different. I agree that more initial timing will help an overly rich condition, which is someting you will no doubt have at idle if you remove the front PV. I've seen cars that would run best with no more than 32* total, and others that ran best with nearly 50* total. Thanks for your input, but I still stand by my test test results. In my case there was no gain to be had by blocking off the rear PV or by adding the rear jet extensions.

If the secondairy power valve is so worthless, I'm curious as to why people like George Rupert leave them in and others I know of block off the 3rd circuit on carbs that have them and install power valves ?
A friends Dragster runs 4.80's @ 148 1/8 1.06 60' with a rear power valve, no problems with it uncovering. And as far as being more consistant w/o a power valve.. how about .006 varience in 9 passes over 2 days ......
I have 2 9375 1050's, one I am blocking the 3rd circuit on and installing a P/V the other I am leaving alone .. for now .. I plan on tuning both and then doing a same day test to see which one ( if either ) works better.

I definately agree about leaving in the sec. power valve. I had one in every race on carb I ever used with absolutely no problems. Just remember that you will probabily need to go to a 3.5 or so with most semi serious cars.