Double Pumper tuning question

[tjmotter]

Installed a new Holley 700CFM DP last week to try to find a way to get rid of the hesitation I was seeing on the secondaries with my old Edelbrock. Hesitation on the secondaries is gone but I now have a hesitation when I first touch the throttle. When I set the mix rich it pretty much goes away and while this isn't the right answer, it tells me that I need to do some tuning. Ford 302/Edelbrock RPM Heads and Intake, 9.5:1 compression. Also, I am seeing 12 inches of vacuum. I have confirmed with Holley that the 700DP comes with 69 primary and 78 secondary with a 6.5 power valve. My questions are:

1) I know that I need to step down to a 5.5 Power Valve but there are two versions: high flow" (12-155) or standard flow (12-55). which should I choose?
2) Should I look at any jetting changes?

Secondaries seem to work great so I am focused on the Primary side.

Thx
Todd

[carbon fiber]

check the function of the accelerator pump. it provides the initial spray of fuel before the primaries & secondaries. take the filter off, look in the top of the carb with the engine off and "hand" pump the throttle lever on the carb. you should see a spray of gas instantly as you begin to move the lever, if not there's a problem with the accelerator pump. you can do a search for tuning/fix for it.

[tjmotter]

The pumps are working fine. As I read about this more, it seems that the Power Valve isn't the issue. In fact, with a manual transmission, the Holley site says that this is fine. Besides, its role appears to be to add fuel at WOT. Based on what I am reading it sounds like I need a bigger discharge nozzle. They are pretty cheap so I will pick one up and try it.

thx

[skullandbones]

Sounds like you are doing your homework but I have a question, why the 700 cfm? I did a search on the Holley site and used their carb search tool for a pretty aggressively build 302 and 351. It recommeneded a 600 or 650 for the 302 and 750 for the 351. These were street and race applications. There seems to be a little gap there but it does look like you are on the bubble for the sizing of your carb. The reason I was interested was because I have been searching for the optimal sized TB for my EFI 302. From what I can see, the 302 pumps about 550 to 600 cfm per 8 cylinder cycle. The stats out there very a lot depending on who you ask but that appears to be about average. I have only had experience with one double pumper, the 750. It was on a 427 Chevy. My thought is that you might be at a critical point for the 700 where the 600 or 650 might be more in the mid range of their performance bands. Just a suggestion if you continue having problems. Good luck, WEK.

[tjmotter]

700CFM wasn't my first choice but I found an eBay auction from the Holley store that was just too good to pass up. Seems everyone searching eBay centers on 650 or 750 CFM carbs which drives the price upward. The 700CFM seemed to fall between the cracks. I was pleasantly surprised when it showed up and I found that it was brand new so I am trying to see if I can take advantage of it. If I can't sort out the hesitation, I will sell it and use the proceeds to buy a smaller carb.



Sounds like you are doing your homework but I have a question, why the 700 cfm? I did a search on the Holley site and used their carb search tool for a pretty aggressively build 302 and 351. It recommeneded a 600 or 650 for the 302 and 750 for the 351. These were street and race applications. There seems to be a little gap there but it does look like you are on the bubble for the sizing of your carb. The reason I was interested was because I have been searching for the optimal sized TB for my EFI 302. From what I can see, the 302 pumps about 550 to 600 cfm per 8 cylinder cycle. The stats out there very a lot depending on who you ask but that appears to be about average. I have only had experience with one double pumper, the 750. It was on a 427 Chevy. My thought is that you might be at a critical point for the 700 where the 600 or 650 might be more in the mid range of their performance bands. Just a suggestion if you continue having problems. Good luck, WEK.

[skullandbones]

I wish I had more experience with the double pumpers. Mine was more for smaller multiple carb setups. Now I'm trying my hand at EFI. I can certainly understand you taking advantage of a good deal like that. Maybe on of the Holley techs or an expert on the forum will be able to get your 700 tweeked. It just may take a little extra time and effort. On the good side, you probably aren't going to have any lean issues. See ya. WEK.

[edwardb]

Yea, 700cfm is probably a little on the high side for a 302, even with the Edelbrock parts. I have a 650 on my 306, although with AFR 165 heads, which probably breathe a little more than the Edelbrock heads. Runs great and seems to be a perfect match. Started with a 580 -- which clearly wasn't enough -- and went to the 650. I'm a long ways from a carb expert, so can't offer any specific advice on how to tune you setup other than drive it to an experienced tuner. Some pretty good carb guys on the other forum who would probably chime in if you haven't posted there already.

[Mesa Mike]

I had a Holly 650 DP and could never get it 'dialed in' so I went with a custom build carb from Pro Systems. The 780 and runs great. I asked Pro Systems if a 780 wasn't too big for my 302 and they told me they build a carb to the specs of the engine, trans, rear end, wheel/tire size and weight. I also have AFR 165 and aggressive Comp cam, etc. Runs great now.

[pmmjarrett]

I ran 62 primaries on a 302 with 10:1, ported heads and a very agressive 304 duration cam. You litterally have to keep trying jets and checking the color of your spark plugs to see if it's jetted right.

Holly's are a pain to tune. Standard flow accelerator pump should be fine on a 302. The accelerator pump cams and the accelerator pump discharge nozzles are where you need to start tuning it to get rid of the hesitation off the line. I'd start there before swapping out the valve.

Something else you might try before swapping out jets and squirters and cams is take off the carb, set the primaries to expose about .030" of the fuel transfer slot and then adjust the secondaries for idle speed. This usually helps out off the line and low speed drivability.

Good luck.

Also try and increase the initial advance of the distributor with big carbs on small engines. Start with around 16-18* advance at idle if you are not already there.

[carbon fiber]

good point with the timing also, i forgot to mention that.