I'm headed to the dragstrip Friday nite, so I was fooling around with the car today, you know practicing hole shots and such. I'm running a 455 auto with a Straightline cam, 226/238 and a 750 Holley vac secondary. A couple of times I could feel just a slight hesitation in low gear, presumably when the secondarys opened. According to conventional wisdom the spring is to weak so the secondaries are opening to soon. This is occuring about half way through low. Now last year I was running a borrowed 850 DP and the only problem I had was a bog if I got into it to quickly, but once it was moving it ran exceptionally well. I could be wrong of course but I think that a 750 carb is really to small for my car, so the hesitation seems odd, I guess Friday will tell the tale. Any thoughts anyone? o No:
Dave the 750 is big enough . The car will have to be sorted out , meaning jet size ( primary , power valve size , and vaccum secondary spring . Holley carbs and fit a variety of cars . The whole package needs to be considered including timing, and total engine vaccum . I am thinking you have a part throttle problem in either the power valve or the secondary spring . Sometimes it is worth it to have a pro tune the car . If you don't have anyone maybe my friend Duane can look at it for you . Let me know if I can help . I will be in Indy and a family reunion next week . Back in town on the 5th John
I appreciate the offer and may take you up on it. I don't even know if the carb has the stock jetting or not. Stock is 72 on the primaries and the secondary doesn't even have jets, just a fixed metering plate which is the equilivent of 75 jets. The car runs excellent just driving around, guess I'l need a spring kit. Can you put a regular metering plate in this carb, I know the old 780 carb had one.
With the metering plate installed on the vacum secondary carb, with good "G" acceleration, you could be uncovering the secondary holes at the bottom of the plate. Yes their is a conversion kit to put a regular metering plate and jets in the secondary side. I forgot the number, sorry. Also use the jet extensions on the secondary side to help on hard launches.
Rick Question, if there are no jets in the secondary metering plate how do you install jet extensions. Seems like I read somewhere a long time ago that you can enlarge the holes in the metering plate, kind of like putting larger jets in. Also does the vac secondary carb have a power valve?
I'm talking about the retrofit rear metering block with the jets. No It doesn't have a power valve and yes you can drill out the plate or buy other plates from Holley. I don't recommend drilling out passages cause its tough to get them exactly the size you want and equal from side to side. Even on Holley factory jets some have the same size hole and flow differently.
yes i found out you can get a secondary metering plate kit for this carb. I have another question, guy at tha local NAPA told me that guys have a problem with the secondary air valve opening if at all. Said the old 3310s had a piston ball arrangement for the vaccum but now have a fixed orfice that is not big enough. Anybody hear of this?
Man, I messed with the vacuum secondaries for hours and would still scratch my head. I dumped the vacuum and went with mechanical secondaries. One of the best things I ever did for drivability and performance. I don't know why that 850DP would bog on you, my 750DP is sweet.
I was told by a fellow hot rodder a while ago to eliminate the ball. Leave the ball out. Buy a spring kit for your vacuum diaphram. Its a hit and miss as to the spring to use. Move up in increments from the stock spring. I ended up using the purple one. Also the metering block is a good idea. I still have the same carb all though it's on my truck. It works great. John. :TU:
John I think my carb has the fixed orfice, I'll know when I get my spring kit and conversion so it's easy in the future to swap springs. Hopefully it's comming today! I tried reving my engine while watching to see if the secondarys are opening, if they are it's to fast for me to see. Drivability is great and idles 700 in gear. Yardley I'm sure your right, I'd love to have a DP Holley but can't afford it right now unless I sell something or find a nice used one. I borrowed a 750 DP and installed it on the car this morning, adjusted the floats and tried it out. It's awesome but doesn't feel much better than the vac secondary carb out of the hole, but it does feel better on the other end. Got to take it back today though. :ball:
you can't see the secondaries open by reving the engine. if you have compressed air and a blow gun you can check to see if they work by blowing air past the little hole in the right side primary venturi area.
Dave Thyanks a bunch for the info, I knew that the carb could not be using engine vaccum to open the secondaries because when you need it you don't have engine vaccum. It's just the old venturie effect. I have a blow gun and compressor, I'll try that pretty soon. :TU:
You can put a paper clip on the vacum rod, make a pass and then check to see how much it moved (opened).
Rick Sorry, not sure I understand how to do this. The vaccum rod is the actuator rod coming out of the bottom of diaphram, correct? How do you do this?? By the way is there a Sleepy Hollow in Illinois?? o No:
Good news, the guy with the 750 DP I was using is running tonight and when he's done he said he'd loan the carb to me til he needs it again. The last time he ran this car was five years ago, so I ought to be set until I can find a carb. :bglasses:
