Vacume secondary spring.

[bigrob]

I have noticed my secondaries, which are vacume operated, open kind of high in the RPM range, so I have ordered a 7 pack of assorted springs as well as a "quick change" adapter to make swapping them out quite easy. I just want to find a lighter spring that will enable my secondaries to open sooner at a lower RPM. The booklet that came with my carb, says I have the Black spring in it, which is the stiffest one. Is there anything special I need to know? The instuctions & tech tips in the Holley instruction sheet for the spring swap seem pretty thorough, but I wanted some opinions as well!

Thanks!!

[vettestr]

I see you have HP and torque at the rear wheels on your sign line and it is a KC engine. If car was tweaked on the dyno it may already be at the best performance RPM. Many things influence how much and when the secondaries enter the picture so you may want to go slow here. Tweaking on the dyno is the ticket so you can see if you are gaining or loosing ground by opening them sooner.

If you feel this RPM has changed it may be from a vac leak or tune up state/ condition. Take some good checks for a baseline before making changes.

[G_Edmonds]

Since you are getting the quick change kit, you can try every spring without too much trouble. Dynos are about wide open throttle. Vacuum secondaries are more about the transition between part throttle and full throttle. The lightest spring is seldom the answer. It just opens the secondaries right away and bogs the motor without the extra fuel that a double pumper would add. Also, what seems the best/fastest is not always the best/fastest as judged by seat of the pants. In other words, make small changes to see what really works.

[RedBarchetta]

Originally Posted by G_Edmonds

Since you are getting the quick change kit, you can try every spring without too much trouble. Dynos are about wide open throttle. Vacuum secondaries are more about the transition between part throttle and full throttle. The lightest spring is seldom the answer. It just opens the secondaries right away and bogs the motor without the extra fuel that a double pumper would add. Also, what seems the best/fastest is not always the best/fastest as judged by seat of the pants. In other words, make small changes to see what really works.

Well said. Go to the next stiffest spring and work down the opening of the secondaries until it's more in line when the engine's torque curve really wants it. Then hold on.

[Silverback51]

Mechanical secondaries.

[SuperHart]

I agree with both of the above posts. Dyno values are obtained at WOT BUT most of our driving is not done at WOT. Changing the opening point by changing the spring should not affect dynoed performance unless you choose a heavy spring that prevents the secondaries from completely opening when needed. Considering that the purpose of vacuum secondary carbs is to open the secondaries only when the engine demand requires the extra air flow, tuning them with varying spring tension is a relatively easy task. If the secondaries open too quickly your engine will bog when you mash the throttle; too slowly and top end performance will suffer. Since your secondaries seem to be opening late you would want to begin your search with a spring one step LIGHTER than the black one. OH, by the way, don't judge the opening of the secondaries while you are parked and gunning the throttle. The engine must be operating under load for the secondaries to operate properly.

Keeping all that in mind and assuming that you want to tweak the secondaries, the Holley color coded spring kit will do the trick. Install your quick-change vacuum cover and refer to the color coded chart on the spring kit. Try a spring that is one step lighter than the black one in your carb, drive it for awhile and with several banzai attempts, some from standing launch and others while cruising. Keep trying lighter springs one step at a time until you hit one that causes the bog, then go back up one step heavier. That's how it is usually done.

Funny thing is, I did that on my dual quad 351W and ended up back at the stock springs. Oh well.

[vettestr]

For an extra 20 bucks we throw in 100 HP secondary spring !! All kidding aside though the start to finish tune on a chassis dyno would have identified a stumble or proper setting. No problem checking or changing the spring settings and just part of the fun of these Toys. I seem to never be happy and always looking for more myself (member of the Tim Taylor ....More Power school of thought).

[bigrob]

Thanks guys! The car was dyno'd with a different carb on her. I wanted to start with a new, fresh one, so I ordered the same model & re jetted it to my elevation here in Vegas. I am not unhappy with the way the car performs, but if I can tweek it a little and make it a little better feeling, at least to me, that is always cool. I did already think about why did I not get a carb with mechanical secondaries. Most of the ones shown, at least on Jegs, were with vacume operated ones. Is there a conversion kit to change from the vacume to mechanical? I don't think I would go to that much trouble. I'll let you know how it goes!!

FYI The black spring, which is the heaviest & the one I have, shows on this chart, that on a 350 engine, the initial opening point is at 2720 RPM & shows " Not Fully Open" for top RPM! The other 6 springs show initial open at different RPM's as well as for top RPM's. Let me try to attach this chart.... Well, so much for the attachment! You can go to Jegs and find the Holley carb springs & click on the wrench symbol, which is the instruction sheet, you will see the chart there on the second page if you are interested!

Thanks again !!

[SuperHart]

There is no vacuum to mechanical conversion kit for Holley carbs. Mechanical secondary carbs are usually designed for racing applications while vacuum secondary models are considered street carbs especially for heavier vehicles. Because the Cobra is a lighter vehicle you could get away with a mechanical carb but with a price, greater fuel consumption because of the secondary squirter; every time you mash the throttle there goes another 30 cc of fuel. Final thought - although many of us think we can get a lot more out of the engine with a mechanical secondary carb, think about why nearly all factory high performance engines came with vacuum secondary carbs. Could it be that they know something we don't? Just my opinion, but if you already have a vacuum carb, don't waste your money going for a mechanical carb. The difference, if any, won't be worth the expense. Tune the carb you now have. It will give you some fun and perform better in the long run. You don't say exactly which carb you have but if you chose a carb based on your engine displacement and performance level it should be quite close right out of the box. You also don't say whether you richened or leaned the jets and by how many steps when you "jetted to your elevation". Holley states that if you have to change the jets more than 8 numbers you have the wrong size carb. The Holley web site contains a good article outlining how to tune a Holley carb for individual applications. You might like to download it and read it over before making any significant changes from stock. Finally, always write down all changes you make so as not to forget should you need to backtrack. Have fun and don't be in a rush.

How To Choose A Carb
Another How To Choose A Carb
How To Tune Your Holley Carb

[bigrob]

The model I have is 80457S electric choke. In my instructions, it states that their carbs are setup at sea level, & you need to go down 1 size for every 2000 ft of elevation. It came with # 64 size primary jets & # 39 secondary plate. I went down 1 size for both. Vegas is at 2200 ft. I leaned out the idle mix screws very little. I also just added a K&N extreme air assembly, which I believe will let a little more air in. I am gonna do as recommended and go down 1 size spring at a time. Due to the fact that I cannot see my tach very easily, it's hard to say with any accuracy, what RPM my secondaries are kicking in at. I would say somewhere between 4-5000. My redline is 6240 I believe the PO told me. I just bough a nice small tach & am going to mount that baby where I can see it!

Thanks for all the help!!

[bigrob]

Did the spring thing and drove her today. Very cool. First of all, the paperwork that came with my carb says it comes with the black spring, which is the firmest of the 7 springs. When I took the housing apart, the spring in there was a plain spring. The spring in the original carb that was on the motor when I purchased the car, is the same plain spring as well. I purchased a spring set from Holley, the plain spring is 5th on the way up to 7, the black one. I compared the 2 from both carbs to the plain one that came in the kit, they are the same. When I compared these to the black one, the black one was much stiffer. Anyway, I went down one size from the plain which is # 5, to the purple & right in the middle of the 7, # 4. I also found the cork gasket on the vacume housing, must have moved when the carb was assembled, and was partially blocking the vacume hole....nice!! Put a new gasket on PROPERLY and have the new spring and the secondaries seem to be coming alive sooner. Did not use the quick change setup, I did'nt like the craftmenship on it, & I do not plan on doing this often. Once I find the right spring, I am done! I may go one lighter still and see what happens.

Thanks for all the help!!