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Old 10-01-2003, 06:48 PM
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how much jet is a jet... recomendations??

how big of a difference does a number jet make on a holley carberator... say a jump between 72-73... or 73-84.... are there just little bit of a difference or is it a good bit of a difference... also can anyone recomend what goes next?? i have a 355 chevy, holley heads 2.02/1.94 valves i think theyre 190 intake runners.. .502/290 solid roller cam, roller rockers, holley dual plane intake, 650 double pumper with a 750+ proform center section... right now i have 73s in the front and 84 in the back... in park it revs pretty well with a slight hesitation on the back barrels... but under a load (in gear and moving) it stutters a good bit... plugs are very lightly tan... still clean (not too long running it though) any suggestions what would be a good jet combo to start messing around with (proform came with 72 and 84 jets, moved the front up to 73s that i had from the old carb) thanks

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Old 10-01-2003, 09:59 PM
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I don't think jets are your problem. Sounds like your plugs are reading fine so jets are likely proper. The double pumper is too big for that engine. Those carbs are intended for drag racing applications where the engine will be at WOT most of the time and acceleration from idle isn't necessary. When you punch it, the secondaries are opening too soon causing a stumble since the carb loses it's vacuum signal and jets quit flowing gas.

You would see a big improvement with a 650cfm vacuum secondary carb. They are intended for street use and the engine runs as a 2bbl the majority of the time, thus the vacuum signal is high and fuel flow is optimal from idle to max rpm. The vacuum secondaries only open on demand when the engine is drawing more vacuum than the primaries can supply, thus the carb efficiency is maximized over a very wide rpm range.

If you want to keep the double pumper, you will need to increase the size of your accelerator pump to compensate for the bog by flooding the carb with gas during the low signal period.
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Old 10-01-2003, 11:05 PM
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you have a descent cam and heads so a 750 should be about right. huh i ran a 850 dp on a 350 but what do i know....


anyways......vaccumm carbs suck arse imho....as for jets you need to run the car up and down the street,drive it around, ya know, then check the plugs. the hesitation is from the squirter,pump cam etcetc...buy a cam assortment and start changing them around .......you will be amazed...same with the squirters.......buy a kit and change them out....its very very easy to do and you will notice a huge differance in improvement one way and disappointment the other..you will quickly pickup on the way to set up the carb....you can set it up for great throttle response from idle or killer response from freeway cruzin rpm...its one of the best things i have learned in messing with cars ever..

you can make it perform killer and get squat for milage........or the other way.......great milage ya know........

hope this helps man,..... dont be affraid to mess around with it..you can always put it back to the way you started.
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Old 10-02-2003, 07:08 AM
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id have to agree that vaccum seconderies suck... i dont know anyone that uses them... i had a 650double pumper on from before and it had beautiful responce from idle right on up... and the heads and stuff came as a kit from holley that recomended a 750dp...its not a street car as much as a drag car thats streetable... id much rather have the motor and sacrifice a little gas and a little trouble... thanks for your help though
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Old 10-02-2003, 07:33 AM
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Depending on the type of transmission (and stall speed on automatics) it varys as to which type of carburetor mechincal secondary or vacuum will work better. Normally automatic transmission equipped vehicles with less than 3000 stall speed will get better overall performance with a properly tuned vacuum secondary carburetor.

You could be running into a few issues.
First a 750 may be a touch big on this combination.
Next When you put the 750 Center Section into your 650 carburetor did you change the idle feed restrictors in your metering blocks to the size of a 750 ? If not this will cause the carburetor to be lean in the amount of fuel going through the transfer slot which can cause a hesitation.
To help cover up for these problems you will most likely need to go larger on the accelerator pump squirter nozzle size.
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Old 10-02-2003, 08:40 AM
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(32-3500stall not sure how much of it i see yet though) like i said holley recomended the 750 for that combo so i dont think its too much... i didnt change idle feed restrictors though...

i went looking for a holley tuning book this morning but didnt find any at the stores.. anyone recomend a good one for ordering?? more so tuning than rebuilding oriented
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Old 10-02-2003, 11:13 AM
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To answer your question corona, there is if memory serves me a 3% difference in flow rates between jets that differ by one digit. IE: A #70 jet will flow 3% more fuel than a #69 jet. So if you swapped #70 jets for #75 jets you would be flowing approximately 15% more fuel through the carburetor.

Also, if they are not close limit series jets they are only guaranteed to flow within 3% of their number rating. That's why you should always go up or down by 2 numbers when experimenting with jetting. Holley jets for the most part are not rated in actual flow.

Close limit series jets were guaranteed to flow within 1-1/2% of their rating. These became popular when OEM's required stricter emissions requirements/tighter tolernaces when carbs were still offered on new vehicles. It is not unusual to find close limit jets in 2 barrel carbs off of mid 70's cars for that reason. A close limit jet is numbered with -x or -1 can't remember. For example a stock jet would be stamped 70, the close limit series would have been stamped 70-1 or 70-x or 70-xx, anything more than the flow number and it was a close limit jet anyway.

If close limit jets are still available they cost more and are usually harder to find, nothing wrong with the lower tolernance jets just keep in mind that a swap from a 69 to 70 jet may yield little difference if the 69 flow is on the fat side of 3% tolerance and the 70 is on the lean end of the limit, hence the reason to change jetting in increments of 2.
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Old 10-02-2003, 01:20 PM
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There are a few tuning books out there but the only one I recall is Super tuning and modifying Holley Carbs from SA. Wickedfab has a good point get the cam kit they vary the degree of opening and the amount of CC (fuel per shot). Also shooter size dictates amount of fuel delivery. Idea of the sec. pump is to cover up "bog" to much air when the trottle is "stabbed" to wide open. In drag racing we have been lauching at what the limiter is set at in first stage 5500rpm + (pedal to metal already) so the acc. pumps are used up and are "tuned for driviablity" in the pits.
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Old 10-03-2003, 02:54 PM
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went out and got me some jets today... put a 78 in the front and 87 in the back and it fixed the big hessitation... hestitation isnt as bad from a stand still but ive got a 91 im going to try in it to see if i can get it where i want it.... thanks for your help
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Old 10-03-2003, 02:56 PM
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Let me know where you live, i'd love to open up a gas station next to your house! LOL
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Old 10-03-2003, 04:46 PM
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gas?? who cares about gas when you can go fast?? accually ill tell you where you can open a staion... a few friends of mine drive 600+hp 110octane burning 3mile to the gallon cars... at 4.20$ a gallonyou could probly make a good living on just them

oh yeah and i was able to look at the plugs in good light and they were runing a little leaner than i mentioned... right now theyre pretty good from what ive seen but still a little on the lean side
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Old 10-04-2003, 09:03 AM
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Quote:
Originally posted by corona jon
id have to agree that vaccum seconderies suck...
That is a common assumption about vacuum secondaries but the facts prove otherwise. Witness the 950cfm Holley 3-bbl carb I run on the 354 hemi in my Willys. It was made in the mid 60s for NASCAR and was very successful. It was banned by NASCAR and anything they ban is usually too good. It has a HUGE vacuum secondary pot to move that big oval throttle valve in the secondary. No one could argue that carb was made for granny's 50hp grocery getter!

Obviously you can get the double pumper to work on the street, many have including you. However, that method of tuning just masks the underlying problem and actually reduces the power potential of the engine. A double pumper was designed for a special circumstance and is a super carb in that application. Racers needed a carb that ran at WOT, period. Really wild cams and compression don't create a strong and stable enough vacuum signal to run a vacuum secondary properly. Just wouldn't work. Also they didn't need any off idle or mid range performance, so Holley came up w/ the DP. It is a monster on top end, isn't dependent on a non-existent vacuum signal, eliminates a lot of jewelery they didn't need and saves a little on weight and maintenance hassles. Common features on these carbs is lack of a choke mechanism, no power valve, holes in the primary throttle plates to make the idle work - a lot of non-streetable stuff. Unfortunately, to force this carb to perform on the bottom end, you must sacrifice optimal settings and power on the top end.

On the other hand, a vacuum secondary carb provides a much more universal carb that can be tuned to perform all through the speed range of all but the wildest engines. It will produce every bit as much top end power as a DP, yet provides bottom end drive-ability.

I would almost bet Jmarks' next 3 paychecks to a donut that a properly tuned 650 or 750 vacuum secondary carb would knock several hundredths if not several tenths off you quarter mile times compared with your compromised DP if you ran it thru the clocks.

All that being said, far be it from me to criticize someone for wanting a great looking carb on top of their engine. After all I run a 950cfm one on mine!!! Just wanted to put in my HO that vacs can be made every bit as powerful as a DP, with great advantages for the street/strip guy.
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Old 10-17-2003, 11:27 AM
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I had the similar problem running the Proform carb body.
If you do not have a 4 corner idle adjustment you need to change the secondary air bleeds to the smaller air bleeds in all 4 places, "34's are a good starting point, also the Proform does't have the passage drilled so you can not run vacuum advance unless you change the throttle plate to an aftermarket that has the vacuum port or you can do what i did and drill the Proform body for the vacuum path, just use your gaskets as a reference. After changing your air bleeds and straightening out the vacuum advance you should br able to run jets in the low 70's. THe guys at Quick Fuel are a good source for knowledge they use yje Proform bodies on their carbs. Good Luck
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