Hot Rod Forum banner

flat spot in throttle

7K views 24 replies 6 participants last post by  elcaminodragster 
#1 ·
first off the specs.

350sbc 10.25 CR
268h cam
rpm air gap intake
650 avs carb
hooker comp headers

well ive been messing with this problem for to long and i could use some help. when driven around normally i dont have a flat spot only when i punch it hard to WOT. if i slowly increase the throttle to WOT then no flat spot. only happens when i punch it. ive messed with timing, step up springs, plunger settings, and the secondary valve control. im most positive i need a calibration kit. but i have also read that a spacer could help out by adding more volume to the intake. i read in a few posts that the reason for some flat spots is that the air and fuel dont have enough time to mix properly. and in other cases make the flat spot worse. but you only know by trying it out. and they are cheep enough to try out. i realize by adding a spacer it kills low end. but it cant kill that much to worry about if it makes the car run better. i have a 2500 stall speed so that helps me get up in the rpms to get out of the low end. so would it be worth buying a spacer and then calibrating the carb??? or just calibrate it as is???

i realize this is a loaded question ive read a lot about spacers some say the work others say they hurt but thats for there setup not mine. but still a loaded question i know.
 
See less See more
#3 ·
carsavvycook said:
A calibration kit may do you no good. It would help to know which carb # you have and what jets and metering rods you have in it. You might just have to get larger squiters. AVS?
edelbrock AVS (adjustable valve secondaries) thunder series 1805 Manuel choke. main jet 0.095 metering rod 0.068 x 0.047. im most positive i need more fuel because when i adjusted the plunger to squirt more fuel it got rid of some of the flat spot.

http://static.summitracing.com/global/images/instructions/edl-1805_charts.pdf

what about setting the carb to #5 in the or #8 in the chart???
 
#4 · (Edited)
elcaminodragster said:
edelbrock AVS (adjustable valve secondaries) thunder series 1805 Manuel choke. main jet 0.095 metering rod 0.068 x 0.047. im most positive i need more fuel because when i adjusted the plunger to squirt more fuel it got rid of some of the flat spot.

http://static.summitracing.com/global/images/instructions/edl-1805_charts.pdf

what about setting the carb to #5 in the or #8 in the chart???
I would go with #11 just changing the jets to .098 with the .068-.047 rods to richen the carb up. You can also do this; close the choke a 1/4 of the way to see if the flat spot is gone before replacing the jets. This will richen it up close to the same as the jet change.
 
#9 ·
i think im gonna try a spacer. . . because ive read some more about them and it might might be that the air and fuel dont have enough time to fully mix when i punch it. because on the initial punch to WOT from idle or cruise speed it chokes but if i slowly bring the throttle open then it works just fine no choke or hesitation. so i figure a 20 dollar spacer cant hurt itll either help or hurt but either way itll teach me something new :)
 
#10 ·
elcaminodragster said:
i think im gonna try a spacer. . . because ive read some more about them and it might might be that the air and fuel dont have enough time to fully mix when i punch it. because on the initial punch to WOT from idle or cruise speed it chokes but if i slowly bring the throttle open then it works just fine no choke or hesitation. so i figure a 20 dollar spacer cant hurt itll either help or hurt but either way itll teach me something new :)

Makes sense to me. Who ever said "You can't teach an old dog new tricks" is wrong :D
 
#11 ·
sorry i forgot to mention i currently have the 8hg springs in there. i dont notice any real difference. i mean there is a difference but so subtle its almost not there. it seems everything ive tried only gets rid of a tiny little bit. before i did anything to it the flat spot was really bad the engine would die and id have to put it in neutral and start the engine. now the engine nearly dies.

well i get paid tomorrow so ill buy a spacer tomorrow say 1"??? since i have a divided plenum ill have to buy the 4 hole or should i buy the open spacer??? im thinking the 4 hole so i keep the intake divided.
 
#12 ·
I have only installed 1 spacer in the last 36 years, and it was on my own car when going to a Holley 4BL on my Spread bore 351CID 4BL intake(factory Ford), and that was in 1978. It never ran right and acted like it was running out of fuel, so I parked it and put it in "mothballs" in 1985.

That's my project coming out soon!
 
#13 ·
Elky, there's a thread titled "carb spacers" that's recent. In there 1Bad80 posted a link to the wiki that's worth reading. I've been running a smaller carb spacer that's been working really well. (1" aluminum). Lot's of opinions on this subject however I like what techinspector1 had to say in the above-mentioned thread.

Alot depends on the setup. Stock, mildly modified, dual-plane, single-plane, RPM range??? rear gears??

Anyway, happy reading...............I seem to do alot of it.............. :D
 
#14 ·
well i dont need a carb spacer i tossed a set of 0.65 x 0.37 rods in compared to the stock 0.68 x 0.47 rods. that helped out quite a bit but i still have a stumble. b4 the rods it nearly died now with the rods it kinda stumbles "rump RUmp RUMP VROOM!!!" lol thats the best way i can put it. this is making me think instead of dumping more fuel just buy a bigger carb. but i can just keep trying more fuel.

so far the upgrades i have done.

1) 8hg step up springs from the stock 5hg (not much change if any)
2) crane cams adjustable vac advance (timing is dead on cant get better)
3) adjusted the plunger from the middle hole to the top hole (bigger squirt of fuel)
4) rods 0.65 x 0.37 from the stock 0.68 x 0.47

with each change or upgrade its gotten a bit better and all together its gotten much better. from stock everything no changes the engine would die. now it has a "rump RUmp RUMP VROOM!!!"

so i do not need a spacer i need more fuel. can i get bigger (idk how to properly say it because i havent found the part) jets for the plunger squirter. sorry if that doesnt make sense.

or should i think about a bigger carb??? 750cfm???
 
#16 ·
well a bigger carb wont be able to come for a long time. i got plans for a new rear end and a roll cage.

but it dont hurt to start shopping around. im not a fan of holleys at all i havent heard much about any other brand of carb and i love edelbrocks because they are very low maintenance and very user friendly. but i have plenty of time to shop

im just glad i dont have such a bad flat spot anymore it was getting really annoying :spank:
 
#17 ·
In the meantime just go up 1 # in the jets with the metering rods you have then. It should make it some what more responsive with the cfm you have. I agree with you that the edlebrock carbs are more street-able and user friendly. At least you don't have to tie your dad down to the fender to drive it anymore. :D :D lol
 
#18 · (Edited)
You don't need a larger carb and you don't need a spacer. What you need to do is to properly tune the carb you've got. Thousands of 350's are out there burning rubber with a 650 or smaller carb.

Start with the components that came stock with the carb, then customize from there to suit your particular needs. Here's the owner's manual for you to go by.......

http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

From Edelbrock Q & A......
Q: What metering rods and jets do I need to run with my combination?

A: There is no “pre-determined” combination of rods and jets for any given motor, accept for altitude corrective applications, which is 4% for every 3,000 ft of elevation. Every engine is going to require a different fuel curve. The best thing to do is run the carburetor on the engine right out of the box to build a baseline, then check the spark plugs and see how they look. You need to have a light brown, or tan color on the plugs. If they are too white or too black, you need to make adjustments, refer to the tuning chart in your carburetor owner’s manual (download here). Plug reading gives you an accurate idea of what is actually taking place inside the combustion chamber. Please note, today’s re-formulated fuels can leave a lack of accurate colorization.

"Out of the box" means this.......
Metering Jets – Primary .095, Secondary .098; Metering Rods – .068 x .047; Step-Up Spring – Orange (5" Hg). Here are calibration kit part numbers.......
http://www.edelbrock.com/automotive_new/mc/carbs_acc/access_calibrate.shtml
And other stuff including pump shooter nozzles......
http://www.edelbrock.com/automotive_new/mc/carbs_acc/access_thun_perf.shtml

Optimal fuel pressure for these carbs is 5.5 lbs at the fuel inlet port on the carb body. Use a regulator or (preferably) fuel return line to modulate pressure. Mallory #4316 is a bypass type in-line regulator that could work well and be easy to install. Rig up a fuel pressure gauge. If using a wet line, DO NOT mount it inside the car. Mount it on the cowl where you can see it through the windshield. Better yet is a gauge with an isolator fluid. That way, you can mount the gauge in the car.

Start by removing the carb lid and setting the float level before you do anything else.
 
#19 ·
techinspector1 said:
What you need to do is to properly tune the carb you've got. Thousands of 350's are out there burning rubber with a 650 or smaller carb.

Start with the components that came stock with the carb, then customize from there to suit your particular needs.
thats what im trying to do. customize the carb to fit my needs.



carsavvycook said:
In the meantime just go up 1 # in the jets with the metering rods you have then. It should make it some what more responsive with the cfm you have. I agree with you that the edlebrock carbs are more street-able and user friendly. At least you don't have to tie your dad down to the fender to drive it anymore. :D :D lol

ill look into the jets in the next week or 2 i got an electrical problem i gotta take care of first. ill report back once i do the jets. they are cheap and cant cost to much like the rods (10 bucks). ya my dad was getting tired of cruise nights on McHenry the rope was digging into his hips :D lol.

well thank you very much and i shall report back after the bigger jets are installed.
 
#21 ·
techinspector1 said:
"thats what im trying to do. customize the carb to fit my needs."

I understand that and I'm trying to help. :)

i thank you and apologize if i came off rude. ive just been dealing with this problem from day one with this carb. so im a little on edge about it. i have a copy of the manual in front of me and a calibration chart for my carb in front of me as well.

i saw that on the link for the nozzles it has 3 sizes 0.024 0.033 0.043. what size do i have??? i cant find it anywhere. i have a #1805. what ever size i have in there i want to go 2 sizes bigger. because i have the pump on the highest of 3 settings so i can set the pump lower if needed after the nozzles are upgraded.
 
#23 ·
Chevrolet4x4s said:
Welcome to the wonderful world of Eddelbroke carburetors :drunk:

Shane

well im just now starting to really mess with the tuning of it. but at least im not rebuilding a holley every other week :p lol.

my sisters little no performance 289 has a 600cfm edelbrock and i didnt change anything with that carb all i did to it was set the mixture screws for the idle and the idle speed. havent touched it since. . . .aside from cleaning it with some carb spray.
 
#25 ·
1380ken said:
Do you have a fuel pressure regulator. Have you checked your fuel pressure?
Sometimes the fuel pressure will fluctuate when you hit the throttle.

i have the edelbrock performer series street fuel pump that is built to go with this carb. no need for a regulator. and there is no fuel pressure drop. ;)



my dad told me that just b4 we pulled over to tighten the rope around his waist :D
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top