I actually have a 670 street avenger(not a 750). Has 64's in the main, 69's in the secondaries, 6.5 powervalve, 30cc accelerator pump. The carb is not opening the secondaries and I am getting a bad stumble under partial throttle. I checked the diaphram and spring for the secondaries and it all looks fine but I have to manually open them to get fuel in my rear barrels(I know it needs a engine load for it to happen but I drove the car, parked it, and the rear barrels are bone dry).

Under partial throttle I am getting fuel spitting out of the main jets.

 

Use a lighter spring on the secondary side. The carb should have come with a handful of springs. What is the fuel pressure? Check the accel pump arm to make sure it's set right.

You need to get that huge exhaust off. Your killing off the scavanging effect of the exhaust.

 

I am going to go with reducers and a 2.5 inch o/r H dumping at the rear end. Fuel pressure is 6 or so psi the last time I checked but I am going to have to buy a spring for the secondaries. The acc pump arm looks to be setup correctly, I did notice that if I back the nut off of the spring loaded rod(thus making the throttle push the acc pump open more) the stumble has subsided somewhat but it is still there. I dont know, just going to take me messing with it, still like to hear others opinions.

Thanks for your input.

 

No need to waste $$ replacing sections of that exhaust to reduce its ID, you are not increasing the scavenging effect by doing so.

 

Yeah right, They should just make 2.5 inch header tubes and dump them into 4 inch exhaust on everything. E7 heads into 1-3/4 headers and into 3 inch exhaust has very little exhaust velocity and won't pull the charge from the next cyl firing so it hurts power. You amaze me sometimes

 

Here we go.... the exaggeration/generalization card again....

So you need "velocity" all the way to the exhaust tip with a smaller ID exhaust?..... a 44% decrease in the flow cross section area past the header will cause an increase in the exhaust velocity and not hamper scavenging past 3000 rpm? ... 4500 rpms? And the ideal exhaust velocity is?

I wonder why they (even stock 302's with E7 heads) perform better with open headers allowing more A/F to be added ....... Oh that's right........ why do I get this notion of the scavenging to be a pulse/wave based phenomena adjusted/tuned for at the header/collector/extension, after which, a better flowing (as in no backpressure inducing to increase "velocity") exhaust system would allow to improve the inhaling breathing cycle of the engine?

John.... if you don't want to build on the better exhaust system you currently have.... YCYDYP, pay to add restrictions to the breathability of your current setup....

 

Joel, there is more to street cars than Wide Open Throttle. This seems to be where all your misconception generates from. The engine must first get to 6000+rpm. Do you argue that a properly sized header and exhaust system won't out perform one that is too large from 1500 rpm until the limits of the engine? I use the exaggerated generalization because it just takes your way of thinking to the next level of wrong. So in your world you can neither over carb or over size the exhaust? Lets all get a pair of dominators on a tunnel ram and put them on a stock 302 with 2.5 inch header tubes and 4 inch exhaust. Yes it's a bit extreme but compared to what you come up with at times it's just a few levels up on the wrong scale. While my exaggeration may be on level 13 of wrong you are still on about the 8th floor.

 

I think there are some misconceptions on what the car is used for. I do drive it on the street but as long as the car will not over heat and will not stall out at redlights I could care less about it manners and low end torque on the street because the only place it will get pushed is at the track. I have a C5 vette for street manners. With that being said I plan on running the 2.5 in and some cutouts right up near the headers. The car will get spray some time down the line and maybe a novi 2000. After I get tired of playing with the 302 I am going to step up to a taller deck 351 and stroke it to a 393 and run a set of GT35R's on it.

 

Misconceptions?...... let me see.... so it's easier for any engine to rev up to 6000+ rpms by choking it? I thought the more and quicker it flowed in and out = the faster it would accelerate.... .... damn those misconceptions.

A "properly sized" header and exhaust system...... based on? Some HP table generalizations? I guess a 302 with a -6° overlap @.050" has the same exhaust requirements as a 302 with +6° overlap @.050", and anything in between, regardless of the actual valve events timing...... right?

"So in (my) world (I) can neither over carb or over size the exhaust? ..... love those dicto simpliciter conclusions..... anyway, over carbing a 302 is not with a 700 or 750 cfm carburetor, too much exhaust is not a 3" full exhaust system...... why/how do I know it?..... BTSTDT many times. Are they "torqueless turds down low"?.... nope. Do they make anything below 16 MPG when daily used?...... nope. Do they accelerate better than with a 2.5" exhaust system from idle? ... after properly configured and tuned...... yep..... damn those misconceptions. ....You're right, it's definitely better for engine efficiency and performance to restrict the engine flow, rather than optimize the engine to utilize the better flowing components...... Where do I get these "misconceptions"?.......

 

Ok I am thankfull for all the input but this is getting so far off topic that it is bypassing the original questions. I know that the carb more than likely needs the 50cc accelerator pump upgrade from Holly. I have the silver spring for the vaccume secondaries but the carb spits fuel at partial throttle. I dont fully understand how a 3 inch exhaust is the reason for my fuel problems with the carb.

Not being a smart *** guys or anything like that so please dont take it that way, but would rather stay on topic.