Gun Jam

Hello

I have a Holley 670 street avenger I've only had it a short time it was purchased new. I have been tuning it and it runs great except for a lean bog that occurs when power is applied slow and smooth up to fairly quickly. I'm not totally sure what it would do if I simply mashed it....


Im going to blame the accel pump at this point because it seems to have degraded in performance both visually and by the presence of lean stumble. But since the carb is practically brad new I dont know how the accel pump shot use to/should look so I'm hoping some one can compare my results to your results.

1) If I rapidly slam the throttle open using the throttle linkage I get a good solid stream from both side of the shooter nozzle

2) If I open the throttle closer to normal opening rates say 7deg a sec I get very little to no pump shot the streams are broken and appear more like drips it even appears to spit air

Shouldn't the accel pump produce clean solid streams even if the throttle is moved slow?

shouldn't the volume of fuel pissed out be equal regardless of speed at which the throttle linkage is moved?

Gun Jam

https://www.youtube.com/watch?v=dtoE...1&feature=plcp

Here is a video that shows what I was trying to describe...

Is this or is this not a suitable pump shot...

musnicki

I have a 670 as well with the same problems. I flipped the cam for a more aggressive pump shot (just unscrew it and flip it over, it should have two holes in it) and installed a different nozzle that looks like the one below. I went from a size 31 to a 35 with the brass extensions as shown in the picture. If you're going to flip the pump cam then you'll need a bigger nozzle otherwise you'll prematurely wear out your pump diaphragm.

This fixed my initial bog. I run a 302 with Performer RPM heads, cam and intake, match set from Summit. When I do floor it, say going uphill, merging with traffic onto a highway, I don't notice any difference from say 1/2 way to 3/4 way. When I take off, say for a street race, I feather it a little coming out of the clutch and I have no problems. I just took it as how carbs a just a second behind EFI. It could also be that I still don't have it tuned very good either and since no one else drives my car I never get any intelligent feed back.

Old one:



New one:

musnicki

I just watched your video, the experts can chime in but when you have sufficient vacuum those little drips of gasoline will turn to vapor and get sucked into the motor just as good as the full streams...still, you might want to invest in a pump nozzle like I posted above.

I jetted the secondaries up from 65 to 68 and up sized the primaries to 70 and then 72 and brought them back down to 68...they might even be at 65 right now...I forget.

James

rbowmar

The accelerator pump is dependent on the rate that the throttle is opened. If you slam it open, there is no way the regular jets can ramp up their gas output to match the engine's immediate need, so it gets a squirt from the a-pump. If you open it smoothly, the usual jets can ramp up gradually to keep up with the demand.

Adjusting the jets and cam will allow getting the acceleration "shot" needed to get the car going right away! See the appropriate Holley bulletin. I set mine open so it works best for the low rpm slam full open scenario, which means it will be a tad rich if i only sort of goose it.

I hope this is useful

Gun Jam

I think the accel pump is working correctly... based on what a holley tech said.

As for this momentary lean stumble that occurs when I apply easy power under 2400rpm...I think im stuck with that.

EDIT: Initially I had been using 68 primary jet and that ran a bit rich but then again it was a 100 deg out too so I switched to the 67 and that was just right with little to no stumble...then it cooled off 40deg and I think that's where I started to run into issues so I put the 68 back in and I think it more or less solved the stumble issue and runs about right too... slightly on the rich side of the scale.

Thanks guys

-Gun

rbowmar

Gun-
I am glad you got it working!
A general comment that applies: Back in the day, the old guy who taught me about tuning carbs said, "If in doubt, go rich. It won't cost that much and you sure as blazes don't want to burn your valves running them too lean!"

Simple physics: Gas being denser than air can't accelerate as fast as the throttle opens throwing the engine into a lean condition. This is solved with fixes like the accelerator pump and power valve. As the throttle is closed, the gas can't slow down as fast as the air giving a momentary rich situation and off-throttle burbles or backfiring. Those kinds of operation are one of the reasons for fuel injection and oddities like throttle-by-wire.

Imagine trying to keep the carb adjusted as it wears out or gets gunked up with gas deposits!

If anyone is following this with an eye to tuning their own carb, Don't forget Gun was using Oxygen sensors to give him feedback on the exact fuel-air mixture. It can be done without them, but getting it right will go a lot faster. Avoiding a lean condition that might lead to burnt valves or greater tendency for pre-ignition can easily pay for the the sensors. The economy alone is worth it if you drive the car much.

I hope this is useful.

musnicki

Yea, I wish that I had O2 sensors. I also err on the side of rich. I use smell and of course the backfire method. I know it's too rich if my wife tells me that the car fumes are making her eyes water or if I backfire when I'm going through a tunnel while letting off the throttle a bit. And then I use good old car response. How does it take off from stop, how about powering up a hill or power from say 50mph to 70mph.

I came in to the car thing a bit late in life, I was 32 when I bought my Mustang, I'm now 38 and attribute most of my knowledge from this forum. A lot of good people have come and gone on this forum and luckily new people continue to join this forum and provide great input to our troubles.

Gun Jam

Yes the forums are a great source of knowledge.

Thanks for everyone's help

-Gun