New Holley Street AVenger 670 idling problems

[klinton994]

1971 Mach 1

New rebuilt 351C, top loader trans.

Holley street avenger 670 carburetor

I started my new motor last weekend, along with help from my wife. I adjusted the curb idle screw so that the motor would run a 2000 rpm for cam break in.

After running the motor for the cam break in period, i began to back off the curb idle screw to get the motor down to an idle. At approximately 1200 rpms the motor died.

I have spent the last five days attempting to find why the motor will not idle below 1200 rpm's    To start the motor, I need to turn the curb idle screw in several times. This will allow the motor to idle at 1200 rpm's when it starts. When starting, I get a lot of spitting out of the carburetor. If I do not set the idle screw as described, the motor will not start. I have also backed out the high speed idle screw on the choke side of the carburetor to be sure this wasn't contributing to the problem.

With the idle at such a high speed I am unable to time the ignition, or adjust the air/fuel mixture screws.

At 1200 rpm's if i disconnect the vacuum the advance the engine dies.

I have opened the float inspection screws on front and rear bowls and verified fuel level.

I have gently bottomed out the fuel mixture screws and backed them both out one and one half turns.

Thinking I may have a vacuum leak I have done the following.

1. Removed all vacuum hoses from the carburetor, with the exception of the vacuum advance.

2.Capped off all vacuum ports on the carburetor with rubber plugs.

3. Held one hand over the pcv hole in the valve cover, and the other hand over the crank case vent hole in the other cover. No change in idle

4. Sprayed carb cleaner completely around intake manifold, carburetor base gasket, and entire carburetor.

 

[Don C]

Have you tried looking at the timing, just to see where it is at 1,200 RPM?

 

[jpaz]

Could the accelerator pump arm be set to tight? Just a guess.

 

[stanger73]

I think you are missing allot of details, before you start randomly looking at causes for a high idle and engine stalling. A large lift, short duration cam will result in an engine that will not idle well. Poorly adjusted valves will result in a poor idle. A poorly seated intake will cause an internal vacuum leak at the base of the ports, and result in an engine that won’t idle well. Spitting out of the carb is generally a timing and or lean condition. All could be impacted by what what I just described, and there are many other contributing factors besides these.

So- Give some details about the engine build. Cam size? Hydraulic or Solid? Is Valve train adjustable - if so - Valve lash? Open or Closed Chamber Heads? Ignition- Points? Duraspark? Aftermarket?

I know you said you cant set timing. You can at least get a ballpark by unplugging the vacuum advance and revving to ~3000 rpm. Should be in the neighborhood of 36 degrees if dealing with an aftermarket distributor (MSD, Pertronix, etc.). If it is a factory distributor, the curve might be slower and full advance night not come in until closer to 4000 rpm.

What do the plugs look like after the break-in run? I would suspect that you have some, if not all that are snow white - even after the short run in period.

For what it’s worth, I have the same 670 Avenger Carb on my 73 Mach 1. It is jetted very lean out of the box - jets factory were 65/68 and 6.5 PV. I am at 72/72 and an 8.5 PV now. I went through allot of the same issues that you are describing when I rebuilt my Cleveland 3 years ago.

 

[stanger73]

I think you are missing allot of details, before you start randomly looking at causes for a high idle and engine stalling. A large cam will result in an engine that will not idle well. Poorly adjusted valves will result in a poor idle. A poorly seated intake will cause an internal vacuum leak at the base of the ports, and result in an engine that won’t idle well. Spitting out of the carb is generally a timing and or lean condition. All could be impacted by what what I just described, and there are many other contributing factors besides these.

So- Give some details about the engine build. Cam size? Hydraulic or Solid? Is Valve train adjustable - if so - Valve lash? Open or Closed Chamber Heads? Ignition- Points? Duraspark? Aftermarket?

I know you said you cant set timing. You can at least get a ballpark by unplugging the vacuum advance and revving to ~3000 rpm. Should be in the neighborhood of 36 degrees if dealing with an aftermarket distributor (MSD, Pertronix, etc.). If it is a factory distributor, the curve might be slower and full advance night not come in until closer to 4000 rpm.

What do the plugs look like after the break-in run? I would suspect that you have some, if not all that are snow white - even after the short run in period.

For what it’s worth, I have the same 670 Avenger Carb on my 73 Mach 1. It is jetted very lean out of the box - jets factory were 65/68 and 6.5 PV. I am at 72/72 I went through allot of the same issues that you are describing when I rebuilt my Cleveland 3 years ago.

 

[red351]

Had a 670 avenger that I could never get it run right even after many $$$ and time. Tried a 770 later that was an improvement straight out of box, but still needed some work. It needed the 50cc acc. pump, bigger squirter, different cam profile. jet changes and the list goes on. It finally would work with the mod. parts that came off the 670 that never works

My opinion is the avenger series aren't good carbs for just everybody. Some say they had good luck with them, others no. I finally went with a used Holley 650 DP because it is a stick car, and has worked since day one. The 670 is now just garage art.

 

[Stanglover]

For what it’s worth, I have the same 670 Avenger Carb on my 73 Mach 1.  It is jetted very lean out of the box - jets factory were 65/68 and 6.5 PV.  I am at 72/72 I went through allot of the same issues that you are describing when I rebuilt my Cleveland 3 years ago.

  Strange that you say the 670 S/A is lean out of the box. On my 71 M code with CC heads, it was quite the opposite right out of the box. Holley's are known for being on the rich side.

I liked what you said on the rest of his possible issues, that made sense to me. Just because the motor has been rebuilt, doesn't mean it was done right.

 

[Stanglover]

Had a 670 avenger that I could never get it run right even after many $$$ and time.  Tried a 770 later that was an improvement straight out of box, but still needed some work. It needed the 50cc acc. pump, bigger squirter, different cam profile. jet changes and the list goes on. It finally would work with the mod. parts that came off the 670 that never works

My opinion is the avenger series aren't good carbs for just everybody. Some say they had good luck with them, others no. I finally went with a used Holley 650 DP because it is a stick car, and has worked since day one. The 670 is now just garage art.

 red351, I KNOW what your saying. The 670 S/A is not the best out there for sure. The only way I got mine to run right was to drill the primary plates with a 3/32" hole and set the transfer slots square, secondary's just cracked and the idle screws at 1 1/2 turns out. It also needed a 1 size larger squirter. Now it runs like a champ, no idle issues, no rich smell, no bogging. Timing may have an impact on how it runs also.

 

[boilermaster]

Klinton 994,

Warm the engine up, lowest idle possible (vacuum advance disconnected) find the rpm where your advance quits advancing (hopefully 3000-3200 rpms and set the timing to 36 degrees btdc at that rpm.

Do this with an assistant if possible so you don't have to hold the throttle and adjust timing at the same time.

Did you have to advance or retard timing to get to 36 degrees btdc at max advance ?

if you had to advance timing at max timing, you should now be able to lower your idle to a respectable spec.

When you go to set your final curb idle you should be using the idle mixture screws to do that and NOT the idle stop screw.

Tell us where your timing is at after doing this at lowest possible idle speed you should be 14-16 btdc @ less than 1000 rpms.

Boilermaster

 

[klinton994]

Have you tried looking at the timing, just to see where it is at 1,200 RPM?

Yes. It is set about 13 d.

 

[boilermaster]

Klinton994, try bumping to 16 degrees and see if that allows you to lower the idle speed, then if you can get it lower, recheck timing again at the lower rpm and try resetting timing.

Even a stock distributor should show some mechanical advance at 1200 rpms.

Boilermaster

 

[Big Red Mach 1]

My Street Avenger becomes garage art tomorrow.

 

[Big Red Mach 1]

Your title says "New" Street Avenger. Are you in the return window from where you bought it?

 

[klinton994]

Klinton 994,

Warm the engine up, lowest idle possible (vacuum advance disconnected) find the rpm where your advance quits advancing (hopefully 3000-3200 rpms and set the timing to 36 degrees btdc at that rpm.

Do this with an assistant if possible so you don't have to hold the throttle and adjust timing at the same time.

Did you have to advance or retard timing to get to 36 degrees btdc at max advance ?

if you had to advance timing at max timing, you should now be able to lower your idle to a respectable spec.

When you go to set your final curb idle you should be using the idle mixture screws to do that and NOT the idle stop screw.

Tell us where your timing is at after doing this at lowest possible idle speed you should be 14-16 btdc @ less than 1000 rpms.

Boilermaster

First, I must confess, I'm not much of a gearhead. When the engine was on the stand, we static timed the engine. My friend made a mark at about 13 degrees. I'll need to do some paint scraping on the harmonic balancer before I can look for total advance.

 

[klinton994]

Klinton994, try bumping to 16 degrees and see if that allows you to lower the idle speed, then if you can get it lower, recheck timing again at the lower rpm and try resetting timing.

Even a stock distributor should show some mechanical advance at 1200 rpms.

Boilermaster

When we static timed the engine, my friend made a mark at 13 degrees. At this time, I can not adjust the timing enough to bring the 13 degree mark up to the pointer.

The engine begins to run poorly. Another possible issue, at 1200 rpm's I feel a fair amount of vacuum on the carburetor vacuum advance port. Could this be an issue. I need to purchase another vacuum gauge tomorrow.

 

[Don C]

Are you sure the mark is 13° before TDC, and not after? When you say you can't change it enough to get to the 13° mark is the problem the engine dies or the vacuum advance hits something?

Another possibility on not being able to lower the idle speed is the bottom of the throttle lever is hitting the intake manifold.

 

[Don C]

Is your harmonic balancer new? Just wondering why you would have to scrape it off? I always mark the balancer marks with white paint before installing it.

If it's not new, the outer ring may have slipped.

It really sounds to me like the timing is way retarded.

 

[stanger73]

Sooo..... Do you have specs on your engine build. Hard to provide areas to look at with only knowing you have a Cleveland with a 670 Street Avenger. My gut tells me that the intake wasn’t properly seated (ask me how I know) and that it won’t idle any lower because it has a massive vacuum leak internally (pulling air, and likely oil) into the combustion chamber. This would cause a massive lean condition at idle. The plugs would tell you that immediately. Again, I am only speculating without having more details.

I have have been wrenching on Clevelands since the late 80’s. I have had my fair share of learning moments - blown a couple up, and built my most recent about 3 years ago. It is in my 73 Mach 1.

 

[klinton994]

Thank you to all and your suggestions. A friend came over yesterday and diagnosed and fixed the problem in less than twenty minutes. I am obviously no mechanic.

I had distributed the plug wires from the distributor based on my Ford 1971 shop manual. I had the firing order correct. My friend googled Ford firing order and received a result of 1-3-7-2-6-5-4-8

We switched the spark plug wires around to this firing order, adjusted the distributor a tad, and BAM!  started right up and idled.

He told me that some cam manufacturer will change the firing order. I didn't have any notice of this in the information I received from Comp Cams, or the machine shop failed to give it to me.

Thanks guys