351w Rough Idle

[Nuzzlet 0]

Hi. So i've been having some issues with my engine running smooth. I recently cleaned the carb and re-tuned the floats and air/fuel mix. It has excellent throttle response and doesn't randomly stall. Problem is, it seems the idle jumps around quite a bit. It fluctuates between 600 and 900 rpm. Occasionally dropping as low as 500, although it has never stalled out. It actually was idling perfectly before it was warm but as it approached temp it started to jump around.

Here is a video: https://photos.app.goo.gl/1r2GnW2hwmxB13D4 Note the temp gauge is not currently working.

How smooth should the idle be? What can I try to fix it? It has a holly carb and is a manual. 

One more note, it seems the ventures only squirt to one side of the carb, not both - but it doesn't seem to bog when i step on it. Not sure if it is related. Thank you!

[Mach1 Driver 560]

It shouldn't bounce around that much- there are several very qualified people that can answer this, but they (notice I didn't say me) will need to know the engine specifics: size, carb type, any other recent changes, big lopey cam, what isn't stock, etc. With any luck Barnett will chime in.

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39 minutes ago, Mach1 Driver said:

It shouldn't bounce around that much- there are several very qualified people that can answer this, but they (notice I didn't say me) will need to know the engine specifics: size, carb type, any other recent changes, big lopey cam, what isn't stock, etc. With any luck Barnett will chime in.

Pretty sure it's what they call the "E" cam from Ford Motorsports. It's a Holley 600 with vacuum secondaries. Edelbrock performer dual plane intake and Duraspark 2 electronic ignition. I hope that is enough, if not ill explore further. 

Car / engine has basically been sitting for 10+ years and driven occasionally in that time. Don't think it's ever ran right. The manual choke is disconnected, and there is no fast idle. 

All I've done so far is clean the carb, use some fuel cleaning additive, adjust idle speed, floats, and air fuel mix. Combine the thing ran substantially better and sounds way quieter and smoother than before. The fuel additive actually seemed to make the most audible difference which really surpised me. 

Thank you for thinking in though, and hopefully I'll get some more responses soon. Appreciate it :)

[jmlay 80]

When did it begin running poorly? I would start with the basics, investigating the fuel supply. Is the carb receiving clean fuel? What does the filter look like? Any garbage in the fuel bowls? Are the needle and seats free moving?

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45 minutes ago, jmlay said:

When did it begin running poorly? I would start with the basics, investigating the fuel supply. Is the carb receiving clean fuel? What does the filter look like? Any garbage in the fuel bowls? Are the needle and seats free moving?

I'll verify all these things later on. I just let it warm up again and I put an infared temp sensor on each of the cylinders' exhaust outputs. The left half of the engine was between 200 and 250 and the right half was showing around 300 and even 375 on one of them. I immediately shut the car off.

Here is a video of the idle before it's warm: https://photos.app.goo.gl/ibmm8eiL5k6ARn5Q7

I hope I'm measuring these temps wrong... This may not have to do with the carb at all

[1969_Mach1 333]

The Ford Racing E303 cam will have kind of a rough idle.  Although slightly smoother in a 351W than a 302.  It's not a real long duration cam, but the lobe separation angle of 110 degrees is at the smaller end increasing the valve overlap for a street cam.   I'd first check for vacuum leaks.  If all is okay there might be a plugged or partially plugged idle restrictor in the metering block, or a vacuum leak within the carb itself.  With it idling cover the air horn of the carb with your hands and listen to see if the idle quality improves.  Some people will spray a small amount of carb cleaner down the throat of the carb with it idling instead and listen for improvement in the idle quality.  Careful though, keep the carb cleaner away from the distributor or coil.  I've seen fires start when spraying carb cleaner down the carb of a running engine.  Either way, if the idle quality improves it is too lean and caused by either a vacuum leak, or something wrong inside the carb.

A lean condition will also cause high and uneven exhaust temps.  Ignition timing too retarded will also cause high exhaust temps, but they will be more even.

It will never idle smooth with the E303 cam, but it should be fairly stable and not fluctuate up and down.

When you mention only one side of the venturies sprays fuel.  I'm thinking you are referring to the accelerator pump nozzles and not the booster venturies.  Either way, it sounds like the carb has some plugged internal passages.  Might be worth opening it up again and carefully cleaning every passage.  Make certain you torque the bowl screws and base plate screws correctly.  Holley has torque specs on their website.

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On 1/20/2020 at 7:06 PM, 1969_Mach1 said:

The Ford Racing E303 cam will have kind of a rough idle.  Although slightly smoother in a 351W than a 302.  It's not a real long duration cam, but the lobe separation angle of 110 degrees is at the smaller end increasing the valve overlap for a street cam.   I'd first check for vacuum leaks.  If all is okay there might be a plugged or partially plugged idle restrictor in the metering block, or a vacuum leak within the carb itself.  With it idling cover the air horn of the carb with your hands and listen to see if the idle quality improves.  Some people will spray a small amount of carb cleaner down the throat of the carb with it idling instead and listen for improvement in the idle quality.  Careful though, keep the carb cleaner away from the distributor or coil.  I've seen fires start when spraying carb cleaner down the carb of a running engine.  Either way, if the idle quality improves it is too lean and caused by either a vacuum leak, or something wrong inside the carb.

A lean condition will also cause high and uneven exhaust temps.  Ignition timing too retarded will also cause high exhaust temps, but they will be more even.

It will never idle smooth with the E303 cam, but it should be fairly stable and not fluctuate up and down.

When you mention only one side of the venturies sprays fuel.  I'm thinking you are referring to the accelerator pump nozzles and not the booster venturies.  Either way, it sounds like the carb has some plugged internal passages.  Might be worth opening it up again and carefully cleaning every passage.  Make certain you torque the bowl screws and base plate screws correctly.  Holley has torque specs on their website.

 

On 1/19/2020 at 9:13 PM, jmlay said:

When did it begin running poorly? I would start with the basics, investigating the fuel supply. Is the carb receiving clean fuel? What does the filter look like? Any garbage in the fuel bowls? Are the needle and seats free moving?

 

On 1/19/2020 at 5:41 PM, Mach1 Driver said:

It shouldn't bounce around that much- there are several very qualified people that can answer this, but they (notice I didn't say me) will need to know the engine specifics: size, carb type, any other recent changes, big lopey cam, what isn't stock, etc. With any luck Barnett will chime in.

Alright I have the carb apart. All the gaskets look like they need to be replaced. Idk if maybe one of you guys can see something I don't or suggest something. I have pictures in this post.

I should also note that turning the idle mix screws all the way does not shut the engine off.

Anyone have any ideas? Or should I just go buy a new carb and not deal with it.

 

Thank you!

[1969_Mach1 333]

I don't see anything odd in your pictures.  The most common cause of the idle mixture screws not being effective is the primary throttle plates are open too far at idle.  This exposes too much of the transfer slot which is not controlled by the idle mixture screws.  Thus, there is enough fuel passing through the exposed portion of the transfer slots to keep the motor idling.  With the carb upside down you want about 0.020" to 0.040" of the transfer slot exposed at idle.  That is just enough so they're initiated at idle eliminating any delays as the throttle is opened but not so much that idle mixture screws will not work.  If you cannot obtain the correct idle speed with the primary throttle plates adjusted to that position, you then need to open or close the secondaries a little either let more or less air into the motor.  Thus, increasing or decreasing the idle speed.

I found the above diagram.  Hope it helps.  After you have the carb upside down and get the primary throttle plates adjusted correctly.  You can get an idea of how much you can turn the idle speed screw and still maintain correct transfer slot exposure.

Here is a picture I found showing the location of the adjustment screw for the secondaries.  The preliminary adjustment is 1/4 turn past when the screw just starts to open the secondaries.  I usually end up closer to 1/2 turn on my Holley carbs.

I think the list number on your carb is 3310-6.  Is that correct?  If so, it's a 750 vacuum secondary carb.  A little big for a 351W but should work okay.

 

 

 

 

 

 

 

[jmlay 80]

With all of the vacuum ports on the carb plugged if one smothers the carb blocking all air the engine should die. If not you have a vacuum leak somewhere, may or may not be in the carb.  

[Nuzzlet 0]

2 hours ago, 1969_Mach1 said:

I don't see anything odd in your pictures.  The most common cause of the idle mixture screws not being effective is the primary throttle plates are open too far at idle.  This exposes too much of the transfer slot which is not controlled by the idle mixture screws.  Thus, there is enough fuel passing through the exposed portion of the transfer slots to keep the motor idling.  With the carb upside down you want about 0.020" to 0.040" of the transfer slot exposed at idle.  That is just enough so they're initiated at idle eliminating any delays as the throttle is opened but not so much that idle mixture screws will not work.  If you cannot obtain the correct idle speed with the primary throttle plates adjusted to that position, you then need to open or close the secondaries a little either let more or less air into the motor.  Thus, increasing or decreasing the idle speed.

I found the above diagram.  Hope it helps.  After you have the carb upside down and get the primary throttle plates adjusted correctly.  You can get an idea of how much you can turn the idle speed screw and still maintain correct transfer slot exposure.

Here is a picture I found showing the location of the adjustment screw for the secondaries.  The preliminary adjustment is 1/4 turn past when the screw just starts to open the secondaries.  I usually end up closer to 1/2 turn on my Holley carbs.

I think the list number on your carb is 3310-6.  Is that correct?  If so, it's a 750 vacuum secondary carb.  A little big for a 351W but should work okay.

 

 

 

 

 

 

 

Firstly, honastly I don't know what the specific model is, I will dig for that though. They are vacume secondaries.

 

I don't know how well you can tell from the pictures but the throttle plates don't seem to be adjusting. I found one screw that only would move a little less than a full turn, didn't want to force it (for the secondaries) and I loosened the stopper on the primary throttle yet the mechanism wouldn't rotate closed any further. Should the throttle plates be perfectly flat and at no angle during idle? 

[1969_Mach1 333]

The throttle plates are at an angle when fully closed, never flat.  After you get the base plate cleaned, before assembling anything, one check to do is make certain the throttle plates are evenly centered.  With both the primary and secondary idle speed screws backed out and the throttle plates closed as much as possible, shine a flashlight under the bottom side of the throttle plates and look at the light seeping around the edges from the top side.  You most often won't see a nice even light seeping through the cracks.  An that's okay.  But it should be relatively equal for each primary plate and equal for each secondary plate.  They are not off too often unless somebody replaced throttle plates or throttle shafts and didn't take the time to center the throttle plates.

Also, make certain the throttle shafts move nice and free before assembling the carb.

I zoomed in closer on one of your pictures, the list number I saw was 3310-6 which is a 750 CFM vacuum secondary carb.

You might want to replace the floats as well during the rebuild.  It looks like somebody added a metering block to the secondary side, which is fine.  So you can use brass floats on both the primary and secondary side if you prefer.