Holley base plate swap between 4780 and 870

[68charger440]

I have a 870 street avenger that has a very loose secondary shaft.  My idle mixture screws have no effect. 

My first question is will a loose leaking secondary shaft cause the idle mixture screws to be ineffective?  The butterflies are set correct with squared up transition slots.   
My second question is can I swap the base plate from a 4780 onto the 870 street avenger?

[Ghoste]

A loose shaft will cause problems, its kind of like a vacuum leak.

[68charger440]

A loose shaft will cause problems, its kind of like a vacuum leak.

But will one of the problems it causes be "no response at the idle mixture screws"?

[Ghoste]

In effect yes, you would have more air coming in than you can compensate for.

[68charger440]

In effect yes, you would have more air coming in than you can compensate for.

Thanks... I'm trying to decide if I should buy a base plate assembly for $150 in the hopes that it will fix the unresponsive mixture screws or just buy a new carb.  I have had good luck buying used carbs in the past and rebuilding them, but maybe this time it is just not going to work out.

Would you care to take a stab at my odds of a new base plate assembly fixing it?  50/50 or less?

[c00nhunterjoe]

You are wasting money imo based on your other thread. You have a tiny cam in a huge stroker with a mild carb and your issues are stemming from that. You need more cam and a 950 carb if it were my car.

[68charger440]

You are wasting money imo based on your other thread. You have a tiny cam in a huge stroker with a mild carb and your issues are stemming from that. You need more cam and a 950 carb if it were my car.

I realize that the cam is way too small, and that I am leaving a lot of HP on the table.  I do want to change all of that as well as the heads when I can save he money to do it right, but how would my current setup make the idle mixture screws unresponsive?

[Paul G]

I realize that the cam is way too small, and that I am leaving a lot of HP on the table.  I do want to change all of that as well as the heads when I can save he money to do it right, but how would my current setup make the idle mixture screws unresponsive?

The primaries are are not even being used at idle under the conditions you are experiencing (air leakage getting past the secondary shaft). I would think you could block off the primaries completely and the engine would still idle. You can close the butterfly on the secondaries until the engine dies. Have you experimented with that yet? The secondaries will get enough air just from the leakage getting past the worn out shaft in the baseplate.

[68charger440]

I realize that the cam is way too small, and that I am leaving a lot of HP on the table.  I do want to change all of that as well as the heads when I can save he money to do it right, but how would my current setup make the idle mixture screws unresponsive?

The primaries are are not even being used at idle under the conditions you are experiencing (air leakage getting past the secondary shaft). I would think you could block off the primaries completely and the engine would still idle. You can close the butterfly on the secondaries until the engine dies. Have you experimented with that yet? The secondaries will get enough air just from the leakage getting past the worn out shaft in the baseplate.

Yes, I did close off the secondaries to the point that none of the transition slot is exposed on the secondaries, but the mixture screws were still unresponsive.  It actually idles pretty good, but I need to be able to adjust the mixture. 
There is not enough air getting through the shaft that I can close off the primaries completely though. 
Do you think if I replace the base plate assembly, I will gain back the mixture screws functionality?

[Ghoste]

Maybe try sealing the top of the secondary air horn with your hand and adjusting them to make certain that is the problem.

[68charger440]

Maybe try sealing the top of the secondary air horn with your hand and adjusting them to make certain that is the problem.

I'll try that.

[68charger440]

Maybe try sealing the top of the secondary air horn with your hand and adjusting them to make certain that is the problem.

I'll try that.

Can't get that to work because I don't have enough hands and my wife is a sissy about getting close to a running motor.  Go figure!
I did notice that if I put clockwise pressure on the secondary shaft  (Trying closing it further) it lowers the idle to 800 rpm, but it still won't allow the mixture screws to work.  I think the loose shaft is preventing the secondaries from getting a good tight seal. 
My options at this point are:
1. Buy the base plate assembly for roughly $125 on the assumption that the leaking shaft is the problem
2. Use a Carter competition 750 I have.  It runs OK, but there is a very noticeable drop off in power.
3. Use the original Holley 750.  This carb has no option to install a electric choke, and I would rather not rig a choke cable inside the car.  This is also a bit small for a 500" stroker.
4. Rebuild a Holley 4780 800 Double pumper I have in the garage.  Would need to convert it to electric choke, and it may also be a bit small.
5. Buy a new carb.  Ouch!

I am a little afraid to keep pouring money into the 870 Street Avenger because I have read about some horror stories of people trying in vain to get them dialed it and finally giving up.  I know most tuning problems are people who don't know what they are doing, but I have also read a good number of posts from people who were obviously very knowledgeable and yet still couldn't get it dialed it.  I may not be a Holley guru, but I am no idiot either, and I am very meticulous about my work, so at this point I'm pretty sure this carb can not be tuned as is.  I think the problem is the leaking secondary shaft, but you never know until you spend the money and time to swap it out.

[BSB67]

My first question is will a loose leaking secondary shaft cause the idle mixture screws to be ineffective?


1) Hard to imagine that the throttle shaft on the secondary side of a carb being too wore out, pretty unlikely, and very doubtful that it is the reason for your ineffective mixture screw adjustments.
2) Adjust the rear throttle plate/shaft to get it more closed if you think that is the problem or what the motor wants.  A worn shaft will not prevent you from making this adjustment.
3) Check the throttle bores, I believe that the 4780 is 1 11/16 and the 870 is 1 3/4.

You probably have a big ole vacuum leak somewhere.

[68charger440]

My first question is will a loose leaking secondary shaft cause the idle mixture screws to be ineffective?


1) Hard to imagine that the throttle shaft on the secondary side of a carb being too wore out, pretty unlikely, and very doubtful that it is the reason for your ineffective mixture screw adjustments.
2) Adjust the rear throttle plate/shaft to get it more closed if you think that is the problem or what the motor wants.  A worn shaft will not prevent you from making this adjustment.
3) Check the throttle bores, I believe that the 4780 is 1 11/16 and the 870 is 1 3/4.

You probably have a big ole vacuum leak somewhere.

Do you mean a vacuum leak within the carb?  I checked the intake, and I can,t find a vacuum leak there.  I have between 13 and 15" of vacuum at idle in gear depending on idle rpm.
I did check the throttle bores, and your correct, so that is out of the mix.
Right now the secondaries are closed to the point where there is none of the transfer slot showing, and the primaries are square.  What else could be causing the innefective screws?

[Paul G]

I think it is leading back to a blown power valve. No idle change with adjustment of the idle mixture screws is the classic symptom of a blown power valve. If the secondary throttle blades are closed, idle mix screws all the way in, the engine must be getting fuel and air from somewhere or it cant run. The power valve, when blown, will dribble fuel in to the intake just under the throttle blades. I would swap out that power valve, or at least make sure it is threaded in tightly enough. Unless there is reason not to. 

A vacuum leak at the intake, or anywhere in the vacuum lines will still need to have fuel to burn, to run the engine. Sounds like that carb is ready for the bin in my opinion. Something is goofed up with it. Especially if the other carb works good.

But then again, I am no expert.   

[68charger440]

I think it is leading back to a blown power valve. No idle change with adjustment of the idle mixture screws is the classic symptom of a blown power valve. If the secondary throttle blades are closed, idle mix screws all the way in, the engine must be getting fuel and air from somewhere or it cant run. The power valve, when blown, will dribble fuel in to the intake just under the throttle blades. I would swap out that power valve, or at least make sure it is threaded in tightly enough. Unless there is reason not to. 

A vacuum leak at the intake, or anywhere in the vacuum lines will still need to have fuel to burn, to run the engine. Sounds like that carb is ready for the bin in my opinion. Something is goofed up with it. Especially if the other carb works good.

But then again, I am no expert.   

I might try some new power valves or maybe I'll just shoot the street avenger just to get some satisfaction out of it.  How can I test the power valves? 

[firefighter3931]

I think it is leading back to a blown power valve. No idle change with adjustment of the idle mixture screws is the classic symptom of a blown power valve. If the secondary throttle blades are closed, idle mix screws all the way in, the engine must be getting fuel and air from somewhere or it cant run. The power valve, when blown, will dribble fuel in to the intake just under the throttle blades. I would swap out that power valve, or at least make sure it is threaded in tightly enough. Unless there is reason not to. 

A vacuum leak at the intake, or anywhere in the vacuum lines will still need to have fuel to burn, to run the engine. Sounds like that carb is ready for the bin in my opinion. Something is goofed up with it. Especially if the other carb works good.

But then again, I am no expert.   

I might try some new power valves or maybe I'll just shoot the street avenger just to get some satisfaction out of it.  How can I test the power valves? 

Paul is correct ; if the power valve is blown you will be able to keep the engine running with the idle mixture screws completely seated/closed. This sounds like what is happening based on your posts ?

I'd pop a new 8.5PV and PV gasket in that Avenger before condemning it 


Ron

[68charger440]

I'll give it a shot. I can always take out my frustrations and feed it to the shredder later!

[68charger440]

OK I tried two new 8.5 power valves front and back.  Same results.  Let me see if I can stir up a firestorm here with this question. 
How about drilling the primary throttle plates?  The original Holley that came on the 440 had the holes drilled at the factory by Holley, and that was on a bone stock 440 motors.  No radical cam or low vacuum on those motors either.  Mine being a 500" motor, it might benefit from more air without having to open the plates to get it, and since I am about ready to shoot this carb anyway I'm not to worried about ruining it. Thoughts anyone?

[firefighter3931]

This carb sure is being stubborn !!    

There has to be a leak somewhere. It won't hurt a thing to drill the throttle plates so give that a try. Could there be a leak at the vacuum pod for the secondaries ? I think there's supposed to be a cork or rubber gasket there ?

What other carbs do you have to play with ? I saw you mentioned a #4780 which is an 800dp. I would focus my attention on that one if it's servicable  


Ron

[Paul G]

I have learned a lot about Holleys from Ron and others on this forum and doing my own internet research. So to solve some of my own problems in the end, I have a 750 Holley sitting on the shelf in the garage that is not happy. Not a happy carb, but I am so much more happy with that carb on the shelf and Holley 670 on the car. Just a word for the wise. 

And, BTW, most Holley owners have lots of Holleys sitting around the garage. Something I learned doing research on how to fix my Holley 750. It was not repairable, found that out after spending way to much money on repair/rebuild parts. My 670 works great, but I want another 750. Probably buy one used again that someone else gave up on. After I spend way more money on it than it is worth, trying to make it right, it will end up on the shelf with the other Holley.

Might be a good idea to just buy a new one.   

[68charger440]

This carb sure is being stubborn !!    

There has to be a leak somewhere. It won't hurt a thing to drill the throttle plates so give that a try. Could there be a leak at the vacuum pod for the secondaries ? I think there's supposed to be a cork or rubber gasket there ?

What other carbs do you have to play with ? I saw you mentioned a #4780 which is an 800dp. I would focus my attention on that one if it's servicable  


Ron

Well, I know better than to declare success, but I will declare tremendous progress!  I went ahead and drilled 1/8" holes in the throttle plates, and presto-magico the idle mixture screws both work like a charm now.  I can kill the motor if I turn them in and increase the idle if I turn them out.  Now I can at least attempt to tune it properly.  I have been tempted to drill the throttle plates for some time, but I read too many articles warning not to do it.  I just had to get to the point where I didn't care if I wrecked the carb, and when Ron gave the green light to take a shot at it, well that was all I needed.  I was afraid you guys would yell "DON'T DO IT"
I'll post more after I attempt to tune it next.
Thanks to all!

[firefighter3931]

Good stuff ! 

I know most will recommend against drilling the throttle blades but i've done it many times with great results. 

The Holley ultra hp carburators now come with the throttle blades drilled directly from the manufacturer. That should tell us something. 

Keep working on it....you'll get there 



Ron

[BSB67]

Good stuff ! 

I know most will recommend against drilling the throttle blades but i've done it many times with great results. 


Ron

Certainly a time and place for it.  Not sure this is it.

Ron, do you know that he has a cam with 204°/214° at 0.050"

[Nickrc3]

68CHARGER440, can you provide a pic of the specific drilling location on the primary throttle blades? I have a similar situation with limited tuning range on the four idle-mixture screws with a new Proform 750 carb.
Thanks.