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  • 1 Post By Sailfish

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  #1 (permalink)  
Old 06-24-2020, 05:16 AM
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Default Holley 770 Questions

Hi guys, I am looking for some help with a couple of issues. Roush 342 with Holley 770 CFM Carburetor w/ vacuum secondaries and electric choke. I've had the car about 3 months.

1) I've noticed lately, and out of no where, that when the engine warms up and you take off at any speed, when you go to shift that the RPMs will not drop back down to idle. They will stay at 3k. If you tap the throttle, RPMs will then reduce down. It's almost like there is a cruise control built in. Something is sticking somewhere? This issue was not there when I first took the car. (EDIT I guess it is more likely to be a throttle cable issue and not part of the carb... but wasn't sure if there could be some kind of vac leak, etc)

2) There seems to be a bog at around 2k RPMs (going more off of sound than a lack of power) and once speed increases it goes away and you are at what feels like full power. I have changed the plugs, but not the wires. I was thinking it was more of a carb setting than anything else?

Thank you for your help!

Last edited by Mattsvtcobra; 06-24-2020 at 06:11 AM..
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  #2 (permalink)  
Old 06-24-2020, 10:48 AM
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I am not sure if it is the throttle side or maybe the auto choke interfering? There is play in the pedal and I canít see where it would hang up at all as it goes in and out smoothly.
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Old 06-24-2020, 11:55 AM
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Now I have a Holley 670CFM with vac secondary. I see two things in your picture: no secondary idle mixture screw (the small lower hole on the base of the secondary float bowl). There is also one on the other side (4 all together) I had to use a spacer (1 inch) below carb to clear both sides linkage. How much play in throttle cable? If too much the hard cable housing will jump out of the fitting of the cable bracket, or at least catch on it when decelerating quickly. I attach 3 photos of mine. Hope this helps.
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Old 06-24-2020, 12:22 PM
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I am new to the carb world, so I will look into what it means to be missing the ones in the back. The pass. side also doesn't have one in it.

The only stuff I can see as far as markings go

8106B
4100
10569

The 770 was on the engine when it was installed 15 years ago, but I have to assume this is the same one.
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Old 06-24-2020, 01:17 PM
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I don't know if this will help much, but I'll throw it out there.

You mentioned the engine was installed 15 years ago. I owned a cobra with a very similar engine (342-s/n 105 with a 770 carb). I purchased the car new in 2007 and experienced all the same issues you're experiencing. I contacted the engine builder (Roush) and asked for help (the car was still under warranty). Roush was looking into it, and in the meantime, I made numerous modifications to the carb (new jets, new squirter, new cam, etc.). Ultimately, we were able to fix the vast majority of issues. Shortly after, Roush called and validated my carb was defective (a paint code on the carb verified it was part of a defective batch). Roush sent me a brand new 770 replacement carb, at no cost, and it worked like a champ. I put over 20,000 miles on the car and never made a carb adjustment.
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Old 06-24-2020, 01:42 PM
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Wow. This 342 is # 047. Seems strong! I am actually over in West Orlando! Looks like I have some digging to do on why the secondary idle screws are missing, etc. Can't get through to Holley as the hold times are crazy and I just sit on hold.
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Old 06-26-2020, 05:36 PM
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Unlikely to have secondary idle mixture screws missing.
First up, the engine would not idle, and there would be fuel stains at the side of the metering blocks where the screws would go.

So i would say you have a earlier 2 corner idle carb.

Being electric choke, I would look at the fast idle cam falls down under the it's own weight, since if it flips back under acceleration etc, AND doesn't fall down, you get a higher than idle engine speed.

Also make sure the carb is even torqued, no thick gaskets etc.

Please check the front of the choke tower for the list number.

Gary
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Old 06-26-2020, 06:56 PM
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Disconnect the throttle linkage to the carb and see if it still hangs. If so, it is not the cable.
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Old 06-27-2020, 04:30 AM
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Like gaz64 stated, I think it’s a fast idle cam issue. As your carburetor looks spotless, I don’t think the fast idle cam is binding. Maybe the spring in the choke housing has changed over time? Anyways, I think the choke needs adjusted, as the fast idle cam is connected to the choke linkage.

Here’s how I do it. The engine has to be cold. Take the air cleaner off. On the passenger side of the engine, Loosen the 3 screws on the perimeter of the choke housing, That are clamping the adjustment plate tight. Grab the throttle linkage and manually open to full throttle, and the choke should spring into the closed position. While holding the throttle fully open, with a standard screwdriver, adjust/ twist the loosened choke adjustment plate to where you see the choke plate start to open. I usually adjust it so there’s about a 1/4” gap between the edge of the choke plate and the carburetor housing, so the choke is almost, but not completely closed. Let off the throttle, and tighten the 3 choke plate hold down screws. See if that helps.

Regarding the engine bogging, pull the distributor cap and check for corrosion
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Old 06-27-2020, 03:41 PM
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Do you have a mechanical timing advance on the distributor?? MSD. I noticed my RPMs were hanging up while lifting my foot. I cleaned and lubed the exterior of the carb (AED) and it would still happen occasionally. I had an epiphany and thought what if the mechanical timing on my distributor was hanging up. That would increase RPMs. So I popped the cap, everything looked good but I gave it a shot of wd40 where the weights and springs move. Problem gone so far. BTW- I have no vacuum advance on the distributor.

Hope this helps...
John
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Old 06-28-2020, 10:58 AM
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John, I think you are on it, if the advance springs are soft for a quick curve and the weights stick a little the engine will stay advanced and hold the RPM up. I have experienced that myself a while ago. I would make sure everything is working freely in the distributor.
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Old 06-29-2020, 11:59 AM
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I appreciate the suggestions! I will try these when I can and get back to you.
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Old 06-29-2020, 03:48 PM
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Hi Matt ... I have a 770cfm Holley on a Roush 427SR (even though the valve cover is 427R !!).. my Carb is identical to yours.. 2 mixture screws in the front (passenger & driver side) .. none in back... That's Normal.. nothing to worry about.

Note .. I have a solid rod linkage so my pedal moves precisely when the carb accelerator moves.. The suggestion to ensure your Return Spring is Stiff Enough to always return the carb accelerator is a geat suggestion.

Additionally as several already mentioned if you have a Mechanical Advance clean the springs and levers. I just replaced my MSD Disty Cap & Rotor as they were in terrible shape. 21k+ miles on them and never replaced by the prior owner! In the process I cleaned the Mechanical Vacuum springs/arms under the Disty Cap as a preventative measure ....

Good luck and hang in there..
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Last edited by Moritz55; 06-29-2020 at 03:57 PM..
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Old 06-30-2020, 07:11 PM
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I see youíre in Florida so corrosion in the distributor is a good possibility.
My Roush 427sr exhibited similar symptoms. I tried the heavier spring, lower idle speed, disconnected the choke and cleaned and adjusted the distributor. Didnít fix it. Finally got rid of the problem by replacing the entire carb base plate. That did it.
And if you want to get rid of that bog, consider a nice 750cfm with mechanical secondaries. I swapped mine and it woke up the engine. Vacuum secondary carbs are not the best choice for our light cars. Even the Holley website will not recommend a vacuum secondary carb for a light car.
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