Holley 770 Questions

[Mattsvtcobra]

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!

[Mattsvtcobra]

I am not sure if it is the throttle side or maybe the auto choke interfering? There is play in the pedal and I can’t see where it would hang up at all as it goes in and out smoothly.

[DesertMK4]

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.

[Mattsvtcobra]

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.

[Sailfish]

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.

[Mattsvtcobra]

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.

[Gaz64]

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

[Texasdoc]

Disconnect the throttle linkage to the carb and see if it still hangs. If so, it is not the cable.

[Anthony]

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

[MaSnaka]

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

[6TNCRZY]

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.

[Mattsvtcobra]

I appreciate the suggestions! I will try these when I can and get back to you.

[Moritz55]

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..

[jhv48]

I see you’re 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. Didn’t 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.

[Mattsvtcobra]

Sorry for the delay and thank you everyone so much for the suggestions! I have a few follow up questions.

1) mechanical springs in the distrib cap. Just open it up and squirt a small shot of wd-40 on them? Any warnings here? Is there a cleaning procedure, or is it all inclusive and wd-40 only?

2) Carb base plate- What do you think the issue was there?

3) A couple of people have suggested going with a new carb and I think that would be a good choice. I'll have to start walking down that road some and see what the options are.

4) The air cleaner is resting or pushing on the back of the distrib cap. No clue why it is set this way but is this an issue? I'm sure I can find a small riser piece to get it up off of the cap.

[cycleguy55]

Originally Posted by Mattsvtcobra

Sorry for the delay and thank you everyone so much for the suggestions! I have a few follow up questions.

1) mechanical springs in the distrib cap. Just open it up and squirt a small shot of wd-40 on them? Any warnings here? Is there a cleaning procedure, or is it all inclusive and wd-40 only?

2) Carb base plate- What do you think the issue was there?

3) A couple of people have suggested going with a new carb and I think that would be a good choice. I'll have to start walking down that road some and see what the options are.

4) The air cleaner is resting or pushing on the back of the distrib cap. No clue why it is set this way but is this an issue? I'm sure I can find a small riser piece to get it up off of the cap.

WD-40 is more of a solvent than a lubricant. If you're only looking to clean it, then great. Otherwise I'd look elsewhere for a better lubricant.

Have you confirmed the throttle cable is not binding in any way, at any point in its travel? As one who has experienced a stuck throttle in a Cobra I'd look there first.

[jhv48]

Not sure why the base plate replacement worked but I can only assume that there was a restriction or leak in one of the bushings or the throttle plates were sticking. I think it cost about $150 to replace the base plate but if I were you, I would Save the money, sell the 770 and get a nice mechanical secondary 750. You won’t believe the difference.

[Mattsvtcobra]

Originally Posted by cycleguy55

Have you confirmed the throttle cable is not binding in any way, at any point in its travel? As one who has experienced a stuck throttle in a Cobra I'd look there first.

I have tested and looked over it a couple of times. Seems okay.

[Mattsvtcobra]

Originally Posted by MaSnaka

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

Gave mine a shot of wd-40 last night before bed per your suggestion. Just got back from taking my son to a friends house and we took the Cobra the RPM's increasing on me is gone! They fall back down to idle as they should. Thanks for replying and helping me!

Originally Posted by Anthony

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

When I took off the air cleaner, I noticed that the butterfly was open. The car has not been started for about a week. When I started your procedure, it closed and was up just a touch. Should the butterfly be open like that when the car is turned off?

[cycleguy55]

Originally Posted by Mattsvtcobra

When I took off the air cleaner, I noticed that the butterfly was open. The car has not been started for about a week. When I started your procedure, it closed and was up just a touch. Should the butterfly be open like that when the car is turned off?

Yes, that's the way an automatic choke operates. It will be fully open from when it was last run - the heating element in the choke pulls it open. After it's shut off the bimetallic coil cools and applies spring tension to the choke mechanism. The first touch of throttle will release the spring tension and pull the choke plate closed tight. Once it's started engine vacuum operating on the choke pull-off mechanism will pull the choke open to the set point. After that the bimetallic coil will continue heating while the ignition is on, allowing the choke plate to open further and further until fully open. There's also an interaction with the fast idle cam to keep the idle speed higher until the engine is warmed up.

More here: https://forums.holley.com/entry.php?...rel-Carburetor

This article has good instructions on adjusting carbs. It shows a pull-off adjustment you won't find on Holley's official pages - it's the adjustment I used on mine to make it run properly.
https://www.chevelles.com/threads/ad...#post-11389045