Slant Six Forum :: engine won't idle below 1500 rpm

Sometimes, not all the time, when a new carb is installed the installer forgets to lengthen or properly adjust the throttle cable so that it becomes too short on the new carb and holds the throttle open. I only say that because I have done it myself and spent several hours chasing down why the carb won't drop to curb idle numerous times.

Thanks for all the replies. I'm leaning against a throttle linkage problem, as I just made up a whole new one (because the current carb sits 90 deg from where the bbd sat) with a bellcrank in it. Nonetheless, I'll defintely check that. I think it was probably a vacuum leak, but now I have to see what the elctrical problem is. It was running last night and just cut off like someone turned off the key and now I get so spark. Thinking it's maybe the HEI module. Ughhh...Anybody know of a good way to test them (ex. should have 12 VDC between these two terminals or xxx many ohms between these two terminals, etc.)?

Thanks

I replaced the module and it fired right up and idled good. I guess the vacuum leak was coming from the carb to manifold seal (or lack thereof) I used a .25" x 2" x 3" piece of aluminum for the heat sink for the module. I put the bolts though the module through the heat sink through the firewall. Should I be using something different for the heat sink. I tried to follow the HEI article on here pretty close and I think that's what the author said to use.

Anyways, she runs and even drives now! I drove for the very first time tonight in over six months. Felt good to get behind the wheel of the old clunker again. I was not hard on it in the least, but could still easily tell it had more power than before I rebuilt it. Here in a few weekends I think I'll drive up to Arkansas or some other close state to get my first 500 miles on the motor. After that I can have fun with it... : )

Now, back to the real world of wrenching on the truck: what's the (best) way to go about tuning a new motor? I know approximately what the timing needs to be, but have no idea on the fuel mix. Is there a systematic/scientific way to get hesitation out of the motor when you step on the gas, or is just all in adjusting something and then going and driving it and seeing how it does and if need be, adjust more? Is this correct: you can use a vacuum gauge to find the highest vacuum (highest power) by adjusting the fuel mixture?

Thanks,
Stephan

Author:	pinkiedog64 [ Fri Jan 23, 2009 5:01 pm ]
Post subject:	engine won't idle below 1500 rpm
Playing with the motor more today and still can't get it to idle below 1500 rpm or maybe even a little more. The timing is at about 12-15 at roughy 1500 rpm. Carb is Edelbrock 500 mechanical secondaries, head milled .100", big valves, ported head, erson .465" 280 deg cam. It runs above that rpm, but when left to "idle" it sits at about 1500 or a little less for a few seconds then peters off and quits. I can keep it going by gassing it, although when I do, it revs up and stays up for a few seconds (even after I've let off the gas) and then come back down. I've played with the fuel mix, idle screw, and timing and can't seem to make too much progress. Any ideas on what the problem is and what I can do to fix it? Thanks, Stephan

Author:	Doc [ Fri Jan 23, 2009 5:04 pm ]
Post subject:
It sound like a vacuum leak. DD

Author:	Aggressive Ted [ Fri Jan 23, 2009 6:30 pm ]
Post subject:
Big time vacuum leak...........check your intake manifold gaskets. Did you use a RemFlex gasket??? or the Aussie gasket? Better get sprayer out and do some testing. Might as well spray around the base of the carb too!

Author:	56 Dodge [ Fri Jan 23, 2009 6:47 pm ]
Post subject:
I agree I had a similar problem be sure ALL unused ports on your carb are plugged. The gaskets both Remflex and the Aussie are the only way to go I used Remflex on the intake/ exhaust and Aussie on the Hot box so I could have the best of both worlds.

Author:	pinkiedog64 [ Fri Jan 23, 2009 8:19 pm ]
Post subject:
I went out and checked the carb gaskets (the ones between the carb spacers and the manifold). Turns out the spacer right on top of the manifold wasn't near as tight as it could have been-enough to where I could easily slide a .013" feeler gauge through. I tightened it down and now there is no gap. As for the manifold-to-head gasket, I used the one from Clifford that came with the headers. Bad choice? What's the best way to check for vacuum leaks?

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engine won't idle below 1500 rpm https://slantsix.org/forum/viewtopic.php?t=33465	Page 1 of 2

Author:	Reed [ Fri Jan 23, 2009 9:04 pm ]
Post subject:
Sometimes, not all the time, when a new carb is installed the installer forgets to lengthen or properly adjust the throttle cable so that it becomes too short on the new carb and holds the throttle open. I only say that because I have done it myself and spent several hours chasing down why the carb won't drop to curb idle numerous times.

Author:	madmax/6 [ Fri Jan 23, 2009 9:54 pm ]
Post subject:
spray wd 40 around base,if it changes,its a vacume leak,thats how I start to check it for leaks,mark

Author:	MiDi [ Fri Jan 23, 2009 10:03 pm ]
Post subject:
Quote: Sometimes, not all the time, when a new carb is installed the installer forgets to lengthen or properly adjust the throttle cable so that it becomes too short on the new carb and holds the throttle open. I only say that because I have done it myself and spent several hours chasing down why the carb won't drop to curb idle numerous times. i just got done fixing that on my car, i switched to a new throttle linkage and i set the cable too tight, and i was wondering why i couldn't get the damn idle down. after playing with the cable i found the problem and now i idle sweet n low. i also did that with my manual choke cable, i was wondering why it was sticking i put a tiny bit of slack in it and problem fixed!

Author:	pinkiedog64 [ Sat Jan 24, 2009 9:23 am ]
Post subject:
Thanks for all the replies. I'm leaning against a throttle linkage problem, as I just made up a whole new one (because the current carb sits 90 deg from where the bbd sat) with a bellcrank in it. Nonetheless, I'll defintely check that. I think it was probably a vacuum leak, but now I have to see what the elctrical problem is. It was running last night and just cut off like someone turned off the key and now I get so spark. Thinking it's maybe the HEI module. Ughhh...Anybody know of a good way to test them (ex. should have 12 VDC between these two terminals or xxx many ohms between these two terminals, etc.)? Thanks

Author:	THOR [ Sat Jan 24, 2009 5:43 pm ]
Post subject:
Quote: Thanks for all the replies. I'm leaning against a throttle linkage problem, as I just made up a whole new one (because the current carb sits 90 deg from where the bbd sat) with a bellcrank in it. Nonetheless, I'll defintely check that. I think it was probably a vacuum leak, but now I have to see what the elctrical problem is. It was running last night and just cut off like someone turned off the key and now I get so spark. Thinking it's maybe the HEI module. Ughhh...Anybody know of a good way to test them (ex. should have 12 VDC between these two terminals or xxx many ohms between these two terminals, etc.)? Thanks Make sure you have an adequate heatsink for the module... otherwise they fry... ~THOR~

Author:	pinkiedog64 [ Sat Jan 24, 2009 7:36 pm ]
Post subject:
I replaced the module and it fired right up and idled good. I guess the vacuum leak was coming from the carb to manifold seal (or lack thereof) I used a .25" x 2" x 3" piece of aluminum for the heat sink for the module. I put the bolts though the module through the heat sink through the firewall. Should I be using something different for the heat sink. I tried to follow the HEI article on here pretty close and I think that's what the author said to use. Anyways, she runs and even drives now! I drove for the very first time tonight in over six months. Felt good to get behind the wheel of the old clunker again. I was not hard on it in the least, but could still easily tell it had more power than before I rebuilt it. Here in a few weekends I think I'll drive up to Arkansas or some other close state to get my first 500 miles on the motor. After that I can have fun with it... : ) Now, back to the real world of wrenching on the truck: what's the (best) way to go about tuning a new motor? I know approximately what the timing needs to be, but have no idea on the fuel mix. Is there a systematic/scientific way to get hesitation out of the motor when you step on the gas, or is just all in adjusting something and then going and driving it and seeing how it does and if need be, adjust more? Is this correct: you can use a vacuum gauge to find the highest vacuum (highest power) by adjusting the fuel mixture? Thanks, Stephan

Author:	THOR [ Sun Jan 25, 2009 1:24 am ]
Post subject:
Quote: I replaced the module and it fired right up and idled good. I guess the vacuum leak was coming from the carb to manifold seal (or lack thereof) I used a .25" x 2" x 3" piece of aluminum for the heat sink for the module. I put the bolts though the module through the heat sink through the firewall. Should I be using something different for the heat sink. I tried to follow the HEI article on here pretty close and I think that's what the author said to use. Sounds good to me, but did you use thermal paste on it? If not, it will help if you do. Arctic Silver is the name of the one I use on my computer processors. I have also seen the heatsinks mounted to the outside of the module, and another piece of aluminum on the underside. Are there fins on the heatsink you have? They would be a great help, but I've seen people get away with not using them. Just keep an eye on it, and an extra module in the glovebox.. Quote: Anyways, she runs and even drives now! I drove for the very first time tonight in over six months. Felt good to get behind the wheel of the old clunker again. I was not hard on it in the least, but could still easily tell it had more power than before I rebuilt it. Here in a few weekends I think I'll drive up to Arkansas or some other close state to get my first 500 miles on the motor. After that I can have fun with it... : ) Awesome! Quote: Now, back to the real world of wrenching on the truck: what's the (best) way to go about tuning a new motor? I know approximately what the timing needs to be, but have no idea on the fuel mix. Is there a systematic/scientific way to get hesitation out of the motor when you step on the gas, or is just all in adjusting something and then going and driving it and seeing how it does and if need be, adjust more? Is this correct: you can use a vacuum gauge to find the highest vacuum (highest power) by adjusting the fuel mixture? Thanks, Stephan If you have a hesitation when you first step on the gas, and the timing is around 8-10 BTDC at idle, you most likely have an accelerator pump issue. What carburetor are you running? You should have a nice healthy "uninterrupted" shot of gas when you open the throttle. Pull the air cleaner off, look down the throat, and operate the throttle from idle, to WOT a few times and listen for air bubbles, or gaps in fuel flow. Make sure if you intend to start the engine immediately afterwards, that you put your foot all the way to the floor until it starts then let off, otherwise it will be a pain to get going. Flooring it will cut off additional fuel from getting into the engine, and thus, allow most of the fuel you just pumped into the engine, to burn without fouling the plugs. They should clean up after the truck starts. Using a vacuum gauge is the best way to do it as far as I am concerned. If you are running a carb with 2 idle mix screws, turn each individually until you attain the highst vacuum reading between the 2 of them. Then set the engine around 800 RPM (this is where I set mine), set base timing, and then set curb idle. If you are running an automatic trans, you should be setting curb idle to give you approximately 650 give or take in gear. Your transmission and Torque converter will change how much the RPM drop is. If it's a manual, I set mine at 700. Then go drive! Great to hear you got it going! ~THOR~

Author:	pinkiedog64 [ Mon Jan 26, 2009 1:27 pm ]
Post subject:
THOR, I am running an Edelbrock 500 with a VERY good accelerator pump. Two very healthy shots of fuel get poured into the motor upon hitting the gas. I don't have any hesitation when I push down the pedal (I used to with the old carb before the rebuild), I was mainly curious. I will do like you said to find the highest vacuum. The module isn't getting hot (I touched it when the motor was good and hot after a drive and I can barely feel anything at all-it certainly seems nowhere hot enough to fry it) I used the white grease that came with the module as per instructions. So, here's the mystery goblin: once the motor is good and hot, it is though someone just turns off the key-no spark. This has happened twice in a row. I'd be driving around and it the motor is up to temperature and running well and then the spark is gone. If I try to start it again, it doesn't start. If I wait till the motor (and everything in the engine compartment, too I guess), then go and try and start it, it starts right up. It did this last night and I came out this morning when it was 40 degrees out and it started right up. Since the module doesn't seem to be getting too hot, what else could it be? My guesses are: coil, wires (not spark plug wires) getting hot enough to get high enough resistance to stop doing their thing, or something along the lines of those two. I have felt around in the engine bay immediately after it has shut off and nothing seems too hot. No wires are touching the head (only the valve cover and it doesn't get too hot) and the relay and module seem fine, too. Any ideas? Any way to test this? Thanks

Author:	Aggressive Ted [ Mon Jan 26, 2009 1:36 pm ]
Post subject:
A thought..... Something in the engine wiring harness must be shorting out or nearly fried.....you might have to take the wrap off an do a through inspection. I ended up replacing the wires and the connectors in the plug blocks. No problems after that. Maybe a bad connection somewhere. Check voltage at the HEI, power wire or at the coil. Should be a full 12 volts coming to the module. You might have to run a separate power circuit.

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Author:	Reed [ Mon Jan 26, 2009 1:39 pm ]
Post subject:
Check the coil and the picku pin the distributor. As far as tuning a new motor goes, I would approach this differently. If you have the original distributor and haven't recurved it, I would try timing the motor to the factory specification. If you have a different distributor or have recurved the distributor, you will need to pull the distributor apart to see how much advance the mechanical advance governor and the vacuum advance pod give you. With those two numbers you can ballpark what your base timing should be. After that, you will need to fine tune the advance curve with the springs and by adjusting the vacuum advance can. Tuning in the carb is a bit easier. Set the curb idle to where it should be. Set the idle mixture screws an equal number of turns out. I like to start at two. If the idle is rough, back each screw out 1/4 turn until the idle smooths out. Once the idle is smooth add another 1/4 turn to each mixture screw. If the idle is already smooth at two turns out, turn the screws in until the idle gets rough. Once the idle gets rough, back the screws out 1/2 turn. That sets the curb idle mixture. Beyond curb idle mixture, you are going to have to drive the car and check your plugs and start adjusting jets, metering rods, step up springs and power valves to fine tune the motor.