Rough Idle

[1974stepside]

I have a Slant 6 with single barrel Holley 1945 carb. It starts and runs fine but after awhile it seems to idle high. I notice when stopped at a light or in traffic.

Adjusting the curb idle and mixture screws do not seem to do much, I've done this in neutral like it is supposed to be. I want to get the idle as low as possible so that I can adjust the valve lash. I havent rebuilt the carb yet either (trying to postpone this until neccesary). Any ideas on what can cause this high, erratic idle? Also I did have it set perfect a few weeks ago but it went back to idling high.

As far as vacuum leaks where should I check? Does the air filter housing need to have a good seal when put on?

 

[805moparkid]

for doing lash just heat past normal operating temp and then shut it off and lash...

as for the leaks check all the vacuum lines to start. creeping like that tho could be from to high of idle... should idle @ 750rpm

and the air cleaner just has a little rubber/cork seal under it but no, it won't affect anything

 

[RustyRatRod]

The carburetor screws are notorious for working loose on the one barrels and causing vacuum leaks, which would 'splain your problem. Grab the carburetor and see if you can twist it side to side. Even if you cannot, it would be worth your while to remove it and check all the carburetor screws for tightness.

 

[1974stepside]

805 - I thought setting lash on a slant 6 needs to be done while running? Can I accurately do it by hand cranking it over? And how do I check the rpm, what tools do I need and how do I do it?

StrokerScamp - Are you referring to the mounting bolts working loose or the idle screw? I tried to twist the carb but it seems pretty solid. I figure if I get it idling properly I might put a dab of RTV on the idle screw to keep it from moving.

 

[805moparkid]

805 - I thought setting lash on a slant 6 needs to be done while running? Can I accurately do it by hand cranking it over? And how do I check the rpm, what tools do I need and how do I do it?

StrokerScamp - Are you referring to the mounting bolts working loose or the idle screw? I tried to twist the carb but it seems pretty solid. I figure if I get it idling properly I might put a dab of RTV on the idle screw to keep it from moving.

some people have done it running (ive tried it)... u better hang on... and also ur beating the crap out of the feeler gauge ur woeking with... no so accurate anymore. heat the engine up to about 200 and then lash it. just bump the starter over until the rocker has no lift applied and the cam is @ 0 lift... cant explain this morn sorry...

digital timing lights have a tach on them

 

[67Dart273]

Trying to set valves running is an exercise in futility. Warm it up good, and set em

"Learn" yourself a little ditty, and you can set valves on ANY engine

EOIC

This means you START when the E xhaust O pens (and adjust the intake)

You FINISH when the I ntake is almost FINISHED C losing (and adjust exhaust)

So take no1, , fer xample. You bump the engine until the exhaust valve just starts to move open and set the intake.

Then you bump the engine until the intake comes open, and is not quite closed, and set the exhaust.

THE REASON you do this in this manner is that this method insures ON ANY camshaft--that you are setting the valve while it's on the "heel" or backside of the camshaft.

 

[RustyRatRod]

805 - I thought setting lash on a slant 6 needs to be done while running? Can I accurately do it by hand cranking it over? And how do I check the rpm, what tools do I need and how do I do it?

StrokerScamp - Are you referring to the mounting bolts working loose or the idle screw? I tried to twist the carb but it seems pretty solid. I figure if I get it idling properly I might put a dab of RTV on the idle screw to keep it from moving.

The screws that hold the carb body to the base. They always work loose on the one barrels. Also, on the valve lash, that is easy peasy. I don't know why everybody gives crappy advice here...."do it runnin"...."heat the motor up"....it's all bullshit. All you need to do is adjust cold so you don't burn yourself and with the engine off of course. Simply add .002" to your cold adjustment and you're done. I don't give a rat's butt what anybody else says, I've been adjustin solid valvetrains that way over 25 years and it works fine. Let the arguement ensue.

 

[805moparkid]

The screws that hold the carb body to the base. They always work loose on the one barrels. Also, on the valve lash, that is easy peasy. I don't know why everybody gives crappy advice here...."do it runnin"...."heat the motor up"....it's all bullshit. All you need to do is adjust cold so you don't burn yourself and with the engine off of course. Simply add .002" to your cold adjustment and you're done. I don't give a rat's butt what anybody else says, I've been adjustin solid valvetrains that way over 25 years and it works fine. Let the arguement ensue.

haha no argument here! i just do it hot but my step dads mustang says to do them cold or hot...

 

[vynn3]

some people have done it running (ive tried it)... u better hang on... and also ur beating the crap out of the feeler gauge ur woeking with... no so accurate anymore. heat the engine up to about 200 and then lash it. just bump the starter over until the rocker has no lift applied and the cam is @ 0 lift... cant explain this morn sorry...

You guys are nuts. Adjusting the valves with the engine running is WAY faster and more effective. The only way you can hear the effects of the individual valve adjustments is to do it on the fly. On all of my slants, each valve usually needs to be a little tighter or looser than spec to make the engine run it's best, not all set to the same specific measurement.

Why do so many guys fight this? Adjusting with the engine running just isn't that big of a deal. It just takes some practice and patience. "beating the crap out of the feeler gauge... not so accurate anymore"? Seriously??? Professional mechanics have been doing it like that for 40 years, and for what it's worth, my feeler gauges look fine...

Do it the way you want, but I'll stick to procedure in the service manual.

 

[67Dart273]

............. Professional mechanics have been doing it like that for 40 years,............... I'll stick to procedure in the service manual.

I've known plenty of pro mechanics who don't adjust valves running, and when I was a kid there were still plenty of engines running around that had mechanical tappets. The service manual I have say NOTHING about adjusting them running. What they says, is,

My '70 manual says nearly the same thing, except 190*

 

[slantsixdan]

Trying to set valves running is an exercise in futility.

I'm surprised to see you say so; anyone coordinated enough to use a computer keyboard can do it, and it's a lot less hassle than doing it with the engine stopped. Perhaps you're doing it wrong. See valve adjustment procedure.

Original poster: Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

 

[vynn3]

I've known plenty of pro mechanics who don't adjust valves running, and when I was a kid there were still plenty of engines running around that had mechanical tappets. The service manual I have say NOTHING about adjusting them running. What they says, is,

My '70 manual says nearly the same thing, except 190*

Yep, that's the manual alright. Now, I've read it several times, and it says NOTHING about shutting the engine off. Since it doesn't mention anything about turning the engine over by hand or using the starter to bump it over for each valve adjustment, sounds like you're reading a lot more into those instructions than I am.

So, you adjust one valve, start the engine, listen to it, shut it down, and adjust again? Or you just adjust all the valves to spec and call it a day, no matter how it runs?

Ever rejet a carb, or do you just assume that whatever jet is in it is right for your particular engine? Same principle. Some of my valves are set at .020, some at .021, some at .019 (Erson cam). Purrs like a kitten. All set the same? Not so much.

You wanna do it the hard way? Great. But I wouldn't recommend that method to a newbie.

 

[1974stepside]

So, I had the truck all warmed up and took off the valve cover to start adjusting the valve lash but now it wont start. The taillights, reverse and brake lights do not work, when i try to start it i get a single click. The headlights, fuel guage, dome light and wipers still work.

I replaced the ballast resistor in hopes that it would be a quick easy cheap fix but no go. Engine bay wiring is pretty hairy and not sure where to start. any ideas?

And yes, i ended up setting the valve lash without the engine running and cold, cut the towers off an older dist cap and hand cranked it, setting the lash .02 more than specified hot. Plan on doing it again while hot once someone helps me figure out this issue.

Thanks for everything so far...

 

[ESP47]

Start with your battery, it's probably dead. Get a jump and see if it starts up. The ballast resistor is usually only the culprit when you can only get the car to run by turning the key all the way in the 'start' position. Letting it go will cause the car to stop running.

Vynn, the problem is that it doesn't even tell you to shut the engine off to install the valve cover. Probably one of the reasons why everyone gets confused. I think the writers of the manual just assumed we would know what they were talking about. Either way as a very amatuer mechanic at the time, I was able to adjust my valves running with ease. Sometimes it takes a second to get the wrench on but then its easy from there. I think the people that fight the idea are the ones that have never tried it. They probably assume oil is splashing all over the place and the valves are popping way too fast to get a wrench in there. In reality once you idle the motor down, they aren't moving very fast at all.

 

[vynn3]

vynn, the problem is that it doesn't even tell you to shut the engine off to install the valve cover. Probably one of the reasons why everyone gets confused. I think the writers of the manual just assumed we would know what they were talking about. Either way as a very amatuer mechanic at the time, I was able to adjust my valves running with ease. Sometimes it takes a second to get the wrench on but then its easy from there. I think the people that fight the idea are the ones that have never tried it. They probably assume oil is splashing all over the place and the valves are popping way too fast to get a wrench in there. In reality once you idle the motor down, they aren't moving very fast at all.

Agreed on all counts.

 

[cudamark]

You guys are nuts. Adjusting the valves with the engine running is WAY faster and more effective. The only way you can hear the effects of the individual valve adjustments is to do it on the fly. On all of my slants, each valve usually needs to be a little tighter or looser than spec to make the engine run it's best, not all set to the same specific measurement.

Why do so many guys fight this? Adjusting with the engine running just isn't that big of a deal. It just takes some practice and patience. "beating the crap out of the feeler gauge... not so accurate anymore"? Seriously??? Professional mechanics have been doing it like that for 40 years, and for what it's worth, my feeler gauges look fine...

Do it the way you want, but I'll stick to procedure in the service manual.

X2.....but either way will work as long as the engine is hot. It's just faster to do it while the engine is running and a bit more accurate. The expansion rate for the valves is different between intakes and exhausts so doing it cold will only get you close, which is fine when you've just done a rebuild or valve job and are firing it up for the first time but they should be readjusted once the engine is fully warmed up.

 

[1974stepside]

So I did the valve lash and reconnected the wires that disconnected at the bulkhead and swithced the wires on the ballast and I definately notice the improvement with power. The only problem now is it still idles very high when in park, at a stop sign or red light. I backed the Idle adjustment screw all the way out as far as it can go but still runs very high. Any ideas?

If not I guess the next step is to rebuild the carb.

Thank you for your suggestions.

 

[slantsixdan]

Sounds like you have got a vacuum leak. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. Tune-up parts and technique suggestions in this thread.