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By Mike. - 7 Years Ago
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Hi all. I'm having a nightmare with my 312, I just can't get it to idle smoothly.
Here's the history..
Fully rebuilt 312, everything new, done by myself ( a professional engine builder) . Isky E4 cam, pertronix ignition in a new distributor, new Edelbrock 500cfm carb.
Right from the first run , it would not idle properly. Eveything has been swapped. Complete ignition system, new coil, new leads, new plugs, all wiring checked for voltage. 2 different carburettors. Valve lash is 20'' hot. Ignition timing 10 to 12 degrees.
The engine will only idle rich to the point it stings your eyes at the tailpipes. The motor shakes as if it's misfiring. Sounds like it is but all ignition system has checked out good on a Sun machine. I renewed everything again just to be sure.
Compression test shows 180 psi on every cylinder . Cylinder leak down test showed 5% leakage on all cylinders.
Above idle speed motor runs smooth and pulls strong.
Anything I have overlooked? Spent all day on it today and got nowhere.
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By Talkwrench - 7 Years Ago
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Sounds like your doing everything right. I know Rick has a similar 'setup' as you and was running a 465 Holley I think he tried and tried with standard jets [about 58's?] It wasn't until he jumped right up to low 60's might have even been 64's then it ran really nice, hasn't looked back. See if you can scab a 600Holley and try , don't be shy on going up jet sizes.
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By Ted - 7 Years Ago
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The leak down test sounds high for a new engine. The leak down test itself should have helped to clarify where the leakage was occurring. Some of your symptoms points to worn valve guides.
Here are some other ideas. Were the valves hand lapped in place after doing the valve job? Were the ring end gaps checked? Cast or moly rings? Was the cylinder wall finish appropriate for the rings being used? Were the heads cc’ed? There’s the possibility of one cylinder head having larger combustion chambers than the other. Where is the camshaft installed in relation to TDC? I’ll assume the camshaft was degreed in and not simply put in at 12 links between the dots. I’ve found camshafts off by as much as 18° when simply installing the timing chain sets without actually degreeing them in. Easily fixed during the build process but can be a tuning nightmare if off significantly and in the vehicle.
How’s the manifold vacuum reading on a gauge? If not steady and high, then start looking at the valves, guides, springs, and lash settings first. The richness from the carb is potentially from too much initial idle setting thus exposing too much idle transfer slot. This could be symbolic of not enough ignition timing, a carburetor that’s too small for the application, inadequately sealing intake valves, or improper valve lash settings. This is assuming the float level in the carb is set correctly and/or the fuel pressure is not excessive. Edelbrock carbs are sensitive to fuel pressure variances.
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By Mike. - 7 Years Ago
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Thanks Ted, a lot to think about there. I could definitely hear valve leakage into the intake on the leak down test, on every cylinder. I didn't do the heads myself but I trust the guys that did them. They were new valves and new guides.
I will check the cam timing also, I can't remember what I set it to, it was 2 years ago. I have lost the cam card, what should I be looking for timing wise?
I have 2 carbs here, a 390 Holley which it ran fine on before the rebuild and a brand new 500cfm Edelbrock performer. The engine behaves the same whiscruber carb is fitted.
Thanks for your help .
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By petew - 7 Years Ago
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Vacuum leak ?
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By Mike. - 7 Years Ago
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That's what i thought , but I can't find a leak.
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By 62bigwindow - 7 Years Ago
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Wouldn't a vacuum leak cause a lean sittuation?
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By Sandbird - 7 Years Ago
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Did you check your spark plugs to see if they are getting sooty that would be evidence for a rich condition? I have an E4 It idles around 800 to 850 w/std transmission. The cam has a bit more overlap than a stock cam which I attribute the rough idle I have. If you do have sooty plugs It could be due the additional overlap the vacuum is fluctuating enough at idle that the power valve is opening too early or the pistons to the metering rods are doing the same on the Edelbrock. That being the case a change in that area will help, The Isky E4 I put in straight up and it was checked on a degree wheel to be accurate with the 12 pin factory setting could have been a luck of the draw. At that setting it runs good and has a good pull in high gear. My Isky cam data calls for .015" hot for the valves.
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By Mike. - 7 Years Ago
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Thanks everyone. I've just put a timing chart on the front pulley, tdc it, and checked at the inlet valve pushrod with a dial guage and it's come out at 106 degrees. I don't know what it should be as my cam card is misprinted and is unreadable. Is it ok at 106 or should I start tearing the front down?
I think the problem could be as you say, carburettor. I have fluctuation on the vacuum guage. I expected a slight rumpiness at idle with this cam but it's not a consistent lope. It's more of an erratic misfire which occasionally smooths for a couple of beats then misses again . It's enough to shake the car. Listening down the Venturi the air doesn't seem to flow smoothly , I can hear gurgling.
Off idle the engine is a smooth as silk and pulls strong.
Plugs show soot around the base of the thread but the electrode itself is a nice mid to light brown.
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By Sandbird - 7 Years Ago
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Mike,
My Isky data shows Intake open 22 deg BTDC and close 58 deg ABDC. That would put the intake centerline at 108 deg ATDC. Depending on your accuracy you're real close and because your reading is close you must be fairly accurate. If your reading is perfect you are 2 deg advanced which is fine. This is assuming your intention was to determine the intake center. With the compression
readings your getting my guess is that you are a bit advanced. I don't think your cam position is your problem.
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By Mike. - 7 Years Ago
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Thanks. I emailed Isky and they confirmed intake lobe centreline is108. I'm happy at 106. I think my problem is carb related, I can hear it spitting and gurgling down the Venturi, it's not a smooth flow of fuel and air. The car runs absolutely spot on apart from idle. Thanks for your input 👍👍
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By Dave V - 7 Years Ago
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I have about the same setup that you have Mike. A 312 with Isky E4 cam, Pertronix ignition, but a 600 Holley. I installed the cam 2* advance putting it at 106*, giving it 180lbs compression. Idles at 800 rpm with a standard transmission. I found that it runs best advancing the timing a lot more. I recurved my distributor to run 22* initial with 14* mechanical to have a 36* total. 16-17" vacuum. Runs incredibly well. DaveV
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By 56Roger - 7 Years Ago
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I have worked on three brand-new Edelbrock carburetors in the last few years. They all had been installed on very well done rebuilt Y-Blocks. None of them performed well out of the box, no matter how much tuning. In each case nothing left to do but take them apart and find out what was going on.
Each of them was a mess inside. As if they had been merely assembled from the necessary parts with no consideration of any adjustments or settings. One had the floats stopped only by the top of the carburetor. One of them had machining debris in the bowls. The closest to being somewhat usable would still sting your eyes and give you a headache even when tuning with the back end of the car hanging several feet out the garage door and a big fan blowing on the radiator. None were close to having the right jets and rods for the application.
If you have not already done so I suggest getting the proper (for your model) carburetor calibration kit from Edelbrock, study and follow the instructions closely. Then you will know just what you have and what to do if/when more needs to be done.
You absolutely cannot assume anything about your carburetor. Any brand. New or old. Especially new these days. Get the right stuff, set it up right, and enjoy life with no mysteries.
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By Mike. - 7 Years Ago
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Thanks guys. I don't like Edelbrock carburettors at all. I bought it because it was easily available . I scored a brand new 600cfm Holley on eBay for a great price. I've just picked it up from the seller. I'm gonna install it tomorrow.
My set up should be identical to yours then Dave 👍👍👍
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By 56Roger - 7 Years Ago
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I will still suggest opening up the Holley to determine that everything is as it should be prior to installation. You can record jet sizes, check the floats, check/clean orifices, and all the other little things that need to be known (should be known) while you're at it. If adjustments are necessary later you will already know where you stand before going any further.
Of course maybe you are just lucky. I'm not. I don't know if I'm any smarter after doing this stuff for over 50 years. But I do know I am less ignorant. Learnt that the hard way.
When someone says, "Well did you check this and then this while you were doing that?", it is best to already have the correct answer. Working for a tough Service Manager at a Ford tractor dealership taught me that a long time ago. Doesn't make you feel good if you don't. The right answer always holds up, even if you are working alone which is my usual situation these days. "Dammit! I knew I should have..." then "Now I have to..."
Easier, not harder, to do it right in the first place.
Had one other very good boss later on who was the same way. Always made absolutely sure of what was actually happening during a project. And could prove it. Very good lesson to learn.
Having the information and not needing it is better than needing some information and not having it.
Edit:
Even worse yet. Needing some information, not having it, at the same time not even knowing you need it. That's the ignorance part I try to avoid. A lot of struggles get easier or disappear altogether when the right information is had and applied.
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By Talkwrench - 7 Years Ago
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Try a green pump cam after you get your Holley on .. ; )
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By Mike. - 7 Years Ago
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Hi guys. Thanks all for your awesome input. I have finally solved the problem. It was the carburettor all along. I put on the new Holley and straight out of the box it idled. It idles smooth now, the bouncing vacuum gauge is steady, pulling about 15/16 HG, just got to set the floats now and reset ignition timing and idle mixture. As we say in the UK.....' I'm well chuffed' 😀😀😀
Thanks again everyone for your help👍👍
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By 56Roger - 7 Years Ago
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Well good for you. Run it for some bit of time and take a close look at your plugs the proper way after that. The great thing about Holleys is they are easily adjustable just about every which way. With the full kit there are many options. The bad thing is they very likely need to be checked/adjusted every which way for optimum performance. Pretty much the same as any other carburetor. Don't settle for out of the box. Likely much more to be had. Mileage, power, all of it.
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By Mike. - 7 Years Ago
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Thanks Roger. I did pull the float bowls off and check inside before I fitted it. I gave it all a thorough flush out with carb cleaner. All was well inside. Not sure if I will need to change the main jets yet, but at least I know what size these ones are.
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