High Idle problem

[Oldbuilder]

Okay folks,,, I've got a idle problem and just can't get it figured out so here's the best I can give ya.

Here's what it's doing,,, won't idle below 1500-1700 RPM and when shut off it will backfire after a few seconds or sometimes quite a few seconds,,, that's the one that scares the crap out of me cause I'm not expecting it LOL,, I get relaxed when it doesn't happen within a few seconds.

350 SBC engine,,,
Engine was rebuilt almost back to stock. I added an upgraded hydraulic cam .488 lift and roller tip 1.6 ratio rockers, an RPM Performer dual plane intake non-air gap, and a Holley 670 Street avenger carb.

Heads are 1968-1970 cast steel 487X from a Corvette with 2.0 X 1.6 valves. Header exhaust into flow master pipes and mufflers. So nothing outrageous here. At least I don't' think so.

When setting the valves, was told to tighten the nut until the push rod was making contact with the lifter then 1/2 to 3/4 turn,,, I think that was wrong and they were too tight. Also while driving they backed off and were rattling pretty bad. Not sure if it's flattened a lobe or two or bent a valve. Will check all that later this week with a 1" travel indicator if I can figure a way to mount it.

Got everything set the best I knew and fired up the engine. Couldn't get it to idle below 1500-1700 RPM. Even after doing all the adjusting I know about, this isn't my first rodeo but it's been a while since I've run into any problems.

Went back and disconnected all the vacuum lines and plugged off the open ports. That seemed to help for a while but the back firing after shut down started. Somewhere there has to be an ignition source I'd think.

Anyways, this last time I pulled the valve covers and checked everything there was no give (cushion) in the lifters so I set them all with a .010" feeler gage and fired it back up. The first few times I shut it off all was good on the back fire but idle was still high.

Part of me thinks there has to be a vacuum leak somewhere and I've heard it can be on the inside of the lifter valley with aluminum intakes not fitting right. ma I hope not.

So chime in crew and don't be shy,,,, it's been a long battle and I'm about to shoot something if you get my drift ha ha ha .

Later Ya'll,
Grego

 

[railroad]

The first thing I noticed was your valve adjustment. You need the valve to be on closed side of the cam to adjust it. You can find the procedure for positioning the valves while rotating the engine by hand. With the valves properly adjusted and the lifter pumped up, it is not likely you will have ANY slack in the valve train.

You are right in thinking a vacuum leak could be the problem.

The backfiring is a possible result of either. A lean condition can cause back firing, but should not cause a high idle.
Backfiring after shut off??. Is this out the exhaust or the carb? If it is out the exhaust, you are probably passing unburned gasoline. This could be valve adjustment or too rich. If it is out of the carb after shutting off, valve adjustment and or hot spots in the cylinders. If the carb is new, it should be good, but a few quick checks will eliminate the question. Check the float level, make sure the butterflies are free moving and closed against their stops. You can back the idle adj screw off until it does not open the butterflies and then, just enough to unseat the primary butterflies, for now.

I hope the cam has survived. Worst case, lobes wore down and opened valve lash. Easiest possibility, the rocker arm nuts are backing off. If the cam is gone, you might was well stop now. Change oil and if it metal flaked, it is bad.

Most of the time your issues are just small over sights. The simple things that you think do not need checking usually cause the problems.

Good luck, post back what you find. We are going to get advise about adjusting the wheel bearing to changing brake fluid, but try to stay with the basics.

 

[Screaming Metal]

And going along the Lines of RR, run a compression check on all cylinders. This will verify that the valves are closing and sealing. Do all RR said check, its all good.
Let me add this also, backfiring after ignition off could be either carbon build up, hotspots from sharp metal edges from machined parts, etc. If its on the exh. side, could be raw fuel in the headers.

After you get it running, spray your intake manifold fittings with carb cleaner. once you find a leak, the motor will try and run away. If your leaking from the lifter valley, you should look down into your intake runner with a borescope, at the intake/head sealing surface, you'll see a oily residue at the parting line.
What I do, is mix some engine oil dye with the oil. If theres a leak at the intake, you'll see it.

 

[Screaming Metal]

Usually the Parts places, if you get alot of parts, they might let you borrow one. Some shops will rent you theirs. Also some rental places have them. If you work and build your cars, your gonna eventually need one, go get the cheap ones at Harbor Freight for about $100 or a little more. My SnapOns are quite expensive, I have 2 of them, one in my main tool box, one goes on the road with me. But I have a few cheap ones that I can put into harms way, should I need to. Especially around blower drives....they'll eat a borescope in a second....

 

[Oldbuilder]

I used this method to set the valves. Don't remember where I read or learned this method.
Brought the #1 cylinder to TDC on compression. Set those valves, rotated the crank 9o degree's (by hand) and went to #8 same there, 90 degrees then to #4, 90 & #3, 90 to #6, 90 to #5, 90 to #7, 90 to #2. Now I was having to eyeball the 90 degree thing, I could have been of as much as 5 degrees each time. Probably is the reason why when I came to the end and rotated the last 90 it wasn't on #1 TDC.

The really old school way was to get the engine running and set all the valves while it's running, you know, back off the nut till it taps then back in till it quits. Surely you guy's have never done it that way ha ha ha .

I'm off Thursday & Friday and hope to spend a bunch of time working this out. Will all that's been suggested too,,, man I hope there's no shaving in the oil,,, what a bummer if it is.

 

[Oldbuilder]

Forgot to add,,, then carb was/is brand new,,, the back fire is out the exhaust. I don't think there's any coming out the carb but can't be sure until I run without the filter on,,, that's next. I suspect there is unspent fuel in the exhaust and the exhaust could very easily have hot spots,,, it's new and you know when tubing is cut it has sharp edges that will glow once heated. Let me do a real quick start & kill to see if it backfires when barely warmed up.
Will get back on & let you know.

 

[railroad]

All good news!! I forgot to ask what kind of choke the carb has on it. If manual, I am sure you can handle that, if auto make sure the fast idle cam is not stuck or out of adjustment. Also, you can loosen the choke cap and turn it max lean to get it out of the way. Popping out the exhaust after shut down is just unburned gas.
Any idea on fuel pressure. If you are running a regular mechanical fuel pump you should be good.
I like adjusting the valves on a running engine. I use a heavy rag or pieces of cardboard to deflect the oil. I also have a set of sheet metal valve covers with the top cut out for access to the adjusters. We are getting there.

 

[Oldbuilder]

Okay,,, just did the cold test but before I started I tied the choke to the full open position (with a tie wrap),,, had to hold the throttle open a bit so it wouldn't die till it warmed up just a little. It's electric choke and turning to max lean is next up on the list. Anyway,, when I shut it off after a short run,, no popping out the exhaust or carb,,,, let it set about 30 seconds and did it again,,, no popping,,, again and no popping,,,, things are looking up

Is the fast idle that little plastic piece?? If so where should it be,,, up or laid over? Don't remember where it was when I started all this.

It would idle around a grand at first then crept up to 1500,,,, I think getting a bore scope is gonna have to happen but funds are tight at the moment.

Compression check is up too,,, I have a gage setup (somewhere I think) that lets you put air in the cylinder and then read gages,,, its' used on aircraft engines to check compression and bleed down. Hopefully I didn't sell it when I stopped working on planes. Otherwise it's the normal way with a set of Snap On gages and spinning the crank. Either one is fine but inputting air lets you watch bleed off,,, only thing is if the crank spins it can hurt ya if your in the wrong place when it happens LOL.

Fuel pump is mechanical original replacement. I don't think pressure is a problem. Maybe I can borrow a gage and check,,,, what should it be??? or what would be too much???

 

[Screaming Metal]

I also have a set of sheet metal valve covers with the top cut out for access to the adjusters. We are getting there.

Hahaha.....I got about 10 pr of those damn things! See, it was your choke causing the hi rpms....Those chokes I loosen off the screws, turn the cap so the choke plate is wide open, then tighten screws. Wanna start a cold motor, pump the pedal about 3 times and hit the key....
I usually have a manual choke lever rigged up so I can raise the idle rpm till the motor gets warm, then I push in on it and let it idle normally!

 

[railroad]

The plastic step cam is the fast idle cam. If you open the throttle by hand the cam should fall to the rear and down. When you let the throttle close the screw that rides on the cam steps should not be touching the cam.
Be sure to loosen the choke cap, do not remove, rotate toward lean until the choke blade is standing open with some force. Compression check and bore scope are good tools, but I think you are getting close to getting the idle down where you can do some tuning. Let us know what you find. If you happen to let the choke cap get out of place, pull it off and re align the thermal spring loop on the post inside the housing. It has to be on it.

 

[Screaming Metal]

What He said!!!!

 

[Oldbuilder]

Hey guy's,,, didn't get to work on it today but when I get home tomorrow I'm off till next Tuesday. So the engine will get lots of attention. I hoping the local parts store has a bore scope I can borrow to check for intake leaks using the method mentioned above. Don't know why I didn't think about the evidence of oil at the seem,,, you guy's are real CSI types. Will post the outcome tmrw.

 

[Mike]

The problem with using old head like those (which are much better than the light castings, to be sure) is that they have probably been milled two or three times over the years. And, depending on how much has been cut off them, then you may have some misalignment issues with the intake runners. Which can cause an internal vacuum leak, but when that happens, you will generally see one starting to burn oil.

If you cannot lay your hands on a scope, there is a more labor-intensive way to see how things are aligning. Pull the intake, clean all the mating surfaces and then use a flat intake gasket, without any sealing beads (something like a 101-B Mr. Gasket) and bolt everything back up again. Pull it back apart and you will see an impression of both the intake face and the head face, on their respective sides of each gasket. If things are misaligned, remember you can get intake gaskets of varying thicknesses, which will let you make some minor adjustments. I would forego the use of end seals at that point, and just lay down a generous bead of silicone sealer at the end rails. When using silicone, cleanliness if next to Godliness.

The leakdown tester is a great way to check things. Just be sure to bring the engine up to temperature before using it, as a cold block can give you some wonky readings. I like that you get the benefit of listening for where air is going, if you do have a hurt cylinder. Hopefully, that will not be the case in this instance.

railroad, I got a good laugh when I saw your comment about wheel bearings and brake fluid.

 

[Oldbuilder]

Hey Mike,,, I got a good deal on these heads when I found them. When I took them to the machine shop they too looked them over and said they had probably never been milled or just barely because the area where you can tell was still pretty thick. I assume there is some misalignment just because I'm using an after market intake. I did use a gasket with he bead already built in. Fel-Pro 12 something if memory isn't too far off. I may have a piece of the packaging hanging around someplace that has the number on it. I don't remember if I added an extra bead or not. There was some concern about the intake not fitting really flat so I may have added extra sealer. I do remember putting more around the water passages to avoid it getting into the lifter valley and the sump if it leaked. Sooo,,, tomorrow it's garage time and taking it slow and methodical. Will post the results.
Later Crew, Grego

 

[Mike]

Yeah, the SBC intake gaskets in the high performance line are all 120x numbers, until you start getting into some of the more exotic combinations. I'm guessing you likely used 1205, unless those castings had been ported.

One thing I really recommend to everyone is a cheap, three-ring binder and a box of plastic page protectors. With nearly everything having a camera on it, these days, take plenty of pictures. At the very least, save them to a hard drive on your computer, but print out images you might need to access straight away. Tape them to a sheet of paper and write notes on the paper. The palest ink is better than the best memory. I even do that with Web sites. I've a thread in a private forum on here, that has posts with information on scripts I've added to the site, dates when they were added, dates when they were upgraded, etc. My remembery isn't what it used to be, so I try to make notes on everything.

With an engine, I recommend recording every piece of data you can. Clearances on each bearing, wall clearances on pistons, piston to valve clearances, all of the camshaft specs, ring end clearances, ring part numbers, bearing part numbers, gasket part numbers, everything you can think of. What oil pump did you use? How much oil is required for an oil change? What spark plug did you use and where did you set the air gap? How many times have the head and main studs or bolts been pulled to torque? What jets are in the carb. Did you roll the camshaft, or is it in straight up? How much coil bind clearance do you have on the springs? Valve lash? What fan belt/s did you use? When you are recording the information, it seems like drudgery, cleaning your hands to write your notes, but when something goes awry, all you need do is pull out your trusty notebook and you will have everything at your fingertips.

Creating another check-off sheet is a good thing, too. Main studs torqued? Check. Oil pump torqued? Check. Rod bolts torqued? Check. That way, you will always be able to look at your build sheet to know what you have and have not done.

Just some food for thought.