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post #1 of 22 (permalink) Old 10-23-2018, 12:18 PM Thread Starter
 
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chasing engine problems

I have a 66 GTO. It's nearing the end of a 10 year restoration. The engine is a 400 out of a 68 GTO and I had it freshly rebuilt by an engine shop. Unfortunately that engine shop retired before I ever got a chance to start the car and now I can't get it to run right. I've tinkered with it quite a bit, had a mechanic friend look at it who is pretty "old school" with carbs, and been on various tech support lines for advice.

The engine has ram air 3 heads on it running about 10.5:1 compression. The cam has quite a bit of overlap to allow it to run on pump gas. It starts ok, doesn't like to idle, gets worse as it warms up and eventually dies most of the time. Although sometimes it idles fine even after it warms up but still won't drive. When I drive it around the block it won't go past 10 mph and jump up and down like the firing order is wrong. Pretty much everything on the engine is new, including the carb. The carb is a holley street avenger 570. It's a little small for the engine but holley tech support and myself don't think that would cause idle problems

Things I've tried with no change:

-Changing the carb to a bigger edelbrock
-check the vaccuum. 9" steady which seems low to me but my research suggest that is normal for the engine specs it has
-Changing the power valve on the holley to match the vaccuum
-trying two different distributors
-putting an MSD box on it and taking it back off (when I changed to a petronix distributor)
-checking for leaks on the intake, despite not finding any changing the gasket anyway
-checking the firing order a dozen times over
-trying several different timing positions including locking it out
-checking for spark with a spark gauge
-running it on 100 octane
-taking compression 170+ in all cylinders
-check the ignition timing timing

Next things I need to try:
-putting fresh gas in it (gas isn't that old so I don't suspect this but the tank could have been contaminated.)
-swapping out the stock exhaust manifold for headers. I wouldn't think it would make a difference at idle but who knows
-swapping heads
-swapping cams
-double checking my valve springs to see if they go with the cam and head.

today I thought I reach a revelation and thought the engine builder might have swapped in one of those 4/7 cams where they swap the firing order but my paperwork on the cam says otherwise and I don't think they make them for the pontiac anyway. Any idea what I might be forgetting?

Last edited by [email protected]; 10-25-2018 at 03:31 PM.
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post #2 of 22 (permalink) Old 10-23-2018, 01:06 PM
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To me, it sounds severely retarded....either base timing, cam timing, or both. I would verify the mechanical base timing of the engine and the positioning of the timing gears. Don't waste time with headers, and swapping a bunch of parts just yet. You need to do one thing at a time and go from there. Start with verifying your cam position, and then verify that #1 cylinder is at TDC and check your timing pointer and distributor rotor location. Lots of info on this if you dig around a bit. My gut tells me your cam timing is probably off.....it happens....BTDT. Keep in mind, also that a 'large' cam with a lot of overlap doesn't compensate for being able to 'get away' with high compression on pump gas. Sure, you'll get less cylinder pressure at low speeds, but you'll end up paying the piper when the engine 'comes on the cam' at increased RPM and fills those cylinders as designed. With 10.5 CR and iron heads, you will need to run higher octane fuel than is available at the pump.
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post #3 of 22 (permalink) Old 10-23-2018, 03:18 PM Thread Starter
 
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Thanks for the reply. At one point I had to take the timing cover off. It did reveal the timing chain at zero degrees with on the power stroke for #1 the marks on the timing gears were pointing away from each other rather than lined up. I was convinced this was the problem and I had the car firing on the wrong cycle. I thought it might be running poorly because it was firing on the exhaust stroke. So I turned the timing back 180 degrees thinking this was the issue. The car didn't start but did in fact make a loud boom under the hood.......... Afterwards I checked my work by taking the valve covers off and seeing what valves opened when and I was right the first time. The engine builder just had the timing marks pointed away from each other at 0 degrees. I assumed he put the timing gear on the cam 180 degrees backwards lined up the marks at TDC for the exhaust stroke and didn't verify they lined up at the power stroke instead.......... But if he messed that up maybe he got the cam degrees off as well and instead of being 180 degrees off he's an extra tooth or two off and is actually 185 or 190 degrees off.

As for the overlap. I talked to the tech support at comp cams. They told me that cam is specifically designed to bleed off heat and pressure in the cylinders to prevent detonation on pump gas. Are you saying this is contrary to that advice?
 
post #4 of 22 (permalink) Old 10-23-2018, 03:24 PM
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+1 on the timing, especially if it's not running the stock points distributor. Take that factory initial timing spec and throw it out the window - it no longer applies. If you don't have a timing light capable of measuring total advance, get one (or put an accurate timing tape on the balancer). Also since there's a lot about the engine that's in question it would pay you to verify that TDC on the timing marks is actually real TDC. Worn factory balancers can slip, and aftermarket ones are sometimes not marked correctly. Verifying the marks can be done without tearing the engine down or removing the heads, but you have to be very careful how you do it. You can use a "spark plug hole" piston stop like one of these
Spark Plug Hole Piston Stop
but don't even THINK about installing it before you remove both rocker arms on the cylinder so that the valves cannot move, otherwise you run the risk of bending a valve as it contacts the stop as you're turning the engine over. If your heads have the Pontiac factory rockers and studs, reinstalling the rockers is simple - just reinstall and torque them down to spec (the factory system isn't adjustable). If you happen to have an adjustable system with poly-lock rocker nuts (the ones that have hex socket set screws in the middle) then hold the outer nut with a wrench, back off the set screw, and then carefully count the turns needed to back off the nut so that you'll be able to reinstall at exactly the same spot before re-tightening the set screw.

The best way to install the stop is to REMOVE BOTH ROCKERS (can't emphasize that enough) on #1 cylinder then turn the engine over "forward" by hand (use a big socket and lots of leverage on the balancer bolt) until the timing mark on the balancer indicates TDC, then keep going until it's a couple inches PAST TDC.
Thread the stop into the spark plug hole, screw down the adjuster until you feel it touch the piston, then back if off a few turns. Now with the stop installed, carefully turn the crank in "reverse" direction with a wrench on the balancer bolt until you feel the stop, make a chalk mark on the balancer that lines up with the TDC mark on the timing cover. Don't go "he man" on it, you're just trying to find where the stop is. Next turn the crank in the 'forward" direction until you feel the stop again - make another chalk mark. TDC will be exactly halfway between the two marks. If this isn't in the same spot as the mark on your balancer, then either it's marked incorrectly or it has slipped on you and should be replaced.

Here's an example of a timing light that can measure total advance:
Bosch Advance timing light

Process:
After you've verified TDC is TDC, disconnect and plug the hose for the vacuum advance canister, loosen the distributor hold down bolt enough so that you can turn it but still somewhat tight so that it won't turn by itself.
Start the engine, have a helper hold and maintain a steady 3300 rpm. Using the advance feature of the light, set the timing to 34 degrees. Shut the car off, lock down the distributor, then repeat the process just to make sure nothing moved on you (if it did, fix it). Now, start it and let it idle --- still with the vacuum line disconnected and plugged. Use the light to find out what the initial timing is and make a note of it - this you're doing just so you will be able to return to this setting without having to go back through setting it at RPM.

Reconnect the vacuum line now and drive the car - let us know how it does.

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post #5 of 22 (permalink) Old 10-23-2018, 06:58 PM
 
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I agree with geeteeohguy & BearGFR that is does sound like a timing issue. However, my experience with a really retarded distributor timing is that the engine will heat up really fast on you - you can watch your temperature gauge climb. You did not mention that you experienced this problem?

I know you have double & triple checked everything, but lets run through some basics just in case.

First, have you observed the rocker arms with engine running to make sure nothing obvious like rounded/worn cam lobe? No rockers have fallen off or gone sideways and bent a pushrod?

Roller cam or flat tappet hydraulic cam? Assume flat tappet?

Firing Order - Counter Clockwise on a Pontiac. 1-8-4-3-6-5-7-2

#1 spark plug at the distributor cap should be at the 5 o'clock position as you look at the distributor from the front of the engine - or pretty close to it.

What do your plugs look like when you pull them? Wet, fouled, metal flecs, etc.? Correct heat range plug?

Do you have a new harmonic balancer? The outer ring is bonded to the inner section and on an older balancer, it is not uncommon for the rubber to deteriorate and the outer ring to slip - giving you an incorrect timing reading. And, make sure that harmonic balancer bolt gets torqued to 160 foot pounds or the collar that goes onto the crank can crack/break.

Does the engine have the 8-bolt water pump or later 11 bolt pump. Each has the different timing cover and needs the matching harmonic balancer. There are 2 sizes of balancers with the 8-bolt water pump cover using the smaller diameter. If it was swapped over to the 11-bolt cover and the smaller 8-bolt balancer was kept, timing marks on the balancer will not match correctly the timing scale on the timing cover.

Noted you have done a Pertonix Conversion. Needs to have the correct air gap. Needs to have the correct matching coil, and you need 12 volts to it. The 1966 uses a resistance wire for points and drops the voltage to 9 volts while engine is running. If the key has been left on for any long period, I have read that this is one of the biggest reasons why the Pertronix conversions failed on their earlier units. They have supposedly corrected this on later versions.

Stock manifolds - does the manifold still have the "butterfly valve" with bi-metal spring? These were used to send hot exhaust gasses through the intake manifold exhaust crossover for faster warm-ups in cold weather. As the engine got to temp, the bi-metal spring opened up the valve and exhaust gasses exited as usual. If it is frozen shut, it'll cause problems and choke the engine. Exhaust pipes not damaged or plugged along its length?

What intake? Noted the Holley and swap to the Edelbrock (AFB?). If using a stock Q-jet intake manifold with these carbs, you would need an adapter. It has happened where the adapter was hanging just enough over the edge, but not really visible, to cause a vacuum leak because the gasket did not seal as it should have. The best intake is a dual plane and not a single plane intake. The single plane intake, if used, can be problematic at lower RPM's.

Have you checked fuel pressure at the carb? Could be a fuel pump, air entering the system via a split or bad rubber line, the sock on the tank's pick-up tube plugged up, wrong style of gas cap (non-vented vs vented) not allowing the gas to be drawn from the tank, kinked line, plugged fuel filter.

9" of vacuum is very low and indicates a fairly radical cam, but indeed may be the overlap in the cam. The factory cylinder pressure for the 1968 GTO 400 with 10.75 compression is 185-210 PSI @ 155-165 RPM's. Overlap on the "068" GTO cam is 63 degrees. I know that your cam has more overlap, but that cam will be poor down in the lower RPM range and won't perform at its best until you hit mid-upper RPM's. So if you have a stock torque converter or running 3.55 gears or lower - that engine will be bucking. With a radical cam more on the side of a race cam, you need a high stall converter and most likely 3.90 gearing or better to work with the cam.

PVC valve hooked up and working?

If you have power brakes, they may not work so well with 9" of vacuum. If you have power brakes - I would also check the check valve that comes off the hose from the carb/manifold that goes to the master cylinder brake booster just to make sure it is not bad and sucking air. This has happened to another member here and was causing problem. Valve appeared good and seemed to work, but it was worn out and not working as it should.

Lift on the valves? Any chance they are hitting the piston tops? Springs matched to the cam so the valves are not bouncing?

You did not mention if you have a stock valve train, ie torqued the rocker arm nuts down to 20 ft pounds, or have the adjustable poly-locks. It is possible that your valve are adjusted too tight in either case and being held open just enough to be a problem. Not saying this is the case as all your cylinder pressures seem consistent, but could be something to check.

If all this checks out, then I am onboard with a timing issue - whether cam or ignition.

Keep us posted and we will get it solved.
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post #6 of 22 (permalink) Old 10-23-2018, 07:06 PM
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9" of vacuum is SUPER low unless you're running a pretty nasty cam.

What are the specs on your cam? (Duration at .050 tappet lift, lobe separation angle, etc) and what kind of cam is it? Hydraulic, solid, flat, or mechanical?

As a point of reference, the cam in my 69 *is* a reasonably nasty solid roller, and my car makes 10" of vacuum idling in gear at about 850 RPM, 14-15" idling in neutral at 1100 rpm.

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post #7 of 22 (permalink) Old 10-24-2018, 02:25 PM
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Originally Posted by [email protected] View Post
Thanks for the reply. At one point I had to take the timing cover off. It did reveal the timing chain at zero degrees with on the power stroke for #1 the marks on the timing gears were pointing away from each other rather than lined up. I was convinced this was the problem and I had the car firing on the wrong cycle. I thought it might be running poorly because it was firing on the exhaust stroke. So I turned the timing back 180 degrees thinking this was the issue. The car didn't start but did in fact make a loud boom under the hood.......... Afterwards I checked my work by taking the valve covers off and seeing what valves opened when and I was right the first time. The engine builder just had the timing marks pointed away from each other at 0 degrees. I assumed he put the timing gear on the cam 180 degrees backwards lined up the marks at TDC for the exhaust stroke and didn't verify they lined up at the power stroke instead.......... But if he messed that up maybe he got the cam degrees off as well and instead of being 180 degrees off he's an extra tooth or two off and is actually 185 or 190 degrees off.

As for the overlap. I talked to the tech support at comp cams. They told me that cam is specifically designed to bleed off heat and pressure in the cylinders to prevent detonation on pump gas. Are you saying this is contrary to that advice?
Yes and no. Cams with a lot of overlap bleed off cylinder pressure at lower rpms. This can allow you to 'get away' with high compression on less octane at cruising speeds and lower. But when you stand on the throttle, that cam will do what it was designed to do: fill up the cylinders quickly to make horsepower. If it didn't do that, your engine would be a real slug. Running any iron head that gives you a compression ratio of over 9.5: 1 with a Pontiac engine will generally need 95-100 octane fuel to run without detonation. I agree that 9" of manifold vacuum is too low for anything but a race-only engine, if caused by camshaft profile.
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post #8 of 22 (permalink) Old 10-24-2018, 05:07 PM
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Quote:
Originally Posted by [email protected] View Post
Thanks for the reply. At one point I had to take the timing cover off. It did reveal the timing chain at zero degrees with on the power stroke for #1 the marks on the timing gears were pointing away from each other rather than lined up. I was convinced this was the problem and I had the car firing on the wrong cycle. I thought it might be running poorly because it was firing on the exhaust stroke. So I turned the timing back 180 degrees thinking this was the issue. The car didn't start but did in fact make a loud boom under the hood.......... Afterwards I checked my work by taking the valve covers off and seeing what valves opened when and I was right the first time. The engine builder just had the timing marks pointed away from each other at 0 degrees. I assumed he put the timing gear on the cam 180 degrees backwards lined up the marks at TDC for the exhaust stroke and didn't verify they lined up at the power stroke instead.......... But if he messed that up maybe he got the cam degrees off as well and instead of being 180 degrees off he's an extra tooth or two off and is actually 185 or 190 degrees off.

As for the overlap. I talked to the tech support at comp cams. They told me that cam is specifically designed to bleed off heat and pressure in the cylinders to prevent detonation on pump gas. Are you saying this is contrary to that advice?
You were actually right both times. Think about it - the cam gear turns exactly once for every two revolutions of the crank, so if you line the gears up "mark to mark", or both of them at 12 oclock - it's exactly the same thing. Install the gears "mark to mark", then turn the crank exactly one revolution and look - now they'll be both at 12oclock. So, installing the gears either way is correct. What matters is making sure that the distributor rotor points to #1 cylinder terminal at the same time that cylinder #1 is at TDC on the compression stroke, not TDC on the exhaust stroke.

Cam timing/valve timing which is what you're dealing with that involves the timing chain and gears, is separate from ignition timing - which is what we suspect is causing your problems.

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post #9 of 22 (permalink) Old 10-25-2018, 02:01 PM Thread Starter
 
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Thanks for all the advice. Lots of questions asked and things brought up I never thought of. I have the cam specs at home i can post tomorrow but don't know off hand. Harmonic balancer is something I didn't think of. Although I would think the timing would be moving around quite a bit if that was the case but it stays pretty consistent and verified it is at TDC to line up with the marks. I've also tried many different timing settings from many different recommendations and nothing seems to make a difference for running better or worse. I have a good timing light and I feel like I've ruled out ignition timing pretty well. The spark plugs look pretty normal but I haven't run it enough to indicate much just yet.

Valve timing might be a different story though. I'm starting to think the engine builder got the cam off by a few marks. I'm also thinking the cam is too hot for the car. I might end up swapping the heads and putting in a more mild cam. I really don't want to run race gas or even premium for that matter, top end power is completely useless to me, I want a cruiser not a racecar, and it's a convertible that is WAY too loud anyway. I would rather have something I can put 87 octane in and cruise around and get decent mileage (at least for an old muscle car not by toyota prius standards) than a fast car. It's a 4 speed convertible going fast is not what I want this car to be all about. I told my engine builder that but I think he heard me say, "build me a nascar engine." I'm also wondering if the 9" of vacuum isn't enough to make the venturi on the carb work right. Holley tech support said it would be ok but they also didn't seem to sure of themselves.

I know where I can pick up some machined iron heads for pretty cheap. My ram air 3 heads I could probably sell and that would be a wash. I might even consider aluminum heads. Or keep it simple and run a thicker gasket. Has anybody ever done this to lower compression? I can't seem to get a consensus if this is a good or bad idea. I also like the idea of a roller cam but not sure I want to spend that much converting it.
post #10 of 22 (permalink) Old 10-25-2018, 02:57 PM
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Running thick head gaskets to deal with a compression problem is a bad idea, do not do it. The reason is because that will kill the quench characteristics in your combustion chambers. You want the tops of the pistons to be coming within about .040-.050 of an inch to the tops of the cylinder heads because this helps to promote turbulence in the chambers. Turbulence in the chambers is good for helping you avoid problems of detonation. If you don't have good quench the engine will actually be more prone to detonate even though compression may be lower. Fat gaskets are a solution of "last resort" only after you've done everything else to address the problem, including changing pistons.

Also just because your timing mark isn't jumping around tells you absolutely nothing about whether or not the mark is correct. All that means is that you don't have a lot of excessive slack in your distributor drive gear or timing chain, and also that the outer ring of the balancer isn't loose and moving. It's quite possible for the marks to be off and still be steady in the light. You won't know until you actually verify it. The only way you're ever going to find out what this engine is actually doing is to start verifying and checking things. Or you can keep randomly changing parts, spending money, and just hoping that you happen to get lucky and stumble over it. It's your choice.

Also, when you set your timing are you doing it with the vacuum line to the advance mechanism disconnected and plugged? If not then that's a problem.

I just reread your original post and paid attention to what you said you've tried and what you didn't mention. One thing jumped out at me having to do with carburetion (yes, that 570 Holley is WAY too small) but I really want to know what your cam specs are before chasing it too far. I'm guessing that you're having to open the idle speed screw in quite a bit to get it to idle, so much so that it's actually idling on the transfer slots and not on the idle mixture screws. Here's a quick way to tell: turn your mixture screws all the way in just so you determine their current settings, then reset them back to where they are now. Start the engine and let it warm up some. Start turning the mixture screws in. Can you kill the engine just by turning those screws or do they seem to have little to no effect? If they don't do much of anything to how the engine runs, then you're running on the transfer slots. This is a problem that needs to be addressed before you try much of anything else.

I also just reread your post about when you checked the timing marks yourself with the timing cover off. You said the marks were pointed "away from each other" --- what exactly did you mean by that? If the mark on the crank sprocket is at 6'oclock and the mark on the cam gear is either at 12 o'clock or at 6'oclock, then that's definitely wrong --- but I'm not sure the engine would run at all with it like that. If things are correct, then when the crank sprocket is at 12'oclock then the cam sprocket should be at either 6'oclock (pointed directly at the crank mark) or also at 12'oclock. If you're not positive about the orientation, then it would be a good idea to pull the cover again and make certain. With the crank sprocket at 12'oclock, you should be able to line a straight edge up with the center of the balancer bolt hole, the mark on the crank sprocket, the mark on the cam sprocket (either 6'oclock or 12'oclock - doesn't matter which), and the center of the cam bolt hole.


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Last edited by BearGFR; 10-25-2018 at 03:55 PM.
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