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1967 corvette 327/300 automatic, overheating issue due to timing and tune
First off, let me just say this is my first post on here so be gentle please. I know there’s a million posts on here about timing and tuning this car but for every post I find with that information, there’s at least ten other posts that contradict it. So let me just tell you my situation and see if anyone can help me get this corvette back on the road. I got this numbers matching(except the auto trans) 1967 c2 327/300 with a Holley 3810 about ten years ago and when I got it, it threw a rod after about a 100 miles. I pulled the engine and had it professionally overhauled. They resleeved two cylinders(1 and 3) and bored all the cylinders 40 over. I reinstalled the engine and ever since the overhaul it over heats at idle. At cruising speed, it’s stable around 160, once I come to a stop it’ll reach 210 in 5 minutes, after another 10 minutes it approaches 250. I’ve verified the coolant temperature sensor and gauge with an calibrated IR gun and measured resistance at set temperatures in a calibrated electronics oven. I’ve installed a Dewitt’s aluminum radiator(PN32-941), an edelbrock high flow water pump(PN8810-2), a new fan clutch, a 6 blade radiator fan, numerous 180* thermostats, coolant flush using dexcool 50/50 antifreeze, 16lb coolant reservoir cap, wired open the heatriser, blocked off the hot slot, replaced all coolant hoses with repro hoses with full length metal coils inside, installed a pertronix ignitor 3 and new spark plugs (NGK PN XR4 5858), pulled the heads and had them cleaned and magnafluxed, and installed new head gaskets(felpro PN501SD). I’m about at my limit now, I would love to keep the original engine but those new crate gen v LT engines are becoming very attractive to me at this point.
Before I decide to go down that road, I’ve decided to give timing and tuning one last shot. I did a running valve adjustment today which didn’t really change anything but at least I know that’s good. My distributor has a B1 vacuum canister installed and I verified that it’s still functioning as advertised. I figured I’d cover all my bases and just purchased a B28, B26, and a B22 last night and they should arrive tomorrow. The idle rpm in gear was about 600 with vacuum about 16-17”hg (vacuum advance hooked up vacuum gauge connected to choke vacuum port). Is there anything that I’ve missed? Should I try to tune to the factory settings of 500 rpm in gear? Any input would be great as long as it’s not something I’ve already done (like replace the thermostat)lol. I’ve tried eliminating the fan clutch and that improved cooling but didn’t totally solve the overheating issue. I also don’t want to go the electric fan or electric water pump route, if the engineers in 67 didn’t need them I shouldn’t either. Thanks for reading any advice in advance.
Wow, that's quite a shot with the parts cannon. I'm not sure it's a timing issue. Unless the timing is severely retarded it shouldn't hit 250°. How does it run in general -- ease of starting, idling, acceleration, steady cruising at town and highway speeds? If all of those seem normal, then I'd focus elsewhere.
Have you flushed the radiator and the heater core to ensure they are freely flowing?
How old is the water pump -- could the impeller blades be corroded away?
On the fan clutches, there should be just a little resistance to spinning when cold, as in, when spun by hand they probably shouldn't make a full revolution and definitely not more than one. Likewise when the engine is hot and turned off, the fan should come to a stop within a revolution or two at most.
Do you own a MityVac? If so, hook it up to your vacuum advance canister, pull the distributor cap and then pump up the MityVac while watching the vacuum advance mechanism move inside the distributor. Note when the vacuum advance is pulled all the way in. What is the vacuum reading? It's probably going to be somewhere between 18"-20" hg. Your engine is producing 18" hg at idle and, therefore, your vacuum gauge is doing nothing to give you full advance at idle. The appropriate vacuum advance can for your application should be the B22. Again, using your MityVac on the B22 you already have bought, check to see what your reading is when the advance is pull all the way in...should be somewhere between 14"-16" hg. You want a vacuum advance that is all in 2" less than the vacuum your engine is producing. In other words, you want the vacuum cannister to fully deploy your idle vacuum advance.
I hope this makes sense. Duke, Lars, Avispa and many, who are much smarter than me, would probably explain it more clearly.
I'm a huge...trying not to make a pun here....proponent of fan clutches in cars that had them from the factory. In every GM I had which was equipped with them, in which the car overheated when stopped, it was the fan clutch and I replaced three of them in one vintage car.. which sounds like a condemnation of fan clutches. It isn't, that took 31 years and 375,000 miles. Anyway. Cold- hard to spin, hot easier to spin, as described above. I bet you know that, you obviously have more than a passing idea of what you're doing.
Over-bored engines can have cooling issues.I hate to say this, but if the engine didn't overheat before the overhaul, and ever since the overhaul it does, there can be a cause and effect relationship. Did you ever talk to the builders about this when it was first done?
Now that I spoke doom and gloom, if it overheats only when stopped it's likely an 'airflow through the radiator' issue to my way of thinking.
Wow, that's quite a shot with the parts cannon. I'm not sure it's a timing issue. Unless the timing is severely retarded it shouldn't hit 250°. How does it run in general -- ease of starting, idling, acceleration, steady cruising at town and highway speeds? If all of those seem normal, then I'd focus elsewhere.
Have you flushed the radiator and the heater core to ensure they are freely flowing?
How old is the water pump -- could the impeller blades be corroded away?
On the fan clutches, there should be just a little resistance to spinning when cold, as in, when spun by hand they probably shouldn't make a full revolution and definitely not more than one. Likewise when the engine is hot and turned off, the fan should come to a stop within a revolution or two at most.
That's all I've got.
Sadly yes my troubleshooting turned into a shotgun approach but at least I did it in a methodical way. The first thing I did was check the coolant sensor and replace the thermostat and cap. Then when that didn’t work I took the fan clutch off and tested its functionality by putting it in a electronics oven and spinning it every ten degrees Fahrenheit up to 260 which it passed. The fan clutch maybe spins 30 degrees of rotation when I spin it cold and it stops immediately when hot. After that I tested the coolant system pressure. Nothing crazy, it held pressure for 2 hours. Then I took the radiator out and had it professionally cleaned and flushed. Then I replaced the radiator hoses thinking they may collapse at hot temps because they didn’t have the metal coils inside. That’s when I started shotgunning the coolant system parts. It started with the water pump then the radiator. I’ll be honest I have not checked the heater core, I’ll do that tomorrow but can you explain to me how that could effect engine overheating. Am I missing some sort of theory of op behind the heater core? The car seemed to run fine, no hesitation on acceleration or braking, throwing it into gear caused a slight tendency of low rpm like it wanted to die but that was it.
Do you own a MityVac? If so, hook it up to your vacuum advance canister, pull the distributor cap and then pump up the MityVac while watching the vacuum advance mechanism move inside the distributor. Note when the vacuum advance is pulled all the way in. What is the vacuum reading? It's probably going to be somewhere between 18"-20" hg. Your engine is producing 18" hg at idle and, therefore, your vacuum gauge is doing nothing to give you full advance at idle. The appropriate vacuum advance can for your application should be the B22. Again, using your MityVac on the B22 you already have bought, check to see what your reading is when the advance is pull all the way in...should be somewhere between 14"-16" hg. You want a vacuum advance that is all in 2" less than the vacuum your engine is producing. In other words, you want the vacuum cannister to fully deploy your idle vacuum advance.
I hope this makes sense. Duke, Lars, Avispa and many, who are much smarter than me, would probably explain it more clearly.
Ah the infamous 2” rule. Lol. The reason I purchased every vacuum canister that was mentioned on this forum. I do own a mighty vac and I played around with it today. I didn’t go the approach you suggested, instead, with the engine running and the carb vacuum advance port plugged I hooked the mighty vac to the vacuum canister and and noted when the engine timing started to advance which was just about 8”hg, at 12”hg timing was advanced 10* for a total of 16*(6*btc static) and at 18”hg timing was advanced to 28*. For all I know this test could have been nonsensical and pointless but I felt cool doing it. Lol I’ll try you way tomorrow. Side note timing at 5100rpm was 28* with vacuum advanced hooked up. I think I’m going to try and tune the carb once more tomorrow. Just to see how close I can get to 500 rpm’s in gear while noting how high the vacuum can sustain. At 6*btc static of course.
I'm a huge...trying not to make a pun here....proponent of fan clutches in cars that had them from the factory. In every GM I had which was equipped with them, in which the car overheated when stopped, it was the fan clutch and I replaced three of them in one vintage car.. which sounds like a condemnation of fan clutches. It isn't, that took 31 years and 375,000 miles. Anyway. Cold- hard to spin, hot easier to spin, as described above. I bet you know that, you obviously have more than a passing idea of what you're doing.
Over-bored engines can have cooling issues.I hate to say this, but if the engine didn't overheat before the overhaul, and ever since the overhaul it does, there can be a cause and effect relationship. Did you ever talk to the builders about this when it was first done?
Now that I spoke doom and gloom, if it overheats only when stopped it's likely an 'airflow through the radiator' issue to my way of thinking.
Nice '67by the way
I never got a chance to speak with them, I was deployed overseas when they finally got around to overhauling it, I had a family member pick it and it stall it before I got back. When I called them after the fact they didn’t keep record of it at that point.
You mentioned your timing at 5,100 RPM was 28*, I assume that is mechanical advance all in? A 327 timing should be ALL IN or fully advanced by 3,000 RPM. 36* is a known safe total advance number. So the advance should come in smoothly from idle to 3,000 and then be all done. This part is set with the vacuum disconnected and plugged off so no vacuum leak. Make sure the advance comes in smooth each and every time the same from idle to 3,000, rev it higher than 3,000 just to make sure it does not keep advancing. Many will run as much as 38* in a 327 but the power difference is minimal, probably less than 5 HP so for set up purposes play it safe. The timing setting at idle is really not important. Set it so your mechanical is correct at 3,000 RPM and if your idle timing is between 4 and 12* you can live with it. Some go with 2,800 for full mechanical advance, some go as high as 4,000 RPM but most common today is to have full mechanical advance in by 3,000. 5,200 is way too high. If your car pings or bucks you may need stiffer distributor springs to slow the mechanical advance but you need to set it and watch it to be sure you have smooth advancing mechanical before you start tuning. Sticky plates in the distributor can sometimes give a advance curve that is not smooth or consistent. They should be clean and free of rust to advance smoothly and consistently.
Once your basic mechanical advance is set correctly you can start playing with the vacuum advance which can improve your idle and around town drive satisfaction. Many cars including some factory Corvettes ran a full life without vacuum advance so do not panic to get that set up first. Set up the mechanical so it runs great and then play with your vacuum advance to make it better.
I wish MikeM was still here to offer advice on your fan clutch, we lost MikeM last year unfortunately. Mike always said a good working fan clutch when engaged should blow your hat off if you lean into the engine compartment. That is pretty much true. Engaged, the fan clutch should be able to spin the fan blade 80-90% of engine speed. Dis engaged it should spin the fan blade 10-25% of engine speed depending on the brand and type of clutch. If you are parked and the engine is heating up the clutch should engage fully and be NOISY as it pulls all that air which will cool the radiator quickly. If yours is not doing this it is not working. The Corvette tilted radiator is hard on the fan clutch. There is too much room between the radiator and the fan clutch so many do not engage as they should. Standard automotive fan clutches need to be played with to make them engage fully. I think some of the factory number match units are made to look correct without enough effort to make them perform correctly. Engaged a thermal fan clutch will howl as it pulls a lot of air over the radiator. If your engine is going past 210* it should be fully engaged. There are posts on this forum mentioning changing the clock spring on the front of the fan clutch to make them engage earlier. A good design fan clutch has fins all over the front plate of the fan clutch facing the radiator. The fins cool the silicone inside that engages the fan. If the silicone gets hot it gets thinner so you lose fan speed. The fins help keep it spinning near engine speed when needed. As the engine cools off you hear the fan speed slow down. That is normal operation for a thermal fan clutch. Your clutch has no fins on the face so it has no extra cooling where it is needed most. It may meet NCRS standards but it might be letting you overheat.
The other thing many have had to do is make sure the factory shroud is fully intact and many have added pieces or somehow worked to make the shroud tight so air has to pass through the radiator rather than pass around the radiator. Due to the tilted Vette radiator the spring on the front needs to open at a cooler temp. Usually the spring is set to open about 20* under the desired radiator temp on normal passenger cars and trucks. On a Vette with the tilted radiator that spring might need to open 40* under the radiator temp. Example: The Engine hits 210* and you want the fan clutch to engage. The spring in a C2 will need to be open at 170* because the air is cooler when it reaches the spring compared to normal cars like a Malibu. When the spring gets hot it opens the valve allowing silicone to enter the clutch area and engage the fan blade bringing it up to engine speed. If the valve never opens you never have the fan clutch help cooling the engine. The two fan clutches in your pictures are different, one uses a traditional clock spring, the other uses a flat spring, both work but you see clock springs on most units that have cooling fins on the face. You do not want the spring to engage too early, this will make the fan clutch engage all the time which will cost you power and gas mileage for no gain and they are noisy when engaged so have it set to come on and keep your temp close to the thermostat temp. You mentioned you have a 180 thermostat, the fan should engage around 195 or 200 ideally with the 180 thermostat. Hope some of this helps and is not a repeat of things you already addressed.
As an engine builder told me the more you take off the cylinder walls the closer it gets to the cooling jackets. So now that they are closer they will cool better.
The VAC you are using is not the right model. Since your car is an automatic the correct VAC you should be using is a B26 not the B-1. When you take your vacuum idle reading (600 rpm) you should take it with the trans in Drive with the parking brake on.
I would concentrate on what Westlotorn discussed about timing as that’s the only area I can see that hasn’t possibly been fully addressed. With vacuum advance hooked up and mechanical advance all coming in by 3,000 rpm you should be seeing around 50 - 52 degrees total advance at and above 3,000 rpm. At idle, with vacuum advance (manifold vacuum) hooked up I would hope to see at least 24 - 26 degrees with 10 - 12 of that initial timing.
As an engine builder told me the more you take off the cylinder walls the closer it gets to the cooling jackets. So now that they are closer they will cool better.
...and the more you get concerned about core shift. RE: closer to the water jackets, now the rest of the cooling system needs to deal with the additional heat transfer. Personally I think this is the majority of "overbore caused overheat" scenarios. More efficient heat transfer in the block, same ability to cool in the rest of the system.
I've heard it both ways from pros I have contracted for machine work, and from people in my family who are mechanics and raced super comp.The overbore is more than just a change in diameter to the system as a whole and the whole has to be considered as always. I know before I paid for a 30 overbore I had my block sonic tested first. Of course there was no problem and the engine never went over T-stat temp.
This is why I hoped OP had spoken to the builders. They might have a recommendation for what his cooling system will need.
In the 1970’s most 327 overhauls were taken to .040 over at production machine shops. They went straight to .040 because they did not want to risk having to send the block back through because it did not clean up at .,030. It is a standard oversize for the 327, it would be rare that it could cause any issue. The fact that he runs cool on the highway but not in stop and go pretty clearly shows he has no cooling at idle. Either flow of coolant or lack of air flow over the radiator., Early Dexcool was pretty toxic and caused issues. The newer stuff, maybe even the past 15 years it has been much improved and I use it all the time now.
In the 1970’s most 327 overhauls were taken to .040 over at production machine shops. They went straight to .040 because they did not want to risk having to send the block back through because it did not clean up at .,030. It is a standard oversize for the 327, it would be rare that it could cause any issue. The fact that he runs cool on the highway but not in stop and go pretty clearly shows he has no cooling at idle. Either flow of coolant or lack of air flow over the radiator., Early Dexcool was pretty toxic and caused issues. The newer stuff, maybe even the past 15 years it has been much improved and I use it all the time now.
Yes I am a firm believer that 30 or 40 thousanths of an inch of less cylinder wall thickness would cause these problems. I have been doing this stuff over 50 years and just havent seen it.
Heavy guage aluminum foil is just about .030
I have not checked the heater core, I’ll do that tomorrow but can you explain to me how that could effect engine overheating. Am I missing some sort of theory of op behind the heater core? The car seemed to run fine, no hesitation on acceleration or braking, throwing it into gear caused a slight tendency of low rpm like it wanted to die but that was it.
Heater core: SWAG because you've looked at everything else! If the heater core is not freely passing coolant perhaps it's impeding free flow in the rest of the system.
... Cold- hard to spin, hot easier to spin, as described above.
<whispering> I believe it's the opposite: cold easier to spin, hot hard to spin. </whispering>
The fan clutch works off engine temp so that it engages as the engine heats up. That's why it should come to a fairly quick stop when shutting down a hot engine but may freewheel a few revolutions on a cold engine.
Fan clutches are odd things. Properly working they will engage when hot. Shut the engine off and grab the fan blade and turn it. It will have a lot of resistance for a little rotation and then almost free spin. As you turn the blade manually by hand it pumps the silicone back into the reservoir and frees up the blade. The clutch needs speed to pump the silicone into the clutch area. Shut the engine off and turn the fan by hand and you dis engage the clutch by pumping the fluid back into the reservoir.
When tested like this cold it will resist more because the silicone settles to the bottom of the clutch when parked and allowed to get cold. This silicone at the bottom is now thick and cold so it will hold the fan clutch engaged longer as you turn it by hand.
Cold would be below 35* F. In the summer when the mornings are 60* or warmer this effect is much less. On cold winter mornings a fan clutch will be engaged on the cold start for 30 seconds to 1 or 2 minutes until the friction warms the silicone and frees up the clutch.
Anyone who drives a full size truck with a fan clutch has heard them growl on cold starts with the fan clutch fully engaged. Drive it down the street and you will hear and feel it dis engage. This is normal when you know how they function.
If you look at a fan clutch and see streaks from the center to the outer you know the clutch is leaking the silicone out. Time to replace. Silicone when hot is thinner than water and will leak if it finds a hole but it works well in the fan clutch. The air flow and cooling fins keep it working under overheat conditions that is the cooling fins doing their job. A fan clutch with the stamped steel front plate is not nearly as effective as the aluminum finned front plate designs. The stamped steel front can't pull away the heat as fast as the aluminum finned front plate can.
Fan Clutches are a simple mechanical device that work really well if properly set up with the right clock spring. In the 1980's the company I worked with owned Kool Klutch they were a major producer of fan clutches and tranny coolers. Later they sold off to Hayden and Hayden picked up and used a lot of the Kool Klutch technology to make their line better. The engineers at Kool Klutch were a smart group and good to work with based in Texas at the time, I am only sharing what they taught me, I was not an engineer I just remember what they taught.
08-07-2023, 10:21 PM
1967 corvette 327/300 automatic, overheating issue due to timing and tune
