FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: 390owner on August 07, 2019, 08:26:19 PM
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I have a rebuilt 390 in my 79 bronco. I have the spark advance unhooked for the time being. I had a tank full of 93 octane gas and have the timing set at 11 degrees. The total timing is about 34 degrees. I was going down the interstate about 70 and stepped on the gas and it labor knocked like crazy. What is the deal. I was having trouble with this before that is why I unhooked the spark advance. I have a one wire dist and the engine has around 2000 miles on the rebuild by a machine shop. It has a small comp cam. If I slow the timing down any more it is sluggish
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Have you checked the timing curve? Maybe it is coming on too fast? What RPM is 70 mph?
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What compression do you have? If it's over 9.75 with old iron heads and a small cam your cylinder pressure may be to high
for pump gas. Retarding timing will cause overheating issues too.
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Slipped outer ring on the balancer? A common issue with old balancers.
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Can your tell us more about your setup heads carb ect by the way a full size bronco with a fe is as good as it gets.
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2600 at 70 mph. I have not checked the timing curve. As far as the balancer slipping,maybe but if I time it by ear I really cant go any slower because it is sluggish. It has stock heads with some exhaust valve work for the cam, a cast iron 4 barrel intake, bored 60 over and a 600 edelbrock carb and headman headers. Yes it is a good engine for a 79 bronco. It tows my 24 foot camper well
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2600 at 70 mph. I have not checked the timing curve. As far as the balancer slipping,maybe but if I time it by ear I really cant go any slower because it is sluggish. It has stock heads with some exhaust valve work for the cam, a cast iron 4 barrel intake, bored 60 over and a 600 edelbrock carb and headman headers. Yes it is a good engine for a 79 bronco. It tows my 24 foot camper well
I would check initial, total, and what RPM you get to total. Could be a broken spring, could be that you have too much advance, who knows
One point though, although you are right that being sluggish "by ear" can indicate too little initial, it doesn't tell you what total is doing. That's why we recurve, stock distributors are meant to have a ton of advance. If you have access to a dial back light, mark TDC clearly, and check every 250 RPM or so until it stops advancing.
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Now I know a term to describe how my body feels when I go to work.
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Total is 34 at 3000 rpms
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So your total is just over your cruise RPM. Is this with no trailer? You might need to tighten it a touch to move it up a little higher.
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No way 34 degrees total at 3000 should knock, in my opinion. Something besides the timing is wrong. Sorry I don't know what it is.
paulie
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I agree 34 degrees shouldn't knock on anything . I wonder what jets and Metering Rods are in the Edelbrock ?? What model number # 600 carb is it , Econo calibration or Performance Calibration ?
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I think mr Philpott is right its lean edelbrock 600 is a good carb but are set up for good mpg get a kit it has new springs and rods its easy to work on.I had a 69 f 100 with a 65 390-300 with a small cam stock cj intake and a 3310 holley.It never knocked even with a car on a trailer I bought a 89 bronco that knocked so bad you almost could not drive it on 93 octane. The 302 should have run on squirrel piss it took a long time to find it the holes in the intake from the egr valve was pluged bad. Had to use a drill to open them up after that it never knocked on 87 octane it was lean even with injection.
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The carb is a 600 (1406) with the electric choke. The primary jet is .098 with 75 x 47 rods and the yellow springs. I think the secondary jet is 95. So you think maybe to lean at full throttle. It does labor knock when pulling my 24 foot camper but not to bad usually when I just start up a hill.
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Edelbrock site has a chart to help you tune it i think i would start with a clear step up spring and see if that helps. If you have one if it works then you can go to a softer spring when it starts to knock go one spring stronger as a place to start you may not need to change rods.
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I changed to a richer setting this morning by changing the rods. It still knocked a little while climbing a hill. I slowly pushed the gas to the floor and it labor knocked at wide open. May try the springs next
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I changed rods again to a richer setting. Same thing at full throttle going up a hill????
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I think a wide band would be the only real way to know how lean it is. Unless you just throw some really big jets in it and see what it does. You may be taking too small of steps?
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What do the plugs look like? Mainly are they all the same color?
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I changed to a richer setting this morning by changing the rods. It still knocked a little while climbing a hill. I slowly pushed the gas to the floor and it labor knocked at wide open. May try the springs next
what metering rod did you start with and which one did you change too ?
The spring change is really to help keep the Vacuum in control of the metering rod so it will pull the metering rods down for idle , in other words big cams with less vacuum need lighter springs to keep the rods in the correct down position at idle otherwise the metering rod will be "up" in the off idle position while it's still idling
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What plugs are you using!? That could be part of the problem here
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The plugs all look the same. They are light tan. The rods I started with are 075/047 then went to 073/042 then 070/037 I think the plugs are autolite
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I really doubt a/f mixture based on info so far, are you sure TDC is right and know curve? If yes maybe too much compression for cam or some other build issue
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Checked how the vaccum advance works?
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Checked how the vaccum advance works?
Good point! I've seen engines behave badly when the advance plate gets sticky.
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The vacuum advanced is unhooked
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I think BattlestarGalactic is right put the big jets in and see how it runs if the plugs are tan its not rich now. If that does not help its not lean.What rear gear do you have and how big are your tires if some how you have 3.25 gear and 38 inch tires the 390 is working hard when pulling a trailer.
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3.50 gears and a set of 32 inch tires
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I still think a piston stop, verify TDC, then map out the entire curve...sure mixture contributes to detonation, but when you take an action and it doesn't change something, you might be barking up the wrong tree
Additionally, if you are able to list your specific cam, where it was installed (ICL not location LOL) your actual compression (or the parts you used to include heads) many of us can see if it's a parts mismatch issue
FWIW - It shouldn't be fighting you. Either too much compression, cam too small or too early, or curve isn't what you think it is, might have a little lean spot somewhere but would like to see what your parts are before throwing fuel at it
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Bored 60, stock heads, Had some kind of head work for the cam. Not sure what. I have not check the compression. You are probably looking for 10 to 1 to 9 to 1 I have no idea what it would be. The cam is a comp cam with this number 33-234-4. The engine is sluggish until I get about 1500 rpms. It seems to me this cam would like a lower rearend gear. Once it gets wound up it will really go.
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Well, there we go...
That's a 256 adv duration cam, and likely the quench is above .070 and compression is a bit high for the cam .
I'd estimate your compression to be near 9.4 or so, could even be more, and with a cam that short, you'd need to be at 9:1 in a truck, essentially with your tire, weight, gear combo make it about equal to a 2.87:1 geared car with street tires. There is quite a load on that 390 and likely it's fighting itself inside with loose quench.
It would be nice to take a good look at the build, heads off, and see what you really have. Which head gaskets, which pistons, how deep in the hole, what the chambers measure, but the behavior seems accurate
It really depends where you are in terms of static compression, but my gut tells me cheapest way out is a 4 degree retard of the cam to 110 ICL, (assuming the compression isn't above 9.4 or so) and then set the distributor up to match the build, to include vacuum advance, and you'd likely be better, have more torque and be easier on fuel. The cam is small enough to still have plenty of torque with 401 cid
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That number equates to a Comp Extreme
Advertised Duration 256/268, Lift .487/.493, Ford, Big Block FE
Seems kinda stout for a long legged, big tire truck with a tight converter.
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The 390 is in my 79 bronco . It also has a 4 speed transmission
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Read what I wrote, if your compression was lower not a bad match, but my guess is just too much compression for that cam, combined with a likely loose quench.
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The 390 is in my 79 bronco . It also has a 4 speed transmission
Oops, sorry, forgot that detail. Been a few weeks since I saw the first page LOL!!!
The gearing is still not real optimal on a truck 3spd(creeper, correct?). So I'm sure it falls out of powerband easily.
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427 I did read what you wrote. Thanks. I may just have to live with it for now. This is a complete rebuild and I am not going to tear the engine down. Thanks for all the replies
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You need to find the problem. An engine knock will break a piston or ring in short order.
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427 I did read what you wrote. Thanks. I may just have to live with it for now. This is a complete rebuild and I am not going to tear the engine down. Thanks for all the replies
If you aren't going inside, I recommend you graph the timing curve as it is now, from idle to max timing, then two choices, retard it overall by running less initial, or recurve it to have a little less where you are seeing the ping
However, a Saturday project you could retard the cam 4 degrees and retune and it'd probably be happy