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Discussion Starter · #1 ·
I took temps on all tubes after 10 min of running. Here's the numbers I got


278 (8) 268 (7)
370 (6) 250 (5)
238 (4) 230 (3)
200 (2) 198 (1)

As you can see it's cooler towards the front, That makes sense. What do you think about cylinder #6?

Note...All the ignition is good, plugs good, compression test good, Intake gasket good(changed)carb good (new), can't find vac leaks.
 

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Don't know if I would rejet without first reading the plugs.

To answer your question; the reason one tube is hotter than another is because of a difference in fuel distribution between the cylinders. This is the reason Barry Grant introduced his new four shooter carb. And the reason dyno cells have thermocouples on every tube of the exhaust header. You can adjust the jetting to compensate, or go to two fours on a different manifold, or convert to port injection EFI as possible means of correcting the problem. Most don’t worry about it so long as it isn’t so lean as to make a tube glow red., and the motor pulls strong.


Larger Dave
 

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If this is on a dual plane manifold, put this in prespective with the distance of the carb and what ports are feeding what. Generally the ports that share one side of the carb that are furtherest, can get the hottest. IMO. But if you have a vacumn leak somewhere, this will also have impact. Think about it.
 

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Discussion Starter · #5 ·

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Discussion Starter · #6 ·
If this is on a dual plane manifold, put this in prespective with the distance of the carb and what ports are feeding what. Generally the ports that share one side of the carb that are furtherest, can get the hottest. IMO. But if you have a vacumn leak somewhere, this will also have impact. Think about it.
Yes it's a duel plane. But why would it only be on 1 cylinder? wouldn't it be on say #6 AND #5. I replaced the intake gasket already. I'll look for leaks AGAIN! Could anything with the valves do this?

Remember I have been trying to find a low RPM miss and a little loss of power.
 

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ok... after re-reading the other post... it seems to make all the sense in the world that the high egt on 6 and the miss is related.

2 ways to prove it. Best way would be to hook it up to an old fashioned sun machine, the kind with the big osciloscope display? That will show in a heartbeat what is going on.

Shadetree way would be to remove (and ground out) each plug wire (one at a time) and verify all but # six gives you a new miss.

I was gonna say AH-HAH and blame the power brake booster, but that hooks into #8 so never mind.

So what causes a high EGT? Could be a lean misfire. Could also be burning fuel in the exhaust port.

Do you still have the issue where one idle screw will kil the motor but the other one won't?
As was said, that is a big clue.

What does your vacaum guage needle look like?
Does the needle flutter?

Still bothering me that the compression is perfect on this engine.

With each primary feeding 4 cylinders, it's hard to blame this on carb, but the ignition had been ruled out.

Think I'll go help the wife get dinner on the table, and ponder this some more later.
 

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Discussion Starter · #8 · (Edited)
ok... after re-reading the other post... it seems to make all the sense in the world that the high egt on 6 and the miss is related.

2 ways to prove it. Best way would be to hook it up to an old fashioned sun machine, the kind with the big osciloscope display? That will show in a heartbeat what is going on.

Shadetree way would be to remove (and ground out) each plug wire (one at a time) and verify all but # six gives you a new miss.

I was gonna say AH-HAH and blame the power brake booster, but that hooks into #8 so never mind.

So what causes a high EGT? Could be a lean misfire. Could also be burning fuel in the exhaust port.

Do you still have the issue where one idle screw will kil the motor but the other one won't?
As was said, that is a big clue.

What does your vacaum guage needle look like?
Does the needle flutter?

Still bothering me that the compression is perfect on this engine.

With each primary feeding 4 cylinders, it's hard to blame this on carb, but the ignition had been ruled out.

Think I'll go help the wife get dinner on the table, and ponder this some more later.
I'll try to awnser best as possible..
-Idle screws work right, Both will kill the motor one at a time
- Vac gauge is around 11 inches ,slight flutter maybe 1/2 inch. Prob cam related.
- Grounded out wires and they all sound the same.

Also I am noticeing , I may be going crazy because of all the test I've done, but right around that cylinder I hear a clacking sound. Not a loose rocker sound or a rod knock, kinda K K K K , EVER SO SLIGHT. I tried putting my trusty sprinkler hose around and I couldn't hear anything. Maybe I'll pull the cover again and take a look.
 

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Discussion Starter · #10 ·
Do you have a vacuum fitting on that runner of the intake? If so chances are there is a vacuum leak somewhere between the fitting and the end of whatever is hooked to it? Maybe a brake booster?

No, nothing
 

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Yes it's a duel plane. But why would it only be on 1 cylinder? wouldn't it be on say #6 AND #5. I replaced the intake gasket already. I'll look for leaks AGAIN! Could anything with the valves do this?

Remember I have been trying to find a low RPM miss and a little loss of power.
Manifold I am looking at, 5 & 6 do not come off same plane level. With a plenum divider and two front idle circuits in front, thats one circuit out of two , that will have to feed 4 clys, those clys closest to idle circuits get the strongest vacumn signal, Cly 5 is furthest away on top plane. On lower plane #6 is even further and gets the weaker vacumn signal as being further away than 5 coming from a higher plane. .
The port furtherest away will prolly get less atomized fuel or it will lay on the floor of the manifold thus not have very much velocity to get to cly thus being somewhat lean? Wonder what happens to temps when rpm is set up higher? Would spark plug indexing help here? You must be generating more heat for some reason and being lean is my first guess.
I would look at richening up the idle circuits. . Perhaps you also have less cly leakage on that cly also, thus running a higher dynamic compression? Could a valve be too tight or loose? What PV are you running? What Carb?
 

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Also possible sticking valve from heat? Intake could also still be the culprit. Plug off all vacuum to the engine so nothing is drawing vacuum to anyplace.

Plug breathers with a rag. Stick a can of cab cleaner with the red small tube into one of the closed off breathers and start spraying. If you get an increase idle just as you would looking around the base of the carb or hoses etc you have an internal vacuum leak. This leak will most likely be your intake and #6 can be a trouble to seal due to thin seal at the FWD edge.

If you find an internal you might also try spraying around the intake at all sealing areas that are external locations after trying this to see if you have air leaking from anyplace else. Make sure you have the breather you were spraying into covered and plugged and everything else plugged.

On you dual plane intake you could pressurize you engine and see which side of the intake air comes from. I would not use more than about 10 to 15 PSI. I have done this before but do this part at you own risk. Rings and seating can be affected.

Joe
 

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Discussion Starter · #14 ·
Manifold I am looking at, 5 & 6 do not come off same plane level. With a plenum divider and two front idle circuits in front, thats one circuit out of two , that will have to feed 4 clys, those clys closest to idle circuits get the strongest vacumn signal, Cly 5 is furthest away on top plane. On lower plane #6 is even further and gets the weaker vacumn signal as being further away than 5 coming from a higher plane. .
The port furtherest away will prolly get less atomized fuel or it will lay on the floor of the manifold thus not have very much velocity to get to cly thus being somewhat lean? Wonder what happens to temps when rpm is set up higher? Would spark plug indexing help here? You must be generating more heat for some reason and being lean is my first guess.
I would look at richening up the idle circuits. . Perhaps you also have less cly leakage on that cly also, thus running a higher dynamic compression? Could a valve be too tight or loose? What PV are you running? What Carb?
New holley 670 Avenger. 6.5 power valve. I'll double check the valves tommorrow. I readjusted them 3 times in this whole process. The idle/miss never changed.

When I had the RPM set at 1500 for 5 min that cylinder was the hotest also. I didn't write down the numbers but I think they were all in the high 300's and that was in the 400's
 

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ok your sure the carb is ok
next question is if its not the carb a vacum leak is possible on that intake runner.
I chased an internal leak to the intake manifold gasket that didnt seal on the underside of the intake causing the mixture to lean out that cylinder and make the noise you mentioed (somewhat).
the oil smelled a bit like gas in worse case senerio like mine was.
after 2 sets of gaskets i found the intake (which I bought at a swap meet as new) was the culprit and wasnt sealing due to improper torque sequence.
Im betting you have the same problem.
BTW just cuz the carb is new doesnt mean its good.
I returned a few NEW carbs for various problems
 

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Don't see anything wrong with your Holley unless one side is not delivering fuel or enuff of it. May try what Joe is talking about, I have heard this work more than once. Its hard to diagnose problems like this , 1500 miles away. Just got to check and recheck.
 

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Discussion Starter · #17 ·
ok your sure the carb is ok
next question is if its not the carb a vacum leak is possible on that intake runner.
I chased an internal leak to the intake manifold gasket that didnt seal on the underside of the intake causing the mixture to lean out that cylinder and make the noise you mentioed (somewhat).
the oil smelled a bit like gas in worse case senerio like mine was.
after 2 sets of gaskets i found the intake (which I bought at a swap meet as new) was the culprit and wasnt sealing due to improper torque sequence.
Im betting you have the same problem.
BTW just cuz the carb is new doesnt mean its good.
I returned a few NEW carbs for various problems
I hear everything your saying. If you read my other thread this happend all of a sudden. I replaced the intake gaskets once and made sure they sealed GOOD, just like I did the first time. I know what your saying,but I don't think it's my gaskets. Who knows. I really don't want to change them again.I torqed them correct in three stages. My old carb was the exact same way. Nothing changed with the new one.
 

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I feel your pain.
Not trying to undermine any of the good advice anybody is giving you, but don't read too much into those exhaust temps.
I work on industrial V-12 and V-16 engines( from about 3000 to 10,000 cu in + ) that have pyrometers. You ignore the damthings until the engine has a monster load on it. Unless you have a dead hole. It's not uncommon for an idling engine to have 3-400 degrees difference between cylinders. It just means they aren't working yet. 1500 RPM fast idle temp test, no sorry, you still just barely have the throttle open. It might come around some, but I wouldn't expect it to be consistent yet.

A trick for detecting a sticking or burned intake valve is to shoot the temperature of the intake manifold runner to that cylinder. I guarantee it'll show.

I am, unfortunately, siding with the gentleman telling you to not expect your new carb to be a good carb. I once had a pretty strong stock 350 with an Edelbrock carb. Ran great but every 20 or 30,000 miles it would lose a little manifold vaccuum and get a little flutter in same. Drop from like 16" to shaky 12" if I remember right. Rebuild the carb, Good for another year or so. Never saw a damthing wrong with the carb. Got to where I just took it apart and blew air through all the passages to heal it.

Failing in that I would rip the intake and heads off and be done with it.
 
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