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Old 07-23-2010, 04:27 PM
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Narrowing down on unusual misfire, but still need help

Some of you may have noticed some of my recent posts on a rare misfire and sparkplug pics. The misfire I get is odd and I'm going in so many different directions, it's time to start a new post with my latest clues.

The engine runs perfect until after about 45 minutes running in hot weather.

Then it misfires. If it's hot enough out, it happens a lot. Never when cruising, just when accelerating fairly hard. Typically it is an engine misfire, although the last time there must have been excess fuel in the exhaust from a misfire and it made a much louder bang. This was the driver's side, so cylinder #4 wasn't to blame in this case (for those of you who saw my plug pics).

The water temp comes up to 180F normally, but can reach 190 when idling around town.

The oil temp continues to climb and once its at 210 or above (its never gone over 220F), that's when the misfires start when I step on it.

I had this happen about an hour ago, so I shut the engine off, raised the hood and felt around. The carbs are only warm, but the fuel pump is too hot to touch for more than a second. The fuel line is also hot, but gets less hot the closer to the carbs it gets. The water pump is also hot, and a lot of heat must be in the engine as the water temp went up to over 200F just sitting there not running.

This engine was rebuilt as the balancer outer ring slipped and I inadvertly advanced the timing too far during maintenance. A piston broke due to detonation. So all my carb jetting, etc. remains the same.

My only real change is the timing. Way back in 1996, I had it set for 12 deg at idle and 38 max. After the rebuild, I had a some bogging, etc., and through some experimenting, I found it ran best at 20 degrees intial while keeping the max at 38 degrees.

I also ran with a vacuum guage hooked up so I could see what was going on as I did change the Power Valves from 5.5s to 6.5s. Cruising vacuum is higher than I thought.....around 18 inHg. Even on light hills, it doesn't drop below 15 inHg. Seems to me way back when I had the timing at 12 degrees intial, the vacuum was lower....somewhere between 12 and 14 inHg.

I'm starting to suspect my ignition timing is generating a lot of heat and that's why the oil temp is getting so much higher than the water temp. However, I always thought retarded igntion generated more internal heat.

I know after the rebuild, I started at 12 deg intial and I knew something weas wrong due to the heat coming off the headers was higher than I remembered. increasing the timing did seem to drop the header temps.

Some pertinent specs on my engine are:

Ford 428 with 2x4 single plane divided plenum intake and twin Holley 600 cfm carbs

Crane Fireball 294 camshaft set at zero degrees

Mallory dist, no vacuum advance, Pertronix Unit installed, Accel 9.5 mm low resistance wires, Autolite 45 plugs

Timing is 20 deg BTDC at 750 rpm and 38 deg max coming in somewhere around 3000 rpm. Cranking speed has a somewhat lower advance due to the stock Mallory advance springs.

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Old 07-23-2010, 04:56 PM
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Went back and had to read your first thread to get an idea of what was going on. The #4 cyl. I suspect is either guides or rings (maybe intake gasket leakage?). The PCV was mentioned, but if it's between #3 and #4...#3 should be oil soaked also.

The carbs you have are vac. secodary? Progreesive throttle linkage? I'm trying to figue out why #1 and #8 are lean, if the front carb. is feeding the engine...right next to #1...but #8 is lean also (which probably should be from being so far away from the primaries). You probably have multiple problems, that need to be taken care of one at a time.
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Old 07-23-2010, 05:09 PM
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Quote:
Originally Posted by a1supersport
Went back and had to read your first thread to get an idea of what was going on. The #4 cyl. I suspect is either guides or rings (maybe intake gasket leakage?). The PCV was mentioned, but if it's between #3 and #4...#3 should be oil soaked also.

The carbs you have are vac. secodary? Progreesive throttle linkage? I'm trying to figue out why #1 and #8 are lean, if the front carb. is feeding the engine...right next to #1...but #8 is lean also (which probably should be from being so far away from the primaries). You probably have multiple problems, that need to be taken care of one at a time.
Not sure about #4 yet, but I suspect an intake gasket leak. It is a little wet with oil on the bottom corner of the gasket where the block, head and intake all meet, plus the gasket looks a little less compressed the closer to the block you go. It's not the PVC because I ran it today with that system disconnected and still had the problem. Some of todays misfires were definately from the driver's side, so #4 cylinder isn't the only issue.

That #1 and #8 stuff is misleading as every time I do a plug check, different ones look white and others look gray.

Yes, vacuum secondaries and progressive linkage.

All I can say is this combination worked great for me for years, but now I have an issue. Oh....I can say more....LOL.....don't forget, this combination is currently working ...dare I say it?....."awesome"...until the engine is completely warmed up and the weather is hot. And I don't mean once the water temp is fully up.....it's sometime after that as the oil temp gets to the 210-220 range (pan holds 9 qts by the way).
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Old 07-23-2010, 05:12 PM
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I think the lack of vacuum advance is hurting you, and leading to the higher engine temps and oil temps. I'd go one heat range colder on the plugs also, 45 seems hot to me.

Have you tried mixing in some race gas, just to see if it makes a difference?? Your other post said that cranking compression was 215 psi IIRC, that is awful high for pump fuel, to the point it won't matter what you do with the timing when the motor is hot and ingesting hot air. The FE combustion chamber shape plays a part in this intolerance also.

Sounds as if you may also be on the verge of vapor lock, could it be lean misfire when it is hot?? Not quite vapor lock but close to it?? Any way to get a fuel pressure reading when it is hot?? Maybe a temporary guage set-up??

Fuel line from tank close to headers?? This is an late 60's Mustang IIRC??

I'm not a big fan of Accel wires.

Just tossing out ideas as they come to me.
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Old 07-23-2010, 05:33 PM
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Quote:
Originally Posted by ericnova72
I think the lack of vacuum advance is hurting you, and leading to the higher engine temps and oil temps. I'd go one heat range colder on the plugs also, 45 seems hot to me.

Have you tried mixing in some race gas, just to see if it makes a difference?? Your other post said that cranking compression was 215 psi IIRC, that is awful high for pump fuel, to the point it won't matter what you do with the timing when the motor is hot and ingesting hot air. The FE combustion chamber shape plays a part in this intolerance also.

Sounds as if you may also be on the verge of vapor lock, could it be lean misfire when it is hot?? Not quite vapor lock but close to it?? Any way to get a fuel pressure reading when it is hot?? Maybe a temporary guage set-up??

Fuel line from tank close to headers?? This is an late 60's Mustang IIRC??

I'm not a big fan of Accel wires.

Just tossing out ideas as they come to me.
Oh....keeping tossing out ideas. I'm out of them.....LOL.

This is in a Shelby Cobra kit car. I built it back in the mid 90's and never had this problem before.

Pistons were custom made by FPP. They are supossed to be 9.5:1 CR. My former pistons before the rebuild were teh same, but 0.030 over and I now have 0.040 over. With the old pistons, I was getting 195 to 200 psi with no trouble.

I was running a colder plug, RF9YCs. Stock, the correct P/N is RF11YCs which is hotter. The Autolite stock plug is supposed to be a #45. That's all I know about plugs for this engine.

I am thinking vapour lock too as the fuel pump gets so hot, but the carbs are only warm, so I'm not sure about it yet. But if it is, the cure isn't to insulate everything in sight, it's to figure out why there's things are hotter than they used to be and fix that.

No, the fuel lines arn't close to the headers. If the fuel is getting too hot, the pump is the hottest part the fuel goes through that I can tell by feeling around.

I have a fuel guage on the carb inlet lines, but of course I can't see it under the hood. Not sure I want to connect somthing up to read inside the car in case I get fuel all over the place and a fire starts, but it may come to that (hooking up a remote pressure guage, not having a fire!)

I've only run 91 octane. I'd like to hook up a vacuum advance, but all the ones I can find have too large a dist cap and it will hitthe front carb, or the rad header tank or both. Another Mallory with vacuum advance will fit, but I'm not happy with Mallory distributors. Their gears are a loose fit on the dist shaft and the split pin can break. I had to modify the one I have now as this did happen to me quite a while back.
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Old 07-23-2010, 06:07 PM
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OK, Cobra Do you have an oil cooler, like the originals did?? It would definately lower the oil temp. We did this with a friends hot running 351W, it took 10 off the water temp also.

I'd try mixing in some race gas, can't hurt and might give you some direction.
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Old 07-23-2010, 06:29 PM
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No, I didn`t think I really needed one in my climate. I`ve been doing research on the Cobra forum and igntion timing is common with other owners. Sort of mixed with respect to oil temps. Many owners have coolers and their concern is oil temps that are too low. Others are getting more or less the same temps I get.

I did have the heads resurfaced during the rebuild. They claim they only took off 10 or 12 thou, but my compression is up about 15 to 20 psi from before. I have no idea where to get race gas (would be nice to try it), but if I can get the temps down, my problem may go away. On the other hand, it may be something failing once it gets hot that I have yet to identify.

When I first had this problem last year, I put the points back in and it seemed OK, so I bought another Pertronix Unit and installed it. I thought everyting was fixed, but all the point testing and the new Pertronix unit testing were done near the end of the summer when it wasn`t so hot out.

Hey...thanks for the thumbs up. Here`s some pics:



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Old 07-23-2010, 07:22 PM
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As far as I am concerned the AC/Ford Cobra 427 is one of the most beautiful sports cars ever built, just drop dead sexy . I also love the Cobra Daytona Coupe, I think it is Factory Five that is doing a kit version?? Who did your kit?? Building one or the other is on my "bucket list".

As far as you engine, the head cut and overbore will raise the compression, maybe it puts you over the edge in hot weather?? seems we are having the hottest summer in a long time so far this year. I'm in south central lower Michigan, we are getting pounded by that freight train weather system that stretches clear to Iowa right now .
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Old 07-23-2010, 09:35 PM
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thye 200+cranking compression is of concern to me and beyond what trust on "91 octane" in hot conditions. you are riding the edge for sure.

heat and mechanical fuel pumps can be very pesky.
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Old 07-24-2010, 07:45 AM
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I would try running an electric fuel pump to get the fuel supply away from the engine heat. Your current setup may be pumping bubbles when you reach full operating temp--???
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Old 07-24-2010, 07:56 AM
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Yes, that's my number one thought at this point, however I am going to try some other thoughts first as I'm not sure where I can find an electric pump.

No need for an expensive new one for testing purposes, so maybe I can get one from the junkyard...of course modern electric fuel pumps are for fuel injection and run at way to high a pressure for carbs.

So it's a great idea, just not sure how I'm going to implement it easly. But I will once I've exhausted a few other ideas that are more easily tried.

I appreciate your post. Going in circles like I am, it's nice to read thoughts that confirm some of my thoughts arn't so crazy.
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Old 07-24-2010, 02:27 PM
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It's fixed.....sort of....

I replaced the Pertronix Unit with the dual-points and just got back from a test drive. No misfires at all. I made sure the engine reached max water and oil temps by idling around town and having it sit on the hot pavement for 10 minutes at a time.

Not sure why this second Pertronix Unit is doing the same thing as the old one. If I can figure it out, I'll post it here. This is why I said "sort of fixed" as I would like to get away from points. Choices are limited, as from what I can tell, only a Mallory dist will fit between the front carb and the puke-tank due to its small dist cap.

On another note, #4 sparkplug was in good shape after yesterday's drive. Not sure if the PVC being disconnected was it, but I think having a hotter plug plus the Autolite design is what did it. Just a slight off-white colour to the porcelin.

I won't be 100% sure of all this until I get some more time in with the car, but so far, so good. Thanks to all who gave me ideas on this.

Sidednote: I am still wondering why the Pertronix has failed me. If the electronics really died, then I should have had complete ignition failure. One would think it's a problem with the car as two seperate Pertronix Units both screwed up. I checked the air gap and got 0.025", which should be fine. I did notice that the section of the plastic magnetic sleeve that holds the magnets is partially below the pickup unit.....i.e. too far down the dist shaft. It may also be, as suggested above, I am losing some voltage to the coil when things get hot. Enough to screw up the Pertronix Unit, but not enough to affect the points system. That would have to be a wiring connection, igntion switch or a problem with the fuse panel, as there is no ballast resistor installed.
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