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Author
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Topic: miss-miss-miss
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 10-15-2001 10:43 AM
   
Hi everyone,
sorry I havent posted in a while but life is busy. I have got a question. I built a basically stock 69 351W 4v engine for my Mach 1 and I have been chasing a MISS since I built the engine. this miss is not a dead miss I mean which ever cylinders are missing they dont miss all of the time I first thought I might have an ignition problem so I bought a pertronix ignitor and installed it no change (I also installed an MSD 6AL ignition with blaster 3 coil) no change. my next thought was spark plugs the first set were bosch platinum plus so I replaced them with autolite standard plugs no change next I thought I might have carburator problems I changed to a different carb again still missing the next step was to check for a vacuum leak I used the carb cleaner method and I found no leak I also plugged off all accesories same same. so now I am thinking I have an exhaust valve hanging open but this valve train is non adjustable. any suggestions?
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franklin
Gearhead Posts: 128
From: VA
Registered: Nov 2000
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posted 10-15-2001 02:21 PM
Only thing I can think of, is did you use manifold gaskets that blocked off the center exhaust heat riser passage. This would make it run rough, especially in cool weather. What kind of carb are you running now?
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 10-15-2001 05:43 PM
I am 99% sure that the gaskets are correct and I am using an edelbrock 1405.
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kcode
Gearhead Posts: 2623
From: alvaton,ky,usa Suburb of Bowling Green, M&M #79, MCA #29208
Registered: Jun 99
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posted 10-15-2001 10:12 PM
Next step, I would run a compression test and a leakdown to possibly isolate a weak cylinder. I had a perpetual tick, which I thought was a lifter. Turned out to be a bent valve trying to recenter itself.  Mike
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 10-16-2001 08:13 AM
I dont have the equipment rightnow to try a leakdown test but I will try to get it. I should also include that when I built this I had the valves ground and lapped new pistons rings ect. basically new from the pan to the carb I dont know how to check the valve lash the book shows a special tool to grip the rocker arms and colapse the lifters the check between the rocker and the valve tip. well I dont have this tool and I cant find any feeler gauges that are thin enough to go between rails on the rocker arm. should I consider changing the rockers to a 289/302 type so that I can adjust them and can I even do this?
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unklshu
Gearhead Posts: 682
From: San Bernardino, Ca
Registered: Jul 2001
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posted 10-16-2001 09:44 AM
Is the miss coming from the same cylinder all the time?
2 ways to check this:
1. pull one plug at a time and look for the engine to stumble, if you pull a plug and nothing happens check that cylinder.2. Use a long screw driver with wooden handle and touch the tip to the heads and put your ear on the other end, listen for tapping noises, follow noise to the cylinder that is acting up. I had to do this on my 68 to find out my num 7 & 8 cylinder had collasped 1 lifter each. ------------------
joe
2001 SVT Cobra
68 GT Fastback
98 F-150
90 Escort GT (smokes the tires in all 5 gears, or just smokes through all gears)
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 10-16-2001 07:20 PM
I have tried both of these methods with no avail but maybe my hearing isnt as good as it used to be haha the miss seems to be coming from the right bank of cyl. but that is as close as I have been able to narrow it down
Dennis[This message has been edited by dhearn (edited 10-16-2001).]
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kcode
Gearhead Posts: 2623
From: alvaton,ky,usa Suburb of Bowling Green, M&M #79, MCA #29208
Registered: Jun 99
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posted 10-16-2001 10:20 PM
Dennis,
I assume you have hydraulic lifters. Valve adjustment kinda depends on the type of rocker studs you have and the year of your heads. Ford used two different studs for hydraulic lifters. Most 65-66 used a straight shank, the threads were the same diameter as the stud. Then they changed to studs with a shoulder that were non adjustable. To adjust hydraulics a feeler gauge is not needed. If this is you situation let me or us know. Mike
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SteveLaRiviere
Administrator Posts: 33988
From: Saco, Maine
Registered: May 99
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posted 10-17-2001 08:04 AM
I've seen bad spark plug wires cause intermittant misses. Try watching your engine run in the dark and look for spark jumping. Sometimes you can see a real light show! ------------------
'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC Be American, Drive American!
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 10-17-2001 08:32 AM
1. my plug wires are new but I did check the resistance on each wire and all checked the same.
2. I did run the engine in the dark what I saw was with the new original type coil and pertronix ignitor I had fire jumping between the positive lug on the coil and the center post I then changed to a msd 6al ignition and msd blaster 3 coil which has a long center post with ceramic/plastic rings to reduce the chance of jumping. after this I did not see any fire jumping.
3. my lifters are hydraulic and my heads are stock 69 351w and they are non adjustable but I did have the valves ground and my thinking is that maybe the push rods are too long or too short due to maching the valves and heads (milled the heads just enough to clean them up) thanks for the input guys
Dennis Hearn
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 10-20-2001 05:30 AM
Are all the mechanics on vacation 8)
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franklin
Gearhead Posts: 128
From: VA
Registered: Nov 2000
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posted 10-20-2001 08:11 PM
You keep hinting around about the valves, but I would think if a valve was misadjusted or leaking, you would have a pretty steady thump thump miss on certain cylinders. If you have a random wavering shake to the motor, I would say you have a mixture problem or ignition. It's going to be hard to diagnose without listening to it. Do you have somebody you trust locally who could look at it?
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 11-18-2001 04:49 PM
I have some new info on my miss problem. I just bought and installed a new holly 4150 avenger carb the engine starts really well with this carb and idles good too but I still got some problems I warmed up the engine then started trying to diagnose the problem. I have narrowed my miss down to ignition but I dont know what to do to fix it. what I found was that if the engine is missing at idol rotate the dist. clockwise and the miss would quit but at this setting the engine misses really bad at 1/3 throttle and more when I hold it at about 1/3 throttle I can turn the dist counter clockwise and the miss will go away. I guess before when I had that junky edelbrock carb on there it wouldnt run well enough to trouble shoot the miss. help me please.....
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franklin
Gearhead Posts: 128
From: VA
Registered: Nov 2000
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posted 11-18-2001 05:58 PM
Where do you have your vacuum advance hose hooked to? Manifold vacuum or ported? I would try ported. Also, at the idle speed you have it set at, do you have vacuum at the ported vacuum connection? If you do, this would be a sign that the throttle plates are open too far. The throttle plates opened too far would also cause too much of the idle transfer slot to be exposed. You can take the carb off and look at the primary throttle bores. If more than .040" of the transfer slot is exposed, it will cause a rich idle, the flat spot you were talking about, and you will lose alot of your adjustment capability with the idle mixture screws. Your initial timing adjustment, idle speed, and idle mixture can all be changed around and one will affect the other.
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dhearn
Gearhead Posts: 227
From: Owenton Ky,USA
Registered: Nov 2000
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posted 11-19-2001 06:58 AM
I think I have the vacuum advance hooked to ported vacuum. I connected it to a vacuum port on the side of the primary metering block this port was suggested by the instructions that came with the carb.
Dennis
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