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engine miss
I have a miss most noticable at 2500-3000 rpm. I have installed a new distrib., plug wires , rotor with no help. Could it be a bent push rod? 351W
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Did you also replace the plugs?
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The plugs have 1000 miles on them.
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Most misses are ignition related if they occur around the same rpm, but under varying conditions (wot, cruise, decel). Do you by any chance have an msd box? The msd shifts from multiple spark to single spark at around that rpm as the capacitors can not charge fast enough to deliver multiple sparks.
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lean spot in the carb.
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Light throttle miss or heavy throttle miss. Put the car in a higher gear and find a hill to climb. Lug it a bit as you climb the hill. Still missing? Next, find a level road and floor it while going through the gears. Any missing at high rpm's?
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Plug may have broken porcelain etc. They are not all born perfect. What do the plugs look like?
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Is there a lack of power also? Does it clean up past 3K? Could it be a vacuum leak?
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Quote:
Camshaft, compression, carb etc. A compression test should be done to rule out anything that could be wrong with the engine. The spark plugs at only 1000 miles old doesn't mean rule them out either. I've had a spark plug fail at 300km, and I thought the same thing until I found it. |
Ok, Let me go through this. I was adjusting the timing on the engine. I set the advance at 12 degrees initial. I started the motor and as I was manually working the throttle I noticed the dist. was wobbling a little so I thought aha, I have found the problem. So I bought a refurb dist. The wobbling stopped but still had a miss. I took the dist. apart and found the advance weights were way to big(40 degrees advance). I changed the weights to 20 degrees plus the intial
12 so I would have 32 degrees total. Took the car out and jumped on it. Run like a scalded rat but at high RPM misses still. The car seems to lope at lower RPM. So I bought new wires (Ford Racing 9mm), a new MSD dist, cap and rotor. Same thing happens. Maybe I got a bad Dist? I did not change the plugs. I guess I'll try that next. Just thought it might be a bent push rod. I think the miss is there at lower RPM but just not as noticible. I don't know the cam profile but it is fairly mild. The carb is an edelbrock 650 that I recently rebuilt recently. Hope someone out there is smarter than me (not hard to do). |
What type of plug wire separaters do you have? And how are the wires routed? Any pics?
So as I count you are on 3rd dist? without removing a valve cover or doing a compression/leakdown test? Correct firing order??? |
On second dist., have not done a leak down yet. Wires are separated well with Taylor nylon horizonal screw downs. I can do the leak down, thanks.
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Ford had a cross fire problem on the FE engines when they came out in 58 so they separated the consequetly firing #7 and #8 on the left bank to where the wires wer routed in the separaters #7,5,6,8 which solved the problem---however the right bank was
#1,2,3,4 and when a person got a tuneup the mech often put the wires back on in order of the separaters which was rt side 1,2,3,4 but the left side they put #7 on #5, #5 on #6, #6 on #7 and #8 on8. Thus there would be 3 cylinders with wrong wires--always resulted in missing, crossfiring engine-- And of course there has been a firing order change on the engines later so wondering if maybe you were using wrong order> |
Checked firing order 3 times and seems in order.
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Well I pulled the valve covers. All seems in order. Leak test looks good. Compression Ok on all cylinders. Guess its back to ignition. Could the duraspark box cause this or does that usually result in total ignition failure?
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Check intake gasket, I had a miss and chased it with all the normal stuff wires,plugs, cap, roter even wen't through carb nothing. Even sprayed intake gasket for vacuum leak nothing. Found intake gasket was leaking from the under side. Take carb off shine light and mirror down runners and if any are dark where they mate with the head there is likely your problem. Some engines were installed with gaskets that diden't work well with the heat cycles of aluminum heads and intakes.
Bill |
it's cheap, change the plugs
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Changed the plugs but have to wait till tomorrow to fire it up, neighbors wouldn't be happy tonight. Thanks for the tips, will be back after testing.
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Has it always had the misfire or did it 'appear' after something else was changed, ie like after you had the carb rebuilt, if thats the case then look there..sometimes while installing a 'new' kit the PV calibration gets changed etc..
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still think it is the carb, the edelbrock 650 will run lean. i have one on a 351 crate engine which is 9-1 copression 385hp and it ran lean and missed on light cruise. drilled the ifr passage and it helped.
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