ignition miss or valve problems
Thread Starter
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Freshman User
Joined: Aug 2004
Posts: 28
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ignition miss or valve problems
78 f 150 2wd c6 tranny
351m, about 3000 miles rebuilt with a mild rv cam(I dont know the exact one)
ford rebuilt 2150 2bbl carb
edelbrock intake with 2bbl adaptor NO EGR
rebuilt distributor, 6a msd ignition module along with new msd wires,and plugs.
timing is set to 14 btdc.
vacuum reading at idle 14-16 (jumping)
vacuum reading at high idle increases steadely to 18
My problem is with a (miss) at idle that i cannot locate. I have replaced spark plugs, wires, coil, cap and rotor. I do not hear any sparks jumping anywhere, but what i do hear is a slight taping on the passenger side heads area. I have adjusted my carb and my plugs were clean before i replaced them (all of them) my compression readings were between 110 and 115 on all cylinders. and I tried pulling individual plug wires while running to see if any differences in idle occurred,,,,,nothing.
I'm thinking it is something with the valves since, it has not idled right since the rebuild ( I did not do the rebuild myself)
And the worst thing is that I only get 8 MPG
The engine otherwise has good power, throttle response is great, except when i come to a stop. I CANT get a smooth idle!!!!!! It is a very consistent miss, Not eradic at all... ANY suggestions would be appreciated......
thanx in advance
351m, about 3000 miles rebuilt with a mild rv cam(I dont know the exact one)
ford rebuilt 2150 2bbl carb
edelbrock intake with 2bbl adaptor NO EGR
rebuilt distributor, 6a msd ignition module along with new msd wires,and plugs.
timing is set to 14 btdc.
vacuum reading at idle 14-16 (jumping)
vacuum reading at high idle increases steadely to 18
My problem is with a (miss) at idle that i cannot locate. I have replaced spark plugs, wires, coil, cap and rotor. I do not hear any sparks jumping anywhere, but what i do hear is a slight taping on the passenger side heads area. I have adjusted my carb and my plugs were clean before i replaced them (all of them) my compression readings were between 110 and 115 on all cylinders. and I tried pulling individual plug wires while running to see if any differences in idle occurred,,,,,nothing.
I'm thinking it is something with the valves since, it has not idled right since the rebuild ( I did not do the rebuild myself)
And the worst thing is that I only get 8 MPG
The engine otherwise has good power, throttle response is great, except when i come to a stop. I CANT get a smooth idle!!!!!! It is a very consistent miss, Not eradic at all... ANY suggestions would be appreciated......
thanx in advance
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
The 4v to 2v adapters are notorious for vacuum leaks especially on the Edelbrock manifolds.
The compression looks low also. It should be ~140# for 8:1 slugs. Compression readings should be taken with all the plugs out and the carb blocked open.
Check your vacuum gage readings here:
http://www.users.bigpond.com/ergoff/vac1.htm
The compression looks low also. It should be ~140# for 8:1 slugs. Compression readings should be taken with all the plugs out and the carb blocked open.
Check your vacuum gage readings here:
http://www.users.bigpond.com/ergoff/vac1.htm
Last edited by Torque1st; Apr 27, 2005 at 12:08 AM.
Lead Driver
Joined: Nov 2000
Posts: 6,640
Likes: 349
From: Albuquerque
You can check for a burnt valve by listening closely to the exhaust. If it has a spitting sound you may have one. Try holding a light rag in front of the exhaust tip. If it gets sucked back toward the pipe in a pulsating manner it indecates a burnt valve.
Vacuum leaks are common in rebuilds and will cause idle problems.
Does your initial timing (14) include having the vacuum advance hooked up to full time vacuum? 14 wont be enough if all the hoses are connected.
Vacuum leaks are common in rebuilds and will cause idle problems.
Does your initial timing (14) include having the vacuum advance hooked up to full time vacuum? 14 wont be enough if all the hoses are connected.
Thread Starter
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Freshman User
Joined: Aug 2004
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Well, i've checked for vacuum leaks everywhere, sprayed all around vacuum, intake and carb, no changes. I even dissconnected and plugged all of them, nothing,
according to the manual, timing is to be set without the advance hose hooked up. If i put full vacuum, then set the timing, won't it be way off and missfire?????
I'm going outside now to check for the exhaust "spitting"
according to the manual, timing is to be set without the advance hose hooked up. If i put full vacuum, then set the timing, won't it be way off and missfire?????
I'm going outside now to check for the exhaust "spitting"
Lead Driver
Joined: Nov 2000
Posts: 6,640
Likes: 349
From: Albuquerque
With a mildly modified motor you probably want to see at least 18 degrees advance at idle with everything hooked up. I run as high as 24 (high altitude)That will vary with compression ratio and altitude.
