Reading Spark Plugs and rough idle?
10-17-2006, 09:54 AM
#1
2nd Gear Member
Thread Starter
Join Date: Feb 2006
Location: PA
Posts: 225
Reading Spark Plugs and rough idle?
Engine: 1971 351W stroked out to 408. Edelbrock Performer RPM power package which includes heads, cam, intake, and 750 cfm carb. Comp Cams roller rockers, Pertronix distributor (not just the conversion kit), JBA 9mm wires, breathing through a JBA exhaust system. All sitting in a 67 Mustang fastback.
Problem/Question: The engine idles a bit rough when down in the 800-900 idle range. Although the power package has a healthy cam, I would have expected a smoother idle. (however my expecatations may be wrong) One thing I do find is that the spark plugs towards the front of the engine tend to foul with a heavy, but dry carbon deposit. Most notably in the 1st and 5th cylinders and tapers off from there to nearly a clean plug on the 4th and 8th. I am using the Edelbrock recommended Champion plug RC12YC, and plan on increasing the heat range up to the RC14YC plug hopefully over the next two-three days to see if this helps. This is a newly built engine and hasn't seen the road yet, as there have been other things being worked on aside from the engine. Note: All typical vacuum leaks have been checked. The engine sounds incredibly strong and smooth at higher RPMs.
1. Can anyone offer some advice in regards to the plug readings? I have tried two different sets of plugs with the same results.
2. Could the rough idle be due to an imbalance with the crank assembly? (it was done many years ago before "kits" came out) If so, is there a harmonic balancer out there that could be bolted on the crank to eliminate the imbalance? (possibly newer technologies out there)
Any advice or direction would be appreciated!!
Problem/Question: The engine idles a bit rough when down in the 800-900 idle range. Although the power package has a healthy cam, I would have expected a smoother idle. (however my expecatations may be wrong) One thing I do find is that the spark plugs towards the front of the engine tend to foul with a heavy, but dry carbon deposit. Most notably in the 1st and 5th cylinders and tapers off from there to nearly a clean plug on the 4th and 8th. I am using the Edelbrock recommended Champion plug RC12YC, and plan on increasing the heat range up to the RC14YC plug hopefully over the next two-three days to see if this helps. This is a newly built engine and hasn't seen the road yet, as there have been other things being worked on aside from the engine. Note: All typical vacuum leaks have been checked. The engine sounds incredibly strong and smooth at higher RPMs.
1. Can anyone offer some advice in regards to the plug readings? I have tried two different sets of plugs with the same results.
2. Could the rough idle be due to an imbalance with the crank assembly? (it was done many years ago before "kits" came out) If so, is there a harmonic balancer out there that could be bolted on the crank to eliminate the imbalance? (possibly newer technologies out there)
Any advice or direction would be appreciated!!
10-17-2006, 07:01 PM
#2
3rd Gear Member
Join Date: May 2006
Location:
Posts: 622
RE: Reading Spark Plugs and rough idle?
Hey Dan,
No expert mind you, but I would be thinking about general tuning and set up issues before you start worrying about the crank. Look at the most likely stuff first.
Regarding the rough idle, I would think you may just not be used to the bigger cam. Did you have the carb properly set? Timing etc. I would think someone at Edelbrock customer service would advise you since you have the whole set up. At least they should give you recomended timing advance and Carb bench set up and idle and mixture procedures specific to their carb. You might consider paying to have the engine dyno-tuned. A seasoned professional with the right equipment is worth the money and the peace of mind, especially when you have obviously put some money into the engine.
Spark Plug Diagnosis:
The spark plugs ideally should have a light tan dusting to them.
Wet/Black is usually valve stem seals leaking down over plug.
Black soot (like yours) engine running rich.
White, too lean/hot.
These are to the best of my memory, someone please correct me if I am wrong...
Good luck, sounds like it will be a powerhouse when you get the bugs worked out!
.
No expert mind you, but I would be thinking about general tuning and set up issues before you start worrying about the crank. Look at the most likely stuff first.
Regarding the rough idle, I would think you may just not be used to the bigger cam. Did you have the carb properly set? Timing etc. I would think someone at Edelbrock customer service would advise you since you have the whole set up. At least they should give you recomended timing advance and Carb bench set up and idle and mixture procedures specific to their carb. You might consider paying to have the engine dyno-tuned. A seasoned professional with the right equipment is worth the money and the peace of mind, especially when you have obviously put some money into the engine.
Spark Plug Diagnosis:
The spark plugs ideally should have a light tan dusting to them.
Wet/Black is usually valve stem seals leaking down over plug.
Black soot (like yours) engine running rich.
White, too lean/hot.
These are to the best of my memory, someone please correct me if I am wrong...
Good luck, sounds like it will be a powerhouse when you get the bugs worked out!
.
10-17-2006, 08:00 PM
#3
5th Gear Member
Join Date: Jun 2006
Location:
Posts: 4,089
RE: Reading Spark Plugs and rough idle?
I'd recommend getting rid of the Champion plugs. I've never had good luck with them, even in lawn mowers. Usually it's the back plugs that will foul first and not the fronts.
I'd second getting the carb tuned properly and the ignition set. It's the most likely cause for a rough idle
If you had an imbalance in the crank, you'd notice it at higher rpms more than at idle.
I'd second getting the carb tuned properly and the ignition set. It's the most likely cause for a rough idle
If you had an imbalance in the crank, you'd notice it at higher rpms more than at idle.
ORIGINAL: shelbymustangdan
Engine: 1971 351W stroked out to 408. Edelbrock Performer RPM power package which includes heads, cam, intake, and 750 cfm carb. Comp Cams roller rockers, Pertronix distributor (not just the conversion kit), JBA 9mm wires, breathing through a JBA exhaust system. All sitting in a 67 Mustang fastback.
Problem/Question: The engine idles a bit rough when down in the 800-900 idle range. Although the power package has a healthy cam, I would have expected a smoother idle. (however my expecatations may be wrong) One thing I do find is that the spark plugs towards the front of the engine tend to foul with a heavy, but dry carbon deposit. Most notably in the 1st and 5th cylinders and tapers off from there to nearly a clean plug on the 4th and 8th. I am using the Edelbrock recommended Champion plug RC12YC, and plan on increasing the heat range up to the RC14YC plug hopefully over the next two-three days to see if this helps. This is a newly built engine and hasn't seen the road yet, as there have been other things being worked on aside from the engine. Note: All typical vacuum leaks have been checked. The engine sounds incredibly strong and smooth at higher RPMs.
1. Can anyone offer some advice in regards to the plug readings? I have tried two different sets of plugs with the same results.
2. Could the rough idle be due to an imbalance with the crank assembly? (it was done many years ago before "kits" came out) If so, is there a harmonic balancer out there that could be bolted on the crank to eliminate the imbalance? (possibly newer technologies out there)
Any advice or direction would be appreciated!!
Engine: 1971 351W stroked out to 408. Edelbrock Performer RPM power package which includes heads, cam, intake, and 750 cfm carb. Comp Cams roller rockers, Pertronix distributor (not just the conversion kit), JBA 9mm wires, breathing through a JBA exhaust system. All sitting in a 67 Mustang fastback.
Problem/Question: The engine idles a bit rough when down in the 800-900 idle range. Although the power package has a healthy cam, I would have expected a smoother idle. (however my expecatations may be wrong) One thing I do find is that the spark plugs towards the front of the engine tend to foul with a heavy, but dry carbon deposit. Most notably in the 1st and 5th cylinders and tapers off from there to nearly a clean plug on the 4th and 8th. I am using the Edelbrock recommended Champion plug RC12YC, and plan on increasing the heat range up to the RC14YC plug hopefully over the next two-three days to see if this helps. This is a newly built engine and hasn't seen the road yet, as there have been other things being worked on aside from the engine. Note: All typical vacuum leaks have been checked. The engine sounds incredibly strong and smooth at higher RPMs.
1. Can anyone offer some advice in regards to the plug readings? I have tried two different sets of plugs with the same results.
2. Could the rough idle be due to an imbalance with the crank assembly? (it was done many years ago before "kits" came out) If so, is there a harmonic balancer out there that could be bolted on the crank to eliminate the imbalance? (possibly newer technologies out there)
Any advice or direction would be appreciated!!
10-17-2006, 08:23 PM
#4
I ♥ Acer
Join Date: Oct 2004
Location:
Posts: 17,565
RE: Reading Spark Plugs and rough idle?
Try changing the plugs to Motorcraft B-45's. However, since you have black on a few of them, you may want ot check the valve openings on those particular valves. Readjust your valve openings.
10-17-2006, 08:28 PM
#5
Gentleman's Relish
Join Date: Feb 2005
Location: AZ
Posts: 13,090
10-17-2006, 09:48 PM
#6
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Join Date: May 2005
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Posts: 2,445
RE: Reading Spark Plugs and rough idle?
I second Colorado's suggestions.
I'm not much of an AC Delco fan either (personal experiences). That said, the end cylinders will run slightly cooler than the middle. Especially when the engine is all ready running rich. And you haven't put the engine under a real load to help clear things out.
Imbalance causes a vibration not rough idle. Sounds like you need to get it tuned. Get that puppy on a dyno and have them set the timing curve and fuel ratios. Then go drive and have fun! It is money well spent.
Good luck with it. Should be a monster when you are done!
I'm not much of an AC Delco fan either (personal experiences). That said, the end cylinders will run slightly cooler than the middle. Especially when the engine is all ready running rich. And you haven't put the engine under a real load to help clear things out.
Imbalance causes a vibration not rough idle. Sounds like you need to get it tuned. Get that puppy on a dyno and have them set the timing curve and fuel ratios. Then go drive and have fun! It is money well spent.
Good luck with it. Should be a monster when you are done!
10-17-2006, 10:50 PM
#7
2nd Gear Member
Join Date: Nov 2005
Location:
Posts: 182
RE: Reading Spark Plugs and rough idle?
I just went through something similar, and it turned out to be initial timing. pull the hose off the distributor, plug the line and put a timing light on it. At idle, set the timing for about 6 to 9 degrees - on the little pointer thingy... 6 is really smooth, but lacks launch power. 9 is a good compromise between launch and idle. 3 little tiny degrees makes a LOT of difference in the quality of the idle. Also, if you just rebuilt the engine, it could just be a little on the tight side. I wouldn't rush to any judgements or worry till you have about 2000 miles on it. [&:]
10-18-2006, 09:46 AM
#8
2nd Gear Member
Thread Starter
Join Date: Feb 2006
Location: PA
Posts: 225
RE: Reading Spark Plugs and rough idle?
Thanks to all for the good information and guidance!!! I will certainly check into each suggestion and also try and locate a reputable engine dyno tuner in the southeastern PA area.
Thanks again!!
Dan
Thanks again!!
Dan
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