• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Help spark plug reading

C

cabledog

Member
Local time
2:41 AM
Joined
Jan 20, 2019
Messages
24
Reaction score
16
Location
Las vegas
C699E03E-E7C2-4E6A-B0FB-C3568390AC25.jpeg
B8E37330-6CC3-4E39-AD18-45C1112F29E7.jpeg
Hi all new member here. Need a little help with some spark plug readings. The second photo is the plugs that have been in the car for a long time. I replaced them with autolite 85s. The new plugs to me look lean or hot. The only two things that changed are I put primary jetting to 76 from 72 and power valve from 6.5 to 2.5 as I have 5 inches of vacuum at idle on a holley 750. It was idling very rich and had a 6.5 pv, holley said to drop to 2.5.


Is it possible the rich idle was masking a lean wot or mid band issue?


I also have black soot on the fender and it smells rich already so I'm wondering how the plugs look so lean.

I don't know much about the particulars of this car, or cars in general since it was my dad's since 71 and he just passed it to me this year. I'm still learning so I apologize in advance if I can't answer some questions but I'll try.


Any ideas?


1971 super bee charger

383 Magnum with performance cam

Holley 750 secondary vacuum

76 primaries and 2.5 pv
 
Kinda lean looking. What is the timing?
 
oldbee said:
Kinda lean looking. What is the timing?
Click to expand...

Still trying to find that out as well. I'm doing compression test tomorrow to see what that is. I'm also going to change power valve to 4.5 tomorrow since I did vacuum tests under load today and I rarely get under 3 unless I'm near wot.
 
Maybe #78 pri. jets. Everything interacts, maybe a 6.5 pv?
 
oldbee said:
Maybe #78 pri. jets. Everything interacts, maybe a 6.5 pv?
Click to expand...

It had a 6.5 in it already but holley said I should make it 2.5 since I only have about 5 inches at idle. She ran really rich at idle before and stalled a few times but idles better now. I'm thinking 3.5 or 4.5 to keep them from opening at idle but not so low as to create lean condition. Am I correct that assumption?
 
How about pics of your car till you sort this out.
 
Very nice ! 5" at idle ? What's the engine build ? That seems extremely low, unless its got a hefty cam in it ..... you're not taking the reading from above the throttle plate are you ?
 
http://4secondsflat.com/Spark_plug_reading.html
This will help on the plugs but first and foremost, you have to get the timing right. You should be 16-18 btdc on the initial with all in around 34. Fixing that vacuum advance will help a lot. Once you get the timing set, if you are still low on engine vacuum, you might call Don at 4secondsflat, he sells vacuum cans that will come in at a lot lower vacuum than stock. A stock can doesn't come in until around 14" of vacuum
 
It would have to have a pretty big cam for 5" vacuum at idle - what cam is it? Besides timing I would be suspicious of a vacuum leak somewhere.

If the cam really is that big - then your rich situation at idle could be due to a stock Holley. Once your have made sure the engine is properly timed and the vacuum advance is working again - if it still is idling rich and especially if you don't get any response out of the idle mixture screws or they are backed way out I would pull the carb and look at it. Turn it over and with the throttle closed to curb idle position look at the exposed portion of the transfer slot in the primary throats. The blades should be closed so that the exposed section is roughly square or about .020" exposed. If it's opened too much your transition circuit can be dumping fuel in at idle and really mess up your A/F mixture. You can gain on it some by opening the rear secondary curb idle setting screw (also on underside with slot head down in a hole) an equal number of turns as it takes to close the primary down. But - you don't want to expose any of the transfer slot in the secondary throats. If it's really a big cam and the secondary plates can't be opened enough to compensate for closing the primaries down - you may have to drill small holes in the primary blades to allow more air and get the transfer slot exposure reduced enough to be able to adjust the idle mixture. But do that only as a last resort.

By the way - Holley usually jets a 750 vacuum secondary carb with 70 primary jets and 76 secondary.

Nice looking car by the way.
 
Last edited:
Cool car and even cooler story behind it. The power valve will have no effect on the idle A/F mixture.
 
cabledog said:
It had a 6.5 in it already but holley said I should make it 2.5 since I only have about 5 inches at idle. She ran really rich at idle before and stalled a few times but idles better now. I'm thinking 3.5 or 4.5 to keep them from opening at idle but not so low as to create lean condition. Am I correct that assumption?
Click to expand...
Beautiful car. It's very important for us to know "about" what cam you have in that motor. 5" vacuum at idle is very, very low. It points towards either a vacuum leak OR a really big camshaft, which is why everyone is asking.

In general, you want the power valve number to be 1/2 of your inches of vacuum at idle....so, for a true 5" at idle, a 2.5 power valve is appropriate (1/2 the number). I think we're betting you have a vacuum leak.....so, if you fix it & get say 12" vacuum at idle, then a 5.5 power valve (or a "6" if they made those) would be more appropriate.
 
Thank you everyone for the responses! As far as vacuum leaks I have found and plugged off all leaks I believe. In fact I only have 1 vacuum line hooked up right now and it goes from the back of the carb into the right side of engine block.

It does have a pretty hefty cam in it I believe. My dad said it's a racing cam but cannot remember specifics as it was put in the 80s.

I'm going to check out the other suggestions and I'll let everyone know how it's going. Thanks again!!
 
hunt2elk said:
The power valve is not part of the idle circuit.
Click to expand...
You are correct in that it is not designed to be but it can if the engine has really low vacuum.
The number stamped on a power valve, such as 65, indicates the manifold vacuum below which the power valve is operational.
In this case, all manifold vacuums below 6.5” Hg, the power valve is operating. Generally a 65 power valve is sufficient for most
high performance applications that have a manifold vacuum of 12” Hg or higher. However, some problems can result with
radically cammed machines equipped with automatic transmissions. These vehicles often “idle” at 2000 rpm, approx. 6.0” Hg.
At this point the main nozzles are starting to feed and richen the mixture (supplied by the power valve) and the engine will
probably “load up”. To correct this problem, install a 45 or 35 power valve. If the engine has a manifold vacuum of 12” Hg or
less, a good way to determine power valve size is take the manifold vacuum at idle and divide that number by two. The answer
is the power valve size. This will provide idling and proper fuel flow under wide open throttle conditions when manifold vacuums
seldom rise above 1” Hg.
https://www.holley.com/blog/post/holley_power_valve_tuning/
 
cabledog said:
Thank you everyone for the responses! As far as vacuum leaks I have found and plugged off all leaks I believe. In fact I only have 1 vacuum line hooked up right now and it goes from the back of the carb into the right side of engine block.

It does have a pretty hefty cam in it I believe. My dad said it's a racing cam but cannot remember specifics as it was put in the 80s.

I'm going to check out the other suggestions and I'll let everyone know how it's going. Thanks again!!
Click to expand...
1: eliminate any possibility of leak
2: timing
3: carb adjustments
 
You must log in or register to reply here.

Similar threads

J
Spark Plug Tubes Fitment Issue
Replies
3
Views
695
jreynolds
J
HAHNAGON
BIG BLOCK ENGINE STUMBLES ONLY UNDER QUICK/HARD ACCELERATION???
2 3
Replies
59
Views
4K
69_A12_6BBL
69_A12_6BBL
Twalker
Engine performance troubleshooting help. Lean stumble when hovering just off idle.
Replies
12
Views
2K
6872n73
6
R
Need help 440 dodge moterhome
Replies
4
Views
749
zyzzyx
zyzzyx
M
Yet another timing problem!
2
Replies
32
Views
3K
mr_spock
M
Auto Transport Service
Back
Top