Mysterious plug reading

[Mr. Land Yacht]

ok so I am trying to get my jetting and/or mixture right and I am stumped by what my plugs are telling me.

so at idle my Vac is 15-16inches..so that is ok.

but my plug readings are making me scratch my head....

I had my jets at 78...and I drove it around at about 40-45mph in D.
Vac gauge is reading around 18-19inches with very light pedal(just enough to maintain the speed) and after driving a short distance...like 1/4mile or so and then shutting it off, pulling it over...my plugs are white..not even close to Tan.
so, I figure it's running way lean...so, obviously I up the jets to 80.

driving in the same conditions with the Vac at 17inches...the plugs are exactly the same reading.

so I decide to go for it and up the jets to 84.
now at idle my Vac is 15-16inches...driving, it's about the same.
I drive again and get a plug reading..SAME...FRICKIN...COLOR.

here's a pic..primary jets at 84 and Vacuum is around 15inches.


is there something about colder plugs that don't give you a reading or something?

oh and I can tell by the smell of my exhaust that it is richer...so I tuned the mixture to get the most Vac at idle...I got it to about 15.5-16inches.

just for kicks, I let it run at idle for a min or 2 and then took a plug reading..still really white.

I'm totally confused.

[DuceAnAHalf]

to get an accurate read you need to go WOT and then pop it in neutral and shut the motor off. if you let it run after the WOT while you slow down the color can change

[torquelover]

to get an accurate read you need to go WOT and then pop it in neutral and shut the motor off. if you let it run after the WOT while you slow down the color can change

+1. Put some gloves on if you pull some rear plugs, too. Last time I did it, I yanked all 8 at the gas station I coasted into for the 15th time that day and found out that not all plugs read the same.

[Mr. Land Yacht]

but if I go WOT then the secondaries will open and I wouldn't know if the primaries are doing what they are supposed to. yes/no?

and when I was cruising in D I shut the car off right away and coasted..then checked.

what really shocks me is that the other plugs I had in there that were the wrong ones changed color like normal and looked fine...why would these give essentially no reading?

[Mercmarquis]

the primaries are still being used at full throttle so having them jetted correctly at WOT makes sense lol. IIRC the demon you have should have a metering plate on the secondaries which you cant adjust anyway. So do this at wot. Go to a test n tune night at the track and do the same lol. There should be an area at the end of the track you can pull plugs at.

[Mr. Land Yacht]

ok then.
I will get out to Lapeer dragway saturday and see what it tells me at the end of the track.

[Mercmarquis]

Until then you can still do the lonely country road test to make sure you arent running it lean or pig rich to baseline it. Fine tune it at the track tho

[Mercracer]

You can not go by plug color for a daily driver. You would need to make a gross change to get a color change... Do yourself a favor and find a chassis dyno to help you tune your carb if you are trying to get ideal. Why are you running colder plugs???

[Mr. Land Yacht]

turbo's recommendation.

[turbo2256b]

Without a air fuel ratio meeter a plug reading can be taken at any speed. The issue is the engine must be shut off at the speed of the desired reading. After shut down the engine should not be turned over it must be out of gear. If it is idling to a stop then you are geting the idle jetting reading.

[1987cp]

hmmmmmmmmm.............. wonder where I could conduct such an exercise in the Holt area .......

Would it be most ideal to start each segment of such a test with a fresh set of plugs, or would it be safe to assume that an existing set will alter its appearance as needed even if they're previously a bit soiled or whatnot?

[turbo2256b]

hmmmmmmmmm.............. wonder where I could conduct such an exercise in the Holt area .......

Would it be most ideal to start each segment of such a test with a fresh set of plugs, or would it be safe to assume that an existing set will alter its appearance as needed even if they're previously a bit soiled or whatnot?

Yes its best to have new plugs. The silver coated ones are easyiest to read.

Heat range, idle jetting, cruse jetting, initial timing, total timing, detonation, proper plug gap can all be read from a spark plug.

Easyest to have several new plugs and only change one plug each test read it and replace for next run.

[torquelover]

Colin, what gap are you running?

[Mr. Land Yacht]

.050"

[turbo2256b]

ok so I am trying to get my jetting and/or mixture right and I am stumped by what my plugs are telling me.

so at idle my Vac is 15-16inches..so that is ok.

but my plug readings are making me scratch my head....

I had my jets at 78...and I drove it around at about 40-45mph in D.
Vac gauge is reading around 18-19inches with very light pedal(just enough to maintain the speed) and after driving a short distance...like 1/4mile or so and then shutting it off, pulling it over...my plugs are white..not even close to Tan.
so, I figure it's running way lean...so, obviously I up the jets to 80.

driving in the same conditions with the Vac at 17inches...the plugs are exactly the same reading.

so I decide to go for it and up the jets to 84.
now at idle my Vac is 15-16inches...driving, it's about the same.
I drive again and get a plug reading..SAME...FRICKIN...COLOR.

here's a pic..primary jets at 84 and Vacuum is around 15inches.


is there something about colder plugs that don't give you a reading or something?

oh and I can tell by the smell of my exhaust that it is richer...so I tuned the mixture to get the most Vac at idle...I got it to about 15.5-16inches.

just for kicks, I let it run at idle for a min or 2 and then took a plug reading..still really white.

I'm totally confused.

Driving around at those speeds and that vacuum your probably running on the idle circut which should color about a 1/16 wide strip around the radius at the tip of the porclin