Plug readings at cruise speed

MilesnMiles · Post by **MilesnMiles** » Tue May 12, 15 7:47 pm

Since fitting my new heads I've been using a 670 HOlley.Feeling it lacked a bit at WOT so, I refitted my 750 Street Demon (similar to Thermoquad ) and the car runs much faster by some margin.
But....
At any light throttle cruise speed (50,60,70mph) I have a really rich reading on my wideband of mid11s whereas it should be nearer mid 13s. I did a few miles at 70 mph and pulled a plug. I'll try to get a pic up but I was surprised to see the tip of the plug was greyish and the electrode more a mid brown.
Pulling the plug at home after low speed and idle round town saw a darker plug all round.
The plugs are NGK BKR6E
Any interpretation of what's happening is welcome.

Pete · Post by **Pete** » Tue May 12, 15 8:28 pm

Vac Sec? DP?

Primary Mains too big?

MilesnMiles · Post by **MilesnMiles** » Tue May 12, 15 9:05 pm

Carb is mechanical. Based on thermo quad. It's definitely running on the primaries at cruise. It has those edelbrock style metering rod and spring assembly. When I add gas the numbers cleanup nicely and when I stomp on it for WOT it riches as you'd expect.
It's just strange that it's cruising rich given that it's an out of the box carb.
Ran it to the Nats and back last year without issue! but didn't check the plugs either.
Could the plugs be too cold for the application?

MilesnMiles · Post by **MilesnMiles** » Tue May 12, 15 9:45 pm

Might have found my answer.
I googled 'what plugs for Indy/RHS heads and found this reply

'If your making 375HP or more, the FR5 is the plug to run. FR4 is a little hot and can cause detonation on pump gas with aggressive timing curve and compression.
I use the FR5 up to 500HP then I look to use the BCP6ES but only for WOT...the 6 will foul quickly just cruising.'

Looks like I should fit FR5s and be done.

MilesnMiles · Post by **MilesnMiles** » Thu May 14, 15 8:14 pm

Changed to a warmer plug and no change. Still rich at cruise. If I take my foot off it shows good idle figures of 14-15.

Once I'm off idle at light throttle what circuit am I on?
Could the squirter be an issue? Bit in the dark here.[/url]

Mick70RR · Post by **Mick70RR** » Thu May 14, 15 9:18 pm

Once off idle and up to around 2500rpm on a light throttle your on the primary main jets.
Find a primary jet that gives you the A/F ratio you're after while cruising.
Once you are happy with the cruise A/F ratio open just the primary throttle plates and accelerate. Vacuum will drop and the power valve will open adding fuel to the mixture. If it's not rich enough you have to drill the power valve restrictor until it is.
Finally give it full throttle and the secondary main jets come into play. Adjust these until you have the same A/F ratio you had when just the primaries were fully open.

Sounds like you need smaller main jets to lean it out at cruise but you may have to drill the power valve restrictors to compensate when you open it up, that's what I had to do.

MilesnMiles · Post by **MilesnMiles** » Thu May 14, 15 10:45 pm

Mick, thanks for the suggestion. However, it's not a Holley, it's a Street Demon 750, basically a quadrojet so no powervalve.
Ironically my Holley 670 ran great but just lacked sufficient cfm, so I used the Demon carb

MilesnMiles · Post by **MilesnMiles** » Sat May 16, 15 9:44 am

Reading your post again Mick, maybe I need to fit leaner metering rods?
Contacted Demon tech line but no response

autofetish · Post by **autofetish** » Sat May 16, 15 10:09 am

Very intresting good post Mick

MilesnMiles · Post by **MilesnMiles** » Sat May 16, 15 10:51 am

The irony is that I had my Hollley 670 exactly where I wanted it by changing the power valve to line up with Micks advice above.
However, it doesn't pull hard enough under heavy throttle compared to the 750. Iil need to work on this.