New sbc 400 blueprint engine keeps fouling plugs with gas

[Grumpy]

First, many engine builders dyno their engines without an air filter, so having one that is even a little restrictive will cause the engine to run rich - think recreating the conditions they had at the dyno and make your adjustments from there.
An A/F gauge is such an inexpensive tool these days, I fail to understand why they aren't commonplace on this site.
Is your ignition system running at peak efficiency? Poor wires or incorrect wires/plugs can be the source of plenty of brain damage.
Pro Tip: Obtain every spec you can find and write everything down, then verify them as accurate. Make all adjustments and changes ONE AT A TIME! Write everything down as you go as well as all the results. If you have to backtrack, all that is required is to refer back to your records - leave your memory out of the equation and anyone who helps is able to review your notes.

 

[potent rodent]

need more info on the carb , what size jets what size power valve make sure u have the right power valve no one here said any thing abought that if its blown that will cause the fouling plugs

 

[mrdap]

While it may be remotely possible to tune the 850 CFM carburetor to marginally work with your 400 in³ engine, it's really too much carburetor for such a small engine.
I would recommend 650 CFM.
Possibly (but probably not) 750 CFM at the most.
You'll have a much more drivable engine, more throttle response and more torque on the low end with a smaller carburetor.
The only time you would need a 750 is if you're winding it up past 7,000 RPM or even 8,000.
850 would be overkill even for full race on the drag strip

 

[potent rodent]

your eng is supposed to have afr reading on the dyno sheet did u look and see what they are ?

 

[Rusty Nail]

850 is way too big.

 

[Snoots]

That's what she said.

 

[80BrownK10]

I run a 454 small cam, low compression, 750 carb. Primary jets 73, secondary 83. The 73 run down the road A/f ratio about 13.8, the 70 primary ran 14.6. Secondary started at 80, now 83, about right at 12.8-13 WOT. I have an A/F gauge in the truck. This 400 sb at 6500 rpm, 90% volumetric efficiency would require a 677 cfm carb.

What tach did you put in the lower left spot there? I was thinking if I could find something to put in the clock spot, a tach or something else

 

[Craig Nedrow]

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I did a write up on the dash mods to get the gage to fit and work. Not hard, but takes some time, but for me worth it. I also use an A/F ratio gage, best tuning tool one could have. Cluster mod etc start on page five. Hope this helps!

https://www.gmsquarebody.com/threads/1973-k20-350-to-454-swap.29840/page-5

 

[potent rodent]

850 is way too big.

yes way to big for the street and that cam is pretty nasty also for a street driven vehicle, not saying it cant but it would be a pita

 

[HEG77k10]

need more info on the carb , what size jets what size power valve make sure u have the right power valve no one here said any thing abought that if its blown that will cause the fouling plugs

72 primary’s and secondaries not sure on valve but I just opened up carb and the diaphragm looks good. Has backfired out of the carb twice and it looks like they’ve held up

 

[HEG77k10]

Af gauge will be here tmrw, changed primary’s in the meantime to 68s. I don’t see why jetting the 850 down wouldn’t work but I really don’t know what I’m doing so we’ll see what the gauge says. What afr should I be trying to shoot for??

 

[Frankenchevy]

Somewhere around 15:1 at high vacuum (cruise) and somewhere around 12:1 at WOT. These are all questions I’d be asking blueprint. They know that engine better than just about anyone. They build many, many engines with the same specs.

 

[Matt69olds]

Shoot for 13.5 at idle, mid 15 or so at cruise, and 12.5ish at WOT. A big cam will typically need a richer idle than a smooth idle cam.

By the way, there is no such thing as too big a cam, just not enough engine!!

Don’t get too hung up on exact numbers. The important thing to remember is give the engine what it wants, not what you think it needs. A wide band is a tuning tool, it will help you determine exactly what the fuel curve looks like.

If the distributor has vacuum advance, you can go a lot leaner at cruise than most people would believe. Ideally, you want to keep jetting leaner and leaner to where you just start to feel a lean miss at steady cruise on a typical day, then jet a couple sizes richer. Since the engine isn’t under a lot of load at cruise, there is no concern about burning pistons or valves like some people would have you believe. Once you have the primary side of the carb dialed in, leave it alone. Jet the secondary side for max power at WOT

 

[JBswth]

850 CFM sounds too big for the street. You should try Tri-Power or dual quads instead of a single giant.

James

 

[potent rodent]

72 primary’s and secondaries not sure on valve but I just opened up carb and the diaphragm looks good. Has backfired out of the carb twice and it looks like they’ve held up

what size power valve is in it , did u test the power valve to make sure its not blowed out u cant tell just by looking