Black plugs
 

Here are the spark plugs from my stock 29 Model A RPU. As they are very black and carboned-up, it must be running too rich - right? However I always close the GAV fully when driving. When idling, the motor stops "rolling" as soon as I rotate the GAV to the driving position, indicating that it is not running too rich. So what's going on here? Hope you can help.
Mac

Re: Black plugs
 

Try a hotter range plug.

Re: Black plugs
 

https://www.fordbarn.com/forum/showth...lug+heat+range
Do a thread search. The above is larry blumfield's list of spark plug heat ranges.

Re: Black plugs
 

I dont see that changing plugs will fix your problem, get the carb fixed first, when the carb is set up properly and idle speed where it should be 500 - 600 rpm there should not be any influence from the GAV.

Re: Black plugs
 

Re: Black plugs
 

So what is likely to be wrong with my carb?

Re: Black plugs
 

Most of the jets available from vendors are not quite the right size. Do you have a set of flow tested jets in the carb? If not, a set might fix this. I'm sure others will chime in here too. Renners is a place to get the good jets.

Re: Black plugs
 

Hello Mac.
I see you are not new to your truck, is this a problem that as just started?, there are many suggestions that can be given depending on which carburetor you have and your abilities to follow them, let us know what as been happening and go from there, several excellent sites for Zenith carburetors here on the Fordbarn.
http://www.modela.org

Re: Black plugs
 

No the problem has been occurring for a long time and I think it probably arises in the carb. I run a standard Zenith carb which has not been flow-tested so my next move will be to get it flow-tested. I have a friend here who flow-tests them, so I will talk to him.
Thanks for your help.
Mac

Re: Black plugs
 

You did not say how often this occurs. Model A plugs foul and require cleaning. Very easy to do. Check them and when black and fuzzy, just clean them.

Re: Black plugs
 

Carburetor jets and an air filter will make the fuel air mixture too rich. Also the GAV needs to be 1/8 to 1/4 turn open when you setup the carburetor. Doing so gives the GAV a wider range of fuel-air mixture adjustment.
Another habit that carbons-up spark plugs is long periods of idling with a rich mixture.

Re: Black plugs
 

I have found a good carb cleaning, take it apart but don't remove the jets, especially given todays gasolines, seems to cure lots of problems which can be traced back to carb issues. I clean and gap the plugs once a year, clean the carb every two years, as part of my standard yearly maintenance along with a minor tuneup - points, timing, etc.

If that doesn't work, then start looking for other more insidious things.

Re: Black plugs
 

When everything is good-- mixture, spark, engine, the plugs do not get black, same 3X plugs, not cleaned in35 years except by driving --- got a set of black fowled ones out of the trash, put them in my car and after 20 miles look white and new( ran somewhat lean to cook off the black)

Re: Black plugs
 

I agree with Kurt. I never clean my plugs and they burn perfectly clean. And I use W14 plugs. Actually I wonder why I don't get better gas milage. I get about 17-18 MPG on the hi-way.

Re: Black plugs
 

your mpg could be any number of things: altitude, compression ( lack of), how hard you drive, what speed you drive, rear end ratio, always diving into the wind, tire psi, tire softness, alignment, dragging brakes, engine temp too cool, gav adjustment, carb tuned right, proper use of timing lever, etc:p

Re: Black plugs
 

On my gas milage I drive mainly near sea level, compression is 55-57 on all cylinders, drive about 45-50 with a Mitchel overdrive, Rear is a 3.76, tires at 35 lbs, alignment right on, brakes don't drag. My engine temp may be a little cool with a 160 thermostat. i run 30* total advance. Carb is not flow tested but the plugs look spot on. Only 17-18 MPG.

Re: Black plugs
 

It's the ethanol.

Re: Black plugs
 

It does seem like you have everything right to get at least in the low 20's for mileage.
A 5.5 head should help some.

Re: Black plugs
 

I used to get no better than 15-17, then I replaced the worn camshaft, put in valve seats to get the valves out of the hole (too many seat grindings), and put in a thermostat, on a long trip I averaged 23 at 55 plus over 1500 miles much of it at 65, before the work top speed was 52

Re: Black plugs
 

No it's not.