Runs Rough at Speed

bobgreco

bobgreco

Senior Member
For the first time since the painting project, we took the truck out for a longer ride at speed, about 10 miles. On the way home, it started to ride very rough around 40 mph and was making kind of a popping or pffft noise from the engine. It also seemed to have a bit less power. Slowed to 35 and it settled down so we made it home OK.

Any thoughts on what this could be?

Thanks for your time!

Bob
 
GAV closed too much
Points closed too much
Too much dirt in fuel screens
Crusty goo on tips of main jet from fuel evaporation while sitting

These are various solutions I have used on my car, I consider "at speed" to be 55

Another possibility is a slightly sticking valve
 
sounds like insufficient fuel flow to the carburetor. Check everything from the gas tank outlet to the carburetor screen for particulate or gum or a fuel line extending too far past the ferrule.
 
Another possibility is a plugged fuel filter. Did you happen to install one of those slick little spring-supported "micro" filters in the sediment bowl? I understand that many folks get about 10 miles out those before they plug up enough to cause problems. In my experience (see photo), the only filter that didn't cause trouble was a now-obsolete fiberglass model (the red one in photo). This is one area in which individual results vary tremendously--my theory is that fuel filters work great for folks whose systems are so clean that they don't really need them.

Steve

post2007MARCrepairs036.jpg
 
I put a new screen in my glass sediment bowl over a year ago. I've never had a fuel flow problem with the screen alone, plus its fun to look at the debris the screen filtered out. In a year, I haven't accumulated enough debris in the glass bowl to make it worth the effort to clean it out. A very well engineered system. Can't understand why they replaced it with a cast iron thing amagig! Gravity keeps the debris stuck to the bottom of the glass bowl and when I get enough garbage in there I will clean it out. , .
steve s said:
Another possibility is a plugged fuel filter. Did you happen to install one of those slick little spring-supported "micro" filters in the sediment bowl? I understand that many folks get about 10 miles out those before they plug up enough to cause problems. In my experience (see photo), the only filter that didn't cause trouble was a now-obsolete fiberglass model (the red one in photo). This is one area in which individual results vary tremendously--my theory is that fuel filters work great for folks whose systems are so clean that they don't really need them.

Steve

post2007MARCrepairs036.jpg
Click to expand...
 
Cast iron came first, then the glass, then back to cast iron. Cast is easy to drain, but glass lets you view the junk.
 
I prefer the cast iron sediment bulbs over the glass type. The gaskets can be a real problem whenever you decide to remove the glass bowl. I had a model A burn in the driveway back in 62 because of a leaking gasket on a glass bowl type sediment bowl. I figure that I really don't need to see the sediment I just drain them periodically and the lead gaskets have always worked good.
 
Well, I finally got the chance to get the truck out on the road and up to speed. I checked the fuel screen and the carburetor and all looked well. I opened the GAV a little more and fully advanced the spark when I got going and it ran better than ever! 52 mph on flat ground. A new record for me. It now cruises nice at 45 mph.

As always, thanks a bunch to all of you that helped!!!

Bob
 
One suggestion
Remove the carb jets and check to see if they are at spec
If not originals, repros in my experience, some were not even close to spec.
Solder them closed and re-drill
 
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