I was hoping I could get a bit of troubleshooting help here. 51 F-1 with the 226 flathead inline 6. When I start the truck cold after it has been sitting a few days, the idle is super rough. I can play a little with the choke and throttle to get the idle to be a bit smoother, but it still remains pretty rough until the engine is warmed up; at which point, any roughness in the ilde goes away. The truck runs and drives fine once warm (i.e. no hesitation or roughness in the engine).

As a bit of background, I began to notice a bit of a skip/slight miss in the idle when starting the truck up a couple of months ago. However, I could tweak the throttle and choke such that it would go away when warming up and be gone when fully warm. This little intial skip has deteriorated over time to the rough idle it has now. The truck would smoothly upon start-up prior to the initial development of the skip.

Any thoughts on troubleshooting this? Please keep in mind I'm pretty new with engines in general here. Thanks.

 

Timing could be slightly off, possibly.

 

The colder weather could also be aggravating a problem that up until now is very slight. I have a number of thoughts

First is that your compression may be low on one or two cylinders, and until the engine warms up, it's loose enough to affect the run quality.

Also, with the colder weather, you might have a head gasket going and when the engine gets cold and shrinks, it's allowing either a compression leak or coolant to enter.

Check your idle mixture - timing too as Ilya stated. If you have been able to "fix" this by manipulating choke and throttle I'll almost guarantee your idle mixture is off.

If it were my truck, I'd put a vacuum gauge on it to test all these things....could be a dash of any or all!

 

I'm with Julie. Reset your idle mix screws. Mine is definately different in summer than it is in winter.

If that's no help, then check you plug colors. If one is way off or has lots of deposits, then the head gasket idea could be true.

Have you tried searching for a vacuum leak? Maybe it cures itself when the operating temp gets high enough.

Jag

 

When was the last tune up?

 

Yep, check the plug gap and electrodes clean too!

 

I'm always a proponent of trying to isolate the cause of things. I wouldn't mess with the carb first because you don't really have exact specs. Carb settings can vary from vehicle to vehicle. However there ARE exact specs for point gap, ignition timing, sparkplug gaps, etc., which are all easy to check. I would make sure these are all OK first. Make sure your spark is lively. A compression check is relatively simple, yet it can be a very revealing test. If you don't have the time for that, at least do a visual on your plugs before you gap or replace them. Also, do you have an inline fuel filter--if so, is it clean? Check these out, make sure they're OK, and then proceed to the carb if needed. The idea being you've locked the tangible stuff down correctly before you begin tweaking the less finite variables.

 

I do not know when the last tune-up was... I've had the truck for about a year now and I haven't done one (bad old truck owner, I know). As far as narrowing this down, regarding the weather for the card idle mixture, I live in San Francisco and it is, literally, the same temperature all yeard round at my house... between 55 and 65 everyday. There just isn't any real temperature swing between the seasons except that it'll lean to the warmer side for a while (warmer being 65) or the colder side for a while (55 or so). I'm thinking spark plugs or spark plug wires, but just wondering why the rough idle would go away when warm if the cause were something in the plugs? Vacuum leak could be a possibility as the hoses on my truck are not generally in the best of shape.... would this be something along the lines of the hose expanding to close the leak as the engine warms up? Thanks for your help everybody.... I do appreciate it.

 

I would do a tune up first as it is in need of that. Than see how it ran. Replacing the vacuum lines if old is always a good idea and part of what I do as a tune up on these rigs. As well as a compression check and timing reset after the tune up parts and during the carb adjustment.