Hey guys, I have a question for you, concerning a slightly rough idle:

I have a 1976 Ford Country Squire, I know this is not a truck / Bronco, but since some of you have the same engine in your trucks, I was hoping to find some advice here...

It stil has the original 400m engine, fairly low mileage car. However, some time in its past, the carb has been changed for a remanufactured one. I don't think it's the original one, since it did not have a PCV port at the back. I installed one myself there, and hooked up the PCV valve hose again, so that's working again.

Also, it says 1.08 on the carb bowl. I've read the 351m400 engines should have 1.21 (or 1.23, I forget?).

As soon as I step on the gas, it runs super smooth and accelerates great. Just what I would expect. (Don't know about wide open throttle; this is just a sunday cruiser, top end performance is of no importance at all in this particular case)

But the idle is just a little bit rough, not too bad but would like to get it a bit smoother.

What would the sypmtoms be, on this big 400 motor, of having the 1.08 carb ( if it originally had a 1.21) ? Would that show only in lack of wide open throttle power, but not have any affect on idle smoothness?
Or could it also be the cause of the slightly rough idle? Again, the motor runs great otherwise, no complaints in handling, no hesitation...

- Other then that, I had a classic ford mechanic check the adjustments; and he also said ignition timing was "almost OK", not too far off, if any, but he could not rotate the distributor last week, it's stuck in the block. I'm dripping some oil around the base now & then to try & soak it over time, we'll try that again later. So I don't have any actual timing setting to give to you for now, other then he said it's probably not too far off, if any...

- Also, I noticed the EGR valve vacuum line goes straight to the intake manifold vacuum tree. Shouldn't it go to the port at the back of the carb? (there is a port capped off, at the back just above the PCV port location...) I still have to try to route it there and see if that makes any difference. (it would have to be a straight route there, for now: my engine has no temperature-controlled vacuum valve) ...
There are no open (uncapped) ports that I can find which could cause false air.
- It does not smoke at all from the tail pipe.
- It has a special oldtimer registration so smog/emission is not an issue anymore here.

Anyway, I would appreciate your thoughts on the 1.08 on a 400m engine: If this can not be the cause of a rough idle, is it OK to leave this carb in place? (it runs great and smooth otherwise; I'll just have to keep looking for other things that could cause slightly rough idle)


Thanks in advance for your information,
Best regards
Werner

-Greetings from Belgium, EU-

 

The 1.08 should be fine and will not cause a rough idle. I would look for vacuum leaks, especially the carb base, the carb throttle shaft and, if you have the spacer for the PCV, that as well.

The EGR should be checked for leaks and function as well.

This is the spacer, not sure you have one, but that is what the PCV plugged in to on my 289s.

http://i.ebayimg.com/00/s/NzM5WDg3NA...T27ot/$_35.JPG

 

Alright, thanks for the quick reply, that's already one thing solved :-)
I'll keep looking.

 

The smaller carb will cause a loss of power at full throttle and high RPM but it should be fine any other time. However, the problem at idle is probably caused by a lean mix due to carb design. Most carbs have restrictions in the idle circuit, and since yours wasn't expecting the significant air flow from the PCV valve, and it was intended for a smaller engine, it would have small restrictions. So you may not be able to get enough fuel to the engine at idle to let it run smoothly.

Have you been able to adjust the idle mix and actually get it too rich? By that, I mean back the mix screws out to the point that the engine slows significantly? If it wont't slow that means you can't get too much fuel and it is still lean. Try pulling the choke on slightly at idle to see if a bit more fuel helps.

 

Aha... that could be... I'll give that a try as well, thanks for the advice guys.

 

. My 351M has always idled rough, I think it's the crap cam duration/lift and retarded cam timing...