New , slight miss at very light throttle..... ethanol related ??

[Geoff 2]

With idle vac at 12*, you need a lot more idle timing. A 440 I worked on today idles with 44*, best idle quality. Vac is 13-14". It was much lower with less timing. Reads the link in post #18....again. 35-40* for a mild cammed engine. Your stumble is possibly caused by the engine not making enough HP to carry the load at that instant. I would like a dollar for every off idle stumble I have fixed by simply increasing idle timing....

What more idle timing does.....

 

[Canadian1968]

I understand what you are trying to say but I really don't feel that is my problem. 44* initial timing ? On a street car ? Almost sounds like a locked out drag race car ! .

Why would the car run absolutely amazing the previous year with the exact same tuning set up . Then suddenly develop a slight ( and inconsistent) miss under light throttle only.

The car has always run at this amount of vacuum with out problem. I understand your suggestion if I was starting from scratch but this is a motor that has been running more or less flawless for that last few years. Great street driven manners and laying down impressive number at the track ( for what it is).

I have a new rotor and new set of plugs coming tomorrow. Will swap those out. Was harder than I thought to find the ngk plugs I have been running .

 

[68 Sport Satellite]

I understand what you are trying to say but I really don't feel that is my problem. 44* initial timing ? On a street car ? Almost sounds like a locked out drag race car ! .

Why would the car run absolutely amazing the previous year with the exact same tuning set up . Then suddenly develop a slight ( and inconsistent) miss under light throttle only.

The car has always run at this amount of vacuum with out problem. I understand your suggestion if I was starting from scratch but this is a motor that has been running more or less flawless for that last few years. Great street driven manners and laying down impressive number at the track ( for what it is).

I have a new rotor and new set of plugs coming tomorrow. Will swap those out. Was harder than I thought to find the ngk plugs I have been running .

I think the 44 degrees refers to total timing with vacuum advance connected, which is a good ballpark # if you have your initial timing advanced to 12-18 degrees at idle. Point being, you may want to try increasing your initial timing as a test.

I wonder if it could also be possible that your small vacuum hose connecting your carb to your distributor vacuum can has a small leak that could be causing this miss?

 

[Canadian1968]

I'm not running a vacuum advance. Never have either.

I'm picking up the plugs and new rotor tomorrow... I will toss them in and take it for a drive. If the miss is still there I will try bumping the timing up .

 

[Geoff 2]

I do not know what has changed, but NOT running VA on street car is a bad move?

You think 44* idle timing is high? My engine runs 48*. One I tune for drag racing ran 53*. Below is about ign timing at idle, but it is in a Holley carb book. Think about why idle timing is in a carb book....& mentioned more than once.

 

[Canadian1968]

I think I finally got it!

Replaced all the plugs and put in a new rotor I checked each wire for resistance .

Fired up and took it for a drive . I immediately noticed the car idling better in gear. I went for short drive, enough to get the car up to operating temp and conditions when the car would miss.

The car didn't miss once , I tried to replicate what caused it before and could not get it do it again. Not only is the miss gone but the car idles COMPLETELY different. So much smoother and now a tad bit faster.

I'm a little dumbfounded to be honest. I think this entire time there something off . I was just putting it down to the nature of the car / engine combo. But I think this whole time I probably had a crappy spark plug ? It just got to a point where it was making itself more noticeable . When I pulled all the plugs none of them really stood out to me as being noticeably different . Although my spark plug reading is a beginner at best!

I need to obviously take it out again and longer drive , but I'm pretty sure problem is fixed !

 

[oldkimmer]

That’s great! Thanks for the update.