Solved-Carburation mystery

[Curtis]

A quick check of the points wire might be in order. The wire likes to break inside the covering and could be intermittently making a connection during timing advance. I have also had a brand new rotor with leak path to the distributor shaft causing an intermittent spark short.

Hope that helps.

Cheers
Ray B.

Yup, had this problem and you couldn't tell the wire was broken until you wiggled it around.

[RBMann]

I removed the distributor but didn't find anything wrong. Some surface rust but nothing broken and everything moved where it should. Cleaned and lubed all moving parts. It still ran the same.

At the last tune-up I noticed the tab no longer contacted the service screw so I looked for info on dog-bone length. I found this thread by SVWILBER that sounds like the issue I am having.

viewtopic.php?f=3&t=21085&start=60

I backed out the service screw to the minimum then shortened the dog-bones and did a completely synch/mixture tune. It might run a little better but the throttle is still very abrupt at small openings. It still does the jerk in low gears on the transition from no throttle to light throttle unless I am very subtle. It feels like it over reactions by rapidly opening and closing the throttle when I really just want a small amount of throttle.

I give up for today and plan to take the days off for motorcycle ride.

[GeoffM]

I'm not sure if i read your post correctly, but I wanted to say that the service screw should not be engaged at all under normal conditions. Idle should be set with the individual idle adjustment screws on the carbs itself, once the dog-bones are set-up correctly.

Cheers!

[Curtis]

Looks like you're running an electronic distributor. Any chance that is going south?

[Gregs672000]

Looks like you may be on to something with regards to the linkage issue. I've often suggested a review of how that works or is working when discussing carb balancing, but clearly there are mechanical operations here that need to be right for drivability as well.

[C.Costine]

You have miss-matched springs in your advance. The one that is the lower in the pic is lighter and is partly extended while the upper one is a heavier wire and is not extended at all.

[notoptoy]

You have miss-matched springs in your advance. The one that is the lower in the pic is lighter and is partly extended while the upper one is a heavier wire and is not extended at all.

These springs are not a matched pair from the factory, so it is correct that they are different, however they do not appear to be the correct springs for a non-smog advance. The advance curve looks to be much greater than 7.5 based on that view of the breaker cam, that looks like maybe a 17.5 degree curve.

[RBMann]

I'm not sure if i read your post correctly, but I wanted to say that the service screw should not be engaged at all under normal conditions. Idle should be set with the individual idle adjustment screws on the carbs itself, once the dog-bones are set-up correctly.

Cheers!

I should have said the service screw was nowhere near touching the tab even when turned all the way in. Completely useless for its intended purpose.

Over the years and after replacing the head recently it seems I have unintentionally kept making the dog-bones longer and longer which moved the tab away from the service screw.

[RBMann]

Looks like you're running an electronic distributor. Any chance that is going south?

Possibly. Six years ago on the way home from my first DROPS meeting the pickup coil started failing. During bench testing it would good completely open circuit when I waved my heat gun at it and it reached 185ºF. New pickup coil and matchbox from O'Reilly's were DOA out of the boxes so I used another source. I now have a spare set so I can try them.

[RBMann]

You have miss-matched springs in your advance. The one that is the lower in the pic is lighter and is partly extended while the upper one is a heavier wire and is not extended at all.

These springs are not a matched pair from the factory, so it is correct that they are different, however they do not appear to be the correct springs for a non-smog advance. The advance curve looks to be much greater than 7.5 based on that view of the breaker cam, that looks like maybe a 17.5 degree curve.

You have me here, I don't know what the springs should look like but they have been working well for six years. There is a '15' stamped on the shaft assembly under one of the weights. My understanding is that is correct for the non-smog EI dizzy. When I checked the timing there was 16º of centrifugal advance.

[jrusso07]

Did you switch out the fuel pump back to the old one as you mentioned in your initial post?

I assume the electric pump provides a constant pressure but it almost sounds like it does not or something is restricting fuel as the engine can't make power with little throttle. Have you checked/replaced all fuel filters including the screens in the banjo bolts? Your carbs are so clean it hard to imagine that the screens would be partially clogged but some crap may have gotten into the gas tank...? Just taking some SWAGs.


When the car is parked and in neutral I assume it runs and increases RPMs just fine...it's only under load that it stumbles...correct?

[RBMann]

Did you switch out the fuel pump back to the old one as you mentioned in your initial post?

I assume the electric pump provides a constant pressure but it almost sounds like it does not or something is restricting fuel as the engine can't make power with little throttle. Have you checked/replaced all fuel filters including the screens in the banjo bolts? Your carbs are so clean it hard to imagine that the screens would be partially clogged but some crap may have gotten into the gas tank...? Just taking some SWAGs.


When the car is parked and in neutral I assume it runs and increases RPMs just fine...it's only under load that it stumbles...correct?

Thanks Joe, I had already forgotten about the fuel pump. It won't hurt to check the things you suggest. Yes, it runs fine when not moving. I would not say stumbles because to me that implies poor running. If I floor it when it starts lurching then it accelerates just fine.

Maybe it is as simple as me losing sensitivity in my foot. I give it too much gas which throws me back in the seat causing my foot to come off the gas and chop the power which then throws me forward gassing it again only to repeat the sequence. The problem could just be me but why it it so overly responsive at small throttle opening?

It will probably be something simple and sort of stupid in the end but right now I am not smart enough to figure of the stupid thing. Hopefully my subconscious mind will find the issue soon.

[jrusso07]

Sure is a strange one...but it's always fun to be thrown back in the seat...just keep that pedal down and all will be well!

[rwmann]

The “driver induced oscillation” behavior you describe is sometimes caused by a mis-adjusted (initial floppy throttle) or completely missing throttle pedal return spring. This is less likely to occur if you really have to mash the throttle to overcome the pedal return spring.

[RBMann]

I still haven't found anything wrong. On a lark I put 75W-90 in the dashpots to see if I could calm things down and because I didn't have anything heavier than ATF. It seems to run fine and is more drivable. The issue is still there but much harder to trigger. I am done looking for the time being while distracted helping a friend do an engine swap.