I've got the midrange hesitation blues

[Mastiffen]

Yes, I've got the ol' midrange hesitation blues
'Cause my baby said she won't drive my Starfire
Until I loose the midrange hesitation blues
(Melody: Any 1940's delta blues guitar track)

Back to the scheduled program! I can't get rid of the hesitation I'm fighting with. I'm quite sure that it has to be the ignition because I am sure I have done the carb correct. The problem is on midrange RPM and careful pedal. Idle is OK, and full pedal is great after having done the carb (I have actally burned rubber unintentionally twice when driving out of my garage after doing that!). It's worse when the engine is cold, it helps a bit to drive a few minutes.

I think it's related to what I did a few weeks ago, because I can't remember having that before (but then I hardly drove it from October and until then because I didn't have plates, which I got a week after the ignition work, so I can't be 100 % sure). I changed the ignition cables (and I had to change to yet another kit since I was clumsy and damaged one of the cables), the rotor, the distributor cap and some module that's flat and metallic, with two connections. It's at the bottom of the distributor. A few weeks before that I changed the spark plugs (mostly to get rid of idle problems, that turned out to be a couple of faulty vacuum connections in the carb, so it helped to rebuild the carb and glue those tubes in place).

I then did a totally wrong timing job (with the vacuum hose still on the distributor - haven't done anything like that in 10-15 years!), and that made this problem hysterically bad, with something that sounded like backfire in the carb. Correcting that made the hesitation a lot less pronounced, but it's still there, and it's a bit too annoying for my wife to drive the car. It's not that important to me, I could probably get along nicely with a button instead of a gas pedal - I mostly drive full on or full off!

[heinz057]

is the vacuum for the distributor hooked up to ported or manifold vacuum?

[Mastiffen]

It goes directly into the carb, next to the one to the canister. I checked all vacuum tubes after the instructions in a routing scheme from the forum.

[heinz057]

if you disconect the hose at idle does the rpm drop?

[Mastiffen]

Yes. When I connected it after the timing job, the RPM's went up.

[heinz057]

thats manifold vacuum. you should try having it hooked up to ported vacuum. thats vacuum only when you give it gas.to see if that helps . also did you check to see if the machanical advance is working? when it hesitates does it back fire?

[Mastiffen]

Thanks! I connected it according to this diagram:

What hose should I trade places with? And if that's what you mean, I did see the ignition timing advance when I increased the pedal pressure. And yes, it did backfire (or rather frontfire, it came from the engine, not the rear exhaust pipe) a lot when I had the timing all wrong. Now I have heard it do that once or twice in the driving I have done (probably half an hour all together).

[heinz057]

try to hook it up to a port that has no vacuum at idle. also popping out the carb can be from being to lean

[Mastiffen]

OK, I will try to find such a port. Any clues to where I may find it?

And would that lean mixture be corrected by changing the float level? Because the mixture screws are only for idle, right?

[heinz057]

to find one start the car and pull 1 off at a time you should here if it has vacuum. if you have a vacuum gauge that would help. if the float is to low it can run to lean. but i would also check the accelerter pump to see if is working. but first try the hose . only do 1 change at a time.

[Mastiffen]

OK, thanks! I actually have a gauge! How accurate it is, I have no idea (part of a cheap set with a multimeter, ignition strobe and a compression tester). But it should work. I'll let you know tomorrow how it went, it's a bit late to start on that now.

[heinz057]

good luck

[gerbsinmd]

Guys, Would the EGR valve cause problems like that? Either one that is too active or stuck?

[Mastiffen]

I have no idea. Anyway, no chance to experiment today, we had a lot of snow over night. And tomorrow there's a storm coming, so I guess I won't be able to work outside until the weekend. Gotta get myself a garage with an exhaust outlet!

[acstestinc]

I think that your engine is running lean rather than having an ignition problem.

If I understand correctly you get a hesitation when you are cruising along at a steady speed and then gently give it more throttle. If you step on the gas hard there's no hesitation. You've also replaced everything on the ignition side (which is pretty straight forward), and the engine idles OK.

Sounds like a vacuum leak or a defective device that runs off engine vacuum and is leaking vacuum. You don't hesitate when you step on it hard because the accelerator pump on the carb gives it an extra shot of gas. The engine idles ok because you probably have richened it up enough to compensate for a vacuum leak. Check your idle mixture screws. They should only be open about 2 full turns under normal circumstances. If they are more than that (3 or more turns) then that re-enforces the vacuum leak theory.

The EGR valve is supposed to close when you step on the gas and the vacuum drops slightly. The EGR doesn't really lean the fuel mixture. It dilutes the air/fuel mixture by introducing some exhaust gas, which is basically inert, back into the intake in order to lower combustion temperatures and in turn exhaust emissions. But having said that, it is a vacuum operated device so you could have a leaking diaphragm in it.

I suggest:

1)That you disconnect and plug, at the carb, all the vacuum lines , one at a time, except for the distributor vacuum and test it and see if it disappears. If it does then you know where to look further for the source of the leak or the defective device. Do the power brake booster line as well, if you have one, but be careful because you won't have power brakes (the pedal will be really hard to push down and the car won't stop as quickly.

2) Leave the distributor vacuum connected as per the diagram. Street engines need vacuum advance at idle for part throttle acceleration. Race engines don't because the gas peddle is mostly used as an on/off switch.

3) Check the appearance of a spark plug against the chart at (http://www.dansmc.com/Spark_Plugs/Spark ... talog.html)
to give you an indication of a lean air/fuel mixture.

4) If you really want to check the operation of the EGR valve you can make up a block off plate from some 3/8 aluminum or steel ( a little cutting and drilling required) and install that in place of the EGR.

A couple of questions:

Does the engine also stumble or hesitate when you accelerate easily from idle? If it does that's a sign of a partial blockage of the main fuel jets.

Does it start easily? By that I mean 1 or 2 pumps on the throttle and it starts right away.

Any black smoke from the exhaust?

Good luck, hope this helps.
Paul