Any ideas?

[Bonne61]

Hey all,

This is a bit long-winded, but the issue is becoming increasingly annoying so I'm putting it out there to see if anyone may have a possible solution. I'll list all relevant details below...

Car has been running really well, but lifters were a bit noisy so checked into my mechanic for an adjustment. Motor has done @ 6,500km's since rebuild. At the same time it had a new set of plugs installed, & a tune up. Mechanic drove it just prior to me picking it up, & said it was going well.

On the way home, it seemed to be missing a little but I put it down to the heat & wasn't too concerned at that stage.

Next morning, it was a bit rough at cold idle & was definitely missing on immediate throttle opening, and at slow cruise speed. Hard acceleration it seemed all good. Car ran pretty poorly, so I stopped in to fill the tank (took 80l, so wasn't empty enough to have dragged any shit into the system). Seems a little better after that, but not much.

Anyhoo...60' 389 rebuilt to 400 (all the details & specs I have but don't think will help here?).

New distributor, fuel sender, fuel pump, carb. Plugs have been checked & cleaned, lifters have been re-adjusted, carb has been stripped, checked & cleaned and retuned, so dirt in there isn't the problem. To be honest, the car ran exactly the same after this was done! Coil & leads have been been replaced since also, to no avail (although the new leads look good😃).

As stated, missing occurs just on throttle opening, & slow cruise (idle/primary circuit)? Will pull easily to 80mph & beyond with no problem, if you boot it.

Thanxx 🤷‍♂️

 

[Frosty]

My first thought could be a possible  vacuum leak, perhaps at the base plate, or a line is not plugged to the carb or manifold.

How is the fuel filter? is it clogged at all?

 

[64 kiwi boni]

paul, justA go through the basics... one thing i note is you havent changed the plugs ? what brand are they ? 

i would be replacing them !

i am using these ngk 

also, check your points, and same there, if in doubt, replace them 

 

Edited November 14 by 64 kiwi boni

[Bonne61]

Hey @Frostyand @64 kiwi boni,

Thanks for your replies-fuel filter is clean, carby has been stripped & cleaned, base gasket is new, plugs (BP5S) were replaced, & have since been pulled & checked a few times now (I even swapped the them over in case that might highlight something), points look as new (as Is distributor, rotor & cap), there is no arcing or witness marks inside the cap or on the rotor, the vacuum lines from the brake booster & distributor vacuum advance are both in place & secure.

Anything else I could be missing (no pun intended)?

[Last Indian]

3 hours ago, Bonne61 said:

Hey @Frostyand @64 kiwi boni,

Thanks for your replies-fuel filter is clean, carby has been stripped & cleaned, base gasket is new, plugs (BP5S) were replaced, & have since been pulled & checked a few times now (I even swapped the them over in case that might highlight something), points look as new (as Is distributor, rotor & cap), there is no arcing or witness marks inside the cap or on the rotor, the vacuum lines from the brake booster & distributor vacuum advance are both in place & secure.

Anything else I could be missing (no pun intended)?

Ok, a couple things. First is it a miss or is it a stumble/ hesitation? They can seem similar, but the causes are very different. I would tend to agree with Frosty. Your description sounds more like a vacuum leak. Why? A miss doesn’t go away. Doesn’t go away on a power increase usually. But a vacuum leak can be like that! You need to get a can of spray ether. Than methodical check your intake gaskets & the carb gasket. There are a lot of variations in carb gaskets & I have seen a lot of carb gasket leaks that cause havoc to people because they never suspect that gasket. Also I would suggest checking your fuel pressure. Just because you replaced it doesn’t mean there isn’t a problem with the diaphragm. I have seen new fuel pumps have a small hole in the diaphragm.

[64 kiwi boni]

last indian, is so onto it !!

And i am going to sound like fitzy here paul ! 

dont trust anything aftermarket as being "good" 

[Bonne61]

4 hours ago, Last Indian said:

Ok, a couple things. First is it a miss or is it a stumble/ hesitation? They can seem similar, but the causes are very different. I would tend to agree with Frosty. Your description sounds more like a vacuum leak. Why? A miss doesn’t go away. Doesn’t go away on a power increase usually. But a vacuum leak can be like that! You need to get a can of spray ether. Than methodical check your intake gaskets & the carb gasket. There are a lot of variations in carb gaskets & I have seen a lot of carb gasket leaks that cause havoc to people because they never suspect that gasket. Also I would suggest checking your fuel pressure. Just because you replaced it doesn’t mean there isn’t a problem with the diaphragm. I have seen new fuel pumps have a small hole in the diaphragm.

Thanks for that-I’ll chase the vacuum solution.

As for fuel pressure-prior to this issue the pump (Holley mechanical) was working perfectly. Also, if it was fuel pressure wouldn’t it deteriorate rather than improve as the revs/speed increased, due to poor delivery?

[Fitzy]

Paul - are you using qualiy points & condenser? My mechanic changed the garbage Chinese supplied items that came with my brand new distributor because he said he 'couldn't get a good spark.' For the record, he has put Bosch ones in and also a Bosch GT40 coil. Use NGK plugs. There seem to be numerous quality issues with everything these days. As Kiwi suggested, be methodical. Start with fuel (the simplest) and check everything - all components. Then move to electrical. Are you getting 12V to the coil? You're using points like I am, so voltage should drop to 6 or so once engine is running, due to resistance wire feeding the coil, or you might have a ballast resistor. Vacuum leaks are notorious for poor running. A possible telltale of a leak is the engine not idling properly - speeds up and almost dies, then repeats. Any timing 'flutter,' where if you observe the timing marks as the engine is running, are they stable? Might be a dodgy vacuum diaphragm. 

As simple as old engines are, they will all drive you crazy with the most irritating symptoms that are seemingly unfathomable to diagnose. Good luck.

[Last Indian]

9 hours ago, Bonne61 said:

Thanks for that-I’ll chase the vacuum solution.

As for fuel pressure-prior to this issue the pump (Holley mechanical) was working perfectly. Also, if it was fuel pressure wouldn’t it deteriorate rather than improve as the revs/speed increased, due to poor delivery?

In general that can be true, but I have seen a hole in a diaphragm leak at low speed because of the slower movement of the bellows/diaphragm, but not at higher speeds. Because the speed of the diaphragm movement is enough to overcome the supply issue, for a while anyway, but over time the hole will deteriorate & get larger. 
If by chance the issue is electrical, & I doubt it is, it could be either + or - issues. Sorry, but if you are running a 12 volt system with a 12 volt coil you better have 12 volts at the supply side of the coil when the engine is running & somewhere in the neighborhood of 30,000 volts or more coming out of it. Otherwise you have problems, but they would not be intermittent in the way you described. 
The issue sounds like fuel, which can be caused by a vacuum leak. Which puts you on the lean side of fuel& can even cause missing & backires!

[Frosty]

Staying on the vacuum leak theory for a moment, worst case scenario would be a leak of the intake manifold gasket. Obviously that is a new set of gaskets, RTV, and absolute pain-in-the-arse to fix. So try spraying some starter fluid or flamable brake clean over the manifold and see if the engine revs at all.

[Bonne61]

Well @Frosty @Last Indian @Fitzy @64 kiwi boni,

In summary below, your combined advice and my actions (if any):-

* vacuum leaks - sprayed BrakeCleen around carb vacuum fittings, base gaskets & inlet manifolds. Thought I detected a very slight rise in idle from the back of the carb (may also have been wishful thinking😬). Removed carb, sealed base gaskets with HT grease, removed & sealed vacuum fitting with thread tape & Permatex. Road tested, no change🙄

* Power supply to coil - this one seems a bit strange to me...I'm getting @6.5v with ignition on (not running), & @9v at idle, which doesn't follow with what you guys have said😯

* Fuel pressure - I'm unable check this (seems to be a strong supply volume though). 

* Points/condensor poor quality - unknown

So, it's off to the mechanics again on Monday, & hopefully it won't come back until it's right🤷‍♂️

 

 

[Fitzy]

When you are running points, this is what is supposed to happen: 12V is routed through the ignition switch when you start the engine. Once it's running and you release the key, it springs back slightly to the 'run' position which now uses a different contact on the back of the switch and sends voltage to the coil but through a factory 'resistance wire' which drops it down to 6V or so, to allow longevity of points. Constant 12V to points will make them burn out quite quickly. So, with engine not running and ignition 'ON' you should have 12V at coil + terminal. Once it's running, you should have 6 or so. I suppose it'd be prudent to check that your earth lead from coil to distributor is also in good condition. A good earth is vital. If you ever peered into your engine bay and wondered why GM fitted so many little earth straps, they are all there for a reason.

If you happen to ever go to electronic ignition, then you'll need constant 12V to the coil + which means running a nice new wire from the ignition 'START' feed so as to ensure 12V. Phew!

[Bonne61]

20 minutes ago, Fitzy said:

When you are running points, this is what is supposed to happen: 12V is routed through the ignition switch when you start the engine. Once it's running and you release the key, it springs back slightly to the 'run' position which now uses a different contact on the back of the switch and sends voltage to the coil but through a factory 'resistance wire' which drops it down to 6V or so, to allow longevity of points. Constant 12V to points will make them burn out quite quickly. So, with engine not running and ignition 'ON' you should have 12V at coil + terminal. Once it's running, you should have 6 or so. I suppose it'd be prudent to check that your earth lead from coil to distributor is also in good condition. A good earth is vital. If you ever peered into your engine bay and wondered why GM fitted so many little earth straps, they are all there for a reason.

If you happen to ever go to electronic ignition, then you'll need constant 12V to the coil + which means running a nice new wire from the ignition 'START' feed so as to ensure 12V. Phew!

So, earth lead from distributor to coil is new, as distributor is also new.

Any ideas why I should have 6.5v with ignition on, but 9v when its idling? This seems backwards doesn't it?

[Last Indian]

Paul, first off the electrical. Irrelevant of what any of us say, you can find out in 5 minutes how your coil/distributor system works with respect to voltage. That said, what Peter is indicating to you is you have two wires that feed the + side of coil. One is a resistance wire the other comes from the ignition, the other can come from the ignition or the starter, both are acceptable. What Peter indicates occurs is correct. On starting the non resistance wire supplies 12- 9 volts depending on battery condition, ground wire condition, etc. once started the starter switch springs back as he says to the resistance wire. In my earlier post said running, I was rushing not paying attention & it should have been starting. So the point is by what you described as happening something is not right. It’s actually backwards in occurrence of voltage. The 9 volts on starting is acceptable, 6.5 is not, but 9 volts at idle while not what you want should not be causing the problems you are describing. So I still have to question the vacuum leak! While brake cleaner can work to chase vacuum leaks it’s not nearly as effective as ether/ starter fluid. Furthermore grease on the base gasket of a carburetor won’t seal anything. You need a vacuum gage connected to the engine than you chase the vacuum leak & watch the gage. Also don’t assume it’s an engine gasket. It could be a hose or a diaphragm. Do you have a vacuum advance distributor? 

 

[64 kiwi boni]

Or a vacuum hose to the trans ? 
what transmission do you have Paul ? 

[Fitzy]

You may as well check the brake booster vacuum hose too - another potential (but unlikely) source of a leak, although having said that my old booster diaphragm was shot and needed to be replaced. Whilst shot, I still had power brake assistance but the trans wouldn't shift of it's own accord.

You say you have a new distributor? Mine is a Cardone brand - unfortunately like everything else is made in China. The crummy points & condenser that came with it were no good.

Your reported voltage scenario sounds very strange. The 12V to coil at ignition 'ON' is fed from the starter motor contact, and the running voltage is fed from the ignition switch. Why you don't have 12V for a start is a mystery. You need to check for a good solid connection from the starter terminal to coil first.