Slant stumbling on acceleration

[SD_R/T]

Ok this one's starting to get to me. Hang on, this will be a lengthy run-down:

I must preface this with the fact that my knowledge of the history of the PO's work (or lack thereof) on this car is basically nil.

So a while back I start noticing a very 'slight' stumble on acceleration. Nothing major and only occasionally, so I don't think much of it. Over time it gradually becomes more noticeable. It's about this time I find that I start having a flooding condition and realize the float in the 1920 carb is probably a goner. Seeing that the replacement costs for the 1920 float is pretty steep, I find a recently rebuilt (by owner, not rebuild house) 1920 on ebay for about the price of a new float. So I swap it out. Flooding condition resolved!

But the stumble remained? Hmm. This made me think it wasn't carb related. So I replace the fuel filter in case it was clogged. The Wix metal filter I pulled off looked pretty clean. That didn't fix it.

So I start thinking vacuum advance. I see that the old vacuum advance hose is in pretty rough shape and looked pretty worn down by the distributor. No problem. That's a cheap/easy replacement that probably should've been done, just like the fuel filter. Nope, that didn't fix it.

Ok, that vacuum advance pod looks WAY old and caked in oil over the years. Swap in a new vacuum advance pod. That doesn't do it either.

Now I'm scratching my head. The stumble I have is not at very low throttle. It's if I start to punch it a little that it bogs and wants to die. Opening it up further to WOT will sometimes get it past the bog. It idles just fine and I can punch the accelerator in Neutral and there is no bog. Only under load.

The condition is worsening this entire time. Only now I start to notice a wrinkle I hadn't noticed before: It's only happening after a couple minutes after warming up a little. Hmm. Temp-related? I look at the coil and it looks as worn as the vacuum advance. I look up the part number on the side and it's some old GM and tractor coil from the 60s or 70s. My multimeter doesn't have batteries to test it but I figure what the hell and replace the coil. Nope. That doesn't do it.

So that's where I'm currently at. Basically stock 225 with Holley 1920. Will pull the plugs this weekend and take a look at their condition and also check compression while I'm at it.
Any ideas where to look next?

- Timing? I don't have a timing light but will try to get my hands on one. If the timing has become retarded, wouldn't I have issues at other times besides just part-throttle?
- Still the carb? I had originally eliminated this when I swapped to the second 1920 with no apparent change, but now that I see that it's somewhat temperature-related I'm still wondering about this. I should note that the issue seems to be there regardless of the mixture screw position, 1.5 turns, 3.5 turns, same issue. Float position looked correct on the second 1920 carb. There is a nice healthy shot of fuel on the accelerator pump.
- Valves? I adjusted the valves (.10, .20) only about 500 miles ago.
- Vacuum leak? Didn't see/hear anything noticeable. Sprayed cleaner around the carb base with no change. Can't feel anything.

I'm in the process of getting my super six conversion together so if it's just the carb then I'm inclined not to spend the effort farting with the 1920 extensively since I'll just be abandoning it soon. Then again, if it's something else I want to make sure it's resolved before I do the conversion.

 

[krazykuda]

Why not do a wet and dry compression test and see what condition your cylinders are in? Let's make sure that there isn't a low cylinder or two first, then build up from there.

Are you running points or electronic ignition? Details...

A stumble can be caused by many things. Let's make sure that your base "core" is good, especially if you are going to change the intake soon...


What are your timing settings? Initial advance? Total advance?

Also look at the spark plugs and see if they can tell anything... Rich... Lean....

 

[SD_R/T]

Why not do a wet and dry compression test and see what condition your cylinders are in? Let's make sure that there isn't a low cylinder or two first, then build up from there.

Are you running points or electronic ignition? Details...

A stumble can be caused by many things. Let's make sure that your base "core" is good, especially you are going to change the intake soon...


What are your timing settings? Initial advance? Total advance?

Also look at the spark plugs and see if they can tell anything... Rich... Lean....

Good point . I didn't address ignition beyond the coil. Found that the PO had removed the points and installed a Pertronix I. Swapped in new NGK's when I had done the valve adjustment.

Agreed about the compression and plugs. As I mentioned above, I'm hoping to look at that this weekend. I also can't comment on the timing at this point since I don't have a timing light (yet). Will be trying to get hold of one.

 

[krazykuda]

If you have an extra new or old plug that you can clean up, you can use the plug to test the spark. Clean it and gap it to spec. (usually .035").

Disconnect a wire from a spark plug and then put the wire on the extra plug. Hold the bottom electrode to a ground (the engine will do) and crank it over and look at the spark. Is it strong or weak? Blue or orange? You may want to move the spare plug from one wire to the next to make sure that you are getting spark through the wires to each plug and how strong it is.


If you pull your plugs, look at them. Are they white, gray, tan, black, oil black, wet fouled black, Carbon build up??? Google search spark plug reading charts and keep track of which cylinder each plug came out of and what condition that it shows.

 

[TheMoparMan]

First thoughts would be to get a timing light and check timing also test the coil. Coils going out can have have intermittent issues as they heat up. The coil issue can mask itself as a carb or timing issue sometimes. It's a least worth a look.

 

[SD_R/T]

If you have an extra new or old plug that you can clean up, you can use the plug to test the spark. Clean it and gap it to spec. (usually .035").

Disconnect a wire from a spark plug and then put the wire on the extra plug. Hold the bottom electrode to a ground (the engine will do) and crank it over and look at the spark. Is it strong or weak? Blue or orange? You may want to move the spare plug from one wire to the next to make sure that you are getting spark through the wires to each plug and how strong it is.

I could stand to look at the wires, certainly. I'll even see if I can get my multimeter working again and test the resistance of each of them. Could a bad wire cause a stumble at a particular throttle opening though? As I mentioned in my original post it will eventually make it through the 'rough' part of acceleration and get back to full power.

 

[SD_R/T]

First thoughts would be to get a timing light and check timing also test the coil. Coils going out can have have intermittent issues as they heat up. The coil issue can mask itself as a carb or timing issue sometimes. It's a least worth a look.

I thought about the coil/temp thing too but swapped in brand new coil with no change.

Appreciate all the feedback. Due to work I won't have much chance to fool around with it for a couple days so I was just trying to line up a plan of attack and prioritize a list of things to look at.

 

[krazykuda]

As I mentioned in my original post it will eventually make it through the 'rough' part of acceleration and get back to full power.

Yes, we have had cases here where the spark plug wire rested on the exhaust and not fires on v-8's. We've also seen brand new wires out of the box that were no good either.

 

[nm9stheham]

What year and model, please?

Anything ignition related can casue a stumble; the amount of spark voltage needed to fire the compressed fuel/air mixture in the cylinders wil vary widely with load and RPM; low RPM, high load is typically a good set of condtions for the spark to need to be 'hot', and it can misfire if not 'hot'.

What coil does this ignition system have? We need the specifc part number and type. Some coils with the Pertronix should have the ballast removed; is it there or not?

Measure the voltage at the 12v feed to the ballast and/or coil at idle and at fast idle; this is likely a dark blue wire but the year/model needs to be know to be sure . We want to make sure that there is not excessive voltage drop to the ignition system. It ought to be close to the voltage at the battery under the same idling conditions.

I would also go back to your 'new' carb and reset and recheck the flaot level. The wet check procedure for older 1920's is to remove the economizer valve and measure from the top of the housing where that bolts in to the fuel level when the car is idling; that should be around 11/16".

Check under the rotor cap for anything odd with the rotor or cap; probably not it.

Also as a last resort, check the fuel pressure when accelerating to make sure that it is not dropping at low RPM but with high fuel demand.

And if I may comment, low compression usually does not cause the type of load/RPM sumble that you seem to be describing.

 

[SD_R/T]

Ok, a little more to report. Due to time constraints and test equipment failure wack I did not get to look at as many things as I wanted. Things I did get to:

Wet float check: Measured right at 27/32, which should be correct according to the FSM I have.

Plug examination: This one was interesting. #1 and #6 were totally black & fouled. #5 had evidence of soot on the insulator but not the tip (like maybe it had been fouled at some point?). I know there are folks much better at plug analysis than I am, so here's a picture of them in order looking from passenger side, #6 on far left and #1 on far right. I did make sure the 1/6 wires weren't switched.

View attachment IMG_20140809_115251.jpg

Things I didn't get to:
Timing: The timing gun I got hold of was a POS. Should've known better with Harbor Freight.
Plug wire resistance: What I thought were dead batteries in my multimeter (Craftsman, not HF!) was not the case. A fresh set of batteries and no dice. It appears to have crapped out too. Which means I couldn't check the voltage at the coil either. Looks like I have some shopping to do.


Regarding the info I left out earlier, my bad. Car is 68 Valiant. The original coil was an old GM coil and the PO left the ballast resistor connected. I had replaced the ballast several months ago (mainly because I didn't trust it, not because it had started acting up). The new coil I put in is the Pertronix 3.0ohm. The ballast is still hooked up although it's my understanding I can bypass it with the new Pertronix coil? (I noticed there were 4 wires, two at each terminal, on the ballast. What are the other two going to?)

Hopeful I can get a little extra time later today to look at it some more.

 

[Pawned]

Sounds like an accelerator pump in the carb.

 

[SD_R/T]

Sounds like an accelerator pump in the carb.

One of the first things I looked at. Nice healthy stream (on both carbs) with a nice Pssshht!


Ok so I was out running around. Managed to pick up another timing light. Had a few minutes here to check the timing. Lo and behold, timing was at 8-degrees retarded. #-o Was running out of time again so quickly reset it to 5-degrees advanced and took it for a very quick swing around the block. Bog disappeared!

So while I'm happy to have that symptom addressed (for now), I'm still curious about a few things:

-What caused this? Timing chain? It was a gradual thing and not an overnight thing. Maybe tomorrow I can get out there and see if there's any delay on the rotor while turning it over.
-Why in the heck did some plugs get super fouled and others not?

 

[nm9stheham]

Glad you posted a pix of the plugs....those don't look to be the right ones at all. The core is waaay too long for standard 225 plugs and will run veeery cold being so long; that would easily explain the misfirirng and the sooting. I just looked at the NGK site and here is a pix of the ones they list for a 1968 225 /6, which is PN GR4 and look pretty much like the Autolite 66's I have in mine, also shown below. I am pretty sure the Pertronix 3 ohm coil should be run with no ballast; check the installation instructions their site for your model.

 

[SD_R/T]

Glad you posted a pix of the plugs....those don't look to be the right ones at all. The core is waaay too long for standard 225 plugs and will run veeery cold being so long; that would easily explain the misfirirng and the sooting. I just looked at the NGK site and here is a pix of the ones they list for a 1968 225 /6, which is PN GR4 and look pretty much like the Autolite 66's I have in mine, also shown below. I am pretty sure the Pertronix 3 ohm coil should be run with no ballast; check the installation instructions their site for your model.

The plugs were the NGK ZFR5N with washers removed. I understand these to be a preferred plug for the slant. Based on my recent findings with the timing I'm thinking they were fouled due to the timing being off. Running retarded would also foul them, correct?

Yes, I believe the Pertronix can be run without the ballast. But what's with the four wires at the BR?

 

[nm9stheham]

Unless there is someting structurally very different inside these cores, they are going to run cold; that is a pretty long core. I assume the extended core is to get the spark initation point at a better point in the chamber. Just retarded typically does not cause this type of sooting.

Edit to add: And I must be pretty tired..these long cores will tend to run HOT, not cold.... never mind! I guess I was enamored with the idea of the sooting being due to cold plugs.....

Where is the plug recommendation coming from? Here is a related thread: http://www.slantsix.org/forum/viewtopic.php?t=47779&sid=ff2994087b205be175b47bade3d7bf6b
Look at the last post.

You have a '68? That uses a 2 lead BR. Sound like you have a later 4 lead BR for the 5 wire mopar ECU ignition; it is 2 resistors in one package. You can use the right one half, but I would not do so with the 3 ohm Pertronix is they say don't. The normal BR+stock coil resistances total to 3 ohms or less, so if you are running a 3 ohm coil plus the BR, you are not getting the energy into the coil that you should.

 

[SD_R/T]

Unless there is someting structurally very different inside these cores, they are going to run cold; that is a pretty long core. I assume the extended core is to get the spark initation point at a better point in the chamber. Just retarded typically does not cause this type of sooting.

Edit to add: And I must be pretty tired..these long cores will tend to run HOT, not cold.... never mind! I guess I was enamored with the idea of the sooting being due to cold plugs.....

Where is the plug recommendation coming from? Here is a related thread: http://www.slantsix.org/forum/viewtopic.php?t=47779&sid=ff2994087b205be175b47bade3d7bf6b
Look at the last post.

You have a '68? That uses a 2 lead BR. Sound like you have a later 4 lead BR for the 5 wire mopar ECU ignition; it is 2 resistors in one package. You can use the right one half, but I would not do so with the 3 ohm Pertronix is they say don't. The normal BR+stock coil resistances total to 3 ohms or less, so if you are running a 3 ohm coil plus the BR, you are not getting the energy into the coil that you should.

Yeah, I'm pretty sure there were a few different places I found recommendations for those particular plugs. Both here and .org I'd have to start digging around to find them.


As for the BR, it's got two terminals. A blue & blue/white in a male plug on one terminal and dk blue & brown in a male plug on the other terminal. I think you're right about the coil being better off without the BR, but haven't had a chance to trace all those wires (although I do see the dk blue at the coil).

 

[nm9stheham]

Look on mymopar.com and you can find your car's schematic. The dark blue/brown wire end of the BR is the one going to the coil +; the dark blue goes to the coil + and the brown comes for the IGN 2 position of the ignition switch to supply +12v direct to the coil when starting. You should just jumper the 2 BR leads together and you are done. Some guys have written on here that they put a wire in place of the resistance winding on the back side of the BR to keep a stock look but still short the 2 together; pretty clever.