UPDATE - Stumble/Miss at Low Cruise RPM

[stu in wichita]

I recently finished my ’61 Ranchero and have about 1500 miles on it. The little 170 runs great except for one problem.

When cruising in 4th or 5th at VERY LIGHT throttle, the engine will stumble or miss momentarily. It doesn’t happen off the line or when accelerating, and isn’t noticeable in any other gear. But in the higher gears, when the throttle is just barely off the idle position, it jerks pretty hard. Shifting up or down seems to take care of it, or I can give it more or less throttle to make it go away. The engine isn’t being lugged, but it definitely happens at low load, low rpm, about 35-40 mph. It's surprising what a light throttle this small engine needs to scoot down the road.

The carb is a Holly 1908 (I’m pretty sure), which I rebuilt to original spec. The ignition is the original Load-O-Matic, although I don't think it's an ignition problem.

This has probably been going on from the start, but now that I’m driving it regularly, it’s becoming an issue I need to fix. Since it seems throttle-related, I wonder if adjusting the float might help, but I have no idea whether to adjust it higher or lower, or what else I might try.

Any help will be greatly appreciated.
-Stu

 

[thesameguy]

I don't know much about carbs, but my understanding is that a stumble off idle, given everything is in proper order, is generally caused by insufficient ignition timing or a jet that's too small. If the problem was at tip-in rather than at cruise you could also look at the idle mixture or accelerator pump, but since we're talking about light throttle cruise it'd probably be one of those first two items. I think. If the 1908 is built to spec, I'd probably look at ignition timing - what distributor are you running, and what's your base timing set to?

 

[stu in wichita]

The ignition is stock Load-O-Matic, set at about 12 degrees initial. Not sure what total advance is, since LOM is a little hard to know when that happens.

It seems like a fuel issue, since it only stumbles once. If it was ignition, wouldn't it continue to miss and stumble? Maybe not.

Anyway, thanks for the ideas.

-Stu

 

[FalconSedanDelivery]

disconnect the loadamatic line set the initial to 25 BTDC ( you will need a dial back light ) , and post how it runs then

 

[thesameguy]

I was gonna suggest a slightly more conservative number (like 16 or 18), but I'd definitely start with some more advance and see if that cures the issue.

You might also check the vacuum diaphragm on the LoM - I had a damaged one on my car when I bought it and sometimes it would misbehave. I think the rubber bits had gotten old and hard, and as they shifted around in the inside of the can I'd get different symptoms - not holding vacuum, or getting stuck at varying degrees of advance. As engine vacuum would change it would adjust and sometimes right itself or sometimes get worse. After I replaced it, idle & low engine speed performance changed for the better.

Actually, I'm also reminded that my original line from dizzy to diaphragm was leaking at the distributor end causing an intermittent vacuum leak. After fixing that, I think the increased vacuum signal killed the diaphragm. But, a subtle vacuum leak like that could cause fuel issues at those sensitive times when there is little air flow through the carb.

 

[stu in wichita]

Okay, it sounds like the advance may be the culprit, and I was so sure it was a lean condition as the carb left the idle circuit.

I wondered if FSD was serious about 25 initial, but he knows his stuff so I can work up to that. I guess it'll be fine until I get some pinging. I don't have a dial back timing light, but I'll just measure the existing marks and extend that scale a little further around the dampener. I assume I should re-connect the LOM line once the initial is set.

The diaphragm is new and holds vacuum well. The metal lines are also leak free.

I'll play with it this weekend. Thanks for the suggestions.

-Stu

 

[FalconSedanDelivery]

Dead serious , you wont have any problems if you disconnect and plug the line

 

[stu in wichita]

I'll try it. Thanks.

I have a Duraspark which I'll eventually use, but right now it's important to me to log some miles on the engine before I change much. After I read some info on the LOM, I was kind of fascinated with the concept. I just wanted to experience it.

I think I'm experiencing it.

-Stu

 

[rbohm]

8) since you dont have a dial back timing light, you can advance the timing a little at a time until you get detonation, then back off a bit until it stops. lock the dist down at that point, and like FSD said, plug the vacuum line off. if you decide to swap to the DSll later on, you can use the same timing technique, but i wouldnt do 25 degree, but more like 10-14 unless you also lock out the mechanical advance, or at least limit it severely to no more than about 8-10 degree in the distributor.

 

[FalconSedanDelivery]

The 25 degree suggestion is just a test to see how bad the loadamatic is , although even with a modern dist I still recommend Initial in the 16-20 Btdc Range ( with the total limited ) / Recurved

 

[stu in wichita]

Well, I advanced it to about 23+. I don't have a dial back timing light so I made an extension of the timing scale. The cast scale goes up to 14 degrees, but by the time I got to about 22, I could no longer see the extended scale due to the water pump pulley. Anyway, I'm close and the engine is noticeably more powerful. From a standing start, It's just smoother and more powerful at even low rpms - from a stop, I don't have to give it much gas and barely slip the clutch. It feels better in every gear, and no hint of pinging. I'm not sure how the mileage will be affected, but unless it drops bad, I'm leaving it this way.

On the downside, the stumble is still there. It's not something I can't live with - at about 35-40 mph, the throttle is BARELY CRACKED to maintain that speed in either 4th or 5th, just every so often, I feel it jerk, either right before or after I upshift or downshift. I'm going to play around with the choke while I'm driving it to see if a richer mixture has any effect. Not very scientific, but it might be a clue.

Another downer this weekend - I tried to upgrade to a Duraspark distributor. No go. I have the small 1/4" oil pump drive shaft. Guess I'll live with the LOM for now.

Thanks for all the help.

-Stu

 

[thesameguy]

Check the idle mixture. At slight throttle you may not have enough airflow to get out of the idle circuit. Maybe try 1/4 or 1/2 turn out & see if that changes things at all.

 

[stu in wichita]

I'll try that and report back tomorrow after I drive it to work. Thanks for the suggestion.

-Stu

 

[stu in wichita]

Check the idle mixture. At slight throttle you may not have enough airflow to get out of the idle circuit. Maybe try 1/4 or 1/2 turn out & see if that changes things at all.

Okay, we're onto something here. I drove 15 miles this morning and it's waaaaaaaaaaaaay better! There was just a hint of a stumble a couple of times. Before, I could make it happen at will, just be feathering the gas off idle in a higher gear, and it would jerk noticeably.

My initial tuning of the carb wasn't tuned as great as I thought, or maybe it has changed. When I backed out the mixture screw last night, it picked up almost 100 rpm, which to me indicates it wasn't right in the first place. I normally adjust the idle screw to get the highest rpm, which is where it is now. I may back it out some more to see if I can totally clear it up.

I'll stick with the higher initial advance of about 23 degrees, or even more - so far, no pinging, and it seems to run stronger.

Thanks for the ideas.
-Stu

 

[ludwig]

You might consider tuning the idle with a vacuum gauge. Hook it on to the manifold vacuum source.

If everything is good, you will pull 18" of vac at +/- 750 RPM. Screw the idle mix in until the engine starts to stumble. Then back the screw out until it hits the highest vacuum. Then add a 1/4 turn. If your idle speed rises, then set it down a bit.

It is easy to miss the sweet spot but the vacuum gauge helps you find it reliably. Vacuum gauge is cheap, BTW. Maybe $20 or so.

 

[stu in wichita]

Thanks for the reminder. Ludwig. I have a vacuum gauge, but I don't use it as much as I should. It's a real versatile tool so I'll try it for this.

I still can't get over how little throttle this engine requires for normal driving. The 170 isn't a powerhouse, and I figured I'd mostly use 4th gear around town. But even in 5th at 40mph, it doesn't lug and can easily accelerate or go up mild inclines. On the highway, I figured I'd need to downshift to pass or go up a hill, but it runs like a much bigger engine. It doesn't accelerate like one, but it sure cruises like a one.

I couldn't be happier with the set-up - definitely worth the time it took to build. Thanks again for the help. This is a great site.

-Stu

 

[thesameguy]

Thanks for the reminder. Ludwig. I have a vacuum gauge, but I don't use it as much as I should. It's a real versatile tool so I'll try it for this.

I still can't get over how little throttle this engine requires for normal driving. The 170 isn't a powerhouse, and I figured I'd mostly use 4th gear around town. But even in 5th at 40mph, it doesn't lug and can easily accelerate or go up mild inclines. On the highway, I figured I'd need to downshift to pass or go up a hill, but it runs like a much bigger engine. It doesn't accelerate like one, but it sure cruises like a one.

I couldn't be happier with the set-up - definitely worth the time it took to build. Thanks again for the help. This is a great site.

-Stu

I am perpetually blown away by how completely drivable the small six is in modern traffic. It took me quite some time to get my car running right (came to me with a bad carb and a 0psi #6 cylinder) and throughout the entire process I was convinced I was going to have had dumped mountains of time and a fair amount of money into it, only to be totally dissatisfied. On the contrary, the car is a *pleasure* to drive at anything up to freeway speeds. It's really only higher RPM passing power that sucks, and I'm pretty sure that's more a function of 2-speed automatic than anything else. The slightest throttle rockets away from stops and lower engine speeds are incredibly strong. I very much see the attraction to these motors where I was previously sure I'd see none.

I still suck at carb tuning and I'm positive I'm giving up something somewhere, but it's pretty darned good right now and I'm afraid to mess with it further. I will say that having a narrow band A:F gauge and a vacuum gauge in the car was critical for me. I simply lack the experience to know what sounds or looks right or wrong, but I can certainly tell the difference between not enough vacuum or too many lit "rich" LEDs.

 

[stu in wichita]

That's a cool set-up. I agree the gauges eliminate guesswork, no matter how experienced a guy is.

Older cars respond to meticulous tuning efforts, and little issues can ruin the experience for some people. Fortunately, I have a fascination with these old cars and enjoy maintenance, tuning, etc. (breakdowns and major repairs, not so much, LOL).

Throughout this episode, I've double-checked lots of details, and found that even in the couple thousand miles I've put on it, things have changed. Point dwell has increased slightly, and, as I mentioned, my idle mix had changed a little. Changing weather will require some additional tweaking. I've found a couple of loose fasteners and other minor issues that could become major issues. Since I'm driving my Ranchero a lot, I need to develop a routine for keeping things working right. I also have a '68 AMX, but it's only driven a couple thousand miles a year - the annual once-over seems to keep things under control. The Pertronix unit stays hidden under the cap for years at a time.

I'll be starting a new thread on another issue related to engine tuning, so I hope you'll share your knowledge and experience again.

-Stu

.

 

[thesameguy]

FWIW, switching at first to Pertronix and then to EDIS was probably the best thing I did for the car. Not having to deal with points that don't last was significant, and now having the timing change automatically based on engine temp, load, and engine speed mostly eliminated seasonal maintenance requirements. Isolating ignition performance from engine vacuum (or vice versa, maybe) also made tuning much easier, which was helpful for me. Since I finished that stuff last winter, I haven't popped the hood once save for an oil change in June. The Falcon has turned out to be one of the most reliable, predictable cars I own. I feel like as an old car I should be paying more attention to it, but it doesn't appear to need it.

 

[stu in wichita]

Update - I thought I had it fixed, but after driving it a couple of days, our weather changed back to 100 degree temps and the problem is still there. At least now I know it's likely heat-related, so there's another piece of the puzzle.

On the positive side, I took FSD's advice, advanced it further and added a fuel pressure regulator. It runs beautifully in the cool morning air. On the way home yesterday, it was 97 degrees, but the car seemed to run pretty good, with just a hint of a stumble. I'll be playing with it a bit more this weekend, but I can live with it until I get my new distributor. I have the small oil pump drive shaft, so the DUI is about my only real alternative to the LOM. Fortunately, another forum member has one for sale - I hope to pick it up next weekend when I'm at the Falcon regional meet in Kansas City.

Thanks for all the advice - I'm learning a lot and will post more results as they come up.
-Stu