Loss of power

[triracer67]

I've been having a weird loss of power/miss on my 1969 351W(4barrel intake and carb, headers, pertronix, all else stock) for the last few weeks, which almost always occurs on the same section of road(it's not the road). While driving, just as the thermostat is opening up, I get a miss, which doesn't seem like a cylinder, but all. The tach will drop about 200 rpms, and then continues running. It only seems to happen when I'm cruising, not on, or off the gas. I'm usually at 3,000 rpms at this point, but, it's only when I'm steady on rpms. After a minute or two, it goes away. I've adjusted accelerator pump, and air/fuel mixture (I've adjusted with vacuum gauge). I really think it's running lean, but, not sure. I don't think it's Pertronix, or plugs/wires, since it's dropping power all together. The carb is Edelbrock 1405. The motor runs great, and starts right up, all the time. It's definitely not backfiring, just seems like power cuts of instantaneously, and comes right back.

 

[SteveL]

Is there an elevation change where it happens? Back when the 66 had an fab carb it would act up at the same spot going up the 15 thru devote. I got used to pulling off at that exit and adjusting the mixture. Doesn't do it now with a q-jet

 

[triracer67]

Sorry, no elevation change. It does happen in the morning, and normally 30 degrees cooler.

 

[blubuckaroo]

Does the engine have aftermarket heads?
Most aftermarket heads haven't had the exhaust heat riser port drilled out when you get them. In cold temperatures it can act like it's lean because the fuel isn't atomizing properly until the engine gets warmed up good. The electric choke will mask it, but the choke usually opens before the engine is completely warmed up, and you'll get a bog.

 

[triracer67]

Stock heads, no choke on carb. I did change rod/spring this summer to stop a little acceleration lag. It seems to be temperature sensitive, which leads me to believe I'm running too lean.

 

[blubuckaroo]

Maybe you could try the next stiffer metering rod step-up springs?

 

[DirtDonk]

I would not expect it to be accelerator pump related, since you say it happens while cruising and not when you're trying to accelerate. And the pump is not doing anything unless the throttle lever is moving, so it has no bearing on any lean/rich mixture issue overall. Just when pushing the pedal.

Is the Pertronix in a stock distributor body? Have you had the whole distributor out lately to make sure that the body is making a good connection with the inside of the engine block? That's where the ground for the trigger comes from, and a dirty, oily or loose fitting distributor might have an issue now and then.

Next with the ignition, did you verify the gap between the reluctor and stator (Spinny bumpy thingy and stationary pickup thingy)?
On the original Ignitor I I believe it's .030" or about the thickness of a matchbook cover.

And what did you set your plug gaps at? Can run more than a stock '69 had originally, but should not open it up as far as modern engines can be set. I like the .039 to .042" range, but not higher.

If it's a lean thing, do what buckaroo suggested and start messing about with metering rods, springs and even main jets if it comes to that.

Paul

 

[triracer67]

Pertonix gap looks good, spark plug gaps at .40. Plugs are white, lean condition? Springs are pink, 7-in hg, and rod is 7052. Chart says silver? Which is it?

 

[jckkys]

I would suggest you look at the step up springs too. My AFB had a similar problem but in hot weather. It was as if the pistons that pull the rods down get stuck in their bores holding the rods in the lean position. Stiffer springs fixed it. The other things mentioned like the intake or headers couldn't cause this. It has to be fuel (carburetor) or spark related.

 

[triracer67]

Stiffer spring seems to help, but, won’t know until next cold drive. I also put a quarter more air/fuel rich. Funny story, don’t try and test spring pressure with fingers, they may go flying!

 

[DirtDonk]

I also put a quarter more air/fuel rich.

In what regard? Changed some physical aspect to the rods and jet? Or turned the idle-air screws?
If the screws, they really "shouldn't" have any effect on what you're experiencing. They pretty much go away when your throttle is at any position above idle.
Their effect is still there in the mix, but it's such a small percentage of the overall mixture that changing them does not alter higher end performance.

Paul

 

[blubuckaroo]

Pertonix gap looks good, spark plug gaps at .40. Plugs are white, lean condition? Springs are pink, 7-in hg, and rod is 7052. Chart says silver? Which is it?

Your 1405 came with orange springs. Your pink springs are the next stiffer. The silver (plain) one is your next and last stiffer spring.

 

[blubuckaroo]

Pertonix gap looks good, spark plug gaps at .40. Plugs are white, lean condition? Springs are pink, 7-in hg, and rod is 7052. Chart says silver? Which is it?

Did you set up the jets and metering rods, or did you get it this way?
According to the manual, you're set for 1 stage lean on the power mode and neutral on the cruise mode.
Maybe you should go back to the factory settings and start over.

 

[triracer67]

After reading the posts, I got up this morning and swapped back to original rod and spring. I realized I never changed a jet, and sent it too lean. Back to square one, another learning lesson.

 

[triracer67]

Thanks for all the help, Bronco is back to normal.

 

[DirtDonk]

Ok, great... But don't leave us hangin' in the wind here!!!!;D
What did you do to fix it? We gots to know...
Or is this just the first time you've had a chance to drive it since changing the rods?

Glad it's better though.

Paul

 

[triracer67]

update

Sorry about the lack of updates. Much like most Broncos, there is a fair amount of trial and error with changes. I spent a lot of time narrowing it to the step-up springs in my Edelbrock carb. I swapped to blue this weekend (3 hg), which should make it richer during load. Well, the ride this morning was great, and no issues. The ride home will be a little more testing with some hard accelerations. I'm sure sustianing 3,100 rpms down the highway doesn't help a lean condition.