Higher RPM Stumble - help

u0061914 · May 6, 2011

Ok guys, I finally got my 67 on the road this week. I worked through all of my suspension issues and have a bit of drive time.
Now, the issue I need to work out is a miss stuble at higher RPMs. I don't have a tach on it yet, but I would say up over 3500 RPMS, there is a miss/ stumble. It is present when revved up in Park and also when under a load driving it. Starts excellent. Idles great. No issues at lower RPMs.

- What I have done to it so far:
1981 - 302. I don't know the complete history of the engine, but I bought it from a guy who had it in his 67 Fastback. It is not stock. I know it has a cam with a 351 firing order. No noises, great oil pressure. I drove his car with the engine and it ran excellent.

- Converted to duraspark
- New/rebuilt Distributor
- New Ford racing plug wires
- New Plugs
- Rebuilt Edelbrock 1406 carb ( from the Carb Shop in WBL)
- new battery
- new alternator
- new fuel pump
- New headers/exhaust

I have played around with the timing, with no change. I'm carb illiterate, so except to the two idle screws on the front, I have not touched it. Clear fuel filter in line, so no issues there.

Suggestions? How to I figure this out?

I'm too stuborn to drive it to s shop and have someone correct it.

Thanks,

19stang68 · May 6, 2011

Here are few thoughts off the top of my head:

1). Check to see that one or more plug wires aren't grounding themselves out.

2). Is the coil in good shape? Providing the correct voltage? Is it cracked or arcing at higher RPM (check in the dark to see it the coil is arcing)?

3). Is the choke fully opening?

4). Fuel pressure? High enough or is the fuel pump weak? Obstruction in lines like faulty fuel filter? Is there an inline filter in the carb (I'm not familiar with that carb).

5). Even though it is rebuilt, maybe the secondaries aren't coming in or aren't providing enough fuel. Maybe too much fuel? Does it smell like gas after reving it up?

Craig

u0061914 · May 6, 2011

19stang68 said:
Here are few thoughts off the top of my head:

1). Check to see that one or more plug wires aren't grounding themselves out.

2). Is the coil in good shape? Providing the correct voltage? Is it cracked or arcing at higher RPM (check in the dark to see it the coil is arcing)?

3). Is the choke fully opening?

4). Fuel pressure? High enough or is the fuel pump weak? Obstruction in lines like faulty fuel filter? Is there an inline filter in the carb (I'm not familiar with that carb).

5). Even though it is rebuilt, maybe the secondaries aren't coming in or aren't providing enough fuel. Maybe too much fuel? Does it smell like gas after reving it up?

Craig

Thanks

1. wires are not grounding out. Brand new
2. I tried swapping the coil with another one - no change
3. Yep, choke is opening
4. Tried a new fuel pump, no change. Has a clear inline filter that is clean. Still could be a fuel pressure issue though.
5. I pulled out the manual for the carb and did some reading . I'm wondering if I need to do some tuning. Metering rods and/or larger secondaries?

CarMichael Angelo · May 6, 2011

u0061914 said:
Thanks

1. wires are not grounding out. Brand new
2. I tried swapping the coil with another one - no change
3. Yep, choke is opening
4. Tried a new fuel pump, no change. Has a clear inline filter that is clean. Still could be a fuel pressure issue though.
5. I pulled out the manual for the carb and did some reading . I'm wondering if I need to do some tuning. Metering rods and/or larger secondaries?

It could definitely be the carb, just not a metering issue. Based on what you say that it does it revved up in park. The carb will never open it's secondary circuit regardless of how high you rev it until it sees a load. AND it never sees a "load" by revving it.

You need to rule out the two things that you are taking for granted that are good, based on their "newness"

#1 That Carb. The whole carb. Replace it w/ a friends if you have one.

#2 That distributor. Same thing.

This sounds like a fuel/spark issue to me. I would never take a rebuilt carb as a "ruled out" factor when diagnosing a problem like this.

Yeah, pulling a distributor, and getting a new one w/ all the clocking/timing issues involved is sometimes a P.I.T.A., but what you're dealing w/ is bigger than that.

u0061914 · May 6, 2011

Good info about the secondaries not being used when reving up when in park. Learned something new today!

I do have access to another carb. I have a Holley on my 75 bronco. I can swap them next week.

The good thing about this is that I do enjoy a challenge.

u0061914 · May 6, 2011

Don't remember the specs on the plugs and gap. I'm heading out of town this afternoon and will be back Sunday. I'll post the answers then.

Thanks guys!

68RCodeConv · May 6, 2011

Sounds like a lean condition in cruise mode. Change out the metering rods for ones with smaller diameters which will richen it up. Pull the rods (takes about 60 seconds, STUFF A RAG IN THE CARB BEFORE REMOVING THEM), get a magnifying glass and read the numbers on them. They will be something like 73/47.

Drop the big number about 2 - 3 points and see if it helps. Leave the small number alone for now. If

You can get metering rods in just about any comboe you want. Check with Summit.

u0061914 · May 9, 2011

OK. New issue.

Did not get a chance to pull the carb off my 75 bronco this weekend, to isolate the original problem, however it seems I have a new issue. Went for about a 3 mile drive. After the ride, I pulled into the driveway, and the car stalled. It flooded out. Pulled off the air cleaner and the primaries were full of fuel and fuel was pushing out the top of the carb at the accelerator pump. Is this a boiling fuel issue? I tired to route the fuel line away from the headers the best I could. This is a 67 6cyl car converted to a 302. The metal fuel line stops just past the firewall. I have a rubber line to the fuel pump. Then a metal line from the fuel pump to fuel filter, just about at the carb.

Or something like a float level issue.

Mike

Ricks 70 stang · May 9, 2011

If you have the duraspark with stock ignition box. Check the box. I had this problem and the back side of box was melted.
Rick

CarMichael Angelo · May 9, 2011

u0061914 said:
OK. New issue.

Did not get a chance to pull the carb off my 75 bronco this weekend, to isolate the original problem, however it seems I have a new issue. Went for about a 3 mile drive. After the ride, I pulled into the driveway, and the car stalled. It flooded out. Pulled off the air cleaner and the primaries were full of fuel and fuel was pushing out the top of the carb at the accelerator pump. Is this a boiling fuel issue? I tired to route the fuel line away from the headers the best I could. This is a 67 6cyl car converted to a 302. The metal fuel line stops just past the firewall. I have a rubber line to the fuel pump. Then a metal line from the fuel pump to fuel filter, just about at the carb.

Or something lie a float level issue.

Mike

More like stuck needle and seat. Actually not new issue, same issue but now more of it. Need to seriously consider swapping out this carb.

OOOOOhhhh you no use rebuilt carb, anymore crazy american.

Percolating fuel will look just like it sounds,...so hot it's boiling out of the vent or where ever. If you don't have a insulator gasket under the carb, there may be a heat related issue, but I don't think so.

Again, Not a metering rod issue, they don't come into play under static driveway tests that you said duplicated the problem.

Do yourself a favor, and get the Bronco carb and try it. If the problem persists, you've ruled out the carb at least.

u0061914 · May 9, 2011

Yes, I do have a spacer under the carb.
Just for fun I pulled out the metering rods and they are moving freely. I'm going to check the float level, etc. tomorrow. A buddy is is going to swing over with his carb so at least I can narrow it down to the carb or something else.

CarMichael Angelo · May 9, 2011

u0061914 said:
Yes, I do have a spacer under the carb.
Just for fun I pulled out the metering rods and they are moving freely. I'm going to check the float level, etc. tomorrow. A buddy is is going to swing over with his carb so at least I can narrow it down to the carb or something else.

Well be careful, A stuck float w/ gas bubbling out of the vent tube is the quickest way to burn that 67 to the ground.

horseballz · May 9, 2011

Sounds like a too much fuel pressure to me. Those Jimmy Carter/Edelbrock carbs have always been highly susceptible acting up with too much pressure. Is your fuel pump electric. Many folks run a fuel pressure regulator with those carbs.
My $.02,
Gene

u0061914 · May 11, 2011

OK. The new flooding issue is resolved. One of the floats was stuck, actually the needle was stuck open. I pulled the car apart, cleaned it thoroughly and set the float levels, now it idles excellent.
I'm going to grab a fuel pressure guage tonight and "T" it int the line, so I'll have a fuel pressure number tomorrow.

What is optimal fuel pressure for an Edelbrock 1406. Around 5ish I assume?

Also borrowed another carb from my buddy last night, so I can try that also.

horseballz · May 11, 2011

You Asked:
"What is optimal fuel pressure for an Edelbrock 1406. Around 5ish I assume?"

I'm not sure, as I've never owned one, but from what I've read/heard especially the older ones (just after Edelbrock took over Carter) and the actual Carters are happiest with 4psi or a little less. It would be nice if someone a bit more familiar with the older ones & Carters would chime in to confirm. Your problem may also be timing/vacuum advance related. Try removing the vacuum advance hose from the distributor and plugging the hose, then do a test drive. If problem goes away or changes, get back to us with the details.
HTH,
Gene

u0061914 · May 11, 2011

I did a little work on it tonight.

1st, I put in a fuel pressure guage. It sits about 5.8. Maybe a bit too high for that carb? If someone can confirm, that would be great. I bought a regulator, so I can throw that in line if need be.

Next, I swapped out the metering rods from 75 to 73 (I think the 2nd number was 47). I was only able to go for a very short ride, but I think this helped quite a bit.

I need to set my throttle pressure cable back up for my AOD before I drive it anymore, but it seems as if I am maybe on the right track here. Weather permitting I will mess with it some more tomorrow evening.

Very much appreciate the assistance here. Sure is easier with the ineternet than it was 20 years ago, when I built my 1st Bronco!

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