dead spots and backfiring

[chitwnvw]

Took the bus for a 15 mile spin last night. Went pretty well. I had a few times where it seemed flat, followed by a few backfires when I tried to accelerate. But over it felt pretty good.

Took it to work this morning and it was real flat until it warmed well, more backfiring out the carbs. (Intake valve still open, is it firing too soon?) Once it was warm, it ran a lot better.

Then coming home it ran like crap. Flat, a lot more backfiring, idle wouldn't come up to a decent speed, and the oil pressure light would come on unless I gave it a little bit of gas.

I was thinking timing, but why would it get so much worse, so quickly?

I'm going to give it the once over in the morning, any ideas on what to look for?

[vdubyah73]

Chilly and humid ? Carb icing. Not chilly and humid ? Kinda normal for a couple of minutes after a cold start up, should improve rather quickly as engine warms up. Idle jets plugged? Happens alot with new carbs especially if you didn't pop the tops and clean the bowls. Plugged idle jets are pretty easy. just unscrew 'em, don't lose the o-rings,I use hardware store o-rings now . Carb cleaner,use the red straw. Make sure the jet itself is clear and then put the straw right in the the idle jet hole and give it a blast. put the jet back in and off you go. To find out what jet it is just pull plug wires one at a time till one doesn't make as much difference thats the jet to clean. Don't forget you have no choke so warm up driving has pops sputters and complete refusal to respond to a heavy foot when cold.
Let us know how it goes.
Bill

[chitwnvw]

Checked torque on intake bolts, they were a little loose. Checked dwell 49-49. Checked timing, was 6 degrees btdc, set it to 7.5, about 28-30 at 3000 w/o vacuum. Seemed a bit better, I'll take it on more than a once around the block later.

The idle had gone way slow, so I had to give each of the idle speed screw a 3/4 turn in, to get it over 800. I suspect my throttle butterflies are now open a bit, and I'm cheating. Does this reinforce the idle jet theory? Got the carbs from aircooled.net, and supposedly John sets the float level, I assume he'd hose them off with the carb cleaner while he's there.

I may have to remove the carbs anyway. I noticed a little gas moistness on the carb where I had to turn the gas inlet around, seems to be the bolt on the bottom, which I didn't mess with, oh well, we'll see.

[vdubyah73]

Keep fiddling you'll get there. Carbs may have been handled by baboons during shipping, check floats when you take 'em out. Check and see if the problem happens at the same throttle position or not. If it's at the same position most of the time it may be that your linkage isn't synchronized right. Linkage synchronizing isn't the same as carb synchronizing.

Bill

[chitwnvw]

I fixed my big problem. The duct tape that I put over the brake booster nipple on the manifold would come off, when I pushed on the gas, enough to cause a vacuum leak when I pushed on the gas. Whacky. I got off my behind and rigged up a tube to bridge between the vacuum booster hose and the nipple on the manifold. And WOW you don't have to push very hard on your brakes when you've got the vacuum assist. I felt like I was driving a Cadillac.

Went for an extended drive, not bad, although, I think I had the tuning closer before I started fiddling with it this last time. Oh, well, I'll get it.

Then on the highway, it started to bucka bucka, with the power cutting out, and the alternator light coming on, then it would be fine, then it would come back. WTF. Got off the highway, into a well lit parking lot. Open the engine compartment. I check the vacuum hoses everything ok. I rev the engine and see some blue sparks... what the! Shit the coil has totally come out of it's bracket and is arcing against the fan. Well that might be a problem.

Gotta give everything the once over. I was under the bus today working on the heat and a bolt was just dangling ready to fall out and the pin that holds the no return flap on the fan had worked out. Yikes. All hell is breaking loose.

[chitwnvw]

Fixed my occasional backfire problem, a nut on one of the studs between the muffler and heat exchange boxes loosened up, and it was leaking. On a close inspection it appears that the flange on the heater box is a bit bent back, making this a hard to seal. Had to apply more than the usual amount of torque. Is it ok to double stack gasket here? Or is that some sort of sealant that will hold up their? To give me a better seal.

I keep tweaking at the tuning of the carb and linkage. There's still some spots where it feel like I have suddenly been hit by a strong head wind. If I push down on the pedal it responds, it doesn't bog. I guess I am expecting a smooth arc and power over the rpm band. What I am getting is good off of idle, then about 2/3 to 3/4 of the way before I shift, one of these strange spots. It's the worst in 2nd and then so so in 3rd. Then when I get past it, again fine. Also it doesn't happen 100% of the time, more like 50-60 %.

[Amskeptic]

Is it ok to double stack gasket here?

about 2/3 to 3/4 of the way before I shift, one of these strange spots.

Best to file flanges flat. They bow because of over-tightening and old age.
Flat spot sounds like lean surging. Check main jet sizes/float level. Definitely map out your timing and make sure vacuum and centrifugal are working correctly. You will be a Master Mechanic by the time this car decides to give you a smooth run.
Colin

[Amskeptic]

Is it ok to double stack gasket here?

about 2/3 to 3/4 of the way before I shift, one of these strange spots.

Best to file flanges flat. They bow because of over-tightening and old age.
Flat spot sounds like lean surging. Check main jet sizes/float level. Definitely map out your timing and make sure vacuum and centrifugal are working correctly. You will be a Master Mechanic by the time this car decides to give you a smooth run.
Colin

[chitwnvw]

Best to file flanges flat.

Added it to the winter "to do" list.

Flat spot sounds like lean surging.

Was driving it last night and discovered that if it's driven more aggressively, I didn't go crazy, but if I revved it a bit more, that I didn't have these flat patches, or at least they weren't as noticeable. Is this consistent with lean surging? Is this because I am using the main jets and not the idle jets, that the idle jets get to the top of their RPM band and can't supply enough fuel?

Also, sometimes it seems to have an out of balance feeling. Its fine at idle, but then and I’m guessing a bit here, between 1000 and 2000 RPMs it feels like engine isn’t firing on all cylinders, then it balances out again. It’s not a huge unbalance, just a little rumbling-wobbling. This isn’t constant, it comes and goes…

[vdubyah73]

See if it's more related to gas pedal position than rpm. If it is then your linkage is a little askew. Meaning your linkage isn't opening both carbs evenly. I never get mine quite right either but I can choose where it happens. I let it happen just off idle, about 1100 rpm no load .

Bill

[chitwnvw]

I'm going to hook up a tach tomorrow.

So pedal position is linkage, rpms something else...

[vdubyah73]

Yeah, thats kinda how I figure it. I didn't have any one around here with dual two barrel experience when I started messing with them 6 years ago. Now I'm the guy on the Cape. I don't know how the "experts" figure out and measure the opening rates and the geometry of the down links with the hex bar linkage. Maybe thats why a lot of them like the bellcrank style.

Bill

[chitwnvw]

Ran a wire up front from the coil and hooked up my dwell/tach. Seems like the bad spot is in the 2500-3000 range and only when accelerating. If I go to 3000 and then back to 2800 and cruise, it's fine.

[Ryno]

This might be a dumb question but I was reading your other thread and I have to ask. Did you ever get a GOOD set of plug wires?

[chitwnvw]

I swapped the wires out. I'm getting a blue white spark from the wire.