My '73 440 in a street vehicle generally runs great and starts fine after sitting for 1 or 2 days, but if it sits for a week it is very hard to start, backfires and runs really bad for 5 or 10 minutes before "cleaning up". Once clean it runs great again. It has been doing this for close to a year, 5000 miles, but I put it down to poor tuning as I was doing other things.

Now the Thermoquad is great, after a lot of help from this site, and the problem still exists.

Some information, bottom end is old original (200,000 miles +)but still sound, heads were rebuilt with all new parts a year ago, moderately flowed and ports matched. Standard cam. Vacuum is fine with about 17 - 18 ins at 2500rpm cruise (65mph). The low garage temperature is about 50F and when good the car starts without needing choke.

I have one thought but would like to hear what people think could be the problem.

Thanks,

that is a strange one! Nothing is hitting me at the moment. is it backfiring out the ex or up thru the carb?

Sure the carb isn't leaking down and flooding the engine after a few days?
Is the carb empty when this happens?
Is it "making oil"?

Another idea that pops into my head is that will 200k on the short block, your rings/cylinder walls are likely a little loose. Do you maybe oil up the plugs causing the engine to run a little rough until the oil burns off? I have no clue why it wouldn't do that EVERY time though.

Super weak fuel pump? Maybe after it sits and the gas evaporates from the carb bowls it's taking a long time for the pump to full refill the bowls enough for it to run well? If that was the case I would expect it to run out of fuel at the top of first or second gear when you floor it from a stop.

Thermoquads are notorious for leaking fuel through cracks in the plastic body. Sounds like it's letting all the bowl fuel run into the engine. Try pulling the fuel line to the pump while idling, have a plug or pinch tool at the ready, and letting it totally run out of fuel. Hook it back up and walk away for a week. Start it up and see if it's better or different. It will take a little cranking to get the fuel back up to the carb. There are other problems besides cracks that can allow fuel to run out of the bowls while sitting. I just had a cracked body that was befuddling me. Carb worked great though.

Thanks for the replies, below for comments and more info;

- Backfiring thru the carb
- I haven't specifically checked if the carb is leaking down and flooding but it is NOT making oil, and accel pump works immediately after a week - I will check this one further. I had the same problem with the AFB before the TQ went in during the last year.
- Yes I'm sure the rings are a little "loose" it does use oil - more below, I also can't see how this would foul the plugs stationary.
- Interesting on the weak fuel pump, but no issues at WOT and the 5 to 10 mins can include a drive of a couple of miles before it clears so I believe unlikely.

Some other points;

- Engine does use a lot of oil (300 miles a quart) and got worse some time after the rebuilt heads
- All plugs look good after carb fix, although numbers 1 & 2 do look 'richer' but nowhere near fouling
- cylinders 1 to 6 have OK compression - can't get my press-in gauge into 7 & 8
- Bore Inspection gauge shows no dramatic issues like bore gouges from broken rings, but all obviously well used.
- Twin exhausts, both banks put dirty (oil?) smoke on the car after a run but don't smoke visibly once warm.

My thought was that the valve seals could be leaking and fouling the plugs and ports but I'm not sure how this could continue while sitting.

Comments?

Thanks Dave, I got this after my last post. Does seem possible, so I will give this a try, I could also pull the carb top after a week and see if the fuel level is still good? (I need to do this anyway). Doesn't explain why I had the issue with the AFB but my tune at that time was poor so maybe confusing the issue.

Ray

Am I reading this right, that the eng acted the same with the AFB?

Correct Robert, same problem with the AFB after the head rebuild. Before the head rebuild it was generally running 'poor' and not a good starter.

Ray

before the head work it ran poor & was hard to start & after the head work now it is really hard to start (plus the posted symptoms in the first post?) then a carb swap (to the AFB) & no real change? I wonder if there is a vac leak from the metal valley pan & it runs poor cuz it is lean cuz of the rich(er) mixture required when cold makes it sneeze then when it warms up, a leaner mixture suffices & it is pretty much alot better (tho I wouldn't think a vac leak would be spot on). just thinking out loud. I ain't thinking ign cuz they (when bad) can usually run OK when cold then act up when hot. Keep after it & keep us posted. I love the toughies.

Is it clacking and making valve train noises when it runs like this? I had some heads done on a 440 years ago and the head guy put some crappy guides in it that would seize up when cold and hold the damn valves from moving!!! Clack on for about 30-45 seconds and they'd start to let go!!! I wasn't happy to say the least but he did fix it for nothing. Just asking from experience and your "head work" comment.

Thanks Dave/Robert,
Dave no clacking noises.
Robert, if there was a vacuum leak i don't see why it would be worse after a week than 1 day.The climate here is very mild right now, 50 min and 70 max so min temp is always reached by the next morning.
Also you have the carbs wrong way around - I ran the AFB for a long while before and after head work then changed to the TQ 5 months ago. My perception is that the problem has existed all the while after the head work/rebuild, I also think it was there to a lesser degree before the head work but not to the same level.

To put it in perspective, today I drove the car after driving yesterday and it started first push, idles well immediately and can drive straight out the garage and up our drive with no issues.

Yesterday I started it after about a week of no driving and it was very difficult to start, backfiring through the carb. I made it idle for about 5 minutes before I could back out the garage and was still backfiring as i drove up my drive and for a couple of miles. I have noticed that it misses the most between about 1200rpm and 2200rpm and seems to be OK if I can get through that until it fully clears.

My son has mentioned water in the gas? but I have a hard time thinking it could be that as the problem is so consistent. I have also wondered what would happen if one of the valve area drain passages was blocked during the rebuild - could oil puddle around some valve stems and weep down into the cylinder and foul plugs?

If you have any other ideas I'd be glad to hear them and I need to make a list of things to check before and after a week break to try and find a cause.

Ray

the thing where the symptoms are more extreme after a longer period of sitting is weird. BUT, to me what you are describing when the motor is cold sounds like the carb is tuned pretty lean and while it's cold it is really lean. that's why it's backfiring through the carb until it warms up.
idle mix and transition circuits are probably leaner than the mains and thats why it clears up after 2000 rpm while cold.

is the choke on the carb operating?

fyi my carbs are tuned lean enough that the motor acts grumpy like this when it's cold. since it's starting to get colder here i'm probably going to richen up the idle/transition circuit a step so that it's a little more driveable in the colder weather...

Ray what about blocking the metering rod tree up so it is on the power circuit then starting it & see how it acts & see if that enrichment helps things (not permanent of course but to see if it is way lean).

A VERY long time ago, I heard of a guy with a similar problem. Of course, I may be mistaken...because this happened 25+ years ago!

If I can recall correctly, I believe he discovered he had a bad distributor cap. It was cracked and would allow moisture to slowly build up inside the cap. Moisture would cause cross firing, etc. When the engine was warmed up, the heat would dry the cap and the engine would run fine.

I would examine the distributor and distributor cap very closely.

Thanks guys, some interesting thoughts.

- on the fuel side, intuitively it does seem like a fuel issue but some counter points.
-- I'll emphasize again that I don't see how leaving a week affects normal cold start - I checked this morning after sitting for 1 day and every part of the engine was at room temperature - it cannot get any colder sitting for a week.
- On the mixture I have an Innovate AFR gauge and idle I aim for 12.7 which seems to be the sweet spot.
- Choke is basically manual only and if I leave it on for more than 15/20 seconds after starting the engine rich stalls. On a normal good day it is fully off after starting and then good to go.
- The idea of richening up the transition on the rod tree sounds good and easy to do, if only to try and isolate the issue then find a permanent fix.
- Quick Dodge - sounds like an awesome idea!! cross-firing might describe what I am getting as sometimes it feels like a kickback on the starter. The humidity out here is always above 80% currently so can see how this could happen. Easy to try - take the cap off after a run and leave the car for a week, clean and reinstall and see if the problem is gone.

I now have a whole lot of ideas to work through which will take some time. I'll re-post when/if I get it fixed, glad also to hear any other ideas.

Ray

this is gonna be an interesting discovery, tho I'm sure it will be simple once uncovered. (We'll be waiting). Hurry!

i had a 390 ford wagon decades ago that acted that way,turned out it had a cracked dist cap!