A few carbernator setting questions ;)

[8v-of-fury]

Hey there GENTS! I got a few q's to pick your brains with, dealing with my 1975 Thermoquad.

Here is the thing. First thing in the morning when I go out to start it up and drive to work, the ambient is about 55F in the mornings. I push the pedal down to set the clutch, half a pump will do for the pump-shot it seems. What is happening, is that it will fire-off right away and rev up to the high idle set by the choke, at that point it will chug way down to about 500rpm for about 2 seconds and then it will rise back up to the high idle.

What is happening? The choke pull-off seems to be set correctly, with the choke a little over a 1/4" or whatever open. I am unsure what the loss of RPM is about right after starting up cold.

SIDE NOTE: I moved the car out of the driveway this morning to allow my GF to leave, and moved it back in. It did not warm up in anyway, i shut it off. About 9 hours later this evening, ambient at 43F I went to go somewhere in the Chrysler. It has a remote start/lock installed, so i tried it for shits and giggles... what does it do but first bump of the starter make it fire right off and idle away (albeit low, but steadily) without choke or pumpshot !?

 

[rcmaniac791]

mine does that too sometimes. Granted, I still have the 2bbl, but my novice idea is that the accelerator pump dumps fuel right into the carb, then when it first lights off, it's firing off of the initial prime, and then it takes the jets a second to "catch up" and everything to start running right. Like I said, this could be completely wrong, but it's just my idea.

Do you have any drivability issues?

 

[8v-of-fury]

One day it will drive away fine after a few minute warm-up, other days it will stumble trying to accel of a stop light when still cold after the same warm-up period. Carb's are rather odd in their completely sporadic behaviour and I quite hate them for it... lololol

Yes, but not all carbs do that "catch up". Most just fire and run at their high idle (if they have it) without the weird lowering of the idle rpm.

Tomorrow morning maybe I will try like 3-4 pumps and note the difference if there is any.

 

[rcmaniac791]

I do no less than 4 pumps in the morning. Even then, it'll start and die and I have to pump it again. After that it's fine.

 

[8v-of-fury]

Oh wow. I have been starting each morning with a single half-pump!

 

[Aspen500]

Don't have any insight into the cause of the problems but something to keep in mind,,,,,,,when set up exactly to stock spec, they won't necessarily have good cold driveability. Cold engine stalling and/or stumble was considered "normal" in the early to late '70's. Restarting a car 4 or 5 times to get out of the driveway was considered routine procedure.

 

[8v-of-fury]

Restarting a car 4 or 5 times to get out of the driveway was considered routine procedure.

Oh how the times have changed haven't they? LOL! Entirely unacceptable today..

What are your thoughts on the engines ability to fire off first bump of the starter after sitting the whole day (and not being at op temp before) without a pump shot or the choke set?

 

[6thGenImperial]

No kidding. In my Imperial (EFI) manual it actually states brief (approx 1 second) spark knock on initial acceleration is considered normal. Now I dunno about multiple cold restarts being considered essentially normal, but we did refer to a couple of our 70's Dodges as cold-natured, as in a bit fussy in the morning. Subsequent starts of the day, on all but the coldest of days were uneventful. I had two different regimens for starting my 318 Charger (no choke in place). It fired right up with the single point ignition, but on cold days I'd have to plan on a couple minutes holding the throttle down to expedite a quick warmup. I had an old aluminum EdelbrockSP2-P intake that no-doubt helped matters. I have been on another forum for Dodge Trucks for a while now, and I get a kick out of having some kid who just inherited paw-paw's old truck. First question is usually, "What's a carburetor and where are the injectors?" Oh, and it does kinda sound like possibly there's a modest loss of prime to the carb and the stumble might be that moment just before it catches up, as has been mentioned.

 

[volareandgtcat]

Yup ... I've had no problems other than self induced(forgetting to hold the pedal down) spring summer and now fall .. but this winter like last, it'll be at least 2 starts just to get out of the driveway, golden the rest of the day.

 

[jasperjacko]

Oh how the times have changed haven't they? LOL! Entirely unacceptable today..

What are your thoughts on the engines ability to fire off first bump of the starter after sitting the whole day (and not being at op temp before) without a pump shot or the choke set?

you could actually be leaking fuel while just sitting there, hence why it doesn't need a "pump" to start when

 

[8v-of-fury]

Ambient was 45* this morning, basically winter for you American boys . I tried three pumps this morning to see what would happen and how it may react differently. It was a perfect cold start. It fired on the first compression stroke and was at the high idle immediately, where it was running absolutely perfectly. Perhaps the stumble I experienced before was too little fuel on the pump before it was able to draw a signal through the choke circuits.

Thanks all for the replies! This car a few years ago when it was a proper running 2bbl leanburn, would fire off amazingly even on the coldest of winter days (-30f).

Also, here is another line of questioning;

Mileage of these old dinosaurs.. ? The last fill of 40L netted me 300km on that tank, which is 13L/100KM or 18MPG. This tank was 90% highway at 75-80mph, and two days where I raced a friend up to 100mph.. We went by his speedo, as ours stop at what is it? 85... LMAO. It was also one day of purely in town running around. So an average of 18mpg, I feel is pretty spectacular for this old girl. I wonder though, what could make it better? obviously slower speeds on the highway, but what else could really get that number up and maybe over 20mpg?

ps. it has new balanced tires, and all wheels turned freely (no brake drag). It could probably use an alignment though..

 

[8v-of-fury]

you could actually be leaking fuel while just sitting there, hence why it doesn't need a "pump" to start when hot

Could it just be a well set carb? Drawing a good signal through the idle circuit upon drawing air in? I have heard this from a friend. Who knows.

 

[6thGenImperial]

My single point fired '69 Charger with 318 routinely turned in 19mpg on combined driving. It had a 727 T'flite (with B&M shift kit), 2.76 open 8.75 rear, running 15 wheels with 275/60's. The 318 had a CompCams 260 High Energy cam with matching springs with the SP2-P intake (it was free, and it worked!), Holley 600 (later changed to Edelbrock 750), cheapo headers and duals with muffler shop "turbo" mufflers. With all that's available now (this was late 80's), I'm sure it'd easily top 20mpg. On yours, even opening the exhaust from the stock manifolds back might put you over. I'd definitely look into transmission upgrades down the road. Higher quality clutch material puts more power to the wheels. My Charger got around a lotta cars it really shouldn't have, and the mileage was great. I credit that to having a better transmission. The others tended to sink money into the engine and did nothing to the transmission.

 

[8v-of-fury]

Yes sir! ultimately, the plan would be like.. a 5.9 magnum and a 5 speed behind it shoe-horned in there.. But I feel you on the 904 upgrades.

I think the exhaust is really hurting it too.. three cats and a muffler through like a 2" pipe. That is one hell of a restriction. A more obtainable dream is a good cam and some aluminum heads on this block. It is lower miles, and still has good compression. Bump the CR up to like 9.5 with the chambers, and a good sized cam to allow enough duration as well as headers and duals. boom, beauty power. Basically, 400hp with the right parts as I've read in many mags about SBM's.

Currently it is the stock 904, stock 7.25 with 2.20 gears and the stock engine with the TQ up top on a Eddie Performer. It does very well for what it is.. honestly. Like 4000lbs and Honda HP numbers.. she does alright. lol

 

[Cordoba1]

Here's what's happening: Your float valve is not closing 100%. When your car sits, some of the bowl flows back down the fuel line (gravity at work.) There's enough fuel in the bowl for the accelerator pump to prime the carb, but not enough in the bowl to keep the mixture at the correct ratio. It takes 5 or so seconds for the fuel pump to re-fill the bowl.

 

[8v-of-fury]

That is a very likely situation. Hmm. Perhaps I may have to install a check valve in the fuel line.

However, it did not do it this morning.

 

[AJ/FormS]

Just my 2cents;
I inherited an 84 Fifth Avenue from dear old dad when his due-date came up. That 318/904 tank always did 25mpgCan on the highway for him, and he was forever bragging on that. It was a tired engine in 2010 when I got it. He bought it very early in it's life, maybe late 80s. He sure loved that car.

As for your TQ, the vacuum break may be a little on the big side. But since you didn't measure it, it would be hard to say.

 

[rcmaniac791]

Like 4000lbs and Honda HP numbers.. she does alright. lol

It's because these 'teeners put out a pretty decent amount of low end torque, enough to twist up that Honda.

funny story actually, I was coming home from a show, and I was on the highway doing 70-75 mph. The area I live in is pretty mountainous, so I had to give about 1/2 throttle or so to maintain speed. My dad was following me in his Civic (buffer car before he gets his 2500 Cummins) and he said he had his foot to the floor trying to keep up, and I was still pulling away. hahaha jap-scrap

 

[jasperjacko]

Here's what's happening: Your float valve is not closing 100%. When your car sits, some of the bowl flows back down the fuel line (gravity at work.) There's enough fuel in the bowl for the accelerator pump to prime the carb, but not enough in the bowl to keep the mixture at the correct ratio. It takes 5 or so seconds for the fuel pump to re-fill the bowl.

It won't be the bowls draining back to the tank. The needle and seat are at the top of the bowl. Cannot physically do that once fuel is below the needle and seat. I have seen however, many small cracks on float bowls that would eventually empty the bowl. And if a carb could be"so well set-up" that it didn't need a choke at start-up, something is compromised. All internal cumbustion engines need fuel enrichment on cold startup, even e.f.i.

 

[8v-of-fury]

JAsper, very true.. I dunno about not needing ANY fuel enrichment. I just dunno lol. I'll try a cold start tomorrow morning with a video going to show you what happens. Who knows haha.

Good point on the needle and seat, i too forgot they were at the very top of the carb. The fuel is much lower than the needle anyway isn't it?