Rebuilt 350 - Abysmal Fuel Econ

[MadOgre]

I don't know nothing about motorcycle carbs. I was referring strictly to automotive carbs ( primarily 4 barrels )

 

[rich weyand]

If you notice with the engine off the choke plate will be pretty much fully closed. Once the engine starts the choke plate will open 1/8-1/4 inch. This is where you get that airflow from. But before that happens there are small air bleeds on all models that allow the engine to have enough air to be able to start.

And yes I agree that 12 BTDC should be about the limit for his setup

OK, so we got to the same place on the carb thing. And the OP has his choke hooked up not. And Skweegle89 has some wires to find.

On the timing, yeah, I would rather see the OP with less vacuum advance and more base timing, but the setup he's got with the bigger vac advance and 12* BTDC base timing will protect him from any knocking problems if he gets a load of off-spec cheap gasoline.

Hey, Nalanthi, you listening? MadOgre and I agree! You should probably back off to 12* BTDC base timing with that big vac advance you've got in that distributor.

Or, if it's an adjustable vac canister, you can keep the base timing where it's at and dial the vac advance back so you are showing 25-27* BTDC at idle with the vac advance connected.

 

[Driver4r]

Unhooked? As in just flapping there? What's to keep it from getting sucked shut by engine vacuum? I'm trying to remember where the pivots are. Probably get sucked open.

I completely disagree on an engine "pulling it shut" The force of air coming in is gonna keep it open, its the linkage that's gonna keep it closed or close it.

 

[flyboy1100]

There has to be an air path into the carb's primaries for some air even when the choke plate is closed or the engine won't run. Full choke is "minimum air", not "no air". You can do it with bleed air passages, you can do it with holes or notches in the choke plate, you can do it with stops that do not allow the choke plate to close all the way. From my motorcycle days, some of the carbs had holes in the choke plate, and we could tune the choke air by enlarging the holes. Just google "choke plate" and hit Images. My Edelbrock 1406 electric choke doesn't close all the way, and I thought that was the choke-air metering, but if the manual choke closes all the way, there has to be another minimum choke-air path in the carb body.

Anyway, I think the choke being on all the time is the OPs mileage problem.

Now I am worried that he has a non-stock distributor, giving him much more than the stock vacuum advance. I'd like to see about 25* BTDC with the vacuum advance all in. That would come out to about 45* of advance at highway cruise with both the centrifugal and the vacuum advance all in. With 35-36* with the vacuum advance all in, he's looking at more like 55-56 degrees of advance at highway cruise, and that's 10 degrees more than what I aim at and right at the outer bounds for an SBC.

With a stock vacuum advance of 7.5*, base timing at 17-18*BTDC would get him to 25* BTDC with the vac advance all in, but some of these "performance" distributors have 20* vacuum advances, allowing lower base timings to keep the engine away from the detonation threshold.

That's why I want to know which distributor he is running. With 18* of vac advance, and I'm guessing 20* of centrifugal advance, I think he should set the base timing at 12* BTDC so that he doesn't exceed 50* of advance at highway cruise. I think he would be better off with a smaller vac advance and higher base timing, but if he doesn't have an adjustable vac advance on that distributor and doesn't want to screw with changing out the distributor or the vac advance can, then I think that's the better course. All-in timing over 50* BTDC is a little edgy for me.

I thought the stock can was good for 20* advance, could of sworn I read that somewhere

Sent from my SCH-I605 using Tapatalk

 

[rich weyand]

I thought the stock can was good for 20* advance, could of sworn I read that somewhere

No, the stock cans are much tamer than that. Most GM stock advance cans in that era were in the 7-10 degree range. Here is the info for the one on my 1978 K10 350; per the parts book it is a 1973482, with 7.5 @ 11-12.5:

Application
VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

Descripition
Brand：GM
Part Number(s)：1973482
ProductName：Vacuum Advance

Cross Interchange Parts
Factory Number
AIRTEX 4V1054
CARQUEST 57-7563
STANDARD MOTOR PRODUCTS VC-236
WELLS DV1853
GM 19138053
ECHLIN VC1853

 

[flyboy1100]

No, the stock cans are much tamer than that. Most GM stock advance cans in that era were in the 7-10 degree range. Here is the info for the one on my 1978 K10 350; per the parts book it is a 1973482, with 7.5 @ 11-12.5:

Application
VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

Descripition
Brand：GM
Part Number(s)：1973482
ProductName：Vacuum Advance

Cross Interchange Parts
Factory Number
AIRTEX 4V1054
CARQUEST 57-7563
STANDARD MOTOR PRODUCTS VC-236
WELLS DV1853
GM 19138053
ECHLIN VC1853

this article you linked too, i guess it isn't 20* but a lot of reference to 15* advance with vacuum. but that might not be stock vacuum either.

 

[Nalanthi]

Yeah, sorry - I'm listening.
I've just been tied up extremely hard and haven't had a chance to read/respond 'til now.

I'll back off to 12 initial and re-check.
My dizzy is a really clean stock Delco-Remy I scored from the JY.


...makes me wonder why I wasn't knocking at 18 degrees. I test-ran it up to 4500rpm in first going up an incline before shifting down.
I can't quite grasp how much advance I'm getting from a supposed stock can. Maybe I should re-check it.

Edit: But I do have a bad ear for a knocking engine.

 

[rich weyand]

Well, when you put your foot in it, you lose vacuum and the vacuum advance backs out, so the heavy advance on the can won't cause knock.

What kind of vehicle (model, engine) did you pull it out of? Some hi-po vehicles did have bigger advance curves. Or the advance can in the one you pulled could have been replaced somewhere along the line.

Probably should re-check it. I would expect more like 25-26 degrees than 35-36.

 

[MadOgre]

Yeah, sorry - I'm listening.
I've just been tied up extremely hard and haven't had a chance to read/respond 'til now.

I'll back off to 12 initial and re-check.
My dizzy is a really clean stock Delco-Remy I scored from the JY.


...makes me wonder why I wasn't knocking at 18 degrees. I test-ran it up to 4500rpm in first going up an incline before shifting down.
I can't quite grasp how much advance I'm getting from a supposed stock can. Maybe I should re-check it.

Edit: But I do have a bad ear for a knocking engine.

Pre detonation can occur with out you being able to audibly hear it.

 

[Nalanthi]

So I warmed her up this afternoon when I got home from work and proceeded to back it down to 12.
I also noticed that my timing light isn't the best in the world, and advancing the dial to 12 degrees is really closer to 10-11 degrees.

My guess is that I overcompensated my reading on what the can was putting out when checking yesterday, so here are today's figures:

The light at zero should be spot-on, so... I advanced it to 12 on the physical timing mark. When the can is connected it shows 34, but when compensating for the potential error in the dial's advance, my best estimate is that it's running more around 26-27 total.


I'm sure that this is a bit more spot-on.
I re-tweaked the idle to run about 700-ish RPMs.

 

[Nalanthi]

Well... After having Re-tuned, nothing seems to have changed. I couldn't wait for Monday to roll around, so I found an excuse to take a trip.
The results? 5.2 mpg.

I was hoping for the best but honestly am not surprised in the least...

Edit: i think i may have a lead on this - and it's sorely obvious. The original inline 6 had no fuel return line hence i blocked the return line on the 350's fuel pump return. The filter is filled to the brim with gas. I figure that i may be pushing too much fuel into the carb - enough to leak by, but not enough to cause a flood condition.

Does this sound like a plausible explanation as to why my econ sucks? I'll plumb a return line tomorrow and check it...

 

[Stewzer55]

It sounds like a good explanation, and probably a fuel leak since that filter (the charcoal canister?) is filled with fuel.

 

[MadOgre]

Im not sure. There are two styles of mechanical pumps one has a return line the other doesn't. If your needle and seat are working properly it shouldn't be an issue. I would think if it was an issue you would have some noticeable performance issues besides just fuel consumption.

Im really wondering what kinda shape your carbs are in? Perhaps they are your problem?

You should get the correct pump with the return as that eliminates that possibility.

How are your spark plugs? If your not igniting the fuel that would be a good way to waste fuel.

You should take a couple pics from under your hood and post them up

 

[Nalanthi]

Thanks, Ogre.
I'll snap a few pics when I get a chance.

The carb is tip top - it's a 1406 from the wife's K5 and was purchased brand new last fall (I scored an ever newer 1406 and that went on her ride - one of those 'can''t pass up deals').
Since she went with an electric fuel pump, I stole her old mechanical fuel pump when I was completing my build. Like the carb, there wasn't a single problem when running it in the K5. I tuned it up back then and it ran like a boss with the same comps that went on mine.
That fuel pump was hooked up to a return line on her ride.

My plugs are new and gapped at .060. I've heard conflicting stories on how they should be .045, and may just regap them to .045.


Stewzer: We have no emissions here, so I'm not running any emissions comps at all. No charcoal can - just a line from the tank to the pump to the carb - and it was installed new last weekend.

 

[MadOgre]

Well the eddy carbs might be finicky with fuel pressure. So you might have an issue there. I think 5.5-6 psi is about the limit for them.

And ya .045 is the ticket.