Rebuilt 350 - Abysmal Fuel Econ

[MadOgre]

I just left it unplugged because that's how it was when I got it. I plugged it back in awhile back but nothing changed other than the choke light coming on. I got tired of looking at it so I just unplugged it again. I have done some research on some old posts on here and thought about changing out my oil pressure sensor to see if that fixes it. The fuse for it isn't blown.


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Im curious if that works. never heard that before but ive never had a choke light issue before either. Would be good to know if that fixes your light issue.

As I said earlier though you probably wont notice any difference on hot days. for the last week my manual divorcred choke hasn't kicked in at all either. Cause in reality you only need a choke to keep your engine running when its cold.

 

[farmerchris]

wonder if you should be worrying about mileage until your engine gets broke in.nice new engines are usually tight until everything wears in together.after breaking it in,mileage usually go up significantly.

 

[rich weyand]

Those of us who are answering questions on a given topic, and not asking them, like me and MadOgre, have already got our trucks sorted. We love to get another guy running. We may not agree on the answers we give, but, hey, you can't have everything!

And like you say, there are multiple approaches. I have some buddies I rely on who have done engine tuning or been GM auto mechanics or whatever, so I ask around, and I search the net, and I try things, and I see what works for me, and then I give the best advice I can. MadOgre probably does much the same. We just sometimes come to different conclusions.

Back to your truck. Higher vac is good. Being on the idle circuit is very good. The power gain is from running less rich, because extra gasoline beyond what you need takes up space that could be air -- it just passes through unburned -- so your working charge running rich is somewhat smaller. The big thing will be to see what mileage you get.

You don't mean 35-36 in Hg, though, I don't think. There's only 30 in Hg in one atmosphere.

You should now check to see if the choke plate is full open once the engine has run for five-ten minutes. You should also set the choke cap adjustment per the paragraph on the top of page 16 (as marked on the page; it is page 17/41 in the pdf) in this manual.
http://www.edelbrock.com/automotive/misc/tech-center/install/1000/1404_manual.pdf

As for gratitude, the best thanks is to help out others on the forum when they get caught by something that you know something about. It might be me!

To close the thread , let us know how your mileage works out for future reference by others searching the archive.

 

[rich weyand]

Ya im not sure what holes your talking about ?

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"SOME other carbs..." not "ALL other carbs..." Some use holes in the choke plate (primarily small carbs), some have a different air source. I don't remember offhand if the electric choke Edelbrock completely closes at full choke or not, but I don't think so. Of course, you can set up a manual choke to do whatever you want.

Here's the diagram for the Edelbrock 1406 choke plate setting:

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I think there may be a difference in the manual and electric choke versions on this. When I look up the manual choke versions, I see this:

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When I look up the electric choke versions, I see this:

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In any case, the difference between closed and open is pretty obvious. Just don't think there is something wrong if there is a small gap remaining and the electric choke plate isn't completely closed. That's all I meant.

Anyway, we got the OP squared away on his mileage problem, I think, and that's all that matters.

 

[rich weyand]

wonder if you should be worrying about mileage until your engine gets broke in.nice new engines are usually tight until everything wears in together.after breaking it in,mileage usually go up significantly.

Also, I think your vacuum will go up once the rings seat in well.

 

[Nalanthi]

lol Rich - you're right.
I typed "vacuum gauge" and was talking about advance, but the word vacuum was lingering in my head, so i accidentally typed Hg. What I meant is that the vac advance hooked up throttles the advance to 35-36 which is (correct me if I'm wrong) a good working vac advance.

I'm thinking/hoping that I just had a horrible tune on this truck and that's why my econ was so horrible and that *fingers crossed* this should put me in the ballpark. I'll keep this thread updated with results after I am able to run her next Monday.

 

[rich weyand]

It means the vac advance is working, but that's a lot of vac advance. Which distributor is on the truck? The stock distributor that came on the truck should have maybe 8 degrees of advance. You're looking at more like 18.

Did you check what the advance is if you have someone hold the throttle at 3000 rpm, to see 1) that the mechanical advance is working and 2) how much it is?

 

[MadOgre]

"SOME other carbs..." not "ALL other carbs..." Some use holes in the choke plate (primarily small carbs), some have a different air source. I don't remember offhand if the electric choke Edelbrock completely closes at full choke or not, but I don't think so. Of course, you can set up a manual choke to do whatever you want.

Here's the diagram for the Edelbrock 1406 choke plate setting:

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I think there may be a difference in the manual and electric choke versions on this. When I look up the manual choke versions, I see this:

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When I look up the electric choke versions, I see this:

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In any case, the difference between closed and open is pretty obvious. Just don't think there is something wrong if there is a small gap remaining and the electric choke plate isn't completely closed. That's all I meant.

Anyway, we got the OP squared away on his mileage problem, I think, and that's all that matters.

Yes Rich they are one and the same. The first picture shows the choke plate fully closed , where as the second shows it opened partially about a 1/4 inch and the angle the picture is taken at makes it so you cant really see that.

But there are no carburetors that have holes in there choke plate, that would totally defeat the purpose of the choke plate. The Carter style carbs have holes in their secondary's vacuum plate, Holly's don't have secondary vacuum plates and Qaudrajets have solid butterflies. These are not to be confused with choke plates.

The choke mechanism controls how much the choke plate opens on all carburetors.

And yes that is what I was getting at earlier that the difference between the choke plate being closed and open is pretty obvious, And the choke plate must be fully open in order to properly set the idle speed.

 

[Skweegle89]

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Anybody know what color the wires are that go to the oil pressure switch? Was about to replace mine and discovered that neither wire is to be found anywhere. I assume this is why my choke light stays on.


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[rich weyand]

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.

 

[MadOgre]

[]

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[MadOgre]

Dark blue and white it says

 

[rich weyand]

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Anybody know what color the wires are that go to the oil pressure switch? Was about to replace mine and discovered that neither wire is to be found anywhere. I assume this is why my choke light stays on.

Should be LT BLU (choke heater), DK BLU (to choke light), and PNK/WHT (to choke fuse).

 

[Skweegle89]

I guess I will go wire hunting after work today.


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[MadOgre]

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.

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