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Topic Review (Newest First)
02-24-2012 01:15 PM
chevydragster83
Quote:
Originally Posted by F-BIRD'88
Rich idle and just off idle:

is the PCV installed and functional?
If not , get one.
No PCV creates a rich idle.

What is the idle base timing and max mechanical advance?
May have to slow the motor down to read the base timing. (no vacuum advance)
The PCV is installed and functional.

I don't remember what the timing is set on, but the guy that installed the curve kit set the vacuum advance and the timing when he done the heads and cam swap. I haven't messed with it because I don't seem to be very good with the ignition system. As soon as I get a chance I'll check to see what it's set on.

What about the WOT, is it pretty much safe to say it needs leaned down, or should I double check the ignition first?
02-21-2012 10:52 PM
chevydragster83 I took it out today and drove about 12 miles at different speeds and the gauge showed to be in the 11s at idle and just off idle. It staid in the 12s until I got up to about 55 and it leaned down a little. It seemed to be staying right on 14 to low 15 between a quarter and half throttle, like pulling a small hill. At WOT it dropped into the high 10s for a second and then staid in the mid 11s. From what I could tell it was showing rich everywhere but at moderate throttle.
02-20-2012 07:17 PM
bentwings It takes a good half hour of driving to get the motor completely warmed up to operating temp. The AFR reading will be all over the place so do be too concerned.

Be sure to get the dist setup correctly first. By all means use a vac adv if you want good mpg.

Download the Logworks program and make some runs to establish a base line. Make sure you note what you did on each log.

You will need a set of number drills from about 40-80 to measure the restriction and bleeds and jets.

Mark all of these down so you have a base line here too. Make sure you do not have any exhaust leaks before the o-2 sensor.

Also make sure yo
02-20-2012 06:39 PM
chevydragster83
Quote:
Originally Posted by F-BIRD'88
Ya at idle it is richer than it needs to be. You need to get the ignition curve in order first, before you dig into th4e carb.

Is the PCV hooked up and functional.?

What is the AFR at different part throttle cruise conditions say 30mph, 45mph, 60mph steady driving.

Once you have recurved the distributo as I outlined earler, the carb will want readjustment Idle speed and idle mixture screws. This itself will be a big clean up.
Use a manifold vaccuum gauge. THE ENGINE MUST BE FULLY WARMED UP before any real meaningfull assesment and adjustments.

Best to take it out and drive it around to get it all warmed up, then set the carb
idle up using the vac gauge and AFR meter ( the vac gauge is the priority at idle.) .

Next is to get the part throttle cruise dialed in. ( primary jet and rod choice.)
Then tune the step up spring tension ( start with the 4# spring)
Take it one step at a time.

The idle spark advance is critical. 24ish+/- BTDC base timing at idle is required
This does not include vacuum advance (yet)
This distributor curve needs modification first. You cannot get the desired curve by just swappping advance springs.

Get a manifold vacuum gauge.
I have not driven it yet. I plan to test it out tomorrow.

The guy that installed the heads and cam for me installed a curve kit in the distributor when he done the swap.

I calibrated the gauge as the instructions explained to do. I ran a ground directly to the battery to avoid any noise.

I have a manifold vacuum gauge also.

I'll update after I do some test driving.
02-19-2012 06:45 PM
chevydragster83 I got an Innovate MTX-L wide band AFR gauge. I installed it yesterday and let the engine warm up. It was staying in the low 12s then it dropped as low as 11.3 at idle after it had gotten warmed up. Looks like I have a pretty major rich condition. I am going to change the oil and take it out and see what it does at different driving styles this week before I do anything to the carb. That probably explains the terrible MPG and low power wouldn't you think?
02-03-2012 04:03 PM
chevydragster83
Quote:
Originally Posted by bentwings
In order to make the AFR work or better yet to use it properly, you must have a very good understanding of how the carb works. You need to know how the idle circuit, transition power and cruise circuits affect the AFR.

Best bet is to get the Holley book with the cut away view of a 4500 on the cover. I don't have mine handy but just check Amazon.

Above all don't get "drill happy" before you understand what you are doing.

The AFR gage simply tells you what is going on. It can be misleading if you don't understand the carb. It's not hard learning you just have to put your mind to it.
I have read the manual that came with the carb and I have some understanding of how it works maybe. It show's how to calibrate it 1 stage lean, 1 stage rich etc. I would think I could at least get it to stob loding up on gas and smelling like its flooding with the manual, calibration kit and a AFR gauge couldn't I? Even if I can't get it right my self and have to take it to someone who can it would still be a nice thing to have.
02-03-2012 01:54 PM
ap72 just tune the carb you have. Edelbrocks deliver some of the best mileage you can get from a carb, and are dirt simple. Even with it in perfect tune at best you're only going to se about 9 mpg's with your combo.
02-03-2012 03:28 AM
tjet IMO, the things that are killin your MPG are:

* the 230 deg cam
* the tq converter
* the 315's wide tires on the rear
* the 750 carb

I would start with the cam. Get something with around 220* at .050, and 112 LS. This may wake up the motor because you will have a compression increase. When you do the cam, also install a 2 piece timing cover (much easier to swap out the cam later on)

Next I would replace that TQ converter to match your new cam. Get something with a low stall. As much as I like that Gear Vendors Unit, I could never pay that much. Maybe one day I'll find a cheap one on ebay.

You will need at least a 31" to 33" rear tire to offset the 4.11's. Just keep it around 11" wide

I would sell that 750 carb and get a rebuilt 625-650 cfm carb. The smaller carb will "work" better with the air velocity increase & the new cam. You can find great used carb deals all day long on line. The good news is that your 750 carb has a good resale factor, so you might make out good on that

The o2 meter kit is a good idea. If you cant afford to get one now, at least order an O2 bung & cap from Summit when you get the cam - like 7 bucks

With the cam & torque converter change, you are looking at around $450.
You can also find a good used vac advance dist on-line (vac advance = MPG)

Not counting the tires, the changes above should give you about 12-13 MPG

02-03-2012 01:20 AM
bentwings In order to make the AFR work or better yet to use it properly, you must have a very good understanding of how the carb works. You need to know how the idle circuit, transition power and cruise circuits affect the AFR.

Best bet is to get the Holley book with the cut away view of a 4500 on the cover. I don't have mine handy but just check Amazon.

Above all don't get "drill happy" before you understand what you are doing.

The AFR gage simply tells you what is going on. It can be misleading if you don't understand the carb. It's not hard learning you just have to put your mind to it.
02-02-2012 06:59 PM
chevydragster83 [QUOTE=bentwings]I thinkthe best thing would be to install an Ait Fuel gage (AFR gage) These run a couple hundred bucks. Innovative motorsports is very popular and they have an active forum. This will help you with the tuning. I get 15-16 mpg with the blower so you should be able to get the same or better....this is on 87 gas too. Also install a vacuum gage. These are a great load sensor....low vac means low mpg.

I'm sorry it has taken me so long to reply. I have been looking at some air/fuel ratio gauges. That sounds like that would be great for tuning. It would definitely help for calibrating the carb wouldn't it? I would think it would be pretty easy to do just basic tuning with one wouldn't it? Is the gauge and the sending unit all I would need?
12-25-2011 01:05 PM
bentwings I thinkthe best thing would be to install an Ait Fuel gage (AFR gage) These run a couple hundred bucks. Innovative motorsports is very popular and they have an active forum. This will help you with the tuning. I get 15-16 mpg with the blower so you should be able to get the same or better....this is on 87 gas too. Also install a vacuum gage. These are a great load sensor....low vac means low mpg.

Next you will have to ditch the gears. Hoosier makes a Pro street tire in the 31 inch range. I have these and they have 18k miles on them now. They work great in the rain...even in the light snow last week. With a good suspension they will hook quite well too. These run about $700 a pair and work tubless. I'd gear for about 2500 rpm at 60 mph +/- a little what ever it takes to get this.

An OD trans is shaky at best....JMHO I cruise with over 150 guys and there are a lot of OD's in the group. There are a lot of them in the shop for repairs too. You will spend $1500-1800 for anything that will hold up and even that is shaky at best. If you had factory EFI the story might be better. Bottom line is any OD is going to be relatively expensive. I like my T-350 without a kickdown. I just manually drop it down a gear if I really want to charge on the highway otherwise I just step on it.
12-25-2011 12:42 PM
chevydragster83 the carb I have now is the Edelbrock 1407. I have the calibration kit for it. Do you think I could calibrate it to get better MPG?
12-03-2011 11:56 PM
DoubleVision I don`t like Edelbrock carbs, not a big fan of Holley`s either. Most holley`s come with straight leg boosters, these aren`t as bad as down leg boosters however, as dog legs are completely worthless.
A annular booster carb is perferred. It gives it better throttle response and real nice manners. However, annular booster carbs are more expensive.
I perfer a Quadrajet carb and they are pretty much all I use.
Not to say a holley won`t work for you as it will. So if you want to go holley by all means do so. The only cheap carb with annular boosters I know of is the summit racing carb, however you have to get the carb that`s a mechanical secondary which would work fine for what your doing.
How to improve mileage, first off, with a 3000 stall and the low gear good mileage isn`t to be expected, however you should be getting better than 5 miles to the gallon. What`s the timing set at? What plug wires are in use?
First off, if you haven`t already, get the process started by packing the front wheel bearings with synthetic grease. Then fill the rear with royal purple or other synthetic lube if you haven`t already. If your not running synthetic oil swap over to it. Lowering resistance is the first step in the right direction. To get decent mileage you have to keep the revs down which is difficult with your rear gears. You could somewhat defang the engine by going with a smaller cam such as the comp 268H which will still give it a lopey idle, but it`s duration figures are on the mild side. With the heads and combo you have it would still have good power. Then you could switch to a lower stall converter.
12-03-2011 03:15 PM
chevydragster83
Quote:
Originally Posted by chevykid90
That's about what my Firebird got with the old carb on the 454 BBC. I rebuild a different carb of the same model, dialed in the timing, and spent hours tuning it and I now get 11 MPG's average.. And that's with a very healthy 454 pumping out probably just above 500 HP/TQ. So to me your small block only getting 5 MPG's doesn't seem right. And I am also using a 750 CFM carb, but its a Holley with vacuum secondaries.
I have had problems with this carb ever since I got it. My engine has always loaded up on gas and has never run as good as it should since I got this carb. I have heard Edelbrock carbs were just not that good. Do you think maybe selling this carb and buying a Holley would be a good idea?
12-03-2011 07:27 AM
75gmck25 You can cut the effect of the 4.10 axle by going to larger tires. A 33" tire should fit on your truck without any mods. Something like a 285/70/17 or 285/75/16 would work. A very standard truck size is 235/85/16, but its a little shorter (32"). The cheapest choice is usually to use 16" wheels, since they were offered stock on many trucks, but the stock GM wheels are only 6" wide which is is really too narrow for anything more than about 255 or 265 width tires.

You could swap an Edelbrock 1406 600 cfm carb and maybe pick up a little mileage. You would need to tune it for your cam, since the stock jetting is probably too lean. The stock step-up springs will also create an incredible bog with a high lift cam because it will transition from cruise to power mode way too late.

I have a very mild 350/TH350 in my '75 GMC truck, and I can't get much more than about 10 mpg if I drive it on the highway at about 55-60. If I kick the speed up to 70, my 4.10 gears drop the gas mileage down even further. These trucks are built like a brick, and its hard to overcome the drag. I'm running stock height 31" tires, but I haven't wanted to spend money on trying any other combination.

The swap to a TH700R4 is fairly easy and would help highway mileage, but you will have to spend money on building a good one if you plan to still race it.

Bruce
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