I can't find a helpfull thread.

ok. I have a 77 ranch w/400. edlebrock 1405 man choke carb, 1" carb spacer, performer 400 intake, msd 6a ignition, 48K volt accel coil 1.73 roller rockers and running out dual exhaust.

Problem:
off of idle or below 1500 RPM there is a dead/stumble spot when pedal is quickly applied to half throttle(before secondaries open). also when quickly applying WOT it dies even more.

Facts:
either times the engine does "catch up".
if I repeat quick WOT it sometimes backfires out the carb.
I tried using 8" metering springs with little to no change. smaller springs did seem worse.
moved accelerator linkage to the lowest hole for more fuel with little to no change.
when slowly applying gas their is no problem, smooth rev.
plugs are new and at .055" gap
cap and rotor are good.
the engine has no timing pointer, so timing??
cam is stock.

I just got a vacume gauge to get more info but I'm not sure where to get my readings.

any help here. thanks

 

Connect the vacuum gauge directly to the intake or the right port on your edelbrock carb. As a suggestion did you check the float level before you installed it? If not you might wanna check it who knows maybe it is not set right, I have had similar experience with a holley and it was the float level but that was due to someone else trying to adjust it. You might wanna figure out your timing, if it is off then everything else is gonna be hard to determine. Third, you might just be lean. Might wanna go to ref#15 in your edelbrock book and try those corresponding jets and metering rod and see if the conditon changes given the other two are okay. Something to think about

 

If you havn't changed the jets in it since you've owned it, it comes with #98 jets, which are perfect. You can go down to #94's for a small fuel economy increase, but I like the power and similar milage I get with the #98's. It really sounds like the float level is too low, but Edelbrocks will not have that problem unless you've messed with them. Your transition circuit is probably too lean (accelerator pump), and your secondaries are late coming on. I also suspect late ignition timing, like JWT said. You don't need timing marks, just loosen the 1/2" distributor bolt down and rotate the distributor body CCW just a tad, then tighten the hold-down again. If this helps, then advance it some more. Backfires are easily caused by late timing, but sometimes they can manifest from a lean fuel condition, or some combination of both. Cheapest solution first: bump the timing up just a bit by rotating your distributor CCW, and see what that does.

Edit:

your transition circuit is also effected by your idle mixture. If your idle mixture is very lean or very rich, you will have an off-idle stumble, because the idle circuit still provides fuel/air the whole time the engine is running. Start with this baseline: turn the idle mixture screws CW until seated, then back them out exactly 1.5 complete turns. Once you have the vacuum gage, plug it into the small vacuum port on the front right side of the engine as you look at it from the front. You can then fine-tune the idle mixutre screws to get the strongest vacuum reading.

You are running the vacuum advance from the left side port, aren't you? if you have the distributor vacuum advance line plugged into the right side port then that is what's causing the backfire. That port is for Chevy engines whose vacuum advance mechanism works the opposite of the Ford design.

 

Torqueking reminded me of something you might wanna look out for. You may have adjusted the air screw out to get your rpm correct. You may have turned it too far out and your timed vacuum port is drawing vaccum all the time. So the first thing I would do is hook the vac gauge up to the left port and trun on the vehicle if it reads anything outside of the black or area shaded listing zero vacuum then you need to turn the air screw in and bump the igniton up until you have 650 or so rpm,then see if your rpm gauge moves at all if it does then turn your idle screw out a little and then the other one a little until it is steady. Double check that the idle screws are turned 1 1/2 turns out. This is edelbrocks "base idle setting". Hope some of this helps later

 

ok. it's not the floats, checked that. I am already about two turns out on the mixture screws.

I bumped the timing back(thats what you meant right?TK) which did seem to help some.

Vacuum is at 20"hg on the tree from the back of the intake.

I'm going to check the vac advance port and vacuum screw on the dist next. it makes perfect sense. it could be advancing too soon or too quick.

thanks for the help.

 

Here are a couple more things to add to your check list.

The accelerator pump is a plunger in a well. If the plunger seal is damaged then it will not push as much fuel. Pull the top off the carb and carefully examine the seal area of the accelerator pump.

The vacuum reading you got sounds pretty good, but you could still have a vacuum leak. With the motor idling take a can of carb cleaner or starting fluid and spray all around the base of the carb, the base of spacer, along the union of the intake manifold and cylinder head (on both sides of the motor), and along all vacuum lines. If the engine speed drops during this procedure then keep going back to that point where it occurred to confirm that it is a vacuum leak. You could have multiple leaks so don’t stop if you find one. Also, pull the line off at the carb that goes to the distributor and suck on it. If you get air then the vacuum advance diaphragm is bad (vacuum leak) and needs to be replaced.

Also pull your plugs and see how they look. There are guides in many manuals and on the Internet that will help you understand what to look for on the plugs. Overly lean will be very light colored and overly rich will be black.

TK's comments on timing were to advance the timing meaning to cause the spark to occur earlier. You should see an increase in the idle speed when it is advanced and a decrease if it is retarded.

Good luck,

Mark

 

ranchero77
I asked this before but I could see how what I was saying might be worded incorrectly, did you take a reading (with vac gauge) at the carb on the left port to see if it was drawing vacuum at idle? You may have just expected the carb to be close out of the box. I did and when I had a few ?'s pop up with the carb I found the air screw was too far out causing vacuum at idle and a messed up ignition thinking it needed to advance. Check that out and see if it makes a difference. You will need to adjust your distributor to bring the rpms at idle back up should you turn the air screw in. Let us know, Later

 

messed with the car at lunch time and I brought the RPMs down to about 700 and I only have about 15" of vacuum now.
I was running a little high(900rpm) due to the 4" crank pulley I have on it. power steering was powerless.

but anyway, using the guage, I was able to fine tune the mixture srews till vac was peaked and steady.
the plunger in the pump should be good I just replaced it last summer.

I have yet to check the carb vac ports. I did however check for leaks with carb spray and their is a small leak on a spot at the carb spacer and manifold but I had to use a lot of spray to make it do much.

I'll get more done later today. thanks for the help so far.

 

You are probably right the accelerator pump may well be good if it is that new, but if you have the top off it would not be a bad idea to carefully look at the edge of the plunger and make sure it has not gotten torn.

It sounds like you may have a pretty good vacuum leak with only 15”. Is it the same when you hook to the driver’s side port of the carb instead of the rear of the manifold?

I recently found a significant vacuum leak on my motor (back of carb spacer at manifold). When I installed my Edelbrock I removed the EGR spacer plate and replaced it with a 2” spacer. I used the gasket that came with new spacer and did not notice that it did not completely cover all the openings in the manifold. Therefore, I had a vacuum leak that was on the driver’s side of the carb and thus mostly effected the rear four cylinders. Pulling my plugs revealed it in that the front four were rich and the back four lean. Once fixed (with a four hole carb gasket and not an open one) I had about an inch more of vacuum – from 19.5 to 20.5”.

Additional sources of vacuum leaks are power brake boosters and any HVAC vacuum operated controls.

Mark

 

what's up
just my hood. so here is the latest.

accelerator plunger is good.
vacuum adv port off the carb draws 1" at idle. I think thats ok
turned IN the vac advance srcew on the dist about 3 turns.
vac adv in the dist is working and I cant suck and air from it.
also replaced the gasket at the spacer and manifold. picked up about 1/2" vac.
the idle is getting smoother but that bog down/stumble thing is still there with quick revs.

I am going to give the carb a good look through while I have the top cover off it. I have a next size bigger from stock primary jet so I think I will try it.

 

It should draw zero vacuum from the left timed port at idle or it will potentially throw your timing off, turn the air screw until the timed port shows zero vacuum and adjust your timing until it comes back up to 650 rpm or so. let us know how the larger jet works you did get the next metering rod as well the metering rods are specific and are only used for one application where the jets are varied by a couple of metering rods? Other wise it may run very rich. Just trying to help out, later

 

I lost something. how can I measure the vacuum from the timed port adjusting the vac screw? I would have to hook up the gauge inside the distributer to see if the screw setting I made is stopping the effect of the 1" draw from the carb.

here is something that might help someone else if they need it. it better explains the vacuum chart in the back of the chiltons repair manual.
http://www.users.bigpond.com/ergoff/vac1.htm
my gauge is reading like the worn valve guides and leaky valve pic. it only shakes within 2 vac points.

to add to the mix I noticed a small thin cloud of blueish white smoke from the exhaust if I rev the engine over 3K. it doesn't do it when I first start the engine.
the engine has always had a bit of a miss in idle. the engine is only about 5 yrs old and only having about 40k miles. I don't remember if the rebuilt block had heads on it or if my old ones were reused. I had it done at a shop. do you all think the heads could be related to my problem?? this is starting to get deeper than I thought

 

The air screw adjusts the throttle which adjusts the butterflies if you will and if they open too far you begin to draw vacum through the timed port, if this happens you've gone too far. You don't want the timed port to draw vacuma at all at idle what is happening your throttle is open enough to begin to draw vac through the timed port and By drawing vacumm your distributor is already out of time at idle and you are starting to trigger other functions of the carb that aren't needed at idle you want zero vacum on timed port at idle. This exact thing happened to myself with my carb and I wasn't happy until I figured it out. The smoke sounds like worn rings or valves. Do a compression test while the smoke is not desired you might still be okay just using a little oil. Your gauge reading of having worn valves sounds fairly accurate. Still trust me on the no vacum at idle on the timed port it is easy to do hook up the vac gauge to the timed port turn the air screw on the throttle linkage in until it shows zero then rotate your distributor until you get the rpm back up then begin the idle mixture screw adjusting you should see a difference, trust me I am not blowing smoke, this is what I encountered with my edelbrock carb and most of what I am telling you came directly from edelbrock as I asked them many many questions. Try this forget about the distributor completely unhook the vac line from distributor to carb at the carb get the vac reading from the carb timed port down to zero then plug the timed port with a cap turn the entire distributor until your rpm is back up to 650 secure it then reconnect the vac line from carb to distributor adjust the idle mixture screws start out 1 1/2 turns out turn on vehicle adjust one screw out until the rpm stop climbing then adjust the air screw in until the rpms are back down to 650 then do the other one the same way reset rpm then slowly turn one screw in until the rpm just barely drops then reset rpm with air screw then do the other one the same way. Now you can begin to set your timing a little more advanced if you are not happy with the power of off idle response. ONe way is to turn the 3/32 hex screw inside the vac canister all the way in turn distributor a little at a time drive it and keep repeating until you get a little detenation then begin to trun the vac hex screw out until the detenation goes away. Depending on the weather hot/cold you may periodically hear a ping you have two choices keep the power and start richening the fuel mixuture larger jets/metering rods or rotate the distributor just a little at a time until it stops. I hope this helps I really have or at least appears I have been through what your experienceing if my post seems forward don't take it as telling you what to do I am really only providing my experiences and how I resolved them, I say don't take my word for it call edelbrock and talk to one of their techs they will more likely than not tell you something similar. Good luck

 

I was mixing up the "air screw" with the "vacuum advance screw". I think we are on the same page now. yeah, stuff like this will drive me crazy till I figure it out.
thanks

 

I can relate to that, let me know if you have any luck getting it to run better. Later