HI I have a 69 Z/28 302 with a Holley 4150 Carb on it. Last year I had distrubutor issues so the car hasn't been driven in 2 years. I got the car running again this year and basically the car will not run between 500 and 1500 RPM. I rebuilt the carb myself same issue. If I constantly pump the throttle I can keep the car running. What is very strange is that when I let up on the throttle the engine feels like it will stall. It then catches again then almost stalls again. The goes on 2 or 3 times before the idle is maintained. It is responsive to the idle mixture screws. The power valves seemed ok from a visual inspection I did. I drove the car like this and it takes of and runs but stopping at a light is an issue. If anyone can tell me where to look I would appreciate it.
69 Z/28 Engine stalls coming off idle
njbob
1 - 19 of 19 Posts
HI I have a 69 Z/28 302 with a Holley 4150 Carb on it. Last year I had distrubutor issues so the car hasn't been driven in 2 years. I got the car running again this year and basically the car will not run between 500 and 1500 RPM. I rebuilt the carb myself same issue. If I constantly pump the throttle I can keep the car running. What is very strange is that when I let up on the throttle the engine feels like it will stall. It then catches again then almost stalls again. The goes on 2 or 3 times before the idle is maintained. It is responsive to the idle mixture screws. The power valves seemed ok from a visual inspection I did. I drove the car like this and it takes of and runs but stopping at a light is an issue. If anyone can tell me where to look I would appreciate it.
What exactly was your distributor issue from last year? Any vacuum leaks? That will bring things down in a hurry. Check the brake booster and the vac advance can on the distributor.
What is your initial timing and what is your total advance? Your advance should be all in by 2600-3000 rpm.
How did you check your power valves? Strong smell of gas? What do the plugs look like?
There are a lot of possibilities including a misalignment of the cam timing sprocket as one TC member with a Z recently found out.
You just have to start eliminating the obvious and making sure ignition and carburetion are up to spec.
alan
What is your initial timing and what is your total advance? Your advance should be all in by 2600-3000 rpm.
How did you check your power valves? Strong smell of gas? What do the plugs look like?
There are a lot of possibilities including a misalignment of the cam timing sprocket as one TC member with a Z recently found out.
You just have to start eliminating the obvious and making sure ignition and carburetion are up to spec.
alan
Thanks for responding. I lost spark, I have an Acell distributor and I replaced the module, Now I have spark. All mechanical adv no vacum. The only Vacum on the car is th PB booster, I will plug that see what happens. I checked the power valve just by looking at it. I thoght if it was blown out due to a backfire I would have seen that. Not sure how else to check it. I will have to check the adv. But I think it was set up for 24 degrees and I know last time its all in by the RPM's you mentioned. The speed shop guy set up the curve.
It is just that the business of the thning reving up to say 200o RPM the satlling and catching again is very strange.
And I thought if it was a vacum leak I could not get it to idle at all, the plugs loogk good.
I do have an original carb for it I was thinking of getting that one rebuilt, since I want to do that anyway, then restore the pcv system.
I have owned the car for 35 yers this is the first issue. Will check the timing later today. Thanks You.
It is just that the business of the thning reving up to say 200o RPM the satlling and catching again is very strange.
And I thought if it was a vacum leak I could not get it to idle at all, the plugs loogk good.
I do have an original carb for it I was thinking of getting that one rebuilt, since I want to do that anyway, then restore the pcv system.
I have owned the car for 35 yers this is the first issue. Will check the timing later today. Thanks You.
Joined
·
662 Posts
It sounds like your idle circuit is too rich at idle and too lean at light cruise. If you suspect the powervalve this makes we wonder when the last time this carb had a rebuild kit was. Maybe it is time? It is extremely rare for a powervalve to blow out these days.
Start with setting your primary butterflies so the transition slots have .020" showing when looking from the bottom of the carb. This is important or any other changes you make will be pointless. Get the primary butterflies set and then use your secondary throttle screws to adjust your idle speed. You may need to take the carb off to get to the secondary screw. It uses a small screwdriver so use the right one and don't strip it.
Set your float levels so they both just barely drizzle out at idle. Then Get a vacuum gauge if you don't have one already. No Holley owner should be without a gauge. Set your mixture from the mixture screws for highest vacuum. Keep both (or all 4 if it is 4 corner) screws turned out the same amount. Keep a not of how many turns out it will be because I will ask later.
Get this baseline down first and then write back.
That is one identifiable problem. That is not normal even for a Z. I can attest to the fact that when a power valve fails, the car won't idle or run below 2000 rpm worth a spit and you will have a strong gas smell. Let's take this stepwise. Follow Luciano's advice and we will go stepwise on this. For the record here, when a power valve fails, the diaphram is leaking. As my high school auto shop teacher used to say, "did you kiss the power valve?" It should not leak any air across the diaphram.
alan
Joined
·
1,750 Posts
What Number Power Valves are you using? (their function is to dump fuel on the transfer when the Vaccum drops... if you ease into it and the Vaccum never gets low OR if your PV's are too low then there is never enough drop in vaccum to open the PV's and it will be lean on the transition from idle to mains, thus your stall)
What is the Vaccum at idle? (if you are seeing 10" at idle then you need a PV that opens 2" below the idle...or a 8" PV)
The constant pumping comment also makes me think that the PV's are the problem because when you pump it you are using the accelerator pump and squirter(s) to overcome the lack of fuel.
What is the Vaccum at idle? (if you are seeing 10" at idle then you need a PV that opens 2" below the idle...or a 8" PV)
The constant pumping comment also makes me think that the PV's are the problem because when you pump it you are using the accelerator pump and squirter(s) to overcome the lack of fuel.
Joined
·
1,750 Posts
Yep just put the PV to you lips and inhale... watch to see if the plunger goes in.
How big of a carb do you have there. You say it's a 4150, but what is the CFM rating? If you're carb is too big, you can end up with an off-idle dead spot. If your cam shaft doesn't give you good idle vacuum the problem can be made worse with a fat carb. Timing plays a big roll here especially at the off-idle spot. Vacuum advance will help.
Well, in his first post he states "the car will not run between 500 and 1500 rpm. First off, no Z will run at 500rpm. But again, for the quick fix, with a big gas smell and no idle capability, I would first look at the power valves. And once that is checked out, he needs to heed Luciano's advice on the setting up transition slots.
I must admit, I have never heard of setting the primary transition slots and then setting the secondarys to get your idle, that is a new one. I do know when I got my new 4053, I had to reset the secondary stops as they were open too much rendering the idle adjustment screws inoperable (no effect as the secondaries were open too much). And that from the Holley factory no less!!!!
You said you have owned this car for 35 years, does it have the original carb on it still? If so, when you take it off, wiggle the throttle shafts to see if they are worn. This will cause problems if they are worn as they will leak air.
alan
I must admit, I have never heard of setting the primary transition slots and then setting the secondarys to get your idle, that is a new one. I do know when I got my new 4053, I had to reset the secondary stops as they were open too much rendering the idle adjustment screws inoperable (no effect as the secondaries were open too much). And that from the Holley factory no less!!!!
You said you have owned this car for 35 years, does it have the original carb on it still? If so, when you take it off, wiggle the throttle shafts to see if they are worn. This will cause problems if they are worn as they will leak air.
alan
Joined
·
662 Posts
The main reason for the transition slot setting is that the t-slot is a bleed just like the IFR and IAB (idle feed restrictor, idle air bleed). The transition slot is different thought because it is a variable restriction that changes with throttle position. Since our carbs idle and cruise off of the primaries, the primaries take precedent over the secondary throttle position. This is especially important in a 2 corner idle carb because the primaries have play in the mixture screws where the secondaries are dependant on bleed area alone basically.
At idle the .020" sets a baseline for your idle mixture because is draws the correct micture to start with from the t-slot while also allowing the richer progression deeper into the t-slot. The idle circuit feeds all the way to WOT. At some point depending on a lot of things, the idle circuit's influence is lost and the mains take over. The mains start point varies with boosters, emulsion hole area, and the MABs (main air bleeds). The IFR IAB relationship will bridge that gap if set in the proper ratio. From Holley or Holley clones, this relationship is set pretty rich to give a "universal tune". This ratio in most cases can be lowered, but only tuning will tell. The IFR affects the idle and cruise. The IAB affects the top of the t-slot more.
The mixture screws meter a combo of fuel and air. The IFR and IAB relationship make that combo. If the IAB/IFR is set to rich it becomes necessary to close down the mixture screws to a point where they become a restriction themselves and won't let the volume of mixture by. If your carbs sets up with 1/2 turn or less on the screws, the carb can't deliver the mixture it needs to.
The mixture screws contribute more to the idle mixture. Once you open the butterflies higher into the t-slots, the IAB/IFR take over and the mixture screws have less influence over your mixture. That is the point where most of you are running into problems because you can't enrich or lean that higher t-slot area and your not into the mains yet. You can richen or lean the mixture screws at this point but then you will negatively affect the idle setting. You'll notice Holley or BG say to raise the float a little or back the mixture screws out slightly. This is because they know most people are incapable of drilling and tapping their IFR/IABs and getting their relation right. So they fix one problem and cause another. Again an over rich idle will run better than a lean so they err to the side of the transition enrichment.
Once you get the mixture right into the transition, you have to get the mains coming in at the right point. This is done with the emulsion holes and the MABs. From there you set your primary jets for higher RPM cruise only (street driven vehicle) and your secondary jets more toward cruise and WOT.
The powervalve needs to come in at the point where load increases enough to need extra enrichment. This point is very hard to find. Your idle transition and main cruise have to be set right with the circuits overlapping where they should first. Then there will be a point where the load increases enough to need the extra fuel switch (the powervalve). Most carb makes say to either go 2" below idle or half the cruise vacuum. This will be close, but in a lot of cases not correct.
If you are willing to drill and tap your restrictions, you can get this set right. An AFR gauge will make all of this a lot easier. It can be done without though, but you really need to know your stuff. There is a lot involved in the process. You need the right drill bits and taps, brass set screws, and to know how to do it all. I got all my info from the Innovate Motorsports board. It took weeks of reading through their posts, but was well worth it. Tuner, Shrinker, and others will answer questions as they come up. Those guys know more than the engineers at Holley, BG, or other clone companies.
At idle the .020" sets a baseline for your idle mixture because is draws the correct micture to start with from the t-slot while also allowing the richer progression deeper into the t-slot. The idle circuit feeds all the way to WOT. At some point depending on a lot of things, the idle circuit's influence is lost and the mains take over. The mains start point varies with boosters, emulsion hole area, and the MABs (main air bleeds). The IFR IAB relationship will bridge that gap if set in the proper ratio. From Holley or Holley clones, this relationship is set pretty rich to give a "universal tune". This ratio in most cases can be lowered, but only tuning will tell. The IFR affects the idle and cruise. The IAB affects the top of the t-slot more.
The mixture screws meter a combo of fuel and air. The IFR and IAB relationship make that combo. If the IAB/IFR is set to rich it becomes necessary to close down the mixture screws to a point where they become a restriction themselves and won't let the volume of mixture by. If your carbs sets up with 1/2 turn or less on the screws, the carb can't deliver the mixture it needs to.
The mixture screws contribute more to the idle mixture. Once you open the butterflies higher into the t-slots, the IAB/IFR take over and the mixture screws have less influence over your mixture. That is the point where most of you are running into problems because you can't enrich or lean that higher t-slot area and your not into the mains yet. You can richen or lean the mixture screws at this point but then you will negatively affect the idle setting. You'll notice Holley or BG say to raise the float a little or back the mixture screws out slightly. This is because they know most people are incapable of drilling and tapping their IFR/IABs and getting their relation right. So they fix one problem and cause another. Again an over rich idle will run better than a lean so they err to the side of the transition enrichment.
Once you get the mixture right into the transition, you have to get the mains coming in at the right point. This is done with the emulsion holes and the MABs. From there you set your primary jets for higher RPM cruise only (street driven vehicle) and your secondary jets more toward cruise and WOT.
The powervalve needs to come in at the point where load increases enough to need extra enrichment. This point is very hard to find. Your idle transition and main cruise have to be set right with the circuits overlapping where they should first. Then there will be a point where the load increases enough to need the extra fuel switch (the powervalve). Most carb makes say to either go 2" below idle or half the cruise vacuum. This will be close, but in a lot of cases not correct.
If you are willing to drill and tap your restrictions, you can get this set right. An AFR gauge will make all of this a lot easier. It can be done without though, but you really need to know your stuff. There is a lot involved in the process. You need the right drill bits and taps, brass set screws, and to know how to do it all. I got all my info from the Innovate Motorsports board. It took weeks of reading through their posts, but was well worth it. Tuner, Shrinker, and others will answer questions as they come up. Those guys know more than the engineers at Holley, BG, or other clone companies.
Joined
·
1,750 Posts
A good many home rebuilders do not check the flatness of and if needed mill the main bodies... I have also seen guys torque fuel bowls with a socket wrench... and tighten them so much as to actually cut the gaskets like a die cutter would! This does nothing but pull the threads out and WARP the main body, creating air leaks everywhere! Not a good baseline for trying to dial a carb in for sure!
BTW you can check for a leaking throttle shaft by starting the car and with the air cleaner off spray some Carb cleaner where the throttle shaft goes into the throttle plate... if the idle picks up then you have worn throttle shafts that will need to be bushed.
BTW you can check for a leaking throttle shaft by starting the car and with the air cleaner off spray some Carb cleaner where the throttle shaft goes into the throttle plate... if the idle picks up then you have worn throttle shafts that will need to be bushed.
I know first hand what you are saying. But how did the factory get a DZ engine to idle at 900 RPM with only 4 deg of initial? The butterflies had to be way up up on the slots.
njbob, I will assume you have a stock built 302 with 30-30 cam and 11:1 compression. I have been there tuning my 302 to run well. Your carb is probably gummed up from the last fuel vbefore it sat for 2 years or has developed leaks in the gasket. Possibly the front metering block may be partially plugged in one of the idle ouylet ports like mine was when I initially started to tune. Have your carburetor gone thru by a competent rebuilder. There are a couple of them here on this site. Yes you can do it if you spend a lot of time yourself, but they can do it right blindfolded. Have a blowout prevention kit put in so power valves don't get blown out. If your carb is a 750-780 CFM like the factory get #72 jets for the front and #76 for the back. Make sure there are 6.5max power valves installed. You will never see only .020" of transfer slot if you can't get to 12 to 14 degrees initial timing. Maybe 8 to 10 degrees will do it. But factory 4 degrees will not. Once the timing is set it is just a matter of adjusting the idle screws equally for 9 to 10 inches HG on a vacuum gauge. These settings will get you very close for a stock engine.
Joined
·
662 Posts
If you pass the .020" up the mixture screws will loose their effect and there will not be enough enrichment left for transition. I don't know how they would have done it on the stockers. Passing up the .020" the secondaries isn't as big a deal. If needed, open the screws to get to your needed idle. If you get the idle circuit tuned right, you won't need to idle that high anymore anyway.
Thanks to all the replys. I am still having the problem bought a vacum gage but cannot keep the car running long enough to effectively use it. The reading just drops to 0 and it stalls.It seems like the carb is just constantly pouring gas in, extremely rich fouling plugs. I still need to check the timing on the distributor. Another thing thats hard to do without a steady idle.
I do have another carb, a 4053DZ that I bought used. I think I will rebuild that and see if anything changes.
Plese forgive me from not posting right away. My job is very demanding and I can only work on the car over weekends.
I was wondering if the gas could be that bad too? I have been mixing Sunoco 100 octane racing gas the pump premium. The stuff smell like mothballs. Maybe its shelf life is even more limited.
I do have another carb, a 4053DZ that I bought used. I think I will rebuild that and see if anything changes.
Plese forgive me from not posting right away. My job is very demanding and I can only work on the car over weekends.
I was wondering if the gas could be that bad too? I have been mixing Sunoco 100 octane racing gas the pump premium. The stuff smell like mothballs. Maybe its shelf life is even more limited.
Wow I cannot thank everyone enough. I was incorrect when I said 500 to 1500 RPM i have always had the idle set at 8 or 9 hundred. I intend to send the used carb out to a rebuilder now rather than do it myself. I will let everyone know how I make out.
If anyone can recommed one I'd appreciate it. I emailed Holley and they have not gotten back to me.
If anyone can recommed one I'd appreciate it. I emailed Holley and they have not gotten back to me.
Joined
·
1,750 Posts
Here ya go... there are alot of guys on the
yenko site who use Eric for their carbs and I have heard nothing but great things about his work.
http://www.vintagemusclecarparts.com/
yenko site who use Eric for their carbs and I have heard nothing but great things about his work.
http://www.vintagemusclecarparts.com/
1 - 19 of 19 Posts
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
