72 Lt-1 stumble on take off
Recently I adjusted my lifters, changed spark plugs and wires, and timed the car. I read an article by Lars Grimsrud about total timing and used his procedure. The car runs much better and quieter but it did not totally solve the reason I did the work for. At take off from a dead stop the car stumbles at which point I have to put the clutch back in and play with the gas to take off. Sometimes it is worse than others. If the car is rolling it usually is fine. Is this a common problem with this year Lt-1? My 71 Z-28 never had this problem.
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Re: 72 Lt-1 stumble on take off
sounds like you did a lot of tuning, I'm glad the car is running much better. You might want to check the operation of the accelerator pump on the carburetor.(I am enclosing a link to an article that may be of help to you)
I don't have any of the stumbling off of idle and I know that these plastic cams that control the initial shot of fuel can be adjusted just by changing them out. Did this problem just start to happen, or have you had it before you made the tuning adjustments? hope this helps
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Re: 72 Lt-1 stumble on take off
My '72 LS-5 had a sumble similar yours, tried most of the things you tried and I fixed it by "accident". While searching for an answer I found that the VAC was leaking, therefore not operational. With the help of many on the forum I rebuilt the distributor and put it back to factory specs. Then installed it and timed it the way the factory did, instead of using the total timing method you describe (I had it that way prior to the dist. rebuild). Stumble disappeared and hadn't run that well since I owned the car.
I read somewhere, maybee here, "90% of all carburator problems are electrical" (meaning the ignition system) In my case I believe they were right. Good luck!!!Comment
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Re: 72 Lt-1 stumble on take off
Thanks for your replys. The car has had this stumble for lack of a better word since I owned it. I checked for vacuum leaks took the intake off and replaced the gaskets and just last week did the timing and valve adjustments. I hooked the vac to full manifold vacuum. After timing the car and hooking up the vacuum advance the timing jumped about 6 degrees so I assume its functional. The carb was rebuilt not to long ago by a local rebuilder. The distributor might be a bit worn and I would like to have it rebuilt. So I will check out that accelerator pump link and take ti from there.Comment
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Re: 72 Lt-1 stumble on take off
James I would recheck the accelerator pump first as you had said it happened during initial acceleration. On all carb cars this is when you need additional fuel to compensate for the added air flow if the pump is not adding the correct amount of fuel the car will stumble. Also the best troubleshooting process is to work on one item at a time this way when the problem is cured you will know what cured it. So with that said work on the fuel side first when you are reasonably sure all things things on that side are working normal and the problem is still there move to the electrical side and so on. Paul 18046Comment
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Re: 72 Lt-1 stumble on take off
James,
What are the specs on the vacuum advance control you are using, you want to be sure it's fully deployed at idle vacuum if hooked to a manifold source. Look in the archives for good info and you will understand..
Checking the accellerator pump for proper adjustment is good, there is also good info in the archives about the pump ball check valve that should be looked at, it's located under the accellerator pump diaphram. Very easy adjustment and easy to check, it can cause the problem you have.
Make sure the pump is adjusted properly and there is fuel shot with slight movement of the throttle blades, outside of that the problem is somewhere else.
Sometimes the accellerator pump will cover up problems, if you turn the idle screw does the engine speed up without a lean stumble in a normal way. This test will tell you if there is a transistion problem with the carburetor circuits. Make sure safety first, chock wheels..
Many times, carburetor problems are actually ignition problems so make sure you work is correct there before $$ on carburetor. Has anyone that you recall ever messed with the carburetor, is it in it's original state, air bleeds, jets, etc.
I once had a hesitation problem and it turned out to be the vacuum advance hose egged from heat where it plugged to the vacuum advance control.Comment
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Re: 72 Lt-1 stumble on take off
The OE VAC requires more vacuum than the engine generates at idle to fully deploy, which can cause various problems. You need to change the VAC to a B28 when you change the signal to full time manifold vacuum. Lots of discussions in the archives on this issue.
DukeComment
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Re: 72 Lt-1 stumble on take off
Just to add another item for you to check:
The carburetor and linkage was designed to operate at idle with the vacuum slots exposed on the primary side. If you disconnect or bypass the TCS (Transmission Controlled Spark) solenoid you will now have vacuum to the distributor vacuum advance actuator at idle -- from what you have said it sounds like this is your situation.
To compensate for that vacuum advance and the resultant higher idle rpm the primary plates have to be readjusted -- that is what happens when you adjust the idle speed screw. After you do that the idle vacuum slots are no longer exposed.
Some folks cure this "problem" by drilling holes in the primary plates. My point of view is not to create the "problem" in the first place, but WTFDIK. Try putting your TCS system back together (post here and we can tell you how to check that it is working properly), readjust your carburetor, and see if that cures your problem.TerryComment
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Re: 72 Lt-1 stumble on take off
The carb is stock . I had to change the fuel bowls because the float adj. screws srtipped, and the re-build was stock. Should the fuel shot when the throttle plates just start to open be a trickle or a spray?Comment
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Re: 72 Lt-1 stumble on take off
James,
Within reason the fuel starts as a trickle because pump movement is mechanical. The point here is that there is no linkage slop in the adjustment. Proper adjustment is holding the primary throttle plates open there is approx. .015 clearance between the pump lever and nut on the adjustment screw. This means with the throttle plates closed the lever is already pushing the pump so there is instant fuel shot.
You mention changing the fuel bowls, did the car hesitate with the old bowls?Comment
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Re: 72 Lt-1 stumble on take off
The only way the fuel gets to the accelerator pump receives fuel is gravity feed through two small holes. What contains the fuel in the accelerator pump cavity when the pump is being activated is a rubber valve that closes off the two holes of the fuel intake. If this rubber valve is not doing its job of sealing properly when fuel pressure is against it, the accelerator pump will force fuel back into the fuel bowl. This will have effect on the accelerator pump forcing/pumping fuel to the carb.Comment
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Re: 72 Lt-1 stumble on take off
Jim,
On these old Holley's the check valve is a check ball retained by a small alumnium strip. The clearance between the ball and strip is .010. Excessive clearance will cause the condition you mention.
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Re: 72 Lt-1 stumble on take off
The fuel does start to trickle as soon as you move the throttle. I will double check the clearances on the pump. I would be surprised though if they were not right on because the guy that re-built it is pretty good. It had the problem even with the old float bowls, as I have had the car 19 years and it always had a bog on take off. What I have noticed is it gets a little better when the car is good and warm, also I did notice that when I was doing the total timing the mark would jump about a 1/4 inch side to side ,would not stay steady at about 3000 rpm. Know of any reliable distributor guys?Comment
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Re: 72 Lt-1 stumble on take off
any chance a faulty power valve could cause this?
Also - what if the spring in the vac. pod for the secondaries is weakened? would it not allow them to slide open to quickly causing a bog?Comment
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Re: 72 Lt-1 stumble on take off
I think a faulty P/V would create a rich condition at idle. I would check the dwell at 30* and inital timing 12* BTDC and make sure there are no visable vacuum leaks. If the old TCS system is operational use it until you find the problem.
I have learned that factory carburetors are calibrated for specific things on a engine and when you dismantal them strange things happen. What is the idle vacuum of the engine and did you set the curb idle emulsion screws to highest vacuum at low RPM??
Under the accellerator pump discharge nozzle there is a check needle and also the check ball above the pump diaphram. That would be my starting point with the carburetor. Also, are you sure the float levels are high enough?
If you have the factory vacuum advance hooked to full manifold vacuum that may be a problem as you mention only 6* more advance when you plug it in. You need to install a different control like Duke says to match idle vacuum.Comment
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