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77 Slant 6 idles, but won't rev up
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Author:  Doctor Dodge [ Sat Sep 04, 2021 5:42 pm ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
Quote:
...The distributor pickup is polarity sensitive. If it's connected backwards the timing will change with RPM...
Did you confirm that the electronic ignition distributor polarity is correct? (swap the wires to test)
"Runs but will not rev-up" is often a result of this problem.
DD
Author:  k7seh [ Wed Sep 08, 2021 1:58 pm ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
I will take a look at that possibility. This morning I used a camera placed in cylinder 1 to watch the piston position and compare it with the timing mark on the Harmonic Damper. I was bumping the crankshaft with the starter. After about 8 or 10 complete revolutions of the crankshaft it appears that the timing mark is in the correct position. When the piston is at the top of its travel the timing markis at 0 degrees. The only other thing I have not compared is to rotate the crankshaft and look to confirm the valves are both closed when the timing mark is at 0 degrees. First I will swap the wires to the stator in the distributor. They are connected through a two-pin automotive connector from which I should be able to remove the wires and swap them in the connector. Some time back out of curisity I did look at the output of the stator without the distributor on my oscilloscope using a screwdriver to activate the pulse and trapping the trace on the scope. The positive and negative pulses are equal in amplitude and their duration which is about what what I expected to see.
Author:  k7seh [ Thu Sep 09, 2021 7:44 am ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
Last night I took Guru's suggesstion and replaced the connector on the pair of wires going to the Stator in the distributor, swapping the polarity of the signal to the Ignition Module. The engine started right up and revs without heasitation. Why the difference it time between the positive and negative pulse from the Stator makes a difference beats me, but it definitely does. Many thanks to Guru, and Jushua, who made an earlier suggesstion I did not try at the time. Next comes figuring out how to put a door glass into the trough on the window regulator, and a couple of minor things I have not paid any attention to yet. Thank you all for your suggestions.
Author:  k7seh [ Thu Oct 07, 2021 2:20 pm ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
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A lot has gone on since I reversed the Stator wires and the engine would rev at higher RPM's. But now it won't run at low RPM's. It likes about 2000 RPM @ about 18-19" of vacuum. I can get it to idle near 1000 RPM @ 10-12" of vacuum. Also @ 2000 RPM the Timing Light would flash on all cylinders. At 1100 RPM it would only flash on 4-6. ??? Along the way I found a BBD-8137S carburetor, and after cleaning it up and replacing the accellerator pump I installed it. I think this is a much nicer carb than the one on the truck when I got it. It is also a Super 6 version as well. Next I decided to get back to basics. A breaker point distributor. Near my home is one of those wrecking yards you see in the distance off the road as you speed down the hiway. I figured out how to get there, and the first distributor I saw on the shelf was what I wanted. $10 took it home. A new set of points and condensor, it started and I ran it for about a half hour tring to get it to idle down. At 1000 RPM the timing light blinks on all 6 cylinders now. The next time I tried to start the engine it wanted to blow smoke back through the carb and would not start, just blowing smoke back through the carb using starting fluid. Is the Damper slipping giving me a wrong timing setting (5 degrees BTDC)? Today I put bought a socket for the damper bolt and put a camera probe in cylinder 1. Previous attempts always saw a black piston top. Using the bump starter method it was hard to see the piston movement in slow detail. Today all 6 plugs came out to help pull the crankshaft around, and they came out clean for the first time, and the piston and cylinder walls were cleaner and easy to see on the screen. I pulled the crankshaft through the timing marks and the marks on the damper are right where they should be. My suspician at this time is that when the head was removed for rebuild they did something stupid and put the old manifold gasket back in. Is there an issue with manifold gaskets? When it was running on the breaker points I could see some smoke around cylinder 4. I don't think it was from fluids burning off. If I get it started again I have a bottle of soapy water to spray around the gaskets to see if anything changes. I am wondering, can you replace ithe manifold gasket without removing the manifold assembly from the tailpipe?
Author:  volaredon [ Thu Oct 07, 2021 5:37 pm ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
A 77 slant 6 would have been plain 4 pin ECU electronic ignition not lean burn or any sort of computer. By putting a points distributor in, that's a definite step backwards. And if it's been apart any time sorta recently the exhaust pipe should come right loose. It's only 2 bolts. Even if the exhaust pipe would move enough to pull the manifold stack back far enough to get the gaskets past the studs, it would be more of a pain in the azz to fight it to save taking those 2 bolts out.
I doubt they reused the manifold gasket, for the $6-7 a new fel pro would have cost, and spraying something in the joint to listen for change indicating a vacuum leak is certainly a good idea and a common troubleshooting step but you're not looking for a propane leak or a nail in a tire. Soap and water wouldn't be my choice there.
Author:  Jase [ Fri Oct 08, 2021 6:53 am ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
Quote:
It likes about 2000 RPM @ about 18-19" of vacuum. I can get it to idle near 1000 RPM @ 10-12" of vacuum.
Try to focus on one issue at a time. Write a list of suspected issues and start at the top. I would begin with checking for a vacuum leak.

For the moment I would not look at the distributor, or plugs or any of that. Make sure the intake system being the carb, manifold, PVC system, distributor vac advance system are not contributing to a vacuum leak which will not allow your engine to idle.

Seeings how it would only idle before, and now it will only run at high idle, I am guessing there is additional air getting ingested. Check and re-check every system that has been touched, to find out what has changed.
Author:  k7seh [ Fri Oct 08, 2021 11:34 am ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
At this point I think I have reduced the options to a vacuum leak. The carburetor is clean and working as it should, the distributor is a basic breaker point system that is working now. The ignition system has been fully serviced and all new parts installed. I have checked and cleaned the PCV valve. The ball is free to move and seals the air flow when closed. The Damper timing mark is confirmed to be in the correct position. Cylinder compression is the same on all cylinders. The head has been recently rubuilt with new valves and springs. I cannot see a shiny new gasket between the head and manifolds. The vacuum actuators function on a hand held vacuum pump and hold pressure. I think I am looking at a leak in the manifold gasket. I will try to get it running again and try to confirm the gray smoke coming out from the manifold/head junction is a leak or not one more time before I change the manifold gasket. All other leak sources, including the power brake amplifier. which is the only vacuum accessory on the truck, have been tested or plugged, and have new hoses installed.

Starting the engine with starting fluid (I have not installed the choke thermostat that arrived yesterday) air keeps being blown back through the carburetor. The air blown back is gray. The same color as the air I see coming from between the head and manifolds around cylinder 4.

Installing the breaker point distributor removed any questions about the GM-EFI ignition. The way I built the wiring for the HEI all ll I have to do to go back to the HEI is to change an 8" cable harness section and change the distributor. The new harnesses are connected using modern automotive harness wiring connectors. Once the engine is running correctly I may do that, but for now the HEI is out of the question.
Author:  mcnoople [ Fri Oct 08, 2021 2:32 pm ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
A good simple method for checking for vaccum leaks is to spray the suspected leak with something flammable and see if the engine responds. Something like brake cleaner works good or a propane torch that is NOT burning just oozing propane vapor. Just remember that the exhaust manifold heats up very fast and CAN ignite liquids, so this works best on a cold start.
Author:  k7seh [ Wed Oct 13, 2021 5:22 pm ]
Post subject:  Re: 77 Slant 6 idles, but won't rev up
I have learned a couple of things about how to remove and install the manifolds, and also noticed that the gasket is so thin, like paper, it could be cut easilly with a knife. Out of curisoty I did a VIN search online and found a 5 year-old for sale ad showing the rebuilt head bolted to the engine. In the picture you can see the mnifolds were pulled back from the head and not separated. The mileage is very close to what is on the truck now, After watching a couple of YouTube videos I am pretty certain keeping the manifolds loosly bolted together is probably important to be certain the fit to the head is tight and properly sealed. Checking to see how the manifold studs were troqued I found on nut that would not torque to 10 Ft Lbs. The threads are stripped. All of the studs will be replaced with new when I put the new gasket in.

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