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74 400 block 4x heads won't start

 
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68ragtop



Joined: 19 Nov 2013
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PostPosted: Mon Jun 01, 2015 10:27 pm    Post subject: 74 400 block 4x heads won't start Reply with quote

Looking for a little help. i traded a guy for a 1939 pontiac sedan. It had a 74 400 with 4x heads and a 2 barrel carb on it Ran good Always wanted a tri power found a 66 tri power rebuilt the carbs installed on block fired up one time about 1400 rpms tried to idle down motor stopped went to fire back up and just turned over and was blowing exhaust out oil filler tube and breather. Found out had 74 gaskets and needed 66 gaskets and also filled in the holes about the exhaust crossover.
Ok tried to start and it just turns over has spark/ fuel So today buddy ask what the compression was we pulled #1 was about 70 pulled #3 and that was about 70 pulled #5 and the motor fired right up with 3 spark plugs out 1/3/5 put the plugs back in and all it does is crank over and will not start?
Any input???????

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Larry
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Joined: 01 Jun 2006
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PostPosted: Tue Jun 02, 2015 1:55 pm    Post subject: Engine Problems Reply with quote

Seems like a number of unusual things going on here.

When I put a '66 tri-power on my '70 engine, I don't remember having to be specific with the intake gaskets; perhaps '66 and '70 were the same. However, if the '74 requires different intake gaskets than the '66, then there could be a problem with proper alignment/sealing between intake, exhaust, and ever water. Obviously the best rule is to carefully examine any gaskets you are using to make sure everything lines up properly.

As for blowing through the oil filler, it was probably an ill fitting gasket that was actually letting exhaust gas into the crankcase. Sounds like you cured that with the new gaskets.

The exhaust crossover has little do do with anything other than carb preheating in cold weather.

As for not starting except when three plugs are removed, that is strange. I might wonder if the plug wires are mixed up such that when those are removed, the engine isn't fighting itself and runs (probably pretty bad).

The compression is nothing to write home about, and can be from rings, valves, guides, or all of the above. The engine will run with 70psi, but you won't be winning any races with it.

Go back to the basics and check your plug wires (firing order). Aside from that, this is hard to diagnose without seeing. If you have more info, please share it.

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68ragtop



Joined: 19 Nov 2013
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PostPosted: Tue Jun 02, 2015 6:13 pm    Post subject: wont' start Reply with quote

Been beating head against wall when we tested it on mon the guy that helped me knows motors in and out he was baffled with it. Talked with a pontiac buddy and he showed up today we pulled the valve cover #1 made sure valves where opening/closing. We then put #1 plug back in and tried it fired up put # 3 in same thing #5 in and the motor runs???????????? still wondering what little gremlin is gone for now. going to pull plugs and do compression check on all 8 and go from there
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78SAFARI



Joined: 07 Oct 2008
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PostPosted: Sat Apr 09, 2016 11:04 pm    Post subject: 400 Engine with Gremlins in it? Reply with quote

4-9-16 Did you get the engine problem figured out. I happened to be reading about this and was coming up with some of the same answers as the other fellow.
I believe the key here is, when you said that the engine ran O.K., you then swapped the 2 bbl intake for a tri-power? WHY?
Your engine was not built to handle heavy carburation. 70 psi on several cylinders? Way to low in any ones book. May as well have a 6 banger under the hood.
I would like to garner an answer here and see if I am right?
1) As I said the key is the intake swap. You applied a 1966 intake to a 1974 set of heads. Bad mistake. Did you take into account needed milling of the heads and/or the intake itself? The intake could be very warped or bowed.
2) Did you use a very good straight edge and place a light behind it while dragging that straight edge across the intake ports? I guarantee you that you would have seen light and probably a lot of it if you had done that.
3) Next thing did you place a trouble light with a 100Watt or larger bulb in it and place in the lifter valley after removing the valley cover? If not, mistake #3
4) Looking at the tri-power set up, do you have, in the center of the intake on both sides, an open area for the 'crossover' of the heated exhaust? Is it a 'round' or a 'D' port? Next, while looking at that crossover, on the intake - are there 2 what are called "cut thru's', like someone took a hack saw and cut either side of the crossover ports, on left and right sides? If so, then you do have a pre-1968 intake.
5) Is the intake solid cast or is it cast in such a way that you could drop a wrench from the top side and it would drop down onto the top of the lifter valley pan? If so, that is a 1968-1969 and newer intake.
6) Here we go - The next thing that is the most critical - Have any and all intake gasket material removed. The heads and intake surfaces must be completely clean. Carefully lay the intake down on the surface of the heads. Be sure you have 4 bolts installed. 2 in far front and 2 in far back. Do not tighten them. Just finger run them in about 3 turns, so that the intake will not slide around on you. Next, examine that crossover and see if there are any gaps on either side or the bottom or top of the heads.
7) I am sure that you will find a gap there. With the wrong gaskets, that is why you were getting exhaust pressure being pumped up through the wrong gasket and out it easiest route, where ever that may be. That is what you saw. Then you said that you covered the crossover hole(s)? You were in the right ballpark, but by blocking that crossover it will give you terrible warm up characteristics and will cause irregular pressures in the engine itself.
Cool If you can get yourself a quality set of intake gaskets, lay the right ones down on eh heads and compare for any areas that get covered up or, are left open when they should be covered. Then, do the same with the intake. Note any drastic changes in the coverage of the gaskets. Make notes then go back to heads and use a black marker, draw or make notes of areas really different from the notes you wrote of differences when laying the gaskets on each surface. I bet you will find that you have a major leak such as the intake is sitting to low and the gaskets are covering up some areas that may well have a major leak in it and you cannot see it as the intake surfaces may be to shallow or low, or too high up on the heads.
9) You may have the intake not sealing flat against the head and may very well be sitting on the bottom but the upper portion of the surface is wide open, or, the reverse. That intake MUST mate with the surface of the heads and also - align itself front to back on each intake and especially the crossover openings.
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