GM V8 (250HP, 350ci/5.7l, Carb) troubles

Guys,
this may not be too well suited in a "cars" forum, but I assume this is where most "people who know" gather. We have the above engine in our
soring winch (what we use to pull our gliders to an initial altitude of somewhere between 300 and 600m, depending on the wind), but it has recently started to give us trouble. Basically, it reproducibly dies when power is alpplied and there's actually some load. It will also misfire at times. I'm not the engine guy at the flying club, but I thought I'd see if I can find anyone here who could give us a tip. The guys have basically taken the thing apart and put it back together. They have even changed some bits of the valve drive (those elements that are supposed to correct the valve tolerance, don't ask me for the English name, I know the German word for it: Hydrostssel). The engine runs fine for the first few take-offs and then starts to become rougher and rougher and eventually will die upon applying power. Again, it's a 350ci/5.7l GM V8 with a carburettor and roughly 250HP and no electronic ignition system.
Thanks in advance
pj
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Again, it's a 350ci/5.7l GM V8 with a carburettor and roughly 250HP and no electronic ignition system.
Thanks in advance
pj
Hydrostssel is most likely 'hydraulic valve lifter'. (And the word is 'soaring', rather than 'soring'. Soaring is 'flying'. Soring, if we used that word very much, might describe how you feel if you stop soaring, and crash)
Since this happens when the engine has been running for a while, I will assume it has something to do with the heat generated. Heat can cause engine loss of power and roughness: (1) If you begin to evaporate the gasoline in the fuel lines or carburetor. We call this 'vapor lock'. Fuel injected systems seldom if ever experience this since the high pressures tend to keep the fuel liquid. With a carburetor, the liquid pressure is less, and if the fuel boils in the line or carburetor, the engine will lose power or stop. The fuel can even boil out of the carburetor bowl, stopping the engine. If this is a problem, you may have to rebuild or adjust the carburetor , cool the fuel lines, or increase the delivery pressure.
(2) If you have an electrical part or system which malfunctions when hot. You have a very minimal electrical system here, so I would suspect any electrical failure would have to be in the ignition system....coil, capacitor, ignition control module, sensor...whatever you have installed on that engine.
(3) If you lose oil pressure, and the hydraulic valve lifters cease functioning as they should, you can experience a loss of power, clattering of the valves, etc.
(4) On some of the older engines, the ignition advance curve was controlled by vacuum from the manifold and by mechanical weights. The weights seldom (but occasionally) gave problems. The vacuum advance system required that the vacuum diaphragm was working properly and that you were connected to the correct source of manifold vacuum. Sometimes it was at the base of the carburetor, and sometimes there were more than one ports which were selected by a thermostatically controlled valve. Be sure you understand the ignition system you have and that it is working to specification under operating conditions.
These some of the things that have come into my mind. Maybe others will recognize the problem exactly and give a better answer.
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IIRC, I've seen similar malfunctions in the 350-Chevy's when the camshaft is worn out--it would seemingly 'pop back' when the engine was put under load, yet run quite smoothly at idle or under part throttle. Hope you find the problem without anyone being airborne! s
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hmm.... well worth looking at. If the engine dies when we're up there, we have no issues, it's only getting there ;-) We don't really want it to fail while we're taking off.
Thanks for the answers so far.
pj
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As I remember, if you had any worn lifters you had to replace the cam and all the lifters to really fix these engines.
New lifters on an old cam quickly wore out any good lobes that might be left.
As suggested, I would look at the fuel system including the pump.
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hehe, thanks, for the words. Yes, indeed it should be soaring, I was probably typing away too fast with my head in the clouds ;-)
Hmm.... Heat would probably not really be the problem because the engine is usually only run for a short while (about two minutes, one warming up, half a minute for the tow itself and half a minute to retract the cable and wind down) with pauses as long as an hour (today) between. Plus it was a rather cool day today. Water temp was around 60C. Item three was one that I tought about as the engine guys said that the valve tolerance (Vetilspiel in German) would actually increase over the course of the day. The overall oil preassure seems to be ok, is there any chance that just the hydraulic valve lifters don't get enough oil pressure? Also, Item four is one that I'd ask the guys to look after.
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Item three was one that I tought about as the engine guys said that the valve tolerance (Vetilspiel in German) would actually increase over the course of the day. The overall oil preassure seems to be ok, is there any chance that just the hydraulic valve lifters don't get enough oil pressure? Also, Item four is one that I'd ask the guys to look after.
If you have hydraulic valve lifters, they must be properly adjusted. Now, there is a range of opinion on what 'properly' really means. I have found that some of the techniques given in the books don't always work out right.
The following is what I do: - I get the valves adjusted well enough so that the engine starts and runs.
- Then, with the engine running, I adjust each valve by loosening the adjustment until the engine starts to miss. I mark that point. (This mark is where the valve fails to open at all.)
- Then I tighten the adjustment until the engine again starts to miss. (This is the point that the valve starts to stay open all the time) I mark that point.
- Next I adjust the valve dead center between the two marks. You can clearly see the logic.
I may go around the engine several times, improving and checking the adjustment.
Sometimes, when I have tried to use the instructions from the manuals, they suggest adjusting to a certain point, and then tightening or loosening a certain number of turns. I find that such a method may put me into a really bad performance range.
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With all due respect....no, I don't see any logic in what you are doing. In fact, your method is inaccurate. You are measuring the half way distance between the lifter being completely bottomed out, and some other distance which is past where the lifter is sitting at the top of it's bore. That's not correct.
If you "really" want to adjust valves while the engine is running, there is no need to back the adjustment off until the valve doesn't open. You only need to back off the rocker arm until you hear ticking (you know now that you have "more" then zero back lash), then adjust it back until it goes quiet. Now you are at zero lash, ie: the lifter is sitting at the top of it's bore. The whole idea with hydraulic lifters is to center it in it's bore so that it can adjust "both ways". Most hydraulic lifters (GM) use two turns of the adjusting nut to go from top to the bottom of the bore. So 1 turn would center the lifter.
If you understand what the adjustment is actually doing, it makes it easier to do the adjustment properly.
I've used the static method of adjusting valves and it's "always" worked. Adjusting valves while the engine is running was a messy way to do it, and the static method works perfectly.
Ian
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Well, I respect your abilities immensely, Ian.
But I have not always found the static method alone to work.
The method I use has always worked for me, but I have made mistakes before and this could be one of them.
I alway adjust the valves statically to get the thing to run, and then readjust them after it is running and warmed up.
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