74 bus sudden slight loss of power, #4 valves both tight

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Steve
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Re: 74 bus sudden slight loss of power, #4 valves both tight

Post by Steve » Fri Sep 11, 2015 2:25 am

I have a few questions for the original poster. These are based on my feeling that the engine has been over-heated due to hard demand.

1. Is every piece of your cooling system in place?

2. Is your thermostat system functioning properly?

3. How many miles passed since a full tune up with dwell set and valves adjusted?

4. How often do you drive that way? Most of the time, or were you in a hurry that day?

5: During that drive, were you buffeted by headwinds constantly?

I am asking these questions because I feel (based on what I have read and not having wrenched on your bus) you may have suffered collapsing seats and/or stretched valves due to cylinder overheating. There may also be scorching or cutting around the valve seats.

I wonder if the curled or bent tang of the spark plug is a result of overheating. What do the other spark plugs look like? Spark plugs don't just get bent, and it is unlikely that it would be installed bent like that.

Replacing the now defective spark plug and readjusting the valves would probably cure the symptoms. But valves don't just get tight without cause. Your first warning was a loss of power. You noted that up until you changed out that plug, you still seemed to have the symptoms when under load. Isolating fuel/air and electrical ignition questions leaves you with mechanical questions.

Now would be a great time to establish a baseline of valve tip height so you can calculate valve creep for future considerations. Remove the rocker arm, and with a machinist scale or fine steel straight edge, lay it across from one far edge of the head to the other, along the edge that the valve cover seals. You will need to elevate this straight edge to clear the tips, use hardware for this on either end.

Center this straight edge over the valve tips, and one at a time stack your feeler gauge set between a valve tip and the straight edge. Record measurement and do it again to eliminate sloppiness. Do each valve.

This is a tedious, tiring process on your forearms and wrists, but I have found it to be about as accurate an indicator as you will get without pulling the heads off in a shop environment.

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Amskeptic
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Re: 74 bus sudden slight loss of power, #4 valves both tight

Post by Amskeptic » Fri Sep 11, 2015 4:25 am

Steve wrote:I have a few questions for the original poster.

Now would be a great time to establish a baseline of valve tip height so you can calculate valve creep for future considerations.
This is a tedious, tiring process on your forearms and wrists,
We have an easier method in IACLand. Record any valve adjustments that are "off". If any valve is off in the same direction three adjustments in a row, we call that a "trend" that must be monitored.

Steve, you can spot valve "creep" right at the adjustment screws. If all screws on one side of the engine are all sticking out four threads past the locknut, for example, we can blame the architecture of the engine build. If all screws are two threads past the locknuts, but there is one valve with four threads sticking out, we call that evidence of possible recession.

My Type 1 engine now has two exhaust valves at one turn further out than the intakes. I have monitored since the day I installed the heads.
My original build Type 4 engine at 110,000 miles has an intake that is sticking out, but that is typical of German 2.0 heads where they just did not put enough aluminum around the intake ports.

Mitch . . . drive nice, now.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

Steve
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Re: 74 bus sudden slight loss of power, #4 valves both tight

Post by Steve » Fri Sep 11, 2015 6:13 am

I like your idea due to its convenience and simplicity. The process I described allows for actual measuring using feeler gauges, yet that is most likely not all that important unless you are doing critical measuring/trend. The important thing is to be able to register change, which your method allows.

Were one to change out peened screws to new ones, the change in thread count between the worn set and the replacement set would also need to be calculated and recorded. Your method would require keeping a matched set installed, which is typical anyway. My method allows for a field examination and recording of valve height differences should you be shopping junkyards or swap meets looking for potentially useable heads that have no rockers.

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