Compression Testing 77 Mark IV 454 Revisted

Well I finally think I've done the compression testing on my 77 Suburban's engine properly. Earlier testing was done improperly (was not
even opening the secondary throttle plate). This time I blocked open both plates in the secondary, pulled out all the plugs, and started pulling the plugs etc with the engine as hot as I could. Wet readings done with two teaspoons of 20 weight oil and five seconds cranking to distribute. Cranked four compression strokes for each reading. Each average below is the result of three readings. So here are the results:
Driver side bank (starting with cylinder closest to radiator)
Dry Wet Difference 1 170.3 178.3 (8.0) 2 173.3 183.0 (9.7) 3 174.3 186.3 (12.0) 4 171.3 179.3 (8.0)
Passenger side bank (starting with cylinder closest to radiator)
Dry Wet Difference 1 168.0 180.7 (12.7) 2 169.7 185.3 (15.7) 3 170.7 181.3 (11.3) 4 171.3 179.7 (8.3)
My 1977 (150k miles) Suburban's power has been steadily declining for the past 8 months. It feels like it's going through 6 inches of mud and it gets about 5 to 6 miles a gallon(used to be 10 to 12). It has a distinct chugging at lower to mid rpms. It has new AC Delco Rapidfire #2 plugs, new dist cap & rotor. Plugs seem to be pretty clean with a nice medium tan but have oil on threads. Found valve cover bolts 1/4 to 1/2 turn loose and tightened them but I don't see how this could get oil on the plug threads unless it drips on during plug removal. I guess I will know next time I pull the plugs.
Today it will get new premium spark plug wires. Defective vacuum advance and transmission vacuum modulator were replaced and helped but did not stop the decline of power. Both vacuum and mechanical advance are correct acording to factory specs although I have not checked the curves. Carb was rebuilt about 50,000 miles ago so that probably needs attention. It has new dual Flowmaster 40 Delta Flows with 2.5 inch all the way. I installed a vacuum guage and these are the readings I get when it is at normal operating temperature:
idling - 16 - 17 inches light acceleration - 10 to 15 inches moderate acceleration - 5 to 10 inches hard acceleration - 0 to 5 inches cruising steady 45 mph - 20 inches decceleration - 20 to 22 inches
A number of people have recommended replacing the carburetor with a Edelbrock Q-Jet (my model would be the 1902, about $460) or rebuilding the Rochester 4MV. Whereas I'd love to get a new Edelbrock and be done with it I have to have a good level of confidence that it is at least the main contributor to the problem and the rest of my engine is in decent shape.
Is there anything else I should check/eliminate before I take the plunge for the new Edelbrock or rebuild the Rochester 4MV. Are my compression and vac readings decent. Thanks to all the people who have given their advice. Any help would be most appreciated as this beast is sucking me dry at the gas pump (Phoenix 195/gallon regular).
Thanks, Bob
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Your compression is fine, replace the carb.
Doc

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Before buying that carburetor you might want to look at the valve train. Cam, gears and chain. A severally worn cam can cause a big decline in power, so you'll have your foot in it just get it going. That could be the gas usage, as you say the plugs are a nice tan color I would doubt that the carburetor could be that bad. Especially if it was rebuilt correctly 50,000 miles ago.
Brian

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Interesting you should bring that up! I talked to a guy at Basko Engine Service, a local place. He said the compression was plenty good but the oil on my spark plug threads indicate that my valve seals and possibly guides had probably worn out and perhaps the oil control rings were leaking. But as far as the mileage and power loss goes he thinks it may be my timing set. "With 150k miles on it I can guarantee your chain is loose" He said I should turn the damper back and forth and have someone watch the distributor cap. There should be no more than 2 degrees of slop between the two. So tomorrow I will check it out. When they are new there should be zero slop.
I don't know if the teeth in the timing sprockets are anything like what was in my 73 Nova but if so I bet there is lots of slop. My Nova had the chain jump at 70k miles several years ago and took some valves and guides with it and tattooed a few pistons. When I had it fixed the guy told me the heat out here in the desert kills the sprockets which were made of some sort of nylon or delrin. He said they went to that to reduce engine noise. Hmm little extra noise vs $700, nice call Chevy!
His second guess was the carburetor. I will do something about that anyway as it has a fair amount of varnish stains (bad seals) on it and the accelerator pump piston has some blowby. Rod's Carburetor here in town wants $165 to rebuild it no matter what parts need to be replaced. Depending on if the timing set is the main culprit and how much the set costs I will feel a little better about spending some money on rebuilding the 4MV or buying a new Edelbrock Q-Jet. I just can't afford to drop $460 on a Edelbrock Q-Jet and then wind up having to have the top and or bottom end rebuilt. The ole budget just won't handle that at this time.
Well thanks for your input, Brian. Tomorrow should be interesting. Bob

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The results are in. The timing chain slop is 10 to 11 degrees. Thats a large amount acording to the people at Basko. I am thinking about an Edelbrock Performer-Link Tru Rolling Timing Set which he said was an excellent set. They make two sets for my engine one with a single keyway and another with 3 keyways. The three keyways allows one to set up the cam for +4, 0 or -4 degrees of advance. The guy at Basko says I should go with the three keyway set and the advanced setting. He says that will give better performance but I don't know if thats legal for a street machine. I guess I better get it right or I will have to pull it out. Anyone have any experience with the different settings? Any suggestions would be appreciated.
Thanks, Bob

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Robert, I'd be careful with changing the cams timing with the keyways. When you advance the cam timing the engine will have more low end power, but will loose a little top end power. And the reverse conditions for retarding the cam timing. Also a double roller chain will stretch just as much or more than a standard steel chain and gears. They are very resistant to breaking but will wear at the sprockets faster than an OEM style steel gear set.
I've built a lot of Chevy motors in my time and always use the OEM steel gears on motors that I want to stay together a long time or will run on the street. On a motor built for speed and racing the engine isn't expected to live as long as a street driven motor. Hope This Helps Brian

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Hello Brian,
I thought the Edelbrock true rolling sets were supposed to have almost no stretch and signifcantly better life? Is that mostly Edelbrock hype? Are the OEM chains of the non-roller bearing type? I guess the OEM timing set would be a dealer only item?
As far as running the advance setting goes I wouldn't mind having more power on the low end as I usually keep my rpm below 3500 and most of my cruising about town is 0 to 55mph (2200 rpm). Then again the 454 already had pretty good low end when running right. I wish I could try both settings but I don't think I want to do the whole job two or three times. I have seen some warnings about checking valve piston clearances before running in the advanced setting. Hmm don't know how thats done but sounds like a hassle. Decisions, Decisions...
It will be interesting to see how much of the power comes back either at standard setting or advanced. I guess as the sprocket/chain wear the cam lags behind so that would tend to retard the timing. It seems like 11 or 15 (assuming I go advanced) degrees might make a considerable difference?
Thank You for your inputs, Bob

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True they won't stretch under heavy loads, they just aren't designed to go 100,000 miles like OEM style. Otherwise Chevy would put them in the Corvette or SS pickups if they lasted as long as the OEM style.
You can get the OEM style at any auto parts store. I prefer Cloyes as they are the manufacturer for the original components for Ford, Chevy and Chrysler. http://www.cloyes.com /

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