Compression Testing 77 MarkIV 454

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Either way will result in the same reading. The engine should be relatively warm to get correct readings (pistons and rings swell a bit with the heat and will result in higher compression readings).

Should be 150 psi, with each cylinder within 20 psi of all others. So, you're within spec, but the 137.5 cylinder are definitely costing you some horsies.

Valve guide seals are probably seeping a bit. For each cylinder, run the test dry. Using an oil can give each cylinder 2-3 squirts of oil, then repeat the test. If compression goes up, rings are worn.

If you had a head gasket leaking, you'd have one or more cylinders REALLY low (like 80 psi) and would likely have overheating problems, loss of coolant, etc.

Sounds more like a carb problem than anything else. If she needs a rebuild or adjustment, might be running slightly rich, causing the poor fuel economy and loss of power.

Doc

I think these are all pretty low. Like 180-190PSI would be my best WAG

My instructions say to take them all out, and to record where they came from. Just to see which may or maynot be fouling. Don't know if it makes a difference though.

Sorry, no idea.

Threads just may mean you have oil all over the outside of the engine. My 88 S10 covers the plugs in oil but they have nice burn patterns. Black = too rich/burning oil. Black/wet = too rich. Dark/Light tan evenly distributed = optimal. White or perfectly clean = too hot.

Yes. Drop 1 table spoon of some thick oil (80/90 hypoid grease) into the cylendar, and crank. THEN attach the compression tester - this distributes the oil and also prevents oil getting into the tester. Thats the test for rings. The test for seals/guides is to drive it. If when you first stop on the gas it blows blue smoke, and then after a bit it clears up, it's your seals. When the valves are cooler it will alow them to leak, and when they warm up they will expand and close the seal.

Take these as suggestions - I'm just a backyard warrior.

~KJ~

You should take all the plugs out and disable the ignition. You want the engine to spin at a consistent rate.

I agree with Doc, it sounds like a carb problem. I would also check the mechanical and vacuum advance.

Al

My guess would be along with Doc"s: Carbureator, mainly power valve if it's a Q-Jet.

Also, if it's been running on the rich side for a while, have a look at the EGR vale for the excessive power loss. It might be stuck open a bit.

I hope this helps!

Refinish King

================= Power valve in a Q-jet? Its been a while, but I dont recall that.

Stuck open EGR would cause bad idle....

It is possible the EGR is opening too much if its a feedback type, but not stuck, I would guess a carb problem too. But it really sounds like the poor thing is just getting old.....I dont know what the compressoin should be but it seems it should be higher than 130.....Have you givin the thing a major tune-up lately?

I have factory heavy duty emissions setup and therefore have no EGR valve. In fact all that I have for emissions is thermal vacuum switch/vacuum delay valve(which is bypassed due to broken part, unable to find replacement) a PCV valve and crankcase rebreather.

As far as my carb it is a Rochestor QuadraJet 4MV. I see in the factory manual illustration a power piston. Is this the so called power valve or is the power valve associated with this piston. Would problems with this part be remedied with a carb rebuild? I am a bit suspicious of my accelerator pump as a little gas seems to be leaking from the rod hole when I push down on it. That would be remedied in the rebuild for sure. Any suggestions as to who makes good quality QJET rebuild kits?

Thanks to you, Doc, Scott, Big Al and kj for your suggestions. It is much appreciated. Bob

switch/vacuum

Don't rebuild it; buy the Edelbrock Q-Jet replacement.

Doc

Get a professional rebuild and recalibration on the quadrajet and distributor and you should be happy with the truck for many more years.

I have a 1977 G-30 van with the HD emissions 400 CID engine with the 4MV quadrajet and little emission controls. I rebuilt the engine about 3 years ago and it had been losing power in 2003. WOT seemed to give less power after a downshift to 2nd than to pull in 3rd. After the 2nd gear WOT upshift to 3rd gear, with lower RPMs the engine ran stronger. In October, I took it in for a carb. & distributor overhaul and dyno tuneup. They found the vacuum rods in the carb had dropped out and the bushings in the distributor were worn. After rebuilds and calibration for 14.7:1 air/fuel at idle and cruise and 12.7:1 air/fuel at WOT the van runs better than it has in the last 15 years. Suprisingly, the Calif. emissions tests have always showed a clean running engine that passed on the inital test. I was glad I spent the $930 for the work at a shop that does street and drag racers.

Sadly, 2 weeks ago an 83 year old lady in a Volvo 240 sedan ran a red light in front of me and I demolished the trunk area of her car and bent the right. front of the van. The first body damage in the 27 years I owned the van. Now, Farmers Ins. is pretending that the $7,700 I have spent restoring the van over the past 3 years doesn't make it worth any more than about $2,200 and my repair bill is $4,000. I want the van repaired because it has been my fun truck for 27 years and I have yet to find a new truck that seems a more comfortable and fun ride.

switch/vacuum

Sorry to hear about your van. I am going to check with my insurance company to find out what their policy is on vehicles that are undergoing/have been restored.. The metropolitan Phoenix area is known as the red light running capital of of the US. It is common to see at least one car run a red light half the time at intersections. In left hand turn lanes there are usually 2 to 4 cars that run the red. Idiots...

As far as my Suburban goes I want to restore it but at the current time don't have alot to spend on it. I've never rebuilt a carb before and from what i hear the Rochester four barrels are a bit complicated to do right. A carb rebuild kit runs about 30~35 bucks but I don't know what parts they don't contain that may be bad. Floats aren't that expensive but I don't know what other parts that might be worn out are not contained in the kits. A new Edelbrock #1902 Q-Jet (4MV replacement) runs around $450. I've heard some people say they are bolt on and go. Seems to me there would be some adjustments I would have to make to set it up properly for my engine.

Did you see bobble in your timing mark above 2000 rpm before the distributor overhaul? Did this go away after you had the distributor bearings replacement. My timing marks jump around in a band of about 1/4 inch above 2000 rpm. Did they set up the centrifugal weights and springs?

Can you tell me what you had done in the engine overhaul, what they repaced and what the total cost was? If my problems end up requiring a bottom end overhaul that may force me to put the restoration on hold. That should be answered by a compression wet test. I'm wondering if I might have a valve(s) hanging when when the engine gets fully warmed up. Sometimes when warm, the exhaust note seems to change a bit (sounds sort of muted) and the performance drops off. Usually happens after moderate acceleration. Almost sounds/feels like it is not breathing correctly. I might be able to afford overhauling the heads depending on what they find.

Good luck on getting you van fixed. Bob

I had the same thing happen to my 88' C1500, two weeks after I finished a full restoration. They wanted to give me $2,000 and total it out, damage was $4500. I fought with them for two months, had the truck privately appraised by three GM dealerships, sent them copies of EVERY receipt I had from restoration parts, found three similar vehicles in my area in similar shape selling for over $5,000 (sent them the ads), and finally they gave up, sent me a check for $4500 and left the title clean.

It was a huge PITA, but well worth it. Took two months of daily harassment, but in the end I won. Don't give up; beat them at their own game.

Doc

When you have a classic:

Or something you restored, get a stated value insurance policy. Then your insurance company has the appraisal right and pays. Then subrogates against the at fault party.

There are many companies that independant agents can offer, such as: Condan & Scully and many more.

Geico also has stated Value policies available, so check into it, and the cost is surprisingly comprable to the standard market rate.

Refinish King

I would advise against this. A factory Q-jet is calibrated precisely for the engine it's on. No way can an aftermarket carb give you the same drivability and economy. It has to be obvious that there is a reason for the hundreds of factory carb numbers.

Al

The Edelbrocvk Q-Jet replacements are APPLICATION SPECIFIC, meaning they are calibrated the same way the various Quadrajunk's were right at the factory. Edelbrock (and Holley for that matter) has many, many different models for Q-Jet replacement based on year of original and CID of the motor. If you actually looked into the A/M replacements before slamming them, you'd already know this.

Doc