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79 620 Has Developed a Misfire


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At 35 PSI just about anything will make the compression jump up even a little oil. If you were at 150PSI the same oil would do very little. I'm thinking it may be the head. To break a ring you would have to really overheat it and drive it till it stopped. If there was a hole in the piston you'd be spraying oil and burning it. If the piston melted from detonation the plug would show it. If a piece broke off it would have hit the electrode and bent it.

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Before the miss began, there was a normal valve tick at idle, but nothing too extreme. Right before the miss began there would be a single pop from intake, if the truck had sat 2 days over the weekend not being started. When driven daily didn't pop. Now after all the lash adjustments quiet. It does idle, but with a miss. It will smoothen out a bit around 2500 - 3500 once moving. I'm going to pump air into the cylinder at TDC and listen for air leakage. Have to work today, so I'll post as soon as I attach an airline to it.

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Find another W58 head and throw new valve seals on. ($16) While springs are off chuck end in a electric drill and lap the valves quickly, assemble and put on your truck on a weekend. Gasket ($25) Keep old head and rebuild when ever.

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Cheaper that having one rebuilt and waiting 2 1/2 weeks without a daily driver. The W58 (although a totally fine head) is also called a 'smog' head. The only real difference between it and a U67 is the round exhaust ports with liners. It's not in demand.

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I see now that you have a '79 but with an A87. In that case get a U67. If you have the water cooled/warmed intake just drill two 3/8" holes and away you go.

 

 

 

L16%20exhaust%20005.jpg

 

This is my '76 710. It came with a U67 head and the co-joined intake and exhaust. It used hot exhaust to warm the intake to help evaporate the fuel. Problem is it warms but when it's really hot out there is no coolant at 180-F flowing through the intake to cool it either.....

 

So I tore it all off and put an L16 exhaust on and a later '79 L20B intake that has the water cooling in the runners. The U 67 head just needs two holes drilled to let the water out and a connection to the thermostat by pass hose.

 

Get the more common U67 and drill two holes and it should be basically the same (square exhaust) but larger ports and valves.

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I think it's a good head. I just didn't want to put a square manifold on the round ports.. so I found an A87 closed.

 

I've got plenty of other stuff to take to Bob.

H5WAGON & I are both running same head on square port exhaust manifold, no issues, he's been running that setup for couple years and he drives the shit out of it.

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H5WAGON & I are both running same head on square port exhaust manifold, no issues, he's been running that setup for couple years and he drives the shit out of it.

 

I've done it too. No issues.. but for flow I wanted to match em. That being said...... head I have now has small intake ports and I didn't port match. So.. I'm still no better off. 

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  • 1 month later...

Update, I finally took it apart and brought the head into the local machine shop, Wednesday last week. The shop just called, 4 Bent Exhaust Valves. Probably occurred when the timing chain was too loose. The only visible sign was a small impression in the top of cylinder 1, the same cylinder that had lost compression to only 35lbs. I wonder why only cylinder 1 had low compression though...

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Ask for the old valves back. To bend the valves they have to hit something (piston) and that would leave a mark. Intakes are much larger diameter and more likely to hit a piston than the smaller exhaust.

 

A loose chain would not change the number of links between the cam and the crank so the valves wouldn't open or close any later.

 

To hit a piston top maybe #1 had a loose valve clearance and the lash pad jumped out and jammed between rocker and spring retainer increasing the lift. 

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Thanks for your input Mike, I have already told them to save the bent valves, seems fishy to me. We'll see, supposed to have it ready on Thursday this week. Just relieved not to see a hole or cracked skirt  edge or crack in the head...

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