Trouble tuning 340.

-

cmcknight

Member
Joined
Feb 28, 2015
Messages
24
Reaction score
10
Location
massillon, oh.
Hello all. I'm having issues getting this motor tuned up. 1971 340 small block with 360 cast iron heads. Heads have original style rocker arms with adjusting nut on top. I'm not sure what kind of cam is in the motor, but it has been changed. Motor has solid lifters. I lashed the valves cold at 24/26 after reading through some of these threads. I'm only getting about 110 psi compression on each cylinder, which I think is very low. The difference in compression between the cylinders is minimal. I have to increase the lash to an unreasonable amount to get about 130 psi compression (I'm talking like almost 1/16"). Vacuum is about 14 or 15 in Hg. Should I run the valves this loose to get better compression? Seems like it would beat everything up. Not sure if it's relevant, but timing is 19 degrees advanced. First time ever doing this *rookie*. Any help is appreciated.
 
Lash specs on that cam are probably something on the order of .018 and .022. I wouldn't run them very loose trying to get cylinder pressure out of it. Was the cam degreed? If not, you can probably pick up some cranking pressure by doing that, but there are lots of other things you can do before you go that far to get it running better. What kind of timing curve do you have on it?
 
Lash specs on that cam are probably something on the order of .018 and .022. I wouldn't run them very loose trying to get cylinder pressure out of it. Was the cam degreed? If not, you can probably pick up some cranking pressure by doing that, but there are lots of other things you can do before you go that far to get it running better. What kind of timing curve do you have on it?
If I adjust the lash to 18/22, I'm only getting about 100 psi compression. Not sure about the cam.
 
I don't see increasing lash affecting cylinder pressure.
 
If I adjust the lash to 18/22, I'm only getting about 100 psi compression. Not sure about the cam.

That's probably about right. One more time. Follow along. You do not adjust valves trying to affect cylinder pressure.
 
I don't see increasing lash affecting cylinder pressure.

Absolutely it can. But to use it trying to make a big difference in cylinder pressure.....especially adjusting clearance to very wide specs ain't the best way to go about it.

Of course here we are again on a thread trying to help someone whose given the least amount info as possible. When will we learn?
 
Year's ago I had a mechanical cam with hydraulic lifters and had some similar results that the OP has.
Put a hydraulic cam in and all was good.
 
Absolutely it can. But to use it trying to make a big difference in cylinder pressure.....especially adjusting clearance to very wide specs ain't the best way to go about it.

Of course here we are again on a thread trying to help someone whose given the least amount info as possible. When will we learn?
Maybe a couple pounds, but not 30 like the OP is stating.
 
Absolutely it can. But to use it trying to make a big difference in cylinder pressure.....especially adjusting clearance to very wide specs ain't the best way to go about it.

Of course here we are again on a thread trying to help someone whose given the least amount info as possible. When will we learn?
While I appreciate your help, you are under no obligation to help me. I stated everything that I know in my original post. I am a young guy who is doing this for the first time. I tried setting my valves with preload as if they were hydraulic lifters, but got zero compression (car wouldn't even start). I've looked to this forum for reference many times but hesitate to ask for help because I don't have the knowledge that you guys do.
 
I’m going to ask a few questions to try to help out. There is no judgement here but asking may help us seasoned veterans get the info we need to give you that “ah ha” moment. And get this engine fixed.

1) how many miles on the 340?
2) have you adjusted valves before with success?
3) can you remove the intake and send us a picture of the top of the lifters.
4) what else do you know about this engine.
Let’s start there. Syleng1
 
I’m going to ask a few questions to try to help out. There is no judgement here but asking may help us seasoned veterans get the info we need to give you that “ah ha” moment. And get this engine fixed.

1) how many miles on the 340?
2) have you adjusted valves before with success?
3) can you remove the intake and send us a picture of the top of the lifters.
4) what else do you know about this engine.
Let’s start there. Syleng1
 
Thank you. I'm not sure how many miles are on the 340. From what I am told, the engine was possibly rebuilt about 15 years ago. The motor was pulled out of the car when I received it but the car was running ok prior to this. The valve play seemed pretty loose when I first looked at the valvetrain before I did any adjustments. My dad thought the motor had hydraulic lifters and told me to adjust them no lash plus a quarter turn. I did this and motor wouldn't run. He then told me that the motor probably had solid lifters. (He lives 1,000 miles away). I adjusted the valves cold 24/26 and car runs but not as good as it should. I don't know if it is supposed to be this way or not, but my reality is that the more I tighten the adjusting nut on the rocker arm, the less compression I get in cylinders. I can't provide any photos of the motor with the intake removed.
 
While I appreciate your help, you are under no obligation to help me. I stated everything that I know in my original post. I am a young guy who is doing this for the first time. I tried setting my valves with preload as if they were hydraulic lifters, but got zero compression (car wouldn't even start). I've looked to this forum for reference many times but hesitate to ask for help because I don't have the knowledge that you guys do.

Well throw us a bone bud. What does the engine sound like? Does it have a smooth idle like stock? Does it have a thump to it? Everybody starts somewhere. But we caint read minds. Tell us everything you know even if it's layman's terms.
 
Thank you. I'm not sure how many miles are on the 340. From what I am told, the engine was possibly rebuilt about 15 years ago. The motor was pulled out of the car when I received it but the car was running ok prior to this. The valve play seemed pretty loose when I first looked at the valvetrain before I did any adjustments. My dad thought the motor had hydraulic lifters and told me to adjust them no lash plus a quarter turn. I did this and motor wouldn't run. He then told me that the motor probably had solid lifters. (He lives 1,000 miles away). I adjusted the valves cold 24/26 and car runs but not as good as it should. I don't know if it is supposed to be this way or not, but my reality is that the more I tighten the adjusting nut on the rocker arm, the less compression I get in cylinders. I can't provide any photos of the motor with the intake removed.

Yeah, you kinda can. Get the camera in there between a couple of intake runners with the valve cover off. Shine a light in there and you will light up the intake valley and a few lifters. You can see them right through the head.
 
The guys responding here have hundreds of years of experience. Post some pics of the valve train, engine compartment, distributor.

Do you have a timing light, and know how to use it?

I’ll tell ya, the 318 in some broke junk 73 Dart swinger I got a few months ago just got started for the first time since I got it. It sounding like **** turning over, and would only sputter and pop a little, the distributor was way retarded and in 180 out ( still is, #1 is facing the fire wall, changed the wire order.) after messing with it all after noon and wiring in a MSD box, all hot wired, it runs, like ****....:lol:.

So the point is, be a detailed as you can in describing your issue and post some pics...
 
Last edited:
Here’s a pic of junk

E3773064-FE69-4824-B0F1-4C3455516BCC.png
 
The guys responding here have hundreds of years of experience. Post some pics of the valve train, engine compartment, distributor.

Do you have a timing light, and know how to use it?

I'm pretty confident that I'm close with the timing (rotor at #1, no backfire). Car starts and runs but idles rough when put into gear. Car ran good before exhaust was installed. I was told that I would need to re-adjust things after exhaust was put on. After reviewing threads about compression, I thought mine was too low. My vacuum also is low. Will post pics tomorrow when it is light out.

I’ll tell ya, the 318 in some broke junk 73 Dart swinger I got a few months ago just got started for the first time since I got it. It sounding like **** turning over, and would only sputter and pop a little, the distributor was way retarded and in 180 out ( still is, #1 is facing the fire wall, changed the wire order.) after messing with it all after noon and wiring in a MSD box, all hot wired, it runs, like ****....:lol:.

So the point is, be a detailed as you can in describing your issue and post some pics...
 
The guys responding here have hundreds of years of experience. Post some pics of the valve train, engine compartment, distributor.

Do you have a timing light, and know how to use it?

I’ll tell ya, the 318 in some broke junk 73 Dart swinger I got a few months ago just got started for the first time since I got it. It sounding like **** turning over, and would only sputter and pop a little, the distributor was way retarded and in 180 out ( still is, #1 is facing the fire wall, changed the wire order.) after messing with it all after noon and wiring in a MSD box, all hot wired, it runs, like ****....:lol:.

So the point is, be a detailed as you can in describing your issue and post some pics...
The guys responding here have hundreds of years of experience. Post some pics of the valve train, engine compartment, distributor.

Do you have a timing light, and know how to use it?

I’ll tell ya, the 318 in some broke junk 73 Dart swinger I got a few months ago just got started for the first time since I got it. It sounding like **** turning over, and would only sputter and pop a little, the distributor was way retarded and in 180 out ( still is, #1 is facing the fire wall, changed the wire order.) after messing with it all after noon and wiring in a MSD box, all hot wired, it runs, like ****....:lol:.

So the point is, be a detailed as you can in describing your issue and post some pics...
Sorry, I accidentally posted my reply in the middle of your previous comment.
 
Maybe the cam timing is off ! If someone put another cam in, and didn’t degree it in properly or at all, that can cause the compression to be low on all cylinders ! Or maybe put the timing chain and gear on wrong... Which would also cause the cam timing to be off ! I don’t know ! Just throwing my 2 cents in !
 
Here is OP’s response:


I'm pretty confident that I'm close with the timing (rotor at #1, no backfire). Car starts and runs but idles rough when put into gear. Car ran good before exhaust was installed. I was told that I would need to re-adjust things after exhaust was put on. After reviewing threads about compression, I thought mine was too low. My vacuum also is low. Will post pics tomorrow when it is light out.
 
If your car has hydros, you should be able to crank adjuster to zero (finger tight on the pushrod) then turn it even more and the valve will not move as there is .060 preload minimum on those hydros with no oil pressure. Solids will move once snugged. Check TDC on balancer to #1 rotor phasin and plug wire order. Simple to swap a pair of wires when your rigging it up or just drop the drive in 1 tooth off. Even some solids will have a snap ring so thats not 100% unless you see no snap ring at all, thats a solid lifter for sure.

Well now, we got more info. If it idles poorly in gear, your mix is off. If you installed headers, its gonna lean it out, if you went back to manifolds your probably fat. IF you got an exhaust leak (318 manifolds on a 340 head) its gonna run like crap and sound terrible. Same as a vacuum leak, may run but youll be chasing the idle. Start spraying the manifold gaskets and carb with water from a spray bottle and listen for the idle to change.
 
Hello all. I'm having issues getting this motor tuned up. 1971 340 small block with 360 cast iron heads. Heads have original style rocker arms with adjusting nut on top. I'm not sure what kind of cam is in the motor, but it has been changed. Motor has solid lifters. I lashed the valves cold at 24/26 after reading through some of these threads. I'm only getting about 110 psi compression on each cylinder, which I think is very low. The difference in compression between the cylinders is minimal. I have to increase the lash to an unreasonable amount to get about 130 psi compression (I'm talking like almost 1/16"). Vacuum is about 14 or 15 in Hg. Should I run the valves this loose to get better compression? Seems like it would beat everything up. Not sure if it's relevant, but timing is 19 degrees advanced. First time ever doing this *rookie*. Any help is appreciated.
Those heads are known to be 72cc.

No wonder its low.
 
-
Back
Top