|10-07-2012 09:21 AM|
The fact that the compression is low is not as concerning as how much it varies. If you bought this rebuilt I would ask the builder to explain this. Have you put a vacuum gauge on it. If the needle bounces it indicates tight valves. If you are absolutely sure the valves are not to tight it would apear you have a more serious issue. A smog engine should be 110-117 compression. Most were in the 7.5 to 8:1 compressing area so that would put you in the 110-117 at sea level. You have some cylinders in the proper range. I would concentrate on the low ones if you are working on valve adjustment.
once you are sure the lower end is healthy, a set of vortec heads and a good intake would probably make a significant difference.
Do you have casting numbers for the heads
|10-07-2012 05:41 AM|
There are plenty of hei's on boats. Like everything else on a boat, it will have the word marine in the title. So get a "marine hei".
Newly rebuild 79 350 engine will still have crappy heads, low cr, and a tiny cam if rebuilt to stock specs. 150hp, maybe. So, don't expect much power unless you change some stuff.
|10-06-2012 10:17 AM|
|10-06-2012 09:09 AM|
It absolutely Must have ignition protected electrical components,...
A GM Hei, Ain't...
'n, it's supposedly a new Rebuilt motor...
|10-06-2012 05:44 AM|
A 79 350 from an impala was rated about 150hp (maybe lower). Low compression, very poor heads, and a tiny cam.
Get some 64cc performance street heads and a mild RV type cam and you can double the power.
Yes, retarded cam timing can lower the values obtained during a compression test. What timing chain set was in the motor?
Did you check compression with the throttle open? Makes a difference.
For a low compression smog engine, I would set up the timing by revving the engine to 4000 rpm and set max timing to 40 degrees. Rev it up, check the max timing, if it only makes it to 32, then add 8 more to the initial, and check it again.
Get a HEI distributor, please.
Are you sure you have a 350 and not a 305?
The engine could have been sitting a long time and not stored correctly which could have messed up the cylinders and/or rings.
|10-06-2012 04:49 AM|
Repump up the lifters, as described over at iboats, 'n try again...
Yer tryin' to remove the valve lash, as in Up/Down motion...
If ya tighten 'em, til the rotational motion stops, they're Too Tight...
|10-06-2012 12:15 AM|
|gfackrell||Ha ha! I know For a fact that i paid 2000 bucks for a rebuilt engine! I know it showed up looking all pretty on the outside. Beyond that, I'm at the mercy of the shop that sold it to me. I'm kicking myself for not using my local trusted machine shop and assembling it myself. You guys have given me some good info. I'll do some more thorough compression testing wet and dry tomorrow and post the results.|
|10-06-2012 12:00 AM|
An engine w/retarded cam timing might have lower than normal compression if the amount of retard was enough- but you'd also expect the compression to be a lot closer from cylinder to cylinder on a fresh engine.
But then you say there's no increase in compression w/oil added to the cylinder. The oil should be shot to the top of the cylinder to get the best result. If the oil was sufficient for testing (a couple teaspoons worth is enough) and no increase was seen, this points to a loss of compression due to valve seal.
A leakdown test is the next thing I would do. If you have the ability to pressurize a cylinder at TDC compression stroke, you can listen for a loss of pressure at the carb and/or exhaust (would be valves), or if you hear hissing through the valve cover or breather you could blame the ring seal. This isn't as good as a leakdown test but can point to a problem if it's present.
|10-05-2012 11:42 PM|
|10-05-2012 10:54 PM|
I think it can be done by getting the cam specs and checking it with a dial indicator while verifing the piston is at top dead center. However I suspect the issue is elsewhere. For your compression ratio figures to be varied like that you either didn't perform the test correctly, or it's got a bore or ring issue or it's got some partially burned valves. You said the engine has been overhauled. How many miles since the overhaul? What type of oil did you use in the break in process? Were the heads reworked? Is it using any oil?
When you performed the compression test it's supposed to be done with the power wire off the distributor, all spark plugs removed and the throttle set to it's wide open position.
Standing in front of the engine pop the right side valve cover, cycle the engine to Top dead center and see if both valves are closed. If they aren't or if one is partially open then I would say the timing is off on the chain.
In the past I've seen guys replace the timing chain and install it 2 teeth off and the car would still run good. If it's off too much then it won't run at all. Also I wouldn't go by the oem timing spec. SBC's like 12 degrees before top dead center with vacuum advance disconnected and plugged. Once it's set at 12 degrees, reconnect the vacuum advance to a manifold vacuum source, one that pulls vacuum full time including at idle. Then recheck timing, it should be 18 to 24 degrees before top dead center. Do this and it'll pick up considerable more power, use less fuel and will have far better low speed cruise manners. What size camshaft did you use?
|10-05-2012 08:24 PM|
sbc 350 cam timing off?
Can a SBC run with the cam timing off? What would the symptoms be, and how can I test to see if that's my problem?
I will quickly detail my situation.
installeda '79 350 rebuilt engine in my boat. put all the original external parts back on it. Carb, distributor, alternator, etc. It's an old points style distributor, and I'm pretty sure I timed it correctly.8* btdc at idle. Spec from the oem manual.
I am not getting good power, and my compression is strangly low on all cylinders. I have re adjusted all valves to make sure they aren't too tight, and still getting low compression.
Compression is as follows:
This engine has 23 hours on it, it starts pretty well, and runs fine. Just underpowered.
I read a post where someone thought this was their problem but wasn't. A guy said that with the cam specs and a dial indicator you could check to see if the cam was lined up with the crank. I would like to find out how to do that if possible, before I pull the front cover off to visually check. Can anyone help me with this?