Engine Problem...Excessive Blow-bye / Low Compression

[j2hang]

I was brave (crazy) enough to build my first motor ,'69 302, and I have not been happy with it because of its lack of power. I have confirmed the correct timing and corrected my initial mis-adjustment of the valve lash . Neither of these has solved my problem. Finally I hooked up a vacuum guage and can't pull more than 13" Hg. The needle is fairly steady but I beleive I need to have 17-22" of mercury. From reading I beleive the problem to be low compression, but there is also a good (bad) amount of blowbye coming from each valvecover breather.
Any ideas / recommendations before I pull this think back-out? ???
Thanks,
Jon

 

[cjjhalfcab]

Don't really know, there could be some variables. You didn't force any stubborn pistons in, did you? If a ring popped out of the compressor when you were installing pistons (happens a lot) and you forced it, you could have broken a ring. Did you make sure that the ring gaps weren't lined up with each other? In both of those cases, she might smoke out the exhaust, too. I'd run a compression test and see if all cylinders are down, or just one or two. Did you use chrome rings? If so, they can really cause blow by until they are seated, which in some cases is never. I have a 351C that I put chrome rings in 6,000 to 8,000 miles ago, and it still blows oil out of the breathers. So it is going to get a do-over with a quick hone job and cast rings. My 302 in my Bronco has cast rings, and is good as gold. Just don't know if they would cause a drastic reduction in compression. I'd still take one more trip through the valve adjusting, just for peace of mind.

 

[CodeeB]

One of the easiest mistakes on first time rebuilds is not staggering the rings on the piston , like cjjhalfcab suggested. My dad warned me about that on my first rebuild.

 

[75MIKE]

A super high lift cam will keep you from pulling a lot of vacuum at idle also.Not knowing what cam you put in , it's a just a thought.

 

[ASE-73]

Do a leakdown test ... this will help establish the source and extent of the problem. If ALL cylinders have equivalent leakdown, then you may not have seated the rings or staggered properly (not enough detail to indicate).

 

[j2hang]

Thanks for the info. To answer a few unknowns:

I did offset the rings as specified
It does not smoke out of the exhaust
I do beleive the kit I used had "moly" rings
The cam is 204 / 214 a "torquer"

Is there a chance I bent pushrods or a valve?????
My mistake was I did not set each cylinder to TDC when adjusting the valves. I just adjusted then to tolerence without any consideration of cam position...Then I started my new motor for 15-20 min. Never drove it that way because of the valvetrain noise. I then bought oil stoppers and adjusted the valves while running.

Lastly, Which is better to do a leakdown or compression test? How would I do a leakdown test?

Thanks for all your help,
Jon
Castle Rock, CO

 

[wildbill]

;D ;D ;D Time to build a nice 351W and have ready to drop in when the old one takes a dump good luck. ;D ;D ;D Bill 8) :

 

[ASE-73]

[quote author=j2hang link=board=5;threadid=16278;start=0#msg124013 date=1050166956]
Thanks for the info. To answer a few unknowns:

I did offset the rings as specified
It does not smoke out of the exhaust
I do beleive the kit I used had "moly" rings
The cam is 204 / 214 a "torquer"

Is there a chance I bent pushrods or a valve?????
My mistake was I did not set each cylinder to TDC when adjusting the valves. I just adjusted then to tolerence without any consideration of cam position...Then I started my new motor for 15-20 min. Never drove it that way because of the valvetrain noise. I then bought oil stoppers and adjusted the valves while running.

Lastly, Which is better to do a leakdown or compression test? How would I do a leakdown test?

Thanks for all your help,
Jon
Castle Rock, CO
[/quote]

First validate timing and vital adjustments. A leakdown is far superior to a compression check. It is essesntially a pressure check ... 100 psi applied through the spark plug hole into to each cylinder with the piston positioned @ TDC. Air escaping indicated leakage ... and will create air leaking sound that will indicate where the leakage id coming from. Do SEARCH for "leakdown test" ... it is explained in detail in an earlier post.

 

[Skuzzlebutt]

[quote author=j2hang link=board=5;threadid=16278;start=0#msg123955 date=1050140101]
.... there is also a good (bad) amount of blowbye coming from each valvecover breather.
Any ideas / recommendations before I pull this think back-out? ???
Thanks,
Jon
[/quote]

???"Each Breather?" ???
Where is your PCV? Any blow-by at all will be excessive without a PCV.
How many miles on engine? Vacuum, compression, blow-by will improve as the engine gets broken-in.

 

[j2hang]

Skuzzlebut...PCV valve? I just ripped it off and got a shiney aftermarket breather. Was that a bad idea?

ASE...Thanks for all your help I will do a leakdown test tomorrow and let you know what I find.

Very encouraging there Wild Bill

 

[Skuzzlebutt]

The PCV isn't an emission control thing. Your engine was designed to have the damaging gasses that are created under normal conditions removed by vacuum. And the valve isn't just a one way valve, it is a calibrated device that varies the size of the opening according to intake vacuum. The vent on the other valve cover, usually in the filler cap, is to allow clean air into the crankcase.
Don't take shortcuts unless you absolutely have to.

 

[BoltBuster]

yea place the PCV back- not sure if thats your problem.

I would beg, borrow or rent a compression check before I pull the motor back out-

 

[j2hang]

Ok, Putting the PCV valve back on tommorow morning. Uh, will that somehow increase the vacuum? I live near Denver and was told not to expect 17-22" mercury. So I checked my father-in-laws Bronco with a 351 and he was only pulling 15" mercury...any comments? The needle was dead steady on his though and mine fluctuates just a little bit 1/2" high and low.

I did a compression test and didn't have any cylinders above 95 most where 90psi.

NEW FINDING:
I was double checking the valvetrain and setting the valves when I decided to just remove the lifters and check the PUSHRODS. I believe that 4 of the 8 on that side were slightly BENT. Tomorrow I will replace all 8 then try a leakdown test on the "fixed?" side verses the side I didn't yet check.

Any other ideas?????????????????????? thanks, again.

 

[j2hang]

ASE-73.

After replacing half of my pushrods, they were slightly bent from one degree to another. I adjusted the valve lash while running.

The next day:
I did the leakdown test and air was coming out of the oil fill / breather holes on the valve covers at about 90psi-100psi on every cylinder. I did miss a step though and did not bring the engine to operating temp. before testing. From the article that would indicate my rings aren;t holding well, but I was told the rings may take 500 miles or more to properly seat and not go nuts until afterwords. The other warning a friend gave was that I may have messed up valve guides or the valves themselves.

Any advice???
Thanks,
Jon
Castle Rock, Colorado

 

[Lttrbox]

The other warning a friend gave was that I may have messed up valve guides or the valves themselves.

Im not convinced of this. I have a cam very similar to yours. The only problem I ran into was that the stock valve springs werent enough to seat the valve. BTW, the heads were worked, right. I am still running the stock pushrods and rockers. That valve lash seems a bit odd, my 302 ('73) required the rocker nuts to be bottomed and torqued slightly.

I guess I would check those rocker studs and see if some of those are pulling out. Im not sure why the bent pushrods unless there is really something else going on. Did you replace with stock pistons? I believe most these are valve recessed versus higher compression ones may not.

One other thing, elevation here is just short of 5000 and mine got 17" (no miles) and rock steady.