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1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive

92K views 157 replies 14 participants last post by  cobalt327 
#1 ·
Hi All,


I have a 1977 Chevy truck with a stock crate 350 engine and a 3-speed auto transmission. It seems to be suffering from power loss when in Drive. I drove the truck yesterday to move for the street sweeper and it gave no trouble.


This morning, the truck started up just fine, but when put into Drive, it had a serious loss in power. At first I thought it hadn't warmed up right and gave it more gas but it was more of the same. So I pulled over, shifted into Park, and gave it some gas. The engine revved smoothly with no issues.


So I shifted back into Drive and started driving and still no power. For reference, it was getting to 25 mph in over 10 seconds and there was no chance at going over 35 mph. It also was making a bhup-bhup-bhup noise. I tried shifting manually but that was of no use either.


I haven't had a chance to pop the hood and look at anything, but could it possibly be that the vacuum line to the transmission has come loose?


Thanks in advance, :thumbup:
 
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#127 ·
lt1silverhawk said:
I inspected the vacuum line at the intake as well as the modulator. It was dry at the intake, but covered in oil externally at the modulator (along with surrounding parts). And although I've tightened it it up before at the modulator, the small rubber hose was a bit loose again. I removed the small rubber hose connecting the modulator to the metal line and found no ATF inside.
No ATF is a good sign, but the piece of rubber tubing needs to be replaced w/new rubber tubing, not the old hardened or mushy-from-ATF piece that's on there now that's taken a set. Use a long enough piece that it's not pulling the hard line when connected, but not SO long that it crimps shut from any bends that you might need to put in it. Straightening the hard line so that it's directly in line w/the modulator vacuum port will help matters. I think we discussed this previously.
 
#128 ·
123pugsy said:
You got to get those oil leaks under control.

The oil is making your vacuum hose gooey.
I know, and its bugging me that I can't find this leak. I've tried everything from wiping down everything to keeping a tray under the truck to figure out where its all coming from. I swear there is a Bermuda Triangle behind the engine where bolts disappear and oil mysteriously leaks. :(




cobalt327 said:
No ATF is a good sign, but the piece of rubber tubing needs to be replaced w/new rubber tubing, not the old hardened or mushy-from-ATF piece that's on there now that's taken a set. Use a long enough piece that it's not pulling the hard line when connected, but not SO long that it crimps shut from any bends that you might need to put in it. Straightening the hard line so that it's directly in line w/the modulator vacuum port will help matters. I think we discussed this previously.
Yes, you did bring this up on page 2. I've actually been meaning to remove the metal line as well and inspect it for any damage, but it became quite obvious today it won't be an easy task. I'll be replacing the rubber portions soon.
cobalt327 said:
That vacuum line to the modulator valve- bend the hard line so it's pointing directly at the valve so the rubber hose connection is straight. It might be the photo, but the hose looks like it's becoming delaminated where it's bent. If there is any question, replace that length of rubber hose w/a new piece.


I will take another look at the spark plugs' electrodes next to see if there is any discoloration. I will also replace every vacuum hose as well as the PCV valve and grommet. I'll try to get to the valve stem seals this weekend.
 
#129 ·
I wouldn't remove the hard vacuum line to the modulator. Instead, cap one end and draw a vacuum on the other end. If the vacuum holds- and the line is clear- (no kinks or obstructions, check for this by blowing through it or pull a vacuum unplugged, it shouldn't hold), it's OK.

Some modulator lines went to a fitting on the intake that had a small orifice that acted as a sort of delay valve. If you still are using one like this, be sure the orifice isn't clogged up w/carbon, etc., or the tranny might shift weird.
 
#130 ·
cobalt327 said:
I wouldn't remove the hard vacuum line to the modulator. Instead, cap one end and draw a vacuum on the other end. If the vacuum holds- and the line is clear- (no kinks or obstructions, check for this by blowing through it or pull a vacuum unplugged, it shouldn't hold), it's OK.

Some modulator lines went to a fitting on the intake that had a small orifice that acted as a sort of delay valve. If you still are using one like this, be sure the orifice isn't clogged up w/carbon, etc., or the tranny might shift weird.
I'll try that method then. I'll also check for the delay valve, thought I don't recall seeing anything of the sort.




Here are some close up shots of the spark plugs. The were wiped down after removal. I might have the order backwards (put away in a hurry), but they are the correct side.





 

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#131 ·
Those plug's condition look pretty bad, all in all. The photos are clear enough, but it's the electrode and ground strap and the porcelain that has the info; it helps to be able to see down inside the end of the plug- but in this case it's really not necessary.

I'd say considering how well the compression test went, that it may be the guides passing oil.

Is there one side of the plugs that is fouled more or has heavier deposits?

The one plug that looks most normal is the top right (circled).


Driver (L) 1-3; (R) 5-7


Passenger (L) 2-4; (R) 6-8
 
#132 ·
Those plug's condition look pretty bad, all in all. The photos are clear enough, but it's the electrode and ground strap and the porcelain that has the info; it helps to be able to see down inside the end of the plug- but in this case it's really not necessary.
I was actually trying to get the camera to focus on the electrodes, but just didn't work out that way.




I'd say considering how well the compression test went, that it may be the guides passing oil.
I had to look up the procedure for replacing them. That's machine shop duty. :pain: Or will replacing the seals take care of this?




Is there one side of the plugs that is fouled more or has heavier deposits?
I'd say the passenger-side seems to have more fouling/deposits.




The one plug that looks most normal is the top right (circled).
Thank you for pointing that one out. Now I have something to compare against in the future.




I want to get an opinion: I am thinking of cleaning these plugs off thoroughly and using them one last time just to see where the oil/deposits end up. Or do they look just too far gone?
 
#135 ·
Got to do some work on the valve stem seals.




I ordered a Powerbuilt valve spring compressor (presently going for $20 + free shipping on eBay) and a Lisle valve holder. The valve spring compressor is very similar in design to the units offered by Moroso, Summit, Jeg's and Probuilt but at a much cheaper price.







I popped off the passenger-side valve cover and found the source of the leak: a small piece of an older valve cover gasket. Can't believe I missed that. No more working at night. Lesson learned.







I hooked up the valve holder to the compressor. There were no directions included so I set it to 100 psi. I had a few moments of frustration because the valve spring compressor shaft that bolts onto the rocker stud was too big. The packaging said it was for both 3/8" and 7/16" studs. Turned out the the shaft has different threads on each end and has to switched. That required popping off a c-clip and reassembling it.







Once the compressor was correctly set up, I removed the retainer, spring and seal from the #2 cylinder exhaust (?) valve (the very first one on the right). Now, I need help identifying what I've got. Based on what I've seen and read so far, I believe these are the positive stop seals.


 

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#137 ·
That is a positive type seal, although the valve stems are set up w/the same double grooves (one for the keepers, the other lower groove for the small O-ring type seal) and there is a tin oil deflector, like has has been used since day one, pretty much.

I don't have a part number, but the seals will be the same as those used on a 1987 Camaro 305 w/TPI, for example. Some sources cite the GM factory positive seals as fitting a 0.561" SBC guide boss. Measure the guide boss OD to compare, most aftermarket type positive seals have a smaller OD (=/< 0.0530") to allow an aftermarket/double spring to fit over them. You should be able to find the seals at any parts store, just inspect them first for having similar dimensions as the ones you have.

BTW, that is an exhaust valve (you will note the spring retainer is also a valve rotator, found only on exhaust valves). The order is (from here):

SBC: front | E - I - I - E - E - I - I - E | rear
 
#138 ·
That is a positive type seal, although the valve stems are set up w/the same double grooves (one for the keepers, the other lower groove for the small O-ring type seal) and there is a tin oil deflector, like has has been used since day one, pretty much.

I don't have a part number, but the seals will be the same as those used on a 1987 Camaro 305 w/TPI, for example. Some sources cite the GM factory positive seals as fitting a 0.561" SBC guide boss. Measure the guide boss OD to compare, most aftermarket type positive seals have a smaller OD (=/< 0.0530") to allow an aftermarket/double spring to fit over them. You should be able to find the seals at any parts store, just inspect them first for having similar dimensions as the ones you have.
Got it, I will remove one and measure and compare it before buying.




BTW, that is an exhaust valve (you will note the spring retainer is also a valve rotator, found only on exhaust valves). The order is (from here):

SBC: front | E - I - I - E - E - I - I - E | rear
Thanks for the wiki. :thumbup:
 
#140 · (Edited)
Just got back from my shopping trip. I asked for seals for an '87 Camaro 305 TPI and brought along one of each seals for comparison. Here's what I got:


- Autozone: Had absolutely nothing that resembled the seals that I had.


- NAPA: Informed me that the seals on the truck are actually high performance. Asked if the umbrella seals they had in stock would work and and was told yes, as long as it wasn't a high performance engine. Bought a set of Seal Power MV-1598C.






- O'Rilley's: Also had nothing that matched, but was offered positive stop intake and exhaust valve stem sets with o-rings from Felpro and was told they should work.






Will any of these work?



EDIT: Picture #6 for the Felpro exhaust seal (side view) is incorrect.
 

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#141 ·
#142 ·
Will one of the sets I purchased earlier be fine, or should I order these instead?
I prefer the FelPro type, and would use them providing they fit the boss and the 11/32" SBC valve stem OD.

I don't know your boss size, but these are what I would use if the FelPro seals didn't fit your boss and these did. The application is for '93- up LT1/Vortec engines, but that's moot if they fit your boss.

Another that may fit is here, they're intake seals; the exhaust seal p/n is given in the text. You will notice these include the small O-rings. GM used both the small O-rings and these positive seals into the '90s.

I had thought the years given earlier would have given you positive seals and not umbrella style seals. Umbrella seals will work, but there are differing opinions on using them as opposed to a positive type seal. Maybe someone else will have something to add.
 
#143 ·
Thank you Cobalt.


I do have the Felpro intake set you listed, but different a exhaust set from one suggested by Summit. The LT1/Vortec seals do strongly resemble the ones I removed, but I think have a wider inner diameter. The valve stems I measured were about 0.341". I will try to get some precise measurements tomorrow. I don't want to install anything that doesn't fit and end up with a set of seals that can't be returned. :pain:


As to the pros and cons of using umbrella seals, I did find two threads on other forums which may be useful for others readers:

- let's talk valve seals - The 1947 - Present Chevrolet & GMC Truck Message Board Network

- http://www.stevesnovasite.com/forums/showthread.php?t=144488
 
#144 · (Edited)
So I measured and remeasured the valve stems as well as the boss guides for cylinder #2.


Intake valve stem: 0.3415"

Intake boss guide: 0.560"


Exhaust valve stem: 0.3415"

Exhaust boss guide: 0.559"




I measured the ID in the Felpros using a caliper, but they seemed a little smaller than the ones removed. I wanted to stop by a Chevy dealership's parts department to see what they've got, but they were all closed today. However, I did find these viton seals on eBay:

- Metal Clad Viton Valve Stem Seals 11/32" x .562" sb Chevy Ford Chrysler sbc | eBay

- VITON VALVE SEALS 11/32 SBC SBF GT40P PONTIAC .520-.565 | eBay
 
#145 ·
The second ones have a more positive sealing area, and might be the better choice for used valve stems. But there can be too much sealing; you want there to be a slight amount of lubrication reaching the valve stem/guides. But in the case of a SBC, the guide is the head itself (cast iron) so there is always "extra" clearance.

Bottom line is I'd expect either to work. The second ones would be a good intake seal, the first ones possibly a better exhaust seal (where you need a bit more lubrication reaching them). If you were only going to get one set, the first ones in conjunction w/the small O-rings, or the second ones w/o O-rings.
 
#150 ·
Hi All,


Just doing a quick check-in and update on the valve stem seals. The viton seals Alex's Parts came in middle around Thursday. They look quite similar to the Fel-pro blue positive stop seals but had a smaller inner diameter (approximately 0.314" vs 0.334). All the new seals can be seen below for comparison.







I first tackled the seals on cylinder #2. I used some scotch tape on the valve stems to protect the seals. Its a bit tricky to make that work because of the oil on seals. But I quickly discovered that, due to the tight inner diameter of the viton seals, the scotch tape was getting stick inside the seal and almost impossible to remove by hand. A gently pull with pliers did the trick.







After I installed the exhaust seal, I tried to install the intake seal but it was the diameter seemed to small for the guide boss. So I moved on to cylinder #4 to see if I would have the same problem and I did. The scotch tape would get stuck inside the seal and it wouldn't seat all the way. So I decided to take a gamble and try the blue Fel-pro seal and, surprisingly, it was even worse. Having already used a seal from the Fel-pro box, I went ahead used the included installation sleeve and it worked like magic. The seals fit flawlessly. I'm gonna have to contact Alex's Parts and recommend they include the installation sleeve with the set.






So, only two cylinders got done. Will get the rest finished this coming weekend.
 

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#151 ·
Sometimes air gets trapped under the seal and (kind of like when you use silicone grease on spark plug boots) the seal wants to pop back up unless the pressure is relieved. Might take a little pinch/squeeze
at the top of the seal to distort the ID of the seal just enough to let air out past the valve stem. Or maybe use a toothpick to slip in between the stem and seal.
 
#152 ·
LOL! I tried the pinch/squeeze method but that was a bit difficult with the metal bands around these viton seals. Then again, I'm always so paranoid about damaging anything, I probably didn't apply enough pressure. The tooth pick I would've never though of, since I'm having to use an installation sleeve to protect the seals.


I gotta say, these seals are really tight and it does take some serious pressure to slide 'em down. But once they are, I can tell there aren't going anywhere. I almost became concerned that the exhaust valves may not get enough lubrication.


Thanks cobalt! :thumbup:
 
#156 ·
Hey guy! Yeah, a dab of lube is a very good idea, dry start up can ruin a seal in short order. I wouldn't bother with putting anything under the seal necessarily, what you want is the seal ID where it rides on the stem to have some lubrication. Motor oil is fine if you're going to run the engine soon, if it's going to sit you can use a smear of lubriplate on the stems- that will stay put.
 
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