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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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#78 ·
Hey Pugsy,
123pugsy said:
Clean it up and reuse the same gasket is OK as long as no chunks came off of it and its not torn anywhere.
Yes, the gasket appeared to be very solid.


123pugsy said:
Does the debris look like the stuff from the fuel filter?

If so, it may be time to replace your fuel line and possibly the tank.
It was similar. There were some red particles that resembled those found in the fuel filter. Also found a small stone which is in the pictures below.




----------




I went ahead and cleaned up the carburetor using carb and brake cleaners and put it back together. I removed some of the epoxy that was coming lose. Where it was still hard, I left it alone, mainly over the plugs. I also did a "light check" on the orifice in the fuel bowl and they looked nice and open.


As always, I forgot a few things here and there and had to redo them, and no project is every complete without me dropping or losing something.On the plus side, I think I can now install the choke pull-off assembly in my sleep.


I also installed a fuel filter in the carb.







Got everything bolted up, closed and then turned out the idle mixture screws to six full turns, and started the truck. Let it run for quite a while as it approached operating temperature. The idle sounded very strong the entire time and never dropped below 1,500 rpm. I temporarily turned it off and it kept it running - dieseling - for a few seconds before stopping with a small bang.


I took the truck for a spin, and ran it well. The temperature stayed at 195 degrees F at all times. No fluctuations. The engine speed in Drive when at stop hovered at 7,50 rpm.


When I finally parked, the temperature was still at 195 degrees, but rising. In park, the idle was still 1,500 rpm. When i killed the ignition, the truck kept going for a good fifteen seconds before stopping with a bang and some smoke from the engine bay. From what I have read, this is very much a timing issue.
 

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#80 ·
123pugsy said:
I guess you're driving around without an air filter?

A stone can't get past a fuel filter.
Yes, I had the air filter off while making adjustments. Lesson learned.


123pugsy said:
The run on is from the high idle.
800 in park is high enough.
Ok, I will try to temporarily lower it to 800 rpm before getting to the distributor and hooking up the timing light.


Slightly off topic, but what are the chances a cheap-looking after-market tach can be off by a few hundred rpm? I ask because when I hooked up the timing light last time, the light was showing about 200 rpm more than what the tach was showing.
 
#81 ·
lt1silverhawk said:
Slightly off topic, but what are the chances a cheap-looking after-market tach can be off by a few hundred rpm? I ask because when I hooked up the timing light last time, the light was showing about 200 rpm more than what the tach was showing.

Odds are good.
My cheap piece of junk fluctuates and only works part time.

When you set your timing , you will have to reset your idle again.
 
#82 ·
123pugsy said:
Odds are good.
My cheap piece of junk fluctuates and only works part time.

When you set your timing , you will have to reset your idle again.
Good to know. I will have to check on this and verify, but I wana say it was cobalt who said in another thread that usually the idle mixture screws on a Q-jet are turned out 4 full turns.

Thanks for the continued help! :thumbup:
 
#83 ·
lt1silverhawk said:
Good to know. I will have to check on this and verify, but I wana say it was cobalt who said in another thread that usually the idle mixture screws on a Q-jet are turned out 4 full turns.

Thanks for the continued help! :thumbup:

Idle mixture screws have nothing to do with setting idle speed.

Idle speed may change however when using your vacuum guage to set the idle mixture screws.
 
#84 ·
123pugsy said:
Idle mixture screws have nothing to do with setting idle speed.

Idle speed may change however when using your vacuum guage to set the idle mixture screws.
Oh, so its strictly a timing issue then.


Btw, I just tried adjusting the idle screws down to 5 full turns ,and then 4 full turns. Both times, the truck started up and revved with no issues and immediately shut off when the ignition was cut. So I am hoping that, at least for the time being, the engine is in an acceptable operating zone in case i need to move it before getting to the timing.




And I also wanted to ask about a part that fell out of the carburetor today while cleaning it. I never saw this part when I did the rebuild. But it seems like fell out where the the key and the rod for the choke plate. Any ideas what it is?

 

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#85 ·
lt1silverhawk said:
Oh, so its strictly a timing issue then.


Btw, I just tried adjusting the idle screws down to 5 full turns ,and then 4 full turns. Both times, the truck started up and revved with no issues and immediately shut off when the ignition was cut. So I am hoping that, at least for the time being, the engine is in an acceptable operating zone in case i need to move it before getting to the timing.
]

If its idling too high, do not adjust the mixture screws.
Idle speed is set by the idle speed screw on the driver's side of the carb.
 
#87 ·
123pugsy said:
If its idling too high, do not adjust the mixture screws.
Idle speed is set by the idle speed screw on the driver's side of the carb.
So that screw doesn't have anything to the do with throttle linkage then? Because I noticed that if screwed in all the way, pushes the throttle lever back. This would be my error when adjusting the idle speed and timing by ear last summer. I was messing the idle mixture screws more than the idle speed screw. I will mess with this next then.




123pugsy said:
KRYPTONITE?
Lol! Considering how strong the truck runs and idles, that may very well be it!
 
#89 ·
dogwater said:
Im impressed with your closeup photography.looks pretty good.I used to do alot of photo work.
Thanks man! Not bad for a 12 megapixel Sony point-n-shot, huh? I've been shooting away at car parts for over a year now and also do some amateur photography. I gotta say that practice does make perfect. Hopefully these pictures will come in handy for someone else. What kind of photography did you do?
 
#90 · (Edited)
The red cam-shaped plastic piece is the eccentric that lifts the secondary hangar/secondary metering rods in relation to the air valve opening. It has to be replaced, Ruggles has the kit, $12 to your door.

I noticed in one of the earlier photos the little anti rattle spring on the power piston that keeps the primary metering rods from vibrating (thus wearing) inside the jets was out of position. Later photos show it back in correctly, so that should be OK. That spring can be omitted if there's any question as to the condition of it, it's there to insure 100K mile service life under all conditions and many guys leave it- and the small clip I'll talk about next- off the carb altogether. I use them FWIW.

On thing that you have got to fix is the small hook that clips to the needle part of the needle and seat assembly. The hook does not go through the hole in the float arm. This is a common mistake, but it can cause the needle to hang up and the float level can be all over the map. So, hook it over the cross piece closest to the power piston.

I'd remove the epoxy. It's not doing any good and even though it can't fall into the float bowl, no reason to have it going through the engine. I'm guessing it was there for prevention's sake. If the well plugs ARE leaking get back to the forum and we can get into it further.

EDIT- The air bleed that's damaged needs to be opened back up to the same diameter as the undamaged one. You can use a small pick or smooth nail if small enough to resize the opening to match the other one. Dress the air bleed tip so there's no ragged edges, just don't shorten it excessively (excessive would be more than say 0.015").
 
#91 ·
Hey cobalt,

cobalt327 said:
The red cam-shaped plastic piece is the eccentric that lifts the secondary hangar/secondary metering rods in relation to the air valve opening. It has to be replaced, Ruggles has the kit, $12 to your door.
I had to go back to the rebuild thread as well as all the pictures I took last year to see if I had removed and reinstalled it, but nothing came up. So I am going to assume it has been inside the open space where the key goes all along. Ruggles does cover the procedure for removal and installation in his book (chapter 5, "Rebuilding the Carb", section "Air Horn", page 84), so I should be able to go through that.




cobalt327 said:
I noticed in one of the earlier photos the little anti rattle spring on the power piston that keeps the primary metering rods from vibrating (thus wearing) inside the jets was out of position. Later photos show it back in correctly, so that should be OK. That spring can be omitted if there's any question as to the condition of it, it's there to insure 100K mile service life under all conditions and many guys leave it- and the small clip I'll talk about next- off the carb altogether. I use them FWIW.
Yes, that looks out of place because it came off when I removed the airhorn gasket. But it was fine before and I did reposition it before closing everything up.




cobalt327 said:
On thing that you have got to fix is the small hook that clips to the needle part of the needle and seat assembly. The hook does not go through the hole in the float arm. This is a common mistake, but it can cause the needle to hang up and the float level can be all over the map. So, hook it over the cross piece closest to the power piston.
I am confused on this one. Would the cross piece be the pivot arm for the float?




cobalt327 said:
I'd remove the epoxy. It's not doing any good and even though it can't fall into the float bowl, no reason to have it going through the engine. I'm guessing it was there for prevention's sake. If the well plugs ARE leaking get back to the forum and we can get into it further.
Yes, I had used the epoxy as precautionary measure (there was some discussion about them on pages 3 and 4 of the rebuild thread). I'll go ahead and completely remove it on the next dis-assembly. I don't believe the wells do leak, but if they do, I can attempt to install new plugs.




cobalt327 said:
EDIT- The air bleed that's damaged needs to be opened back up to the same diameter as the undamaged one. You can use a small pick or smooth nail if small enough to resize the opening to match the other one. Dress the air bleed tip so there's no ragged edges, just don't shorten it excessively (excessive would be more than say 0.015").
Ok, sounds like an easy enough procedure.



Thanks! :thumbup:
 
#92 ·
lt1silverhawk said:
I am confused on this one. Would the cross piece be the pivot arm for the float?


The well plugs that you have don't leak in the vast majority of cases. The older cup plugs were the culprits that gave the bad reputation about leaking well plugs to the Q-jet- and spawned a generation of guys who needlessly apply epoxy to them.

I personally have never seen an epoxy that wouldn't delaminate after being exposed to heat (and possibly fuel) Although Ruggles has recommended Marine Tex for this, he also sells threaded plugs, IIRC. The only sure-fire method is mechanically plugging the holes w/threaded plugs after the old plugs are removed. There are some fosdick o-ringed plugs that were (are still?) sold for this that are also to be avoided, IMO. The o-rings are not going to stand up to the conditions any better than epoxy, IMO.
 
#93 ·
carb trouble

You can use an oxy-acetylene welding tip cleaner to clean up the air bleeds. Just don't get too vigorous with it, since the tip cleaners are actually little round files and will take off metal.
I hope you sprayed the venturies to get them clean. they looked pretty nasty.
You must adjust your idle speed screw (there is only one and it is on the throttle linkage side of the carb) to get an idle speed of around 500-800 rpm, THEN adjust the idle mixture screws (2, one on each side of the primary float bowl) for "lean best idle". You do this by running them both all the way in, but don't make them tight as you will damage the seat.
Next, with the engine off, turn them both out 3 turns (for a Quadrajet, 1 and 1/2 turns for a Carter).
Start the engine and check the idle speed. Turn the 2 idle mixture screws in or out as needed to make the engine run faster. Turn each one only 1/4 turn at a time, and make sure that they both are the same number of turns out.
Once you have the fastest idle with these two screws, go back to the idle speed screw and reduce the idle back to 500-800 rpm.
Generally speaking, once the idle mixture screw have been set in this manner, they might need only 1/2 turn in either direction to prevent a lean spot just off idle (assuming that the accelerator pump is working correctly).
Again, make sure that you only turn the mixture screws 1/4 turn at a time, turning first one and then the other so that they are both the same number of turns out. If they come out different, then you have a vacuum leak.
If your idle is set over approximately 1,000-1,200 rpm, then your engine will diesel (run on after shutting the ignition off). This is VERY bad for the bearings!
Set the timing now.
After setting the timing, you may have to increase or reduce the idle SPEED screw, but don't touch the 2 mixture screws again unless the engine has a lean bog or miss just off idle.
Good luck.
 
#94 ·
Hey cobalt and barry,


@ cobalt: Thank you for the picture. I thought I understood what you meant but not having the carburetor open made it difficult. I also saw the image you uploaded to your album (currently your last uploaded image). This helps! :thumbup:

As to the plugs, I remember you did mention during the last rebuild that they weren't very likely to leak. I dabbed epoxy over them as a precautionary measure at the suggestion of the video series I was following on Youtube for rebuilding Q-jets. But I have seen no leaking and will remove the remaining epoxy upon dis-assembly when I reposition the needle hook.




barry425 said:
You can use an oxy-acetylene welding tip cleaner to clean up the air bleeds. Just don't get too vigorous with it, since the tip cleaners are actually little round files and will take off metal.
Got it.


barry425 said:
I hope you sprayed the venturies to get them clean. they looked pretty nasty.
I did spray everything I could with the carb and brake cleaners, and then wiped it all down. I didn't pay any attention to the venturis buit I can give them more a more rigorous cleaning during dis-assembly.


barry425 said:
You must adjust your idle speed screw (there is only one and it is on the throttle linkage side of the carb) to get an idle speed of around 500-800 rpm, THEN adjust the idle mixture screws (2, one on each side of the primary float bowl) for "lean best idle". You do this by running them both all the way in, but don't make them tight as you will damage the seat.
Next, with the engine off, turn them both out 3 turns (for a Quadrajet, 1 and 1/2 turns for a Carter).
Start the engine and check the idle speed. Turn the 2 idle mixture screws in or out as needed to make the engine run faster. Turn each one only 1/4 turn at a time, and make sure that they both are the same number of turns out.
Once you have the fastest idle with these two screws, go back to the idle speed screw and reduce the idle back to 500-800 rpm.
Generally speaking, once the idle mixture screw have been set in this manner, they might need only 1/2 turn in either direction to prevent a lean spot just off idle (assuming that the accelerator pump is working correctly).
Again, make sure that you only turn the mixture screws 1/4 turn at a time, turning first one and then the other so that they are both the same number of turns out. If they come out different, then you have a vacuum leak.
Thank you, this helps me a lot. As to the vacuum leak, based on the responses of a couple of members, I am suspecting that I didn't do a good job of sealing the intake manifold gasket on the passenger rise (the plugs came out more fouled on that side than the driver side). There also been some smoke coming out that side at times as well.


barry425 said:
If your idle is set over approximately 1,000-1,200 rpm, then your engine will diesel (run on after shutting the ignition off). This is VERY bad for the bearings!
That's good to know! Unfortunately the tach in the truck doesn't seem to be very accurate so I'll be using the RPM readings provided by the timing light (timing light shows about 200 rpm higher than what the tach registers).I did adjust the idle speed screw a few days back and it seems to be hovering around 750 rpm once at operating temperature.


barry425 said:
Set the timing now.
That is the next step. I did being work on a timing tape. Just need to find time to sit down carefully make the markings. I'm thinking that it doesn't need to be big enough to go all the way around (full 360 degrees). Maybe up to 60 degrees before and after TDC (180 degrees) should be sufficient. Any thoughts?


barry425 said:
After setting the timing, you may have to increase or reduce the idle SPEED screw, but don't touch the 2 mixture screws again unless the engine has a lean bog or miss just off idle.
Good luck.
Got it. I have the screws at four full turns out right now and will leave them be for the moment.




Hopefully by this Friday evening, I will disassemble the carb, clean up the venturis some more and reposition the needle hook into the correct position. Saturday morning I start working on the timing. I will report back as regularly as I can.


Thanks for the continued help! :thumbup:
 
#95 ·
Timing

There should be marks on your dampner and on the drivers side timing chain cover, so you shouldn't need timing tape. Start at 5 degrees before TDC (Top Dead Center) and go from there. If you can find a Motors Manual for the year of your engine/car, it will list the timing information in there. In my opinion, Chilton is a poor second to Motors, and Haynes sucks because the info is hard to find and is sometimes even missing. You can also go to a Chevy dealer and get the specs from them.
The more you advance the timing, the faster the engine will run until you reach a point where it is too much timing and it will begin to stumble and slow down. Moving the distributor CCW (Counter ClockWise) will advance the timing. Check timing with the vacuum hose removed from the distributor and plugged.
Total timing at 3,000 rpm should be no more than 34 degrees with todays lousy gas, if your timing light will work at that rpm.
 
#96 ·
lt1silverhawk said:
I did being work on a timing tape. Just need to find time to sit down carefully make the markings. I'm thinking that it doesn't need to be big enough to go all the way around (full 360 degrees). Maybe up to 60 degrees before and after TDC (180 degrees) should be sufficient. Any thoughts?
Sorry, that should say 120 degrees, not 180 degrees. :drunk:



Hey barry,
barry425 said:
There should be marks on your dampner and on the drivers side timing chain cover, so you shouldn't need timing tape.
There is a tab with marks (image below) on top of the damper. But I am making the tape more as an added assist and peace of mind to make sure everything is right on the money.







barry425 said:
Start at 5 degrees before TDC (Top Dead Center) and go from there. If you can find a Motors Manual for the year of your engine/car, it will list the timing information in there. In my opinion, Chilton is a poor second to Motors, and Haynes sucks because the info is hard to find and is sometimes even missing. You can also go to a Chevy dealer and get the specs from them.
I've been using the Autzone website as a reference and it calls for 8 degrees BTDC. I can stop by the local Chevy dealership and ask.




barry425 said:
The more you advance the timing, the faster the engine will run until you reach a point where it is too much timing and it will begin to stumble and slow down. Moving the distributor CCW (Counter ClockWise) will advance the timing. Check timing with the vacuum hose removed from the distributor and plugged.
This is where I recently hit a trouble spot. The last time I installed the distributor, I did in such a way that it now hits the intake manifold when being turned CCW. So I need to take it out and reinstall it while making sure it doesn't hit the firewall either.




barry425 said:
Total timing at 3,000 rpm should be no more than 34 degrees with todays lousy gas, if your timing light will work at that rpm.
After the last timing light turned out to be a dud, I waited and bought a fancy one (Equus 5568 Pro-Timing Light) and its features include "tachometer ranges from 249 to 9,990 RPM", so I am sure it will do fine at 3,000 rpm.
 

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#97 ·
That's a good light. I have the same one. You can advance it as you pull up the rpm's.

With that light you don't need timing tape. I've never seen a balancer spin. I'm sure it happens sometimes, but i've never seen it.

8* is correct for your truck. My 76 is the same.
 
#98 ·
Outer ring movement.

Just put a sharpie line across the outer damper ring to the inner hub and use that as a visual reference to see if the damper outer ring is moving over time.

If you put the sharpie line near to the damper TDC line you will be able to see it w/the timing light with the engine running.
 
#99 ·
Hey Pugys and cobalt,

123pugsy said:
That's a good light. I have the same one. You can advance it as you pull up the rpm's.
After reading all the reviews of the various timing lights for sale on Amazon, I think its a good one too, and fairly priced. May be a bit advance for a rookie like me. Does the advance feature only works on the new DIS ignitions, or old ones too?




123pugsy said:
With that light you don't need timing tape. I've never seen a balancer spin. I'm sure it happens sometimes, but i've never seen it.
cobalt327 said:
Just put a sharpie line across the outer damper ring to the inner hub and use that as a visual reference to see if the damper outer ring is moving over time.

If you put the sharpie line near to the damper TDC line you will be able to see it w/the timing light with the engine running.
Ok, the sharpie method is much easier. Personally, I don't think the damper is spinning but playing it safe never hurts.




123pugsy said:
8* is correct for your truck. My 76 is the same.
Got it, that's the one I will stick with.


Btw, how much timing does the vacuum advance add?
 
#100 ·
timing

Generally speaking,
Initial timing=8 degrees
Mechanical advance in distributor=26 degrees (at the crank)
Subtotal=34 degrees
Vacuum=14-16 degrees
Total= 8+26+14=48
This may be a little high for todays gas. I would limit total around 45 degrees.
Here is a method for finding the optimum timing settings. It is best if done on a hot day.

INITIAL:
Disconnect the vacuum advance hose and plug it.
Set the initial timing at 8 degrees.
Take off from a dead stop and see if the engine pings right off idle. If it does, then back off the initial timing until it stops.

MECHANICAL:
Make sure that the mechanical advance is correct with the timing light. Then take the warmed up engine out on the road and floor it.
If it is an automatic and it pings just after it shifts into a higher gear, back off the mechanical timing (you may have to buy a spring and weight kit).
If it is a stick, put it in a high gear (like 3rd) and floor it. If it pings, reduce the mechanical advance in the distributor (again, you may need a kit).

VACUUM:
If it doesn't ping, then put the vacuum line back on and drive it up a long hill. If it pings, get an adjustable vacuum pot and back off the total available vacuum timing by the screw/cam adjustment on the inside of the distributor (NOT the screw inside the vacuum nipple).

Continue the above steps until you get it to never ping and you're set.
 
#101 ·
dampner slippage

I have seen several dampners slip. The guy with the sharpie idea has it right. make sure that the mark goes from the timing mark down the front of the dampner all the way onto the hub of the dampner so you can see if the rubber layer is allowing the outer ring to slip. IF IT IS, GET A NEW DAMPNER! THIS THING WILL BE A GRENADE AT FREEWAYS SPEEDS!!!
I always run a FluidDampner on all my stuff. I hate buying radiators, water pumps, belts, air conditioner compressors, fenders, batteries, etc. just because I wanted to save a buck and use a worn out dampner.
The rubber on your dampner looks bad to me. It should be even all the way around, but yours seems to be pushing out in at least one spot.
 
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