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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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#52 ·
it has happened to me that sometimes q-jets never work well again, no matter what I make to them, I have several on the shelf that look brand new but they just don't work well, I think they wear out the internal passages or something like that because even after a through clean up and a new parts kit they just don't work well anymore.

stay away from the late models with the internal aneroid, they are the worst, and the electronic models just won't work, smog crap.

I firmly believe that the best improvement you can do to an old car/truck is buying a brand new carburator, I like holley's 1850 for SBC 350's
 
#53 ·
Augusto said:
it has happened to me that sometimes q-jets never work well again, no matter what I make to them, I have several on the shelf that look brand new but they just don't work well, I think they wear out the internal passages or something like that because even after a through clean up and a new parts kit they just don't work well anymore.

stay away from the late models with the internal aneroid, they are the worst, and the electronic models just won't work, smog crap.

I firmly believe that the best improvement you can do to an old car/truck is buying a brand new carburator, I like holley's 1850 for SBC 350's
The idle tubes need to be removed. There are spaces below each one that can easily become plugged and cleaning w/o removing them is next to impossible. Because these orifices also contribute to the overall air/fuel ratio/fuel delivery, if they're not right drivability will suffer.

Once the idle tubes are removed, the passages for the idle restrictions and the idle tubes (they're next to each other in the main body casting) can be properly cleaned out.

If the idle tubes were removed carefully, they'll go back in using the original collars just fine. Otherwise, Ruggles sells replacements, both pressed (like stock) or threaded.

The aneroid type APT was a '75-only deal for the most part. There may be carbs on either side of '75 that have it, but they'll be rare.

The feedback Q-jet as used w/the CCC emissions system can be used for drag racing by removing the metering rods, then rejetting it using smaller jets w/o the metering rods. An electronic Q-jet carb set up like this will not be a good candidate for street use- just for drag racing/WOT use mainly, although it'll work well enough to get back to the pits after a run.
 
#54 ·
So, here is the game plan:

- Inspect vacuum line from intake manifold to transmission. Reapir/replace as needed.

- Switch vacuum advance line back to where it was previously installed on the carburetor (no vacuum at idle).

- Disconnect the choke, tie open the choke plate and try starting and running the truck.

- Test out transmission modulator.

- If all goes well, then yank the distributor and redo the timing.

- If all does not go well, then its time to revisit an old friend, the carb rebuild thread. Will pay special attention to the idle tubes.

- Fuel pump volume test still recommended?

- Concerns about the intake gaskets were raised. Do they look ok here (#130)?



Looks good?



I probably won't get to work on the truck until Saturday or Sunday, so it will be a day or two before results are posted.



As always, thank you very much for the continued help :thumbup:
 
#55 ·
- Inspect vacuum line from intake manifold to transmission. Reapir/replace as needed.

Good for maitenance, but imo not your problem

- Switch vacuum advance line back to where it was previously installed on the carburetor (no vacuum at idle).

could be an issue, I like to run the one that gives timing at idle...helps out mgp a little

- Disconnect the choke, tie open the choke plate and try starting and running the truck.

could also be it, explains the nasty plugs...

- Test out transmission modulator.

one again...I would save this one for last.

- If all goes well, then yank the distributor and redo the timing.

Dont pull the dist! Just reset the timing by ear, your balancer may have slipped and then you CANT set it right by a light. I urge you to reset the timing, by ear, first.

- If all does not go well, then its time to revisit an old friend, the carb rebuild thread. Will pay special attention to the idle tubes.

- Fuel pump volume test still recommended?

I dont think its even an issue...all my luck with stock pumps tell me they either are good, or shot...no in between.

- Concerns about the intake gaskets were raised. Do they look ok here (#130)?

everything looks fine to me.
 
#56 ·
Thank BigRoy, I'll keep those things in mind.


I've been doing some reading today and have come across two topics that I can use some help on.


Been reading another thread on this forum ("what qjet to buy?"), and there was some discussion on the block off plates for the intake manifold gaskets. As I have mentioned before, this truck does not have a heat riser valve. I did use the block off plates when I redid the intake manifold gaskets. Could they be causing any issue(s)?


I was also looking up information on HEI vacuum advance, how they work and how to recurve them, when I ran across a page this page: "Adjustable Vacuum Advance And HEI Distributor Recurve". The author states that, as part of the timing procedure, Lars Grimsrud recommends setting total timing at 36 degrees, instead of a base timing and going from there: "How to Set Your Timing for Peak Performance ". What are your thoughts on this?
 
#57 ·
lt1silverhawk said:
Thank BigRoy, I'll keep those things in mind.


I've been doing some reading today and have come across two topics that I can use some help on.


Been reading another thread on this forum ("what qjet to buy?"), and there was some discussion on the block off plates for the intake manifold gaskets. As I have mentioned before, this truck does not have a heat riser valve. I did use the block off plates when I redid the intake manifold gaskets. Could they be causing any issue(s)?


I was also looking up information on HEI vacuum advance, how they work and how to recurve them, when I ran across a page this page: "Adjustable Vacuum Advance And HEI Distributor Recurve". The author states that, as part of the timing procedure, Lars Grimsrud recommends setting total timing at 36 degrees, instead of a base timing and going from there: "How to Set Your Timing for Peak Performance ". What are your thoughts on this?

The block offs shouldnt be causing this big of an issue.

on my street vehicles I set the timing to where when the engine is warm, it will start smoothly...just a couple degrees before it starts to lug the starter when it tries to fire.

But the method mentioned is a great one.

I would tie the choke open and try it...just to eliminate that. then concentrate on the timing.
 
#58 ·
cobalt327 said:
The idle tubes need to be removed. There are spaces below each one that can easily become plugged and cleaning w/o removing them is next to impossible. Because these orifices also contribute to the overall air/fuel ratio/fuel delivery, if they're not right drivability will suffer.

Once the idle tubes are removed, the passages for the idle restrictions and the idle tubes (they're next to each other in the main body casting) can be properly cleaned out.

If the idle tubes were removed carefully, they'll go back in using the original collars just fine. Otherwise, Ruggles sells replacements, both pressed (like stock) or threaded.
I tried once to remove them but they were pretty much stuck in there and I feared damagin 'em, what would be a good way of doing it? heating up the casting maybe? what would be a good tool to grab them without crushing?

thanks for the tip, I'm sure this will cure some of my "junked" q-jets
 
#59 ·
Augusto said:
I tried once to remove them but they were pretty much stuck in there and I feared damagin 'em, what would be a good way of doing it? heating up the casting maybe? what would be a good tool to grab them without crushing?

thanks for the tip, I'm sure this will cure some of my "junked" q-jets
If you look closely you can see there is an outside collar and the inner idle tube. Use a flat punch about 0.085" diameter to drive the tube out of the collar. Don't drive the tube down too deep, just carefully tap it down about 1/8" or so. If you go much further the bottom of the tube will hit the bottom of the passageway.

Then take a fairly long (at least 1") wood screw or drywall screw that fits the top of the collar. You can grind the sharp point off the screw so the screw threads bite into the collar before the tip of the screw touches the tube you just drove out of the collar.

I always use a small pencil type butane "torch" to warm the exposed area adjacent to the tubes instead of pulling them cold. 10-12 seconds is all it takes. A good cigar lighter could work well for this- you don't want nor need a big amount of flame/heat. This makes it VERY easy to pull them.

Side cutters work excellent- I use a 2" long x 1" wide x 3/16" thick strip of wood (made from a piece of wood paneling but could be most anything) across the carb body to protect it as well as making it easier to lever the screw/tube out. The strip- when positioned fairly close to the screw extending up from the tube- helps removal by letting the tips of the cutters reach downward from horizontal to grip the screw, then when levering the tubes out the cutters have enough room to remove them before the handles and your hand touch down, if you follow. Then tip the carb upside down and the idle tubes will fall out.

Then you can get into the passageway and flush/scrub it out so there's nothing in there to clog things up. Take a thin stiff wire to probe/clean the idle restriction tube then use compressed air and/or carb cleaner spray to flush the restriction tube. This blows all the crud into the (now empty) idle tube passageway so it can all be removed.

Hopefully that'll make a big difference in how the "unfixable" Q-jets will work for you.
 
#60 ·
BigRoy1978 said:
The block offs shouldnt be causing this big of an issue.

on my street vehicles I set the timing to where when the engine is warm, it will start smoothly...just a couple degrees before it starts to lug the starter when it tries to fire.

But the method mentioned is a great one.

I would tie the choke open and try it...just to eliminate that. then concentrate on the timing.
Block off plates with that style choke dont go together at all. if the exhaust is not allowed to cross over it will take forever for the choke to open.

Lock the choke open, clean the plugs and try it. I think you will find the problem will be gone.
 
#61 · (Edited)
I wired the choke plate open using a paper clip, sprayed some starting fluid, and started up the truck.





The truck started up find but the speed immediately dropped to below 750 rpm. I gave it enough gas to keep it at 1,500 rpm for the first few minutes. I did rev it a few times and then eventually just let it run on its own as it idled close to 1,000 rpm, when the temperature began to rise.







I then took the truck for a spin and saw no difference in how it drove.


Up next: timing. The reason I mentioned that I was planning on pulling the distributor is because the way I last installed it, the canister is hitting the intake manifold at this point, which means the only thing I can do is to retard the timing from where it is at right now.


On a side note, another member has expressed concern that since the fuel did have crud it in, there is a possibility that there is crud in the fuel lines and/or the fuel tank is rusted. I will be looking into the carburetor at some point as well.


As always, thank you for the continued help. :thumbup:
 

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#63 ·
lt1silverhawk said:
While watching the video from yesterday, I just noticed that the fast idle cam wasn't raised up from the very beginning. Would that be because the choke plate was "locked" into an open position?
Yes they are connected. The closing of the choke also pull up the fast idle cam. The choke can open independently due to the linkage design which is why you need to kick the idle down
 
#69 · (Edited)
In regards to timing and fuel issues:

The next step is definitely adjusting the timing and a look inside the carburetor itself. There will be a few days before I will post back with any results on this.




-------------------- On a Separate Topic --------------------




In regards to fouled plugs, intake gaskets and vacuum leaks:

sbchevfreak said:
Is it just me, or do the "even" plugs look to be badly oil fouled? If so, I would be looking at intake gasket sealing on that side....
T-bucket23 said:
Block off plates with that style choke dont go together at all. if the exhaust is not allowed to cross over it will take forever for the choke to open.
T-bucket23 said:
I would just to see if they [plugs] are black again. Could have been the choke fouling them
ztwntyn8 said:
I'd check the plugs first but a bad enough vacuum leak can cause hesitation and I've also experienced hesitation with too little base timing and that will def rob the **** out of your power and mileage.
Based on the above comments, I immediately had two questions:

- Would the intake manifold gasket that is not sealing correctly be causing both the fouling as well as any vacuum leaks?

- And if that is the case, then the intake manifold vacuum line routed to the transmission would cause it shift poorly because there is no vacuum source?



A quick search online led to the following passage:

Vintage Engines:


Automatic transmissions on many older vehicles use a vacuum modulator. It senses engine load by using intake manifold vacuum to tell the transmission when to shift. If the diaphragm in the modulator leaks, transmission fluid will be sucked into the intake manifold. This can produce smoke that might be confused with engine oil smoke, even though the engine may be in perfect condition. One clue is when the spark plugs nearest the vacuum tab on the intake manifold become oil-fouled with automatic transmission fluid (ATF). A leaking modulator diaphragm can also cause rough engine idle because it allows air to leak into the intake manifold. Harsh, late transmission shifts, or no upshifts, are other symptoms.


Be sure to question the vehicle owner thoroughly. The combination of any or all these symptoms can lead an owner to believe that an engine overhaul is needed. The key to diagnosing a faulty vacuum modulator is that the engine is probably consuming ATF and the level drops consistently.


From: "Automotive Engines: Diagnosis, Repair and Rebuilding" by Tim Giles. Page 58. Copyright 2010. Found online on Google Books: http://books.google.com/books?id=VX...ssion shifts poorly intake vacuum&f=false[/I]



Although it is not about intake manifold gaskets, the fore-mentioned quote echoes points raised by cobalt and Augusto on page 2of this thread:
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.
Augusto said:
about the modulator's piece of hose, it's a good idea replacing it for a new one even though I don't believe that's is your problem, but let me tell you that I have seen modulators that fail, they get the internal diaphragm riped and let the engine suck ATF, that makes them smoke and run terrible. easy check, disconect the lline at the carb end and stick a piece of white paper, a napkin or something, it should come out clean, no traces of red ATF.

The transmission's poor shifting is one of the issues. I will be checking to see how the transmission fluid levels look, if the diaphragm is indeed working, and check the condition of the vacuum line. I will be able to report back on this shortly.
 
#70 ·
lt1silverhawk said:
The transmission's poor shifting is one of the issues. I will be checking to see how the transmission fluid levels look, if the diaphragm is indeed working, and check the condition of the vacuum line. I will be able to report back on this shortly.
Have you had a vacuum gauge on this yet? If so, what was the idle reading and needle behavior?
Was a compression test done on this beast? Results?

I hate to say it, but if the timing is right (or close) and the vacuum normal, etc., it may be time to go back into the carb... :pain:
 
#71 ·
cobalt327 said:
Have you had a vacuum gauge on this yet? If so, what was the idle reading and needle behavior?
Was a compression test done on this beast? Results?
Haven't hooked up the vacuum gauge to it, but I thought of that immediately after reading the passage I quoted. No compression test on this "beast" of a crate motor either lol, but I will be able to do that when I check on the plugs again.




cobalt327 said:
I hate to say it, but if the timing is right (or close) and the vacuum normal, etc., it may be time to go back into the carb... :pain:
Well, luckily for me, if there is anything I am now comfortable tearing into, its the good 'ol Q-jet. :D :thumbup: That's two votes for re-tearing into the carb.


I gotta mention that when the timing was done by ear last summer, the truck ran perfectly. No issues whatsoever with driving. I drove it to work and around the block and whatnot. The only trouble it gave was the occasional refusal to start, but that turned out to be only super-old battery and ground cables and weak battery posts, which were replaced.The truck still does that after having been started and run for a while and then shut off. It takes a few attempts to get it started again.


The two reasons why I never adjusted it by using a timing light last summer was because the timing light I bought turned out to be a dud, and I mistakenly thought that in order for me to adjust the timing properly, the truck had to reach the proper operating temperature. Since the thermostat was never installed while doing the "3-6 hour coolant flush", I never bothered with the timing, choosing to wait until the flush was done.


With that said, yes, the timing is definitely in need of adjustment. But I don't see that being the major issue, unless the timing somehow changed that fateful day of February 1st. I did mess with it yesterday by moving the distributor back and forth, which is when it was most obvious that the distributor needs to be reinstalled so it doesn't hit the intake manifold when being advanced. This may or may not be of any significance, but back when I originally did the timing in summer, I had the idle mixture screws turned out a full six turns to provide enough fuel.
 
#72 ·
lt1silverhawk said:
I gotta mention that when the timing was done by ear last summer, the truck ran perfectly. No issues whatsoever with driving. I drove it to work and around the block and whatnot. The only trouble it gave was the occasional refusal to start, but that turned out to be only super-old battery and ground cables and weak battery posts, which were replaced.
What has been changed between when it ran OK and now?
 
#73 ·
cobalt327 said:
What has been changed between when it ran OK and now?
Between when the timing was done by ear last summer and when the truck first began exhibiting these problems, all I did was begin the cooling system flush, an oil change and added a can of Restore. The other stuff was the cables and battery posts. It drove just fine up until the day before the problems began.


After the truck began having problems, I was advised to check the ignition components. I check the spark plugs, the ICM and the coil. The spark plugs were fouled, more so on the passenger side (pictures in post #9). The ICM passed the test at Autozone twice. The coil was tested using a multimeter and while I was able to get a reading for the primary, but not for the secondary. The coil was swapped out under warranty, but the results were the same (post #9). Once I put everything back together, the truck began running just fine again. But there was some light smoke seen from the passenger-side of the firewall. It began a few minutes after the truck was turned off, and was there for a couple of minutes at the most.


Ever since after that, the truck only ran fin the next day. I cleaned out the glass-cased fuel filter in front of the carb which had crud in it (pictures on page 3, post #33). I checked on the choke. I tried running it with the choke plate wired open. Most recently, I moved the distributor back and forth and turned out the idle mixture screws on the carb to a full six turns (same as before). The distributor ended up in the same place as before, with the canister touching the intake manifold. I did not take for a spin after that.


----------



This morning, I started up the truck to move it for the street sweeper. I let it warm up for a about three minutes and then began moving it to park on the other side of the street. As soon as the truck began turning, the "Oil" light came on for a brief moment. I let the truck continue to warm up for a bit and then decided to take it for a spin.


The truck ran pretty good. Not perfect, but the shifting was happening a lot more quickly and softly and there wasn't much hesitation. Definitely no bogging. But I could detect slightly less power. I took it onto the main street and it got up well beyond 45 mph before I had to slow down. I parked the truck and it revved with no hesitation, even though it had been hesitating around the 2,500 rpm mark. The "Oil" came on only once and it was soon as I began driving and made a turn.


I turned off the truck and came back to it about 30 minutes later and took it for a quick spin. As usual, the idle was a lot lower than when it is first started in the morning (1,500 rpm vs 750 rpm). Again, the truck ran good, but not perfect. I parked it again and came back to it about an hour later.


Letting the engine run and leaving the transmission in "Park", I first checked the transmission fluid. Didn't see much on the dipstick. I put it back in and checked it again:






I then turned the engine off and checked the oil. Nothing showed on the dipstick. I then started truck, let it run for about 30 seconds, turned it off and checked again. There was quite a bit of oil after that:






So that's where its at. Some time tomorrow morning, I should be posting results from hooking up the vacuum gauge to the intake manifold, along with a test of the diaphragm. Maybe even check a plug or two. Distributor re-installation tentatively set for Saturday.
 

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#75 ·
123pugsy said:
YOU NEED OIL!!!!!!!!!!!!!!!!!!!!!! :smash:

You don't start the truck to check the oil.
Lol, I know that Pugs, but the reason I did start it was because I was wondering if it was weather-related (been pretty cold here during the nights and mornings for the past few days, well pretty cold for Los Angeles). Yes, it was risky, but I didn't want to overfill without checking to make sure that it wasn't because it was too cold (did that before and had to take some out). I promise it is getting as much oil as needed before the next start up. At this point, I'm gonna assume its been leaking/burning on the passenger side. I'll try the 20w-50 to be safe. The transmission fluid seems awful low too. I got nothing but crud on the towel when I wiped down the dipstick.
 
#76 · (Edited)
Thursday morning, I added more oil and some Lucas to the engine. I used 20w-50 this time. I used 10w-40 during the previous oil change. I also check the cool ant level, which wasn't quite at full. There was also some crud under the oil filler cap. Visually checked the fuel filter before the carb and it looked clean and free of any debris. There was not starting up of the the truck this day.







----------




Started up the truck this evening and let it warm up on its own for a good twenty minutes. It reached operating temperature (about 195 degrees F) and then I took it for a spin. The truck ran pretty good again. No hesitations. No bogging.


This is the second time since the problem began that somethings were removed or adjusted and brought back to the same settings and the truck began working. The first time was replacing the coil.The second time was moving the distributor back and forth and closing up and turning out the idle mixture screws on the carb to 6 full turns. Both the distributor and the screws are at the position as before. No change in position.


The fuel was low so I added about five gallons to the tank.


I decided to take off the carburetor and remove the airhorn to see if there was any debris in the fuel bowl. I also removed the fuel filter for inspection and it came out clean.


Once the airhorn was removed, it was obvious that there was debris at the bottom of the fuel bowl.







I briefly looked at the idle tubes that cobalt mentioned, and they did not appear to be clogged. However, I did notice that the main well bleed tubes in the airhorn appeared clogged. Something to be concerned about?







I also checked the jets using the "light test", and they appear to be fine.


I removed the throttle body from the float bowl/main body, and discovered that the epoxy used during last rebuild was coming off. However, I cannot tell if it is even needed and if leaking is occurring.






Slightly off topic, but does anyone feel this info also should be added to the carburetor rebuild thread for a follow up?


Does anyone feel a complete tear down and rebuild is needed? Or can I just clean it all up using carb spray and bolt it back up? Can the same gasket between the fuel bowl and the airhorn be used? If yes, then i want to get this bolted up tomorrow and move onto timing. I've already begun work on the timing tape.
 

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