help on tune up and carburetor issues

[centuryhouse]

Hi all - thanks in advance for any assistance. I'm doing tune up work on my 72 GMC (350 engine, replaced sometime in the 80s, upgraded to HEI (?)).

Since buying the truck I have replaced the spark plugs, wires, cap, rotor, and replaced the fuel filter. It made a slight difference but not much.

The truck starts and runs pretty smoothly, but does not accelerate quickly from a stop OR when already moving. It is very sluggish. Also, if I punch the pedal there is a quick drop-out, like it has died for a second, and then it revs. If I am already going (say 35 mph) and floor it, it barely accelerates. Occasionally it will backfire when I do this. It does not jump into passing gear at that speed. I am not finding any vacuum leaks.

A few concerns / questions:

1) there appears to be no choke - is this normal?

2) the 'butterfly' was so stiff (in the open position) that I couldn't move it with my fingers. I sprayed it all down and now it does move but is still somewhat stiff.

3) the carb is slightly leaky around the sides.

4) what is this blue part (pictured last) and why is it plugged up? Should it be?

Any thoughts about the carb or issues I mentioned? Should I be looking at replacing the carb yet, or rebuilding? I'm not even sure what I have, other than the brand is Holley.

I have not checked the timing yet because I don't have a timing light. I'll be working on this part.

[Wrenchbender Ret]

Sounds like the timing is slow. Advance the timing a little & try it. To advance it , loosen the clamp screw a little & turn the dist. the direction that the vacuum advance points to. CCW looking down on the dist. If it runs better advance it a little more & try it. If it pings on hard acceleration or kicks back or bogs when starting the engine you have gone too far. Take the plugs with sloted screws out of the sides of the fuel bowls. The fuel level should be right even with the bottom of the holes. If not the float on the corresponding bowl needs to be adjusted. These are some basic things to try. If you need more or instructions on adjusting the float level, reply & some of us will help.

[engineer_gregh]

Looks like the carb could stand rebuilding. Like Wrenchbender suggested I would advance the timing. What gap did you set on the plugs? If that is a glass fuel filter I would get rid of it ASAP.

[Coley]

Is the engine in good condition?....fairly stock or?
If it is, perhaps changing from that well used Holley over to a Quadrajet might be a good move.
That manifold is typical of the late 70's and 80's intake manifolds with all the smog ports, etc that came on them.
If you are looking at getting a little more performance perhaps upgrade to a Edelbrock Performer (dual plane- NOT single) manifold.
Are you pretty mechanically handy? I would start by removing that carb and giving it a very serious (carb cleaner) clean-out and compressed air blow out to get things rolling.
The other issue, as mentioned by the guys, is to make sure your timing is set properly. Do you have a light and the capability to check this?
My two bits.
Coley

[70STOVEBOLT]

There is a choke it is just not hooked up. That carb is set up for a hand choke. The lever on the right (passenger) side with the small hole in it connected to the choke plate. You just need a cable for it.

[centuryhouse]

I don't have a timing light but plan to get one on payday.

I advanced the timing to various degrees, and retarded it as well, but neither resolved the issue. It seemed that where it was originally was the best place, based on idle speed, how the engine sounded, and how the exhaust looked/smelled.

I don't remember the gap of the plugs, but I researched it here when I did it and used the most commonly recommended plugs and gap.

Why get rid of the glass filter? I just paid extra specifically to get one, rather than the plastic or metal kinds I had. I wanted to be able to see what was going on. This filter was $16 instead of $4 for the other types.

The observation that the engine is probably late 70s - early 80s is probably correct, based on their telling me (when I bought it) that the engine had been replaced in the 80s. It makes it hard to buy parts that fit.

I am somewhat mechanically handy, though a little intimidated at the idea of a rebuild. I could buy a new carb. It seems I'd have to be able to adjust the mixtures and fine tune it no matter which course I take, and I don't know anything about doing that yet.

[davepl]

Originally Posted by centuryhouse

Why get rid of the glass filter? I just paid extra specifically to get one, rather than the plastic or metal kinds I had. I wanted to be able to see what was going on. This filter was $16 instead of $4 for the other types.

They're a bit of a fire hazard. Good that you can see, to be sure, but maybe only as a temporary tuneup measure to see what's in the tank. Steel doesn't burn, break, or crack.

(Sound of glass filter on cast iron itake)
..tink... ...tink... ...Crack!... ..splash... ...WOOF!...

At a very minimum keep it securely zip-tied somewhere that it's not banging or resting on anything hard. But I'd replace it once your other issues are sorted out.

[Clemsonhelo]

Had a lot of similar issues with my 70. Aside from the aforementioned timing issue, a carb rebuild would do a world of good. I recently rebuilt my Edelbrock on my 350 and it solved nearly all my issues. A rebuild kit is around 40 bucks, and replacement carb is around 300. So if your keeping an eye on finances like I am its worth a go at the rebuild. Fairly straightforward process, and the instructions that come with the kit are straightforward and pretty easy to follow; at least on the Edelbrock side.

I was also nervous about attempting the rebuild, having never done it before but my thought process was that if I couldn't get everything back together, I wasn't really that much worse than before, and if I did get it fixed then I would have saved myself a few hundred bucks. Just my two cents

[Jrainman]

Holley does have a website that will help you trouble shoot and set up your carb, also holly has videos that will help you.

if anything at all it will get you familiar with the sequence of operation of all the working parts in a Holley carb and help you isolate your issues.

[engineer_gregh]

After re-reading the original post and the comments thus far; It could be that you have a worn timing chain and/or worn cam lobes. Either can cause a hesitation when trying to accelerate and can cause an intermittent back fire. Any idea hows many miles are on the engine? I've attached a couple of pictures for reference; on the passenger side front there should be a number stamped and on the drivers side rear there is a number stamped, those numbers can be googled to tell you what year and size engine you have. I still believe I would either rebuild or replace the carb as a first step. A leaking carb and that glass filter are both fire hazards, just my .02 cents of course.

[centuryhouse]

Originally Posted by davepl

They're a bit of a fire hazard. ... Steel doesn't burn, break, or crack.

Excellent points. Thanks for that.

Originally Posted by Clemsonhelo

...a carb rebuild would do a world of good. I recently rebuilt my Edelbrock on my 350 and it solved nearly all my issues. A rebuild kit is around 40 bucks, and replacement carb is around 300. ..... my thought process was that if I couldn't get everything back together, I wasn't really that much worse than before, and if I did get it fixed then I would have saved myself a few hundred bucks

All excellent points. I'm timid about doing this but as you said, not much to lose.

Originally Posted by Jrainman

Holley does have a website that will help you trouble shoot and set up your carb, also holly has videos that will help you.

Good to know, I'll check it out when I get a few minutes free.

Originally Posted by engineer_gregh

...It could be that you have a worn timing chain and/or worn cam lobes. Either can cause a hesitation when trying to accelerate and can cause an intermittent back fire. Any idea hows many miles are on the engine?

...should be a number stamped and on the drivers side rear there is a number stamped, those numbers can be googled to tell you what year and size engine you have

Hopefully it isn't major things worn out, but it certainly could be. No idea of the miles. The previous owner had basically no info, and they guy he bought it from wouldn't respond to my inquiries about general history of the truck.

Here is a number I saw on the engine - is this the one I should be looking for?

[centuryhouse]

Also - couldn't vacuum issues be effecting this? If the carb is leaking around the base wouldn't that cause issues?

Also, and vacuum related, a guy at work has a 67 Chevy pickup. A thicker hose goes from the brake booster to the manifold on his. On mine, there is some plastic part (almost looks like a fuel filter looks) that plugs into the booster and then runs a smaller hose to the manifold. Can someone tell me what that plastic piece might be?

[engineer_gregh]

That number is on the intake. That particular intake came on 1980-85 305 or 350 engines. You will need to get the number off the pad behind your alternator to verify the engine year. I'm assuming your alternator is on the passenger side.

[engineer_gregh]

The plastic piece in line between your intake manifold and brake booster is a check valve. It could cause a vacuum leak. Does it look similar to this picture?

[Motosharpley]

so one thing I didn't see yet is to check that your getting squirt from your accelerator pump. this would cause stumble on acceleration. Looking at the pics there is a good chance your squirters are clogged or the accelerator pump diaphram could be blown. To check pull your air cleaner and with the engine off work the throttle you should be able to see and hear two streams of fuel squirting from the nozzle. If no squirt the nozzle can be removed and cleaned with carb cleaner it is just a single screw but be VERY careful not to drop it down the carb. Or better yet pull the carb and do it off the vehicle.

[Motosharpley]

And if you don't have one already when you get a timing light get a vacuum gauge it makes tuning a whole lot easier

[geezer#99]

Look here for casting number locations and what the numbers mean.
http://www.mortec.com/

Also look here for suffix code info.
http://www.nastyz28.com/decode.php

[El Dorado Jim]

Originally Posted by davepl

They're a bit of a fire hazard. Good that you can see, to be sure, but maybe only as a temporary tuneup measure to see what's in the tank. Steel doesn't burn, break, or crack.

(Sound of glass filter on cast iron itake)
..tink... ...tink... ...Crack!... ..splash... ...WOOF!...

At a very minimum keep it securely zip-tied somewhere that it's not banging or resting on anything hard. But I'd replace it once your other issues are sorted out.

I used to run those glass filters, until I smelled gas one day, popped the hood with engine running and saw gas spraying out like a fan from a length wise split in the glass, don't know why my truck didn't catch fire...luckily, had a piece of steel tubing with me in my toolbox, so I removed the filter and temporarily installed the tubing to get me home, will never use another glass filter again..

[samlsn]

I was recently in the same boat and while my dist had been upgraded to hei, I still had the original resistor ignition wire running from the hei to back of my fuse panel. Also had platinum plugs and copper core plug wires which both serve the hei poorly (I've been told) correcting those issues did improve my situation, along with the new holley 670 S/A carb

[Gromit]

Accelerator pump diaphragm looks like it is leaking - which could mean it is torn as well.

Also the air filter to carb gasket is missing so if run like that long enough the carb could pull in a lot of dirt.

I'm a fan of these Quick Fuel super rebuild kits - they come with all new bolts and screws and springs..

Here is the page from their catalog - should be less than $50 - I don't recognize your carburetor but it is a vacuum secondary with side bowl floats and balance tubes - maybe someone will chime in but it looks like the list number is ..3343920? (number on the front) But I've never seen one that didn't have the word "list" in front of the number...

Anyway; doing the rebuild isn't that bad; remove, disassemble, (take lots of photos), clean, match up the old parts with the ones in the kit; and reassemble using an inch-pound torque wrench using toque specs attached below in the pdf

There are lots of videos on youtube where they show step by step...

[rcazwillis]

Do the rebuild as suggested on that carb. Your problem sounds like a typical accelerator pump issue. Research how to setup the carb. It is not hard, but makes all teh difference.

[centuryhouse]

Originally Posted by Motosharpley

...check that your getting squirt from your accelerator pump. this would cause stumble on acceleration.

I checked with the engine off, and steady, even jets come from both sides.

[centuryhouse]

Originally Posted by samlsn

....I still had the original resistor ignition wire running from the hei to back of my fuse panel

Can you elaborate on this? What would the original (and I presume wrong) wire be like, as opposed to whatever should be there?

[swamp rat]

Originally Posted by centuryhouse

Can you elaborate on this? What would the original (and I presume wrong) wire be like, as opposed to whatever should be there?

What he means is there is a resistance wire or at least in my 72 K20's case what's ( I thought all our trucks had one) called a ballast resistor in line to that wire that cuts voltage to about half where the newer electronic systems require 12V to operate correctly. It looks very much like what is in this link and its mounted to the firewall above and off to the side the distributor

https://www.motoparts.net.au/cat/ele...ors?&position=

[swamp rat]

Here's a pic of my truck, it shows the ballast in the upper right hand corner. I run a Mallory Unilite distributor with Optical pickup, it still requires use of the Ballast resistor.

http://67-72chevytrucks.com/vboard/s...&postcount=332