Sorting out POs mess

[RADustin]

Hi,

I recently purchased a '76 c20 CCLB with a non original 454(3rd digit L) and a granny 4spd. I haven't pulled the numbers off the engine but I have bought all the ignition parts and a coolant temp sensor for a '76 c20 with a 454 and everything has worked thus far. Engine 'looks' original, but I know it isn't.

Pic-

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I'm having some problems with a random stumble here and there, where the cure is to throttle the pedal a little on and off(mostly off) and the engine will come back smooth and I can continue. Heat and/or runtime definitely effect how often it will stumble. Also, under more load vs less load makes a difference(fuel consumption). All the ignition is new and I have moderate confidence it is good. I may change the HEI module again to experiment.

I'm going to rebuild the carb with the edelbrock 1921 rebuild kit and see how that works out. The truck is a dog on power, so I am hopeful this will wake it up a little. None the less, when I do call on the power it usually stumbles anyways. Entering the freeway has become a chore.

My concern is in fuel delivery. I have a bastardized system, so I know I may not be able to get definitive answers here but maybe I can bounce some ideas around as I investigate further.

• The carb is an Edelbrock Q-jet 1906. Brand new in-carb filter. No other filters present except maybe the socks in the tanks.
  
• The fuel pump has a return line, but it is capped off.
  
• The truck has both fuel tanks, but only a single fuel line goes to either tank. Then on the outboard side of the tanks I can see the fill hose and maybe a vent line? No return line, no wire for the sender on either tank. Fuel gauge does not work. I believe it is stuck past full(sender goes 0 or 90 at full?)
  
• A mechanical 3 way valve is mounted on the drivers floor board that switches the 2 gas tanks to the single pump feed. It seems to work as I have switched back and forth across the tanks several times with success.
  
• The fuel lines are kind of a mess of some original looking metal lines and some 'custom' rubber lines/clamps.
  
• The line from the pump to carb is metal from the pump up to behind the alt of the passenger head. Then it goes to rubber to the carb(I replaced this). I made the rubber line kind of too long so it bends out over the valve cover and goes straight into the carb, instead of laying on the intake mani. My idea was to reduce fuel temps, encase fuel vapor was the problem.
  
• everything on the HEI is new except the vac advance. I'm changing that this weekend. New plugs/wires. When I initially put the new HEI parts in it, a clutch out right off idle stumble cured, but returned. I may put another module in it to see if it cures this. Maybe the truck is eatting HEI modules?

My eventually goal is to shortbed the truck with a factory shortbed tank setup from a later model truck with a drivers side tank and fuel door. I plan to remove all the current mess and probably run some new hardlines up to the front of the truck.

In the meantime, before I shortbed the truck, I'd like to ensure the fuel system is as 'correct' as I can manage- at least cure the stumble

My questions are-

1. Is the fuel pump return OK to be capped off?
2. Is the return needed?
3. could no return cause my stumble?
4. could maybe no tank vent cause the stumble? Is the cap vented?
5. engine has got harder to start now that the truck is in a much hotter environment. (bought truck in NJ, I'm in south LA) Everything kind of sounds like a sort of vapor lock?
6. Does anyone have a diagram for a standard short bed truck setup? I'd like to get an idea of how I will run it in the future.

I'm interested in any conversation I can generate, I appreciate any help or ideas. I'm going to mess with the truck again in the next few days and play with some more ideas.

I'm going to try to inject propane in the carb when it stumbles, see if it cure it to ensure it is a lean stumble. I'll also rebuild the carb and see how that does. I'll report back my findings.

Thanks!

 

[1987 GMC Jimmy]

Hi,

I recently purchased a '76 c20 CCLB with a non original 454(3rd digit L) and a granny 4spd. I haven't pulled the numbers off the engine but I have bought all the ignition parts and a coolant temp sensor for a '76 c20 with a 454 and everything has worked thus far. Engine 'looks' original, but I know it isn't.

Pic-

You must be registered for see images attach

I'm having some problems with a random stumble here and there, where the cure is to throttle the pedal a little on and off(mostly off) and the engine will come back smooth and I can continue. Heat and/or runtime definitely effect how often it will stumble. Also, under more load vs less load makes a difference(fuel consumption). All the ignition is new and I have moderate confidence it is good. I may change the HEI module again to experiment.

I'm going to rebuild the carb with the edelbrock 1921 rebuild kit and see how that works out. The truck is a dog on power, so I am hopeful this will wake it up a little. None the less, when I do call on the power it usually stumbles anyways. Entering the freeway has become a chore.

My concern is in fuel delivery. I have a bastardized system, so I know I may not be able to get definitive answers here but maybe I can bounce some ideas around as I investigate further.

• The carb is an Edelbrock Q-jet 1906. Brand new in-carb filter. No other filters present except maybe the socks in the tanks.
  
• The fuel pump has a return line, but it is capped off.
  
• The truck has both fuel tanks, but only a single fuel line goes to either tank. Then on the outboard side of the tanks I can see the fill hose and maybe a vent line? No return line, no wire for the sender on either tank. Fuel gauge does not work. I believe it is stuck past full(sender goes 0 or 90 at full?)
  
• A mechanical 3 way valve is mounted on the drivers floor board that switches the 2 gas tanks to the single pump feed. It seems to work as I have switched back and forth across the tanks several times with success.
  
• The fuel lines are kind of a mess of some original looking metal lines and some 'custom' rubber lines/clamps.
  
• The line from the pump to carb is metal from the pump up to behind the alt of the passenger head. Then it goes to rubber to the carb(I replaced this). I made the rubber line kind of too long so it bends out over the valve cover and goes straight into the carb, instead of laying on the intake mani. My idea was to reduce fuel temps, encase fuel vapor was the problem.
  
• everything on the HEI is new except the vac advance. I'm changing that this weekend. New plugs/wires. When I initially put the new HEI parts in it, a clutch out right off idle stumble cured, but returned. I may put another module in it to see if it cures this. Maybe the truck is eatting HEI modules?

My eventually goal is to shortbed the truck with a factory shortbed tank setup from a later model truck with a drivers side tank and fuel door. I plan to remove all the current mess and probably run some new hardlines up to the front of the truck.

In the meantime, before I shortbed the truck, I'd like to ensure the fuel system is as 'correct' as I can manage- at least cure the stumble

My questions are-

1. Is the fuel pump return OK to be capped off?
2. Is the return needed?
3. could no return cause my stumble?
4. could maybe no tank vent cause the stumble? Is the cap vented?
5. engine has got harder to start now that the truck is in a much hotter environment. (bought truck in NJ, I'm in south LA) Everything kind of sounds like a sort of vapor lock?
6. Does anyone have a diagram for a standard short bed truck setup? I'd like to get an idea of how I will run it in the future.

I'm interested in any conversation I can generate, I appreciate any help or ideas. I'm going to mess with the truck again in the next few days and play with some more ideas.

I'm going to try to inject propane in the carb when it stumbles, see if it cure it to ensure it is a lean stumble. I'll also rebuild the carb and see how that does. I'll report back my findings.

Thanks!

1. It is if you have two line sending units in your tanks. I'm thinking it should, but you never know. If you can check, do so. It just sounds like someone put a newer style pump on it.

2. Again, it's not necessary if you're sending units have only two outlets instead of three. If they do for some reason, you can cap the return line on the sending unit, too, and that'll eliminate your return, but you shouldn't have it. My '86 model year Caprice has the 1985 fuel system, which is returnless, but they went to a return style in the 1986 fuel system.

3. No.

4. If there's something wrong there, possibly. That truck should have come with a charcoal canister, which mediates fuel tank venting. If that's still there and properly hooked up, maybe try replacing the fuel cap on it. If it's a total mess, deletion is always an option, which you do by disconnecting your from tank vapor line and putting an inline filter on it, but it's non-obtrusive to performance if you can make it work. I'd actually like to see pictures of that if possible. Oh, the cap is not a vent on an EVAP fuel system.

5. Have you checked your timing? I think that's a good place to look. I know you're having some qualms about your fuel metering/delivery situation, and that can be causing problems, too, but the ignition timing could be off. 8-10* BTDC would be a good place to try your base timing. If you are suspecting vapor lock, make sure your makeshift lines you described aren't coming in close contact with extreme heat sources (i.e. exhaust manifold). Also, do you have a way to see engine temperature? I would consider investing in a small non-contact thermometer. Thirteen dollars at the Harbor Freight on Airline.

6. I'm attaching LMC's pre-bent fuel line diagram. There are better service manual style diagrams. @chengny would have that one.

Also, I'm willing to come help you if you want. I live off Broadway Street close to The Boot if you know where that is. I have a timing light, multimeter, distributor wrench, and a bunch of other gadgets. I'm in Mississippi this week, but I'll be back before the start of next week. I also think it's a good idea addressing the vacuum advance canister and checking for vacuum leaks. Have you considered putting a Quadrajet on it? Almonaster Auto Salvage had a good Quadrajet core in a square body a few months ago so it wouldn't be hard to find one. You mentioned something about HEI modules. Are you talking about the HEI coil or the Ignition Control Module? If the ICM was bad, you'd either not be able to start, or it'd die once it got hot. There's a test for the HEI coil you can do with a multimeter, which I'll attach to this. I've found that older ones develop dead spots on the conductive part, but they still work despite this. Those things last a long time. Finally, check this thread out pertaining to your fuel gauge issue. Mr. Jerry very privy to the electronics of square bodies, but he is also adept in their complex engineering concerns and how their systems are supposed to work, which allows for a very tailored diagnostic when someone comes on here with something broken or not working like it's supposed to.

http://www.gmsquarebody.com/forum/showthread.php?t=8926

[yt]SEOdRoeS4mk[/yt]

 

[RADustin]

1. It is if you have two line sending units in your tanks. I'm thinking it should, but you never know. If you can check, do so. It just sounds like someone put a newer style pump on it.

2. Again, it's not necessary if you're sending units have only two outlets instead of three. If they do for some reason, you can cap the return line on the sending unit, too, and that'll eliminate your return, but you shouldn't have it. My '86 model year Caprice has the 1985 fuel system, which is returnless, but they went to a return style in the 1986 fuel system.

3. No.

4. If there's something wrong there, possibly. That truck should have come with a charcoal canister, which mediates fuel tank venting. If that's still there and properly hooked up, maybe try replacing the fuel cap on it. If it's a total mess, deletion is always an option, which you do by disconnecting your from tank vapor line and putting an inline filter on it, but it's non-obtrusive to performance if you can make it work. I'd actually like to see pictures of that if possible. Oh, the cap is not a vent on an EVAP fuel system.

5. Have you checked your timing? I think that's a good place to look. I know you're having some qualms about your fuel metering/delivery situation, and that can be causing problems, too, but the ignition timing could be off. 8-10* BTDC would be a good place to try your base timing. If you are suspecting vapor lock, make sure your makeshift lines you described aren't coming in close contact with extreme heat sources (i.e. exhaust manifold). Also, do you have a way to see engine temperature? I would consider investing in a small non-contact thermometer. Thirteen dollars at the Harbor Freight on Airline.

6. I'm attaching LMC's pre-bent fuel line diagram. There are better service manual style diagrams. @chengny would have that one.

Also, I'm willing to come help you if you want. I live off Broadway Street close to The Boot if you know where that is. I have a timing light, multimeter, distributor wrench, and a bunch of other gadgets. I'm in Mississippi this week, but I'll be back before the start of next week. I also think it's a good idea addressing the vacuum advance canister and checking for vacuum leaks. Have you considered putting a Quadrajet on it? Almonaster Auto Salvage had a good Quadrajet core in a square body a few months ago so it wouldn't be hard to find one. You mentioned something about HEI modules. Are you talking about the HEI coil or the Ignition Control Module? If the ICM was bad, you'd either not be able to start, or it'd die once it got hot. There's a test for the HEI coil you can do with a multimeter, which I'll attach to this. I've found that older ones develop dead spots on the conductive part, but they still work despite this. Those things last a long time. Finally, check this thread out pertaining to your fuel gauge issue. Mr. Jerry very privy to the electronics of square bodies, but he is also adept in their complex engineering concerns and how their systems are supposed to work, which allows for a very tailored diagnostic when someone comes on here with something broken or not working like it's supposed to.

http://www.gmsquarebody.com/forum/showthread.php?t=8926

[yt]SEOdRoeS4mk[/yt]

I appreciate the reply.

I need to get under there with a mirror and a light and try to look at the tops of the tanks. There is no canister anymore- therefore I'm not sure exactly how the tanks are venting. The previous owner(s) may have capped off everything except the feed and maybe that's why after a little driving I'm having fuel issues. I can try to get a vented cap and see what that does.

I haven't checked timing. I'm going to do that maybe this weekend when I finish rebuilding the HEI. I haven't removed the HEI yet- I rebuilt the top of it while still bolted in the engine. I actually did all the work in a Love's gas station in PA on the way back to NOLA. I have a fortune in the HEI at this point, I'm sure I could have bought a brand new one by the time I have pieced everything together from NAPA. But everything is new except the vac advance. I'm going to function check it when it is out with my mighty mite vacuum pump. I believe the timing vacuum is ported, so I shouldn't have to unplug the vacuum line to set initial timing- or maybe I do?? I can also mess with the vacuum on the HEI while its running and watch the timing. I'll do this as well.

The fuel lines only get close to something hot right before the carb, where the line passes up behind the alt and over to the carb. But it's not terribly hot. The fuel should be pressurized a little so I don't think that short distance is going to cause much problems. I have an infrared temp probe. I can check around with it. I replaced the coolant temp sender in the head this weekend and the coolant gauge works now. Truck runs with the needle pointing straight up, right in the middle.

I'll checkout the LMC link, thanks.

The coil is brand new. I changed the ICM as well, but I noticed right after changing everything that the truck drove better- but has sense degraded.

I live over in Lakeview, you're further in the city- but not far. I have plenty of tools- my Dad actually owns a shop out in Kenner, but I do appreciate the offer. I can borrow anything I don't own, but my knowledge is with EFI and late model stuff mostly. Only carbs I've worked on were Holley. If I did get rid of this Q-jet, I'd probably just go to a Holley.

When I redo the exhaust I'm going to add in 02 bungs for tuning- but that won't happen until I short bed it.

Do you ever go to the cruise nights in the Kenner/Metairie area? Be cool to catchup one Saturday afternoon and show you what I have. Maybe you can teach me about what I bought lol.

I'm working on this thing very sporadic right now as I'm fitting it in between work, life, and studying for my professional engineering test. Once I get this test out of the way I'll be much more freed up and will start collecting parts for the shortbed swap. I have many questions about that as well. lol.

Thanks again, we'll definitely have to meet up

 

[CSFJ]

If you're thinking that the lack of venting is creating an issue, drive it without the cap on the tank. Cheaper than a vented cap when you're not positive it the problem.

 

[RADustin]

If you're thinking that the lack of venting is creating an issue, drive it without the cap on the tank. Cheaper than a vented cap when you're not positive it the problem.

this crossed my mind. I was trying to be a little more PC about it. Don't want to freak out the soccer moms.

 

[1987 GMC Jimmy]

Okay, so you're off the 610 bypass by the lake and the park and stuff, huh? I've never heard of Cruise Nights, but I'd like to know more about it. It sounds like fun, and it's probably blasphemy for a Tulane student to say this, but I like Kenner and Metairie more than the city itself so that'd be fun.

 

[CSFJ]

this crossed my mind. I was trying to be a little more PC about it. Don't want to freak out the soccer moms.

It's just a cheap and easy check to eliminate the possibilty of vacuum building in the tanks. Besides, you're on a forum where trucks with diesel VIN's are considered the holy grail, because they'll NEVER see an emissions inspection. Hell, sitting here waiting for my class to start, I have a better chance of some snowflake reading this over my shoulder and losing their ******* mind about the environment, than any of the soccer brigade knowing you're driving around (gasp) without a fuel cap. Lol

 

[RADustin]

It's just a cheap and easy check to eliminate the possibilty of vacuum building in the tanks. Besides, you're on a forum where trucks with diesel VIN's are considered the holy grail, because they'll NEVER see an emissions inspection. Hell, sitting here waiting for my class to start, I have a better chance of some snowflake reading this over my shoulder and losing their ******* mind about the environment, than any of the soccer brigade knowing you're driving around (gasp) without a fuel cap. Lol

I'll fit in well around here.

 

[RADustin]

Okay, so you're off the 610 bypass by the lake and the park and stuff, huh? I've never heard of Cruise Nights, but I'd like to know more about it. It sounds like fun, and it's probably blasphemy for a Tulane student to say this, but I like Kenner and Metairie more than the city itself so that'd be fun.

they are put on by custom cruisers car club. We like to try and make them as much as we can. Fun to get out and hang out with like-minded people.

http://www.customcruiser.net/calendarofevents.htm

Yes, I'm just North of 610 and between Canal and West End.

It just takes work to do things in the city. Parking is also a PITA. I like living close to the action so I have an easy choice of running to mid-city for some food and beers or running right to Metairie/Kenner and work on cars, etc. So I tend to agree with you.

 

[Craig 85]

I had a stumble problem on my 3+3 off idle and acceleration from steady speed. It had been parked for a couple of years prior to my purchase and me installing the new engine. With our crappy California fuel and age, the rubber plunger for the accerator pump in the carb failed causing the engine to momentatily starve. I rebuilt the carb and it was as good as new.

 

[RADustin]

good to hear- going to rebuild my edelbrock quadrajet this weekend.

hoping that fixes it up.

Is there a recommended fuel pump that wouldn't have a return but would work on my 454 with the eddy quad?

 

[1987 GMC Jimmy]

I don't know if you use RockAuto, but I like their site because it shows you all kinds of pictures and stuff. I looked to see what they had available for a '76 C20 with a 454, and all they had were three line pumps. I checked for a 350, and they had both two and three. I figured something that old would have had just had a two line pump, but maybe they did something different with the BBC. I use a Delphi mechanical pump on my '86 model year Caprice, which has a two line pump. It's made in South Korea, but it's never had a problem. AC Delco makes pumps, too, and those would be good. Unless I'm missing something, you may have to just cap that return port.

 

[Craig 85]

Check your clocking of the fuel hose locations. My 79 K15 had a fuel pump clocked at 45 degrees to the frame. When I had to get a replacement the 4WD version was clocked straight at the frame and wouldn't work. I had to use a 2WD version that was clocked forward and lengthen the rubber fuel lines.

 

[RADustin]

I don't know if you use RockAuto, but I like their site because it shows you all kinds of pictures and stuff. I looked to see what they had available for a '76 C20 with a 454, and all they had were three line pumps. I checked for a 350, and they had both two and three. I figured something that old would have had just had a two line pump, but maybe they did something different with the BBC. I use a Delphi mechanical pump on my '86 model year Caprice, which has a two line pump. It's made in South Korea, but it's never had a problem. AC Delco makes pumps, too, and those would be good. Unless I'm missing something, you may have to just cap that return port.

I use rockauto as much as I can. Place is awesome

currently the return pump is just blocked off on the return part. seems to be working right now, but eventually I'd like to find a suitable 2 line pump and redo the pump to carb line in all metal.

Check your clocking of the fuel hose locations. My 79 K15 had a fuel pump clocked at 45 degrees to the frame. When I had to get a replacement the 4WD version was clocked straight at the frame and wouldn't work. I had to use a 2WD version that was clocked forward and lengthen the rubber fuel lines.

currently the pump is straight off the engine. I'm not exactly sure what the application is for.



Update on stumble-

Worked a bit this weekend and got some new parts installed.

• ripped the q-jet apart, soaked it, and installed rebuild kit
• vacuum line on choke pull off was toast. put a new one on. also tested choke pull off and it was good on the pressure it pulled at and the time it took to extend.
• reinstalled carb and set idle A/F ratio
• pulled out the dizzy. checked the vacuum advance, it was working fine and advancing at the correct pressure. full travel was smooth.
• the weights were bound up under the rotor, so fixed them.
• changed the wiring harness that hooks between the cap and the module.
• changed the power and tach pigtail on the truck side. It had some jank spade connectors, now it has a nice latching connector.
• didn't check the initial timing before I started working on the dizzy, but reset the initial timing to 6° BTDC.
• when I initially checked the timing it was around 70° BTDC. I'm thinking this was my stumble. I didn't check the timing before I took everything apart, but I never spun the engine over with the dizzy out of it, or moved the oil pump shaft. So I either put the dizzy right back in how it came out, or 180 out. Pretty sure it was in there exactly how it came out(on the rotor position) and I'd assume the housing to be within a few degrees. So I use the 70° number loosely, but damn it was way off. I can't believe the engine even ran with that much timing. I had to realign the oil pump shaft to drop the dizzy down all the way, but got the timing finally set.

so that's basically what I did. Truck runs like a top. set it to idle at 700rpms. When I drive it, it drives much better with no stumble. I can hear the secondaries open up when I stomp it, so that's good. Truck isn't fast at all, quits pulling around 2800rpms or so- but it is much better to drive.

I'm hoping I'll be fine from here on out- until I automatic swap it and shortbed the truck. When I shortbed I'll fix up the fuel lines/pump/etc.

I did break a part on my eddy q-jet. Its the little bucket that fills the old void left over from the old style accel pump. So I'll be looking for another one to swap it out on a rainy day. I don't think it effects performance.

You must be registered for see images attach

in any event- good weekend. Glad the truck is finally running like I think it should.

 

[1987 GMC Jimmy]

Glad you posted. I don't know if you listen to WTIX 94.3 (this is my favorite station in New Orleans), but they had an advert for the Knights of Columbus Car Show happening this Saturday at St. Mary Magdalen Catholic Church on 6425 W. Metairie Ave. Its going on from 11:30-2:30 on the backfield of the church. All the proceeds are going to the Wounded Warrior Project, and the prices are free spectator admission, $15 pre-registered, and $20 to register at the gate. I'm leaning towards going, but a former coworker of mine just passed away, and I was gonna go to the funeral if it was Saturday or Sunday. Otherwise, I think I'm pretty much there. Here are the email addresses for the registration:

[email protected]
[email protected]
[email protected]

Glad you've got your truck going better!