Initially I thought this was an ignition related issue and replaced coil, cap, plugs, wires, etc. but that didn't help and then I found that the plugs look EXTREMELY lean when I pull them.

1980 4-speed L-48 stock except: Emissions removed, tight tuck headers. All vacuum ports tightly capped (except the ones still in use which are the one for the vacuum secondaries and the one that feeds the brakes etc.). Today I pulled the PCV hose off it's vacuum port and capped that port temporarily just for troubleshooting. I'm currently running 36 degrees total advance, with the (new, adjustable) vacuum advance can currently disconnected but I've tried it both ways. I ran it without the gas cap and there was no change. Tank interior and fuel pickup look pretty much brand new as near as I can tell using my cheap endoscope while the fuel level is low. I replaced the soft fuel lines at the pump and the in-carb filter. Based on looking at the inside of the tank and the fuel lines at the tank end I suspect the tank, sender, and soft lines at the tank end were replaced by the PO but it's a super clean car so I guess it could be misleading. Last week I opened up the carb and blasted carb cleaner through every passage (except the idle circuits which have given no sign of trouble) and it had no effect.

I rebuilt the carb several years ago after reading tons of stuff online including Lars papers and watching a ton of youtube videos. I got the parts for the rebuild from quadrajet power, and also installed their "adjustable part throttle kit". Except for a very annoying but minor momentary hesitation at times when leaving traffic lights and stop signs etc. it ran fantastic for about two and a half summers, then this problem started. More evidence it's a lean condition, adjusting the adjustable part throttle to a richer setting helps, just not enough.

The problem developed over a couple months of driving the car last year. First it very suddenly started running rough and lost a lot of power while passing a slow moving truck I'd been stuck behind on the interstate. it ran rough until i got home then ran fine when I started it again after being parked for 15 minutes or so. After that I started noticing rough engine and loss of power only at interstate speeds, then it started happening on 50mph roads, then got to the point where it happens a few minutes into every drive.

The car starts easily hot or cold. After fully warmed up I go for a drive and the car runs great for the first five minutes or so, then it starts to sound a bit off like a cylinder or two are cutting in and out. This is especially apparent if I try to accelerate hard, and there's a significant loss of power. It gets worse over the next five minutes until it's more like it's constantly running on less than 8 cylinders and can hear and feel it's running that way at any speed or throttle setting, from low throttle and constant speed at 35mph or slower, to wide open throttle at higher RPM at any speed.

If I shut the car off for ten or fifteen minutes it again drives well for the first few minutes. If I shut it down suddenly when it's running lousy and pull off the road it still gives repeated strong squirts from the accelerator pump when I move the linkage by hand.

Any advice welcome!

 

You might be onto something thinking it’s an ignition related issue. It’s great that you have a lot of new components and is set to 36 total, but when do you achieve this amount, and what is your initial?

After reading your other threads it sounds like this isn’t a new problem. Do you still have the commercially rebuilt carb from 4 years ago that was deemed trash?

 

After finding the plugs are the leanest I've ever seen I became convinced it's a fuel issue... correct me if I'm wrong but I don't think any ignition problem is going to give me plugs that look crazy lean!

 

The post you reference is from when I bought the car and before I rebuilt the carb as mentioned in my third paragraph above, after much reading including all Lars advice. The assumption that the carb I had was trash came purely from the general criticism of rebuilt carburetors, but when I took mine apart and inspected everything it was all properly matched parts of exactly the correct type and in very good condition. I did a very thorough and careful rebuild and it ran great for a couple years before this started. The only imperfection was that annoying little hesitation I never did get rid of but it was very minor.

Current trouble came on just driving the car the same as I had been, without any parts or settings being changed.


36 total advance is achieved at about 3,000 RPM, I didn't note the exact point where it maxed out. I set the timing at 3,200 RPM after checking that was fast enough for it to be maxed out and not fluctuating at all. Initial advance I didn't note that exact number either since I set it by total desired and only checked idle reading afterward to make sure it wasn't outside the expected range. I think it was around 14 -16.

 

Your Q Jet has mechanical secondaries. Not vacuum.
Have you considered sending your carb to Lars?

 

I have heard of certain ignition modules failing slightly rather than outright over time. I recently had a hesitation in my car that was fixed by replacing the autozone brand module with a more high end one. Heat from the engine can cause the circuits inside the module to expand and not work as well. Once the car is cooled down all will go back to normal.

The only brand I would use now is DUI. You can find theirs here.

And post a picture of your plugs. If they are lean you can go up a size or two on the primary jets. Which should be 71 on your 17080207.

 

I would be looking at the HEI module and or coil. The GM parts will last a long time but the current aftermarket pieces seem to be less reliable.

Suggest you try a NAPA Echlin module with plenty of heat sink compound. Maybe the Echlin coil too just to rule out the questionable one you have now. What you describe is pretty much textbook for crappy aftermarket HEI parts.

As for the 1980 L-48 carb, they are lean, but thats not your issue causing problems here.

 

Did you look in the gas tank for crud?

 

StingR69
"As for the 1980 L-48 carb, they are lean, but thats not your issue causing problems here."

Interesting post--I have a 78 L48 and would like to know how I can get it checked or check it myself "for LEAN"???

JJ78

 

Mike -
The only way to really check and verify a lean condition is to use a wideband ox sensor and get an actual reading. Innovate makes a "tailpipe sniffer" that can be mounted temporarily and used while driving and testing, but it's a lot of money to spend on a one-time verification.

The 1980 carbs are notoriously lean. When you add headers without making any mods to the carb, they go grossly lean. Starting in 1979, the Q-Jet carbs use the revised "fine thread" idle mixture screws, and they have very small idle fuel discharge holes under the screws. I've seen factory 1980 idle mixture screws set at 9 turns out and almost ready to fall out, and the idle mixture is still in the 15:1 range. Like all M4M carbs, the 1980 uses the large Main Air Bleeds (.120" diameter) in both upper and lower locations, making cruise mixture also lean. But unlike your 1978, which uses #77 primary jets to compensate for the huge MABs, the 1980 carbs dropped the jet size down to #71, running the cruise mixture and WOT mixture super lean. Add headers or other mods to a 1980, and the carb goes dramatically lean unless it's modified to allow some fuel flow.

However, the OPs problem does not sound like a carb problem, since the problem just developed on a carb that was previously running OK. The carb's mixture setup is not going to change from one day to the next. That fact points towards an ignition problem, as suggested by Mark in his post above.

Lars

 

What brand and number plug are you using?

 

I put in a larger jet and I think also changed the metering rods based on advice received after telling that I had put on headers and that the emissions stuff was removed. I'm pretty sure it was you who I asked what jet size to start with. Still now I'm thinking maybe it was running lean all the while and the lean looking plugs were unrelated to the problem described in the OP. I changed the plugs at the same time I rebuilt the carb three or four years ago, and I probably hadn't pulled them to check them until this problem started a couple years later so I wouldn't have known if they looked really lean while the car was running well for a couple years. At that time (last season) I pulled the plugs and that's when they looked really lean.

With that, now you've all got me back to thinking ignition. The way the plugs looked was what made me start thinking the problem was on the fuel side of things and now it seems likely that had nothing to do with what's going on with the car now. I think I'm just going to buy and install a good quality HEI module and coil, and install those before I do anything else since I'm super tight on time and really need to be working on other things.

 

Good plan

 

@lars I just checked my kit and the jets currently in the carb are 73's.

@stingr69 @Piersonpie I just picked up the Napa Echlin ignition module and installed it with high end PC thermal paste, no change to how the car is running. I've got two coils, the one that was in the car when this started and the one I bought and put in to try to fix this but had no effect. They are probably both cheap ones (I know the new one is) but I'm pretty sure I confirmed last time I was messing with this that the car ran exactly the same with either one. I'll probably switch it again just to verify. A good brand name coil is expensive and I'm broke due to surprise major home repairs needed this year including a new roof, and after just spending $50 for an ignition module that didn't help I hesitate to spend even more than that on a coil I'm pretty sure also won't help!

Anyway I'm glad I now have a spare working ignition module to keep in the car now (the one I just took out) in case the new one ever fails and leaves me on the side of the road. I carry that extra coil for emergencies too.

 

What about the power piston? I'm very suspicious of it now.

When I opened the carb up to blast it out with cleaner last week I had a bit of a hard time getting the power piston out. The retaining plastic washer thing at the top of the piston looked a bit deformed and I had a hard time getting it back in so I ordered a replacement that won't be here for a couple of days yet. When I installed it, the piston was moving up and down OK before putting the top back on the carb so I figured it was unlikely to be the problem but yesterday I read something somewhere about (carefully) putting something down the vent tube and feeling around for the top of the power piston then pressing it down to see if it moves freely. I just tried that with an allen wrench and if I'm doing the test right it's stuck down! Nothing down that vent tube is moving as far as I can tell.

I just looked at a rebuild video and based on this image I'd change the description of the one in my carb from "a bit deformed" to "badly damaged"! I installed this part myself when I rebuilt the carb several years ago, and apparently I botched the job since it's now mangled. I guess alternatively it could have been damaged when I opened the carb up last week and struggled to pull the power piston out without pulling too hard on it.

 

Sounds like you need to fix that piston retainer for sure. Not sure how that would get trashed unless the top was off the carb for some reason.

Do the magnetic pickup and wires look OK or corroded? The pickup wires can get brittle after all these years.

 

They look fine, but I'll check them more carefully. Thanks.

As for how the retainer for the power piston got damaged, I can only imagine I installed it wrong or put the carb together with it a little out of place. It looks almost like it melted a little bit but that seems very unlikely.

 

Well, I'm once again at wit's end. I'm now pretty sure the power piston is working properly. The arms were a bit bent, so I straightened them. The car seemed to run better for a while afterward, but not perfect. After a bit of driving it got worse again.

In addition to the new echlin ignition module I installed last week I have now put in a new echlin coil, cap, and rotor. No change to how the car is running.

When I start the car cold and let it fully warm up, it always drives flawlessly for the first few minutes cruising and wide open throttle both, then a couple miles down the road it starts sounding a bit off. Once it starts running rough it does so at any speed or RPM, steady cruise or WOT.

 

I once had a problem that was ignition related, so I changed out all the old parts. The car ran well for a while, but after a while, the car started having a intermittent miss after getting up to temp. Eventually, I checked all the plug wires with a meter and found out that the primary wire--from coil to dist.--was open when it flexed a little. I put the old wire back in temporarily, and it's still running fine after 2k+miles.

 

Another possible pattern just occurred to me but I won't be able to test drive again until next week. The drives where it ran much better were all the ones where I was running with the whole air cleaner assembly left off because I was fiddling with the carb. I put the air cleaner back on yesterday before my last test drive and that's when it got really bad again. It was on today as well.