70 L-46 drivability issues

[Mike F.]

About a week ago I started having some drivability issues with my 70 L-46. All original parts except for points, I have the Breakerless-SE Electronic Ignition Module from Lectric Limited.

The car started to backfire & pop through the exhaust in first gear when trying to get moving. If you babied it, it would go, if you tried using half throttle or so it would act up. Once I was moving in 1st, 2nd, etc, as long as I moved the throttle easily it would run fine, if I was more aggressive than that it would backfire & pop through the exhaust.

The car started easily and idled at 800rpm when at operating temperature. Accelerator pump is functioning correctly, two steams of fuel entering the carb. When in neutral, you can blip the throttle cable, engine responds as it should. The problem seems to be when any type of load is applied to the engine.

The quadrajet was overhauled by Lars a year or so ago and has worked flawlessly.

Yesterday was the first day of trouble shooting using known good extra parts. After each parts replacement, I would take a test drive. Replaced distributor cap, no change. Replaced coil, no change. Replaced fuel filter, no change. Because it was hot out and engine was warm the choke was fully open at all times. Started car about 10 different times with no trouble, idled perfectly, but didn't like when a load was applied.

Todays trouble shooting, engine is cold (as cold as Texas can be), checked resistance on all plug wires. They seemed to be within normal limits ranging from 17k for #1 & 2 and 10k for #7 & 8. Pulled all spark plugs, all looked great, gapped at .035, no difference in any of them.
Re-installed plugs and wires, pumped the throttle, accelerator pump squirts fuel, crank engine, engine fires and starts running on the accelerator pump fuel charge, vacuum is created, pulls choke open 1/4 inch, then engine dies. I've tried numerous times manipulating the throttle, cannot get engine to run after accelerator pump fuel charge is burned up.

I'm just talking out of my butt here, today it seems like I'm not getting any fuel from the idle circuit of the carb, even though yesterday wasn't a problem. The accelerator pump is still working, so fuel is getting to the carb.

What's my next trouble shooting step?

Let me add that I just recently moved to Texas,. We're waiting on our house to be completed, so I'm living in an apartment with a few car related parts/tools and the rest of my stuff still in storage.

TIA,
Mike

[Edward J.]

Re: 70 L-46 drivability issues

Mike, sounds like you have covered most of the bases, I have seen the fuel pump fail, generally what I have seen is when the engine is loaded and requires more fuel the pump cannot keep up with demand. bogging under load and just plain old running out of fuel under hard acceleration. the car can recover while coasting and if the gas pedal is feathered. and will idle as normal, but if run through these cycles again same thing will happen.

[Gene M.]

Re: 70 L-46 drivability issues

Mike start with the easy stuff. Check point gap, dwell and timing. The plugs look all carbon up. Are plug wires carbon core stuff or a good wire core? Do you have another coil to try?

What makes you say not getting any fuel from the idle circuit of the carb? After all you say it idles....?

[Mike F.]

Re: 70 L-46 drivability issues

Mike, sounds like you have covered most of the bases, I have seen the fuel pump fail, generally what I have seen is when the engine is loaded and requires more fuel the pump cannot keep up with demand. bogging under load and just plain old running out of fuel under hard acceleration. the car can recover while coasting and if the gas pedal is feathered. and will idle as normal, but if run through these cycles again same thing will happen.

Is there some type of fuel pump test that I can accomplish?

Because I'm getting fuel to the accelerator pump, the fuel pump must be partially working at least. So in theory it should start and idle. Today I can't get it to run after the initial fuel charge is used up.

[Edward J.]

Re: 70 L-46 drivability issues

Mike, the pump is generally about 5-7 psi just check the spec.s you will need a rubber hose and a fuel pressure gauge, sometimes the old vacuum gauges have fuel pressure also. another thing you can try is get a long rubber hose and a bottle and unhook the fuel line from carb. and put rubber hose onto end and the other end into bottle and have someone turn the engine over while you hold both items, you should see the pump working as you are cranking, a pint bottle should fill in no time.

[Mark E.]

Re: 70 L-46 drivability issues

Gene- Mike said he has a breakerless ignition and he's already swapped coils. I agree that the idle circuit isn't a prime suspect yet.

I like Edward's theory- intermittent/marginal fuel supply or maybe bad gas. I've had similar drivability issues in the past that cleared up when I filled up with fresh gasoline.

So you may try-

1. Fill up with fresh gasoline. If problem persists, then-

2. Look for a vacuum leak, which can cause your symptoms. On C3s, I start by disconnecting and plugging the vacuum source for the lights and HVAC at the manifold. That system is complex with many opportunities for leaks. Then check the VAC, TCS, choke diaphragm and their lines. Other likely leak sources that can be checked with soapy water: power brake, carb base, intake. If no vacuum issues, then-

3. Check fuel delivery. The manual has a procedure for checking fuel volume and pressure (pretty straight forward to do).

If there's insufficient fuel volume or pressure, visually check for leaks on the suction side of the fuel lines (upstream from the pump). The rubber hose sections and their connections are the most common sources of leaks. Remember that a small leak here sucks air in the line (impacting fuel delivery) rather than leaking fuel out, so it can be tricky to detect. Practically speaking, what this means is a visual check and replacing any rubber lines that look the least bit aged, worn, spongy or cracked.

If that doesn't clear things up, then just replace the fuel pump. They're cheap and easy to R&R.

If fuel volume or pressure remains low, then you'll need to look for restrictions in the line... this involves blowing out the lines and checking the pick up screen in the tank (a visual check is possible by peering into a near empty tank with a flashlight).

If the above check out, then we're back to ignition or possibly an internal carb problem.

I hope this helps.

EDIT: Yes, the plugs have some carbon but that's expected after the cold start and short drive cycles you went through. They don't look nearly bad enough to cause your problems... IMO.

[Bill C.]

Re: 70 L-46 drivability issues

I would go right to ignition

1. is it two or one wire ?
check the lead or leads from distributor. look for pinches where they go through the hole in the bottom of the dist.

2. check the advance plate to be sure it is moving freely.

3. check to see if distributor moved.. it may have rotated to a very retarded position.


if no luck
swap out the electronic and go back to points.

in in old school point systems, when the condenser would take a crap it start the backfiring stuff too.

this is where I go looking if it were my car.

bill

[Mark E.]

Re: 70 L-46 drivability issues

I would go right to ignition

bill

I can't really disagree. As they say, most fuel system problems are the ignition.

[Mike F.]

Re: 70 L-46 drivability issues

Day three of trouble shooting. I disconnected/plugged the vacuum line to the headlights and wipers, just the brake booster was hooked up.

I pulled the fuel line and the fuel pump moves fuel like a race horse. All eight spark plugs & wires are firing per the tester.

I brought the #1 cyl to TDC using my finger to block the hole. I did this a couple of times. Every time that my finger was forced off of the hole and the engine bump was complete, the timing mark was at about 30*-40* ATDC. (I thought that was a bit far.)

I tried moving the distributor without loosening the nut and it would not move. I am pretty confident that it has not moved from before when the car was running fine. The vacuum can appears to be in the same location comparing it with past engine pictures.

I was able to get the engine to start and chug along. The timing indicated quite a bit retarded past ATDC. So I loosened the distributor and advanced it quite a bit to get the timing back to 8* BTDC. The car was idling as it should. I let it get up to operating temperature and retimed it using a dial-back timing light. I had to re-adjust the idle screw, moving it out to get the idle below 1000rpm. (Again odd, it's idled at 800 rpm for years without adjustment.) I can rev the throttle and the engine responds while in neutral. I did not drive it yet.

The engine is at operating temperature, timing is at 8* BTDC. The TCS system is functional, I put the car in 3rd gear, the timing advances to about 28* (so the TCS system is functioning), but the engine idle does not increase. It actually sounds worse with the timing advanced.

What is going on with the timing/distributor? If the harmonic balancer had slipped, the car should have still run fine. The problem would have shown up only when you went to time it and used an incorrectly placed timing mark. Correct?

With the timing currently set at 8* (no vacuum) and then engaging the TCS system, the timing advances to 28*. So the vacuum advance seems to be working. Correct? Why won't the engine idle increase with the timing advanced?

Have I finally eliminated the carb/fuel pump as problems?

It's almost like I've had to advance the timing 30*-40* to get back to 8* BTDC and re-adjust the idle screw (even though nothing mechanically had moved on the car) and while it starts and idles it doesn't like vacuum advance.

What am I missing?

Anyone in the DFW area that wants to educate me is more than welcome to come over or is there a recommended shop in the DFW area that I can tow the car to? I'm running out of time for the Lone Star Regional and don't have the space, tools, etc. needed in this apartment to do much trouble shooting or disassembly.

TIA,
Mike

[Timothy B.]

Re: 70 L-46 drivability issues

Mike,

Make sure the vacuum advance control holds vacuum and does not have a small leak. Go for a ride with the vacuum advance plugged just to see if the condition is better then try it connected again to see how it responds, sure sounds like a lean condition and could be a vacuum leak. When the distributor cap is off are the centrifugal advance weights returning to there at rest position and are the breakerless parts snug, (shutter wheel etc.). Make sure your engine and distributor grounds are good.

It's going to be a process of elimination before you start throwing $$$ at the car.

[Mike F.]

Re: 70 L-46 drivability issues

Day four of trouble shooting.

The vacuum advance can functions and holds vacuum. The points plate functions and returns as it should, the timing light proves that. The weights move freely and are returned by the springs.

I replaced the carb & carb intake gasket with spares that I have. No change. It idles at 8*, but drivability isn't as it should be.

The TCS functions correctly, timing advances to about 30* at idle when slipped into 3rd gear, but the idle rpm never increases. Prior to this problem the idle would jump up to 1200rpm when shifted into 3rd.

At idle with all vacuum lines plugged, sprayed carb cleaner around base of carb and along intake manifold. No decrease in rpm.

Why won't the idle speed increase when the timing is advanced by the TCS system? The timing moves from 8* to 30*, how can that not affect idle speed?

So far, I have replaced the coil, distributor cap & rotor, carb and carb to intake manifold gasket. Plug wires ohm out at 17k for the longest and 10k for the shortest.

TIA,
Mike

[Tim E.]

Re: 70 L-46 drivability issues

Mike - One thing that has not been suggested yet is the timing chain. Remove the distributor cap and rotate the crankshaft back and forth with a breaker bar. The rotor should move without hesitation. If there is hesitation, it would indicate a worn chain or camshaft gear. Tim

[Tom F.]

Re: 70 L-46 drivability issues

Hi Mike,
Maybe check the camshaft lobes. Make sure you don't have any lobes wearing out.
Tom

[Mark E.]

Re: 70 L-46 drivability issues

Mike,

It may be time to methodically re-check everything. Reading through the thread, maybe you have multiple and/or intermittent symptoms- first it would backfire under load, then it would not run at all, now it's "chugging along" and responding in an unexpected way when the ignition timing is changed. This means you may have multiple and/or intermittent causes, which is more challenging to work through.

A suggested way to move forward:

- A good question to ask when trouble shooting is: What's changed? Double check those.

- Did you fill the tank with fresh gasoline? As mentioned earlier, old gas can cause peculiar and unexpected drivability issues. I chased my tail a couple of times in my life because of this.

- Did you replace your electronic ignition with points and condenser to eliminate that as an issue? While doing that, double check primary ignition wiring, including the portion of the coil wire inside the distributor and the ground wire in the distributor.

- As earlier suggested, go ahead and completely disconnect and plug the vacuum advance to eliminate that as an issue. When disconnected, the engine should run smooth and steady, albeit with less power and maybe at a higher temperature.

- You said you didn't disconnect and plug the power brake vacuum connection. Do that to check for a vacuum leak there.

- Manifold vacuum leak?

- First it was backfiring, then wouldn't run, now it's chugging along... An ignition problem is usually (but not always) more consistent. Maybe dirt or debris passed through the fuel system? Clean, check and install a new kit in the carb.

- Check timing chain as suggested above.

If all of that doesn't solve it, then do a leak down test to check for real mechanical problems.

If you don't have the time, a great mechanic in the metroplex who is honest and reasonable: Scott Ekbald, the owner of Rallye Auto Service on Hillcrest Road. Paul usually answers the phone; both great guys. See www.rallyeautoservice.com

[Edward J.]

Re: 70 L-46 drivability issues

Mike, Another GM carb issue I have seen on occasion is the primary metering rods get stuck down (due to a piece of debris), this will cause a poor performance on acceleration. almost feels like the engine is running on half the cylinders. does this sound like your driveability issue? I know you said the carb. was rebuilt, these kind of problems will happen out if the blue.