Help on carb/electrical - idle and misfire problems

[Vetteophile]

OK... first time poster here...

New 63 coupe w/340... having trouble with 2 things
a) rough idling: sometimes OK... sometimes rough idle, wants to die.. doesnt come back to idle smoothly and doesnt consistently sit down at 7-800rpm... sometimes 1000...othertimes 450. Seller told me carb has been recently rebuilt (whatever that means) Any tips/tricks/ideas to get this thing to idle smooth?

b) misses when driving down the road ~40mph... OK on power up, but I notice the miss/hesitation/buck when coasting... and, occasionally backfires through carb when I gun it from idle (even though its warmed up)

Any info appreciated.

 

[Silver Blue 58]

First check plug wire orientation for correct order. Inspect each plug wire for condition. Make sure each wire is clearly separated and not crossing each other. If OK then go to fuel...

You will probably have to get into looking at the carb too. Who rebuilt it?
Was it a knowledgeable individual? Trash in the fuel bowl, float level set correctly, fuel line correct and if any rubber hose check out carefully. What type of fuel is being used....ethanol been left in tank for a while? If it has you can bet on carb and fuel line rubber deterioration. Fuel pump pressure OK? 4-7lbs. Remember either fuel or spark.

Just a few places to start. Oh yeah....timing, points and condensor replaced/good how about coil? Ok I will stop now....hope you find out what is going on....

 

[Vetteophile]

OK...thanx much for response.. lots to start with.

First check plug wire orientation for correct order. Inspect each plug wire for condition. Make sure each wire is clearly separated and not crossing each other. If OK then go to fuel.... Plug wires and plugs look new and reportedly replaced when engine rebuilt last spring.. that shop only does Corvette engines so assume experienced. Are several wires that cross each other so will start there. Did check with engine running in dark and saw no sparks.. but agree, symptoms seem like occasional electrical short and still suspect that. why?? missing when driving is one or two sparks at a time... seems too quick to be carb... but rough idle causes me to also question the carb.

You will probably have to get into looking at the carb too. Who rebuilt it? engine builder said he 'sent it out'... so who knows Was it a knowledgeable individual? Trash in the fuel bowl, havent seen any.. float level set correctly, have to check...fuel line correct and if any rubber hose check out carefully. is some rubber line near fuel pump but looks new... What type of fuel is being used....ethanol been left in tank for a while? I'm running premium.. probably contains 10% e-nol.. If it has you can bet on carb and fuel line rubber deterioration. Fuel pump pressure OK? 4-7lbs. have to check this.. Remember either fuel or spark. how about fuel filter... I think there is a bronze metal cone type filter in-line near carb... is this a possible culprit? havent taken it apart yet.

Just a few places to start. Oh yeah....timing, points and condensor replaced/good how about coil? timing checked and reset recently by experienced shop, also new points/condensor... but I suspect coil. Had a transitor ignition installed and shop that did timing took that out cause couldnt get things set right... but, same old Blaster coil still present.. is Ohm possibly wrong on these type coils? that enough to cause this problem? Ok I will stop now....hope you find out what is going on....

Hope that beer was tasty... think I'll have one, too. Thanks for help/ideas..

 

[Hib Halverson]

Based on the OP's symptoms, particularly that idle is unstable and inconsistent and that it's got a sag off idle and it spits back once in a while, I'm going to guess this car has carburetor problems.

No doubt the suggestions to check ignition condition, check for cross fire, check firing order and so forth are good suggestions, but I think, once you determine the ign. sys. is not at fault you are going to find the either the carb rebuild the previous owner mentioned was either done wrong, was done on a carb with worn mechanical parts that the typical rebuild did not address or was not done at all.

 

[Vetteophile]

Thx HH.. agree this is carb problems, at a minimum. Been reading this forum and found several good recomendations. Must admit is very helpful.

Havent had chance to dig more yet... but another quesion. Choke tube is in place and looks reasonalby good. But.. I notice there is no clean air tube present. Is there supposed to be on 327/340hp?? The access tube at the carb is blocked off with rubber seal where the clean air tube would be attached. I assume this clean air tube provides a path for warmed recirc air going thru the choke tube... and back to carb?? Could this be culprit for rough idle?

 

[JohnZ]

Choke tube is in place and looks reasonalby good. But.. I notice there is no clean air tube present. Is there supposed to be on 327/340hp?? The access tube at the carb is blocked off with rubber seal where the clean air tube would be attached. I assume this clean air tube provides a path for warmed recirc air going thru the choke tube... and back to carb?? Could this be culprit for rough idle?

Nope, the upper choke clean air tube missing wouldn't cause any of your issues; there's a calibrated minor vacuum leak through the choke housing that constantly pulls heated air through the choke tube, and the connection at the carb air horn just ensures that the air intake for the tube is filtered.

You have some carb issues, but you also have some ignition and/or ignition timing-related issues. You really need to go through a complete tune-up procedure and make sure the distributor and its centrifugal and vacuum advance systems are working (and are set to the factory specs in your '63 Corvette Shop Manual), dwell and timing are set correctly, plug wires and plugs are OK, cap and rotor are good, etc.

When that's done, you can focus on the carburetor; most "carb problems" are really ignition problems.

 

[Vetteophile]

Nope, the upper choke clean air tube missing wouldn't cause any of your issues; there's a calibrated minor vacuum leak through the choke housing that constantly pulls heated air through the choke tube, and the connection at the carb air horn just ensures that the air intake for the tube is filtered.

You have some carb issues, but you also have some ignition and/or ignition timing-related issues. You really need to go through a complete tune-up procedure and make sure the distributor and its centrifugal and vacuum advance systems are working (and are set to the factory specs in your '63 Corvette Shop Manual), dwell and timing are set correctly, plug wires and plugs are OK, cap and rotor are good, etc.

When that's done, you can focus on the carburetor; most "carb problems" are really ignition problems.

OK... but sounds like I should install a clean air tube to ensure no dirt enters the air flow thru choke tube. I'll order one.

I'll work on the timing, advance/springs, plugs, wiring this week.. might also open up the choke housing as I noticed the choke air plate does not close when I hit the accelerator...possible broken internal spring??

 

[magicv8]

Due to reformulated fuel, I locked the choke at full lean on my AFB at least 10 years ago. The car always starts and runs, easily even in below zero weather with just a couple of pumps on the accelerator pedal.

I also believe that ignition is the fault with timing advance problems. Make sure the underside and pivot holes of the distributor weights are clean, and check that the vacuum can lever has the spacer that takes up slop in the slot in which it connects.

 

[Hib Halverson]

If you adjusted your choke "full lean", you may thought it was because of "reformulated fuel" but you misunderstood what happens with AFR when running on reformulated or oxygenated gasoline.

The AFR, when using gasoline which is reformulated or oxygenated with ethanol and the carb calibration is not changed, gets more lean not more rich. If anything, it would have made the engine need a choke set more rich rather than full lean.

If you had to set your choice full lean, the problem is not ethanol in gasoline.

 

[magicv8]

If you adjusted your choke "full lean", you may thought it was because of "reformulated fuel" but you misunderstood what happens with AFR when running on reformulated or oxygenated gasoline.

NO
I noticed that unlike the fuel that was made in the 60s, todays fuel boils up and smells up the whole garage when you park, but that also makes it light up better when the engine is cold and run a cold engine without a choke.

I set my AFR by changing the AFB jets to the leanest mix my engine can stand and still give me great performance without coating the back of the car with fuel residue. After that, whether the engine is cold is irrelevant, and that's just an observation on my part.

Having said that, I will not post here again about this because that would be hijacking this thread......

 

[Vetsvette2002]

If the timing gears and chain are the original, that is where I would look to see why the timing is changing. This would also cause a change in idle speed.

 

[Vetteophile]

If the timing gears and chain are the original, that is where I would look to see why the timing is changing. This would also cause a change in idle speed.

Agree... yes could be the source, but chain was replaced when rebuilt recently. However, heres what else I've learned... see if you agree.

Vac is low at 7" Hg at idle (500 rpms - heavy cam)... I suspect current can is releasing its advance at about 8-10". Why do I say this? Only a guess, but engine seems to want to idle at about 800... but then ultimately drops off to 500 (I think when advance is released).... which makes it difficult to get a good base advance at idle. And, I can see vacuum bounce when idling, which maybe advance is close to coming back in?

Anyway, I've located a VC 1810 can and am hoping this solves it. It should begin at 4 and be all-in at 7... hopefully thats quick enough to solve the problem.

Then I can zero in on base advance.. plus get right springs for centrifugal... then on to carb. I'll get this thing running right yet!

Think I'm on the right track??

Also... in another thread I asked about ohms coil resistance. Current is Blaster 2 carried over from electronic ignition installed by previous owner. I've since switched back to points... but left the coil. Now I read coil should be 1.6 ohms resistance on Primary, and no less than 11 on secondary. Blaster is .8 and 8.5 (per multi-meter)... frankly, I dont know the significance of these numbers. What are the consequences of leaving it?

Any thoughts appreciated.

 

[JohnZ]

Think I'm on the right track??

See your other thread.