I got this car late last year, my first sports car and first carbureted engine. After two extensive solid test drives (including an hour long drive) and a look-through the underside with it up on the owners lift, I made the deal planning only to deal with the transmission leak. Turned out it was the front seal, so I had the transmission rebuilt and now there’s a significant engine issue, running rough at idle and hesitation during acceleration.

I dropped it off with a slight miss. Transmission shop said it was like that first time they started it, they have a good reputation and I’m inclined to believe them. Further, they offered to pay for any related repair if it would up being due to something their shop touched. This all happened back in December and I parked it for most of the winter while I figure this out.

How it’s running right now. You can see the motor shake more than normal. I’ve also had carbon spray out of the front of the muffler, and just recently even noticed some fuel spray on the tire/ground ahead of the muffler on one side:

https://m.youtube.com/watch?v=TYWJnLJFhvE

So far I've:

• Verified a plug wire on the cap was halfway off! Issue did not resolve after re-seating.
• Battery died while trying to diagnose, it was 8 years old and likely overdue. Still runs rough.
• Replaced the distributor rotor as the old one looked melty (thread https://www.corvetteforum.com/forums...otor-wear.html)
• Verified the vacuum advance diaphragm properly operates

Background:

• 90k mi, original L48 350 and TH400 auto, Holley 4160 carb (80457-10)
• “Bad idle, poor vacuum, lousy performance” cam according to Lars. (specs below). I do not believe the cam is the current issue, as initially the car behaved quite smoothly.
• Transmission rebuilt (front seal was leaking)
• Moved distributor vacuum advance hose connection on carb from ported to manifold vacuum connection per Lars instruction (thread https://www.corvetteforum.com/forums...arburetor.html )
• Hooked up previously open EGR to ported vacuum connection on carb. I may try undoing this and cap off EGR to see if it is faulty, but I don’t expect it’s having this large an effect.

My guess here is I need to verify spark, verify TDC is correct on harmonic balancer and set timing. I need help on a gameplan for what’s next. I am no expert, but I feel it is ignition related, but whether timing or spark equipment, I’m uncertain. I don’t have a jack or ramps.
I’ll need to watch some videos and make a shopping list.

Going to update this shortly with some other pictures.

 

You found @lars. Did you email him for his timing papers? What is the timing set to now?

Is that the original spread-bore intake manifold, and is there an adaptor for your square-bore Holley?

I don't think zip-ties on your ignition wires are doing you any favors, but I'm not sure that's the "aha!" problem, either.

Can you post a similar video, or more photos, with the air cleaner assembly removed?

Here's a handy timing video. You'll need either a dial-back timing light, or MSD timing tape.

 

I have an email thread with Lars started before the biggest issue here, I plan to have him rebuild a Qjet. I put that on pause while I tried to sort this (and take a bit of a break from it mentally). I had not asked about timing yet.

I have every reason to believe it’s the factory intake with an adapter plate for the Holley. And I agree on the wires, it wasn’t a problem at first.

I’ll look for, or record and post, another video with the filter off the carb. I have an Innova 5568 timing light that I bought before this got worse but haven’t yet used it. I also had an issue accidentally deleting my first video from the post, I added it back.

 

The inside of that distributor cap looks horrible. Your plug wires are going through the upper ignition shielding brackets that are missing the rubber 4 way grommets. Likely crossfiring and possibly getting shorted to the steel uprights. Can't see if the lower 4 in a row grommets are there or not holding the wires. Can't see if the forward 2 cylinders on each side have the plug wires routed through the motor mounts and have the proper 2 way clip holding them.
Basically your plug wires are a mess.
Check every wire for cuts and or burns. Replace all the missing correct rubber grommets and retaining clips. Ensure all heat shields above the plugs are present and installed correctly. While your doing that.
Remove all 8 plugs. Lay them out in order. Do not mix them up and take good photos of the tips
post plug photos on here. Any miss firing cylinder should be obvious by looking at the inside tips of the plugs.

 

• Agreed on the distributor cap. I don't believe it looked like that back when I first inspected it for issues, but I can't be 100% certain. There's a photo in this older thread. I've cleaned it out now and will see if the condition returns.
• As for the plug wires, when the issue started, the shielding was in place. No reason to believe they would have touched them when dropping the transmission. I'll have to source some grommets and wire separators. I thought the shield on top was for RF, not heat?
• Plugs; I'll have to get a spark plug socket and it seems like I'm in for a treat in order to get to all the plugs. I won't have the free time to do that until this weekend. I don't have a jack or ramps currently, either.

Updated the original/top post with more photos!

 

H-m-m-m-m-.
Transmission rebuilt.
Unusual burning / dripping of fluid at muffler.

Did anyone replace the trans vacuum modulator, hook it back up or even check it to hold vacuum?
I think you are burning ATF.
And have a vacuum leak at the tranny.

 

The fluid that was dripping at the muffler I assumed to be unburnt fuel (it evaporated quickly). The black spray, though, not certain. I could verify next time I start it up. As for the vacuum leak, I'll have to look up to see where/how to verify. As for burning ATF, I only had the trip home from the shop that was long enough to get warm (30 minutes/17 miles). It's been parked since then.

 

A faulty transmission modulator will allow the engine to vacuum up ATF out of the tranny, onto the Intake where it is burned off in the cylinders.
Almost always smokes out the pipe, will run erratic due to a vacuum leak and usually affects shifts.

You just came from the tranny shop.
Just sayin'.

 

The black exhaust reversion sooting inside all of your carb venturis is indication that you have some severe ignition timing problems. Fix your timing problems before you start talking about any carb tuning. Timing & ignition affects the carb. The carb does not affect timing and ignition. You cannot tune the carb if the timing is all messed up (as yours appears to be).

 

Wasn’t talking any carb tuning! Promise!

Finally had some time tonight to dedicate. Went through all plug wires and verified proper firing order, investigated condition on both ends, and measured resistance. No visual issues on ends, neither corrosion or insulation, and resistance didn’t show any inconsistency in measurements while flexing wires around at all. All measured between 1k to under 3k. Since the readings were low I didn’t write down exact figures, I was more looking for issues upon flexing them.

Trying to find positives, I’ll also consider it encouraging that I was successful in hooking up and learning how the timing light works. I let it idle to warm, plugged both the vac advance can (and manifold vacuum port I took it off of) and after some annoying angles with a 9/16 wrench got the dizzy loose to adjust.

I learned I want a distributor wrench and to just loosen the bolt enough to turn the dizzy with a little effort otherwise the plug wires move it right back when you let go.

I learned the manual for the Innova light sucks and it took me 5 minutes trying to see what each button even does. Next time I think I’ll be able to measure what the timing is at idle instead of just seeing the mark too far off the tab to line up to a measurement.

I think I was supposed to hit the up arrow on the Innova until the mark on the balancer lines up to 0 on the engine block tab/scale. The display on the light will then tell me how many taps it took to get there and that’s the degrees at idle. So many descriptions were confusing or assume a base level of knowledge and just say “adjust until you get to zero” — adjust what? But I think I get it now. It wasn’t sinking in at first that the “line” isn’t moving, the light is flashing at a different time.

I am a little concerned about advancing the timing when the mark is already showing it is fairly advanced… I feel I can’t trust the mark and I have to do what everyone says you should… verify TDC. I’ll pick up a piston stop tomorrow and give it a go. Previous owner said he thought maybe it slipped because he had a hard go timing it and instead “went by ear.” Double confirmation to put in the work to find TDC and mark it.

One last note, the carb has been that dirty since I got it, and at first, the car was seemingly running quite nicely. I picked up some carb cleaner spray but have yet to use it.

yellow circle= balancer timing mark while idling and not adjusting the timing light. Green line = TDC



got lucky and it lined up like this when I turned the engine off once, giving a clear shot of the timing mark

 

timing for these cars is not adjusted at idle. it will be different at idle from engine to engine. You have to adjust it at full mechanical advance. To do this plug the vacuum advance hose after removing it from the distributor and adjust the timing at 3000rpm (or where your distributors mechanical advance has maxed out)for somewhere between 33 and 36 degrees depending on what works best with your head/ piston/ quench combo. (typically you adjust the timing gun settings to read 36 degrees and then it will flash at the 0 mark at 3,000 rpm) if it pings back it off closer to 33 at least thats my understanding of it

The directions already mentioned in Lars paper are the correct was to set up the timing on these cars for performance and not emissions as the priority.

 

Did you email Lars for his timing instructions?

If the dial-back Innova isn't working for you for whatever reason, you can install the MSD timing tape (link above). Then just leave the timing light at zero dialback.

Did you disconnect the vacuum advance when doing this?

 

Millie,

Just a couple observations here:

You don't need to plug the Vac canister when you remove the vac hose. Nothing is going to come out of the canister. Nothing is going to go in. Its empty.

The close-up photo of the plug wire looks terrible to me. Either that was a Do-It-Your-Self-kit, or its a $9.95 wire set.
If all 16 terminal ends look that bad, that could be an issue.
I would look into something of more quality.

When you disconnect the vac hose for setting the timing, a simple golf tee works well to plug the hose. You may notice a drop in RPMs or even stalling out.
If so, be prepared to turn down the idle speed screw.
If no change in RPMs, your vac advance is not working.

 

I'll email Lars for those now that I'm sure this is something I can actually handle. I have a prior thread with him going that I was going to resurrect to have him rebuild an eBay Qjet. Yep, vac can was disco and plugged. Also, afterwards something clicked in my brain and now I "get" how to use the dial back function on the Innova next time.

Yep! Totally got it. I was mainly trying to see how this all works. I had no expectation of actually dialing anything in yet. I know it is to be done at 3k and let initial fall where it may.

1. Ha! Makes sense, I was just being overly cautious I guess.
2. I'll take more photos of the plug wire ends. Perhaps it was bad lighting? Either way, I'm not opposed to a new set. I think the wires are currently these.
3. I used a golf tee! Saw that suggestion elsewhere around here. Works perfectly.
4. Tested vac advance can with a Mityvac, the parts move inside gradually when applying up to 15", though I didn't write down exact measurements.

 

To be clear I was referring to plugging the vacuum hose not canister

 

To be extra clear, I was referring to the O.P. post #10.

 

Millie,

Looking at the set of plug wires you bought, it appears that they are good quality.
They are the do-it-yourself type. But I believe there are not made correctly.

Somethings not right in that photo of the plug boot. Should not be able to see the actual carbon wire, which is visible in the picture.

I do not believe you need a new set; however, I think you need to label each wire, remove, place them on your bench-top and disassemble.
Maybe just nip the ends off and start over. The brass connectors are not fully engaged around the wire / inner core.
Hopefully you have enough slack to do all 16 ends, over again.
Somethings not right there.

 

Hi Millie74,
Do you have the spark plug wire shields [#1] (AKA boomerangs) and have you considered using them?


Your local parts store will have these wires that will fit your 350 Corvette.
AC Delco 9508N

 

I have the coil shield and upper ignition shield arm 12/13 in the diagram above. I don’t believe the lower shields are installed or in my spare parts box.

As for the plug wires, I don’t believe I have enough slack to redo each end, but maybe. I’m going to continue with checking/replacing the plugs and will likely replace wires soon too to clean them up, I’ll have to get new separator mounts for the lower parts that broke off.

Found this and verified it is attached to a vacuum line on the intake between the carb and distributor. Once I chase down the usual suspects mentioned above if I’m still having issues, will come back to this part.

 

Sounds like you have some solid advice and are on the right track.
The only thing I saw not mentioned was the EGR.
Those things can make an engine run horrible.
For testing, I would definately put the hose connection back to however it was when the engine "ran good".
If it was disconnected, hoo-ray, then leave it that way. Just make sure any extra hoses are plugged so you do not have a vacuum leak.