I have a bog/stumble off idle I'm trying to diagnose and it would help for me to have a list of potential culprits. Below are some details.

1972 with recently build 383 (mild build) engine
New HEI distributor, plugs and wires with engine build
Rebuilt Quadrajet by Lars
- Good accl. pump squirts from visual exam
Gas tank replaced by previous owner. No signed of debris
Electric headlight conversion, so only vacuums lines are to the wipers, brake booster and distributor.

I have adjusted the timing to many times to mention and have seen the effects on everything except this bog/stumble.

If I feel confident the carb. is good after Lar's rebuild and have attempted multiple timing settings, what else am I missing? Vacuum leaks are the only think I haven't looked for, although I have eliminated a lot of areas for leaks.

Thanks in advance for any help you can provide.
Keith

 

You mention that you’ve adjusted your timing many different times, what is it at currently? Initial? Total mechanical? Vacuum advance amount? And is the VA properly calibrated to your engine with the 2 inch rule?

 

I stopped keeping track of the timing numbers. I'm around 13 initial. I'm not aware of the VA calibration 2" rule and don't believe my distributor has any adjustment without changing springs.

 

Since you’ve already had your carb rebuilt by Lars you should have his email. I would shoot him one asking for his timing and vacuum advance papers. Chances are with a different cam you’ll need more initial timing for it to run correctly while still reaching a peak of 36.

 

Thanks. I have his papers but I'm starting to question if my HEI is the issue, as it is an inexpensive one. I don't even know what I have, as the engine builder put it on. I'll have to look into it.

Thanks
Keith

 

I would test the distributor, and plot the curve, then get back to us.
Just cause the builder put it on, does not mean it has a good curve.
Almost all bogs or "carb" problems are usually timing related.

Specs needed from you:

• manifold vac at idle? 11" or 15" vac = big difference
• initial timing at idle, no vac: ~13*?
• additional vac * at idle when vac can plugged in to manifold vac: ~10-12*? Many vac cans go as high as 22* & can cause bogs or trailer hitching / jerking
• Vac level at which the vac can is fully advanced, as measured with a mity-vac pump: ~9" or 13" depending on idle vac #. It MUST be 2" below idle vac. If not it can cause bogs.
• Total distributor advance (no vac): ~34 to 36*
• Rpm that distributor advance maxes out at: 2400 min # (or) 3200 rpm - 3000 is about ideal, 2400 can often cause rattling/pinging, over advanced too early also causes bogs

 

really sounds like a poorly curved distributor.
A lot of these aftermarket HEI distributors have a great timing curve in them for grandma's station wagon.
Not so much so for a mild 383.

 

Thank you. This is great info and I'll report back soon. It will be this weekend before I can check it.

 

To truly help you out we're going to need to know things such as what kind of cylinder heads what kind of compression what kind of camshaft what kind of torque converter stall speed and rear end gears.
Personally I would think you could give as much information as you can to somebody like DUI and have a custom-made HEI distributor done up for your car. It's also possible that you could just need more of a pump shot on your squirter in which case you need to talk to Laura's about doing that Holly's seem to be much easier to tune in that regard but I'll get arguments from somebody.
It's frustrating trying to deal with stuff like this and I wish you well with it these are just some of the things I was thinking of.

 

That makes sense. I'll have to dig that info up, as I don't have it memorized.

Keith

 

Here's a little more info:

• Bore 40 over
• Cam - Comp Cam 12-242-2
• Pro Comp 64cc combustion, 190cc intake heads
• B&M 20404 torq converter
• Performer intake
• Stock Quadrajet
• Stock gear (363 I believe)
• Don't know compression

 

Do you have access to the air/fuel ratio readings? I used a AEM Air/Fuel Ratio Gauge inside my C3 while using the carburetor. One of these would be helpful for you showing you whether it is a lean bog/stumble or one caused by a rich bog/stumble. I loved the gauge while I had a carburetor because it allowed me to see what was happening in action. After going to an EFI system the output data became too consistent and read the same numbers or close most of the time.

Another nagging problem that can be tough to diagnose is when the Fuel supply lines get a tiny hole in them allowing air to pulled into your fuel system as the pump is drawing fuel from the tank. If you have a little air going into the fuel it can cause fuel issues at partial speeds.

If you have a Vented Gas Cap that is partially plugged can cause stumbling in the engine when the vacuum starts to build up inside your fuel system the fuel system is compromised. You could remove the fuel cap and drive the car to see if this helps at all. If your car uses a non-vented cap then you would have to start exploring you emissions system for something that is not venting properly.

With today's gasoline and the nasty ethanol in it your fuel lines are being corroded as you use the Corvette. I have seen fuel supply lines that have rusted shut internally, rusted shut partially and even a fuel lines with a tiny pin hole fuel leak. Ethanol can make big problems for the fuel system on our precious Corvettes.

To be sure that your fuel lines are okay it is wise to verify the flow rate is enough for the engine in your C3. I ran a 3/8" hose from the electric (fuel pump mounted inside the fuel tank) fuel pump to the throttle body and then measure the flow volume over time. On your C3 it would be possible to use the same size fuel line going around the car and not installed on the engine.

Vacuum hoses need to be replaced every so often to ensure that they don't leak and still hold a solid vacuum when needed. Sometimes the distributor gets a bit dry and the parts don't move as fast or as smoothly as they should. Cleaning it our with compressed air and keeping the parts moving is important. I apply lubrication with a tiny needle oiler and keep the parts that are intended to move still moving smoothly.

Old gas with ethanol can cause mis-fires and stumbles in the engine. It is essential to treat any gasoline made with ethanol with some sort of ethanol treatment to stop the bad effects of the ethanol in your C3's fuel system before it does the damage. If the gas is 6 months or older I would get rid of it and fill the system with fresh gasoline and some ethanol treatment.

 

My guess if you had an L-82 you are about 10-1 compression the cam you have is a bit milder than mine but has a 110 lsa which helps make use of the increased compression you should have. Have you done anything to your distributor? What total timing do you have? The easy way would be to contact DUI distributors and have them set up an hei distributor for you. Yes you can play with it in the car but it's easier on a machine by far. Anderson few people even have a sun machine much less know how to use it.

 

UPDATE

I had a few minutes to work on it at lunch and have eliminated the bog/stumble with some adjustments to the timing. To accomplish this, I set initial timing to 16 and with the vacuum advance disconnected. The total timing is around 32. I left the vacuum advance disconnected and took it for a drive. A few times the starter had a little trouble turning the engine over. The bog/stumble is gone, but it feels like it could have more on the top end with more fuel or spark. It almost feels like it's being held back.

When I returned home, I checked to see what rpm total timing was achieved and it is really early. 1700 RPMs and I'm at 32. That seems like an issue.

I attempted to tune the air/fuel screws but didn't have a lot of success. I have 11-12 hg vacuum at idle and the needle bounces around between those numbers. When adjusting the air/fuel screws, I screwed them in until I saw vacuum drop and then came back until I hit my max vacuum. Per videos I have watched, seems like the vacuum should hold more consistent and I should have seen more variation in it when adjusting the air/fuel.

So, right now I feel like I definitely have a distributor issue, if I'm coming all in at 1,700 rpm and only have about 16 mechanical advance.

Now, someone who actually knows what they are talking about, since I'm just guessing, please give me your thoughts.

Thank you!

 

You are falling short in total timing you want to be more like 36 total that would make the car peppier. I have personally observed 20 horsepower difference between 32 and 36 degrees on a mild 355. The dyno operator ran.the engine at 32 came up with 390 horsepower and I said you will get 20 more at 36, everyone looked at me like I was crazier than normal but guess what ? Exactly 20 horsepower. Details matter..

 

If I adjust to 36, I end up dieseling. Also, am I coming in to total timing too quickly (1,700 rpm)?

 

It's a little quick. Maybe your timing tab is wrong or your balancer has slipped.

 

Less than 5K miles on the rebuild, so while it's possible, I feel like it is unlikely.

Keith

 

I wouldn't bet against it , parts get mixed up factory defect. Etc. crank it to tdc feel for air to rush out of number 1 spark plug hole to verify.

 

All in at 1,700 is far too quick. I would remove your cap and rotor and take a look at your advance springs and weights. Chances are your springs are too light. With a Mr. Gasket 929G or similar kit using one medium and one light spring should get your total timing all in at 2,800-3,000 RPM.