As noted in his thread titled, "Which port is manifold vacuum?", Dennis has been having some problems getting his newly built engine to idle:
https://www.corvetteforum.com/forums...-vacuum-2.html

The engine builder, naturally, is telling him it's all caused by his carburetor: His 800 cfm L82 High Performance Q-Jet is just "too small" and he needs to have fuel injection installed... In the thread noted above, I have expressed my concern that he does not have a carb problem, but that the engine builder has done something funny with either ignition or cam timing to cause the problems.

To verify this, I asked Dennis to send me the carb so I could give it a good functional test. Let's see if that carb is causing all the engine problems that the engine builder believes it is...

My test engine is a 357 small block with a CompCams XE268, Dart heads, and headers. It's a good, representative "street performance" engine running right at 350 hp, and is cammed very close to Dennis' 270 (advertised) Lunati-cammed L82. If the carb runs well on my test engine, it should run identically in Dennis' engine, if the engine is otherwise correctly assembled and tuned.

I just received his carb a couple of hours ago. Here it is on the workbench for inspection prior to test:


Checking the setting of the idle mixture screws, I found them to be imbalanced, with one set lean and one set rich. I set them both to a nominal setting of 6 turns out from seated:


I also checked the odd clocking of the choke housing. The clocking and setting turned out to be OK:


Carb was then bolted up on the test engine:


Fuel line snugged up to the inlet fitting:


Since I had just completed testing of another Forum member's carb on the engine, and since the engine was up to operating temperature, I applied power to the choke to get it to open and to verify its operation:


The choke opened as it should:


Calibration of the wideband oxygen sensor was verified prior to engine start to make sure I was not going to get any false air/fuel mixture readings:


Carb bowl was filled with fuel to assure instant start:


Engine fired up instantly at the first touch of the starter. The carb running on the test engine:


The hot idle screw was adjusted very high - screw here seen in the as-received position screwed in pretty far to hold the throttle open, confirming what Dennis has stated in his previous post about needing to have the carb set to a high idle speed to keep his engine running. This high idle setting was "forcing" his engine to idle:


This setting had the engine running at an idle speed of over 1500 rpm:


I then backed the idle speed off to see just how slow I could get the engine to idle at a stable rpm:


The carb's idle circuit is working perfectly, and I was able to get idle speed down to a smooth and stable 727 rpm before it wanted to kill the engine. My cam does not like to idle below 800, so I was amazed that the carb would allow a stable idle speed at this low of a setting. The carb is running and idling exceptionally well:


Once verified, I then re-set the idle speed to a more reasonable rpm. Idle is smooth and rock steady:


Note the significantly different position of the idle speed screw at the "correct" idle rpm versus the screw's position shown in the similar photo above:


At a smooth and stable idle rpm of 850, the idle mixture is right on the perfect number at 14.7:1. The carb could not be running better:


Throttle response is excellent, and all aspects of the carb are in perfect operating order. He absolutely, positively does not have a carb problem, even though the engine builder would certainly like him to believe something to the contrary...

Now comes the challenge of finding the actual problem with the engine. But we have now, at least, eliminated the carb as the source of that issue...


Lars

 

Lars,

That was quick! Thank you for verifying what you already knew.......that the carb is not the problem.

My weekend project now is going to be try to take the cam measurements outlined in the other thread. I have a machinist rule as you suggested. Depending on what I discover I will be in touch with the shop on Monday. Stay tuned.

Once again, Lars, I cannot thank you enough.

 

Seems fuel injection is the new go to response for any suspected carb issues. Just a few yrs ago a truck forum mod had issues with me inferring a carb and HEI was simpler than setting-up the engine for computer controlled ignition and fuel injection. Must be a generational thing.
I bet the "Builder" doesn't want to see this thread, nor budge in his attitude, hope I'm wrong.

 

I'm afraid to say that I agree with your prediction...

 

I'm still wondering if the "builder" installed the timing chain one tooth off.

 

Have closely read thru both threads and looked at that cluster of a vacuum hose detour

? where is that Cluster**** Garage located and its official moniker ?

 

That's what I'm thinking, too. Dennis is going to do the rudimentary cam timing check procedure I gave him in the previous thread to see what it might reveal... I'm sure he'll post up results as his quest continues...

Lars

 

Isn't it disgusting to have to realize, and admit, that this is the standard of "quality" that most people can expect to get when they take their old Vette in to a "professional shop" to have work performed? You would simply not believe some of the "workmanship" I see on cars and parts I get here at my shop for repair... It must be very discouraging to classic car enthusiasts who are unable to perform their own work, yet wish to enjoy the pleasure of a cool old car.


Lars

 

It is, indeed.

 

I did the cam timing check and if I am doing it right and measuring correctly the intake retainer is closer to the head than the exhaust by about .050" So this indicates that the cam is in the engine correctly?


exhaust on left, intake on right?


TDC?


rotor pointed opposite #1 terminal

 

Has an intake manifold vacuum leak been considered yet?

 

Yes, I have checked the bolts, sprayed all around where it mates to the head and the carb base. If I cup my hand over the carb the engine chokes.

Thanks for the reply.

 

Stuff the engine in a flat rate box and send it to Lars.

 

...it wouldn't be the first time....
Improperly-built engine received from Chicago in a flat rate box:

Upon teardown, so many problems were found that it probably would have run very similar to Dennis' engine...

Lars

 

Should have done that to begin with, would have been cheaper

 

I think you can measure your valve lift from top of the head all day long and if you are one tooth out on the timing chain, you won't find it there as you will still have normal lift. You still have the exact same valve lift. It's just not at the time you want it. I would remove the water pump, drop the front of the pan, pull the timing cover and check. It's either that or the distributor and if the distributor worked fine zero miles ago, chances are it should work fine now. Your symptoms sound like that's the problem and the mistake is not that hard to make. Don't forget, your camshaft only turns half the rpm of your crankshaft so looking at your timing mark on your damper really isn't going to tell you much.

 

That's why I gave him the check procedure so he could accurately check it. Go to the previous thread referenced above to see the "camshaft installation check" procedure. See my post #16 in the previous thread.

Lars

 

Was that guy a lawyer perhaps?

 

I went back and checked the intake manifold bolts again and four, two on each side in the center were not snugged down. Don't know how I missed them. On one side I could actually see the gasket slightly compress. They are allen head bolts and I'm guessing they couldn't get to them so they just said screw it. Maybe that's not my entire problem but it certainly wasn't helping. When I get the carb back on I am going to start again from scratch on the timing. I also have a different vacuum advance canister on order that complies with Lars' 2" rule. One thing at a time.

 

OK I went back and did the cam check once again and got the same results, the intake appears to be closer than the exhaust. I triple checked the firing order and the correct wire is going to the correct plug. Started it up and it was a little rough but did idle on the fast idle. After it warmed up I blipped the throttle off the fast idle and it promptly died. I once again had to increase the idle screw for it to run. It does run better, it will idle now below 1000 rpm but still not smooth. I'm sure Lars putting the carb back like it was is why it's now better but something other than the carb is still not right. The initial timing is 18* and with vacuum advance hooked up is 32*. But idle below 1000 rpm is only 10" of vacuum. Got to be either ignition or vacuum leak? I disconnected the main vacuum line off the intake expecting that to make it much worse but it didn't seem to have an effect at all. I can feel vacuum at that line.

Could the cam still not be in right even though my measurements, assuming they were made correctly, indicate that it is installed correctly?

I am completely buffaloed and more than a little aggravated. Car ran perfect before.