f1rob

Sorry for the long thread here!

I have a '69 L68 that has been giving me fits trying to tune it. When I bought the car the carbs were leaking and the car ran rough. Since they were the original date-coded carbs, I had them rebuilt by a reputable carb specialist locally. The main bodies had some warpage on the surfaces facing the metering block/ fuel bowls. They were machined as best as possible, but on the centre carb, there still is a slight warp, but no more than 0.005", so that should be compensated for by the gasket.

I have a couple of problems, and some questions. First the problems:

1) Car runs very rich. Tailpipes are both very black. Exhaust reeks of being rich, so bad in fact, that after going for a short drive, I need to take a shower, and my clothes need a wash.
2) After I shut down the car in the garage, the space has a very heavy raw gas smell for 4-5 days. The carb rebuild guy says i might be getting perculation, since the factory fuel line touches the lower rad hose, right by the fuel pump. I have installed a temporary rubber fuel line that does not touch anything solid on the way to the centre carb. I have fuel going only to the centre carb right now, to isolate the problems through process of elimination. Still stinks in the garage. Still running super rich.
3) When the outbound carbs were getting fuel, there would be a small puddle of raw gas in the manifold under them, a few minutes after shutting the car down.

Questions:
1) I have a factory-stock mechanical pump. What could be causing the fuel to dump out of the carbs after I shut the car down?
2) Is the fuel line touching the rad hose enough to cause percolation? I don't think the line is original, but except for trial and error, and buying a few with the hopes that one will not contact the hose, not really sure what to do.
3) Do the outboard carbs feed any fuel at all to the motor at idle, or low rpm, or only at wide open throttle? Seems like to motor is running worse without fuel going to them - even though the car still smells super rich.
4) If the outboard carbs only supply fuel at full throttle, how do you keep the fuel from going stale - how did they do this back in '67-'69?

This is such a pain in the A$@, that I am almost at the point of just tossing the lot, and buying a new set of carbs. My buddies keep telling me to bin the set-up, and get a 4 barrel in a dual plane. I bought the car for the tri power bling in the first place.

Please help with your suggestions and thoughts folks!!

Rob

A10pilot

Rob,
Here's an article for you. It may help in some way. Don't give up yet; there is a lot of knowledge in the Forum !
Mike
https://tech.corvettecentral.com/201...oubleshooting/

A10pilot

Tri Power fuel Line

Tri Power Fuel Line

Nowhere Man

426 Hemi

The outboards do have a small bleed.

bb62

I have a 67 437 - so it's close even though your car has the low-riser manifold
1. The center carb warpage has nothing to do with the rich mixture. With warpage leaks, you would see fuel on the OUTSIDE of the manifold.
2. The extra fuel from the outer carbs inside the manifold would likely be floats not properly set (which is also probably true for the center carb as well).
3. You should not be experiencing percolation. Can you post a picture of the set-up? I would not use rubber fuel lines. Originals were steel for a purpose (some of which is safety).
4. I've had my 435 for 23 years and have never experienced fuel going stale - even through the winter where the car is stored in a "CarBag".
5. Was the center carb tested before it was returned by your "reputable" rebuilder? Are the proper jets being used? Are you getting spark ignition at all the cylinders?

SWCDuke

Do you have a '69 CSM? Have you gone through the tripower section and verified that all the various adjustments of all three carbs are to spec including going through the idle speed/mixture adjustment procedure? What's the initial recommended idle screw position, and where are yours at?

Normal L-68 idle behavior, manual trans idling in neutral is about 600 @ 15". What do you get?

Duke

SW Vette

There are 2 likely areas for fuel leakage with a tripower setup: center carb throttle shaft, and the dozen or so flare fittings in the fuel lines supplying the carbs. If the center carb was properly rebuilt the shaft shouldn't be leaking, but I'd check it anyway. Also look for drips at all the points a flare fitting is used, including at the carbs themselves. Float level setting is also critical to getting the carbs to work properly, so I'd verify that's all good before tearing anything apart.

Edit: .I misread and thought you were seeing fuel on the manifold outside the carbs, so nevermind the throttle shaft and flare fittings. Float level would be a good place to start.

bb62

My 67 manual shows that the automatic idles at 600, the manual idles at 750 rpm for the L68 (and L36).


I agree on the float. That's why I didn't include the other items to look for in my list above. That would explain the excess on inside the manifold, but it doesn't explain the overly rich mixture. This could means the jets are not the right size, not screwed in properly, OR the accelerator pump might be left open. The plugs may also not be correct. If there is a rough idle, perhaps they are not all firing which could cause a lot of gas to go out the exhaust.

PAmotorman

use a straight edge to check metering body for warpage

C2Scho

...

PAmotorman

you can not grind or machine the stock holley metering body flat as there are little hair like projections around the fuel passage areas that make the seal to the gasket that will be removed. the metering bodies must be straightened in a fixture using a arbor press or buy a new metering body

f1rob

Hi A10pilot - Nice engine compartment - all lines appear to be routed the same as mine - thanks for posting!

f1rob

Stock pump is putting out a steady 8psi, so I believe within spec. In the C3 section, another member suggested 6.5psi. Will that make a difference?

Rob

f1rob

Hi bb62 - the floats are adjusted so that the fuel slightly spills out when I rock the car, which is how many articles suggest it be done. In the C3 section, a couple of guys suggested setting the outboard carb floats lower than that. Those are the carbs that appear to be leaving puddles in the manifold after it is shut down.

The rubber line is only temporary while I isolate the carbs.

The centre carb was not tested before I go it back, as the "reputable" rebuilder does not have a flow bench. The jets at "66", which I believe are the stock ones.

I have tested, and am getting good spark at each cylinder.

I'm thinking that I should take the carbs to a different builder, and let him have a go at them...

f1rob

Hi Duke: I have adjusted as per the specs, except the "69 L68 shows idle to be set at 750 rpm for 4-spd. I get about 14" with a bigger cam with more overlap. Holley says start with idle screws out 1-1/2 turns, and go from there. Mine are at about 2 turns out.

SW Vette

If you can adjust the floats so the fuel level is at or below the sight hole, excessive fuel pressure is probably not an issue. I've never heard of rocking the car to adjust float level though. Learn something every day

f1rob

Hi bb62:

When you say "the accelerator pump might be left open", do you mean there is check-ball missing, or zero or less clearance between the lever from the throttle shaft and the pump arm? Now that I think of it, there is no gap between the two - could that be allowing too much fuel through the carb, and into the motor? I had the impression that it only made a difference when the throttle increased.

Thanks,

Rob

bb62

I checked mine and there does not seem to be any real play in the accelerator pump linkage, but yours might be too tight leaving it always open. That would definitely result in too much gas in the manifold. You may wish to back down your accelerator pump settings to see what that does.

As for rebuilding, you could have Holley rebuild them. After rebuilding, they will be tested on a real engine to make sure everything is working as it should. Another alternative is to buy a second set of new Holleys, the same carbs, but without the numbers.

nilsthemis

Have you checked if fuel is dribbling down in the end carbs at idle?
Visible fuel flow indicates throttle blades not closing fully.
Maladjusted closing linkage can cause that, resulting in very rich mixture.
That is the only Tripower problem I've had. (Except getting all the fuel line junctions tight.) Rear carb every time.
Some adjustment procedures in handbooks seems to be based on incorrect GM instructions that results in the rear carb not closing fully.
Make sure the linkage really fully closes the end carbs.