Captain Morgan

I just recently installed Mid America's true dual exhaust, H-pipe, and Flowmaster DeltaFlow 50's on my 79. Before I did this, it would bog around 5500 RPM. I assumed it was because of restrictive exhaust as it felt the same as when the cat was clogged. It wouldn't do it every single time, but quite a bit. I took it out tonight and drove it and it was starting to bog/cut out around 4500 sometimes.

Any ideas as to what is causing this?

QuickVet

ignition or fuel :yesnod:

Stingy74

Fuel delivery problem maybe? It's a shot in the dark, but you have to start somewhere. Might want to look at ignition too.

Sting Raycin

Is your ignition timing set correctly??

Captain Morgan

I was going to check timing today just out of curiosity but got caught up in other stuff. It was fine a couple months ago but I suppose it could have slipped if the distributor hold down bolt isn't tight enough. I'll check it tomorrow night. Heck, it could possibly be ignition or even the fuel pump since they are both original.

Thanks,
Avery


[Modified by sharklover, 10:26 PM 12/30/2001]

Vetterodder

Is it cutting out OR is it bogging? There's a big difference but I doubt that you'r getting a bog at those revs. There's a point that an engine just runs out of power and, if yours is close to stock, 5500 rpm would be normal. But, assuming no other changes than the freer flowing exhaust, I also suspect an ignition or fuel delivery problem. Floating valves would also be a likely cause but an exhuast change wouldn't have affected when it happens. The reduction in back pressure could be causing a lean condition (or making an already existing lean condition worse, which would explain the problem occuiring sooner). A lean mixture can be harder for the ignition to fire. In any case, the leaner mixture that resulted from the reduced back pressure in your exhaust system may be more than your ignition can handle. Your ignition may have been your problem all along or it may be fine but your jetting was too lean to start with. It would be helpful if you gave more info about the engine and the condition of it's components.

Shark Racer

I personally would go with the engine running lean... when I went from my old engine with stock exhaust, threw on a performer intake, a high flow cat and super turbos, my engine started running very lean. Maybe you should have it dynotuned with an air/fuel sensor? (Though this is expensive... ~$100). They could probably help you figure out if it was ignition or carb mixture.
-Steve

MikeE.79

When was the last time you changed the fuel filter? It may be getting plugged!

Mike

vert2go89

I had a camaro with 350 and it did same thing.turned out it was the fuel filter was clogged and wouldn't let enough gas thru at high rpm.

BB68Vett

I had this same problem and it turned out to be valve float. Unbeknownst to me at the time, I had some seriuos valvetrain issues and it showed as a lack of power at high rpm (above 4500 rpm).

Captain Morgan

A little more info -

Engine was replaced in January of 2000, dynotuned and rejetted in Jul 2000. Now that I think about it, it could be running very lean. It was running really rich before the re-jet but that was before the cat was clogged. After punching out the cat, that would probably make it a little more lean. Then I moved from Utah (4500 ft) to Indiana (400 ft) so I'm not sure what affect that would have. I'm planning on having it dynotuned again but not till spring. If it is purely a lean condition, will it be ok to drive as long as I keep revs down? Or is there something I should absolutely do before driving it? Closest shop that will dynotune it is about 1 1/2 to 2 hours away.

I guess you would consider it cutting out - it just falls flat on it's face


[Modified by sharklover, 8:30 AM 12/31/2001]

Captain Morgan

What were the problems causing valve float? What should I look for and how? I do most of the work on the vette if possible but I don't really have experience with engines. I do however, have people around that could help me if I need help.

lars

Even a set of factory lifters aren't going to float the valves at 5500.

Here's what I would check if I were you:

1. Verify that your timing is set right and that your centrifugal advance is working: Put a timing light on it, rev it up, and verify the timing is advancing. Either set it up for a total mechanical timing (with vac disconnected) of about 34-36 degrees, or get it in the ballpark by setting the initial timing (with vac disconnected) to about 14 degrees.

2. Install a new fuel filter.

3. Adjust your secondary airvalve spring to 3/4 - 7/8 spring windup.

4. Check your secondary metering rods. Your mods will lean out your mixture, and your car is set up stock pretty lean anyway. If you give me the letter code off your secondary rods, I'll give you an opinion on them...

Captain Morgan

Ok, checked timing tonight and here is what I have -

Total mechanical (vac hose off and plugged) 34-36 all in by 2500 RPM, jumping around a little (2 or 3 degrees) above 4000 RPM.

However, out of curiosity, I hooked the vac hose back up and checked and it gets up to 40 degrees. Is there something wrong that would cause this?

I'm not that familiar with carbs yet, do I have to disassemble it to check #'s on the metering rods?

I can probably set the airvalve spring, just need to grab the book and read about it.

I know it wouldn't hurt to change the fuel filter, but I'm a little leary about changing it and screwing up the threads. I do intend to send my carb off to you (lars) at some point and figure I'll attempt to change the fuel filter just prior to that so I don't have to send it to you twice.

Avery

Les

Vetterodder said exactly what I was thinking. Since the simplest place to start on a fuel delivery problem is w/ the fuel filter, I'd do that first & then go down the list that Lars gave you. Good luck.

lars

Shark -
Good job checking the timing - sounds like you know what you're doing. Your centrifugal (mechanical) sounds like it's right on. Your vacuum is actually just a little short: You can run a total combined total (max mechanical plus max vacuum) of 52. Sounds like you have a fairly short vacuum curve. This doesn't affect performance, but it's shorting you slightly on gas mileage. Contacdt me if you'd like a good part number for a better vacuum advance unit...

You do not need to disassemble the carb to get the rod numbers. There is a "hanger" with a single screw attaching it right between the secondary airvalves. Pull the screw, and lift the hanger straight up. The secondary rods will come out with it. There is a two-letter code stamper into the rods. This code indicates the size of the rods. Give me that code and I'll give you some info on them.

Here's the procedure for adjusting the spring:

Before you start changing the rods, you want to get the secondary opening rate set up. This is determined by the spring windup.

It is a very common “speed trick” to loosen the secondary windup spring so that the secondaries will open very quickly. This is the single most common cause of a severe stumble or hesitation upon acceleration or transition into the secondaries.

The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about Ľ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ˝ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ľ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening.

Contact me with any questions.

ddecart

What's the liklihood of a mechanical fuel pump not being able to keep up at higher RPMs. I have a similar problem occasionally and it's definitely a fuel starvation problem. Comes on rather suddenly for a brief moment, then seems ok. I'm usually out of the throittle pretty quick when it happens, just in case.

Dave

lars

dd -
The stock mechanical pumps on these cars were designed to provide adequate fuel delivery throughout the usable rpm range, and will usually do a good job of producing trouble-free performance with no fuel starvation. Exceptions are:

* If 2 fuel filters are installed in the system (an in-line filter in addition to the in-carb filter)

* If the "sock" in the fuel tank has become partially obstructed by debris/rust/sediment

* If there is a kink in a fuel line

* If the engine has been severely modified to require a much larger than stock fuel supply

* If oxygenated fuel with very high vapor pressure is used on a hot day the fuel in the suction fuel line can begin to vaporize under high flow conditions, causing a vapor lock problem.

If none of these conditions exist, it's been my experience that the stock mechanical pump tends to provide very good, reliable service.

ddecart

Thanks Lars.

Non-oxy fuel, cold weather.

It seems to be a problem mor if I'm easing into the throttle for the first half of the gear and then I'll go to WOT. It's fine with no bog going to WOT and it pulls good, but then I hit a point where I've got fuel starvation.

If I start at WOT from a near standstill, I'm in good shape through the rpm range.

My carb is a Holley 4010 square bore, mechanical secondary, 2 fuel bowls/inlets, no filter on the carb, but I do have a big inline filter. No clue how the sock in the tank is, the tank is only a few years old, so I hope Bubba at least took care of that when the new tank went in.

My guess is that I'm simply starving the system of fuel while accelerating like that. I probably have enough fuel to make a wot run through a gear, but when I drive at say 1/2 throttle and then go to wot, I've already drained the fuel in the bowl(s) and I don't have enoug left to finish the WOT run.
Did that make sense?

Dave

Captain Morgan

I learned how to check it from your posts and papers so you deserve the credit. Luckily the guy in Salt Lake knew what he was doing when he set it up for me too. Just glad you're on the forum to tell me how to check it properly. :)

I would definitely like the number, we could all use a little better gas mileage. Not that it would help much with my leadfoot...but I'd still like to maximize it for when I'm just cruising.

Would that possibly be the cause of it bogging on launch at the dragstrip? I usually launch between 2k and 2.5k depending on stickiness of track. Just enough to give me slight wheel spin with street tires. Any less and it just won't go, any more and I get wheel hop. However, when I launch, it pops up and then nose dives and then gets going again. That was before the new exhaust, don't know what it would do right now.


Thanks for all your advice. I have been way too busy to check on numbers yet, but I'll hopefully get to it this weekend. Just wanted you to know that I hadn't forgotten. I did figure out how to pull the metering rods from reading the book but it did not tell me how to set up the spring (at least not the same way you described).

I'll e-mail you with codes when I get them.

Thanks again,
Avery