Engine Bog

[Bennett]

I need some help. My engine has a bog when you lay into it even a little I’ve been trying to systematically figure it out but I’m stumped. I’m mechanically inclined but not extremely versed in engines and definitely not with Carbs. With that being said, I’m willing to learn whatever I can.
Let me give you the particulars of my project which I purchased from a guy who was very meticulous but was also unable to solve the problem.
350 Chevy, camel hump heads, 202 valves, tunnel ram, two 390 Holley’s (4160), 400 turbo trans, 3500 stall converter, Camaro rear w/ 2.73 gears…everything is approximately 20 years old.
Starts good cold but a little harder when hot and as I said earlier it has a nasty bog regardless of speed or RPM when I open the throttle under load. I’ve verified constant fuel pressure throughout various speeds and RPM. The timing was set very far advanced and when I corrected it the bog got worse.
Verified vacuum advance is working (at least some anyway). I have read many of the threads on this site that pertain to my problem.
Where should I start to figure out the root cause of the bog and how do I remedy it?
If you’re going to tell me to loose the tunnel ram, please refrain from comment. That would be after I have exhausted all other avenues.
Thanks in advance for you help,
Mike

 

[Screaming Metal]

OK, start with a tuneup, put on a fresh fuel filter, plugs, make sure the air cleaners are clean, set the timing the best you can, and buy yourself a can of carb cleaner. Also, make sure you have fresh gas in your car. If you haven't changed your plug wires recently the last 3 or 4 years, put a new set on. They're cheap. And be sure you have at least 2 1/2 psi and no more than 5 psi at the carbs.
If its bogging right off the bat, either you got a vacuum leak, or your carbs are way off. Now, I need to ask you, are you running a progressive linkage? Are the carbs in good shape? Check your float levels on both carbs, be sure to adjust your float levels so you are barely have a trickle out of the level holes.

OK, well, start your car, idle her to about 800 rpm, might be loping or slightly rough if you have some cam timing there, but if she won't idle, let us know. Otherwise, Idle her to 800. Disconnect the vacuum advance right now, and plug it off temporarily.
Break out your can of carb cleaner, and where the intake bolts to the heads, spray along the mating surfaces. Then where the lower intake bolts to the plenum, then where the carbs bolt down to the plenum. If the motor automatically goes crazy and idles up fast, you've found a leak. I usually find them at the carb base, but they can be anywhere. Usually if theres a leak, its a vacuum plug removed and/ or vacuum line not plugged. Anyway, if you find any, fix the leak.
Now, lets start a check list here. We need to check for flooding, for leaness, for timing issues, for carb actuating issues. Now, take off your air cleaners, idle the car, get a mechanics inspection mirror, look into the throat of the carb, you should not see any fuel dribbling from the boosters. If you do, you have a problem with power valves or floats/needle valves. DO NOT PUT YOUR FACE OVER THE CARB AND LOOK INTO THE THROAT>>>>IT IS DANGEROUS!!!!!
If there is no fuel dribbling from the boosters, on your front carb, you want to screw the front corner screws on the metering plate in, bottom them out gently, then back our roughly 1 3/4 turns listen to you motor rpm idle up or down to maintain the 8oo rpm. Now go to rear carb on each corner screw on the metering plave, bottom the screw out and back out 1 3/4's turns. Now then readjust the rpm to 800, then back off the front idle aduster, you should hear a difference like its laboring, but it should not die. Now you want to idle off 1 Carb, not both!!!!! Make sure the front butterflys on the front carb are closed, if not, turn your motor off and adjust your linkage so it does close. Now, start her up, she should idle better. Rev her lightly, all should be fine. If all is well in your carbs, when you jump down on the throttle, they'll be a bog, which you can adjust out with your power valves, accelerator pump cam, etc, etc. Worse case scenario, a progressive linkage will be needed. Also, the timing will probably need to be reset.

Now, if your still bogging bad, start on your front carb, disconnect the throttle linkage, and squeeze off the fuel line, what we want to see if she'll run off 1 carb cleanly. have the rear carb hooked up to the return spring. See if she starts up, see if she'll run off 1 carb and rev up some....

Get back with us and we'll talk ya thru getting her straightened out....

 

[Screaming Metal]

Let me rehash this....
1. Tuneup
2. Check for vacuum leaks
3. Look to see if carbs are flooding or going lean.
4. Turn your idle mixtures to 1 3/4 turns out, front carb idle screw backed off, idling off the rear carb. Note- even if no progressive linkage, I like running off the rear because that carb will be running warmer then the front, which does affect idle quality.

Once we eliminate that your not washing your rings out or going dangerously lean, we need to get that timing set before we get too far into this....timing affects motor vacuum and everything else!

 

[railroad]

There are 2 things that can be causing the bog. One is the power valve in the primary metering block. It might be opening too soon, or too late. The second, is the timing of the opening of the secondaries. I would guess the secondaries would be most likely. If you want to eliminate the power valves probability, disconnect the secondary arm on the diaphram, zip tie the secondaries closed and drive it like you do trying to create the bog. If it is clean, no bog, put it all back together. If it bogs, post back and we will go from there. Back to the secondaries. Holley makes a spring kit to change the secondary opening timing. The springs are color coded and the instructions tell which to use to delay or increase the opening. Yours are opening too soon and the engine rpm is not high enough to handle the large increase in air. Ideally, in my mind atleast, the two vacuum pots would have a balance tube between them. Holley also has the lids to the diaphrams with the ports already in them. All you have to do is run a small vacuum line beween the two. You can get by without this. After re reading your post, the power valves sound like they may need tuning. If you do not have one, get a decent vacuum gauge. Here is a test that might help. With no breathers, a power valve will open later, bog will be later in the rpm range, less or disappear. With breathers it will occur sooner. If this is the case. Put the vacuum gauge on the intake source, idling in gear and get a reading. If the reading is less than 12, your power valve number should be half the reading. If above 12, reduce the number by 2 for your power valve number. The accuracy of your gauge and what the power valve really operates at my not be the same, so some adjustments may be necessary to be optimum. Maybe I didn't leave anything out. You also indicated the ign vacuum adv was questionable. Check to make sure it is hooked up to ported vacuum, no vacuum at idle and increases with throttle opening. If you have to change this, reset you base timing.

 

[fordsbyjay]

As the others mentioned you really need a vacuum gauge to determine power valve size. They are cheap and everyone should have one.

The previous posts are all leading to too much fuel but a bog can be caused by not enough fuel off of the idle circuit. A couple things to check are the squirters /pump cam/ accelerator pump. When you move the throttle the squirters on top should start immediately. The rate it squirts at is controlled by the position of the pump cam. The volume of fuel is control by the pump cam shape. The accelerator pump must supply the fuel so it has to be working properly as well.

One other common mistake is people have the idle adjustment on the throttle plates to high causing fuel to leak in through the idle transfer slot. This can cause the engine to run really rich causing idle and off idle problems as there is way too much fuel coming out of the carb.

All these parts are normal tuning items. Also, it wouldn't hurt to pick up a book on tuning Holley carbs. That can be very helpful in terminology and how things are supposed to be seems how others have worked on your car before you.

 

[fordsbyjay]

sorry double post

 

[Wild Mango]

The timing was set very far advanced and when I corrected it the bog got worse
AHA. Theres a clue here. Check your TDC mark on the balancer with a piston stop. We do this every tuneup and long ago stopped being surprised at how many instances the marks were out because the damper slips on the hub. Reset the TDC mark and set the idle timing at 12deg with the vacuum line to the distributor disconnected and plugged.

 

[donsrods]

I know you said to refrain from telling you to scrap the tunnel ram, and I know some people run them and love them, but I honestly feel that is at the root of your problem. Yes, if a person spends enough time and effort they can be made to be "ok", but by their very design they are made for drag strip, wide open throttle use, not stop and go driving.

I would bet you a steak dinner that if you pull it off and put an aluminum intake and one (or even two) carbs on it your problem will disappear.

Don

 

[2old2fast]

Bennet , there are many threads on this forum about dealing w/ 2x4 tunnel ram setups. Do some searching [I'm getting lazier by the hour & just don't feel like going over all this again !!]

dave

 

[Screaming Metal]

Well, heres your carbs, put them back stock....and we'll tune them out from there!
Now, let me just say this....is the car and TR setup 20 years old? If so, get a bucket of carb cleaner to soak those carb parts in(comes with a basket)and get yourself 2 kits and put in those carbs or take them to a carb specialist, have them cleaned and rebuilt.
20 years of dirt, crap, bad gas, even if the car has been taken care of, usually T's aren't driven that much, and that is a perfect scenario for fuel to go bad in those puppies. Just think of it as a tuneup for those carbs. OR, at the least, get a good bottle of carb and fuel system cleaner (If the floatbowls look really clean on the inside), and run it thru as we work these problems out for ya. Carb gaskets go pretty quick if you don't run the motors enough. Also, the gaskets dry out and the carb won't run worth a crap without those passages sealed correctly....
We'll be in the wings waiting for ya, do the tuneup and proceed....let us know what ya find at each step.

http://www.holley.com/0-8007.asp

Yep, I Love TR's and have ran many, it just takes getting them set to the motor. Its all in the tuning, but we'll help ya thru.
Or, if all proves too much for ya, put a single 4 on top, (a dual feed 650 doublepumper) just as sstock did. I believe you'd be pleasantly pleased! Just Ask SS!

 

[donsrods]

I'm not saying it can't be done, just that I know more people who have tried and given up on a tunnel ram because of bad idle, hard starting, bogging, etc. IMO even two fours on a low riser are much better on the street. I have two 600 cfm Edelbrocks on the 331 stroker in my 27 and it drives just like it has a two barrel on it, very smooth. I can idle down to 1000 rpms in high gear, step on the gas and it pulls right on up with no hesitation or surging.

I hope you get yours straightened out though as they do look cool.

Don

 

[Bennett]

Thanks for all the advice. I'll get started with the tune-up, verifications, checks, and adjustments you all suggested and get back to you with the results and details. It may take me a couple of days or so as my wife has chores for me. Just kidding.

 

[railroad]

Couple of highlights, or something like that. I think the easiest thing to do first, is to check your accel pump shot as suggested above. Any movement of the accelerator should produce a shot in each primary bore. How did you know your timing was too high? It is probably not your bog, but might contribute. If you think the carbs have been modified, which being a previous owner problem would indicate, you may need to verify what is not stock on the carbs. Instead of buying a book on Holleys, unless you want to, there are some good sites, including Holleys on the net.

 

[grazza]

If your chasing a pig, no matter what you do throughout the chase, you'll still have a pig when you catch it. Take the Screamer's advice and rebuild the carbs before you do one single other thing. Clean them thoughly, replace all the crook stuff with new genuine parts and put them together with new gaskets, seals and bolts/screws. Or more easily go buy two new carbs if you can. At least that way you will know pretty well where your starting point is.

By the way, only place I've seen Tunnel ram manifolds work real good on a street car is under a couple of Propane (LPG) mixers.

 

[railroad]

Nothing wrong with a carb rebuild, and the T is about 20 years old, but have Holley 390's even been around that long. Not that age is the sole factor in rebuilding, but my money says it can be tuned as is. That is just a odds bet, because I have no history on the carbs. People often bring me carbs saying I need a kit put in this. I got rid of my carb cleaner vat, when I moved, so I ask if they just want it to run right, fix it and send them on their way.

 

[Screaming Metal]

Well, heres your carbs, put them back stock....and we'll tune them out from there!
Now, let me just say this....is the car and TR setup 20 years old? If so, get a bucket of carb cleaner to soak those carb parts in(comes with a basket)and get yourself 2 kits and put in those carbs or take them to a carb specialist, have them cleaned and rebuilt.
20 years of dirt, crap, bad gas, even if the car has been taken care of, usually T's aren't driven that much, and that is a perfect scenario for fuel to go bad in those puppies. Just think of it as a tuneup for those carbs.​

OR, at the least, get a good bottle of carb and fuel system cleaner (If the floatbowls look really clean on the inside), and run it thru as we work these problems out for ya.​

Carb gaskets go pretty quick if you don't run the motors enough. Also, the gaskets dry out and the carb won't run worth a crap without those passages sealed correctly....
We'll be in the wings waiting for ya, do the tuneup and proceed....let us know what ya find at each step.

http://www.holley.com/0-8007.asp

Yep, I Love TR's and have ran many, it just takes getting them set to the motor. Its all in the tuning, but we'll help ya thru.
Or, if all proves too much for ya, put a single 4 on top, (a dual feed 650 doublepumper) just as sstock did. I believe you'd be pleasantly pleased! Just Ask SS!​

 

[Screaming Metal]

Only he can tell if his carbs are in good condition, I'm not there looking over his shoulder. But I can say this, you pull the bowls off a holley or any carb thats been run for 15+ years, even if its clean in there, you see that gray looking stuff after the fuel has evaporated, up in the corners and little nooks and crannies....that shit is in the passages as well.
Whatever kinds of filters you have, or HAD, during the last 15 to 20 years, if its been that long since the carbs were new, it needs a kit. Crappy cork gaskets around the bowls. The o-ring around the fuel feed tube between the front and rear.Crappy gaskets between the throttle body and the metering plates....
Crap gets past those filters on a daily basis....Just want the guy to get his car straightened out.

If he doesn't know the history of his carbs, he should look them over good, while trying to get his fuel delivery system straightened out. I do this crap day in and day out, for a living. Its best to know.

 

[Screaming Metal]

You can go to the Holley website to get tuning info....
OR
http://www.ehow.com/how_7619675_tune-390-cfm-holley.html

 

[Screaming Metal]

Replacement Parts# for Holleys....for carb tuneups....

 

[Bennett]

Hi guys,
I'm finally getting back to this issue. So much to do and so little time. I'm going to try and respond to each and every one of you with my findings as I go from your posts. Please be patient with me it may take some time.