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Old 04-02-2004, 11:21 AM
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bogg off the line

i have a 400 destroked to a 377
comp cam with .507 intake .501 exhaust
edelbrock rpm air intake
brodix aluminum heads
timing gears.
and a holley 650 double pumper

my problem is off the line if i floor it it dies, but if i let into it the floor it she flies
adjusting the timing doesnt do anything
im wondering if its the carb?
i just put the carb on and my teacher adjusted it, i dont know much bout carbs but im wondering if its to lean now or something....
any help would be nice

would it have anything to do with the accelerator pump?
or do you think its to lean right now?
i know nothing about carbs

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Last edited by molevolent_creations; 04-02-2004 at 11:21 AM.
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Old 04-02-2004, 11:53 AM
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Your problem is a lean condition. You need to tune your enrichment circuit (pump cams, squirters and accelerator pump).

You may need a 50cc acc. pump to provide enough fuel. First I would try a different pump cam and play with squirter sizes. What you need is more gas, when you snap open the throttle. The problem right now is when you snap open the throttle there is a hole, not enough gas to cover for all the extra air. It is a simple and cheap fix, just take it step by step until you get it sorted out. Different engines like different things. Some want a longer shot, some want a short but fat shot, this is all adjusted with cams and squirters.

Royce
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Old 04-02-2004, 12:03 PM
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You likely have several problems the main one being WAY too much carburetor for that engine. Accelerator pump lever arm is probably adjusted too lose (there should be zero lash), cam is probably wrong etc. The best thing you could do is swap in a 600cfm vacuum secondary for that double pumper.

Read this current thread

General carb info.

Genera 'too big' discussion

Double pumper vs. vaccum secondary discussion.

Another carb size discussion.

Another DP/VS discussion
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Old 04-02-2004, 03:24 PM
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I disagree 100% if anything he does not have enough carb for that engine. A 650 on a 377 (which are usually high winding) is on the small side. It should work but, we do not know enough about the rest of the combo to say either way.

Al he needs to do is tune his pump shot and the engine will run fine. Once he gets rid of the bog (lean condition) he can then read his plugs or make passes to figure out where his jetting should be.

Royce

Edit: How can you suggest a vacuum secondary 600 without knowing the intended use of the car, the weight of the car, transmission type, stall speed, rear gears, etc....?
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Old 04-02-2004, 03:42 PM
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If it were bogging off the line then the carb would be dumping too much fuel when he steps on the gas, not too little.

A lean condition would cause bucking, rough acceleration and back firing. Now as for the carb being too much, not quite, 377X2 equals 754, so he should have at least a 750CFM carb, one of the oldest and easiest tricks in the book to find how much an engine needs.

An even easier way to find if it is running rich or lean is just run a volt meter to the O2 sensor, in fact first thing you should do if you have an O2 sensor. If you dont, then if the car were running too rich you would have some black sut coming out the tail pipe and or blackish fouled plugs. If it were lean you could pull a plug and check for a yellow-whitish color, and or blistered plugs if it were running too long that way.

But what do I know, I love Q-jets remember.

Oh yeah and vacuum secondaries always.
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Old 04-02-2004, 04:01 PM
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87442lover, I disagree a bog can and usually is a lean condition when the throttle is opened up. Too much air and NOT enough gas. Most people think-it is the opposite and end up getting the carb so far out of tune it is ridiculous. While too much gas can cause a stumble it will usually not cause a major bog or stall the car as he mentioned. If he was dumping that much fuel(enough) to stall the car he would not get it started back up, due to wet plugs.

Now I don't think I am a know it all but, I have been through this several times on several different guys cars, most aren't believers until I tune it and prove it to them.

People that don't like mechanical secondaries usually don't because they can't tune them. In a performance application it is hard to beat them. As I said above we don't know enough about what he is trying to accomplish or details about the car to make carb suggestions. He did not ask which carb people thought he should run, he only wanted to clear up his bog. To do this he needs to adjust his pump shot, plain ad simple, If he was anywhere near me I would prove it.

Royce
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Old 04-02-2004, 04:11 PM
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Well I disagree. Too much fuel will stall the car, bog it down, or blow the top of the air cleaner off(done this to my friends mustang ).

Too less fuel will make the car stumble, backfire, backfire up through the intake and cause overheating/ burnt pistons and plugs.

I know, I had the peoblem with too much fuel on Q-jets a many times. Now I know this is a holley and I think all holleys are junk, but I have messed around with enough of them to know if it was bogging than too much fuel is being dumped into the enigne at too low of an RPM so it wants to die for a sec and then it comes back to life REAL fast.

Now I have also caught my friends mustang on fire cause I turned the mixture screws a bit too rich and I moved the throttle all the way back real fast and it went silent then BOOM! hehehe... pretty blue and purple flames.

Now moving too lean would make the car stumble at idle, or slowly accelerate, not bog down. Sometimes you wont even know it's running lean cause there is no bog and it seems fine until you check the plugs, or overheat the thing.

I think he should check his O2 voltage first before anything else, or check for signs of being lean or rich first before anything else.

You also have to remember that too much fuel and it cant burn properly, too less fuel and it burns more effiectly. This is why people "lean" out their carbs before an emissions test.

Last edited by 87442lover; 04-02-2004 at 04:19 PM.
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Old 04-02-2004, 05:28 PM
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Well.... I always look at the obvious before the abstract. Here are photos of a couple of carb size calculations (dozens of these calculators are available on the Internet and they all say the same thing). Note that the calculator recommends a 540cfm carb for a 377 @ 6000rpm at a typical VE. Thus the 650 double pumper is probably perfect for a WOT pass down the 1/4 mile, which is what is intended for. However, molevolent_creations' original question is what to do about a low speed, part throttle bog. The second pictures is of the carb size calculated for a 1500rpm condition which says it needs a 135cfm carb. Thus when he hits the throttle on his double pumper at low speed, all 4 bbls open and instantly the carb is 4X too large. This totally kills manifold vacuum and venturi signal, thus gas flow stops and the engine bogs. Eventually the vacuum builds and venturi flow resumes as the engine consumption begins to catch up with carb size. Yes you can band-aid over the problem with a REO pump and maxed out accelerator pump cam but to truly solve the problem, he needs a vacuum secondary carb that will automatically tune carb size to demand at all throttle positions and engine speeds. A smaller carb would also enhance drive-ability but isn't as important as the vacuum secondaries. The latter just aren't intended for efficient stop & go service and are very unforgiving if the carb is even a little over-sized.



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Old 04-02-2004, 06:05 PM
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LOL, funny stuff. This tells me why you do not like Holleys, if this is how you tune them you will never get one to run right.

It is NOT easy to overcarb an engine. A carb is a metering device it will only supply the air/fuel an engine needs. I could put a 950 DP on a 283 and tune it to run pretty darn good without bogging etc... While it is true for optimum performance a carb should be matched more closely. To say a 650 is too much carb for a 377ci engine is just plain WRONG I don't know any other way to put it.

You can run all the formulas you want on this and they will only get you in the ball park. You have to understand a 650CFM carb will not flow 650CFM.

Someone explain to me how a carb "dumps" too much fuel? If the carb is not broken or way out of adjustment it should not "dump" fuel at any time. The fuel should be PULLED out via the jets and only according to how much AIR is rushing past them to draw the fuel out.

There are several different circuits to a Holley carb and you must tune each one individually.

The idle mixture screws you mentioned do nothing (very little) once you are on the main circuit. The reason your friends car popped or stumbled is because you had them adjusted incorrectly. Don't mistake a poor tune up as the problem.

Lots of peopel think they have worked on Holleys for years only to find out they still don't know what they are doing. Anyone that "hates" them only hate them because they do not understand how to tune them. This is the reason people hate Q-jets or any other type of carb. People tend to frown on things they don't understand.

If you don't know how to tune a Holley why/how can you you give advice on how it is supposed to be done?

I would also like to know where you come up with a 650 being too large for a 377ci engine? Keeping in mind we really don't know much about the application. I still say 650 is the minimun cfm that he should even think about.

The only reason a carb would be too big is if it can flow more AIR than the engine can use, even in this case it would only cause low speed torque/HP loss and should still run fine once up in the RPM range (if tuned right).

I'm not going to argue over this, believe what you want.

Royce
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Old 04-02-2004, 06:17 PM
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They bog just like any other carb cause the accel. pump squirts out fuel when you step on the gas.... Adjusting the mix is one fix for this problem.

I got an answer for the dude, he should check all the signs as I said and get back to the board with the problem. That way instead of us argueing about it all day long we could see who is right in this case.

So like I said, if O2 equiped than check the voltage, go through the RPM range and see if the voltage drops, or climbs.

If no O2 check for sut coming out the tailpipe at idle, and during reving, during the bogging. Check the plugs to see what they look like if they are black and gas smelling, or if they are blisterd, or whiteish.

How bout that?

As for only needing 540 CFM... aaaa... I think my method is better, 650 will work, 750 is better.

Oh yeah and as for the holley he had, you can only tune it by the fast idle screw on the pass. side, and the two front screws. Aside from modifying, or messing with the jets.

Last edited by 87442lover; 04-02-2004 at 06:23 PM.
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Old 04-02-2004, 06:23 PM
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If the engine is only 83% efficient,then the calc is correct.Most built engines are at least 95% efficient or with the heads available now and precise machining and cam timing it can and will be over 100% efficient. So it depends on how the engine was built.

As far as the bog, it is a lean cond when the butterflies are suddenly opened not 2 much fuel. If its 2 much fuel,it will usually just blow black smoke and be lazy until the pump shot is used up and the main jets take over. {if the engine speed is sufficient to burn the fuel being dumped into the inlet side} correct?
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Old 04-02-2004, 06:24 PM
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i agree with camaroman7d when i first built my motor when i floored it the thing would crap out. then i richend up my carb and bam good as gold. it was running way lean.
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Old 04-02-2004, 06:24 PM
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If the goal is a stop and go traffic grocery getter then a vacuum secondary might be right. I don't know too many people that go through the trouble to build a 377ci engine only to drive in stop and go traffic. He was obviously after performance. There is a place for a vacuum secondary carb and I have run them when the situation is right.

If you want the perfect mixture for every situation you need to put electronic fuel injection on the car. Once again this was not the question, he wanted to fix the "bog" which IS caused by a lean condition (just as willys explained). For some reason people that like one thing (vacuum secondary carbs) always seem to think anything else is not right or no good. This is just not the case. If you have no control over your right foot then a vacuum secondary is probably better for you. You don;t have to drive around with your foot to the floor. His bog could be before the secondaries even open, we just don't know enough to prove otherwise.

There is no reason to try to sell this guy another carb to fix his problem when it can be tuned out. Call it a band aid if that makes you feel better but, the fact is even vacuum secondary carbs need tuning (are those also band aids?) no it's called tuning. Nothing comes out of the box and runs perfect even fuel injection needs tuning.

This is one of those discussions that no matter what is said you are not going to change the other persons mind. I don't have anything against vac secondary carbs but, in a performance situation it is VERY hard to beat a mechanical secondary carb. I am sure we all can agree on that.

So like I said many times before it still DEPENDS on the use of the vehicle none of us know what it is yet. So until then I don't see how you can recommend a change.

He asked what caused the bog, those of us that know how to tune will agree it is caused by not enough fuel to cover the throttle plates opening. This in other words is lean for lack of better terms.

Royce
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Old 04-02-2004, 06:28 PM
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dude... he asked a question with out enough information. You DONT know for a fact that it's a lean condition.

There needs to be some more info here and he needs to do some more work instead of guessing at it, I say he needs to do a few more checks.

Plus if the engine is bogging down the engine is pulling in less air NOT causing a lean condition, BUT we will see, he needs to provide some more info!
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Old 04-02-2004, 06:29 PM
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mechanical secondaries are definately better for high performance.
i would be surprised to see anyone disagree with that.
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