The car has been running great all summer and took it for a ride down the valley this afternoon and all seemed fine until I decided to drop to 3rd gear and open her up a little on the straight stretch. Heard and felt the usual 4 barrel kick in the pants and then just totally bogged down then cut out again. Did this two or three times and actually got the tires chirping. So I tried in several more times with the same results except 4th gear where it just stayed "bogged" until I let up on it. Where do I start? My fuel filter used to be yellow and I noticed it seemed dark orange so I thought I'd start with the easy stuff. Where would I be looking for dirt stuck anywhere in the carb or how would I check the fuel pump. It's mechanical. Would like to get her going top notch again.

Thanks,

I had the same thing after I rebuilt mine. It turned out that I had bent the hanger for the secondary metering rods and it would go ultra-rich when the secondaries opened. Check yours to see if that hanger is moving properly.

there's a vacuum diaphragm on the front passenger side. test it to see that it holds vacuum. the link on it goes to the secondary air valve, acts as a damper. also, while you've got the air cleaner off, check to see that the secondary air vale (flapper on top of the secs) is still spring loaded and pops back to the closed position if you open it with your fingers. post back here with results, we'll figure it out.

I'm at work this morning till this evening but will check things out and post back. Come to think of it, I manually opened the vac secondaries last night with my fingers so I could peek down to see if the mechanical secondaries were opening with the throttle and the vac ones stayed fully open on their own. I just tapped them with my finger and they snapped back shut after I confirmed the mechanical ones were opening properly.
How do I check the vacuum diaphram to see that it doesn't have a leak?

Push it in & put your finger over the hole & see if the punger stays in.
It will come out a little but should stay most of the way.
Then take your finger off & it will take about 2-3 sec. for the plunger to come back out.
The air valve has a spring on it that should have about 1/2 to 1 turn of preload on it depending on what your combo likes.
It shouldn't stick open either.

Checked the diaphragm and works as it should. Changed the fuel filter and still the same problem. It seems to take longer to start lately but that might just be in my head now that I'm having secondary issues. I can feel and hear it opening but then it just falls flat. After I get off it things seem to work fine again. Any other thoughts? The vacuum secondaries do spring back shut the way they should but will stick ever so slightly at wide open. What next?

I bet this just started with all the heat we are getting in pa.

it sounds like the air valve on top is opening too fast. this floods the manifold with dry air and causes a lean bog. Are you familiar at all with the procedure for adjusting the preload spring?

Yes, it did just start with the heat but I was hoping I can fix it. I am familiar with the adjustment I believe. On the back of the carb on top and on the passenger side I use a small flathead screwdriver to hold the screw in place while I turn the allen head screw loose. Once loose I turn the flathead screw clockwise for more tension and ccw for less, right? I did try to put more tension on it by turning it one whole turn clockwise with what seemed like worse results. Now that I'm thinking this through as I'm typing, I think I should have turned it the other way making less tension on the spring so it doesn't open as quickly? Can someone set me straight on this?

Hi Scott,I know you had the q-jet tweaked just right and it ran great at the strip and then all the sudden it runs like crap,something all the sudden had to have happen/changed.

I dont think its the sec air valve sring adj because it was working fine and all the sudden changed unless the allen screww was loose.

But even if the sec air valve spring was too loose allowing the upper sec valve to open too fast it boggs like heck but usually fuel eventually catches up in the secondaries and the motor picks back up but his motor doesnt seem to be picking back up,it just stays bogged down so i dont thinks its that.

If your running stock point ign take the points out,clean them up with some 400 grit paper and then clean off with brakleen or contact cleaner.
Then reinstall them and set dwell to 30 deg take a test drive. I have seen dirty out of adj points cause your motor symptoms.

If it runs good you likely fixed it but there is still a poss if it's cooler out when you took the test ride it ran ok your fuel system was in a partial vapor lock situation when it was hotter out when you experienced the bogging issues eralier in the day.

Also,you said your fuel in the filter used to look yellow now its a dark brown,thats a red flag right there which tells me you may have dirt & water or some kind on contamination in the fuel/system.

REPLACE THE FUEL FILTER.

When fuel gets contaminated the motor will run very poor under a load like when you get on it and it may idle poor too if the carb/idle circut get's clogged up with silt/dirt/rust/water,etc.

Pull the fuel line off the inlet side of the filter and then spin the motor over and pump some fuel out into a glass bottle to see if it's truely contaminated with dirt & or water.

If you suspect contaminated fuel after you do this you could replace the fuel filter and then get a long (temporaray)piece of rubber fuel line and hook it on the inlet side of the f/pump. Then put a gal of fuel in the car and drop the temporary fuel line from the inlet side of the fuel pump into the fresh fuel in the car and go for a short ride to see if the bog goes away. Be carefull that you dont spill it in the car and run with windows open for good vetalation.

If it runs good you got a bad dose of fuel with water/dirt in it.

But if it runs poor then it still could be bad fuel but the carb could have some dirt in it too needing to be cleaned out.

If the fuel tests out ok then its been very hot lately,is it possible you were in a partial vapor lock situation?

Test this out by wraping some aluminum foil around all exposed fuel line/s & fuel filter and go for a ride in the evening when its cooler to see if problem goes away.

If it runs fine then you were likely in a partial vapor lock situation but if it still runs poor we are back to possible poor fuel and dirty carb.

Also,the sock on the end of the sending unit in the fuel tank could have also been clogged up with silt/contam causing this problem and i that case would need to be changed.

Another thing is maybe your fuel pump went south so check your oil to make sure the pump diaphram did not go bad and isn't laeking into your crankcase.

If you need a new f/pump get a carter street pump (6901?) thats approx 6-7 psi,its a good reliable pump which i feel is one of the best out there for a mild BBC like yours with a q-jet.

It could be the f/pump is weak and goes into a vapor lock situation very easily when it's hot out and needs to be replaced.

Try some of these things out and let us know how you make out.

Scott

10 bucks says your running out of fuel in the bowl. If it falls on its face and never recovers, it almost has to be out of fuel. If it bogged, then picked back up and ran good, it would likely be the secondaries opening too fast. My bet is the fuel pump or some other restriction, since you already changed the filter. My 2 cents.

That's what is driving me crazy because I have made no changes to anything on the car and it just started running poorly. Since I don't have a choke on it I usually have to pump the gas 6 or 7 times to get fuel up to the carb especially if it has sat for a week. But a few times here in the last couple of weeks it hasn't even started. I kept thinking it was the HEI module or one of the wires at the distributor because it seemed like I had no spark. An hour later or the next day I'd go out to tinker at it and the thing would start right up so I never got to trouble shoot. Wonder if the fuel pump will intermittently just not work one time and then decide to go?
Another thing I've noticed lately that has changed is when upshifting (between shifts) it seems to stumble a little. Used to be a smooth shift but now it seems to "buck" a little as I let the clutch out. Never did that before. Are any of these symptoms of a fuel pump problem also?

Scott,I would start with the fuel filter and see how it is after that.
You might have a dirty sock too

That's what is driving me crazy because I have made no changes to anything on the car and it just started running poorly. Since I don't have a choke on it I usually have to pump the gas 6 or 7 times to get fuel up to the carb especially if it has sat for a week. But a few times here in the last couple of weeks it hasn't even started. I kept thinking it was the HEI module or one of the wires at the distributor because it seemed like I had no spark. An hour later or the next day I'd go out to tinker at it and the thing would start right up so I never got to trouble shoot. Wonder if the fuel pump will intermittently just not work one time and then decide to go?
Another thing I've noticed lately that has changed is when upshifting (between shifts) it seems to stumble a little. Used to be a smooth shift but now it seems to "buck" a little as I let the clutch out. Never did that before. Are any of these symptoms of a fuel pump problem also?

Thats why I would hesitate doing any adjusting on the carb for now. You may very well cause more problems. Hard starting/no starting sure sounds a lot like a fuel pump to me as well.

As far as when shifting, it sounds like the fuel level in the bowl is low enough that when you let off to shift, it sloshes the fuel and causes it to instantly go lean. As Chris said, it could be the sock in the tank, or the fuel pump. Do you have a fuel pressure guage on it? Might be worth doing if not.

Definately won't make any carb adjustments! Only thing I did was put 1 more full turn of tension on the preload spring and I will be putting that back to it's original position. Where would I cut a fuel pressure gauge into the fuel line just so I can check the pressure? That would certainly be a way to check the fuel pump rather than just buying a fuel pump and then not being the problem. Would hate to have to drop the tank to check the sock but certainly will do if not the fuel pump being the problem.
Can I put the gauge in the rubber fuel line before or after the filter?

Definately won't make any carb adjustments! Only thing I did was put 1 more full turn of tension on the preload spring and I will be putting that back to it's original position. Where would I cut a fuel pressure gauge into the fuel line just so I can check the pressure? That would certainly be a way to check the fuel pump rather than just buying a fuel pump and then not being the problem. Would hate to have to drop the tank to check the sock but certainly will do if not the fuel pump being the problem.
Can I put the gauge in the rubber fuel line before or after the filter?

You should be able to get a fitting to put one in a rubber line after the filter.

Didn't get home until late last night but I did have time to check in my "old parts" box above the garage and I do have the original fuel pump that worked fine two years ago when I removed it to get the engine rebuilt. Any pitfalls or harm in throwing that on to troubleshoot the fuel pump? Do they have anything in them that gets brittle after sitting for 2 years or should I just be able to take the old one off and throw this "older" one on? Can't remember if there's a gasket involved or any pointers with install. Any help appreciated.

There is a gasket involved Scott but other than tht it's a pretty quick swap. Certainly no harm in trying it.

Dan

Thanks Dan. I'll try and get a gasket today at lunch and make the swap tonight to see if the problem goes away. I'll keep you posted.

that spring on the sec shaft needs to be adjusted to 3/4 turn tighter from the zero-tension point. don't wind it up a whole turn more, it'll reportedly damage the spring. does the air valve ever stick in the open position?

When it falls on it's face, and stays that way, it's fuel delivery. There is a filter in the carb, behind the big "nut" on the fuel inlet. Change it. If you have another filter, change it. Now test it before you start changing "hard parts" or adjust something till it won't work at all.

You guy's are forgetting in the original post Scott/wolfhunter stated THE FUEL USED TO LOOK YELLOW NOW ITS A DARK BROWN.(RED FLAG!!!)

That right there says to me fuel contamination like dirt/silt/water etc which can easily cause the problems he is having that can glog filter/s and idle/low speed circut in carb etc.

Scott,follow some of the very quicks tests i already mentioned in my prior post before bothering to change out a new fuel pump.

Go after the easy stuff 1st like changing the fuel filter/s and taking a sample of fuel from the outlet side of the pump by spinning motor over and pumping fuel into a clean container. Then pour that sample into a clean clase bottle to see if its contaminated.

Remember,your motor was running fine then all the sudden runs like crap and the fuel went from yellow to drk brown in the fuel filter which raises a red flag right there for contaminated fuel IMHO.

I have seen bad fuel in the past cause these issues so chk it out and get back to us.

Good luck
Scott (OLDER ONE !!!)

Tom,
Sounds like I have the spring cranked a bit too far. The plates will stay open fully if I manually open them but slam shut with just a tap. Sure don't want it stuck wide open so I will back it up one full turn because that is how far I turned it clockwise. Thanks.

Greybeard,
I haven't had a filter in the carb at all but did change the clear in line filter with the same results (bog).

Scott,
I wondered if the fuel additive/octane boost (Kemco) may have discolored the filter a little? The gas I spilled out when changing the filter had a slight orange/yellow tint to it.

I can feel and hear it opening but then it just falls flat. After I get off it things seem to work fine again. Any other thoughts?

My Q-jet is doing this too. If I stay in it when it starts to feel "flat", it will "come on" again just before the shift. I'm thinking mine just has to be fuel delivery. I have a 327 w/ 5/16" line, and this worked OK with the Holley. I figure the smaller fuel bowl of the Q requires a better delivery system. I've purchased 3/8" line and tank sender, and I am going to keep my fingers crossed that this solves it.

Let me know what finally solves your problem, as I am dealing with the same issue.

Scott,i think your chassing your tail and here is why.

When i say this i am assuming you have already checked your entire ign system to verfy its working properly with no loose wires,over advancing or broken mech advance,loose dist cap,etc.

I rememeber helping you dial in your q-jet over a yr ago through many emails and it ran fine all year long even at the strip too.

Then all the sudden one day to the next it 's falling on its face for no reason,something suddenly had to happen.

There is no way all the sudden the secondairy spring went way out of adj being too loose unless the scew was loose or the spring broke.

You have already tweaked it and never stated the adj screw was loose when you went to adj it or that there was no tension at all like a broken spring so thats not the problem.

So again the motor went from running fine for over a year with the current adj/tuning that was done to the carb a yr ago then all the sudden that particular tune setup meaning secondary airvalve spring is now too loose,now way something else is wrong here and thats why the adj you made to the sec spring etc have not fixed the problem because the carb is not the primary problem but may be a secondary problem as a result of the following.

I still think you picked up some dirt/silt/water/rust contam in your fuel because you stated the color of it in the clear fuel filter went from yellow to dark brown or orange which poss screwed up the tank sock,fuel filet,and got fine silt into the carb too. Also,if you dont aloways keep the fuel tank full esp durring winter storrage with stabizer the tank can rust up and start flaking into the fuel turining it a brown/orange color like you described.

I know you changed the fuel filter but that doesnt get rid of the silt/dirt/rust that may have gotten past it into the carb which can happen

I guess your fairly recent new fuel pump could have gone bad but not that likely unless its an edel or Holly pump which i have seen more then my share of defective new and or only 1 old ones go bad but not the carter pumps,they usually go the long hall for the most part.

You really need to get a fuel sample from the outlet side of the pump and compair it to fresh fuel both as seen in a class jar. Also add a small touch of the Kemco booster to some fresh fuel in a jar to see if that changes the color of the fuel like you were questioning. Remember to only put a small amount of booster in the glass jar with fresh fuel because you usually use a qt of booster to 18 gals so its spread to a very thin ratio of boost per gal fuel. (1.8 oz per gal with 18 gal tank) so a few drops in a jar of fuel for the test to see if it changes the color which i think it wont do with Kemco,but with something like 104 booster yes but not the kemco from what i have seen.

Check this these things and let us know how you makeout.

Scott

Well It was stated this just started happen in the hot weather.Sounds to me like a classic vapor lock condition.If a mechanical fuel pump is being used than add a carter 5psi electric pump as close to the fuel tank as you can.I did this in a 383 stroker engine i have in a 74 vette.I just turn the electric pump on when its hot out.Insulate any hot spots on the fuel line that you can as well.Install a heat shield to the carb to if ones available.My car use to fall on its face in hot weather.The best way to put it is it feels like it runs out of fuel.Now it pulls right to 5800 consistently

i would check the mechanical advance in your distributor,,make sure it moves freely,,i had the same problem ,tried about 5 differant carbs same result
better gas did give me more performance but it still bogged.if i eased onto the throttle it did go but if i nailed it i felt i was almost putting on the brakes
i feel bad gas may not give you the desired performance but it still should'nt bog or fall on its face
could also check while your at it the vacume advance
just figured i would add my 2 bits
good luck

Have you considered vapor lock, fuel pump could be pumping enough fuel for normal driving but not enough for WOT especially in the higher gears.

Can someone tell me what vapor lock really is? Does the heat cause pressure to build up in the gas tank causing it to not allow fuel to flow smoothly through the lines or what exactly happens? I'm changing the fuel pump because I have the old one that worked laying around and just want to rule out the fuel pump. But I remember hooking this up once before and I had to plug the fuel line from the gas tank because the gas would run out. When I unhooked the fuel pump the other night, guess what, no gas flowing out. I wonder if I go open the gas cap if it would start running again? I'm going to try it tonight. I do know that my gas cap in not vented and many times it has a lot of pressure in there when I open it at the gas station.

Scott,if you go back to my original post i covered how to test for the vapor lock along with 2-3 other things to look for and check out which you still havent done.

Has the motor gone back to running fine now that it has cooled off,then maybe it was vapor lock as i had already mentioned as a poss problem & how to check for it in my 1st of 3 posts trying to help.

I have suggested some things to try/test that could very well be your problem but if you dont follow through as i noted in my 1st of 3 posts you will be chasing your tail some more re-adjusitng the carb etc when it was already dialed in and working fine for over a yr with that setup. Something else has happened which is what i was trying to determine with the testing suggestions i gave you.

Good luck.

Scott

Scott,
I didn't think the things you mentioned to try in your first post had to be in any special order so I thought I'd try swapping the old fuel pump I had to rule out that being the problem since I was having some problems getting the car started a few times. But I ran into a problem with the pushrod (see other post) and that's holding me up from trying anything else you mentioned. Besides changing the fuel filter which didn't resolve it.

The other thing I did check when I took off the fuel pump was the color of the gas as I left it run in one of my wife's canning jars. I compared it to some gas I had in the gas can for the mowers and they look to be the same color. And a few drops of Kemco doesn't seem to have an affect on color so I'm not sure why that dang filter looks a little darker to me.

I don't think I'm chasing my tail, I call it troubleshooting. Sorry if I didn't do things in the order you would have done them but I didn't see any sense in going out and buying some fuel line to do the gas can thing if I could jsut swap fuel pumps first to rule that out. Then I can do the gas can test if it doesn't resolve it. The only adjustment I made to the carb was messing with the preload spring for the secondaries but I have the original position of that screw noted so it is now back where it belongs. So I caught my tail on that one!

So once I rule out the fuel pump I can move on to buying some fuel line and doing the can of gas thing as well as the aluminum foil thing. Now instead of "jacking me up" which I do need from time to time, what exactly happens when these things vapor lock?

Scott,On the vapor lock,from what i understand its when the fuel is heaqted enough to change if from a liquid state to a gas/vapor which can happen in the fuel line and sometimes in the carb or both at the same time.

The real clue/tell tell sign this is happening is when the motor & outside temp lowers enough the motor will go back to running just fine and would not fall on its face when you open it up like was happenng to you.

Then when the temp gets high enough again it will either go into a partial vapor lock which you may have already experienced because the motor never fully quit running or you go into a full vapor lock where the motor quits running all together and wont refire untill temp drops enough for fuel to go back from gas state to liquid state.

I dont see your f/pump rod post so i cant suggest anything for that.

Sorry,i didnt notice any responces form you stating you had already tested out the possible bad fuel or what was going on with the fuel in the filter changing from yellow to orange or was it brown. Maybe the excessive heat from this heatwave we were in and or possible vapor locking discolored the fuel filter since the fuel looked to be ok when you checked it comming out of the pump and tested the kemco in it too.

Let us know how it runs with the test pump and if it runs ok then test it for vapor lock when it hot again to see if it acts up then.

Scott

Got the fuel pump on yesterday and it's back to running top notch! But of course it's like 10 degrees cooler now than last week so it certainly doesn't rule out the vapor lock possiblity. I'm leaning towards vapor lock being the real problem because I took the vette out on Thursday night to get gas and it acted the same way as the chevelle but under less load. I couldn't step on the throttle even 1/4 without it bogging. Yesterday the vette ran fine without making any changes so I think it was the heat here in Pa. Time will tell if we get some more heat which I of course don't want.

VAPOR LOCK---Had this problem with my 454. Most problems are due to owners like me screwing with what the factory did. All rubber products under the hood need to be in place to tunnel the hot air out of the engine bay. The correct fan and or clutch must be in place. I went back to factory under hood set up and no more vapor lock. For the most part you can leave items out such as splash guards, rubber seals,fan shrouds,correct fan set-up. BUT when it gets hot the DO_DO will start to show up. A 1970 LS6 when new is about 5-8 degrees from boiling the fuel, When everything is new and perfect. I wanted to adjust everything I could find to solve the problem. I now have a stock cooling system, runs around 200-205 in town with A/C on 195 on interstate with A/C on. HOT weather makes all things turn to S##T. Get up at 3 am and go for a ride. This may help zero in on the problem BAD carb or just to D@M hot. My 2 cents. Bob