Help!! The car falls flat on its face at 4K RPM. The motor pulls real strong until that point. It doesn't matter if I roll into the throttle or if I punch it, she still goes down around 4K. If I ease off the throttle it will recover somewhat. The motor is a 396 with a Qjet, 1 3/4 hedders, Accel 32oz points, AC RT43s, solid core AC delco wires. The timing is set at 6 degrees BTDC and the dwell is set at 29 degrees. Plugs are gapped at 35. I took the plugs and points out and cleaned them and they both appeared to be firing. It doesn't miss or stumble at lower RPM. Anybody have any ideas?

What is the number on the side of the Q-jet?
Maybe you have the small version and it's starving for fuel?

Do you know anyone with a 750 Holley you could try?

All V-8 point dwell are 30* period. Ford ,Chevy, Mopar, whoever. Now, having said that, what the heck are points doin' in your car?! If this is a relatively new development, I would say the spring in the points is going bad, allowing them to "skip". If the motor sounds fine but just flattens out, check all the usual stuff, plugged fuel line is a biggie. Especially if the limiting rpm starts to creep down. say, to 3800, then to 3600 and so on. I had a plugged fuel line do this once. DO NOT change the timing unless this is a new motor that has run this way from the start. If it's been running good till recently, and the distributer hold down isn't loose, IT"S NOT THE TIMING! Like I said, check the simple stuff first, it's probably something dumb. How 'bout the secondary metering rods on that quadra-bog? Just trying to give you some ideas. Maybe someone else has some more ideas. Good luck.

If you could provide a little more information it might help.
1-Is this a new problem?
2-What changes have you made recently?
3-Have you changed cams and did you install matching springs?
4-Have you checked your fuel lines all the way to the tank for cracked or leaky hoses?
5-Do you have a vacuum gauge that also reads pressure that you could install in the fuel line just ahead of the carburetor and mount it temporarily OUTSIDE the car using hose clamps on all connections so you can see if it is a loss of fuel pressure that is causing the trouble.
6-Do you know that the mechanical advance is working and what the total timing is?
Mike

The only thing I've done to the motor recently was remove it from the car, change the rear main seal and camshaft expansion plug, paint the motor and put it back in. I pulled the distributor but when I put it back in the motor fired right up. I also changed the starter too as it craped out on me. The carb is reworked with larger primary jets and secondary metering rods. Cam is a Comps K kit so everything matches.

I have a mityvac that I can check the mechanical advance with but no fuel pressure guage. The fuel system is stock with a stock fuel pump, canister filter in the front of the motor, and return lines.

The car improved when I cleaned the points and plugs and changed the cap and rotor but still bogs. I reset the timing back to stock and checked the plugs and points to get a base line. I don't know what my total timing is as I took the timing tape off the damper when I painted the motor.

I think its something simple but I'm terrible at diagnostics (and spelling too!). Thanks everybody for the responses.

try advancing your timing

Likely a fuel issue but many other things can do this too.

1) But 1st things 1st,If your running an aftermarked perf cam of any kind the 6 deg initial your running is too retartded for sure but is not your problem with the power drop after 4k rpms. But putting that asside for a moment you need to run approx 16-18 deg btdc inital with most afrermarket perf cams but only a bone stock cam from GM would use 4-6 deg btdc initial timing. So bump up the inital if your running a perf cam but watch that total doesnt exceed 38,if so have the mech adv in the dist limited to 20 deg max or you may damage the motor from detonaiton solisten for it.

2) Since your running a q-jet do the following and i bet your problems will disappear. I have encounterd & fixed this problem with mild to fairly hot BBC many times on there peoples cars and mine too. This is esp true when running a q-jet on a mild to fairly hot BBC which is more prone to fuel startvation issues due to restrictive in carb fuel filter,small fuel bowl,and small .110 needle & seat in stock form.

* Remove small restrictive in carb fuel filter and install a larger inline hi flow filter. You could use the stock type large inline filter used for 1969 chevelle 396/325 & 350hp motors if you want to look stock.

* Many q-jets come with a .110" or .125" inch needle & seat which are not lrg enough (esp the .110") for some BBC perf motors putting out more power,rpm, etc. Have the carb rblt for a stage 2 rbld that includes a larger .135 or .145 high flow needle & seat. Also have 72-73 primjets & 45b rods along with AX or BG secondary rods installed. This is for 1973 oroled carbs. Sean (from SEAN MURPHY INDUCTION.COM) can do this for you,he is honest & does good work so check his site out if you need it done or want to buy the parts to do it yourself.

* Install a carter street f/pump rated for 6-7 psi max because anything over 7 psi is not good for q-jets.

* Even though its advertised that the lighter fuel pump rods are not required under 6k rpms dont believe it. I have run into mult times over the yrs where some perf pumps and esp stock pumps arms floated at above 4k rpms which results in fuel starvation issues so install one with the new pump.

* Not likely but possible is the sock/strainer in the fuel tank on the fuel pickup sending unit being clogged up restricitng fuel flow when you demand more fuel with your foot in it.

Also,if your car has a non vented tank & cap maybe the return line/tank vent is not operating propely creating a vacuum in the tank with a non vented cap reducing the pumps ability to suck fuel out too but its very unlikely but possible.

But from my 35 yrs of 1st hand experience with these motors if you overlook one or more of the above mentioned things you will still run short of fuel when running a q-jet on a BBC thats been modified even mildly from stock form turning considerably more rpm etc. If you leave the fuel delivery system in basically stock form which is marginal at best your destined for top end power drop offs or the motor totally falling on it's face when you modify/increase the perf level even just a little bit from stock.

* Could also possibly have an issues with the dist when you open the throttle and the intake vac drops the vac adv also deactivates moving/retarding the breaker plate which moves the wires for ign. If any of the wires are loose or worn/broken inside the insulation when the breaker plate moves it can cause an open power wise during this breaker plate movement and kill the ign & motor when the power drops. Then when you get out of the gas and intake vac comes back the breaker plate advanced again moving the wire/s reconnecting to get the ign power/voltage back & wala,the motor comes back to life.

* A sloppy dist shaft & breaker plate along with an inaccurate dwell meter could do this too. If the dwell meter was reading over what the dwell actually is along with a sloppy shafts that can stop the motor too. What can happen is if the dwell meter is reading approx 30 deg but its acutually 20 deg the points are already wider then they should be. Then when you rev the motor up with a sloppy dist shaft/breaker plate things get even worse and the points open a little more enough so to be too wide for the ign system to function anymore and the motor will die and fall its face untill the rpms drop enough for the point gap to close enough for the ign to work again.


Check this stuff out and let us know how you make out.

Scott

Your Mityvac will allow you to check your total vacuum advance but you will need another timing tape to check your total mechanical. You should be able to use your MityVac to check fuel pressure check the information here: http://www.mityvac.com/pages/info_service_fp.asp
I wouldn't change a thing until you have made a proper diagnosis this is the biggest mistake people make,they start changing this and ordering that and they really have no idea what the problem is and all along the way they're likely creating other problems that didn't even exist when they started. Always remember this Diagnose then Repair not the other way.

Try using a performance type of point (stiffer spring) I had this problem back in the day.Use an aplication that has a high perf motor that has the same dist. like a corvette or camaro or even replacement from accel for your aplication

Do a simple test. You stated your fuel pump has a return line and that the carb is reworked with larger primary jets and secondary mettering rods.
Pinch off the return line with some needle nose vise grips or whatever, then road test it again. Could be that return line is robbing you of fuel causing a lean condition at higher RPM.
You could also try running it up to 4K rpm then rapidly pump the go pedal up and down to see if it keeps pulling instead of bogging down.

Thanks everyone for the responses!! I have to go to Japan for a couple of weeks and when I return, I will review this thread and see if I can fix it. Again thanks!

OK, resurrecting this thread now that I'm back from Okinawa and Christmas is over. To update, I replaced the points with a Crane XR-1 module and rechecked the timing. The car still pulls strong to 4k and then bogs. I'm almost certain its a fuel problem. I'm running an inline stock filter so I'll pull the one out of the carb if there is one. I took it to 4k and pumped the throttle like Angelo 67 suggested and it didn't recover. I suck at carbs so I will have to be very careful pulling the top off it but I have Roe's book so I'm certain to go too far and be over confident (LOL). Anyway, thanks for the previous responses and Happy New Year to all.

Pull the 2nd small restrictive in carb filter out for sure and see if that does it.

If not i have 35yrs of q-jet experience on BBC perf motors and i would bet that your needle seat is the samller .110 one meant for SBC and that you also may have a fuel pump arm float going on too due to a weak pump arm spring and the heavy stock pump rod cusing it to float thus shutting down fuel flow,kind of 2 things at ones causing the problem as i already mentioned way in the begining of your post/problems wks ago.

Edelbrock makes a high flow .145 needl & seat for q-jets used in perf applications and also make sure get a lighter pump arm meant for high rpm use and maybe replace the pump while your at it too with a carter street 6-7 psi unit but no higher on psi for q-jet. Many peolpe will say its not needed for under 6krpms but i have seen more then 1 time where its needed well below 6k rpm causing fule starvation caused by pump arm float so replace it.

If repacing the needle & seat is a pia for you the try the new carter 6-7 psi f/pump & lighter pump arm 1st as this may get it fixed if your running at least a .125 needle & seat. But if the new pump & lighter rod doesnt fully fix it then get the .145 high flow needl & seat from edelbrock and that should finally get you to your top end rpm without running out of fuel if thats in fact whats really hapening here. This seems to be a classic case of fuel startvation issues found when BBC are warmed over for street perf but the fuel systems are not up to the additional fuel delivery requiedr for the higher rpms and deamnds of a few more ponies being put put by the motors which is a very common problem with q-jets.

Keep in mind there is a remote possibility the strainer sock n the end of you fuel sender pickup i the fuel tank is plugged causing issues too but i would go after that one last. This is because i have always been able to correct the fuel startvation problems with q-jet doing the things i suggested and never had to replace a sock in tank but it could happen and wanted to remind you ofit just incase its your problem.

Scott

Help!! The car falls flat on its face at 4K RPM. The motor pulls real strong until that point. Anybody have any ideas?
SHIFT!!........................sorry, had too;)


seriously, Are you sure the fuel pump is good?

SHIFT!!........................sorry, had too;)
I do, at 3800 cause its embarassing to hear it fall on its face!!:o

I'm gonna yank out the small filter tonight. I can't really road test the car as I'm in the great Pacific Northwest where it rains constantly and the car ain't coming out of the garage in this nasty weather!! If its not the filter, I'll check the fuel pump although the pump's only about 3ys old and its a Delco and not a generic rebuild. I am also going to take the carb to the shop that built it and see if they'll change the needle and seat to the larger one. Thanks to all for the responses.

I too have the delco pump on my bbc and it needed the lighter pump rod so yes you should replace it . I also needed the larger needle & seat,mine would fall off at 4500 rpms,now good to 5,600 after i did all the fuel system mods i suggested to you.

If your motor still runs out of fuel after that then have the lager needle & seat installed that you can get from edelbrock. Buy it now so you can take it to the carb shop to have installed & float lvl reset.

Now if your problem is not fully fixed after the new lighter pump rod and larger needle & seat then your f/pump could be defective,the sock on the fuel tank pickup could be partially clogged,or remotely possible could be your fuel pump lobe on the cam is going flat not givning enough pump stroke.

A real out there thing could be when your motor hits approx 4k rpm where stock mech adv is fully activated the breaker plate in the dist moves/turns far enought to pinch or short an ign wire or causes a wire to open or loose contact and then when you let off the gas and the mech adv retardes & the breaker plate moves/returns back to home you get your ign power back and the motor comes alive again which i have run into a couple times over the yrs so chk that out too.

Could there poss be a rev limiter in an aftermaket ign if you have,that would sure do it too.

scott

Resurecting this thread. To catch up, I pulled the small filter out of the carb and went up 2 primary jet sizes from 74 to 76 and went from AYs to DCs on the secondarys. The car runs a lot better. If I roll into the throttle from 2d through 4th, the car pulls strong through 5k. If I start from a stop, the car still bogs at 4200 which is a small improvement. I was thinking of going up 2 more primary jet sizes to 78. Am I on the right track? Does this sound like a resonable solution?

No its likely not the jetting is causing a bog at 4200rpms.

Do you mean it bogs when you hit the gas at approx 4k-4200 rpms when you floor it at that rpm and the secondaries are opening?

Or is it pulling fine to approx 4200 & bogs a little at that rpm and picks back up and pulls fine past that to 5k now like you stated it does?

The DC sec mettering rods you picked are really too rich for your mild setup,you should get AX or BG rod still avail from GM PT#'s 7033549/7034522.
There AX & BG rods are definately richer then the AY's you started with but not as rich as the DC's.

Just removing the small in carb filter like you did and now it pulls better to 5k rpm on top end when it used to fall on its face at 4krpms shows in fact the motor has a fuel delivery issue on top end/high load. Go back to the ealier part of your post to go over again the items (esp on fuel delivery) i suggested you check and go over them again. Fuel pump/lighter rod,larger high flow inline filter,possibly partially clogged strainer on fuel tank pickup,still Need larger needle & seat in your q-jet becasue it has a stock .110" seat in it and needs the larger high flow 135" or 145",etc.

BTY,what color are the ac 43's burning,those are a fairly cold plug for a mild street 396. If they are bruning dark brown or dark tan move up to 44's if requierd for your setup,you want to see med to light tan plug color.

Scott

Scott,

If I start from a complete stop in first gear and floor it, it will pull till about 4200 rpms then bog. If I roll into the throttle rolling in second gear, and slowly press the gas, she will pull real strong thru 5k. From what I can tell, the secondarys are working fine. I will recheck the fuel system and order a new inline filter. Will change the needle and seat too. Thanks for your help.

That fuel filter sock has nailed thousands and it could be a pita even back when the cars were new. All you had to do was continue to buy cut rate gas back then where the station owner was careless about his own pump filters, assuming he had any in the first place.

Its easy enough to replace but you have to drop the tank to get at the sending unit. I would suggest driving out whatever gas you have in the tank minus enough to get you to a gas station when you have it all back together.

Greg,it seems as though your getting the bog when the secondaries are opening but i would think they would be opening a little earlier then 4200 rpms unless the upper secondairy air valve spring tension is set on the tight side.

Then in that case the rpms have to get a little higher before they open and if they open quickily when the motor hits 4200 prms you can get a bog for a few seconds at that point which will be much more noticeable in low/1st gear then in 2nd,3rd,or 4th gears.

But if the secondaries are all the way open at like 3800-4000 then it bogs at 4200 for a moment then quickly picks back up & pulls no problem through 5k thats strange.

If that only happens in 1st gear maybe there is a fuel pickup issue that only shows up in 1st gear when more g's are present and fuel gets pushed back more in tank and carb.

But it doesnt happen in the 2nd through 4th gear when there is a tad less g's pushing the fuel back so maybe something to do with that is your problem at this point.

So check the fuel being pushed back & or secondary opening thing.

Also,is your initial timing still set at 6 deg with an aftermarket cam as you mentioned earlier in this post? If yes its too retarded as i mentioned earlier on. You need to advance it to 18 deg initial with approx 18-20 mechanical for an approx 36-38 total. You never mentioned taking care of this which is why i am bringing it up now because its important.

Beware to watch out if you adv to 18 initial with a stock mechanical adv curve which could easily push the toal into the mid 40's which is too high & could ping. In that case you need to have mech adv curve limited to 18-20 in the dist which can be done at a speed or machine shop.

But your going in the right direction,your motor used to go on its face at approx 4,000 rpms now it will pull to 5k rpms so have patience and you will eventually get this all sorted out/dialed in.

Scott