I have a 70' f250 4x4 with 35" bfg muds that I dropped a 70' 429 into. The problem is the stumble off line when I drop the hammer from idle. It has a Holley 750 double pumper with manual secondaries, and the engine build is something like this...

*429 bored .030 over
*dished pistions to lower comp ratio to 9.5:1
*3 angle valve job, rollers, comp cam springs, hrd pushrods,guide plates
*edelbrock performer/portmatched to heads
*cam: (276-286 adv.dur)(218-228 @.50)(114deg lobe cntr)(509/509gross valve lift)installed strait up.
*MSD ignition with billet distributor.
*Stock exaust manifolds
*Holley 750 double pumper

I have tried different pump cams on the holley and even added larger nozzles in the secondaries. Still has the stumble. Runs like a banshee once it gets going though.

Anyone have the answers? I want that stumble gone! I would sure like any suggestions and yes, i'm going to put some headers in soon.

Also, anyone with a similar buildup get thier engine dyno'ed? I was wondering what my numbers might be...

Thanks, Steve.

 

hey Steve, well, is this a brand new carb? have you been inside of this carb? sounds to me like it could be the power valve. what size is the valve? i don't think putting larger nozzles in the rear will help that much, well it will help drain your tank faster. might want to try a 75 or 85 power valve, should give you quicker responce of the line. your power valve works off engine vacuum, if your engine idles at say 17 inches of vacuum when you stab the throttle the vacuum gage dives towards 0, now your power valves are marked 85, meaning 8 inches of vacuum, 75 meaning 7 inches of vacuum and so on you get the picture? all the way down to 25 meaning 2 inches of vacuum. so as you stab the throttle your power valve is going to react faster at 85 at 8 inches of vacuum compaired to a 25 at 2 inches of vacuum. so if you have around a 25 power valve it's going to create a bog condition because it's waiting the extra few milliseconds for a fuel charge. oh and by the way, how do you set your fuel & air screws? i'm assuming you just have primary screws on the front metering plate. you have a about the same set-up i have in my truck and i run the fuel & air screws at 1/2 turns out from all the in any more and it will start running to rich. let me know what you find out. C.J. later.....

 

I respectfully disagree with the above post. The power valve
adds fuel to the main jet circuit of the carb. When you stab
the throttle, you are right in that vacuum goes to zero and the
power valve opens. But at this time also, you have little air flow through the carb to pick up this extra fuel (or any main circuit fuel) from the main venturi. The accelerator pump
mechanically pumps fuel into the carb to cover this situation
up till the engine can start to build rpms thus airflow. It can
then take advantage of the extra fuel from the power valve. He
didn't say what his finally gear ratio is, but a large truck
with large tires will probably need a vacuum secondary carb.
Mechanical secondaries are for light weight vehicles with
numerically high gear ratios.

 

Tell me this, if you put your foot in it, How hard does it stumble? And, how fast does it recover? Like, *foot in it* then a second or 2 before it recovers, or do you have to feather it out until it revs up?
It sounds like an accelerator pump problem, like your carb not getting the squirt of gas it needs right off idle. You said you tried different cams and nozzles...maybe it's in the accelerator pump itself.
I had the same prob on my 390 w/Holley.. it was the accelerator pump cover. It was warped, and allowed a loss of pressure to the nozzles by letting the gas squirt out the sides of the pump.
Check for gas leaking around the accelerator pump by watching it and running the throttle wide open(engine OFF). If it's your pump, there will be a stream or dripping of gas from the pump..Keep your eyes open, though, because often is hard to spot. If none, strip the fuel bowls/metering plates out and clean all the passages, pull off the nozzle assy.. there may be something clogging it up.
Good luck.

 

CJ, It's not a new carb but in pretty good condition and I had it gone through by a guy at work who races and owned his own performance shop for years. I am confident that the setup was correct, and even checked the settings/linkage before I installed it. The powervalve is the stock that came with the carb...I don't know what it is off hand but I will check this afternoon. I have the air bleed screws out 1 1/4 turns for optimum vaccine but will check that again as well.

Franklin2,
My gear ratio is a 4:10 but with 35's,... who knows. It's probably pretty ugly for a truck that big. I enjoy the cruise speeds but I know that my bottom end suffers quite a bit. Also, it's now sporting a C6 with a shift kit and stock converter. Not optimal, but with a gear change might be adequate. I think the vacuum secondary carb would be great to try, and i'll start looking at the swap meets to see what I can find.

Proudowner,
Reguarding the stumble...it's pretty bad. After replacing one power valve I didn't want to drop the hammer and leave it there to see what happens for fear of toasting another one. I do feather it or just gradually apply to full throttle on the gas guzzling starts.

Guys, thanks for the replies. I'll get into it and mess with the adjustments again. I think my neighbor is going to the racing shop this afternoon so I'll look at a larger acc pump and a different PV. It's only money, right?

Steve.

 

This is gonna sound really dumb but have you checked the idle mixture screws? I've seen several cases of "off idle" stumbles cured by fattening up the mixture a little, not just on Holleys either.

 

I would look at the accl. pump system and make sure that you are getting fuel with when you stab the throttle. Sounds like to me the fuel mixture is going way lean when you step on it. I would investigate trying larger pump discharge nozzles. Changing the cam on the throttle lever determines volume and pump shot duration.

If the engine stumbles right away, you probably need to go to larger pump discharge nozzle.
If the engine runs hard briefly, then stumbles, it is likely the plastic cam selection.

By the way, how does that camshaft make the engine run? That's a Summit cam, right? I almost installed that cam in a 460 that I'm getting ready to put in my Crewcab right now.

Todd

 

Guys,

When you mention the 'idle mixture screws' are you talking about the air bleed screws, for and aft? I haven't seen any other adjustments on the carb. My old edelbrock had them on the front, but the holley 4150 does not.

I think i'm going to try the larger pump nozzles in the front as you mentioned. I think they are 28's in the front and I did upgrade them to 31's in the secondaries. The stumble is off line, or cruising when I apply quick throttle.

The cam runs strong. With the 35 bfg muds, it's hard to tell if any cam would have bottom end... but with my MSD tach output not working-it's hard to tell where in the rpm ban the power hits. I can tell you though, that within just a couple moments of dropping the hammer, the 'pull' is hard and consistant. I am not disapointed in the PAW ssi cam at all. Great idle, maybe a little choppy, but I didn't want to have just chop and loose the low end, so it was a lucky choice.

Thanks for the great advice,
Steve.

 

> When you mention the
>'idle mixture screws' are you
>talking about the air bleed
>screws, for and aft?
>I haven't seen any other
>adjustments on the carb.
>My old edelbrock had them
>on the front, but the
>holley 4150 does not.

Yep those are the ones I was talking about. Carter/Edelbrocks seem to be more sensitive to them but I've seen Holleys stumble when they were too lean at idle. I see now that it also stumbles at cruise so that probably isn't the problem tho'.

 

I aggree that the lack of a vacuum secondaries contributes to the problem of bog in a large tired heavy vehicle. Feathering the throttle is what the vacuum secondaries do until the engine is drawing enough air. In the meantime with the mechanical secondaries, I believe Holley used to offer a bigger accelerator pump kit. It consisted of a spacer and diaphram that gives the pump lever more travel and thus more gas when you stab the throttle. A cam with a high early delivery and bigger accelerator pump squirters also helps. I had the entire set to tune a Holley on my 1978 Bronco. I also had a set of jets. If your main jets and secondary jets are too lean you will get bog.

 

My friend put that same cam in his 87 f350 it ran real strong but roller rockers really added power after the cam was installed. If you dont have them invest in them they really liven up a motor.

 

I have a 390 with a Holley 600 vac sec. and have had the same problem for awhile now. Tried rebuild the carb and even tried replacing it with a known good used carb. Still hat the bog. Just found out what was causing it the other day. It was my Dist. the breaker plate pivot and vac advance holes are worn. The other day it started running really rich (could really smell it) and a short while later it quit. Tried starting it but no luck. Got looking around and found the problem. I opened up the points to .025 instead of the .017 I usally run and the bog was gone. I have a dist on order.


http://www.radarscastle.com/images/ford-tough.gif

 

I would like to respectfully agree with franklin on this, think of a smaller vaccum secondary carb. On a motor with the combination of lower compression and lower rpm, try to keep the primary venturi as small as possible for better atomization and throttle response. An engine as you know functions on air pressure (or lack of in the carburator area called vaccum). When the motor is at low rpm, air moving through a venturi slows down, the larger the venturi the slower the air movement yet still. When air moves slower, fuel drops from an atomized state and will not ignite (hence the stumble)throttle lag is the result. An advanced state of this is a backfire or extreme lean condition, the acceleration circuts are designed to overcome this but only if the air fuel ratio is corect to begin with. The fastest way to go slow is to overcarb. Use a 650 vaccum secondary, buy the quick change kit for the secondary springs and adjust to suit. If that does'nt sound like something that you want to try, another way to make the double pumper work (kind of)in a heavier vehicle is to adjust the secondary linkage to come in late, let the primary cover the hesitive state, let the vaccum catch up before the secondaries open. It's not the right way, but it'll save a couple hundred bucks.

 

Hi, I know this is late but I have almost the same setup, minus the roller cam and 35's(33), but I have a holley 750 vacuum secondary carb instead. I have never had a stumble with this setup. All that was done out of the box was adjust the main jets down one. My problem is to get the truck to hookup, I think I could tear the tires off if I keep my foot in it. One problem I do have is with this setup though is a part throttle ping but that problem will need to be solved later as I blew the engine up awhile back, broke piston skirt I think.