Question: too much carburetor ?

[weegie]

I have a 69, 350,350 hp. 4spd. 411 gears. It was bought with a Holley 750 double pumper carb.Is this over kill ? seems to run rich. I do not race this car but still like some performance out of it.Any input would be helpful.Thanks

 

[Hib Halverson]

I have a 69, 350,350 hp. 4spd. 411 gears. It was bought with a Holley 750 double pumper carb.Is this over kill ? seems to run rich. I do not race this car but still like some performance out of it.Any input would be helpful.Thanks

It is possible, depending on your engine's camshaft profile and cylinder head choice, that a 750 DP might work ok. On the other hand, if you have a mild cam and relatively stock heads, a 750DP is too much and needs to be replaced with a vacuum secondary 650 or 700.

Typically, Holley DPs run rich out of the box. They can be recalibrated such that they run pretty well on the street, but, admittedly, the job is more than just changing jets.

"Auto Enthusiast" magazine published a 2-part series late last year on just that subject...tuning Holley double pumpers for street use. Look in the magazine's web site. It may have previously-published articles on-line.

 

[GerryLP]

Weegie,


If the car was running fine with this carburetor, then it could be a blown power valve.

I have used 750 DP on a 353 before, and they run fine; however, when adjusting it, it will get close to its adjustment limitations. For example, I have had mine adjusted at between 1 and 1-1/2 turns CCw on the idle circuit needles to obtain max vacuum signal. My previous engine ran a best 14.600 at 93MPH on the 1/4 mile, and this may not sound like a whole lot of quickness, except that it was at 5,000' altitude. The general rule of thumb is that the same car/engine combo would run 13.6 at sea level.

Not bad for a naturally aspirated, 9.5:1 engine.

GerryLP:cool

 

[weegie]

to much carb

It is possible, depending on your engine's camshaft profile and cylinder head choice, that a 750 DP might work ok. On the other hand, if you have a mild cam and relatively stock heads, a 750DP is too much and needs to be replaced with a vacuum secondary 650 or 700.

Typically, Holley DPs run rich out of the box. They can be recalibrated such that they run pretty well on the street, but, admittedly, the job is more than just changing jets.

"Auto Enthusiast" magazine published a 2-part series late last year on just that subject...tuning Holley double pumpers for street use. Look in the magazine's web site. It may have previously-published articles on-line.

as far as I know the cam and cylinder heads are stock.I do have hooker headers and side exhaust.I would agree that 650or 700 would be a good fit.The car runs very well and never stumbles or loads up .I can just smell that it appears to be running rich.I found a web site and followed the entire set up for 750 dp carbs and it did get better.But not completely.Thanks for the input.I will let you know if I go to another carb.

 

[lars]

Keeping in mind that the stock factory Q-Jet installed on these cars was 750 cfm, I have never experienced that a 750 Holley is "too much carb." If you have mechanical secondaries, it is possible to "over-carb" the engine if you decide to slam into the secondaries at an rpm range which is too low for the cfm flow potential, but that's a driving technique issue - not a carb size issue. I regularly test carbs up to 1050 cfm on my 357 small block - no problem at all on a correctly tuned carb.

Further, keep in mind that a big carb on a small engine will not run rich - it runs lean due to decreased fuel metering signal to the carb. Any "over-sized" carb can be correctly set up and tuned to run at the correct air/fuel ratio throughout the operating range of the carb.

Although a large carb can be set up to run just fine on a smaller engine, I don't promote oversized carbs as a way of making any more power: A carb that's too small will limit power, but an oversized carb won't make any more power than a "right-sized" carb. On the dyno testing I've done, I've found that a 650 tends to be just a tad small to produce peak power on a good-running 350. Your 750 is probably about right for good power production, but the mechanical secondaries will require proper driving technique and gear/rpm selection before you force the secondaries wide open.

Lars

 

[weegie]

too much carburetor

Thanks for the input Lars.I was hoping you would jump in on that question.I will go over my carb again to insure all is well and work on my driving habits.I really did not want to spend 4 or 5 hundred on another set up.living in Ohio I still have a couple months before I can play on the salt covered streets.Thanks again for the input................................................

 

[tracksnblades1]

TOO much CARB

Lars is spot on. The 70 350 LT1 came factory with the 780 holley, 11-1 comp, solid cam. There are several 750 variants. Some with the dog leg boosters. Some with the annular discharge boosters. Mechanical and vacuum version are only a few. I always like the center hung float bowls dual line mechanicals the best. I didn't really like the metering plate types. I would research what you have. Check your Idle vacuum for your automatic or manual trans. Install the right power valve(s). If shes burning your eyes at an idle and you know what cam is bumpin your valves, find out whats wrong, fix it and jet it correctly. I'm sure Lars can tell you also with the correct jetting, correct power valve(s) selection, correct accelerator pump and nozzle selection on top of a good heart this holley can scream. I know the smaller cfm holleys are cheaper and simple to get out of the box driveability. I like the old 830 and 850 holleys on well prepared 350s. With special attention to accelerator pumps sizing, squirter sizing and pump cam selection. You old guys know, the naked automatic launch, dead idle, stab it to the floor, flash the converter and hold on until it breaks. 4:11s love big carbs.
Good luck tuning her in. AGE QUOD AGIS Do what you do, Well

 

[lars]

I'm sure Lars can tell you also with the correct jetting, correct power valve(s) selection, correct accelerator pump and nozzle selection

Yes, if you give me the list number, I can give you all the specs for that carb. If you drop me an e-mail request, I can also send you some of my papers for overall setup and tuning along with my paper on the "Tuning Tour Tuning Obersvations" paper, which summarizes a lot of the common tuning setup techniques and errors.

Lars
V8FastCars@msn.com

 

[weegie]

too much carb ?

Yes, if you give me the list number, I can give you all the specs for that carb. If you drop me an e-mail request, I can also send you some of my papers for overall setup and tuning along with my paper on the "Tuning Tour Tuning Obersvations" paper, which summarizes a lot of the common tuning setup techniques and errors.

Lars
V8FastCars@msn.com

Thanks for the info guys.Sound like I have plenty of homework to do.I do like the fact that my carb is a good fit.Just need to get it all dialed in.Thanks again.........................................Lou