Can anyone help?

[airkiller]

I have a 68 Road Runner with a 383 and 727. I rebuilt the engine almost 2 years ago and have always had a bad hesitation every time I hit the gas hard. The engine is equipped with a Edelbrock Performer Monifold, Holley Street Avenger 800 CFM carb and a MSD ignition. I have been told the the carb I selected was prone to this problem. Can anyone give me some tips. I have experience on building engines but not trouble shooting performance issues.

 

[sicksatellite]

Try re-jetting

 

[HEMI-ITIS]

Need more info:sign12:Is it a bog with a puff of black smoke,or a hesitation and THEN picks right up and pulls strong through the RPM range?That carb is a single pumper style so it's also possible it might need a larger accelerator pump.Have you checked the advance in the distributor??At what RPM does this occur?




I'll let you reply 1st so we can super tune that bitch:icon_eek:

 

[airkiller]

When this thing stumbles it will recover quickly and then go like crazy. Sometimes it will back fire through the carb as well. If I gently apply gas it will run a little rough for a second and then be fine. I have had it at a performance tunig shop and had it dyno tested and he tried to get it better but it still is not very good. I have been told these carbs are junk and should get rid of it but I can not beleive a manufacture would sell sometigh that did not work, I am sure it is operator/builder error.

 

[airkiller]

The car seems to be running way rich at idle also.

 

[HEMI-ITIS]

When you had it on the dyno,how was the AF ratio??If it was good,my opinion ,go with a 50cc accelerator pump:blob1:

 

[airkiller]

DO you have any idea what the part number or were to get this part.

 

[HEMI-ITIS]

Start with your local parts store and work your way to the speed shop.

 

[HitIt]

Summit sells a tuning kit for these carbs. My edelbrock 750 had the same problems you describe. I ended up going to the largest pump nozzle, firmer rod springs and one step richer rod. Runs good now and the hesitation is gone.

The edelbrock tuning manual is great and if you take the time to read and understand how the carb works, then try one tuning attempt at a time you can tell what change is getting you in the right direction. The tuning manual can be read and downloaded from the edelbrock site.

 

[67 B-body]

Summit sells a tuning kit for these carbs. My edelbrock 750 had the same problems you describe. I ended up going to the largest pump nozzle, firmer rod springs and one step richer rod. Runs good now and the hesitation is gone.

The edelbrock tuning manual is great and if you take the time to read and understand how the carb works, then try one tuning attempt at a time you can tell what change is getting you in the right direction. The tuning manual can be read and downloaded from the edelbrock site.

I believe he has a Holley.

 

[HitIt]

I believe he has a Holley.

Duh. I likez to be reading??



ops:

 

[Meep-Meep]

Why do you have an 800 CFM carb on a 383? If you have a stock or even a mild perf build with a 6000 RPM redline you still don't need 800 CFM. Try borrowing a 600 or 650 from a friend and see how that works. I'm running a 750 AFB on a mild 440 and it's great.

 

[airkiller]

I ordered a 700 CFM carb from Summit and they sent me a 800. I have a pretty agressive cam from Comp Cams and the heads are a set that were ported by a racer here locally. The speed shop sold them to me as the heads I got with the engine (In a box) were not very good. I can not recall the casting number at this moment, maybe 904 912? But they are the small chamber heads. The block was zero decked and the compression is @ 9.65:1. The engine runs like a scalded dog when on step, but just has a little problem getting there.

 

[HEMI-ITIS]

You could check what position the pump cam is in to see if you can adjust the squirt.Sounds to me because it's not a double pumper,that animal is just getting a sip when it wants a gulp.So if the stumble gets better with the adjustment,but needs alittle more,put the bigger pumper on that carb:headbang:

 

[rlsbee]

You mentioned an aggressive cam. Have you tried putting in more initial timing? Aggressive cams can want a fair amount of initial timing. The cam in my car is not terribly aggresive but it has a lot of overlap. It stumbled as you described until I bumped up the initial timing. If you know that your total timing is in the high 30's, be careful that you don't go out and gas it hard after increasing the initial. If it responds to the initial timing you'll have to limit the total somehow. This can be done by welding the slots in the original distributor (best done on a distributor machine) or getting a distributor that has the capabilities to limit total with bushings of some sort. I know you said an MSD ignition but do you have a box and MSD distributor, or just a box fired by the old ditributor?
The suggestion about checking the air/fuel ratio is also an excellent idea.
Also, what kind of vacuum reading do you get?

 

[HEMI-ITIS]

So what was the air/fuel ratio when it was on the dyno??At what RPM does it happen?What ignition are you using?Have you played with the timing?


No one ever said this **** is easy:glasses8:

 

[airkiller]

I have to buy a dial type timimg light to get an exact number. I remember that the speed shop that "tuned" it said that timeinwas on the order of 38 degrees or something like that. I am not up on the finer aspects of tunig a non stock engine so pe patient with me.

 

[airkiller]

I found the packet from the performance shop that built the engine. Here are the particulars:

1) Holley 0-80870 Carburetor, Street Avenger, 870 cfm, Square Bore, 4-Barrel, Electric Choke, Dual Inlet, Silver

2) Eldelbrock Performer 383 INtake Manifold

3) Comp Cams 23-224-4 Cam Shaft (Camshaft, 3-Bolt, Hydraulic Flat Tappet, Advertised Duration 274/296, Lift .488/.491, Mopar, Big Block)

4) Ported and Polished 440 85cc Heads

5) Compression Ratio is 9.59:1

 

[tirefryin-Bob]

If it were me I'd just switch to a Carter / Edelbrock 750... I've never had much luck with holley's myself, Isn't once you backfire a holley you blow the power valve???
Carters are just so easy to tune..... just my two cents.

 

[HEMI-ITIS]

I have to buy a dial type timimg light to get an exact number. I remember that the speed shop that "tuned" it said that timeinwas on the order of 38 degrees or something like that. I am not up on the finer aspects of tunig a non stock engine so pe patient with me.

Need to acuire a "dial back" timing light that is will work with your ignition and a large face vacume gauge.Let's check to timing and vacume at idle to determine the proper power valve reuirement:happy1: