Greetings,

I am having, what seems to be an endless amount of problems with my engine. In my previous posts I thought it to be a carb problem. Now I am not so sure. My engine seems to be bogging down on take off at WOT. I don't know where to begin in ironing out this problem. Let me start by listing some of the specs of my engine....

Its a SBC 350 that is slighly modified:

144 B&M Powercharger (blower) (NOW WITH THE ORIGINAL SIZED PULLEY)
750 CFM Holley (4779) Blower Carb with 40/40 SHOOTERS & 8.5 Power Valve
18in of VAC at 800RPM IN DRIVE
700R4 Transmission
2800 Stall converter
3.70 Rear
Hooker Header side pipes
HEI Distiributor w/ MSD 6A Ignition and GM Distributor Advance Kit
Splitfire Double Platnium Spark Plugs

The bottom end of the engine and the heads are bone stock

I originally thought that my engine was not getting enough gas on take off so I switched from a 650 CFM Edelbrock Thunder Series Carb, to a 750 CFM Holley (4779) Blower Carb.

Now, the last time we left off, I was told by a Holley Tech at the Holley hotline to change the power valve to an 8.5 because of my 18in of VAC at 800RPM while in drive. Didn't seem to make any difference. I also check the timing. At 600RPM its at 16*BTC and at 4000RPM the DIS stops at 35*. Then I moved from shooter sizes 28,31 - 31,35 - 35,40.... which also made no difference. Right now we are at 40 primary and 40 secondary, which did make a noticable power addition when pushing the pedal to almost full throttle starting at about 50MPH.... But the engine overall sounds like **** and has no power from takeoff. It still boggs severely from a takeoff @ WOT or at while already moving to WOT. And I think its DETONATING! I don't know what to do????? I am running out of ideas. I decided to go back to basics and I am going to buy a new set of plugs tomorrow. Maybe I foulled them or something. Other then that I don't know what to do. Do they even make bigger shooters, and if so do I need them,......... or should I buy a cam kit and start changing plastic cams...also, a while back I bought an advance kit for my distributor and changed the spring and distributor curve and saw some benefits at that time. The advance kit is still in. What do I do next,.....

Thanks in advance for the help guys,

Barry (aka "becoming suicidal)

Check your primary accelerator pump linkage if there is any slop check it. As soon as you move throttle the arm should move. Set it by pushing it all the way down and check for .15 clearnce after full travel. I have tuned many cars with hesation and this is one of the most common. Then check you fuel level in both bowls. Gas should not come out while engine idles. Check your fuel pressure and should be about 6 to 7 pounds,. MOST important get those splitfire plugs out and use regular plugs. One guy bought his car over for tuneing he insted on using those split fire plugs we put them in and the car ran ruff. I removed them and put in a set of spare AC Delco R44TS plugs and car ran great and he was smileing from ear to ear. Hope this helps you out.

post a pick of your spark plugs,sounds lean.

You probably have serveral issues that need to sort out. The first and most critical is detonation. Are you using race fuel? If not step up to race fuel at least for the track. Make a few runs with that done before stating to try and track diown the next part. It is best to only change one thing at a time to ensure you are going the right way.

what bbcvelle said: get rid of those plugs first. just buy some plain old ordinary spark plugs and put them in.

what bbcvelle said: get rid of those plugs first. just buy some plain old ordinary spark plugs and put them in.

I posted this question in the Performance section but didn't get a response. I want to go with the original AC Delco plugs, but because of the blower "I think" I need to go with a cooler plug to reduce detonation???

Is this true,... and if so does anyone have a part # or something I can tell the guy behind the counter at autozone. The engine is a GM Performance Crate Engine so I have no idea what year it is.

Thanks,

Barry

What compression ratio is the motor? You need to resolve the detonation before you mess something up. I agree with Redrum, if you plopped a little blower on top of a 10:1 motor, you'll need race gas when the motor is put under a load. Was this a running motor you added the blower to, or was it purpose built? What cam is in it? I'm no expert, but I'm not sure a "motor" cam is a great choice for a blown application. I think blower cams have a wider lobe separation.

Assuming all is matched, I'd think R43's ought to be fine for now. Pull a plug that's in it and check against the 43's. I'd bet it's the same. Check your accel pump adj as well as mentioned above, although if it's too lean it should be popping.

Once the detonation is resolved with the proper fuel, you might toy with your timing a couple of degrees at a time. Personally, I'd start by retarding back to 34 at 3000 and reduce the distributor's mechanical advance as much as possible. With an HEI, I doubt you can reduce it much, but I'd reduce the mechanical advance as much as possible (ie. more initial with the same total). If that helps, retard another couple degrees unit it quits improving. I can imagine it wanting less than 32 total. If it makes it slower, try going the other way (carefully).

Eventually you'll want to ditch the HEI and go with a MSD unit.

I would reconsider the compression ratio. Call holley and find out what pulley is recommended for your application. When I had a 144 B+M blower it was on a 355 with d-dish pistons. The motor was 8-1 compression, and with the pulley I had was supposed to be equivalent to 10.5-1. It might be detonating right off idle.

What makes your 750 a "blower carb"?? Unless it is boost referenced, the power valve will never open up and it will be lean all the time....

What makes your 750 a "blower carb"?? Unless it is boost referenced, the power valve will never open up and it will be lean all the time....


HELP.... Thats exactly what I think I mean... can you give me a little more info please?

Barry

You can always plug the power valve port and jet the carb up to see if that fixes the problem! Add 8 sizes to the primary jetting to compensate for no PV and see if that fixes it!

:thumbsup: Not sure if this will be too rich at idle. It is definately worth a try.

I think I may have found the problem..... unfortunately it may be a little bigger then I had hoped. Hooked up a vacuum gauge today, and found that the needle was rapidly moving back and forth,... appears to be a valve spring problem....

I posted exactly what happened under........ "This is just not good!" Check it out guys,...

Barry

The jetting is in the intermediate circuit of the carb. It will have no impact on idle mixture of a properly operating carb.

Doing what I suggested is not a great solution for a street driven car. It will be too rich under part throttle. It's fine for racing and WOT. That would have been a way to bypass the PV as a check only.

Regarding the valve springs, that could certainly be the case. I'm pretty sure you will get an erratic vacuum signal with a blower though.

I'm pretty sure you will get an erratic vacuum signal with a blower though.

I really hope your kidding..... I just pulled apart the heads!!! Im just lost in general,... at this point, I think that this could possibly be the problem. No power no matter what I change???? Seems to me there is something else wrong other then a problem with my air fuel mixture.

Barry

I may be wrong, but not kidding about what I recalled. It certainly won't hurt to check your spings anyway. If it is your valve springs, I wouldn't expect it to be better at 50 mph upward and you won't be able to rev it squat.

Did the motor run ok before installing the blower? If yes, I'd focus on tuning for the blower. If you don't know and the heads check out ok, I'd focus on (in order) 1)ignition: is it wired correctly?, is it a new HEI install and your using the old resistance wire?, firing order correct?, disconnect the vac adv if connected and make sure the timing is what you want mechanically. I've seen distributors actually retard the timing when revving it up. Winging it in park puts no load on the motor and you can't see what happens when you're driving under load. Does it miss or just make no power? 2) carb: the post above about the PV being boost referenced is correct. If you don't know, buy one of those PV plugs and jet the carb square-ish and see how it does. 75s or so should be in the ballpark. Regardless of whether it's got razor sharp throttle response or not, it should pull good from a 20 mph 2500 rpm slow roll in first gear. Once you know it's running ok, then you can focus on the accel pump and optimize timing, etc.

I know it seems rudimentary but when all logic is out the window sometimes you need to go back and carefully check the basics. It's amazing how much stupid stuff we do in our haste to get her fired up with our newly acquired speed piece!

I may be wrong, but not kidding about what I recalled. It certainly won't hurt to check your spings anyway. If it is your valve springs, I wouldn't expect it to be better at 50 mph upward and you won't be able to rev it squat.


This is very accurate. Thanks alot for the help. Right now I am going to take a look at the valves springs and I will get back to you. Thanks again for all the help with this problem.


Barry