va454ss

iTrader: (1)

When first opening the throttle on my truck, it is really hard.

Once vacuum is broken on the throttle blades, all is well.

With engine off, throttle works smooth and effortlessly.

My engine pulls almost 19 inches of vac at idle. Plus, I have the 2.2 inch bore TB.

Is my problem because of such a large area has vac pulled on it?

I had the shaft holes enlarged and bushings installed. This made the action smoother, but still have the abrupt tip in.

To better define it, it is physically hard to tip in the throttle from idle.

Would drilling a hole in each throttle blade help, or cause other problems. If drilling a hole is the answer, should it be near the throttle shaft or near the edge?

JokerRS

I dont know if this helps but I had a simular situation. After I bored out my tbi and installed home made blades, the blades would bind slightly in some locations because they didn't have perfect clearence all the way around. This only happens when my trottle is closed all the way. I set my throttle open slightly farther with the idle screw. I cant think that its your vacume holding it shut, but I could be wrong. You are still using you IAC arent you? There should be enough air going through theIAC to relieve the trottle blades. If the blades are sticking in the bores they will leave a mark on the bore. Maybe the thing to do is shave alittle off the blades where they are hitting.

va454ss

iTrader: (1)

This problem didn't rear its ugly head until I switched intake manifolds. When I was using the Edel TBI manifold, I had no problems. It started when I switched to the RPM Air Gap, and continues with my single plane.

There are no witness marks to indicate any binding. I also tried setting the blades open further, but to no avail.

JokerRS

Like I said, I didn't know if it would help. What made me wounder was that the tbi had been modded. But like the man said, there are no witness marks so you can definantly rule it out.
Good luck and let us know what you find.

va454ss

iTrader: (1)

I read a post a long time ago when someone had rec'd drilling a hole in the throttle blades, but couldn't remember what the problem was.

I know its hard to believe that vac is having that effect, but the difference between when the engine is running and not is phenomenal.

JokerRS

I havn't had the joy of owning a BBC yet. I hope if I do it will be FI.

Grumpy

Drilling holes in the butterflies, is an old Carb trick to reorient the butterflies to the transfer slot.

On TBIs, it allows you to further close the butterflies. If you look into the TBI with the engine idling you'll notice the cone of fuel from the injector sprays at the outter perimeter of the butterflies, and this causes the fuel to be sheared, and well atomized. So you typically want the min blade opening possible.

If you have everything as right, as you can, you might try releasing one of the throttle return springs. I've had to do that on several healthy TBI'd cars.

While alot more work going to a decreasing radius throttle set up helps and usually adds some to the overall drivibility.

FWIW, 30% of throttle opening is about 60% of the airflow potential. There's a drastic air flow change just cracking the butterflies open.

va454ss

iTrader: (1)

Thanks.

I'll try releasing one of the return springs first.

What makes it so strange is I never had this problem when using the TBI intake, but now I have had it ever since I started using carb intakes.

If I did drill holes in the blades, is there a preferred size to start with and location?

dimented24x7

iTrader: (2)

As you said earlier its the vacuum. Since you pull around 19 inches of vacuum thats about a 10 psi pressure difference going from the outside to the manifold. Given the size of the butterflies that correlates to around 75 lbs of force on the butterflies, which results in alot of friction from the contact surfaces along the throttle shaft. Given that its easy to see why its so hard to move the throttle untill the vacuum is broken. This is also worsened by the force from the cable and return springs. The net effect over time is to cause the casting around the throttle shaft to wear off center in the direction of the net force, which isnt pointing straight down. The bad part of this is that the butterflies will no longer be centered and, in effect, they can lock solid in the bores and prevent the throttle from opening. Doubt you have that yet but I did with my stock one. The gas pedal felt as solid as a brick wall. My present holley has a similar problem. Turns fine untill the car is started. After that, it might as well be encased in concrete...

Edit: There is one cure, lots of overlap at the cam. There, problem solved

va454ss

iTrader: (1)

Actually, I thought about that, but its not an option (yet )

May just be justification enough for that solid roller I've been dreaming about


At least I found someone that agrees with my diagnosis.

JokerRS

are you still using the IAC? I was thinking you could unplug it in the park possition and crack the throttle blades some more to set up the idle. Didn't mean to hurt your feelings about your diagnosis. I just hadnt heard of that happening before.

va454ss

iTrader: (1)

No offense taken

Yes, I'm using the IAC. I've tried resetting the idle several times. Even tried setting to a higher idle speed, but to no avail.

dimented24x7

iTrader: (2)

Setting the throttle to be open more helps when it starts to wear out. Prevents the throttle blades from binding. What would be cool is if it was somehow possible to machine the tbi for greaseless roller bearings. That would be cool having the throttle open effortlessly even when there is alot of vacuum.

Ronny

iTrader: (1)

i am installing the 1995 7.4L GM unit on my car maybe next week(?) now what did GM do to help this issue 454/2.00 INCH tb/ MILD CAM+ LOTS VACUUM. MAYBE THE THROTTLE CABLE mechanical advantage mechanism is different? i have a similar issue that until today never thought much of. TY ! seems to want to open a bit too quickly like really hard to do a mild granny accelleration from stop light. easy to get a chirp out of tires . i thought it was my inability to drive a stick/clutch. clutch new as is hydralics. now that holley 900 looks even better now with sequential openings!!!

va454ss

iTrader: (1)

Well, when the engine and TB were stock, there wasn't an issue. And the stock cam pulled more vac than the one I have now.

As I stated earlier, I didn't experience the problem until I began using carb manifolds.

Immediately, I thought it was an issue with the throttle cable. Replaced the cable to no avail. Sent the TB back to the gent that modified it and he drilled out the shaft bearings and sleeved them. He didn't find any evidence of binding.

Is intake plenum volume the culprit?

I'm game to drill a hole in each throttle blade, but don't want to create another issue, while trying to correct another.

I'm thinking the location should be near the throttle blade; may have less of an impact than being nearer to the fuel cone the closer to the edge you get.

dimented24x7

iTrader: (2)

Could be that a change in geometry of the liinkages made the effect noticable. No matter what the vacuum will cause the shaft to resist turning since the force causes a good deal of frictional resistance to develop. Not much to be done about that, but if the cabling is at a different angle it can increase the force needed to open the throttle.

dimented24x7

iTrader: (2)

A very small hole in each near the downward side is a good idea if the tbi doesnt have them in the casting. The stock ones had small orafices just above the throttle blades that allow a small ammount of fuel and air to bypass the throttle blades when they where fully closed..

va454ss

iTrader: (1)

I considered the angle of the cable also. Originally, I had a cobbled together cable bracket. Made my own from alum with loads of adjustability. Actually, as the cable is now, it is in less of a bind than the original configuration.

va454ss

iTrader: (1)

Not sure what you mean by the downward side. I have a couple of spare TB's on the shelf, so I'll look tomorrow. Never noticed any holes in the blades

dimented24x7

iTrader: (2)

Actually, I was mistaken about its location. By 'downside' I mean that, when the blades are closed, theyre at an angle with one side higher then the other. If you look at the point where the butterflies are the 'highest' on a smaller stock tbi, there are two small holes to let a minimum of air in when theyre completly closed. Like said above it allows for the butterflies to be as nearly closed as possible. MAy or may not be necessary depending on how much space there is between the edges of the blades and the bores of the tbi.

va454ss

iTrader: (1)

Well, all that got me was the throttle sticking at 2K and not returning to idle.

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