Could a larger than normal hole in the throttle plate cause low vacuum at idle and rich idle? I was thinking about ordering a set of non-drilled plates if this would help. This carb has been a living hell trying to tune for idle. The holes that are in it now look to be 3/16" or 1/4".

Thanks,
Chas

 

Anyone?

 

Better to start with a new carb than trying to unbutcher that one.

 

I can buy a set of 4 throttle blades for $12 I figure that would be a good place to start. The guy i bought it from had it on a mild/street 502 big block. I will check the air bleeds and such. That and carbs are not cheap in my neck of the woods.

 

I don't remember the details of your build...but I would get rid of the plates with the holes. I tried that on a carb once and it was a worthless modification that only caused problems.

What kind of idle vacuum? What idle speed? What is timing at idle with the vacuum advance plugged in? Are you adjust the screws on BOTH sides (primary and secondary) to achieve this idle speed?

 

I always think it's cute when people say that car parts are expensive somewhere in the U.S.

Are you positive that the holes are the only modification that have been made?

 

the reason why i say they are expensive is all of the local dirt race guys scoop up every 700+cfm carb as soon as they come available. I don't like to buy things like carbs without looking at them in person. I just wish I knew more about the carb I bought and they guy I bought it from would return my phone calls so I could figure out how he modified it.

No I believe the air bleeds might have been modified. I have been considering buying a new main body for the carb (HP series) to get rid of my choke horn and improve air flow. I was going to have my choke horn machined off but they wanted $90 and for $30 more I can buy a whole new main body from proform or holley. This is just a thought. I believe they also modified the curb idle circuit.

I have been racking my brain trying to get some vacuum out of this carb and I can't for some reason get much at idle. When cruising down the road I pull 10-15 inches.

 

You are pulling 10-15 at cruising speed...but how much at idle? Mine idles at 12" (1000RPM), and at cruise it is closer to 20". That makes me think your idle vacuum is REAL low!

 

I pull 4-5" at idle (1000 rpm) at WOT its around 10". I don't have any vacuum leaks but driving this car on the road is a pain with next to no vacuum for the power brakes. I downshift and that adds a lot of vacuum so I can use my brakes.

I am going to bring my initial timing up to 18* and see if that helps. The duration on this cam isn't helping the vacuum (240/246 at .050")

 

4-5" at idle is real low. Your cam has a good bit of duration...is it a solid or hydraulic? Roller or flat tappet? My cam is 242@.050...it is a hydraulic roller. It still manages 12" at idle. A hydraulic flat tappet with the duration you listed would be a TOTALLY different story!

Are you positive there are no vacuum leakes? If you are, you may need a different carb (with annular boosters) to increase the signal.

 

I am running a flat tappet hydraulic. I am going to plug the pcv/brake booster vacuum line and see if that helps.

My idle mixture screws are out about 4 complete turns on each side and if I try to lean them out the car won't stay running unless I run a 1500 rpm idle. This opens my transition slots too much and I start using the primary circuit and no longer use the idle circuit.

This car has me puzzled. This motor has maybe 2000 miles on it and all of the miles it hasn't gone past 4500rpms (car is too fast for the street.)

 

That is a lot of duration for a flat tappet hydraulic motor. What compression are you running? Have you tried advancing the cam at all? If it were me, I would try to borrow a 750-ish carb with annular boosters to see if that helps. Or, I would contact a carb expert and see what they recommend. I have only once tried to tune a carb on an engine with that low of a vacuum signal (for street use). It wasn't pretty and I gave up!!

Here are a list of things that should help the vacuum signal:
1) Dual plane intake
2)4-hole tapered spacer
3)Advancing the cam
4)More compression

 

Haha I have 3/4, Dual plane intake, 1" 4 hole spacer, cam is installed 1* retarded, and I am running 11.2:1 compression.

 

Have you ever done a cranking compression test? I wouldn't be suprised if it shoes 160psi. If that were the case, I would be looking to advancce the cam. If you can get the cranking compression up, it should help with your idle vacuum and low RPM response of the engine. Have you ever run it down the track?

 

I really couldn't stop laughing when i saw this! I guess my car is too fast for the planet then..LOL

If the holes in the butterflies are that big then i can see why you cant build vaccuum at idle, there's way too much just freeflowing through there. Please dont say all 4 are drilled this way. At this point you probably should try that $12 fix first and change out the butterflies.

 

Haha I am glad someone got a laugh out of it, gotta lighten up the situation a little. I don't think they are all drilled but it wouldn't surprise me. I will take a look tonight.


What should my cranking compression be around? The higher the compression the more cam advance I need?

I did a compression check shortly after the rebuild and it was around 190psi. I will double check tonight.

 

Compression and camshaft design both have a bearing on cranking compression. 190psi would be good. Try finding someone with a useable carb (holley in the 650 to 750 range) that you can try. This test could fix your problems.

 

My compression test is as follows:
8-184 7-180
6-180 5-185
4-185 3-179
2-181 1-180

My plugs were black with some coking on them. I hit them with a wire wheel and cleaned them up and re-installed them. The bumper of my car seems to have a little black on it (not sure if its from burning oil or running rich.)

When I built the motor I clearanced everything on the tighter side. My ring end gap was set up for nitrous so it was a little wider (.024 IIRC) than a the street gap. But I shouldn't be burning any oil.

 

Did you keep the throttle wide open when doing the tests??

If you did, those readings aren't bad. I would still be tempted to play with advancing the cam. First thing though, do you know anyone you can borrow an un-modified carb from? Or, can you get new throttle blades for yours? I may have a spare 750 baseplate laying around in my basement. I know I have a proform center section and some billet meetering blocks. I would want to rule out your current carb before messing with the camshaft...or anything else.

 

Ya they were with blades open. I can pick up some throttle blades for for pretty cheap at a local store. I could see if my buddy's dad would let me borrow his carb 750DP but it has been on his car for 20 years and I doubt he wants to take it off anytime soon, but its worth a try. I'm not entirely sure if its all stock tho.

 

Bit of an update. I was messing around with my idle mixture screws and was able to get the car to idle at 5" of vacuum at 850 rpm. When the car is in gear the idle drops down to 3"-4" and 750 rpm idle I was thinking if I boost my in gear idle up a little bit it would allow me to pull a little more vacuum. The car seems to hold the low idle fine and when I hit the gas she comes right up the rpms no problem. Soon I will be buying a auxillary vacuum pump and won't worry at all about this.