Re: drill holes in throttle plates?
[Re: Stanton]
#3192127
11/20/23 01:19 AM
11/20/23 01:19 AM
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Joined: Sep 2003
Posts: 3,104 Morgan Hill, CA
ackpht
OP
master
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OP
master
Joined: Sep 2003
Posts: 3,104
Morgan Hill, CA
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Yes and yes. Occasionally in order to get a motor to idle you have to open the butterflys past the idle transfer slots so now the motor is running (idling) on the primary circuit instead of the idle circuit and any adjustment to the idle screws is pointless. By drilling holes in the butterflys they allow just enough extra air such that the butterflys can now be closed enough that they cover the transfer slots and allow you to tune the idle circuit. A 1/8" hole in each primary blade is a good starting point and it doesn't matter where you drill them as they're only purpose is to bleed air into the motor. If your car idles on the idle circuit there is absolutely no point to drilling the holes. OK, I follow that. I have an engine with a lopey cam and drilled (not by me) throttle plates that will not idle below 2000 rpm even with the plates fully closed. To me this says too much air is getting through. I figure I can either take some timing out or close the holes with epoxy.
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Re: drill holes in throttle plates?
[Re: ackpht]
#3192134
11/20/23 03:11 AM
11/20/23 03:11 AM
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Joined: Feb 2015
Posts: 2,076 CA
crackedback
top fuel
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top fuel
Joined: Feb 2015
Posts: 2,076
CA
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Make sure the idle timing is in a decent range for operation.
Lopey cam, likely at least 18-25* as a guess.
JMO. Get it started, time it at 36* and see if you can get the idle under control. If the blades are all the way shut and you can't get it to idle down, sounds like a leak somewhere.
Last edited by crackedback; 11/20/23 03:13 AM.
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Re: drill holes in throttle plates?
[Re: ackpht]
#3192217
11/20/23 03:26 PM
11/20/23 03:26 PM
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Joined: Jan 2003
Posts: 15,369 Omaha Ne
TJP
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 15,369
Omaha Ne
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Yes and yes. Occasionally in order to get a motor to idle you have to open the butterflys past the idle transfer slots so now the motor is running (idling) on the primary circuit instead of the idle circuit and any adjustment to the idle screws is pointless. By drilling holes in the butterflys they allow just enough extra air such that the butterflys can now be closed enough that they cover the transfer slots and allow you to tune the idle circuit. A 1/8" hole in each primary blade is a good starting point and it doesn't matter where you drill them as they're only purpose is to bleed air into the motor. If your car idles on the idle circuit there is absolutely no point to drilling the holes. OK, I follow that. I have an engine with a lopey cam and drilled (not by me) throttle plates that will not idle below 2000 rpm even with the plates fully closed. To me this says too much air is getting through. I figure I can either take some timing out or close the holes with epoxy. Question #1 What carburetor(s) are you working with? some have the adjustment on the secondaries while others do not Question #2 have you verified the timing mark (TDC) on the dampner is is the correct spot? Question #3. If so to #2, What is your initial and total timing Question #4 Are you running a vacuum advance? Question #5 If yes to #3, is is hooked up to ported or manifold vacuum? Either way disconnect and plug the port until the idle and timing are sorted out. Question #6 What size are the existing holes? Next read my post 392117 above Might also want to read 392125 AND 392126. You can back the timing off but if the holes are too large you are band aiding the root cause of the issue which has to isolated. Closing the secondaries a bit can also help but do not close them too far as their purpose is to (bleed) keep fresh fuel in the secondary bowl(s) should they not be used for an extended time. As far as timing goes, the initial and total will be determined by your combination. As this is an idle issue I would start at ~ 10-12 initial. Some may argue with the above but it is a starting point to get the idle under control. once that is accomplished Do the above and report back You may also as mentioned in 392134 have a Vacuum leak. I would verify the above first. We can then look at a possible vacuum leak
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Re: drill holes in throttle plates?
[Re: TJP]
#3192247
11/20/23 05:58 PM
11/20/23 05:58 PM
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Joined: Jan 2003
Posts: 21,511 N.E. OHIO, USA
A12
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 21,511
N.E. OHIO, USA
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You may also as mentioned in 392134 have a Vacuum leak. I would verify the above first. We can then look at a possible vacuum leak How about looking for a vacuum leak first instead of going through the ten or eleven steps first? How would you go about trying to find a vacuum leak? Spraying carburetor cleaner around the manifold and carburetor or spraying propane (not ignited of course) around the same areas? I f it were me I would first figure out a quick, safe and easy way to temporarily close those holes in the plates to determine if they are actually a problem or not? At this point it's only a guess if they are causing the problem,
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Re: drill holes in throttle plates?
[Re: TJP]
#3192254
11/20/23 06:16 PM
11/20/23 06:16 PM
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Joined: Sep 2003
Posts: 3,104 Morgan Hill, CA
ackpht
OP
master
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OP
master
Joined: Sep 2003
Posts: 3,104
Morgan Hill, CA
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Yes and yes. Occasionally in order to get a motor to idle you have to open the butterflys past the idle transfer slots so now the motor is running (idling) on the primary circuit instead of the idle circuit and any adjustment to the idle screws is pointless. By drilling holes in the butterflys they allow just enough extra air such that the butterflys can now be closed enough that they cover the transfer slots and allow you to tune the idle circuit. A 1/8" hole in each primary blade is a good starting point and it doesn't matter where you drill them as they're only purpose is to bleed air into the motor. If your car idles on the idle circuit there is absolutely no point to drilling the holes. OK, I follow that. I have an engine with a lopey cam and drilled (not by me) throttle plates that will not idle below 2000 rpm even with the plates fully closed. To me this says too much air is getting through. I figure I can either take some timing out or close the holes with epoxy. Question #1 What carburetor(s) are you working with? some have the adjustment on the secondaries while others do not Question #2 have you verified the timing mark (TDC) on the dampner is is the correct spot? Question #3. If so to #2, What is your initial and total timing Question #4 Are you running a vacuum advance? Question #5 If yes to #3, is is hooked up to ported or manifold vacuum? Either way disconnect and plug the port until the idle and timing are sorted out. Question #6 What size are the existing holes? Next read my post 392117 above Might also want to read 392125 AND 392126. You can back the timing off but if the holes are too large you are band aiding the root cause of the issue which has to isolated. Closing the secondaries a bit can also help but do not close them too far as their purpose is to (bleed) keep fresh fuel in the secondary bowl(s) should they not be used for an extended time. As far as timing goes, the initial and total will be determined by your combination. As this is an idle issue I would start at ~ 10-12 initial. Some may argue with the above but it is a starting point to get the idle under control. once that is accomplished Do the above and report back You may also as mentioned in 392134 have a Vacuum leak. I would verify the above first. We can then look at a possible vacuum leak You may need two beers. The lopey cam is in a gen 2 Hemi, stock inline manifold with mongrel AFBs, jets and rods stock values, blue metering rod springs. Rear AFB went to carb shop for addition of electric choke and they told me it was from a 413. Anyway it comes back with a choke and the drilled holes. Both carbs have throttles closed, set up in a progressive linkage. At first set up with no vacuum advance and about 8 base, car would idle at 900 rpm and 9 inches vacuum (with throttles closed), but top end wasn't great. Watched junkerup's video about using vacuum advance and a light went on. Advanced base a few degrees, hooked up vacuum advance to manifold vacuum (side port on front carb), and it went to about 1300 rpm at 23 degrees. WOT much improved (honest, officer...) What happened to 2000 rpm, I dunno. Pulled a plug (only an hour or so of run time) and it was brown. Ecch. A/F says 14.7 at idle, lean while cruising (about 2200 rpm). Don't dare take my eyes off the road at WOT to read the meter. But it's spending a lot of time at idle in the garage while I mess with the settings. Timing readings a bit erratic as timing light is having problems triggering with this car (ok with others, clip is "pointed" the right way). But right now it looks like 1300 rpm at 25 degrees advance (base 11) and 15 inches of manifold vacuum. If I switch to ported vacuum (port on front of rear AFB), the rpm and advance don't change. I think I should close up the holes on the throttle plates and tweak the mixture screws (all four) for best idle.
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Re: drill holes in throttle plates?
[Re: ackpht]
#3192267
11/20/23 07:39 PM
11/20/23 07:39 PM
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Joined: Jan 2003
Posts: 12,085 Benton, IL.
DaveRS23
Special needs idiot
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Special needs idiot
Joined: Jan 2003
Posts: 12,085
Benton, IL.
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You may well need two beers. The idle and off idle on AFBs can be a royal pain in the a$$ to tune. If you eliminate the other possible problem areas and have isolated the problem to the tunes in the carbs, the AFBs can be much more difficult to dial in than the Holley platform carbs which can actually have adjustable idles and transitions. Unlike the CarterBrocks.
Timing is CRITICAL with cams that have a good amount of overlap. And it can be hard to stabilize lots of initial with the idle bouncing around. Often 25* at idle and all in by 2,000 is necessary to get some combos happy. Then you have to worry about kick back when starting, especially hot. And if the engine likes the timing all in that early when in high gear. Lots of things to consider. And everything affects everything else.
Good luck.
P.S. I have 26* at idle and all in by 1800 in my 540 Hemi. It does hit the starter some when hot, but not enough to back it off and suffer the other consequences. It also took some tweaking of the Dominator carb to get the best idle and transition. That would have been much more difficult with CarterBrock(s).
Master, again and still
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Re: drill holes in throttle plates?
[Re: ackpht]
#3192328
11/20/23 11:48 PM
11/20/23 11:48 PM
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Joined: Jan 2003
Posts: 15,369 Omaha Ne
TJP
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 15,369
Omaha Ne
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At first set up with no vacuum advance and about 8 base, car would idle at 900 rpm and 9 inches vacuum (with throttles closed), but top end wasn't great. hooked up vacuum advance to manifold vacuum (side port on front carb), and it went to about 1300 rpm at 23 degrees. WOT much improved (honest, officer...) What happened to 2000 rpm, I dunno. Pulled a plug (only an hour or so of run time) and it was brown. Ecch. A/F says 14.7 at idle, lean while cruising (about 2200 rpm). Don't dare take my eyes off the road at WOT to read the meter. But it's spending a lot of time at idle in the garage while I mess with the settings. Timing readings a bit erratic as timing light is having problems triggering with this car (ok with others, clip is "pointed" the right way). But right now it looks like 1300 rpm at 25 degrees advance (base 11) and 15 inches of manifold vacuum. If I switch to ported vacuum (port on front of rear AFB), the rpm and advance don't change. I think I should close up the holes on the throttle plates and tweak the mixture screws (all four) for best idle. Switching to ported vacuum with no change indicates and issue with that port as there should be NO VACUUM there UNLESS the primary butterflies are too far open. Also note your manifold vacuum at WOT is likely pretty low ( meaning no advance) as using manifold vacuum typically is only beneficial for cruising ( here come the argments ) if you happen to pull the carbs, measure the hole size, and hold them up to a bright light to see if the plates are centered as best as possible in the bores. If not slightly lossen the screw and center them. An off center plate can hold the throttle open increasing the RPM at idle. You can also check the opening for the ported vacuum which should be closed off and fully above the blades
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Re: drill holes in throttle plates?
[Re: TJP]
#3192534
11/21/23 11:30 PM
11/21/23 11:30 PM
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Joined: Sep 2003
Posts: 3,104 Morgan Hill, CA
ackpht
OP
master
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OP
master
Joined: Sep 2003
Posts: 3,104
Morgan Hill, CA
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I can't really tell what the vacuum is doing under load since all these measurements take place in my garage. With no load the vacuum will drop when the throttle is first opened, then quickly increase again as the engine speeds up and sucks more air through the carburetor. I have to imagine that under partial and light loads on the road the timing contribution from vacuum is significant. I road test the car to see if the total timing is too much, and so far it isn't. The similarity of the vacuum level at the manifold and ported ports I chalk up to the high idle speed. I'm sticking with manifold vacuum for now. The additional advance at idle makes it idle nicer, just faster than I'd like, so I want to stick with vacuum advance. I plugged the holes in the throttle plates and got the idle down a little. Idle mixture screws still effective so I'm not sure the holes were ever needed. Carb shop only had the rear carb. I told them it was a non-original carb from a dual-quad Hemi. I don't know why they drilled the plates, I hadn't asked for it, just the choke. They said the carb was from a 413- maybe the other one is, too. The Hemi manifold is stock AFAIK. The car ('70 Charger auto) is OK on the open road but difficult in around-town traffic because it stalls unless you brake-torque it. It's clearly a work in progress. I suppose the real fix for stalling is either a different cam or a different torque converter. Certainly off-idle and light loads are where the car is not well behaved- power does not come on gradually. If my tuning difficulties stem from having the wrong type of carbs instead of my ineptitude, I'll buy into that explanation. I'd need to know what to replace them with. I am getting REALLY good at taking this air cleaner off and putting it back on.
Last edited by ackpht; 11/22/23 02:07 AM.
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Re: drill holes in throttle plates?
[Re: ackpht]
#3192535
11/21/23 11:55 PM
11/21/23 11:55 PM
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Joined: Jan 2003
Posts: 15,369 Omaha Ne
TJP
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 15,369
Omaha Ne
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I can't really tell what the vacuum is doing under load since all these measurements take place in my garage. With no load the vacuum will drop when the throttle is first opened, then quickly increase again as the engine speeds up and sucks more air through the carburetor. I have to imagine that under partial and light loads on the road the timing contribution from vacuum is significant. I road test the car to see if the total timing is too much, and so far it isn't. The similarity of the vacuum level at the manifold and ported ports I chalk up to the high idle speed. I'm sticking with manifold vacuum for now. The additional advance at idle makes it idle nicer, just faster than I'd like, so I want to stick with vacuum advance. I plugged the holes in the throttle plates and got the idle down a little. Idle mixture screws still effective so I'm not sure the holes were ever needed. Carb shop only had the rear carb. I told them it was a non-original carb from a dual-quad Hemi. I don't know why they drilled the plates, I hadn't asked for it, just the choke. They said the carb was from a 413- maybe the other one is, too. The Hemi manifold is stock AFAIK. The car ('70 Charger auto) is OK on the open road but difficult in around-town traffic because it stalls unless you brake-torque it. It's clearly a work in progress. I suppose the real fix for stalling is either a different cam or a different torque converter. Certainly off-idle and light loads are where the car is not well behaved- power does not come on gradually. If my tuning difficulties stem from having the wrong type of carbs instead of my ineptitude, I'll buy into that explanation. I'd need to know what to replace them with. Run the vacuum guage inside the car or duct taped it to the windshield The Vacuum is doing excatly what i tis supposed to
Your comment on the two ports is likely correct. With the holes plugged and the butterflies lowered you should no longer be seeing any vacuum at the ported openingTry bumping it into neutral when stopping and then back into gear when taking off (Did it for years for the same reason IE: converter to tight for the camshaftusing manifold vacuum for the advance will aggravate the low speed drivability as the advance drops out when accelerating and does not return until cruise. The ported vacuum should climb and add more advance until the throttle is opened beyond a given point. by that time your mechanical should be taking over. Hope this makes senseSad part is I used to live just up the road from you off Bernal and 101
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