Rembrant Posted October 27, 2019 Author Share Posted October 27, 2019 You should be able to kill the engine by closing the throttle plates... Gary, I took the truck to go see my folks today so I didn't get any troubleshooting done per se...BUT...on two occasions today, I was cruising along one time at maybe 45 mph and another time at 70 mph, and when I pushed the clutch in while slowing down the engine would not idle. I had to feather the throttle a bit to keep it running, and then a minute later it idled just fine again. Is that a sign of anything related to my idle issue? Only change made since yesterday was that I dialed the choke back so that it worked again. Also, I did let the truck warm up using the choke, and still nothing showing up on my AFR gauge. Not a proper test really, as it was a lower RPM choke setting, so I doubt the choke plate was closed all the way. I'll get some more tests done this week. Thanks guys. Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 27, 2019 Share Posted October 27, 2019 You should be able to kill the engine by closing the throttle plates... Gary, I took the truck to go see my folks today so I didn't get any troubleshooting done per se...BUT...on two occasions today, I was cruising along one time at maybe 45 mph and another time at 70 mph, and when I pushed the clutch in while slowing down the engine would not idle. I had to feather the throttle a bit to keep it running, and then a minute later it idled just fine again. Is that a sign of anything related to my idle issue? Only change made since yesterday was that I dialed the choke back so that it worked again. Also, I did let the truck warm up using the choke, and still nothing showing up on my AFR gauge. Not a proper test really, as it was a lower RPM choke setting, so I doubt the choke plate was closed all the way. I'll get some more tests done this week. Thanks guys. Did you have the brake on when the engine wouldn't idle? I'm wondering if the vacuum booster has a split in it. Sometimes it seals and sometimes it doesn't? Link to comment Share on other sites More sharing options...
Rembrant Posted October 28, 2019 Author Share Posted October 28, 2019 Did you have the brake on when the engine wouldn't idle? I'm wondering if the vacuum booster has a split in it. Sometimes it seals and sometimes it doesn't? That's possible Gary. I just tried the booster using a Mitivac and it holds 20" vacuum no problem, and seems to hold when you push the brake pedal, but something weird could be happening when it gets warmed up. I kind of dismissed the booster because I noticed last week that it was holding vacuum OK, but maybe tomorrow I'll actually plug the line to the carb as a more definitive test. Thanks for the idea. I'll be doing some more vacuum leak checks this week. It's weird...the AFR gauge seems to work for about 15-20 seconds on start-up, then the numbers rapidly ramp up and the gauge flatlines. I haven't ruled out a bad gauge either though, so that is possible as well. Link to comment Share on other sites More sharing options...
Rembrant Posted October 29, 2019 Author Share Posted October 29, 2019 Did you have the brake on when the engine wouldn't idle? I'm wondering if the vacuum booster has a split in it. Sometimes it seals and sometimes it doesn't? Ok, so I'm back with some news and test results... After doing some of the suggested tests of covering the carb up and restricting air flow, I brought the truck close to stalling several times and I actually did stall it once. AFR gauge didn't budge. Upon start up, the gauge actually reads pretty good...13.5 AFR, sometimes 14:1, etc, and then it rapidly runs up and pegs full lean and stays there. Meanwhile you can smell the stink of fuel... Anyway, fully convinced at this point it's the gauge, I called AEM tech support and they were super helpful. When I explained what the gauge was doing, he immediately asked for the serial number on the back. Once I gave him the number, he confirmed that my gauge is one of a batch of known faulty gauges...in which the issue is exactly what mine is doing, pegging full lean. They're going to send me a new kit asap. As for the vacuum leak diagnosis, still nothing yet. I tried with my cylinder of MAP gas going all around the carb, the base of the carb, spacer, back and forth left and right the full length of the intake manifold to cylinder head seam, and nothing, anywhere. So I'll have to keep hunting. If by chance it's sucking from the lifter valley, I guess there's no way to know without pulling the intake? I've restricted the flow into the carb, and the truck stalls. One observation...and one of you carb guys can tell me if this is OK or not, but with the truck idling, if I cover ONLY the secondaries and close them off, the engine will stall. I thought the thing would breath enough through the primaries...but apparently not? Link to comment Share on other sites More sharing options...
85lebaront2 Posted October 29, 2019 Share Posted October 29, 2019 Ok, so I'm back with some news and test results... After doing some of the suggested tests of covering the carb up and restricting air flow, I brought the truck close to stalling several times and I actually did stall it once. AFR gauge didn't budge. Upon start up, the gauge actually reads pretty good...13.5 AFR, sometimes 14:1, etc, and then it rapidly runs up and pegs full lean and stays there. Meanwhile you can smell the stink of fuel... Anyway, fully convinced at this point it's the gauge, I called AEM tech support and they were super helpful. When I explained what the gauge was doing, he immediately asked for the serial number on the back. Once I gave him the number, he confirmed that my gauge is one of a batch of known faulty gauges...in which the issue is exactly what mine is doing, pegging full lean. They're going to send me a new kit asap. As for the vacuum leak diagnosis, still nothing yet. I tried with my cylinder of MAP gas going all around the carb, the base of the carb, spacer, back and forth left and right the full length of the intake manifold to cylinder head seam, and nothing, anywhere. So I'll have to keep hunting. If by chance it's sucking from the lifter valley, I guess there's no way to know without pulling the intake? I've restricted the flow into the carb, and the truck stalls. One observation...and one of you carb guys can tell me if this is OK or not, but with the truck idling, if I cover ONLY the secondaries and close them off, the engine will stall. I thought the thing would breath enough through the primaries...but apparently not? If it is pulling from the bottom of the intake ports, you will have a lot of vacuum on the crankcase pull the PCV valve out and plug that opening in the valve cover, on the other side, remove the breather cap and try covering the hole with the engine running, normal result will be a gradual pressure buildup, if you have a vacuum under these conditions the intake gasket is leaking around the ports. Link to comment Share on other sites More sharing options...
Rembrant Posted November 2, 2019 Author Share Posted November 2, 2019 If it is pulling from the bottom of the intake ports, you will have a lot of vacuum on the crankcase pull the PCV valve out and plug that opening in the valve cover, on the other side, remove the breather cap and try covering the hole with the engine running, normal result will be a gradual pressure buildup, if you have a vacuum under these conditions the intake gasket is leaking around the ports. Ok, so I finally got back at this today (briefly). I plugged the PCV valve hole, and then covered the oil filler hole with my hand (with a thin leather glove on) and there was indeed a gradual pressure build up. No vacuum of any kind. According to my vacuum gauge, which is by no means a precision instrument, I'm pulling 18" vacuum at idle. That is about the best that I've gotten with it. I seem to recall this being the best vacuum we were able to achieve on the dyno day as well, but my memory could be fuzzy on the topic. The idle speed screw is backed out all the way, and the idle is currently at about 840/850 RPM. I can't find a vacuum leak anywhere. I plugged everything. I've gone around and around with MAP gas spraying on the carb and intake, etc. I plugged the brake booster vacuum line, and it was fine...nothing changed. The only thing that pulls the idle down a little bit is plugging the PCV valve...which I assume is normal? Isn't the PCV valve a controlled vacuum leak of sorts? So...I guess this means I'm back to suspecting the carb again...and the primary or secondary plates not closing properly. With the idle speed screw backed out all the way and with maybe a 1mm gap (0.040"), I can actually close the throttle a little more manually...and the idle does drop a little bit...but I don't want to force it any harder than I have been. I don't know what stops the primary throttle plates travel when they're not hitting the idle speed screw. I seem to recall there being an adjustable stop in there, only accessible from the under side of the carb...one of those things that is "factory set"... Anyway...my new O2 sensor kit shows up on Tuesday and will take all of 5 minutes to install, so I'll finally be able to see what's going on with this thing...at least in how it's breathing anyway. Stay tuned. Link to comment Share on other sites More sharing options...
Gary Lewis Posted November 2, 2019 Share Posted November 2, 2019 Ok, so I finally got back at this today (briefly). I plugged the PCV valve hole, and then covered the oil filler hole with my hand (with a thin leather glove on) and there was indeed a gradual pressure build up. No vacuum of any kind. According to my vacuum gauge, which is by no means a precision instrument, I'm pulling 18" vacuum at idle. That is about the best that I've gotten with it. I seem to recall this being the best vacuum we were able to achieve on the dyno day as well, but my memory could be fuzzy on the topic. The idle speed screw is backed out all the way, and the idle is currently at about 840/850 RPM. I can't find a vacuum leak anywhere. I plugged everything. I've gone around and around with MAP gas spraying on the carb and intake, etc. I plugged the brake booster vacuum line, and it was fine...nothing changed. The only thing that pulls the idle down a little bit is plugging the PCV valve...which I assume is normal? Isn't the PCV valve a controlled vacuum leak of sorts? So...I guess this means I'm back to suspecting the carb again...and the primary or secondary plates not closing properly. With the idle speed screw backed out all the way and with maybe a 1mm gap (0.040"), I can actually close the throttle a little more manually...and the idle does drop a little bit...but I don't want to force it any harder than I have been. I don't know what stops the primary throttle plates travel when they're not hitting the idle speed screw. I seem to recall there being an adjustable stop in there, only accessible from the under side of the carb...one of those things that is "factory set"... Anyway...my new O2 sensor kit shows up on Tuesday and will take all of 5 minutes to install, so I'll finally be able to see what's going on with this thing...at least in how it's breathing anyway. Stay tuned. It does point to the carb. I don't think there's another stop for the throttle, although I've proven that I don't know much about a Holley, so maybe there is. But the plates are supposed to be aligned on the shafts from the factory so both of them close well. However, if you are able to turn the shaft a bit more and get the idle to come down I'm guessing that one of the throttle plates is misaligned and stopping the other one from closing. I don't recommend this as everything I've read says you shouldn't do it. But I've taken the screws out and put them back on many carbs and have gotten the throttle plates aligned nicely each time. However, you don't need to completely remove the screws or plates. You should be able to loosen the screws and shift the plates around to get them to close better or evenly. But the screws are probably staked such that you can't remove them w/o grinding them off - or risk breaking them off. So be careful. Anyway, with the idle at ~850 something's wrong and you've pretty much proven that there's no vacuum leak. Link to comment Share on other sites More sharing options...
85lebaront2 Posted November 3, 2019 Share Posted November 3, 2019 It does point to the carb. I don't think there's another stop for the throttle, although I've proven that I don't know much about a Holley, so maybe there is. But the plates are supposed to be aligned on the shafts from the factory so both of them close well. However, if you are able to turn the shaft a bit more and get the idle to come down I'm guessing that one of the throttle plates is misaligned and stopping the other one from closing. I don't recommend this as everything I've read says you shouldn't do it. But I've taken the screws out and put them back on many carbs and have gotten the throttle plates aligned nicely each time. However, you don't need to completely remove the screws or plates. You should be able to loosen the screws and shift the plates around to get them to close better or evenly. But the screws are probably staked such that you can't remove them w/o grinding them off - or risk breaking them off. So be careful. Anyway, with the idle at ~850 something's wrong and you've pretty much proven that there's no vacuum leak. If the throttle plates have shifted, the easiest way to check everything on a Holley is to actually remove the throttle body. With the throttle body removed, hold it up to the light so you can see if the throttle plates look even in the holes. If they do not, first remove the secondary closing link (J shaped link on the linkage side), with it off and the idle stop screw backed off so it isn't touching, carefully loosen each of the 4 screws on the primary throttle plates. move the shaft slightly side to side to find the centered position. Holding the shaft in the closed position, tighten the screws (I usually press the throttle plates closed and tighten the screws. Recheck the throttle plates against the light, if there is a difference, your throttle shaft may be twisted. On the secondaries, if you do not want to mess with the stop screw, remove the lever the secondary diaphragm connects to, this will allow the plates to fully close. Repeat what was done on the primaries to align the secondaries. While you have the throttle body off, take a picture of the top side for Gary, so he can see the idle circuits. After both sets of throttles are centered and tightened, reinstall the closing link, it should just push the secondaries closed when the primaries are closed. When you reassemble the carburetor, make sure that the secondary diaphragm does fully close the throttles, if not either it is the wrong part (there are two different lengths) or it is assembled wrong (pain it the rear to get right sometimes). If you want a "sanity check" on the secondaries, with the choke and primary throttles open, blow compressed air down the passenger side primary venturii. If everything is right, the secondaries will open. If your carb has an additional hole in passenger side rear venturii, they may not open fully as it will bleed part of the vacuum signal. It is to "accelerate" the secondary opening once there is enough air flow. Some Holleys have it and some don't. Link to comment Share on other sites More sharing options...
grumpin Posted November 3, 2019 Share Posted November 3, 2019 If the throttle plates have shifted, the easiest way to check everything on a Holley is to actually remove the throttle body. With the throttle body removed, hold it up to the light so you can see if the throttle plates look even in the holes. If they do not, first remove the secondary closing link (J shaped link on the linkage side), with it off and the idle stop screw backed off so it isn't touching, carefully loosen each of the 4 screws on the primary throttle plates. move the shaft slightly side to side to find the centered position. Holding the shaft in the closed position, tighten the screws (I usually press the throttle plates closed and tighten the screws. Recheck the throttle plates against the light, if there is a difference, your throttle shaft may be twisted. On the secondaries, if you do not want to mess with the stop screw, remove the lever the secondary diaphragm connects to, this will allow the plates to fully close. Repeat what was done on the primaries to align the secondaries. While you have the throttle body off, take a picture of the top side for Gary, so he can see the idle circuits. After both sets of throttles are centered and tightened, reinstall the closing link, it should just push the secondaries closed when the primaries are closed. When you reassemble the carburetor, make sure that the secondary diaphragm does fully close the throttles, if not either it is the wrong part (there are two different lengths) or it is assembled wrong (pain it the rear to get right sometimes). If you want a "sanity check" on the secondaries, with the choke and primary throttles open, blow compressed air down the passenger side primary venturii. If everything is right, the secondaries will open. If your carb has an additional hole in passenger side rear venturii, they may not open fully as it will bleed part of the vacuum signal. It is to "accelerate" the secondary opening once there is enough air flow. Some Holleys have it and some don't. Dang Bill! I’m sure you’ve forgotten more about carbs than I dare say I know! Link to comment Share on other sites More sharing options...
85lebaront2 Posted November 3, 2019 Share Posted November 3, 2019 Dang Bill! I’m sure you’ve forgotten more about carbs than I dare say I know! That might be true. Preston Carburetion was a Holley Warranty Center, and we also sold Rochester, Stromberg, Carter and Weber carbs. There is one book I would love to find a copy of, it was a book put out by Weber, titled "Technical Introduction to the Master Catalog" It explained in great detail how each part of the metering circuits, both idle and main along with transition circuits functioned. I haven't been able to find one anywhere. Link to comment Share on other sites More sharing options...
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