View Full Version : choke plate help
02-15-2011, 07:36 PM
I have a 85 k10 with a 305. I am having issues with my Rochester carburetor. First off its got about 1000 miles on a fresh rebuild from Mountain Man Carbs (a guy on ebay with 28 years of experience) so i dont think its something inside the carb. However i am having issues with my secondaries. My choke plate isnt opening up at all on my secondaries. My truck is at 160-180 degrees and my primaries choke plate is completely open. I am trying to figure out what might be wrong. From the research i did i understand that the throttle plate is vacuum operated, is the choke plate too? also will the choke plate on the secondaries only open up under a load like the throttle plate on the secondaries?
02-16-2011, 05:44 AM
your Carb is called an air-valve Carb, that second plate is not a choke plate...your secondaires will open the "butter-flys" all the way..but that plate is what restricts and "limits" should i say, the ammount of flow..it kinda works like the secondary diaphram on a Holley vac-sec carb.....its not supposed to open as a "chocke" but a as a air limiter, so how ever much air is needed that plate will open...understand? most rochester carbs flow an ywere from 700 to 820+ cfm but your engine will only use what is needed.
02-16-2011, 10:06 AM
no that makes perfect sense. so any idea why my secondaries arent kicking in (pretty sure they arent working based on lack of power for passing or going up hills, also my foot has to be ON the floor to keep it at 70 on the highway, also no sound difference when i step on the pedal hard) or how to test certain parts that might be causing it? i am not sure if the plates arent opening under load or if there is no gas going into the back barrels. the new carb came with a new choke pull off so i hope that wouldnt be causing it. someone i talked to said it may be a linkage to my transmission that causes it to downshift might be froze up or disconnected is that possibility? i appreciate any help you can give to the novice asking questions. he is from the era of direct injection haha
02-16-2011, 10:20 AM
there is a little "stop, on the choke side of the base plate that should be clear of the bottom secondary shaft when warmed up. Check that 1st, if that hasn't swung clear of the secondary shaft, the bottoms will not open and either will the tops. If that checks out, the next thing to check is the upper casting just in front of the rear most screws. It is common for these to get over tightened and mushroom the casting witch than rubs against the upper air valve plates. This can "lock" the plates closed. Push down on the plates (truck not running) and see if you feel them hang a little (it doesn't take much). If both of those check out, there is a spring on the side of the casting that can be adjusted with a small allen and a real small screw driver. Do not let the valve plates sag. Make sure the spring can shut them completely.
Also, the "chock pull off" also doubles as a meter for how much the secondary air valves open, some let the air valves open faster and farther than others. They are typically real conservative in later models. there is a metered hole mid way up the nipple tube, if you have a strait nipple tube, they can be drilled out but you want to use vary small machinist pencil drills and only go one step at a time, because these are non reversible changes.
Keep in mind that Q-jets are vacuum dependent and the secondary not opening could be a sign of a vacuum leak or a flat cam. They get blamed for those issues all the time. Also, the secondaries are demand sensing and the carb can flow as much as 825 CFM (depending on the casting #) so they rarely open all the way on relatively stock engines
With some minor mods these carbs can be some of the best. I reworked one to run a big cam by putting brass bushings in the base plate, drilling out some of the air meter holes and chaging my metering rods, jets and hanger. There is a great Q-jet book out there but its allot of reading. Its well worth it if you plan to make any kind of power off a q-get. Also, the on-line shop "tin Indian" sells the bushing kit for the primary shafts witch almost all q-jets need by this point in there lives
02-16-2011, 04:33 PM
ok well i had a chance to work on my truck today (it was 60 degrees today vs the -20 degrees it was last week. welcome to the u.p. of michigan i suppose). anyway everything you said i checked out and seems to be in operating condition. i grabbed that little "stop" and gave it a little pull and she perked right up. it was like a new engine, so i know its getting gas. next i undid the vacuum line to the rear break and there was no vacuum whatsoever no matter how hard the engine was working (i.e. wouldnt hold a piece of paper onto the end of the line). tracing it back it was connected to what may be a "deceleration valve." whatever it was it had 3 vacuum lines leaving it. one straight to the pcv valve, one that branched little hard plastic lines (one ending at the carbs front side), and of course one that went back to the air cleaner and the rear break. the first two got vacuum but the one to the rear break doesnt. is this normal or is this valve not working right? also if i bought a vacuum gage should it being pulling a certain psi at idle?
02-17-2011, 05:48 AM
ok well as for your "tri t vacuum" it may be tempture related...or just clogged.ect....as for vacuum..with a vacuum gauge to the engine some were around 22-16 in-hg is what you should have steady at idle ....
02-17-2011, 10:45 AM
It sounds like you have a bad thermostatic vac switch. Is the 3 nipple piece or any other piece along the way, screwed into a water jacket? The one that goes to the air cleaner should be with vacuum when cold and without when hot. It is opening and closing a diverter valve in the air cleaner that is meant to draw air from the outside of the exhaust mani when cold and then to draw normal un heated air once the intake is at operating temp. The problem is, the air cleaner is vary restricting and that hot air stove is even worse. If you are never running with that valve open, the carb can never draw enough air to open the air valves. I always ran my Chevys with a ball bearing in that vac line and the air cleaner lid upside down between smogs LOL
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