Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: 78GMCMH on December 04, 2009, 09:03:43 AM
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Hello, I'm having problems getting my new motor to idle. 72 cadillac 500, .020 over,,507ci. 9-1cr,, comp RV torque cam,, .540 lift, duration intake 233,ex 235,,,,lobe seperation 112. Factory Ex. manifolds. Edelbrock 2115..This is for my GMC motorhome. Its sitting in my shop in the frame, hooked up to the transmission,differential,radiator. Hooked up a temporary contol panel with a throttle that can be locked into place. Also installed an oil pressure gage,water temp, fuel pressure and AFR. Fired up ok and ran for 20 min. at 1800 to 2200 rpms. Small problem with glowing ex. pipes but thats what happens when you leave out the idle air screws. in trying to set the idle I had costant problems with flooding. 1/2" of gas ontop of the butterflys. Had 8.5 lbs with a factory pump. Switched to a .125 needle and seat. Installed a regulator and gage,,,4.5 to 5lbs. Factory manual calls for 8 degrees but it won't hardly idle. have to crank on the idle screw and opon the primarys some. 8-10 inches of vacume. very rough. more advance,,,12 or 14 degrees lets me back of on the idle screws,,,12 to 13inches of vac.,,,,, still idles rough. 700 rpms.Responds to bliping the throttle and it takes alot of sudden throttle quickly to raise the APT. Sprayed ether and carb cleaner around the manifold and carb looking fo a leak. Nothing. IR temp gun shows all cyl. head passages the same temp.,,320 range. Made a smoke feeder out of a paint pressure pot and injected it into the manifold to see if I had a large leak in the valley. Never came out of the rocker covers. 165 to 175lbs. compression. Rmoving one wire at a time makes it idle rougher,,None make" zero" diffrence. It idles better rich,{screws out 5-8 turns} 12.5 AFR on gage. Idle air screws will change AFR to 14/1 but it idles worse. Removing the plugs after warmup and idling, very wet. Checked zero mark on crank hub twice,,and came shaft using the "intake centerline" method. After warm and idling I removed the carb twice and found fuel around the well plugs. Tested for leakes,,none found. Filled bowl with penatrating oil ,,overnite,,no leakes.
Idle tubes .037
Idle channel restrictions .046 loose,no go .052
upper air bleeds .070
Lower air bleeds .055
Idle screw holes .059
Idle bypass air .108
Main air bleeds .052
Jets 73
Rods 54 V
Carb# 17080212
14444 GPW
Carb is off of a 1984 P30 cassis motorhome.,, used a kit from you. Hpe I didn't miss anything, help.,,,,,,,P. Leavitt.
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Hello again; also, i blocked the crossover. In order to eliminate ignition problems, found an HEI off a 77 425, swapped out the Pertronix, no change in idle quality,, thanks,,PL
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OK, I think I know where my vacume went,,,,,wrong cam. I thought I orderd an RV torque grind.
Heres what I got,,,,,,
Duration @ .050 In. 233 Ex. 236
lobe sep. 112
lift .540 .537 I think this explanes why I have a rough Idle and little vac. and also why it likes so much advance. Looking for a new cam now,,,this is HOTTER than comp cams hotest,,,more later,,,PL
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yes that is a big cam. especially for a motor home. I would think 210-220 @ .050 would be more apprpriate. Maybe about .450-.460 lift. Lobe center @ 112 LSA ok though
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I agree, pretty big cam for a towing application, even at 500cid. You'd need up near 11 to 1 compression to smooth it out.
I run a 236/245/112 in the 455 that powers my Ventura, at 11 to 1 compression. It idles OK, vacuum is around 13", but it would not be suiteable for towing at low rpm's.
About 10 degrees smaller duration, on a 112 or even a 114 LSA will really smooth out the big Caddy engine......Cliff
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Like Cliff said big for towing. Did you degree the cam?