ThermoQuad surging issue - ideas?

All

I had a TQ built for my 360, .030 over, 8.5-1, nice lumpy purple shaft cam (specs unknown) TTI headers, M1 dual plane intake. Previously, I had an LD4B intake and an Eddy 600 on it, which ran and idled fine, just a bit rich though. Carb needed a rebuild so I had Demon Sizzler build a TQ and swapped on the M1 intake.

At idle, the carb seems to surge and vary around 300 rpm. I'm seeing the same surge and possibly a light miss at low rpm speeds. When the 2ndaries kick in, it pulls real hard. I'm hearing a whining audible noise from the carb at idle, not sure if that is normal or not. I haven't changed timing, so I am looking for ideas as to what the problem is: vacuum leak? TQ primaries too lean?

Have you PM'ed Demonsizzler with your questions?

I couldn`t get my 340 with a stock thermoquad to idle well with a purple shaft cam. Not knowing how to fix it I put a stock cam back in and my problems were over. I do miss the lumpy sound though. Mike

Could the whine be a whistle caused by a vacuum leak? Make sure your throttle shaft bushings aren't shot.

With a lumpy cam you will probably want to drill a 1/16" to 1/8" hole in each primary throttle blade to get a better idle. This way you don't have to turn the idle speed screw in too far and bypass the idle circuit.

I run a chokeless 850cfm TQ on my 340 with a MP508 cam, RPM manifold & headers. I have 3/32" holes drilled in mine and my idle mix screws are 3 turns out. My jets are 96 primary with 147 secondary and 63/50/40 metering rods. Squirters are drilled out to .032 and metering tree height is 1-1/4 turns.

I love the way the TQ looks and runs on my car.

there's a screw on the metering rod tree you can adjust to compensate for larger cams.
if you run outta adjustment on that, you can use a weaker spring that the tree rides on.
no need to drill holes just yet.

IF, after you CHECK EVERYTHING for vacuum leaks and find none, AND IF you're running a larger than stock cam, and adjusting the metering rod tree doesn't help with surge or idle. Here are a couple items that will fix the problem. First on idle.


Take a numbered set of drill bits and using them as a gauge find the size that fits EXACTLY in the small brass tube in the picture. Drill it .001 to .003 larger. That should do wonders for a thermoquad that won't idle with a larger cam,(richens idle).
IF you have an LM-1 meter the next mod will help you really dial in the lowspeed curcuit.
Pick up some holley bleed blanks and drill and tap lowspeed bleeds. FIRST though use numbered drill set to find out the stock bleed sizes and WRITE them down for reference.
To richen the curcuit up go a couple thousandths SMALLER on the bleed. For the hispeed primary bleed you can also do the same. Hispeed bleed is the other bleed shown in the picture. Dave
After rereading the post and seeing that Demonsizzler did the carb for you, I'd talk to him before doing ANYTHING to your carb. He is the master at modifying thermoquads.
For you other guys that feel adventurous, these mods can be done at home by anyone with a little mechanical ability. I WOULD NOT screw with a numbers matching carb but find another replacement to modify. You don't need to drill holes in the butterflys to get one to idle.

Definately call DS first.

Thanks for the big pics and tech QuickD!

Please keep us updated on your carb tuning.

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there's a screw on the metering rod tree you can adjust to compensate for larger cams.
if you run outta adjustment on that, you can use a weaker spring that the tree rides on.
no need to drill holes just yet.

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The screw on the metering rod try adjusts how far the rods enter the primary jets under full vacuum. Drop it and you lean out the cruise mixture, raise it and it will richen cruise.