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Discussion Starter #1
Hello all. I'm having trouble with a new engine that no one seems to be able to pinpoint, me included and was hoping for some ideas from y'all with more experience than myself.


Been racing this car about 4yrs now. Tore up the motor and let it sit the past two years. Just built the new motor a couple months ago.


Motor idles (slowly lowering rpms like its loading up) and spitters/misses while slowly increasing throttle up until about 2000rpms then revs and screams. At the track it burns out fine (no load) but when I let off the button on the line (5800 off of converter) it pop pop pops for about a second then takes off, then pop pop pops when I shift for about a second then goes.



Im assuming its a lean/rich problem but can't figure it out. The holley carb is well into the transition circuit to keep it idling. Vacuum gauge reads 3-4" at idle. Closing the idle screws makes the engine pick up. I've sprayed carb cleaner everywhere around the carb base and manifold but it doesn't change anything. No pick up or down. If I spray it into the carb it will almost kill the motor.



The catch is I didn't really change anything on this build from the last one and the carb has been on it for 15yrs running fine. I took the carb apart and blew everything out but it was spotless and I didn't see anything wrong.


Here's my setup and everything I've done or tried:


67 Mustang chassis car (2500lbs)
SBC 383 with BTE Powerglide
5.13 Gears 31" tire
12.4 Compression (235-240lbs all cylinders checked last night)
Cam .666 .638 107* in CL 103* 4/7 swap (did change to 1.6 rockers)
Chuck Nyutten Holley Carb
Brodix T1 Heads CNC'd by Sunset Racing (Just rebuilt by brodix)

Eagle Bottom End with Diamond pistons
Timing Locked 38* (where it was on last motor)



Brand new MSD 6AL, Pro Billet Dist, Blaster 2 coil and wires (thought it was electrical at first)
New Fuel Pumps and Regulators (6lb to carb)


Only thing that was replaced on this build is block, pistons and rods. Everything else is from old motor. I've done all I know to do and could sure use some advice on where and what to check next from folks that know way more than me. If you need more info I'll do my best. Thanks in advance for any help! James
 

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Since the car sat for awhile, I'd start with a basic carb rebuild kit. Disassemble completely, clean all passages (spray brake clean through all passages then compressed air), reassemble with new gaskets (apply chap stick to the metering block gaskets on both sides to prevent sticking).

Then I'd look at the fuel cell, foam in the cell, lines, filter, etc. to make sure there is no contamination from "stuff" coming apart.
 

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Discussion Starter #3
Thanks Cole. For shits and giggles I just got off the phone with Chuck Nyutten (his carb) and he said when I dropped a valve on the old motor that he suspects when it had a massive backfire that I "blew the float" then went on to describe several things the motor has done since rebuild. Ordered new floats and kit from Chuck. Will reply back if that fixed it in case someone else has a similar issue in the past.
 

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….. Backfire "blew the float" ??? … power valve , yes … float ? How in the hell ? … 38* on Track 1 heads might be a little much …..
 

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Discussion Starter #5
Yeah, didn't make sense to me either but I've had trouble with the primaries flooding out since I put it back on the new motor. I'm willing to try anything at this point and I know the carb needs attention. I don't know if it matters but the heads have been extensively ported by sunset racing and thats where it made the most hp/torque on the dyno so I just left the timing where it was. Will play with it more when the dang thing idles right.
 

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Oring on needle and seat probably causing the flooding. They dry out and shrink. See how flexible the acc diaphragm is. If it's hard it will need replaced, same w PValve.
 

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Oring on needle and seat probably causing the flooding. They dry out and shrink. See how flexible the acc diaphragm is. If it's hard it will need replaced, same w PValve.
I was thinking the same thing as I was reading this thread.
O ring on needle and seat damaged and leaking fuel .
 

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Discussion Starter #10 (Edited)
ok new carb kit and floats came in today. NOPE not it. Carb has been thoroughly gone through. Needles are holding no problem there. Lets simplify things, lets not worry about running a pass here's whats going on at idle.


Idle (neutral) 1300
idle (in gear) 900-1000)


vacuum 5"
pull vacuum gauge off or close 4 corner idle screws engine rpm picks up. (rich I'm assuming)


Edit: EVERYTHING in carb new



Same carb thats been on this motor (old one) for 15yrs. ONLY thing I changed on motor is .4lbs higher in compression, 1.6 rockers instead of 1.5. Same everything else.



Compression 235-240 all cylinders. Again, any ideas are welcome! James
 

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Discussion Starter #11
Lets talk about the valves and leak down for a second. Had a bent valve replaced at what used to be a good racing machine shop here locally.



would a bent valve or cause this condition? runs great from 2k up?


I'm still learning so can someone explain how to do a leak down check? Thanks guys
 

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Yeah, didn't make sense to me either but I've had trouble with the primaries flooding out since I put it back on the new motor. I'm willing to try anything at this point and I know the carb needs attention. I don't know if it matters but the heads have been extensively ported by sunset racing and thats where it made the most hp/torque on the dyno so I just left the timing where it was. Will play with it more when the dang thing idles right.
Sounds like a stuck needle and seat
 

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With the high idle speed, the throttle blades are probably open too far & idling off the transfer slot. This would explain the rich idle. It might also explain the spluttering if the spark plugs are sooted up from a too rich idle.
 

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Discussion Starter #15
I haven't tried another carb, just don't have access to one without buying one. Live in the middle of nowhere with no other racer "buddies". But might just have to buy one



I'm definitely running off the transfer slots but can't close the throttle plates without it dying. The 4 corner idle screws are 1 1/2 turns out (where it ran on old motor). Could I close them all a half turn to pick up rpm then lower idle screw to close plate back to acceptable range? Would one turn out still be in standard adjustment range?


From the couple things I've changed on the motor I would think (maybe incorrectly) that the car would need more fuel not less. Slightly higher compression and 1.6 rocker. Am I wrong thinking this?


Again, thanks for the help
 

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I haven't tried another carb, just don't have access to one without buying one. Live in the middle of nowhere with no other racer "buddies". But might just have to buy one



I'm definitely running off the transfer slots but can't close the throttle plates without it dying. The 4 corner idle screws are 1 1/2 turns out (where it ran on old motor). Could I close them all a half turn to pick up rpm then lower idle screw to close plate back to acceptable range? Would one turn out still be in standard adjustment range?


From the couple things I've changed on the motor I would think (maybe incorrectly) that the car would need more fuel not less. Slightly higher compression and 1.6 rocker. Am I wrong thinking this?


Again, thanks for the help
Exactly what you need to do. That will get you off the transition slots.
 

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Discussion Starter #18
Exactly what you need to do. That will get you off the transition slots.
ok, thats what I did tonight. 5" vacuum to start. slowly adjusted all down, 1/8 a turn at a time. Vacuum stayed the same. Got down to 3/4 turns out when lean backfire started. vacuum never increased. car sounded worse. again I'm at a loss
 

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Discussion Starter #19
at this point I've been fighting this for 2-3 months. nothing anybody tells me changes anything for the better. I've bought a leak down tester per advice and will try that next. charcoal fluid and a lighter sounds better right now
 
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