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carburetor tuning woes- please help!


hpfiend

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Hey all-

 

Thanks for reading this!

 

The car won't idle below 1000 rpm- have to set it to 1200 rpm in park so it wont stall in gear. AFR mid 14s at idle - still somewhat rough.

 

Terrible dead spot/stumble off idle or light cruise. Happens while free revving in park and under load driving. It seems as if the stumble is lean as the AFRs go way up and then richer than idle then up again and richen back to idle but it could be because the car is stalling out and recovering.

 

2800 lb vehicle

224/224 @ 0.050 Duration .450/.460 lift 114 lobe sep cam (2200-5500 rpm)

RPM air gap manifold (1500-6500)

TH350 with 3000 RPM stall

3.73 rear gear

MSD Distributor, coil, and 6AL (full 18 degree centrifugal advance at 1600 rpm with lightest springs and red bushing) base timing set to 16 degrees. 34 degrees total.

SBC 355 (dished smog pistons)

1.94 valve double hump heads

Holley 4778-2 700 cfm double pumper - 66/76 main jets, 28/31 shooters, 6.5 power valve in front, pink pump cam front and back with 30cc pumps, secondaries barely cracked at idle with very little to zero transfer slot exposed.

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Sounds like the typical lean condition on your idle jets.

 

I really don't know much about those Holley carbs, but when tuning the Webers I had, there was a dead spot between my idle and progressive circuit- my idle jets weren't large enough. I bought larger idles and adjusted the air correctors so my idle was nice and solid, and it helped a steady idle quite a bit. With those larger idles there was a smaller gap per-se between the idle and progressive circuit into the mains.

 

upsizing the pump jets will help with a quick throttle stab, but that's not really the issue you're experiencing with what sounds like a dead spot at partial throttle.

 

just my .02

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No, it never has ran correctly-

Yes- it is a new cam setup- I plan to check the valves with the engine running net week - thanks!

 

I put a smoke machine to it but need to chef the vacuum line to the tranny-

 

 

Anyone know about setting timing and idle with a vacuum gaug?

 

Thanks!

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I had a 750 on there previously with the exact same issue- I tried about every shooter and pump can possible on that carb. I am going to try a 50cc accel pump next... The prob occurs mainly when you smack it to the floor from an idle or a cruise...
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Which cam are you using for the pump? If you're using something soft, then I'd switch the most aggressive cam and see if that helps with your stumble when putting it on the floor.

 

I'd look for a vacuum leak to solve not being able to idle down.

Edited by Nickey427
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I am using pink pump cams on front and rear right now. With the hp750 carb I just took off I had the blue in position 2 in the front and green in position 1 in the back- same dead spot.

 

I readjusted the valves with the engine running, a few dramatically changed the idle when I turned them down-

 

The carb screws were both out about 1.5 turns. The valve adj,

A little more base timing and tweaking the screws with a vacuum gauge got me up to about 15 inHg. The problem is that the needle rhythmically swings from 9 or 10 to 15. The fuel pressure gauge for the mechanical fuel pump swings at the same rate- doesn't that indicate I am down a cylinder or two? I unplugged wires at the cap as many times as I could tolerate the electrocution and found a few wires that did nothing- I even left two off and turned it off and restarted it and it still idled about the same????

 

I am pretty sure I acchieved dot to dot on the cam and crank gears and then rotated to 12 and 12 but i didn't use a plumb bob to line it up- I would hate to take all that apart I see it is correct- this is the second cam I have done but def not a pro

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Did a compression test and got 130-135 in every cylinder but number 3 which read zero. Took it to TDC, backed rockers off, took them both to zero lash and then a half turn and again zero compression- I took the rockers off completely and got 155 after several cranks and 135 after about four which is where the others seem to max out at after four pulses.

 

Looking up EOIC method now that I have used previously...

 

 

Thanks!!

Edited by hpfiend
Typo
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EOIC didn't help-

 

It looks like I got a defective lifter! The exhaust valve remains open at anything beyond zero lash like a solid lifter. I am going to verify I do not lose compression on number one doing the same valve adjustment procedure and then I will have to tear it down and rebuild.

 

Thanks for all of the help! Will post update!

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