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carb assistance

E

ed rex

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Hello, After recently replacing my Torker manifold with a Performer, I decided to replaced my Holley 600 single pump with a 750 double pumper. I've had the 750 on the shelf for a few years, it's not new.

Problem is that I can't set the idle low enough. With the screw backed out fully, it still 'races' at about 1100RPM in Park. I then did some air/fuel mixture adjustments...basically started lean, then richened until more 'richning' no longer raised idle, then backed it down a hair, but still too high on the idle. Timing is set about 10 degrees BTC, dwell is dead nuts on 30. What should I do from here?
 
Pull the carburetor. Make sure the primary butterflies close almost all the way. Make sure that the secondary butterflies close completely. Some Holleys have a stop adjustment for the secondaries and if it's not adjusted properly, you'll be idling off of the secondary throttles and providing fuel from the primary idle circuit.

Other than that, I'd check for vacuum leaks.
 
Thanks Steve! I'll pull it tonight and give it a look-see. Would improperly adjusted air/fuel mix with regards to the secondary contribute to a rough idle? It is a mechanical secondary carb.
 
Unless the carburetor has four corner ifdling, improper secondary mixture should not make a difference. That should only make a difference under conditions where the secondary barrels are opened up, not at idle (unless you have nozzle drip at idle, then you really need to look at the carburetor).
 
Steve! Thanks much. Pulled the carb, flipped it over and there it was. Set screw preventing the secondarys from closing all the way! Backed it out until it closed and was able to get the idle set anywhere I want.

A couple of follow questions. My idle is still rough. I suspect because I was dinking around with the air/fuel screws in an attempt to reduce idle prior to your input. Is there a standard 'rule of thumb' when adjusting a/f mix? One dude here at work says you turn it in (lean it) until the engine starts to stumble, then richen it until the point where richening doesn't increase the RPM's anymore. Another says you lean it until the engine starts to stumble then back it out 1/4 turn. Is there any validity to either of these methods? Also, what about the two screws themselves? Does one effect the 4 cylinders on the passenger side of the engine and the other effect the driver's side? I ask because the tailpipe on the passenger side of the load kind of 'burps' almost like a muffled backfire. (Dual exhaust w/no 'H'-pipe) Could this be an indication of too rich on that side? Or is it more the result of a timing issue?
 
I was going to say all the way in lightly then back out 1.75 turns. Didn't some of the holley have ones that work backwards?
 
Oh man, backwards??? How would I find out real quick-like? Guess a Holley manual purchase is in order.

Does anybody know if there is a 'break-in' period after carb adjustment? It was running like crap.....vibrating, unstable idle, the whole rot. I had to take it down to Sac (about 100 miles round trip from about sea level up to about 3000 ft) and whereas when I left I had reservations of making it, the more I drove it the better it got. Now I have very good pedel response, the 'roughness' has settled down considerably to where now I'm fretting messing with it more! Geez, I tell you......
 
The more I think about it, I think there were some Holley's that the screws adjusted how much air instead of how much fuel went thru that part of the system?
 
OK, broke down and bought the Holley Carburetor Manual made by Haynes so research and readjustment are top billing for this weekend!

As an FYI, I had thought that the secondary air/fuel mix screws on the double pumper had no effect on the idle, so for the past week I've been dinking around trying to get the idle to smooth out using the primary a/f mix screws. WRONG. Just for s's and g's, I turned them. Had just as much effect as the primary ones!! Looked to be the more I turned the front ones IN, the more I had to turn the back ones OUT. Ditto for both sides. Given all this, I went and bought the book!
 
OK I dug out my Holley book and it says "some Holley carbs have a reverse idle system. On those carbs you turn the screws in to richen the mixture and out to lean it". These carb should have a label on the metering block noting this. Just bring this up incase someone ends up with one of these carbs. /forums/images/icons/cool.gif
 
im going to try this carb out.i have one also sitting on the shelf.i think im going to rebuild it first before i even try to bolt it up though.your carb was maybe dirty inside and gummed up.maybe thats why it started running better after you ran it for awhile.how much change did you have from changing from the 600.i have a 600 edelbrock and am going to try and put the holley 750 double pumper on hopefully next week.just wanted to know if there was a horsepower change or if it was too much for the engine.i have a stock 350 with 2.02 corvette heads and 1.6 lift rockers, so im not so sure about if the carb may be overkill
 
Chuliso - I realized a significant increase in HP. Either the 600 I had on was toast, or the 750DP just flat out gets it. I've a relatively stock 454 crate motor. Edelbrock Performer, headers, not much more but it sure does have alot more acceleration. Feels like a BB should now!
 
With a 454 a 600 was just way to small. A 750 to 780 would be better on a 454. /forums/images/icons/laugh.gif
 
I usually use a vacuum gauge to set idle mixture. Set for highest vacuum reading, then lean the screws 1/4 - 1/2 turn (depending on the screw taper and orifice size).
 
Thanks Steve. That's exactly what I did. However, I'm still experiencing some heartburn. The problem with the vacuum gauge (and maybe I'll have to try again) was that I could attain the 'highpoint reading' with different settings. Let's say the highpoint was 19 at idle. Since the 750DP has 4 mixture screws, I could have one primary screw turned in considerably and the same side secondary screw turned out considerably. Or visa versa. Nowhere in the Holley carb manual does it speak to the relationship between primary and secondary mixture screws, so I tried to make all 4 as even as possible and maintain the vacuum 'highpoint'. At this setting, I'm getting a rough spot from 1800RPM - 2200RPM. My latest attempt has been to purchase the air/fuel mixture gauge. The analog one. However, for the time being, I have this mounted in the cab so that I can see the effect of going up and down in elevation.

Which brings on another question, running dual exhaust without an 'H' tube, do I just 'pick a side' to shove the ox sensor 'bung'? Will this be fairly representative of both sides?
 
Even with a crossover, you're going to only read one bank of the engine. If you have headers, you will need a heated oxygen sensor. Just mount the sensor as close as you can possibly mount it to the cylinder head.

Anyhow, reading only one bank of the engine isn't such a bad thing if you are using a dual plane intake manifold since two of the cylinders will be fed off of the top plane and two off of the bottom plane of the intake (left and right sides of the carburetor).
 
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