Carb trouble.

[turtle5353]

Hey guys, I have been having some issues with my carb since I built my 408 C stroker. We have messed with it numerous times, re-jetted it several times, adjusted the floats, replaced power valve, ect. But it still runs a little rough at low rpm, and when you are just touching the throttle. Push the throttle just past 1/8-1/4 and it takes off like a rocket. From 1/4 throttle to WOT it runs PERFECT! In gear at idle and to about 1/4 throttle it misses, bucks, and is just not very street friendly. I am running a Quickfuel 850 or 900 carb...cant remember I will have to look again. But I just assumed this was being caused by the size of the cam and the HP of the build. Just wasnt very friendly when cruising around town. Cruising at 45mph you stab the throttle and it will smoke the tires instantly. But if your just touching the throttle it misses slightly and bucks and lunges constantly. I talked to Roy (TNFASTBCK) on the phone over the 4th of july and he has a very similar build and said he was having the same issues with his motor till he dropped to a 750 CFM Holley double pumper. I did some of the online calculators and that looks like it will cover my engine for street use. I think my engine builder built my motor to more of a race spec for the carb as its at the top end of the recommended CFM spectrum. I have an almost brand new 750 Holley DP in the garage in the box from my last engine build. Will it hurt my engine at all to just bolt it on and try it out?? Will I need to richen the jets?? Or will I be ok to just try it and see if it helps my below 1/4 throttle issues??? I know my Quick Fuel carb is still running pretty rich cause it will burn your eyes if its running in the garage!! Let me know what you guys think. I would call my engine builder but he's on vacation this week. Any help or input would be great! Thanks gentlemen!

 

[Jeff73Mach1]

I'm running a hot 393 stroker with an 850 with no problems. I suggest you remove both power valves and block them off, then increase jet sizes on both primary and secondary sides with 4-6 sizes larger. You'll run a little rich at idle, low RPM, but it should solve your problem.

With large cams, the vacuum signal at idle, low rpm is pulsating so much that the power valves are "bouncing" open and closed and the fuel delivery is inconsistent.

 

[turtle5353]

Jeff, excuse my ignorance, but I haven't messed with power valves much. Do you completely remove the power valve? Do they sell a block off? Thanks for the input!

Also would it hurt to try my other carb for shits and giggles??

 

[c9zx]

It won't hurt to try the Holley. The QF carbs have great adjustability. It sounds like the idle circuit needs to be calibrated. The Idle channel restrictor is changeable as are the idle air bleeds. QF or Summit sells the the parts. To base line the carb you'll have to remove the carb, drain it, turn it upside down an adjust the primary and secondary transfer slots so that about .025-.030 are exposed on both sides (I'm assuming it is a double pumper). If too much of the slot is exposed it will be rich at idle. If not enough is exposed it will have trouble transitioning from idle to light cruise and may have lean surge at light cruise. Set all four mixture volume screws to 1.5 turns out. If you are sure it is rich, change the idle channel restrictors to 2 numbers smaller (less fuel). A little bit goes a long way here as the holes are very small. Adjust the mixture screws for best vacuum. They don't all need to be exactly the same but, they should be within about a quarter turn of each other. If you need to adjust the idle speed change equal amounts on the primary and secondary idle speed screws and keep track of how much you have changed them. If it improves but still a bit rich, try changing the idle air bleeds 2-4 numbers larger (more air) to lean the idle mixture out. I do not suggest plugging the power valves. It will either be rich at idle or lean at WOT. All of the fuel delivery paths are there for a reason. I do suggest getting the proper power valves. Coarse starting point with big cams is 1/2 the idle vacuum with the car in gear, if it is an automatic. It can be a time consuming process, be patient and try to change one thing at a time so that you know what caused any changes. Let us know what progress you make. Chuck

 

[Jeff73Mach1]

Yes they sell plugs to block them off. QF carbs are not anything I've ever played with- they look to be a Holley with some additional adjustability built in. My method will leave you rich at idle or lean on the top end. You want to be rich at idle, just not so much as to load up badly in traffic. Having the secondaries richer than the primaries can help some, but you have to be careful and watch all your cylinders to make sure that you don't have one significantly leaner than the others. A temp gun on the exhaust manifold will help you.

The ability to adjust air bleeds, transfer slot and idle channel restrictors without altering anything permanently is a great option.

 

[scarpent1]

What is your initial timing at? If it is too high it may cause the same issues you mention.

 

[turtle5353]

Thanks for all the input guys. Im not sure what my timing is set at now. I let my engine builder set everything up for me. less stuff for me to mess up.