I have a 1970 350/350. I have not really driven it for ~4 years. I am anxious to get it on the road this year. I am having some tuning problems. It is really stumbling on accelerations...and I'm getting some backfire even at light/moderate acceleration. I've also noticed that it is really slow to reduce RPM down to idle after I take my foot off the gas.

Since this winter (in preparation for driving it this summer), I have recently rebuilt the carburetor (Holley 650 DP)...I did most of Lars' carb tuning procedure...I confirmed the accelerator pump arm/spring is correct...I have confirmed the dwell is at 30 degrees...I put new plugs and wires (Taylor 8mm) on...I set the timing (at idle) to the factory setting of 8 BTDC (I know Lars has a better way, but I just wanted to get it going "reasonably well" for now as I don't have an advance light to make setting at 36 all-in easy). It just seems that at factory specs (8 BTDC), I shouldn't get backfire...no? Idle is set at about 800 RPM.

I then increased the timing to 12 DTDC and it seems to backfire less...and when I turn the car off, it seems to stop sooner (before it would "diesel" a little after turning the car off, now, at 12 BTDC it seems to diesel less).

The previous owner did put a different cam in, but I don't know what it is...and there are exhaust headers as well.

Any help? What would you chase first?

 

ok - I've confirmed again that I am an idiot! I just found wires #5 and #7 switched. I put those where they belong and I am no longer getting backfire out the carb....but....I have 2 remaining issues

1. I still get a small backfire out the tailpipe once in a while???

2. The trottle doesn't want to come off the high idle. So when I accelerate and get it up to 2000-3000, it just hangs out up there...I have to "kick" the accelerator pedal down quick and then it will come down. Almost every time. What could cause this? Is it a carb setting issue, or a cable sticking issue?

Any help would be appreciated!

 

Remove the air cleaner so everything will be visible.
Disconnect the accelerator cable from the carb.....move the cable in/out....it should move very easily with no binding at all.

Move the throttle arm slowly all the way open....watch for linkage that may bind/catch...it may take several tries before you pin-point binding linkage (the accelerator pump will be pumping fuel into the carb!)

Reconnect the accel. cable.

(I found that two linkage parts on the throttle shaft and choke shaft were catching at mid throttle...there were rub marks where the two pieces were rubbing...I used needle-nose pliers to bend each link a small amount that gave 1/16" clearance between the 2 links without affecting the linkage operation)

 

you only mention a Holley DP but not what kind of choke, electric, hot air, manual, etc. Could the choke be not fully opening causing the fast idle cam to be engaged?
you set the timing with the vacuum advance disconnected?

 

"Double pumpers" don't belong on street engines! They are made specifically for for drag and track racing where engine speeds never drop below 4000+ rpm.

 

Hi guys - thanks for the comments.

The carb has a mechanical choke...that is disconnected. I have made sure the choke plate is all the way open. Makes it a little tougher to start and keep idle for the first couple minutes, but then it's fine.

I realize double-pumper is not the ideal set-up...nor is the Edelbrock torker 2 intake. The previous owner was racing the car. My goal/plan is to get a new Edelbrock intake and put the original Q-jet on it. I have the original carb and I plan to rebuild that one this winter.

I'm really curious about the small tailpipe backfire...doesn't happen all the time...maybe once a minute?

 

does the back fire occur only at idle or also if you hold the throttle at 2K rpm? If only at idle have you adjusted the mixture screws?

 

I know for sure it happens at idle...and I THINK it happens while driving as well.

I adjusted the mixture screws to give peak vacuum...but now that I think of it, that might have been when I had #5 and #7 wires crossed. So maybe I need to do that again? Would that make a big difference?

 

A couple of things to check, are the distributor advance weights clean and freely moving? Have you checked for leaking power enrichment valves in the Holley?

 

Thanks doc...

The distributor advance weights probably aren't perfectly clean...in fact, as this picture shows, there was a small amount of red stuff on them that I don't know what it is. Would this affect the backfire issue or the throttle high idle issue?


I don't even know what the power enrichment valve is? I just re-built the carb over this winter, so everything should be clean and good. Is this the "power valve?" If so, I know I replaced it...and this weekend when I took the carb apart, I pushed it with my thumb and it moved freely.

 

Predictable comment- and ridiculous. This has been discussed before at length and the forum members who are most respected for their real world experience have rebutted this very effectively.

OP, follow the advice of others in this thread. If you decide to stick with the Holley get a good book on tuning them and you'll be fine. Those Torker2 intakes aren't a great choice for the street due to issues with vacuum signal to the carb. Edelbrock will tell you so. Good luck.

 

Sounds like a similar situation as my car. It sat for about 3 years before I bought it. You may want to run through the fuel tank and then fill up fresh with some dry gas. After you go through the tank with the dry gas I recommend replacing the fuel filter, spark plugs, wires, cap, rotor and points. I also recommend disassembling the carb and soak clean it. Assemble the carb with new gaskets and power valve (if equipped). Tune the timing and carb per Lars recommendation. Or set the timing at 10* BTDC (it should run fine at this setting). Correct carb set up is crucial, float level, idle screws, choke, etc...