c-1 backfires at low speed

[Mike B.]

'61 270 hp & 2x4's runs like a top except at too slow a speed for any given gear and if you give it too much throttle it will back fire through rear carb. If revs/speeds are high enough there is no problem, runs great. My question and I know this will sound dumb but, can this symtom be caused by the fact that the car has only 3:36 gears and not the 3:70 it should have thus not getting the rpm's up for the demand of fuel ???????? FYI timing is good, valves adjusted correctly, dwell good, carbs adjusted by screws only (I have a vacuum issue causing vacuum guage to fluctuate rapidly at idle with no apparant vacuum leak) Just telling all that I know. All help will be appreciated. Thanks, Mike

[Timothy B.]

Re: c-1 backfires at low speed

Mike,

I would take a closer look at the valve adjustment, if the car has solids there may be a tight valve causing the spit back and vacuum gauge reading. Backfires through the carburetor are from a lean condition where the throttle blades are opened and the fuel emulsion drops out of the air because of lack of vacuum and the engine goes lean for a moment.

Where is the ignition timing set.

[Mike B.]

Re: c-1 backfires at low speed

timing at 12. dual point dwell at 34. I am new to solids and 2x4's. Lifters were set like hydraulics when I got the car. Just did adjust and feel they are correct. I did this before this post. The back fire is the same as before I adjusted them. Again the backfire only occurs if I am too slow for any given gear and give it too much gas. If speed is enough for the gear I am in there is no backfire.

[Duke W.]

Re: c-1 backfires at low speed

Your problem could be due to improper distributor installation. I suggest you verify that the wires are properly indexed on the cap. Then pull the dist. and verify that the dimple on the gear is in line with the rotor tip, then reinstall the dist. properly indexed. I assume this information should be in ST-12.

What is the lowest RPM that the engine will accept at least half throttle in the various gears?

Duke

[Mike B.]

Re: c-1 backfires at low speed

I will double check dist like you suggest. My car tach is inop. Bounces badly. Pulled and Checked and repacked cable and made no difference. Need to send out (not high on my priority list). I did mention that car runs like a champ w/o any miss or stumble or backfire if RPM's are up beyond that backfire point. You could keep your foot in it till it blew up. It is only at the low end. I will figure out a way to hook up a hand held tach and check your last question. thanks, mike

[Timothy B.]

Re: c-1 backfires at low speed

Mike,

One other thing to check is the accelerator pump for proper adjustment. Don't do this with your face over the carburetor..

[John P.]

Re: c-1 backfires at low speed

Hello Mike, If you have the ignition shields, inspect a plug wire shorting to the shield. Under load at lower RPM the voltage demand on the ignition is greater and will cause arcing. Especially inspect the distributor (top) shield for plug wire contact. I have had two cars with similar problems and had to replace the wire and raise the shield mount. Good luck, John

[Mike B.]

Re: c-1 backfires at low speed

thanks for the suggestion but I have the shielding off.

[Mike B.]

Re: c-1 backfires at low speed

I know is squirts but I have put off taking apart the carb until last resort. But I may be there. Since you brought it up, the vent cap on the rear carb has a loud ticking sound (front one does not). Definately can tell that this is where the noise is coming from by using stethascope. sounds like knocking from under side---could this be related to backfire???? Again have not dealt with carb other than adjusting mixture and idle screws (no idle adjustment is set on front carb). by the way the front carb comes in at a little over half throttle and I have disconnected front carb to see if back fire still occurs and it does. thanks, mike

[Joe C.]

Re: c-1 backfires at low speed

If the valves were adjusted like hydraulics before you readjusted them, then depending on how long it was run like that, you might have a few burned valves. Reason I say so is because of your fluttering vacuum gauge at idle, and the pop thru the carb.

Verify your valve adjustment again. They should be lashed at .012/.018, warm. If the vac gauge still flutters, then I suggest you do a leak down test to check for burned valves.

[Mike B.]

Re: c-1 backfires at low speed

thanks and I am afraid I may be headed to that. trying all else before I go there. It just runs too good except for the back fire thru carb. Mike

[Tom B.]

Re: c-1 backfires at low speed

I agree with Joe. With valves set at zero lash or tighter with solids it was holding the valves open all the time to some extent. I'd also check for bent push rods and a wiped cam lobe.

[Steven B.]

Re: c-1 backfires at low speed

timing at 12. dual point dwell at 34. I am new to solids and 2x4's. Lifters were set like hydraulics when I got the car. Just did adjust and feel they are correct. I did this before this post. The back fire is the same as before I adjusted them. Again the backfire only occurs if I am too slow for any given gear and give it too much gas. If speed is enough for the gear I am in there is no backfire.

Mike, how did you adjust the lifters after they were set like hydraulics? They should be adjusted with feeler gauges to the specs. noted, not the same clearance as hydraulics. Tom and Joe have a point.

Steve

[Mike B.]

Re: c-1 backfires at low speed

Been out of pocket for a while. thanks. Yes I did adjust them correctly as directed by the guru here on the site. Does run better but still will die when coming to a stop if you dont play with pedal as stated earlier in post