235 backfire - please help

All,

I can't seem to iron this out. Step on the gas somewhat aggressively (and I'm not talking drag racing) and it pop's with a backfire. Otherwise it idles well and seems to run okay.

the facts:
-1953 Chevy 210 with powerglide
-stock motor
-stock Rochester BC carb (rebuilt)
-paper air filter
-dual exhaust
-new plugs & wires

-timing is correct
-valves are adjusted correctly (hydraulic lifters)
-can't find any vacuum leaks
-accelerator pump seems to work okay (when viewed from above it squirts when the throttle is depressed)

Thoughts?

Thanks,
Carl

 

Jetting and/or accelerater pump paper element may be making it too lean.

 

x-2

if you can change the hole the rod for the accelerator pump goes into give it an adjustment and see how that goes... sometimes that fix it...

 

Does the car die with the backfire? Have you used a vacuum gauge to check the timing? I know on my car if I just time it by the mark on the flywheel it's just not right. I got some info on timing with a vacuum gauge from this forum and it got the car dialed in really nice. A couple of other things I'd double check is that the vacuum advance is working and that the mixture screw is out adjusted right. Having the paper filter and duals would lean out your air/fuel compared to the stock exhaust and oil bath so you may need to richen it up a little. If that's correct you could be getting to much of a shot from your accelerator pump. Just some thoughts...good luck

 

Very helpful guys! I'll look into adjust the timing by vacuum. ...and the other suggestions as well.

~ Carl

 

all the time or once in awhile ??

 

It does it all the time.

 

First choice acc pump. If it does not start squirting with the first movement of the throttle it will pop back.

 

I'll check it again. ..thnx

 

Also be sure all check balls in the acc. pump circuit are in place. Very easy to lose one or more when rebuilding a carb.[don't ask me how I know !!] A missing one will delay the pump shot until the circuit is filled.

 

Ball(s)?? I don't remember their being two in there for the acc pump. ...hmmm

#37 in this diagram, correct?
http://www.carburetorfactory.com/expvw21.html

 

If I remember right they are two different sizes too so you have to make sure you get them in the right spot.

 

check your vacuum and mechanical advance. If they're not working, that could be your problem. Mechanical advance maximum is 10 degrees, vacuum advance maximum is 14 degrees. I have 8 in mine at idle, so I'm running about 32 total. Check it with a degree'd timing light to be certain where you're at. That big nationwide tool chain has them for $40.
Let us know what you find.

 

Here's a good vacuum gauge guide I got from someone else here on the H.A.M.B when I was having trouble with mine. I used the vacuum port below the carb on the intake. I just put a T on that and it worked like a charm.

 

You guys rock - thanks all. ...and Scooby, thanks for the pdf - printed and going on my garage wall!

 

You're welcome...I taped it to the lid of my box LOL....I never knew you could use a guage like that till i got educated here

 

well ive been workin on my 235 for about 30 years... anybody think that the harmonic balancer might not be correct??they do slip, espessially after 55 years or so... just a thought.. seen it happen..... let us know how it goes and find out... if the balancer is off you have to go through tdc indexing aand marking ... cam tuners can help you with this...luck....k

 

235 do not time off the balancer,ball in face of flywheel.

 

Last sumer I bought an old profesional vacuum gauge at an antique shop (while the wife was looking at vintage jewelry) ...well last night I dug it out and sure enough it has marked zones for engine performance. I hooked it up to my "timed correctly via ball bearing in flywheel with timing light method" and sure enough the vacuum gauge pointed at the "timing retarded" zone. I adjusted it and MAN what a difference. The power of the motor was greatly increased and I only had one backfire pop on my test drive. Clearly I still have some tuning and searching to do but things are looking up.

Thanks all,
Carl

 

That's good news Carl,

I'll bet if you back out (richen up) your fuel screw a half turn or so you might be done with the backfire. You may have to adjust your idle abit afterwards. There's lots of stuff out there on these old carbs.

 

The basic timing on any of the stovebolts is pointer and ball. Then you use the octane selector tab to fine tune it. By the way that tab has degrees marked on it. Set it to zero first then loosen the dist. clamping bolt to move the dist. to get the pointer and ball lined up. All of this done with the vac. advance disconected.