502 backfire at 3000 rpm

« **on:** May 13, 2014, 06:19:13 PM »

Hi all, sorry i missed half of you all sat at avi, we drove up while most went north to lazy harrys... but i digress.

my issue is with the rebuilt 502 in my 210 hallet, 850 dp holley twin powervalves 6.5, jet 71p, 75s, mild rv cam. compression is 150 all within 2 points.
checked all wires 3x. new cap n rotor.

when we start out at idle it is fine sounds great feels great. when we get on it to get to plane is shudders abit then at 3000rpm it back fires.
any one with some input would be great,
ps thanks miller n john for the help this weekend!!

« **Reply #1 on:** May 13, 2014, 06:29:16 PM »

Quote from: havasuman on May 13, 2014, 06:19:13 PM

when we get on it to get to plane is shudders abit then at 3000rpm it back fires.

Backfires through carb or exhaust? Single or dual plane intake? This is a square bore carb and not a spreadbore?

« **Reply #2 on:** May 13, 2014, 06:44:43 PM »

Did it just start doing this if so what changed tune up new carb new ignition

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« **Reply #3 on:** May 13, 2014, 07:21:51 PM »

carb is a square bore 4150 holley model, on a dual plane air gap manifold, jim guthrie rebuilt the engine and put it back together, i checked the timing and it is at 32* at 2000 rpm
i noticed with the 750 proform dp prior there was an exhaust popping late detonation in the manifold and now with the 850 it is backfireing through carb at load around 3000rpm.

« **Reply #4 on:** May 13, 2014, 07:24:10 PM »

also after checking compression i noticed the header bolts were bottomed out on half of the manifold so there was alot of carbon soot all over the wires and engine bay.plugs are also very black.

« **Reply #5 on:** May 15, 2014, 02:26:15 AM »

Quote from: havasuman on May 13, 2014, 07:21:51 PM

carb is a square bore 4150 holley model, on a dual plane air gap manifold, jim guthrie rebuilt the engine and put it back together, i checked the timing and it is at 32* at 2000 rpm
i noticed with the 750 proform dp prior there was an exhaust popping late detonation in the manifold and now with the 850 it is backfireing through carb at load around 3000rpm.

Going to throw out a guess and say you are too lean. Those jets seem a little small to me, but this is purely a guess without seeing it. What do the plugs look like?

Quote from: havasuman on May 13, 2014, 07:21:51 PM

i noticed with the 750 proform dp prior there was an exhaust popping late detonation in the manifold and now with the 850 it is backfireing through carb at load around 3000rpm.

This sentence has got me a little confused. If I am interpreting this correctly, sticking valves?

« **Reply #6 on:** May 15, 2014, 10:03:33 AM »

Check to see if u have a good seal around the bottom of the crab

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« **Reply #7 on:** May 15, 2014, 12:33:51 PM »

does it do this wide open too?maybe too lean,
Bump the jets 74 76

« **Reply #8 on:** May 15, 2014, 02:34:57 PM »

Quote from: jyeager on May 15, 2014, 12:33:51 PM

does it do this wide open too?maybe too lean,
Bump the jets 74 76

. Im not a bbc mechanically inclined guy so my uncle handles all my motor issues for my boat but I build 2 strokes for fun that produce a good amount of power and when I tune them and there popping it's because there too lean!! I agree with jake maybe go bigger jetting see how she reacts.

SRC

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« **Reply #9 on:** May 15, 2014, 04:32:12 PM »

Quote from: havasuman on May 13, 2014, 06:19:13 PM

Hi all, sorry i missed half of you all sat at avi, we drove up while most went north to lazy harrys... but i digress.

my issue is with the rebuilt 502 in my 210 hallet, 850 dp holley twin powervalves 6.5, jet 71p, 75s, mild rv cam. compression is 150 all within 2 points.
checked all wires 3x. new cap n rotor.

when we start out at idle it is fine sounds great feels great. when we get on it to get to plane is shudders abit then at 3000rpm it back fires.
any one with some input would be great,
ps thanks miller n john for the help this weekend!!

It shudders then backfires? Is this "shudder" a misfire?

How long since the motor was gone through? Is it a flat tappet cam or a roller? If its a flat tappet, was a proper cam break in done and was zinc additive used?

Only reason I ask is because these symptoms sound a lot like the cam might have went flat. Jmo

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« **Reply #10 on:** May 15, 2014, 10:36:58 PM »

Mine kinda did something like that last year at the big river get together. Thought it was the power valve. Started, ran fine til about 3k rpm or so then suffers and pops thru carb. I thought it may be power valve. Tried thant and it wasn't, cap and rotor, no. Plugs, nope. I fought it for w few weeks and finally found it. It as the positive battery cable loose on the starter. I felt so stupid but sometimes its then little thimgs

« **Reply #11 on:** May 18, 2014, 04:54:39 PM »

thanks for all the input, just back from jim guthrie who built the motor and said like several of you here, way to lean, he gave me the new jets to run and another set of powervalves, (83,85, 6.5pv) the plugs were initially sooty from the previous carb which ran very rich.
i will get a new set of plugs in it this week and check it out.
going to work on it mon night at river, there till sun or mon.thanks again, i hope this will do it!!!

« **Reply #12 on:** May 18, 2014, 05:24:36 PM »

Hopefully that's the issue. We should be in the hood on Thursday.

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« **Reply #13 on:** May 18, 2014, 06:31:13 PM »

Check your water fuel separator and make sure it isn't full too. That happened to me before and was the issue.

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havasuman

502 backfire at 3000 rpm

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