Please help me!- wit's end.

[SeanPaintsFlames]

I really hope somebody can help me out and point me in the right direction. I've got this 65 GMC. It's a piece of junk. But, I've been trying to successfully rebuild the 250 straight six in it. After taking everything but the shortblock apart and replacing or rebuilding it all, it was running okay other than a high idle and some off-idle hesitation. So, to fix the idle issues I replaced the carb with a remanufactured one, and now it has a really really (really!) wicked backfire out the exhaust (or after-firing, so I've heard). Now, I know that cherry bombs are known for that engine-braking pop, but this is retardedly stupid. I have the split headers collected into a pair of cherry bombs and that's it.
If I'm not WOT, it's backfiring. I'm talking fire-out-the- exhaust popping. If I try to maintain a constant speed, it backfires on what sounds like all cylinders until I either let off the gas or put it to the floor. I've tried to adjust the timing, adjust the air/fuel, and no difference. I picked up a timing light today, and when I hooked it up, the timing mark on the crank pulley wasn't even close to the metal tab with the marks on it. It's weird 'cause I've put a good thousand miles on it since the tear down with nothing like this prob, but as soon as I get the new carb, this starts.
So you know, its a 250 Inline six with headers, HEI, and a single barrel carburetor.

Any thoughts?
Thanks so much guys!!

[1964C10]

I have no experience with inliners but on V8s sometimes people put the distributor in on the wrong marks, might that be the problem here? What about vacuum advance and the distributor itself? Are any of the plug wires maybe touching the block and have burnt through causing a short?

[MacAttack]

Originally Posted by 1964C10

I have no experience with inliners but on V8s sometimes people put the distributor in on the wrong marks, might that be the problem here?

^2 (I have limited wit - so it doesn't take very long to get to the end of it.)

Mac.

[padresag]

Originally Posted by MacAttack

^2 (I have limited wit - so it doesn't take very long to get to the end of it.)

Mac.

so does that make you a 1/2 wit?
ron

[MacAttack]

Originally Posted by padresag

so does that make you a 1/2 wit?
ron

HA HA That's funny......Wait, What!?

[fleetsidelarry]

sounds like somebody crossed the plug wires on you when you weren't looking. Seriously, if you didn't touch the distributor, just put the new carb on, and now its way off on timing, I gotta wonder. did it kickback big time when you first tried to start it? Distributor could have moved. Did you pull the cap to see if the rotor was broken? Also, I gotta ask, did you replace the cam and crank gears when you rebuilt the engine? Wait, (just re-read your post) I see you didn't redo the shortblock.

[slepysal]

Originally Posted by fleetsidelarry

sounds like somebody crossed the plug wires on you when you weren't looking. Seriously, if you didn't touch the distributor, just put the new carb on, and now its way off on timing, I gotta wonder. did it kickback big time when you first tried to start it? Distributor could have moved. Did you pull the cap to see if the rotor was broken? Also, I gotta ask, did you replace the cam and crank gears when you rebuilt the engine? Wait, (just re-read your post) I see you didn't redo the shortblock.

x2 Check the firing order

[Deryl]

Not an expert by any means, but I suggest you go back and start with the basics ( It can't hurt and it does't take that long). Verify that your cam and gears are set correctly at top dead center, then make sure your distributer shaft is set right. Then verify that your plug wires are in order. After that make sure you have no vacume leaks. If my Dad were alive that is what he would tell you to do (miss him every day).

Just last week a friend of mine had a small block chevy that he rebuilt under the close eye of a 30 year ASE certified mechanic, who I have complete confidence in, but it would'nt start. It never fired up, but he bent or broke all of the exhaust rods and bent all of the exhaust valves on a new pair of Edlebrock RPM heads. Turns out that it was not on TDC when they set the cam gears even though the cam and crank gears lined up "dot to dot". Goes back to what to daddy said "check and recheck EVERYTHING."

[lakeroadster]

Check the distributor cap. When I was sixteen (circa 1976) I had a '70 Nova with a 230 six banger. Driving it home from work one day it began doing exactly what you are describing.

After my Dad cused me up one side and down the other for "blowing up the engine" my grandfather discovered it was a cracked distributor cap!

thanks for the vote of confidence dad

[padresag]

make sure that it sucking air at the base of the carb as you say it got worse after changing it. manifolds are not leaking? did you mix up any of your plug wires. distr out a tooth?
check your valve clearances and or bent pushrod
ron

[1964C10]

You might want to check these sites for more info:

http://www.inliners.org
http://www.stovebolt.com

I think they will have more information on how to set this up right.

[bullington]

Popping out the carb is a timing issue 90% of the time. Get the rig to top dead center and check your timing. Verify your timing order first. Sticking your finger over the hole isn't going to cut it. There is a compression and exhaust stroke. Do you know which one you were on when you checked it the first time? You can pull the valve cover and see which valve is down to verify.


bullington

[Ham-er]

Yep.
You must make sure #1 is top dead center on the compression stroke!!

You will get some "air" on part of the exaust stroke, but timing wont be correct if you set the dist to #1 on the exaust stroke. SO make darn sure it is TDC on the compression stroke.

Then set the dstributor and also take a look at that "timing mark thingy". Minor slippage/wear of the rubber bonding ring in the balancer - or the wrong timing tab thingy bolted on.

Verify TDC! Then if the marks are not quite right remember that and adjust the distributor according.

[Vernski]

Hi 198+ if I read your 1st post correctly all was fine till you put a rebuilt carb on. Something got disturbed while changing out the carb. Either it's not adjusted right, a vacume leak, base gasket leaking. if all these are fine then it may be timing after it was fooled with or when was the HEI installed? The distributer could be 90* off have a qualified mech set it for you. There is another thing that will make the engine backfire, to thin off a gasket between the header and the head will make it leak and backfire, it could be just a crappy carb rebuild. Good luck---Vernski

[neto]

Have you done a compression check? Do you think maybe something fell in the intake when the carburator was changed? Maybe bent a valve? Was there more space between the intake and the base of the carburator on the old one? Sorry for the questions just thinking out loud. And the TDC as everyone has said must be on the money.

[Jimmy S.]

I bet you got a carb. with a bigger metering jet.

Did you keep the original carb? If you did, remove top of carb.'s to see the number stamp on the jets. If the numbers don't match,use the original jet in the remanufacture carb. See if it runs any better.


Jimmy

[SCOTI]

Keep it simple.....
Put the timing back where it was & put the old carb back on. See if the perf returns to where it was. The carb might be rebuilt.... it might not be rebuilt correctly.

[Alvin Robinson]

Sounds to me like you have a flat spot on the cam

[65Gregg]

Something else to consider, if you rev'd it real hard after you put the carb on it could have spun the outer ring on the balancer which will give you false timing readings no matter what you do. Don't even ask how i know this...