Strange SBC problem.

I've got a 307 motor stock except for the Eddlebrock intake and Carb. It's got no power at all and when the motor winds out it pops back through the carb. The compression is good and the vacuum is good. The timing is set at 2btdc, the timing mark is correct, vacuum advance is working. I have a friend who is an old mechanic, he checked the exhaust for hot spots and found that the passenger side was 600 degrees. I removed the heat riser, now it doesn't get hot. But it's still poping back through the carb.

The mechanic has no idea whats going on and in 50 years of working on cars he has never seen anything like it. I'm gonna assume the motors haunted at this point. Has anyone ever seen something similar?

 

If an exhaust cam lobe is worn off it will pop back through the carb on revs....check the rocker arm lift while cranking.

 

And I believe, I read somewhere that original 307 engines were notorious for having bad camshafts in them. One of our experts on here can confirm it........

 

with the vac advance dis connected how mch total timing are you getting at say 3000 rpms

 

2 degrees BTDC??? Dang!! How about advancing it another 6 degrees for a total of 8 BTDC.....

 

Agreed!!!!!! even more if it can use it.

At first I was thinking flat cam but then I read on to see 2 btdc. Try it out and you might find that is the issue. you have nothing to lose except about 2 minutes.

Keep us posted!

 

i started in a gm dealership in 1976 , we put in a shit load of camshfts in 307 motors . i would bet you a 12 pack you have a lobe going flat . easy enough to pull the v-covers and have a peek .

 

Do you mean 8 degrees before or after TDC?

 

before. it probably wouldn't even start at 8 degrees after.
and check for cam wear.

 

Be sure your timing mark and timing chain cover are correct. Put #1 at TDC and check the mark. Pull the cap and rotor and check your timing counter weights. They may be flopping around with worn pins and/or broken springs.

 

Get that sucker timed to at least 10 degrees before top dead center! It is so retarded that it is firing too far down in the hole, hence the high exhaust temps and no power.

 

i've only seen 1 haunted motor before [very rare]. check the small wire inside the distributor also i had the exact thing happen when the customer put a set of points in that the return spring fell out of the box. this caused point float at higher rpms that wasn't a problem at idle.

 

Ok, so what's the verdict???... Did you git'er done yet?? How's it runnin'???

 

Gee I got mine static timed @ 12 degrees BTC just like all my other motors, I hope that's going to be OK.

If screwing with the timing and looking at the lobe lift on the cam doesn't work out you may suspect that there is a valve or two that needs attention. Sometimes a valve job can work wonders on an old worn engine.

 

Gee, maybe that's why I said "At Least 10 degrees ....)

 

Took mine to dealer back in the day, the mechanic got in and power braked it and it was popping like crazy through the carb, He puts it in Park steps out and say "time for a new cam" I did it myself and on lobe was GONE!

 

Agree...more than likely the cam. usually around the 3rd or 4th lobe from the fuel pump lobe. had quite a pile of them under the bench.

 

Xinfinityon the cam... If that's NOT it, you may have a dead or dying intake valve spring.

 

He beat me to it and make sure that the vacuum advance is working.

Popping back can be caused by a bad cam or it can be the result of a weak accelerator pump among other things.

 

Also on another note I hope you changed the nylon steel cam gear ,,,Do a compression and that should tell you what cylinders are the problem.....

 

I'm pretty sure 8 degrees before top dead center was the stock setting on the little 307... I'd check the books or the net just to be sure... Once the timing is set, re-adjust the idle mixture screws on the carb for the highest reading and also the rpm with a vacuum gauge and a tach.... This could all be done within a half hour including re-checking everything you did and then take it fer a ride to see how she feels... ..... I like to do it with perfection!

 

Check if the outer ring slipped on the harmonic balancer, this would throw your timing marks way off . If not thens it's a soft cam.

 

I pulled the valve covers today, all the rockers are moving like they should. Does that mean that the cam is not worn? I also adjusted the rockers, no change.

The stock ignition timing for a 307 is 2 degrees BTDC at idle with the vacuum advance disconnected. I tried setting the timing a few degrees advanced and a few degrees retarded from the stock position, and in both cases the car ran worse.

I have no clue what else it could be.

 

Before we get to far down the road tearing this thing apart, let's install a new set of plugs. I chased a problem like this a while back and it was two bad plugs in a box of eight. If not, at least, put your timing light on each cylinder to see if it lights. But as long as the covers are off, check cam lift for a bad lobe. That was an issue with these motors.

 

I always set mine at 36-38 total. That's all that matters anyway.

 

How many miles on this motor? If it is high mileage, it needs a timing chain & gears. Does your timing mark jump around with the light on it?

 

Check the firing order leads to the wrong cylinders can do stuff like this.

 

Firing order is right, timing chain is new, 63k on the motor. The timing mark stays dead stable. Could bad plugs cause no power and poping through the carb but still run well at idle and with no load on the motor?

 

Yes.

 

Popping through the carburetor can be caused by many things. Bad or sticky valves, bad distributor cap, bad plug wires (I have had new ones that were leaky), excessive plug gap, I put in a new set of plugs once that would not fire under pressure. Condensation in the distributor cap will do it.