popping out of carb

[windryder]

Help!!! 1966 Ford F-100,302 V-8 bought rebuilt in 1995 from Pep boys.Three weeks ago I replaced the orig.2 barrel carb and intake with a Edelbrock performer 289 intake,and a Edelbrock 1405 carburetor.Truck ran great for 2 1/2 weeks(use to drive to work daily,25miles a day)Out of the blue it started popping out of the carb.(machine gunning!)when I open it up passed half trottle! I've put points,cond,cap,rotor,new distribitor,coil,wires,cleaned& gapped plugs.Pulled intake and installed new gaskets,called Edelbrock,told to change set-up springs to the pink set,I did!Compression test results are:#1-90psi,#2-126,#3-105,#4-100,#5-105,#6-118,#7-114,#8-112psi. Vacuum gauge jumps from below 0 to 20Hg when cold(bounces fast 0-20 back & forth)when it warms up it steadys out to 19Hg.I've timed it 20 times or more.No matter were I set timing it doesn't fix the pop.There is only one vacuum line(vacuum advance)it's good.all vacuum ports are capped.I've tryed carb clearner,starting fluid,propane,all with no sign of intake leak.What do I do? HELP

[peppermrj]

Sounds like you checked all the easy stuff.The vacuum gauge readings would indicate a burnt valve or bad valve clearance according to the book. I would start with #1 since that is where your compression is low. Bring it toTDC and blow air into the cylinder thru the plug hole. Then listen at the carb for the hiss. My "guess" is you might have had a vacuum leak on #1 and fryed the intake valve. Hydralic lifters aren't really adjustable except for adjusting the rocker. I don't know that they could cause the valve to hang open.

The only other things would be a bad head gasket,cracked head,or bad timing chain/gears. All of which have other symptoms that go along with them.
Good luck and let us know how you make out.

[windryder]

Thanks I'll do the leak off test and see what happens..So a leaking intake gasket will burn a intake valve? Will the same happen do to a mass prouduced overhauled motor? I've heard that engines that you buy from Big national chain parts stores are rebuilt replacing only parts that are bad,ie: if a core engine has a bad head,only the head is replaced,then it's started to see if it runs.Then painted and sold with a warranty?Has anyone heard that?

[peppermrj]

Running lean raises combustion chamber temps. Overtime it will take its toll. I bought a rebuild from parts america for a dodge. 3.0 mitsibishe 6. i know they redid the both heads at least. No smoke. But it did suck in a head gasket after about 20,000 miles.

[windryder]

Well I did the leak test.I adapted my compression gauge hose and put air in # 1 cylinder.No air from carb.!Put hose to dip stick tube and the other end to my ear.shot air into cylinder,Heard air from tube.Did compression test again.#1 was at 80 psi(10 lower then before)Put a little oil into spark plug hole and rechecked.It went to105psi.I did same to #2 - #3 making sure valves were closed.#2 had 120psi,w/oil 160psi.Same for #3!! Could bad rings cause the popping through carb.? Not that it matters now,but I'm still just curious!!

[BCMedic]

Originally Posted by windryder

Well I did the leak test.I adapted my compression gauge hose and put air in # 1 cylinder.No air from carb.!Put hose to dip stick tube and the other end to my ear.shot air into cylinder,Heard air from tube.Did compression test again.#1 was at 80 psi(10 lower then before)Put a little oil into spark plug hole and rechecked.It went to105psi.I did same to #2 - #3 making sure valves were closed.#2 had 120psi,w/oil 160psi.Same for #3!! Could bad rings cause the popping through carb.? Not that it matters now,but I'm still just curious!!

Rings are way overdue, you go from 80-160 PSI, I would be curious as to the cylinder wear and valve condition as well.
For now though, have you checked or replaced the timing chain lately? Pull the dist. cap and rotate the crank slowly both ways with a big socket and see how much slack you have.

[windryder]

This Truck is about to drive me mad!! The engine runs so sweet!It will start cold with 3 turns of the starter.It idles smooth,There's no smoke out of tailpipe,It doesn't use oil!The only problem with it is that damn "pop-pop-pop"out of the carb.Is it time for a Chevy?(LOL)

[peppermrj]

Backfiring thru the carb happens when an ignition source (flame) meets the incoming fuel/air charge. The flame is coming from a cylinder, thru the intake valve, into the intake manifold, and finally out the carb. this happens for a number of different reasons.
1 Spark occurs before the intake valve closes.Why.
a The spark timing is off. (spark early)
1 incorrect initial setting
2 faulty vacuum advance
3 burnt or worn points
4 improper dwell
5 bad condensor
6 improper firing order
7 worn loose or incorrectly positioned distributor
8 bad or worn distributor gears (on distributor or cam)
b The cam timing is off. (valve opening late)
1 Bad or loose timing chain
2 Worn crankshaft timing gear
3 worn camshaft timing gear

2 Intake valve does not seal. Why.
a excess carbon build up
b bad valve seat
c worn or burnt valve face
d bent valve
e sticking valve
f weak valve springs

3 Clynder leaks into adjoining cylinder. Why.
a blown head gasket
b cracked head

I would start with #1 since that is where your compression is low.

Key word is start. You may have a leaking valve on any of the cylinders. How much psi are you using. 100 lbs is optimal. Be cautious that if the engine spins nothing (hands/fingers) are anywhere they can get hurt.

Sorry for the long list. That is how i organize my brain. I know you have gone over a lot of these already.
My own experience with cam timing has been if the timing chain/gears slip or the distributor is a tooth off The popping is almost constant.
Good Hunting.

[peppermrj]

forgot to mention that blowby into the crankase is normal to a degree. If I remember correctly, when dueing a leak down test, up to 5%=new, up to 10%=good, and 20%=:o(

[rangerreb]

Try checking for a weak or broken valve spring and possibly even a bad cam shaft.

[highboy73]

you said the vac advance was hooked up...to the right vac port?
have you lift?might be a bad lobe.

[steve longo]

Originally Posted by windryder

This Truck is about to drive me mad!! The engine runs so sweet!It will start cold with 3 turns of the starter.It idles smooth,There's no smoke out of tailpipe,It doesn't use oil!The only problem with it is that damn "pop-pop-pop"out of the carb.Is it time for a Chevy?(LOL)

Check and see if you have a bent push rod or broken rocker arm.If the cam was not broken in correctly,High idle 2200 rpm for twenty minutes the cam could have gone flat. Also bring that cylinder to tdc compression stroke and on the heel of the cam, Loosen the rocker arm and see how much preload each one has till you can feel just as the push rod is about to cease contact between the lifter and rocker. Did you put in a higher lift cam than reasonalbly able because if you did you may have contacted the piston top with the valves and possibly bent a push rod or broken a rocker arm. As for the low compression that may mean the above descriptions ,the cam is too big and possibly the engine was not assembled correctly, but thats going to the worst senario. I hope this helps you out. The more info you can supply the forum the better we can help