Rebuilt 1976 Backfires Through Carb.

[KRH1326]

Hi All,

I have rebuilt my 351W, Holley Carb and outdrive.
I followed all instructions that came with any parts or kits, and followed good practices that I could find in my Ford, Holley and OMC manuals. I took my time ( 1-1/2 years ) and tried to do everything "just right".

These are in a fairly heavy 24' 1976 Winner Flybridge Sportfisherman.
Carb is Holley 4 bbl, vac secs, elec choke.
I first ran and tuned the unit in my shop. I built a steel stringer mock up, and had all units together. I even built a steel drum of water for the outdrive to run in, including circulated cold water for the drive to draw from and a fan vented top to vent exhaust outside.

I ran the boat for the first time and developed this problem:
Backfire through carb at or above 3000 rpm.

It started just fine, idles down ok, temp never breaks 140 degrees, beautiful oil pressure. Timing is 10 btc, gap and dwell are right on, right out of the book.

What can cause this?

From my teenage years of old ford pickups, my experience is to suspect bad timing, though my numbers seem just right OR not enough fuel to air running down the beast's throat. I do have a slightly rough idle <700 rpm, but I think that is a different prob ( I may have a minute vacuum issue that I am trying to track down.)

I am starting to chase my own tail with this, and am looking for a fresh view point.

Any suggestions?
Thanks

 

[Bondo]

Re: Rebuilt 1976 Backfires Through Carb.

What can cause this?

Ayuh,.... It's going Lean,... Look to the Fuel Delivery System

 

[haulnazz15]

Re: Rebuilt 1976 Backfires Through Carb.

Also, you might verify your timing the old fashioned way by pulling the #1 spark plug and getting a screw driver to make sure you find 0-degrees TDC, then back it off to the 10-degree BTDC. That should elimintate that issue since the timing and distributer will be set. The rough idle may just need to have the idle screws adjusted a bit or a vacuum leak at the carb gasket.

I would get a fuel pressure gauge and put it in between the fuel pump and the carb to make sure you have the 6psi it needs (I think it's 6). Other than that, it seems that if you have spark to all cylinders you'll be good.

 

[MikDee]

Re: Rebuilt 1976 Backfires Through Carb.

Make sure your timing (centrifugal advance) is advancing as you open the throttle, retarded timing can cause a backfire also.

 

[havasuboatman]

Re: Rebuilt 1976 Backfires Through Carb.

I doubt it's the timing if it starts at 3000 rpms. I think Dons got you looking in the right direction. Check your float levels, and fuel pump output (volume).
Have these motors been sitting for any length of time, and/or well used?
I had me a jeep once that did this. I went thru the carb (4 times) checked and rechecked timing, fuel pump.
Turned out the valve springs were weak and my valves were floating at high rpms.

 

[KRH1326]

Re: Rebuilt 1976 Backfires Through Carb.

Great Suggestions!! Thanks for that help. A fresh point of view can really help, when frusteration comes a callin'.

BTW- Does anyone know what the actual names for these parts are called?
They are not referenced to or pictured at all in any of the 2 OMC manuals, OMC cd rom, Ford engine rebuilding, and several Holley books that I have referenced for this project.

1) Between the carb mounting flange and the intake manifold, there is a hollow block spacer. It has vacuum access, as there is a hose barb that comes out of it. That point is connected via rubber tubing to the PCV in the rear of the starb head.

2) Once the name of the above part is learned, might help with this. There is an extremely heavy gasket between this hollow block and the intake itself. This gasket must be heat resistant or retardant as it is about the same make up as an exhaust manifold gasket, meaning it has gasketing as well as a metallic side.

Everything I have just shows the carb along with a flange gasket (8R-1398), which would be the gasket that I used to mount carb flange to this hollow block. I had to reuse the heavy gasket (I cringed, but have had no other choice yet, untill I can identify it) between the hollow block and the intake manifold.

1976 OMC electric shift stringer 235 HP 351 winsor Holley list 7163 4bbl

 

[searay3]

Re: Rebuilt 1976 Backfires Through Carb.

Just an observation. Have not worked on a ford of that vintage...BUT....#1, the hose barb in the carb base plate/adapter. PCV? If that PCV is empty, it is sucking air and giving a lean condition....backfiring at hi rpm and bad idle...plug the barb off at the carb base plate and try again. Shouldn't the fuel pump overflow go to that barb?

 

[KRH1326]

Re: Rebuilt 1976 Backfires Through Carb.

The hose barb is ( i am not at boat right now) 3/8 I think. It is exact size of the PCV valve. The hose that connects is a preformed hose, meaning that it was shaped to the sweep and the down turn bend to make the connection to the valve in the gromet. But again, I have not found this illustrated in any reference materials that I have found.

My fuel pump doesn't have an overflow. Just an inlet port, outlet port and, what may have evolved into an overflow, a glass globe with a drain, with a tab inside that reads" if you see gas, diaphram has ruptured...blah blah blah"

 

[a70eliminator]

Re: Rebuilt 1976 Backfires Through Carb.

That carburetor spacer is for the PCV valve in your valve cover grommet. The popping through the carburetor most probable cause is too lean, the float setting on any holley carburetor has to be correct for it to work right, the power valve provides enrichment fuel off idle and you have to use the correct gasket or it won't work, some holley kits supply both style power valve gaskets, the ones with the triangle notches and the plain round ones. You mentioned rebuilding the carb and now your getting lean pops out the carb thats why I'm touching on the carb.