1979 470 backfire at idle

[Rinker captiva 206]

Did an engine transplant with a 470. Ran the engine in the previous boat earlier this spring and engine ran very well. Finally got the engine in the other boat and started it today. It had a hard time starting and when it did it was backfiring out the carb like crazy. When I gave the engine more throttle, it seemed to smooth out. But when I back it down, it backfires again. We are running from a gas can. We rebuilt the carb last fall and it ran great in the other boat. I don't think it is a fuel issue. A almost think that spark is jumping wires, from what I've read on another post. I checked the wires for correct firing order and they look correct. The spark plugs were new and after taking them out they are black in color, like carbon build up. The number 4 cylinder plug looked good light brown in color. Any thoughts about the plug wires? Would a weak ground from the battery to engine cause this problem? I ask this because it previously had a ground to the starter and a strap from the starter to the block itself.

 

[achris]

Did you check the timiing?

 

[Rinker captiva 206]

I did not check the timing because I did not take the engine apart at all, I left everything on, even wires. I did check to make sure when the harmonic balancer is at 0 degrees, the rotor is pointed toward cylinder 1. I also made sure the firing order was correct.

 

[stonyloam]

Do a compression check. The #4 cylinder being a lighter color may indicate that the head gasket could be failing and some coolant is getting in there. A common problem with 470s. In the meantime you could try pulling the plug wires one at a time while at idle to see if you have a weak cylinder. Mine backfired when the head gasket failed at #4.

 

[Rinker captiva 206]

I will do a compression test today. I did a compression test last fall and it was all good. I can't remember what the numbers were, but they were all within 5 psi.

 

[stonyloam]

What causes a backfire out of the carb? For that to happen the intake valve must be open when fuel is ignited. Timing that is too far advanced, or a plug wire mixup will do it. A intake valve that is stuck open could do it. A bad head gasket that lets hot gasses in from an adjacent cylinder could do it. A too lean condition could also cause it to backfire or spit through the carb.

 

[kenny nunez]

Also possible that one of the exhaust valves may not be opening, stuck valve, bent push rod, broken rocker arm or worn cam lobe.

 

[Rinker captiva 206]

Got into the engine today. Found out the intake and exhaust pushrods on number 1 cylinder were bent. Exhaust valve was rusted stuck. Not sure how it ran so well before. Measured a good pushrod at 8.615 in.

 

[achris]

Both on the same cylinder could indicate water. Although it's usually #4 that suffers from that.

 

[kenny nunez]

With a little luck it may be possible to free up the valves. Try to squirt some penetrating oil under the valve seal or just take a pick and rip it out. With a brass drift try to drive the valve down, some times the valve will "pop" back up. If that starts to happen put the piston @ tdc and remove the valve spring. Chuck the valve stem in a drill and spin it while adding penetrating oil. Replace the valve stem seals and the springs. This is a long shot but I have done this several times with success.

 

[stonyloam]

It is a 470 and valve lash is set by pushrods length, so your pushrods may be different lengths, so do not mix them up. You should be able to get new pushrods from your local Ford dealer. They are the same as used in a 460 V8.

 

[Rinker captiva 206]

Instead of messing with different size pushrods, would It be possible to get an adjustable rocker arm kit for a ford 460?

 

[kenny nunez]

Summit or Jegs has adjustable rocker arm conversions but you may have to buy the whole set and will need a regular 460 valve cover for the clearance. Just keep it simple and get new pushrods.

 

[stonyloam]

This may help, lengths are listed on chart. http://forums.iboats.com/forum/engi...-valve-adjustment-on-a-mercruiser-3-7-aka-470 See if there is any color code left on them, or streighten and measure as best you can.