Backfire through carb under load

rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: Backfire through carb under load

sounds like an accelerator pump issue to me.<br /> either the pump arm is not in the correct hole or the check ball in the passage just beside the needle valve tower under the screw is not seating or the vents for the pump passages in the airvalve assy is clogged. if it only backfires on rapid throttle opening but not during a steady speed I would double check the accelerator pump circuits.<br /> on rapid throttle opening that engine requires a vary large shot of extra fuel or it goes "pop".
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Not sure if this helps, but when I was in the water it would pop running at a steady 2000rpm, which is a much more advanced throttle position obviously than in the drive. <br /><br />All accelorator pump circutes are totally clean, I blew all those out this morning, and checked and tripple checked that all flowed clean.<br /><br />Upon quick depress of plunger you can see 2 jets pumping into the carb.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Backfire through carb under load

Mahoney,<br /><br />454 carb on 350 engine.... Unless the jets were changed then you could run into any number of 'incorrect mixture' problems.<br /><br />But it does sound like the accelerator pump is working correctly.<br /><br />Chris...........
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I am going to do some more research and when the rain lets up for a bit, pull the pump, get number, also get number of engine, and do a follow up post with all part numbers to make sure I refit with correct parts.<br /><br />The rain coming back gave me some time, so I think best route since I have no idea what the mechanical history of the boat is, and the fact it was sitting for years, is to replace both pump and carb. Boat was free so this is fine. Good news is the drive worked like a charm in forward and reverse, with no water in gear oil.<br /><br />Before I realized that since the pump housing was cracked, all water pressure and much of the water pressure when engine was running was directing pressure right up through the water pump housing against the seals instead of out to the engine cooling system, contributing to water getting past seals.<br /><br />Thanks to all who helped me in my previous post on that fix it too. Ill post more pics of parts when I get to the boat, so you knowladgable people can point me in the right direction ordering.
 

Reel Poor

Vice Admiral
Joined
Jan 29, 2005
Messages
5,522
Re: Backfire through carb under load

Have you checked the condition of distributor cap and rotor, point dwell?
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Ingnition was all converted with electronic ignition, new nkg v power plugs and new wires. I tested both with new coil purchased from cross referencing original when I thought that might be the problem. Also switched back to original coil, and same popping under load. Tested with different degrees of advance timing due to unleaded gas as recommended in manual, with no change. mechanical advance is working properly(no stuck wieghts) I think we nailed it to the fuel system, so I am going from there.<br /><br />Thanks
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Ha, this post is getting huge! Good reading I hope for people, although the plot thickens. Apperantly through research, I am seeing that this engine(can't find serial # anywhere it should be) might be a ~1967 L48 Corvette/Camero Block.<br />Intake stamp:348250<br />Water Pully Stamp:3927797<br />Crankshaft Stamp:3755820<br /><br />When checking all other V8 fuel pressure numbers above 200hp, it does say 4 - 5.75, which would be within range for the pump.<br /><br />This is obviously a repower for this boat, and I am not sure on the specifics. It is painted that OMC blue and contains valve covers with 225 decals.<br /><br />Any suggestions on this?
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Backfire through carb under load

Find the Block Casting #s....<br />Or, the Block Stamped #s.....<br />Either or both of these are a much Better indicator of What you really have.........<br /><br />The Casting #s are at the very Back of the Port Head, On the Block, at the bellhousing flange......<br /><br />The Stamped #s are at the Leading Edge of the Starboard head, In the Block, on the Deck surface.........<br /><br />With Those #s, Go Here,...... To See What You Have...........
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

I was looking where the manual told me to look and they were not there, but I did some more research and it did say to look where you indicated Bondo.<br /><br />Whats the difference between a casting number and a stamped number? <br /><br />I am anxious to get to the bottom of this, so I am going to brave the rain again and get in there with the flashlight!
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Backfire through carb under load

The Casting #s will indicate What the Block is,+ What is was built for..........<br /><br />The Stamped #s will tell you What the motor was doing when it Left Chevolet(What car, etc.)
 

weatherz

Seaman
Joined
Oct 9, 2004
Messages
56
Re: Backfire through carb under load

Mahoney, I have seen SBC,s that had the same symptoms as yours (backfire under load) that turned out to be two plug wires crossed (5 going to six and six going to five) and since you said the plug wires had been changed...just a thought, and easy to verify the firing order. good luck
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

OK, found some numbers <br /><br />port side bow cast into head:8127<br /><br />starbord side bow cast into block:<br />:W 430084 and another right after it<br />:S11294U<br /><br />I am working on decoding those now.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Backfire through carb under load

The Head Casting #s, Don't have enough digits......<br />The #s cast into the Starboard side Aren't the 1s you need Either.......<br /><br />My previuos post explains Where to Look......<br /><br />All the other #s are No Good......<br /><br />This IS a Chevy based motor isn't it ?????
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Boy this is getting crazy, it is a chevy engine. I am going to take a pic of all this stuff when I get back in there. Those numbers I reported are excactly what is stamped on the head, those were easy to read. I could not find any numbers on the back of the engine, but will look again.<br /><br />About the only number I was able to cross reference was the crankcase pully wheel stamp, so far nothing for the engine. <br /><br />Thanks for sticking with me Bondo, I am about all googled out on these engine identification codes!
 

rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: Backfire through carb under load

hello<br /> look on the pad on the block deck just above the fuel pump, on the front of the block just below the stbd side cyl head. should be a series of numbers stamped in the machined block pad. if you place your skinners on the stbd side front head and move them down you will hit the pad.
 

rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: Backfire through carb under load

hello a broken valvespring and or a sticking valve will do the same. 2000 is a bit low for lean popping. try to find an itake vacum port and it will lead ya to a valve or spring if its bad. a vacum gauge on a carbed motor is invaluable if you study what the readings mean.
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

rodbolt, the numbers I provided in the post a couple strings back are the numbers above the fuel pump and don't seem to cross reference with anything. <br /><br />I did a vacuum test and everything checked out, pretty steady idle at 15lbs. Nothing to indicate sticking valve.<br /><br />When it popped at a little over 2000 rmp, it was under sustained heavy throttle load and not on plane.
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Well its been repowered with a GM 307 Truck block apperantly. GM 3970020. So now I have to look for engine specifications for that, because I am sure my OMC manual does not take that into consideration.
 

Mahoney

Chief Petty Officer
Joined
Aug 2, 2004
Messages
537
Re: Backfire through carb under load

Wow, what a deboccle I am running into.<br /><br />Ok, cross referencing the date code its a 1973 GM 307. All parts listed for this in an automotive application indicate a 2 brl Rochester Carb. Its got a 4 brl quad on it right now. I have no idea what was going on with it, but have been assured from my friend, who gave me the boat, it ran fine prior to storage.<br /><br />Should I be looking for parts indicated for a 307 truck engine, or replace with what is on there?<br /><br />Confused for real now!<br />Shawn
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Backfire through carb under load

Should I be looking for parts indicated for a 307 truck engine, or replace with what is on there?
GM 3970020 indicates a 307 truck block, with 2 bolt mains...... That's ALL it indicates.......<br />That Doesn't mean it's Not the Original OMC motor........ The 225hp badge was hung on the 307s.... I've got 1 sitting out in the shop Now.......With a 4bbl. Carb......<br /><br />Look on This Page , It'll Show you Exactly Where the Right #s Are........<br /><br />I Wouldn't go Replacing Anything.....<br /><br />I Think you've got the Original Motor,+ It's Own Parts.......<br /><br />Looking down on your heads, there Should be Another Casting #,.. With At Least 6 digits... It'll be in-between the Valve Springs.....<br />I Believe you found the #s that you did, Between the 2nd,+ 3rd valve springs... Look on the opposite end of the head, between the 2nd,+ 3rd valves on That End,...... Is there a # There ????
 
Top