Backfire at idle

Engines, Shafts, Steering, Struts, Rudders, etc.
Post Reply
Rockxx
Aluminum Star
Posts: 107
Joined: Fri Mar 06, 2015 12:39 pm

Backfire at idle

Post by Rockxx »

I Had a dead miss a few days ago and found that # 1 cylinder had a bad plug wire. I replaced the wire and replaced the plug and this fixed the miss. Took a short cruise, boat ran fine. Started the engine at the dock this past Friday and immediately had intermittent backfire tru the port exhaust, mostly at idle. More of a "pop" than a "bang". Pulled the new plug and it seemed fine, filed the points as I didn't have a replacement set on the boat. Wasn't going to waste the Labor Day holiday so took it out anyway. Cruised for an hour at 2000 rpm with no backfire, idled back in ok. Went out again this morning and upon starting got the same problem but once again, straightened out while cruising. Anybody got any suggestions for me.
Thanks,
Rockxx
1972 28' express "Safari"
Single Chrysler 318
Center Hill Lake, Tennessee
Rockxx
Aluminum Star
Posts: 107
Joined: Fri Mar 06, 2015 12:39 pm

Re: Backfire at idle

Post by Rockxx »

1972 28' Express
Single 318
1972 28' express "Safari"
Single Chrysler 318
Center Hill Lake, Tennessee
Fastjeff
Site Admin
Posts: 1521
Joined: Sat Jul 12, 2014 5:06 am
Location: Rock Halll, MD

Re: Backfire at idle

Post by Fastjeff »

The usual reason is crossed wires (wires on the wrong plug, which is EASY to do the dopey way they ran the wires). Alternately, the spark can be jumping from one wire to the next. (Old wires whose insulation properties have broken down, will do that.) Separating them with 'snake skin' can temporarily eliminate that problem, but new wires are still needed.

On my old 32 footer, the marina "mechanics" (doing a compression test for the insurance company--long story) crossed a pair of wires. The motor started okay--it even idled moderately decent--but popped through the carb on plane off. (Yes, it effortlessly planned off.) I went nuts before discovering what was wrong, and this is how I did it:

Using insulated gloves and pliers with rubber grips, I pulled each plug wire out of the distributor with the engine at idle. When the motor slowed down I put the wire back in. When it didn't--on two wires--I reversed them and the motor picked up speed. Ah-hah!

Jeff
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington
Rockxx
Aluminum Star
Posts: 107
Joined: Fri Mar 06, 2015 12:39 pm

Re: Backfire at idle

Post by Rockxx »

Thanks Jeff. I'll probably start with a new set of plug wires since I've already had one go bad and go from there.
1972 28' express "Safari"
Single Chrysler 318
Center Hill Lake, Tennessee
martindesign
Aluminum Star
Posts: 104
Joined: Sun Mar 29, 2015 2:07 pm

Re: Backfire at idle

Post by martindesign »

Do a cap and rotor while ur in there.
Machanic, fabricator, carpenter, plumber, electrician, designer, hotrod builder and glutton for punishment. :ugeek: current boat; 1969 Marinette 32 express bridge 440's
Catawba Oh
User avatar
bill
Royal Aluminum Star
Posts: 520
Joined: Sat Jul 12, 2014 7:17 am
Location: Grosse Ile MI

Re: Backfire at idle

Post by bill »

BigM
Also when you have the cap off do the fastjeff flick :D on the rotar to make sure the springs in the distributor are working and not rusted solid. :roll:
bill


Oh and add a signature when you get a chance :mrgreen:
Former owner of
ALUMINATION
Grosse Ile MI
Located on LakeErie
37' F/B Sedan
1975
Twin 360 Chrysler Marine
Raw water cooled
Hydraulic steering both helms
USCG Master Lic. Retired[/color]
Third Owner bill
User avatar
Busia
Royal Aluminum Star
Posts: 406
Joined: Tue Jul 15, 2014 12:21 am

Re: Backfire at idle

Post by Busia »

Check the plug wires go to the correct plug. Check that the rotor is aimed at the nipple for that wire when it fires. ( it can be making the spark when it is not lined up ) check the timing and the advance. If want to, you can make any nipple you want on the distributor be #1. Just make sure #1 is on the compression stroke, not exhaust. Set the crank on the timing mark. Turn the crank in the direction it runs. If you go past the mark, back up way past the mark and come up to it again. ( this is to take up backlash -slop- in the gears. Makes it more accurate ) If the rotor doesn't line up with a wire just pull it out and move it one tooth over when you put it back in and mesh the gears. All pretty simple when you understand it. Secret is you have a 12 volt circuit that makes the spark at the right time. It is the primary or low voltage side of the coil. Then you have the secondary side of the coil that is about 20,000 volts. One end is grounded inside the coil the other comes out the top of the coil, is jumpered over to the distributor cap, goes to the center of the rotor and should be aimed right at a nipple with a plug wire in it. Then you take that plug wire to the correct plug. Put the plug wires in the right firing order. One secret to know which way the rotor turns is that you can turn it a little ways in the correct direction, but not at all backwards. This is because of backlash in the gears. Good luck, Ed
BUSIA
located in Ketchikan, Alaska. Gods country
32 foot Marinette (no fly bridge)
twin 350 Crusader (Chev 350) engines
1:1 Borg Warner velvet drive transmissions
Closed cooling (antifreeze in the engine)
Proud to be retired IBEW and an A+P
TheScottFraser
Member
Posts: 11
Joined: Sat Jan 24, 2015 2:49 pm

Re: Backfire at idle

Post by TheScottFraser »

Fastjeff wrote:The usual reason is crossed wires (wires on the wrong plug, which is EASY to do the dopey way they ran the wires). Alternately, the spark can be jumping from one wire to the next. (Old wires whose insulation properties have broken down, will do that.) Separating them with 'snake skin' can temporarily eliminate that problem, but new wires are still needed.

On my old 32 footer, the marina "mechanics" (doing a compression test for the insurance company--long story) crossed a pair of wires. The motor started okay--it even idled moderately decent--but popped through the carb on plane off. (Yes, it effortlessly planned off.) I went nuts before discovering what was wrong, and this is how I did it:

Using insulated gloves and pliers with rubber grips, I pulled each plug wire out of the distributor with the engine at idle. When the motor slowed down I put the wire back in. When it didn't--on two wires--I reversed them and the motor picked up speed. Ah-hah!

Jeff
it is a good idea to tag your wires also so you always put them back in the correct order. A auto racing tip i carry over to all my multi-cylinder toys is to put cable zip ties on the plug wire. One tie for number one, three for three, five for number five. you get the picture. This way you can yank off all your wires and there is no way to get the order wrong. Home depot or your local hardware store sells them by the bag and they are very versatile to have on board. I use the mini ones for this application and to keep lines coiled neatly. As an added bonus the mini ones can be broken by hand if i need a line fast. As far as the backfireing, time for a full tuneup. plugs,wires, rotor, points, think about an upgrade to electronic ignition. If you dont own a good timing light get one. it is possible your distributor is loose and your timing has slipped. it is also really easy to check and adjust. Let us know what does the trick.
1985 marinette 39AC
1988 Oday 272 sailboat
1986 Boston whaler 13'
And more motorcycles then could ever be deamed appropriate.
The Captain and I live in woodstock, Ga.
Our boat is on Lake Lanier
Post Reply