Backfires and Dies intermittently

H

hblair

Seaman Apprentice
Joined
Mar 26, 2010
Messages
39
I've got a cabin cruiser with an old 888 configuration running a ford 302.
I'll give all the info I know of and if you need more just ask.
New (last summer) sei outdrive upper and lower. Boat runs (did before problem) 37-40mph at 4900-5000rpm.
The engine was rebuilt and has a lunati 61001 cam, an edlebrock 1409 four barrel carb (and cast iron four barrel intake) with stock jets and needles. 10:1 compression. I recently changed the points ignition to an "HEI" style electronic ignition.

Near the end of last season the boat developed a problem, don't know what the cause was, but when accelerating it did fine, would run strong but after a 1/4 mile or so it would either
a) slowly lose power and if you didn't back off the throttle it would backfire and die, or
b) backfire and die immediatley. It did this with the points ignition and still does it with the HEI ignition.
We took the boat out this past weekend, same problem.
We've pulled the carb, no trash, no water. Floats adjusted right.
I've replaced the plug wires and plugs.
We pulled the plugs yesterday to check compression, compression is good but the number 1 plug has a burnt electrode, and number two was just starting to burn. Sounds lean right?

Does a carb that's running lean (or is just too lean for the engine) do this all of a sudden with no valve chatter? This is the same carb setup that early last summer would stumble on acceleration. ( i thought it was too rich then)(that was on points ignition too)

My engine guy said to chunk the edelbrock carb and put a holley 650 on it. He said he's had this problem with the edelbrocks before on street rods.

Before I spend more money on a holly carb, does it sound like we're barking up the right tree?
 
R

Riegfrghtl

Petty Officer 3rd Class
Joined
Jul 7, 2008
Messages
77
Re: Backfires and Dies intermittently

I also have a mercruiser 888 (1972). Mercruiser specs are 3800-4200 rpms. Your rpms seem quite high. You also said that your motor has been rebuilt. The new cam that was put in was it a marine cam? Summit racing seem to say that your cam is for a street motor ( I may be wrong). Also the carb may be a tad too big for your motor. 4bl 302's usually came with Holley 450's. hope this helps..... Tim
 
H

hblair

Seaman Apprentice
Joined
Mar 26, 2010
Messages
39
Re: Backfires and Dies intermittently

Riegfrghtl said:
I also have a mercruiser 888 (1972). Mercruiser specs are 3800-4200 rpms. Your rpms seem quite high. You also said that your motor has been rebuilt. The new cam that was put in was it a marine cam? Summit racing seem to say that your cam is for a street motor ( I may be wrong). Also the carb may be a tad too big for your motor. 4bl 302's usually came with Holley 450's. hope this helps..... Tim
Click to expand...

Sorry for not being clear. We cruise at about 4000 rpm, 30-32 mph. Wow, a Holley 450? I thought I was running lean now. Thanks for the input.
 
picklenjim

picklenjim

Chief Petty Officer
Joined
Mar 11, 2009
Messages
528
Re: Backfires and Dies intermittently

Really need to know what's in the complete build but from what you have said it sounds like you put in a performance cam and a carb. It sounds to me that you may need a better fuel pump. Sounds like it's running out of fuel on the top end. If you have iron heads than 10:1 compression is kinda high. Also if you have stock exhaust with that cam having 112 LSA your flirting with having water reversion issues.Can't only change a couple parts and expect performance improvements. Everything has to work together. You should have an aluminum dual plane high rise intake, improved exhaust and fuel pump to complimant the carb and cam. At this point I would put on a good fuel pump and see if that takes care of it. When they lean out that bad on the top end your about to drop an exhaust valve. Usually they pull real hard the last few hundred rpm's then the fire goes out.Also run a 3/8" fuel line from the pump to the carb. That carb is 600cfm and should be fine as well as your HEI ignition. Also if that thing is truely 10:1 compression with iron heads I would always run premium fuel in it. That compression with iron heads creates a lot of heat and then top it off with regular grade fuel and a lean condition and things get ugly.

Another thing if you improve too much on the engine the drives not going to like it especially in that heavy of a boat.
 
H

hblair

Seaman Apprentice
Joined
Mar 26, 2010
Messages
39
Re: Backfires and Dies intermittently

picklenjim said:
Really need to know what's in the complete build but from what you have said it sounds like you put in a performance cam and a carb. It sounds to me that you may need a better fuel pump. Sounds like it's running out of fuel on the top end. If you have iron heads than 10:1 compression is kinda high. Also if you have stock exhaust with that cam having 112 LSA your flirting with having water reversion issues.Can't only change a couple parts and expect performance improvements. Everything has to work together. You should have an aluminum dual plane high rise intake, improved exhaust and fuel pump to complimant the carb and cam. At this point I would put on a good fuel pump and see if that takes care of it. When they lean out that bad on the top end your about to drop an exhaust valve. Usually they pull real hard the last few hundred rpm's then the fire goes out.Also run a 3/8" fuel line from the pump to the carb. That carb is 600cfm and should be fine as well as your HEI ignition. Also if that thing is truely 10:1 compression with iron heads I would always run premium fuel in it. That compression with iron heads creates a lot of heat and then top it off with regular grade fuel and a lean condition and things get ugly.

Another thing if you improve too much on the engine the drives not going to like it especially in that heavy of a boat.
Click to expand...

Thanks for the information. I forgot to mention the boat does have the mercruiser manifolds but also has thru hull exhaust which I thought would help the exhaust flow. I stayed away from the aluminum intake because I may be putting this thing in salt water at some point.
I have a fuel pressure gauge 6 inches from the carb which never drops below 5 psi. I've been back there and monitored it when it backfires. Never loses fuel pressure.
I have a mechanic friend who insists I recheck the timing. I plan to do that tomorrow.
Perhaps I need to go ahead and purchase the needles and jets to go richer with the carb too?
 
Bondo

Bondo

Moderator
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Messages
71,082
Re: Backfires and Dies intermittently

hblair said:
It was the timing. It's set to 12* BTDC now, runs great. Don't know how we had it so far off. Runs great, no quitting, no problems.

Perhaps I need to go ahead and purchase the needles and jets to go richer with the carb too?
Click to expand...


Ayuh,.... Glad ya found it, but it's still possibly runnin' Lean,.... Have ya checked the plug's color again,..??

The Edlebrock is a Great boat carb,... it's a carbon copy of the Merc/ Webber, AFB Carter clone...

Edlebrock I believe still sells their Strip/ Street Kits,.... It's the Best money you can spend for that whole class of carbs,...
Needles, 'n jets, 'n springs to make that carb, Properly feed, 'bout any fuel map you Desire....
Edlebrock has a Great on-line manual for 'em too...

I'd check the plugs again, 'n see if ya oughta be fattenin' up yer fuel,....
Detonation, in a boat, can wipe out yer motor, Before ya even hear it...

Is that HEI, Marine rated,..??
 
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