Back Fires through carb under load

[PMP]

Hey guys,

I put my boat in the water for the first time yesterday. Just rebuilt the 4.3 merc, Installed a new edelbrock 1409 carb and intake manifold. Put her in the water and brought her over to the dock. once she was warmed up she wanted to idle at 1200 rpm so I turned the idle screws down on the carb and got it down to about 750 rpm's. Then I turned it off pulled out my timing light and grounded the timing wire, Turned it back on and set it to 8 btdc. Took it out or the first test run and as soon as I started to give it a little throttle it started hesitating. It wouldn't go above 1500rpm's and it started to backfire out the carb. I just ordered a new cap and rotor but I don't think thats my problem.. Any other ideas I can check for? The only reason I can think of for it to backfire out the carb is if my timing is off but I checked it like 5 times and its perfect. Do you think I am looking at a new ignition module?

 

[NHGuy]

Re: Back Fires through carb under load

Which year and ignition do you have. The Thunderbolt IV doesn't use the base timing you described, but the V does. If it's a Thunderbolt IV you just set it to the specified angle BTDC. Did you set the carb up too? What about mix screw(s)? Most carbs require you to turn down the idle mix screws til she stumbles (warm motor) and then back em off a set amount. Carbs differ, I know on my Rochester what to do, but I wouldn't tell you how to set the carb you bought. Maybe there's something in the paperwork or online.
If you suspect an intake manifold leak you could shoot some carb cleaner around the seal points (gaskets etc) to see if she picks up RPMs. I wouldn't expect that on a new setup but you never know.
If none of that helps you might have to re "stab" the distributor. That is, reset or check the position of the distributor gear in the motor.

 

[wca_tim]

Re: Back Fires through carb under load

what do you have in it for jets, metering rods and springs?

 

[04fxdwgi]

Re: Back Fires through carb under load

Hey guys,

I put my boat in the water for the first time yesterday. Just rebuilt the 4.3 merc, Installed a new edelbrock 1409 carb and intake manifold. Put her in the water and brought her over to the dock. once she was warmed up she wanted to idle at 1200 rpm so I turned the idle screws down on the carb and got it down to about 750 rpm's. Then I turned it off pulled out my timing light and grounded the timing wire, Turned it back on and set it to 8 btdc. Took it out or the first test run and as soon as I started to give it a little throttle it started hesitating. It wouldn't go above 1500rpm's and it started to backfire out the carb. I just ordered a new cap and rotor but I don't think thats my problem.. Any other ideas I can check for? The only reason I can think of for it to backfire out the carb is if my timing is off but I checked it like 5 times and its perfect. Do you think I am looking at a new ignition module?

What year is it and what ignition module do you have? I have some Thunderbolt IV and V testing documentation if you need it.

Other possibles (since you have done alot of work on her and changed a bunch of things)

1. Fuel starved
2. Vacuum leak
3. Plugs
4. Coil
5. Module
6. Distributor sensor

The list goes on, but I would get a copy of the Thunderbolt module (If you have the Thunderbolt) testing flow diagram and start from there.

 

[PMP]

Re: Back Fires through carb under load

Sorry I Thought I had all the info in there. The motor is a 98 so I believe its the V Please correct me if I am wrong. The ignition module part number is 861251-1 for the 4.3l/lh alpha. It also has brand new plugs, Wires and coil. The cap and rotor are not here yet but as soon as they get here I will post up with results.

I will read though the instructions for the carb and see if I made an error.

WCA tim- I am using the parts that came in the 1409 carb. I have the 1485 kit I was told I might need but I thought that kit would lean out the system more?

 

[Bondo]

Re: Back Fires through carb under load

Sorry I Thought I had all the info in there. The motor is a 98 so I believe its the V Please correct me if I am wrong. The ignition module part number is 861251-1 for the 4.3l/lh alpha. It also has brand new plugs, Wires and coil. The cap and rotor are not here yet but as soon as they get here I will post up with results.

I will read though the instructions for the carb and see if I made an error.

WCA tim- I am using the parts that came in the 1409 carb. I have the 1485 kit I was told I might need but I thought that kit would lean out the system more?

Ayuh,... Is the rest of the Fuel System delivering the fuel like it should,..??
Dirty filters,..??
Bad fuel pump,..??

 

[PMP]

Re: Back Fires through carb under load

Ayuh,... Is the rest of the Fuel System delivering the fuel like it should,..??
Dirty filters,..??
Bad fuel pump,..??

The pump was seized when I first put it back together. I believe the boat was sitting for 8 years. I put some pb blaster down it and hit it with power a bunch of times and it broke loose and pumped like a champ. I ran 5 or 6 gallons though it of water. So I believe the pump is working great. I am guilty of not replacing the fuel filter so I will grab a new one of those also.

Jason

 

[PMP]

Re: Back Fires through carb under load

Ok I took the boat out again today and nothing changed,

It has-
New Coil
New cap and rotor
New Wires
New plugs
Fuel filter

Is there a way to test the ignition module?
How might I test for a vac leak also?

I am still on stock jets and rods in the edelbrock 1409 carb and I am at 4500 ft above sea level on the 4.3 so I am thinking I should be getting more then enough fuel.. I have the other set to put in when I get it running better.

Anyone have any ideas?

Jason

 

[wca_tim]

Re: Back Fires through carb under load

you need to check fuel pressure, and you will need the leaner jets and metering rods. I think I was one rod richer than the kit by the time I was done jetting my 4.3 with the same carb and intake. You may also have to go with a different spring - I believe I had to do that also to deal with off idle lean condition. by the way - you need to know what your vacuum at idle is to know what spring to use... if you don't have the carb owners manual download it and look through the jetting instructions. They're not bad to follow.

Fuel pressure is the first place i would start. edelbrock carbs are very sensitive to fuel pressure. you might check to see that you aren't dribbling fuel down the carb at idle - that will make it run like crap. that's if you have over 6 psi it'll be pushing fuel past the needles, if you have a lot less, then you know that insufficient fuel supply / pump is the problem. You can run a hose directly from a can to the fuel pump to see if the problem is upsrream of the fuel pump. either way, a cheap fuel pressure guage mounted at the carb will tell you a lot.

a vacuum guage would be helpful to see what kind of vacuum you have at "idle". note that you can get backfid re out the carb from a pig rich fuel mixture also... especially if you have an intake valve that isn't sealing all the way.

cheap vacuum guage (or rent from local auto parts store) and cheap fuel pressure guage are probably the best thing to do next...

 

[PMP]

Re: Back Fires through carb under load

Thank you so much for the response! I will do a fuel and vac test tomorrow. What should I set the idle down to btw? Right now I have it at about 900.

Jason

you need to check fuel pressure, and you will need the leaner jets and metering rods. I think I was one rod richer than the kit by the time I was done jetting my 4.3 with the same carb and intake. You may also have to go with a different spring - I believe I had to do that also to deal with off idle lean condition. by the way - you need to know what your vacuum at idle is to know what spring to use... if you don't have the carb owners manual download it and look through the jetting instructions. They're not bad to follow.

Fuel pressure is the first place i would start. edelbrock carbs are very sensitive to fuel pressure. you might check to see that you aren't dribbling fuel down the carb at idle - that will make it run like crap. that's if you have over 6 psi it'll be pushing fuel past the needles, if you have a lot less, then you know that insufficient fuel supply / pump is the problem. You can run a hose directly from a can to the fuel pump to see if the problem is upsrream of the fuel pump. either way, a cheap fuel pressure guage mounted at the carb will tell you a lot.

a vacuum guage would be helpful to see what kind of vacuum you have at "idle". note that you can get backfid re out the carb from a pig rich fuel mixture also... especially if you have an intake valve that isn't sealing all the way.

cheap vacuum guage (or rent from local auto parts store) and cheap fuel pressure guage are probably the best thing to do next...

 

[Fun Times]

Re: Back Fires through carb under load

To find help find you the correct engine specifications for your engiune, what is your Mercruiser engine serial number?

 

[PMP]

Re: Back Fires through carb under load

Serial number is - 0L022226

 

[wca_tim]

Re: Back Fires through carb under load

In thinking about this, it would be really helpful to know a litle more history. I have the impresion that:

This engine sat for 8 years without being run(????). NOw, after the things you outling having done above, you've got it firing and running, but it still runs poorly, etc.. showing the symptoms you laid out above

Was it in good running order when it was "parked"?

Aside from adding a carb and intake (I would probaby have done also and had great results with that same carb and intake on a 4.3...)

Have you checked compression?

What kind of shape was the fuel supply in? (ie, did you completely clean the fuel tank and lines out? did it have fuel stabilizer in it? etc... It might be smart to run a piece of fuel line from the fuel pump inlet to a couple gallon gas can to eliminate the possibility of craopped up fuel system if you have any doubt. Is there any chance you could have water in the gas? note that if you do have a lot of junk in the fuel system, you probably need to clean the carb out again now...).

 

[45Auto]

Re: Back Fires through carb under load

What should I set the idle down to btw? Right now I have it at about 900.

Factory spec is 600-650 RPM. You're not doing your outdrive any good banging it into gear at 900 RPM.

I have the same carb on a '97 5.7L, it idles so smoothly at 600 RPM you can't tell it's running.

You don't set the idle RPM with the idle air screws, they are for fine adjustment of the air mixture. Set them 1 1/4 turns out from closed and you'll probably never need to touch them again. The idle adjustment screw on the side of the carb linkage is what sets the idle RPM.

You have mechanical secondaries, so it is possible to get a lean backfire by opening the throttle too quickly from low RPM. If you open all the butterflies at low RPM, the air velocity will drop so much that not enough fuel can be pulled through the venturis to maintain the correct mixture no matter how rich your accelerator pump is set. It'll go lean and give you a nice bang.

As WCA_Tim suggests, download the manual and read it.

 

[PMP]

Re: Back Fires through carb under load

There is now a lot of confusion in this thread..

Tim, The boat sat for 8 years.. read the first post and it says I just rebuild the 4.3 merc... So this is a brand new engine. Everything else not listed as new is used and sat for 8+ years.

The fuel tank was empty
Fuel lines looked clean
Fuel filter looked good but I have now replaced it as well

45 auto-
I am going to read the manual tonight. I was hopping to test out the vac and fuel pressure but ran out time. I will see if I can get it down to the 600 range. Its not a quick snap to wot its slow and easy and its also not one big boom.. Its a bunch them. It sounds like it is trying to suck in a bunch of air and it also is looses power when it starts to pop and studder.

Jason

 

[Fun Times]

Re: Back Fires through carb under load

Serial number is - 0L022226

Your MODULE-ICM part number is actually 861251T1.

Your serial number (0L022226) shows this engine to be a 1998 MERCRUISER 1 PC W/PS 4.3L (262 CID - GEN +) ALPHA style engine. Your service manual number for your engine is S/M #25, Merc part number 90-8613281.

In that S/M under Maintenance on Page 1B-5 is your Engine and Tune-Up Specifications for your engine. Your engine timing needs to be set at 10* not at 8*. Your idle needs to be set at 650 rpm. usually these engines will idle best between 643 & 648 rpm when everything is adjusted 100% right.

I hope you had this S/M by your side during the engine rebuild??? If not, here it is for you.:facepalm:
http://www.4shared.com/document/dKAM4Vg8/Service_Manual_25.html

MCM (Sterndrive) Model MCM 4.3L ALPHA / BRAVO = 190 hp, MCM 4.3LH ALPHA / BRAVO = 205 hp, MC 4.3L EFI ALPHA / BRAVO = 210 hp.

Number of Cylinders V-6
Displacement 262 cid (4.3 l)
Bore / Stroke - in. (mm) 4.0 x 3.48 (101.6 x 88.4)
Compression Ratio 9.4:1 9.4:1
Compression Pressure Minimum 100 psi (690 kPa) See note#7
Idle rpm In Neutral See note#3 650 rpm See note#5 600 rpm See note#5
Maximum rpm (at WOT) See note#3 4400-4800 rpm
Oil Pressure (at 2000 rpm) Minimum 30 psi (207 kPa)
Minimum Oil Pressure (at Idle) Minimum 4 psi (28 kPa)
Fuel Pressure (1800 rpm) 3-7 psi (21-48 kPa) 30 psi ( kPa)
Electrical System 12 V Negative (?) Ground
Alternator Rating 55 or 65 amp See note#8
Minimum Battery Requirements 375 cca / 475 mca / 90 Ah
Firing Order 1-6-5-4-3-2
Spark Plug Type
AC - MR43LTS
Champion - RS12YC
NGK - BPR6EFS
Spark Plug Gap .045 in. (1.1 mm)
Timing (at idle rpm)See note#4 10? BTDC 8? BTDC, is for the EFI engine.
Preliminary Idle Mixture 1 1/4 Turns DNA
Thermostat 160? F (71? C)
1 Power Rated in Accordance with NMMA (National Marine Manufacturers? Association) rating procedures.
2 Power Rated in Accordance with SAV1 rating procedures. This rating procedure is used to certify that the
engine complies with ?Stage 1? Bodensee and Swiss Regulations. Horsepower differences shown result from
differences in test rpm, allowable test tolerances, and/or installation of special kit components.
3 Measured using an accurate service tachometer with engine at normal operating temperature.
4 Timing must be set using a special procedure as outlined in the appropriate section of this manual. Timing cannot
be properly set using the conventional method.
5 A special procedure must be followed to adjust idle rpm. Consult your Authorized Mercury MerCruiser Dealer
before attempting this procedure.
6 Idle speed on EFI models is not adjustable.
7 Minimum recorded compression in any one cylinder should not be less than 70 percent of the highest recorded
cylinder.
8 Serial Number Break for 65 Amp alternator: OL619083 and above.

 

[PMP]

Re: Back Fires through carb under load

Fun Times,

I can always count on you for fantastic information. Do you know if I for sure have the Thrunderbolt V ignition? I thought I did but I don't have a knock sensor..

Where is there a How to set up on my ignition? The one in the S/M sucks.. It does not even tell me what to set the base timing to. Where did you find the 10 BTDC?

And yes I have the S/M and it was a life saver getting the wiring done.

Jason

Your MODULE-ICM part number is actually 861251T1.

Your serial number (0L022226) shows this engine to be a 1998 MERCRUISER 1 PC W/PS 4.3L (262 CID - GEN +) ALPHA style engine. Your service manual number for your engine is S/M #25, Merc part number 90-8613281.

In that S/M under Maintenance on Page 1B-5 is your Engine and Tune-Up Specifications for your engine. Your engine timing needs to be set at 10* not at 8*. Your idle needs to be set at 650 rpm. usually these engines will idle best between 643 & 648 rpm when everything is adjusted 100% right.

I hope you had this S/M by your side during the engine rebuild??? If not, here it is for you.:facepalm:
http://www.4shared.com/document/dKAM4Vg8/Service_Manual_25.html

MCM (Sterndrive) Model MCM 4.3L ALPHA / BRAVO = 190 hp, MCM 4.3LH ALPHA / BRAVO = 205 hp, MC 4.3L EFI ALPHA / BRAVO = 210 hp.

Number of Cylinders V-6
Displacement 262 cid (4.3 l)
Bore / Stroke - in. (mm) 4.0 x 3.48 (101.6 x 88.4)
Compression Ratio 9.4:1 9.4:1
Compression Pressure Minimum 100 psi (690 kPa) See note#7
Idle rpm In Neutral See note#3 650 rpm See note#5 600 rpm See note#5
Maximum rpm (at WOT) See note#3 4400-4800 rpm
Oil Pressure (at 2000 rpm) Minimum 30 psi (207 kPa)
Minimum Oil Pressure (at Idle) Minimum 4 psi (28 kPa)
Fuel Pressure (1800 rpm) 3-7 psi (21-48 kPa) 30 psi ( kPa)
Electrical System 12 V Negative (?) Ground
Alternator Rating 55 or 65 amp See note#8
Minimum Battery Requirements 375 cca / 475 mca / 90 Ah
Firing Order 1-6-5-4-3-2
Spark Plug Type
AC - MR43LTS
Champion - RS12YC
NGK - BPR6EFS
Spark Plug Gap .045 in. (1.1 mm)
Timing (at idle rpm)See note#4 10? BTDC 8? BTDC, is for the EFI engine.
Preliminary Idle Mixture 1 1/4 Turns DNA
Thermostat 160? F (71? C)
1 Power Rated in Accordance with NMMA (National Marine Manufacturers? Association) rating procedures.
2 Power Rated in Accordance with SAV1 rating procedures. This rating procedure is used to certify that the
engine complies with ?Stage 1? Bodensee and Swiss Regulations. Horsepower differences shown result from
differences in test rpm, allowable test tolerances, and/or installation of special kit components.
3 Measured using an accurate service tachometer with engine at normal operating temperature.
4 Timing must be set using a special procedure as outlined in the appropriate section of this manual. Timing cannot
be properly set using the conventional method.
5 A special procedure must be followed to adjust idle rpm. Consult your Authorized Mercury MerCruiser Dealer
before attempting this procedure.
6 Idle speed on EFI models is not adjustable.
7 Minimum recorded compression in any one cylinder should not be less than 70 percent of the highest recorded
cylinder.
8 Serial Number Break for 65 Amp alternator: OL619083 and above.

 

[Fun Times]

Re: Back Fires through carb under load

Fun Times,

I can always count on you for fantastic information. Do you know if I for sure have the Thrunderbolt V ignition? I thought I did but I don't have a knock sensor..

Where is there a How to set up on my ignition? The one in the S/M sucks.. It does not even tell me what to set the base timing to. Where did you find the 10 BTDC?

And yes I have the S/M and it was a life saver getting the wiring done.

Jason

Yes, your ignition system is considered to be the thunderbolt V ignition. Google the part number for some info.

You will find all the info in the S/M (#25) under the Maintenance section (in the 1B section). Look at both Page 1B-5 & 1B-27, your Engine and Tune-Up Specifications are on page 1B-5 and how to set the timing is on page 1B-27, You need to look at both pages to gather all the information.

Had you had a knock sensor then you would have had a knock sensor module also, usually mounted somewhere near the ignition control module area.

Have a look at the links below for some ideas.
http://www.boatfix.com/merc/Bullet/94/94_02.pdf
http://www.boatfix.com/merc/Bullet/92/92_18.pdf
http://www.boatfix.com/merc/Bullet/97/97_08.pdf

Ignition Timing
Thunderbolt V Models
1. Connect timing light to number 1 spark plug wire.
2. Connect a shop tachometer to the engine.
IMPORTANT: Before starting the engine, connect a jumper wire from the ignition timing
lead to a good ground. This has to be done before the ignition key is turned ON
to lock the ignition module into Base Timing Mode.
3. Before starting the engine, connect jumper wire from timing lead to a good ground.
NOTE: The PUR/WHT timing lead is located towards the front of the engine near the fuel
line or near the distributor, as equipped for your model.
4. Start engine and run at 1300 rpm until it reaches normal operating temperature.
5. Disconnect throttle cable from the carburetor.
6. With engine at idle rpm, adjust the carburetor idle rpm screw to the specified engine idle
rpm.
7. With the engine still at idle rpm, check the ignition timing. If incorrect, rotate the
distributor until timing is correct. Torque clamping screw to 18 lb-ft (25 Nm).
8. Adjust the idle mixture screw. Inward is LEAN, outward is RICH.
9. Recheck ignition timing.
10. Stop engine. Remove timing light, jumper wire and shop tachometer.
IMPORTANT: Timing jumper wire has to be removed or the ignition module will stay
locked in the Base Timing Mode and it will not be able to advance the ignition timing
correctly when the engine rpm is increased.
11. Adjust and reinstall throttle cable. Open and close remote control throttle lever. Ensure
that the carburetor throttle lever is contacting the idle rpm adjusting screw every time.
12. Restart the engine, increase rpm to 1300 then return to idle position slowly and shut the
engine off. Ensure that the carburetor throttle lever is contacting the idle rpm screw.
13. Shut engine off.

 

[PMP]

Re: Back Fires through carb under load

New information,

Ok so I wanted to test the ICM so I started up the engine and let it worm up, Then I took my timing light and took it up to 2000RPM's (where it has the problems) It starts to advance the timing a little then it jumps like 70 deg for one spark then back to normal then jumps again to about 70deg. It does this like every 10 rotations or so but not in any kind of pattern it seems to be truly random.

I then grounded the wire to put it in base timing mode and the timing still moves around and when I get up to the problem area it seems to be advancing a bunch still for random sparks..

Is there a better way to test my ICM? or should I just buy a new one and hope I didn't wast 400+ bucks?

Thanks in advance!

Jason

 

[PMP]

Re: Back Fires through carb under load

I have good fuel pressure coming in at 6 psi. I also tested for vac leaks and found non.