Backfire through Carburator at high RPM

[dnemeth]

Just installed a New GM 5.7L Marine engine from Michigan Motorz in my Monterey 196BR. After it was installed it fired up with no issues but would only run up to about 3500 rpm before it started to backfire through the carburetor, Edelbrock 1409 marine model. The base timing is set at 8 deg as called for and it advances up to 24 deg after the timing shunt is removed and the rpm?s are increased. The fuel filter was original to the boat so I decided to remove it and check for water, sure enough it was full of crap and some water. I replaced the filter and added some fuel treatment and now it runs up to around 4500 rpm and starts to pop and backfire randomly through the carb with a loss of power. I pumped out all the fuel and replaced the filter again and added fresh fuel with a bit of fuel treatment. It seems to run fine to a certain point and then it starts to break up and loose power. I replaced the ICM and still the same issue. My question is if the floats in the carb are not set up correctly or the jets are too small will it starve the engine for fuel and cause this condition. I have run about 20 gallons of fuel through it, by passed the fuel filter and its impossible to open the throttle completely under load or no load without it backfiring. Could the fuel pump be incapable of keeping up with the demand of the engine. Carb is 600 cfm.

 

[John_S]

Re: Backfire through Carburator at high RPM

Usually caused by a lean condition. Did you clean the carb after finding all the crap in the filter?

If carb is clean, I'd check engine compression, fuel line pressure, and engine vacuum, to make sure everything is in good order, before changing the secondary jets to get a richer mixture.

 

[littlebookworm]

Re: Backfire through Carburator at high RPM

I'd also check the fuel hoses and in-tank pickup. If the flexible hose is the old one, replace it. It may be collapsing internally with heavy draw. E10 gas has been killing fuel hoses. I'd pull the fuel pick-up tube from the tank and check the screen at the bottom. It may be partially collapsed or filled with crud. If all that's OK, I'd check the fuel pump pressure, though I doubt that's the cause because your problem is not at all high RPMs. Then, as the Captain, suggested, I'd check the carb for cleanliness and jetting. Hy

 

[dnemeth]

Re: Backfire through Carburator at high RPM

I cleaned the carb and it was in perfect shape, compressed air and blew it all out. I checked fuel flow from the pump before putting the line back on and filled up a 16oz bottle in a few seconds. Its raining will have to wait till tomorrow for compression checks. I did opperate the engine on the trailer and I can get slowly around 4600 rpm but not WOT and I cant go from idle to WOT without backfire. The thing runs great up to this point, no hickups, pulls the boat up on plane, just no WOT. WIll try fuel from a can and bypass the tank.
thanks

 

[John_S]

Re: Backfire through Carburator at high RPM

I mis-read the bypassed fuel filter with assuming you bypassed the fuel tank. As LBW suggested, should test that part of the fuel system.

Given that you can recreate with no load, which requires much less fuel, (could still be a collapsing fuel hose) sounding more like carb or other issue. Is it happening at the point that the secondarys are just starting to happen? There is a particular throttle position that mechanically they open, and then it is up to the air door to open with enough engine vacuum. Are you seeing fuel flow in the secondaries?

Did any crud make it to the carb inlet filter or the carb fuel bowl?

Compression test will verify piston ring and valve sealing to a degree. A vacuum gauge will identify leaks, sticking valves, or other issues. The carb is very dependant on engine vacuum for correct operation.

 

[John_S]

Re: Backfire through Carburator at high RPM

The base timing is set at 8 deg as called for and it advances up to 24 deg after the timing shunt is removed and the rpm?s are increased.

I assume this meant that your base timing is 8 degrees BTDC and at 32 degrees BTDC in the 4K rpm range. What ignition system?

 

[dnemeth]

Re: Backfire through Carburator at high RPM

The ignition system is a Delco Voyager EST and they (Michigan Motorz) call for 8 deg base and a max of 26 deg after the shunt is removed taking it out of base mode, I ve checked it at 3000 rpm and it is at 24 deg. When I cleaned out the carb I didnt find anything but gas, no dirt or FO.No problem recreating the issue loaded or un loaded the only diffrences is that under load it happens at a couple hundrend rpm lower. Where should I try to pick up a vacume on this engine, there are no vacume lines so may have to take one of the plugs out of the manifold and pick up some fitings. Exactly what kind of vacume readings will I be looking for.There is fuel going in through the secondarys, I held the choke closed, not all the way but more then half and it seems to stop backfiring but it bogs down.

 

[John_S]

Re: Backfire through Carburator at high RPM

The ignition system is a Delco Voyager EST and they (Michigan Motorz) call for 8 deg base and a max of 26 deg after the shunt is removed taking it out of base mode, I ve checked it at 3000 rpm and it is at 24 deg. When I cleaned out the carb I didnt find anything but gas, no dirt or FO.No problem recreating the issue loaded or un loaded the only diffrences is that under load it happens at a couple hundrend rpm lower. Where should I try to pick up a vacume on this engine, there are no vacume lines so may have to take one of the plugs out of the manifold and pick up some fitings. Exactly what kind of vacume readings will I be looking for.There is fuel going in through the secondarys, I held the choke closed, not all the way but more then half and it seems to stop backfiring but it bogs down.

Float levels were good?

There should be a plug in the intake behind the carb. Replace it with one that has a vacuum fitting. You can leave it in, just cap it off well. See: http://www.secondchancegarage.com/public/186.cfm Vacuum readings can tell you allot, but mainly checking that you have a good steady vacuum at idle, no vacuum leaks, and valves not sticking or floating at higher rpms. Usually, the 1409 will not need calibration for a basicly stock 350. So, just trying to make sure you are not calibrating to other issues.

 

[dnemeth]

Re: Backfire through Carburator at high RPM

Float levels were good, called for 7/16 and they were with 13/32.

 

[John_S]

Re: Backfire through Carburator at high RPM

Compression check and get the vacuum gauge on there, and reread the tuning guide: http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

Hopefully, if no engine issues, you just have to change the secondary jets. Looks like .107 would be 2 steps richer. While the primary jets/rods/springs are easy to access, the top has to come off to change the secondary jets.

 

[dnemeth]

Re: Backfire through Carburator at high RPM

Ok I did a compression check and here are the results,

cyl
1=188psi
3=188
5=188
7=190
2=190
4=190
6-190
8-190
The vacume test seemed to be fine, started off a bit low (15psi) till the engine warmed up and then wobbled or shook between 18 and 19 at idle.
Fuel pressure was the biggest issue as it was about 3 psi at idle and droped to below 1 psi at higher rpm's. I decided to pull fuel from a can and the pressure was higher at 4 psi but the issue with backfiring and poping was still present and it did drop off to about 3 psi. I wanted to pull the pickup tube but it seems that this may not be possible as the hardware is under the floor, Im wondering if a new one can be installed and caping off the old one but I have no experience with this type of arangement. In any case the biggest concern was the low fuel pressure but even after going straight from can to pump didn not correct the issue. Thanks for the help thus far!

 

[John_S]

Re: Backfire through Carburator at high RPM

Ok I did a compression check and here are the results,

cyl
1=188psi
3=188
5=188
7=190
2=190
4=190
6-190
8-190
The vacume test seemed to be fine, started off a bit low (15psi) till the engine warmed up and then wobbled or shook between 18 and 19 at idle.
Fuel pressure was the biggest issue as it was about 3 psi at idle and droped to below 1 psi at higher rpm's. I decided to pull fuel from a can and the pressure was higher at 4 psi but the issue with backfiring and poping was still present and it did drop off to about 3 psi. I wanted to pull the pickup tube but it seems that this may not be possible as the hardware is under the floor, Im wondering if a new one can be installed and caping off the old one but I have no experience with this type of arangement. In any case the biggest concern was the low fuel pressure but even after going straight from can to pump didn not correct the issue. Thanks for the help thus far!

Compression is great, idle vacuum is good. What did the vacuum gauge do at the poping?

SM says 3-8psi @1800rpms. Your fuel pressure is low and could be causing this.

 

[dnemeth]

Re: Backfire through Carburator at high RPM

I need to go back and check the vacume again when it starts poping, I do know it goes down and jumps back up after the initial surge. I did look up the spec's on the fuel pump and its 4 to 7, edelbrock sugests 5.5 and no more then 6. I need to know whats too low I guess. Im not sure what to do about the fuel tank as the floor of the boat is over the elbow and even if I can get at it to turn it wil I be able to take it out. The other thing is I ran the line from the pump to a gas can and it did not improve the issue that I am having even with better fuel pressure. Looking for tank parts.

 

[John_S]

Re: Backfire through Carburator at high RPM

Less than 1 psi at higher rpms, is what I keyed on. I can't help you with tank access. Mine is right in front of engine with access to all fittings.

 

[dnemeth]

Re: Backfire through Carburator at high RPM

Edelbrock spec's say no lower then 2 psi so we should have solved the problem when I was using the gas can as I kept 3 psi. I will have to come up with a solution for the tank, maybe install another pickup tube in the part of the tank that I can get at and cap the other off.

 

[John_S]

Re: Backfire through Carburator at high RPM

I wouldn't try capping the fuel line off, and installing another. Install an access hatch where you can access the fuel lines. There is no gurantee that this is the sole cause, but you don't want to tune the high speed jets to low fuel pressure.

 

[dnemeth]

Re: Backfire through Carburator at high RPM

Ok cut the floor back a couple inches and got the pickup tube out, was clean. Maybe a new fuel pump, one that can do better then 4 pounds.

 

[dnemeth]

Re: Backfire through Carburator at high RPM

Ok New fuel pump, 5.5psi. Still no improvement. Re-jet the carb 2 steps richer on both Primary and secondary and the problem is better but not gone. Any chance the carb is junk?? The Guys at Michigan motorz sent me the new pump and have been very helpful. Checked vacume at high RPM and while its poping it dosent get up past 5psi. Thanks for all the input.

 

[fossill]

Re: Backfire through Carburator at high RPM

Check your ignition system first.
Then the fuel system and finally the engine when troubleshooting and in that order.

 

[John_S]

Re: Backfire through Carburator at high RPM

Going two step richer and still wants more??? Just doesn't seem right. I'd contact Edelbrock tech support and start discussion. Someone had posted here a while back where Edelbrock had them drill holes in air door counterweight. I haven't seen others with that, but you should find it here. Before making it richer, did you plugs look like it was running lean, as well?

http://forums.iboats.com/showthread.php?t=429895