Mercruiser 5.7 Backfires through Carb.

[Jbs25464]

I am currently working on a 1987 Merc. 350 with a rochester 4 BBL. Carb. Customers original complaint was that it sounded like it was running ruff and wanted a tune up. Did Plug, Wires, Cap, Rotor, Checked timing and cleaned the carb. Went to go test run the boat and runs great. Will run WOT no problem. The problem is if you hammer down at low rpms the boat will studder and backfires some then finally take off. I was told the accelerator pump could be bad in the carb so i replaced it as well as the filter inside the carb and recleaned. I know i need to do a compression check but is there anything i could be missing? Odered a new coil and will be here tomorrow.

 

[greno]

Rusty valves/seats not sealing well. How old are the exhaust manifolds and risers? What did the old plugs look like?

 

[Scott06]

Of accelerator pump is working Float level set correct and would the. Reclean carb seems like it’s lean on putting the hammer down.

 

[Jbs25464]

The manifolds are not orginal but they are not new either. The old plugs were wore out but not rusty. I have the carb idle/air screws backed out 1 and 3/4 on boths sides maybe 2 turns on the stbd side.

 

[greno]

Compression test.

 

[Jbs25464]

i know it. will do a compression check tomorrow. hopefully all is well.

 

[AShipShow]

When you look down the throat of the carb and turn the throttle open the whole way, do you see two solid streams of fuel coming out. Accelerator pump could be okay, but the passages could still be gunked up.

 

[alldodge]

Customers original complaint was

I was told the accelerator pump could be bad in the carb so i replaced it as well as the filter inside the carb and recleaned.

I know i need to do a compression check

Not trying to hammer on you but would suggest maybe change customer to Friend. Someone in business should know the basics and call on help for the hard ones. If you change it to Friend then this would not be noticed. Now if your from an area which customer is used the same as helping a buddy out, I will retract my statement with an apology.

Odered a new coil and will be here tomorrow.

Its almost never the coil, and since the issue is bogging and not high and or low rpm missing it surly is not the coil.

Being a 1987 it should have a Weber or Rochester carb, which one is it?

When the carb was cleaned, was it removed, disassembled and a new kit installed?
Float drop and height set?
If its just being cleaned by spraying carb cleaner that isn't going to get it.

Is timing set correctly to
8 BTDC
Plugs MR43T or equivilent, gap .035

Fuel pressure between 4 to 7 psi?

 

[Bondo]

Not trying to hammer on you but would suggest maybe change customer to Friend. Someone in business should know the basics and call on help for the hard ones. If you change it to Friend then this would not be noticed. Now if your from an area which customer is used the same as helping a buddy out, I will retract my statement with an apology.

Ayuh,..... My 1st thought too,.....

A "Mechanic" that's knows little, tryin' to make a buck, with My knowledge,.....

 

[Scott Danforth]

I wish i had a buck for every coil changed that didnt need it. I would be making millions per year.

 

[Jbs25464]

yes the timing is at 8 btdc and there is mr43t spark plugs installed gaps were rechecked, carb was of course removed and float was not changed so do i still have to set height and drop?. There is two good streams of fuel coming out of top front of carb when throttle is pumped. Found both wires on the coil were loose and tightened. Could the coil have been dameged by that? Or could the coil of engine have been damaged due to fact that the motor was getting over 200 degrees at idle? It very well could have. Customer apparently does work on the boat himself sometimes so i have found plenty of things wrong and found out just today other things that were going on with the boat before hand. When you work for a marina thats not a bad coil to have laying around. As for the work being done it all pays the same customer is neither friend nor a customer to me. yeah im a young marine mechanic trying to make a living and learn as much i can in the process. Just need some advice.

 

[alldodge]

A coil getting hot or loose wires would not necessarly damage it, and 200 degrees for the motor is not high enough to damage the motor. That said, it also depends on how accurate the gauge is, and most boat gauges are not top of the line.

You have a lag when taking off and a backfire, and runs ok at idle and WOT. This issue is either carb, valves or both. So do a compression test to verify the health of the motor. Doing a leak down test will determine what is leaking and where. Then the carb really and truly needs to be cleaned on the inside, which is a rebuild

Rebuilding then all the adjustments of the carb can be set to spec

 

[sjajf16]

Reading this is like reliving my own frustrations. I have a Maxum with the same Merc 5.7, 4 barrel. Had the exact same problem and went through everything, ignition, fuel, carb, etc for two months. Always came back to the same issues, smooth at WOT, well maybe an occasional hiccup, but coming up to power backfire through the carb. Did compression test, #2 cylinder extremely low compression (don't remember the numbers). Popped the valve cover off and rotated engine to close both valves (compression stroke), removed #2 plug, put a compressor through plug hole and had air blowing out carb. Once heads were removed found burnt intake valve on #2.

Good luck, let us know what you find

 

[Jbs25464]

Gauge was replaced first due to improper installion from customer. carb was remvoed then reassembled with a 823426A1 kit. Does not come with everything but enough to hopefully get the job done. will do a compression check. was supposed to just be doing a tune up so this was never done since there was no complaints. im thinking its something to do with my lack of knowledge on setting the float

 

[kenny nunez]

2 things that I think may be the problem. On the carburetor you need to go over the procedure on adjusting the secondary air valve tension spring. If the valve flips open too fast you will get a bog and sometimes a back fire. Some times the spring has to be tightened more than the instructions, you have to experiment. The temperature needs to be addressed if it is really getting that hot. Is this a closed cooled model?
At one time we all were inexperienced young guys grasping at straws. Hopefully you will get it corrected. Quite often a customer with a little knowledge is the worst customer and usually is the cause of the problem.

 

[Oshkosh1]

I wish i had a buck for every coil changed that didnt need it. I would be making millions per year.

Beat me to it...

Don't forget all of the billions of perfectly good plugs which needed only to be cleaned and gapped again, the caps and rotors which needed only a light sanding on the contacts and of course the miles of perfectly good wires because reaching for that meter is sooooo far away and besides...they're dirty. Ewwwww....
!lol!...!Lol!

 

[Jbs25464]

thanks for all the advice. Compression test came up good. All cylinders were between 150 and 160. Spark plugs agian look good. The motor is raw water cooled. The motor stays cool when ran above idle so i know i need to address the issue just not as much of a concern till i figure out the first problem. I will look at adjusting the secondary air valve tension spring. When you first stomp it the motor kind of boggs for a little bit till you see the vaccum diaphram release. could the diaphram be sticking or it something i could have done wrong with the carb?

 

[Jbs25464]

The coil isnt going on the customers boat if it dosent need it. The other parts ( Wires,Cap,Rotor and Plugs) were requested.