Cannot get rid of a Backfire on AQ131D Volvo

[rdengil1]

Not sure what the problem is, been through everything numerous times. Started when the fuel tank pitted and sea water got in the fuel, killed the engine. I repaced the fuel tank and all lines hoses, etc. cleaned the Carb and did a rebuild kit on it. Adjusted the air mixture 9 1/2 turns per the manual, and the idle screw two turns (backing off once it hit the throttle bar). checked the timing and even put in an electronic ignition. the timing is set to 6 degrees at 900 rpm. when i throttle up, it backfires and sputters a bit then goes. Currently this is not under load. (driveway). Is it possible to be off a tooth on the belt? or should I look elsewhere. Also has new plugs, rotor, dist cap. Thanks. Dennis

 

[Don S]

Re: Cannot get rid of a Backfire on AQ131D Volvo

Turn the idle mixture screw to 2 turns out from lightly seated. Then run the engine and adjust the idle mixture for best running. Those specs are for a starting dimension, and I don't think you need 9 1/2 turns out.

 

[dennis461]

Re: Cannot get rid of a Backfire on AQ131D Volvo

Is this the same boat, engine, carb, distributor you had a problem with last year?
Has it ever run OK?
Maybe the engine is shot...

I'd start with all the basic tests again,
compression check
timing belt alignment.
static ignition timing
new points, condenser,
check spark plug gap.
plug wires OK?
check real good for any loose connections in the points circuit, a loose capacitor screw can keep you guessing for a long long time !!
carb float level

 

[billbayliner]

Re: Cannot get rid of a Backfire on AQ131D Volvo

Assuming all else is OK.
Not mentioned is the progressive ignition advance. If this is not doing what it's suppose to be doing, it may be part or all of your issue.

Cut and paste:

BASE advance is BASE advance. We start and idle on BASE only.
The more important aspect of this will be our progressive advance, and quite frankly not enough people bother to check this.
See your OEM specs for ignition advance.
Strobe your timing marks while increasing rpm up to the where the OEM specs suggest that the advance will be Full-In.
See if the distributor is giving the correct advance.
Also note that any distributor shaft wobble will throw the dwell off.
This includes point systems as well as the electric conversion systems.
The distance from the Hall unit to the sensor unit controls dwell just as point gap does.
If the shaft cannot maintain the correct dimension, causing an erratic wobble, dwell will be compromised.

Pertronix kits are sensitive to alignment and will include a shim. See the Pertronix instructions that pertain to this.

 

[rdengil1]

Re: Cannot get rid of a Backfire on AQ131D Volvo

The engine has been giving me problems (haven't even had it on the water in over a year because I don't want to have an issue there) for a while, started when I took water into the fuel tank (tank was corroded on top and took a wave which washed into the tank and killed the engine.) I since have replaced the following: fuel pump, rebuild carb kit and cleaned completely (twice), fuel tank and all associated lines, changed to electronic ignition (hotspark). I followed the carb rebuild to a T by the specs in the volvo manual. Engine constantly will backfire when I throttle forward quickly. If I gently push forward it will increase in RPMs and not backfire. Sometimes when I shut it off, it diesels. I checked the timing and rotated the distributor until the strobe indicated 6 degrees. The engine seems to run ok although it is extremely hard to start, have to constantly pump the throttle and keep turning over until eventually she stays fired. Also noticed that in addition to being hard to start, once it starts, it runs a bit rough at first (vibrating) then it will smooth out after a few minutes. Not sure if this is normal). Idle is at 950 rpm. Sounds good (the carb sounds like it is sucking a lot of air though). I have changed the plugs, will change the wires next (I am not sure how old they are or how long they have been on the boat). When I redid the carb, (I have the later version of the solex) I set the idle screw by tightening until it touched the carb lever, then I turned it two full turns. I then adjusted the idle mixture screw by lightly seating it and backing it out 9 ? turns. Not sure what else I could be missing. My current state is that I think I have everything the way it should be, but it is hard to start and runs somewhat rough. Although as I said, it does start eventually and runs smooth once running for a bit. Should I restart through all the setup again, meaning strip it back down and redo the timing, etc. It didn?t always backfire. There was a time when it ran great. Not sure what else to do. Another note, the other night after getting it running, it ran for about 15 min and then the RPMs dropped until it stalled, hasn?t done that since though. I would like to just get it to run smooth with no backfire so I feel comfortable taking it out. This might sound dumb, but so I am certain I did the timing correctly what is or how do I tell BTDC or do you set the timing at TDC? #1 cylinder all the way to the top. Thanks. Dennis

 

[sqbtr]

Re: Cannot get rid of a Backfire on AQ131D Volvo

timing belt alignment

+1.

 

[rdengil1]

Re: Cannot get rid of a Backfire on AQ131D Volvo

Meaning that my belt is 1 tooth off?

 

[Maclin]

Re: Cannot get rid of a Backfire on AQ131D Volvo

A compression check could show some valve seating or cam timing issues or valve lash issues (if too tight). Cam timing problems affect all cylinders. A compromised intake due to physical valve sealing issues can also cause this, and is cylinder by cylinder. Also valve lash adjustment can affect this.

 

[rdengil1]

Re: Cannot get rid of a Backfire on AQ131D Volvo

Decided that I would tear the engine down again and ensure all the timing marks are lined up and upon teardown, noticed a few things that might have caused the belt to possibly shift. since I do not know if it did or not, i am going to ensure they are aligned. 1st thing I noticed was the tension pulley spring guide was stuck and not actually giving a lot of tension (although it was tensioned some, not sure if it was enough), so I removed the spring guide and will replace with a new guide and stud, pulley was replaced last year and is still good. It was also stuck and did not move left or right, so I WD 40'd it and removed it, will put a little marine grease in the pin hole to allow it to remain free and move left right as it gets tension from the spring. Plugs are new, however, not sure about the wires since they are the same that have been on the boat since I bought it and knowing the previous owner, probably have not been changed by him either, so I'm guessing at least 10 8 years on the wires. So I will start with reseting the timing and cleaning up the block and accessories to ensure they are well lubed and those that are supposed to move do and nothing is stuck. I will also be rebuilding the carb again to ensure that it is clean as well. We'll see if I can salvage a bit of Striper Season

 

[Maclin]

Re: Cannot get rid of a Backfire on AQ131D Volvo

I almost went back to my post and added to take the time and check the tensioner right after I posted, that is a major component in a belt system regarding reliable cam timing, especially as items wear in. It is one item that should be probably be replaced along with the belt if financially able and parts are available. My bet is that is a major contributor to your problems. I think I would plan on also refeshing the head while it is off to ensure good valve sealing for longer life. Look at the tops of the pistons closely as well to be sure no valves hit. When cam timing is off or allowed to fluctuate they can be in the wrong place at the wrong time when the crank drives the pistons to the top. Also check the deck on the block to be sure it is flat. Ensure proper valve lash setting method is used. Stuff like that...

 

[rdengil1]

Re: Cannot get rid of a Backfire on AQ131D Volvo

Well took the engine apart, reset the timing, i am sure it is spot on. cleaned up all the sticking parts and regreased, primed and painted others, engine gleams like new, also replaced all gaskets, looks great, but I still have issues, now it wont turn over and stay running, starts or trys to start, runs for a few seconds and then stalls. Adjusted the timing to exactly 6 degrees, idle is at 900 rpm. not sure what is up, this is just becoming a headache, would really like to get this figured out. here is what I haven't done: Havent replaced the wires. I am using NGK spark plugs (purchased from auto parts store, not sure if this makes a difference). I may go back to the points and condenser and ditch the electronic ignition. I realize this engine is a basic setup, but even so, I just can't seem to make it work. Possible carb issue? I did the rebuild and cleaning, maybe I should do it again. Any help would be appreciated, again, I am positive on the setup of the timing, followed the manual exact. Boat fires, just wont stay running. also, it has always been a bear to start being cold.