1994 454 Backfire problems

[OMARiverRat]

Hey guys, its been a while since I've last posted. I've enjoyed a wonderful summer of boating (Thanks to alot of help from people here), and now I'm looking to upgrade into a bigger boat. The boat I'm looking at is a 1994 Crownline 225CCR w/ Mercury 454 (not magnum) and Bravo 1 lower unit. The boat is in wonderful condition, however it has a mechanical problem. When under load, the engine tends to backfire at a certain RPM... right around 3,000 or so. When in the seller's driveway on the muffs, the boat would rev up perfectly, but would occasionally "pop" out of the carburetor at about 3000-3500 RPMs. Can anybody point me in the right direction on what this may be? The seller is dead-set that this is an electrical problem. He has replaced the plugs & wires, cap & rotor... He believes that he is getting "Scattered spark" in the distributor. A friend of mine believes its either a fuel (Carburetor) problem, or the cam is worn. Any help would be greatly appreciated. I can literally steal the boat for what he's asking, however I don't want to put more than a few hundred into it to get it running good. Thank you for your time.

 

[OMARiverRat]

Re: 1994 454 Backfire problems

One more thing... I should also add that this engine does seem to have a slight noise at initial start-up. Once the engine has been running it seems to quiet down....however once its revved, the noise comes back. The seller performed a compression test on the motor, and according to him it has very good compression. Seller also adjusted timing. <br /><br />Thank you for your help.

 

[Don S]

Re: 1994 454 Backfire problems

It's hard to say about the noise, could be just the lifters at startup. I doubt you have a worn cam as the problem would be all the time, not just occasionally.<br />Check the oil level, I have seen the oil be too full and the crankshaft hits and foams the oil causing air to get into the lifters and cause problems.<br />Your problem does sound electrical though. A full tuneup would be in order to start with. New plugs, cap, rotor, set timing. and be sure to check the advance timing. If the ignition module isn't working properly, or there is corrosion on the terminals you could loose your advance.<br />Also with the Thunderbolt V distributor, you need put it in base timing mode with a jumper wire before setting the timing or adjusting the idle on the carb.

 

[OMARiverRat]

Re: 1994 454 Backfire problems

Don, thanks for the help and quick response. I'll check the oil level this evening. How does one put the distributor in "base timing mode" by using a jumper? I'm unfamiliar with this procedure.... <br /><br />Thank you for the help.

 

[OMARiverRat]

Re: 1994 454 Backfire problems

Hello again everybody...<br /><br />I ended up purchasing the boat I have spoken about above. Now that it's getting warmer, I'd really like to start tracing this problem. As for setting the timing, can anybody tell me how to set the distributor in "Base Timing Mode" by using a jumper, as Don S has described below? <br /><br />Thanks for all of your help!

 

[Don S]

Re: 1994 454 Backfire problems

The first thing you need to do before you start working on it is to get a service manual. All the details are covered for troubleshooting, adjustments, ignition repair,testing, and setting, etc. Without it, you are just using the trial and error (mostly error) method of troubleshooting.<br /><br /> Service Manuals

 

[Bondo]

Re: 1994 454 Backfire problems

Ayuh,...........<br /><br />And, Besides that,..... The Manual will give you all the spec. for the Carb overhaul it needs.........<br />

however it has a mechanical problem. When under load, the engine tends to backfire at a certain RPM... right around 3,000 or so. The seller is dead-set that this is an electrical problem.

He Was Wrong..........<br /><br />The carb is going Lean at the transisition from primaries to secondaries........<br />It Needs a Good Cleanning,+ Rebuild.......<br /><br />Good Luck...

 

[waterone1@aol.com]

Re: 1994 454 Backfire problems

Besides the obvious ( don always nails it) get a service manual. I would suggest to start simple...inspect and replace both fuel filters (water seperating and inside the carb inlet). Check all fuel lines from gas tank to carburator for restricions and loose clamps/air leaks. Anytime I hear of someone "playing with" the ignition timing, I want to make sure it is set properly. Then, I would start thinking about the carb and more serious problems. By the way, did you say that you took the guys word about compression readings ?

 

[rodbolt]

Re: 1994 454 Backfire problems

hello<br /> also carefully inspect the plugs for the presance of water. will usually leave small green flecks or white spots that are tiny but there. the last 454(not to be confused with the 7.4) I had come in with an occasional back fire problems I noted the presance of water deposits on the plugs. the man and his buddies discounted my diagnosis based on the fairy tale that the risers had been replaced before he bought it. they took it someplace else and had a major tune up and carb job. still backfired<br /> 3 weeks later the engine locked due to a riser that had a hole in it. it was in a wellcraft 33 st tropez. v drives and all. what fun I had.<br /> there are only a few reasons why it will back fire though the carb. 1 is the mix in the intake runner got lean and will ignite when the intake valve opens, two is the spark occurs while the intake valve is open. now ya have to find out which one is causing it. a careful anylisisis is in order. all conditions leave telltale indicators.usually if the problem comes and goes you can rule out a mechanical failure. however a broken valve spring,leaking riser or a valve that is sticking can be hard to find. a careful eyballing ofthe plugs and a session with an engine vacum gauge can help. <br /> if you suspect a cross fire its not difficult to set 7 more TDC marks on the balancer and use a timing light with an advance function and watch all 8 marks in sequence. if another mark shows up that is not the number you have on the balancer it just cross fired. works well for outboards too<br />the exact location ofthe other 7 numbers is not super critical. your just looking to see if any other number shows up when its not supposed to. a piston stoptoolis handy but youcan find itclose enough with a plasic straw in the plug hole and moving the engine by hand, no need to worry about which stroke its on all you need is numbers close to TDC. all readings will be based on the advance of #1 at the problem RPM.thats where the timing light with an advance function is a MUST.