Omc 460 back fire through carb!!

[Aircrazy1257]

Ok quick scoop on my boat 1988 sun runner ultra 252 with omc king cobra 460. Boat engine has lagit 400 hours. Biat sat for two years when i bought it. There has not been anything done to this boat. Its a virgin. I pumped out all of fuel replaced filter rebuilt carb which is a holley 4160. Replaced plugs and pionts in dist. Gimble bearing rebuilt outdrive and so on. The boat fires up like a champ! Idles great. No missing or anything. But when i let it get warm which is 150 i begin to throttle into it. I hear a terrible vacuum leak through the top of the carb. And it begins to seem like its pulling the enigine down then as i progress with the trottle it begins to clear up and vacuum leak goes away. Now out of the 6 time sice taken it to the lake ive only been able to get it on plain twice. On land it revs out fine. On the lake 3500 rpms is all i can get most of the time. If i goose the throttle from idle to wot it pops loudly and backfires through carb.? The dist is siezed in the intake. So i dont know how it could have jumped time being its nvr been touched. Ive cleaned the carb three times. From the get go there was nvr any signs of wear in e carb to begin with. I adjusted the acc pump on the carb as well. Its just not getting out the way of itself. The pionts were set and installed by a local boat shop. Why would it be intermiten. But the bck fore is always there no matter how warm the boat is it still pops and carry on. Im about to sink it!!! But let me say it does not miss. Plugs r right wires right. Im getting plenty of fuel she just bogs down right above idle and comes back on but is not producing the rpms itneeds. The prop is right pitch. Likeisaid ivehadthe boatup to 40mph before. Caring system is working right as well. Spark is white bluish as well please help!!

 

[Levinz11]

Re: Omc 460 back fire through carb!!

Are you saying the distributor is stuck and you cannot set the timing? What is at right now?

 

[Aircrazy1257]

Re: Omc 460 back fire through carb!!

Yes. Its seized up. I guess some ob blaster and some coaxing will loosen it. I don't kbow what it is set at right now. I'm picking up a timing light this afternoon. What I am saying is how could it of got out of time? Its obviously pre igniting from the smoke of the back fire. It has good gas new plugs. And a fresh carb. I know low octane is 10btdc and high is 15 btdc. Also if I need to adjust the timing do I adjust it at idle? Or is there a specific rpm to adjust it at.

 

[Maclin]

Re: Omc 460 back fire through carb!!

I am not sure how you can blame the carb yet gotta verify dwell and timing first.


I will throw this out there, that if the carb has too light of a spring in the secondary vacuum actuation chamber it will bog like that. Or if the spring is not there at all due to a quick rebuild that missed some parts. But look at the carb AFTER the dwell check/adjust then timing check/adjust.

 

[Maclin]

Re: Omc 460 back fire through carb!!

Also verify the distributor's mechanical advance is working correctly.

 

[Levinz11]

Re: Omc 460 back fire through carb!!

Yes. Its seized up. I guess some ob blaster and some coaxing will loosen it. I don't kbow what it is set at right now. I'm picking up a timing light this afternoon. What I am saying is how could it of got out of time? Its obviously pre igniting from the smoke of the back fire. It has good gas new plugs. And a fresh carb. I know low octane is 10btdc and high is 15 btdc. Also if I need to adjust the timing do I adjust it at idle? Or is there a specific rpm to adjust it at.

Changing the points changes the dwell which changes the timing. Order of operations: Set dwell...set timing...set idle to factory reccomended (600ish). Set initial timing while at idle then check for advance.

 

[Aircrazy1257]

Re: Omc 460 back fire through carb!!

Ok well i checked the spring and secondary vacuum plunger when i rebuilt it. And yes im not to sure it has anything to do with the carb either. So check the dwell? Is that the space between then piont and the octaganol place on the dist shaft? Listen the guy who owns a very reputable shop in my area charged me 400 bucks to change my pionts and adjust my timing. Im not even certian he changed the pionts, and i know for certian he didnt adjust the timing because before i took it to him i loosened the clamp bolt on the dist myself and saw how seized up it was. I then took the boat to him and when i got it back from him the bolt was still just as loose as it was when i had messed with it. So if yall could maybe explain the dwell settings and piont settings i would be happy as a rat chewing on a cheeto. But if he did change them i guess that would explain the timing issue. All this help is very appreciated!!!

 

[Aircrazy1257]

Re: Omc 460 back fire through carb!!

Also the inside of the dist is extremely clean. Was like that from the start. How can i check the mech advance? The springs move so is there any more way to check that? It seems like it is working properly. I get high rpms in nuetral and on land after i get past that 2k bog it revs out quickly

 

[Levinz11]

Re: Omc 460 back fire through carb!!

Dwell is the amount time the points stay open measured in degrees. I don't know for sure about your motor but mid 20's to mid 30's is normal. A feeler gauge will get you close if you don't have a dwell meter.

It would be tough to tell if he changed the points by looking at them. However, usually the condenser would be changed at the same time and "look" new or clean. But yes...you are correct in thinking that if he changed the points the timing did change some as well...how much, and enough to make it run like crap is anyone's guess.

So you need a timing light with built in dwell and advance functionality to totally check everything out. They can be had for like $75 on Amazon.

Set timing at idle, then set idle to 600 or so. Then give her a rev and watch for the timing to advance. Sometimes these gremlins don't show their face when running on the hose but do when the engine has a load.

The ignition system IMHO is the easiest to diagnose and repair. Get it in good order and see what happens.

Or go back the reputable shop and ask how he timed it if the dizzy is froze??

 

[Aircrazy1257]

Re: Omc 460 back fire through carb!!

Ok thanks alot. Ive got feeler gauges and my buddy has a timing light. So for tonig im going to check the piont gap and see if it dwell is remote. Then check my timing and see what it is. Ill post back when i get that far. Thanks again

 

[Maclin]

Re: Omc 460 back fire through carb!!

You can check the mechanical advance with the cap off by turning the rotor, it should turn some one way then hit a stop, then snap back, the action should be smooth.

 

[Silvertip]

Re: Omc 460 back fire through carb!!

If you paid $400 to have points changed, then you should be back at that shop with the boat and tell the dude to finish the job you were charged for. Dwell and timing adjustment are part of that process. Why are you fussing with this yourself? You have proof it wasn't changed. There is a very rare chance the points were replaced and the dwell and timing didn't actually change but I find that highly unlikely.

 

[bruceb58]

Re: Omc 460 back fire through carb!!

Dwell is the amount time the points stay open measured in degrees. I don't know for sure about your motor but mid 20's to mid 30's is normal. A feeler gauge will get you close if you don't have a dwell meter.

Dwell is the amount of time the points are CLOSED! Always use a dwell meter.

Always check timing after adjusting dwell.

Timing advance is measured with the engine running using an advance style timing light or timing tape.

 

[Aircrazy1257]

Re: Omc 460 back fire through carb!!

Yea silvertip you are right. I did some looking tonight and I don't think he eventouch a dang thing on my boat! He will have to deal with md tomorrow. Anyway. Got a timing light and before I did anything I fired the boat up and chrck my timing. 8? atdc!!!!!! So with some pb blaster and a nice snap on pry bar I got the dist tube free. I adjusted I to 12? being I use mid grade octane. Manual claims 10 for low grade and 15 for prepremium.man its running so amazing!! It starts easy and idle smooth' !as heck! I let her warm up good and the fast transistion is with out bog or vacuum sounding leak. No back fire from idle to half throttle. Thanks everyone for some great advise and taking time to walk me througj this. Its a totally different running engine now. Not loading up or anything. Anyway thanks again. If I missed anything let me know. Also total timing at 3k was 34 or so degrees

 

[bruceb58]

Re: Omc 460 back fire through carb!!

Good job!

I would probably still replace the points. Get a dwell meter to set them.