79 140 backfires-its the carbs but why?

[mnw001]

ok if you've been following my post on this engine I have new info. I installed carbs from a 73 135 that runs good. With those carbs the motor runs great. I can retard the timming as much as I can and I don't get backfires. I can slow it down to 500 rpm with the timing and its smooth enough to troll with. if I set timming to 4 degrees it runs about 800, but this is with air box off and running in neutral in a tub. I can live with that but its easy to make it run slower just by retard a little more. With these carbs my exhaust water is clean and clear none of the oily globs like i had with the stock carbs and the spark plugs look much better on the new carbs <br />So the question is what's wrong with the stock carbs. idle jets on both sets of carbs are size 30 79 carbs are 1 3/8 venturies 73 are 1 5/16. high speed in 79 are 65c in 73 size 73c. its obvious 79 carbs are dumping fuel in motor to make it idle way too fast. The float levels are just a little higher than level with body. Don't have a gauge but I'm going to get one could this make the carbs dump fuel in the engine. I checked the floats and they both float fine. They are fiber and not cork. When I pump bulb on fuel line it gets hard so I think needles and seats are ok and the needles look fine. They are rubber tip with no signs damage.. I need some ideas please, not sure what to check now. The 73 carbs do have adjustable jets but I didn't mess with them. With the 79 carbs it idles about 1500-2000 r's at four degrees, burns with filthy exhaust and blackens lower spark plugs after 15 minutes.. Thanks again for all the help...

 

[R.Johnson]

Re: 79 140 backfires-its the carbs but why?

Did you replace the core plug's in the offending carb? Or did you replace the carb bowl gasket, that also seals around the high speed nozzles? A leak here will give an idle problem? Changing carbs solved your problem! Which one? You are painting your self into a corner.

 

[mnw001]

Re: 79 140 backfires-its the carbs but why?

did not do anything with core plugs bowl gaskets looked fine so I did not replace them. One of the core plugs does have marks as if someone tampered with it at one time.<br />are you asking which carb or which problem?<br />I changed both carbs and the problem I outlined.<br />Thx

 

[R.Johnson]

Re: 79 140 backfires-its the carbs but why?

If the core plug looks damaged. replace it. The bowl gasket also seals around the high speed nozzles which can cause a slow speed, or idle leak. Replace that gasket if in question. Is that gasket black in colour?

 

[ledgefinder]

Re: 79 140 backfires-its the carbs but why?

I think you just about have it now. I wonder if there isn't an air leak in that top carb. Maybe try swapping one carb at a time.

 

[mnw001]

Re: 79 140 backfires-its the carbs but why?

R J yes the gasket is black in color I understand the new ones are red for the new fuel<br />do the cores usually come in a rebuild kit?<br />and do i need the omc sealant with them?<br /><br />L F I don't think I'm getting too much air I believe its too much fuel but the one carb idea is a good one..Thanks

 

[walleyehed]

Re: 79 140 backfires-its the carbs but why?

Fishslayer, have you jetted this engine for your altitude?<br />Some of your problem may be altitude related...maybe.<br />For timing settings, you can throw the book out at your location. we'll see how ya make out after putting gaskets in and what-not, but if problems persist, post back, I think I can add some insight as to set-up for ya at your location.<br />Good luck.

 

[mnw001]

Re: 79 140 backfires-its the carbs but why?

Kenny I'm in denver 5200 feet. usually the guys in the shops say they leave them with the sea level jets in which are what I have at 65c the idle are 30. The carbs off the 73 have 30 and 73c and they run fine and help on that would be appreciated..

 

[R.Johnson]

Re: 79 140 backfires-its the carbs but why?

The OMC carb kit comes with core plug. Use that kit, as they have the best quality gaskets. Now you have me wondering about something else! the 73, 135 did not have adjustible jets.

 

[mnw001]

Re: 79 140 backfires-its the carbs but why?

R J your right about the 73 not having adjustble jets. I didn't think it should have them either. I guess they used the same carb body and just put bolts in place of the needle jets. So does the Sierra kit not come with core plugs. I showed the carbs to a local mechanic and he thought maybe the float needles were too soft. They are rubber but he said they could be too soft. Has anyone experienced this and would that make the carbs flow too much fuel into the engine?

 

[walleyehed]

Re: 79 140 backfires-its the carbs but why?

Fishslayer, we've been running outboards (OMC) in your area and above for years and I will tell ya now, to get it to run as it should, you will have to jet down on the mains .003 from the sea-level jets, and you can safely bump your timming up about 2 degs. from stated max.<br />I'm just accross the Kansas border to the east on I-70, and we run .003 smaller on mains here at 3800ft, and advance about 6 degs. on the V4's (crossflows), but I don't recommend any more than 2 degs. unless you will be much higher.<br />I go at least 2 degs. advanced down to 3000ft, and I ran .003 smaller mains in my '97 115 at 2000ft above sea-level with no problems on one particular 5 day trip. I watch things very closely and wouldn't recommend that to anyone I didn't know.<br />I run .006 smaller at Dillon, and 8 degs advance on the max timing. I also had to drop to a 13 pitch to keep 5800RPM......I ran a 19 pitch at 600ft elevation. When you're above the 4000ft mark, don't go above 87 octane fuel, and 85 octane is best above 5000ft, which is common in your area.<br />This only applies to crossflows, the OIS ignition systems on a looper or newer outboard are very touchy to advances and alot tougher to set up.<br />If you want to get real specific on this, email me at ktsander@st-tel.net

 

[R.Johnson]

Re: 79 140 backfires-its the carbs but why?

I looked at both the 79 parts book, and manual. In the parts book they show a plug that is listed as an idle restricter, it is unique to only the 140 carb. The part number is 318073, so it is a replaceable part. To me, the diagram is not clear where this part is located, other than it is in the carb bowl. The service manual makes no mention of this part. Carefully look over both carbs, and see if this plug is loose or missing. The soft rubber tip on the needle is normal. I have not had good results with the gaskets in Sierra carb kits. Hopefully see one else can shed more light on this idle restricter plug. I would really like to have those two carbs in front of me.

 

[R.Johnson]

Re: 79 140 backfires-its the carbs but why?

Kenny: If you have a 79 parts manual, take a look at that restrictor plug. It's no. 5 on the parts list.

 

[walleyehed]

Re: 79 140 backfires-its the carbs but why?

I'll Take a look R.J....it may be after lunch before I get back.<br />Are you using OEM on gaskets???<br />I've used some Sierra parts and although not a bunch, I haven't had any issues with them until I had a rod bearing come apart after 7 hours on a rebuild, #2 on a V6 Johnny....long story I'll share another time...

 

[R.Johnson]

Re: 79 140 backfires-its the carbs but why?

Kenny: Yes! I stay with OEM on carb kits. That had to be a sinking feeling with that rod bearing. See what you can make of that restrictor, and where it goes.

 

[mnw001]

Re: 79 140 backfires-its the carbs but why?

I will try to take some pics of a carb when Iget home form work today. That is interesting perhaps its missing from one or both of the carbs and that is causing the problem.<br />Thanks again for the help guys

 

[walleyehed]

Re: 79 140 backfires-its the carbs but why?

R.J., The only thing I show are the welch plugs (reffered to as core plugs on the parts list)<br />Those welch plugs give access to the idle circutry as I'm sure you alredy know, and when installing new plugs, a bead of OMC type-M is recommended, and installed in the usual manner.<br />If the plugs are not removed or a leak is suspected, brush a small amount of type-m arond the surfaces that meet. That's all I can find for any plugs.<br />Fishslayer, if any of those plugs are missing, it won't draw any fuel at idle and "could" be the problem with the one set of carbs.

 

[87Aggie]

Re: 79 140 backfires-its the carbs but why?

Don't want to hijack this thead or anything, I have a similar year model 140 hp and have been following along with interest. I'm having an idling problem as well and may need to re - rebuild my carbs. RJohnson, is there a substitute for the OMC type M? I don't have an OMC dealer anywhere close and didn't use the type m or anything when I rebuilt my carbs last year.

 

[walleyehed]

Re: 79 140 backfires-its the carbs but why?

3m products, # 847 "IS" type-M.

 

[mnw001]

Re: 79 140 backfires-its the carbs but why?

Kenny All of the core plugs are in place although one of them looks as if its been tapped in an effort to seal it a better perhaps. My problem seems to be way too much fuel instead of not enough