79 140 backfires-its the carbs but why?

[87Aggie]

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

I appreciate the quick response Kenny, I'm sure I can get the 3M product around here. Just read back through the posts and noticed that you were the one that mentions OMC type M, sorry about the reference to RJohnson. Thanks again!

 

[R.Johnson]

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

This is a quess on my part in regards to what the parts book calls an idle restrictor. With the carb bowl off, you will find an idle passage on each side of the bowl. With the main body of the carb upside down, you will see where the passages' match up. Is there a brass plug pressed into each of the passages with a small orifice through it? They would probably be pressed into the main body, but possibly the bowl. If I am right, see if one is missing, or damaged.

 

[jy118lfd]

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

What Rjohnson is refering to is the idle air bleed if the part with the brass orfice is clogged it will pull all fuel from the bowl not fuel mixed with air. This will cause a rich condition at idle. It goes from the throat of the carb to the passage under the float bowl. It is very hard to see with the choke plate in the way. Look on the carb without the plate and try to reference that. The opening is on the "choke side" of the throttle plates. Make sure and blow it out with compressed air. Do not drill it or use wire to clean it. If you make it bigger it will lean the idle circut as I did on a 235 and found out the hard way that the orfice comes with the main body. I had to replace the carb!

 

[mnw001]

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

Jon and RJ I think i have found the passage that you mention. I have looked at the brass orfice and they all appear to be clear I will make sure that they are using air although I had focused on that area pretty hard the last time I had the carbs apart. The symptom you describe sound exactly like what I have going on.<br />I belive the orfices are in area that was controled by the adjustable air needles on the older carbs. Is this setup specific to 79 do you know? So Jon if I understand you correctly your saying if the brass orfice is closed it won't get air to atomize with fuel going thru the idle jets and into the throat just under the throttle butterflies. I wonder if I might need to remove the core plugs to see if the small holes that open into the throat are clear. I blew thru the idle jet passage and thought I was getting a good stream of air thru the throat.

 

[R.Johnson]

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

My 79 parts manual, which is a factory parts manual shows a plug called an idle restrictor. It carries the part No. 318073, with a foot note, "140" Kenny was not able to locate this part in his referance, and my factory service manual does not show, or make mention of it. What I am trying to determine is just what that part is. The parts manual shows it in the bowl area, not up in the main body around the throat. My quess was that it may be in the idle passage where the main body, and float bowl meet. It may also be in the roof portion of the float chamber, and is a screw in orifice. The parts manual does not show this.

 

[mnw001]

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

very intersting R J I will try to call a dealer tommorrow and see if its available at all..<br />Thanks again for everyones help It would be great if we could figure this thing out..

 

[walleyehed]

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

I'll do some further digging as well...

 

[mnw001]

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

first of all thanks to everyone helping with this thread.<br />Seems I have a few things going on..<br /><br />First I guess float levels were set wrong. I have reset them and don't seem to have the quite the richness problem. I can actually get the idle under control somewhat. were talking about 7-800 with timing around 2 degrees.<br /><br />Second I'm having a lot of trouble setting the linkage to the bottom carb. If I get it set to where it opens at the same time as top carb it seems to be open at idle which ofcourse speeds motor up. I guess I'll just have to keep working on that. I wonder if replacing the nylon retainers would help. <br /><br />Third: If I spray starting either on throttle shafts I get rpm drop at port side bottom carb. I think this may be the source of my backfire. When I watched it closely I did notice it popping out the carb a little at the same time it backfires out prop. I hope to correct this by installing newer idle jets. present are number 30 thinking of going two sizes smaller. Does that make sense..<br /><br />Fourth thing is that with motor tilted slightly up If I close choke and then crank motor. When I reopen choke I see fuel dripping out the front of both carbs with more from the top. only happens with choke closed. If I pump primer bulb it is hard and no fuel flows. Does the choke also prime these motors?<br /><br />Also anyone know if I could put later model carbs on this engine meaning late 80's or early 90's carbs?

 

[R.Johnson]

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

Your idle jets meter fuel. not air! You have to go larger. Some drooling of fuel out the carb throats is normal after using the choke. The choke shuts off air, pulling in more raw fuel. It is normally not referred to as a primer in the true sense. A primer is generally a seperate device That delivers fuel behind the throttle plates. On the 2 throat cards' such as your v-4, or v-6, The float should be parallel with the carb body when held upside down. I'm not surprized at the air leak around the trottle shaft, I always thought that the fit was a little to generous to start with. As for your lower carb not returning to close at idle. You have to correct this. You should have enough adjustment in the linkage to do so. There is a screw on the top carb linkage that you loosen to let the bottom carb snap closed. If you are going to ere on this, do so on the side of the carb being closed at idle, not on that they are going to open exactly the same. Did you locate, or identify that idle restrictor on the 79 carb.

 

[mnw001]

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

RJ I know your right about the idle jets. I wonder how much larger to get rid of a small backfire.<br /><br />The drooling from the choke seems excessive for some reason I could understand a small amount but this seems like a lot. If the engine wasn't tilted forward all that fuel would be running in the engine. <br /><br />I wonder if anyone has ever tried to engineer an oring in the throttle shaft. Maybe by machineing a pocket for it in the side of the carb or a groove in the shaft. Seems like it could help. The side that is leaking is the side with the recoil spring on it I wonder if a thin tightfitting washer under the spring might help. Seems like a lot of trouble just to get rid of a pop at low speed but that would drive me crazy.<br /><br />yes I know about the linkage adjustment its just very difficult to get it right on these carbs. It seems like if I let it just snap shut it opens late so I have to try to hold the linkage a little tighter and then it winds up too tight. There is slop in the nylon bushings which I think makes it a little harder.<br /><br />I talked to two dealers who did not know what the idle restictor was but they said they thought the part could be ordered. Its only like 2.00 so I guess I'l order one tommorrow and see if I can figure it out.

 

[jy118lfd]

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

Just another question? Did you do a compression test and if so what are the results? The pop through the carb, does it sound like a sneeze? The drooling you speak of sounds normal, when the choke plate is closed there is a large amount of fuel that goes into the carb. I think this is why they went to the primer choke system. Are you following a manual the set the link and sync with the carbs? <br /><br /><br />On another note I have a 78 150 and bought a set of 83 150 carbs on ebay that were like new. I just had to get the primer choke solenoid (e-bay)and my fuel line had the nipple for the fuel line to the primer. I got new line and a tee from the dealer and followed the routing in the seloc book. The primer mounts in the same place as the choke solenoid and wires in the same way. Now have an almost brand new set of carbs the new primer choke system and the motor will idle at 600 in the water in gear all day long.

 

[R.Johnson]

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

I have toyed with the idea of a seal, such as an O-ring around the throttle shaft. My thoughts ran more to the small fishing motors. I live in a small town here in Iowa, with a river right on the edge of town. Most of the people I deal with are fishermen, so idle is always their concern. One problem you would run into is that the screws holding the throttle plate is staked over after it is cinched down. You would have to remove this burr with a die grinder to remove the throttle plate, or you would ruin the throttle shaft. Then you would have to restake the screws after making the modifacation. I would not trust loc-tite, if that screw comes out, you could kiss a piston good-by. As you said, there is a lot of play in those nylon throttle link retainers. Why not have the dealer get you some new ones. You may just whip this problem yet!

 

[mnw001]

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

Jon <br />Yes compression test ranges from 115 to 120 which I am told is excellent for this motor.<br /> The pop I'm referring to is really more of a backfire or pop thru the prop alon with a very small amount of oh call it a puff out the carb. not really a sneeze. It wasn't there at all with the 135 carbs on so since the one shoaft has an idle drop when I spray it with either I'm guessing that is my problem.<br /> Yes I'm using a clymer to set linkage and I feel that I understand what needs to be done just having trouble getting it done..<br /> Boy I wish you had told me about the carb conversion a couple of days ago. I just missed a set of late eighty's to early ninety's 115 carbs with reed plate and reeds and primers on ebay. They sold for 41 complete but I didn't know if they would work on my motor because they seemed to have extra hoses and such that mine doesn't. I know the primers require hoses but what about block ventilation and vro stuff can that all be plugged off.

 

[R.Johnson]

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

We're just dealing with an idle problem! let's solve that. If you start changing carb's, manifold, reed cage's, etc, you are going to run into other problem's. That 79, 140HP has 1-3/8 throttle bore. It also has rubber stuffer's around the reed cages. That 115 you missed would not. You live in denver, you could solve the idle problem, and create a jetting problem at that altitude. Let's work with what we got.

 

[jy118lfd]

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

If the carb shaft is where the air is leaking RJ I would say the carb is shot as I dont think you could repair that. If he can get a set of newer carbs and jet them correctly for the altitude then he might end up with the best of everthing. He would not have to use the reedplate from the later motor cause the carb flanges are the same. It is just a suggestion cause carbs seem to be all over e-bay. I just picked up a set of 175 v6 carbs for the low price of 29.95 They are clean and really don't need kits although I will go through them. <br /><br />Fish the other hoses you saw were the recirc system and it could be adapted also as I have that on mine you would need a set of bypass covers to route the hoses to.<br /><br /><br />I also agree with RJ on trying to fix what you have. Lets exhaust all possibilties with the carbs you have first. You reset the floats and almost have your problem fixed. If you can see which throat sneezes then take a spray bottle with premix and when the motor is running spray a little in the throat of that carb and see if that does anything to help. If it does then the idle jet for that cylinder is plugged or too small. You can remove them and check the number on it with the dealer. You canorder a couple of larger sizes and try them. You can also drill them with a pin vise but I would only do this to test then replace it with a proper size jet.

 

[jy118lfd]

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

By the way don't use either on a two stroke unless you want an unexpected "inspection hole" in the block Always use premix in a spray container

 

[jy118lfd]

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

Hey fish I think I have a set of 140 carbs in my shed. I will not be home till wed. Let me look and maybe the shafts are tight. you could then put all the jets in these bodys and try it as a unit one at a time. I will get back to you.<br /><br />Leave me your e-mail

 

[mnw001]

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

Jon <br />email me at mnw001@hotmail.com<br /><br />Thanks again for all the help<br /><br />Local shop told me today that my carbs weren't bad enough to cause a problem. I'm going to test run it tonight with the shaft in question sealed with an oring. Hopefully that will tell me..

 

[walleyehed]

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

Fishslayer, wheel bearing grease works well to seal a thrott. shaft for testing too....<br />'nother "Briggs trick".

 

[mnw001]

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

Good idea Kenny<br /><br />Well after all of this I'm about ready to pull my hair out and I finally think I have found the problem. It appears to be wear on throttle shaft bushings but its not causing the problem I thought it was. When running the motor tonight I realized that if I held the shaft tigtly closed on the bottom carb my sputter and pop would go away. It seems that for some reason this carb won't cloase completley and that is where the motor is getting air and throttle. I believe the recoil spring is pulling the shaft up in an ovaled out bushing just enough to cause a problem. As long as I hold it closed it runs fine if I let up it pops. If I hold it closed enough with the linkage then the one on top opens too.. It seems like a wierd deal but I think this is it. I still need to remove the oring seal that I put on the shaft and try it but I believe the leakage from the bushings won't affect the idle circuit if the butterflies are completely closed. Anyway it looks like I'll need at least one carb although I think I could put a spring on it and it would run but I don't really want to mickey mouse it. Thanks again to everyone's help and I will keep you posted but I really hope I have it this time. Its been a pain in the *** and Jon I'l still be intersted in hearing about your carbs......