more on 79 140 that backfires

[mnw001]

After a lot of work I beleive that the backfire's were caused by my retarding the idle timing too much because I didn't have a timing light. I did this to try and get it to slow down. I have now used one to set slow speed timing to 4degrees. The problem now is the same as when I started working on this motor. The motor won't slow down more than about 15-1800(guess)rpms. Removed air horn and butterflies are closed and chokes are fully open. I have sinked carbs and throttle was not touching throttle cam. <br />some of the things I've done are.<br />Checked flywheel key<br />disassembled carbs which were very clean as if they had been done recently. Have sinked linkage according to book.<br />didn't have float gauge but them about a 32nd higher in front than level with the bowl which is about where they were.<br />reclamped all fuel hoses<br />metered coils and power paks all ok<br />low speed jets are 30 high speed 65c with no adjustment.<br />compression 115-120<br />Also replaced defective rectifier<br />I bought this bought at an auction and have no history on it. The casting finish on these carbs looks different one smooth one rough. They have the number 13/8 on the front of them. I'm stumped would appreciate any ideas on things to check..

 

[ezeke]

Re: more on 79 140 that backfires

Did you check the idle stop with the control in neutral and the warm up lever down? <br />Do you have the OMC service manual? If so, see page 2-16, item 4. If not maybe someone could send you a copy of the page.

 

[Walker]

Re: more on 79 140 that backfires

Are your testing the motor in the water or on Muffs. They will idle a little higher on muffs because there is less exhaust backpressure. Is the airbox installed also for the same reasons.<br />Are you sure your tach is reading right.

 

[mnw001]

Re: more on 79 140 that backfires

The idle stop is the throttle cable and I have the end off and the throttle lever is back all the way, As I said the carb shaft is not touching the throttle cam. <br />I don't have the service manual just a clymers which is not very good, I would appreciate it someone could send me pages. mnw001@hotmail.com<br />Run motor in tub and on muffs same result either way. Same thing for airbox on or off makes little difference Thanks again

 

[ezeke]

Re: more on 79 140 that backfires

If you have the Clymer B736 Manual, you might take a look at page 240, picture 45, "Cam Follower", marked number 3. That roller should be exactly where it shows, on the upper mark.<br /><br />At that point, with the control in neutral and the warm-up lever down, all of the throttle valves should be closed.<br /><br />Any motion from there on the control should begin to open the valves.<br /><br />Your 65C Orifice is stock for the 79 140hp at sea level to 3,000 feet. From 3,000 to 6,000 wanted the 63C and 6,000 to 10,000 feet the 61C. Dealers can get these for you. I don't think I'd fool with the low speed unless you want to go real slowly most of the time.<br /><br />Hope this helps.

 

[mnw001]

Re: more on 79 140 that backfires

Ezeke<br />Yes I have that manual and I've made sure that the throttle vales are closed. with timming at 4 degree the idle is very fast. I am at 5200 feet in Denver but the high speed jets shouldn't affect idle speed should it. Also I'm checking idle in neutral instead of in gear in the water as the book suggest but surely that wouldn't make a difference of 500-1000 rpm's would it.<br />Previous owner just told me he never noticed a high idle problem but i think he just didn't want to tell me about it.

 

[Qld_Kev]

Re: more on 79 140 that backfires

The only other thing I'm thinking is the butterfly linkages sucking air from too much wear??

 

[R.Johnson]

Re: more on 79 140 that backfires

Here is one way you can at least trace it down to the carb that is running lean. Use a plastic squeeze bottle filled with pre-mix. Spray lightly into each carb throat to stop the lean spit-back. Then you will know which carb, or which side of the carb is giving trouble. One place you might ckeck is the soft plugs of the offending carb. With the float bowl off, and the carb upside down youwill see two soft plugs about the size of a dime. An air leak around these plugs will effect the idle. If you replace them, us a light bead of type M sealer around the seat before seating down the plug. When you adjusted the idle pick-up timing, did you do this by adjusting the yoke on the throttle arm? This is the correct way.

 

[mnw001]

Re: more on 79 140 that backfires

Butterflies don't seem to have too much wear, I sprayed wd40 on the shafts while running and it made no diference. <br />R J My problem now is not a backfire or spit assuming that 4 degrees is proper place for idle timming. it just runs fast now but does backfire.<br />and yes I'm adjust yoke on throttle arm.<br />these carb venturies are marked 1 3/8 I read in the clymer manual that some 83 150's came with this size venturie and it was too large and should be replaced with 11/4 venturies. Does anyone know if the 79 140 should have 1 1/4. maybe someone put these carbs on before I got it. The number stamped on the casting of the top carb is 324859 if that helps. Thanks again for the help

 

[R.Johnson]

Re: more on 79 140 that backfires

The 1-3/8 carb is correct for the 79, 140. My manual shows it should be 5 degrees, but I don't think that will make a difference. Did you check the position of your timing pointer to make sure it is in sync. with the piston. With out being there to listen, I'm at a loss as why you can't calm down that idle.

 

[R.Johnson]

Re: more on 79 140 that backfires

Did you try putting the choke to full off manually?

 

[Dhadley]

Re: more on 79 140 that backfires

There is another possibility. If one of the reeds is hanging open or possibly has a broken tip the motor will idle rather high and its nearly impossible to calm it down. <br /><br />I worked on a V4 crossflow that had swallowed that yellow plastic plug thats clipped on the fuel line of an oil injected motor. Anyway the plastic piece had went thru the carb throat and was hung up in the reeds holding one slightly open. Popped it out and away we went.

 

[mnw001]

Re: more on 79 140 that backfires

Rj will check pointer<br />Choke is fully open<br /><br />Dhadley <br />I looked reeds over when carbs were off, didn't see anything. this motor I'm sure has less than 1000 hours but hard to tell. The reeds are steel do they get weak. one man thinks I'm getting too much air somehow but I've looked for leaks and not found any..What about bottom crank seal could that cause this problem and how do I check it. Top crank didn't show any signs of leaking

 

[Walker]

Re: more on 79 140 that backfires

about 15-1800(guess)rpms

You need to put an accurate tach on this motor to verify what rpm its idling at.

 

[mnw001]

Re: more on 79 140 that backfires

Walker I will work on that but I have been around boats for a long time and I can definatley tell when a motor is idling way too fast. I will work on a tach<br />new info: just ran the motor for 20-25 minutes on fast idle. I did check timing pointer and it was fine. I pulled the plugs after running and all four were very wet but bottom two were quite black compared to others. Top two were wet but insulators were a dark tan color. insulators and ground bar were black on bottom two. I was running it in a large tub and the water was filthy as it usually is with this motor having globs of oil floating on top. To reach in the water my hand would have a greasy slime on it. I'm sure this thing is running way too rich but I can't figure out why. Will richness translate to extra rpm's? Slow jets are marked 30 which should be stock for sea level and with current 87 octane and 5200 feet el they are suppose to run ok. I wonder if they could have been drilled or something. I have a 73 135 that runs great. I think I will put those carbs on the 79 and see if it will idle that should tell me something. Anyone know if that makes since?<br />Any other Ideas? if not I'll post again when I know more..Thanks again everyone.

 

[ledgefinder]

Re: more on 79 140 that backfires

Actually, it'd rev high if it were a little lean...<br /><br />Those 135 carbs ought to at least allow you to figure out whether it's carbs or timing.

 

[mnw001]

Re: more on 79 140 that backfires

Walker I will work on that but I have been around boats for a long time and I can definatley tell when a motor is idling way too fast. I will work on a tach<br />new info: just ran the motor for 20-25 minutes on fast idle. I did check timing pointer and it was fine. I pulled the plugs after running and all four were very wet but bottom two were quite black compared to others. Top two were wet but insulators were a dark tan color. insulators and ground bar were black on bottom two. I was running it in a large tub and the water was filthy as it usually is with this motor having globs of oil floating on top. To reach in the water my hand would have a greasy slime on it. I'm sure this thing is running way too rich but I can't figure out why. Will richness translate to extra rpm's? Slow jets are marked 30 which should be stock for sea level and with current 87 octane and 5200 feet el they are suppose to run ok. I wonder if they could have been drilled or something. I have a 73 135 that runs great. I think I will put those carbs on the 79 and see if it will idle that should tell me something. Anyone know if that makes since?<br />Any other Ideas? if not I'll post again when I know more..Thanks again everyone.

 

[Walker]

Re: more on 79 140 that backfires

I think that engine has recirculation valves to each cylinder. Check them as per your manual. They should only flow in one direction.