Backfire/misfire thru carb

[Beno]

the engine clearly has had work, there is that dark gasket material allover the assembly seams. I got it as a “great running engine” some time ago

I pulled the carbs and took the (square?) bowls off.. shockingly clean.

I will get my video scope and try to make sure the butterflys are matched closed.

[fleetwin]

OK, so the engine has run normally for you before, just started messing up now….
With the carburetors installed, remove the black intake cover and have a look at the butterfly valves with the throttle at dead idle… Make sure the throttle cam is not touching the throttle roller though. Both butterflies should be completely closed. Feel free to post pictures of the throttle linkage when the engine is at dead idle. Go ahead and do a compression test as well…

[Beno]

sorry, I meant that the engine was sold as “good running” to me. I have never run it before.

\I have the black cover off, its hard to see because of the choke, but even with the choke open it’s down inside the boat (Sunbird/OMC L-Drive, jetboat) there is no Transom on this style OMC.

the throttle linkage is definitely touching the butterfly controls at the lowest setting, I’ll get some pics tomorrow.

[fleetwin]

Oh boy, you have got quite the set up there. Not OMC’s finest hour for sure. So, how did you mount the older 140hp in that cave? Or, did you just put the older 140hp powerhead onto the existing set up? Some pictures would help us understand just what you have/set up.
Try loosening the screw to the roller, and move it away/off the cam at dead idle speed just to see if that makes any difference. Try to take a picture of the linkage before you make any changes, I realize it is tough to get down in there though….Take pictures of the black sealer you mentioned also.
So, you have never had this engine running before, so we really don’t know what we have here. Try to do a compression test as well..

[Beno]

lol.. no no. it’s all engineered like this from the factory.. years ago i pulled the 1997 115hp omc bubbleback and replaced it with the 1978 140hp bubbleback.

it’s a straight bolt on swap. I just have to mix the fuel now since the oil injector never came with the buy (oil tank did though)

my engine is white…
These is not my engine pics attached but just examples of how a sunbird sizzler Ldrive is constructed

[fleetwin]

OK, so you have the “OMC Turbo jet”. First, I’m pretty sure that this unit has a fairly high idle speed to make sure it pumps enough water to cool the engine. And yes, it will be tough to look inside the carb throats to ensure both butterflies are closing fully.
So, like I said before, try moving the cam roller away from the cam slightly at idle speed..
Try disconnecting the link between the two butterflies, this should ensure both are closing fully provided the roller is not touching the cam….And yes, it is possible that one of the carbs has some sort of binding issue/wear causing its butterfly to stick open slightly, but this is not that likely.
Let me know if either of these steps clears up the idle, we can go from there with the syncronization….

[Beno]

Ok heres what I’m dealing with.. I think the black is just sealant.
upon closer inspection it looks that both butterfly’s are closed at the lowest setting.

[fleetwin]

OK, well I don’t want to be a “downer”, but looks like some pretty shoddy work has been done in the past… A few things of concern in your last picture:
That idle speed screw is cranked all the way in….This usually indicates idle issues that could only be dealt with by cranking up the idle speed
The plastic threaded link is upside down on the throttle cam, the engine will never work correctly this way…

I hope you didn’t pay too much for this, any chance of returning it? What happened to the original powerhead?

[Beno]

more pics

Otherwise I have them all uploaded at
https://link.shutterfly.com/kDOEeP8xg9

[Beno]

interesting.. I didn’t notice that plastic linkage reversal.. I’ll get it fixed, but I think that would just be a position thing.

the original 115 had a cylinder turn into a meatball about 12 years ago. I sold the long block to a guy who was going to rebuild it in lake champlain.
couple years later I found this 140hp bubbleback on a electric transom, the lower unit had self destructed.
The seller said the powerhead had been bored .30 over and was “a good runner”.

I accidentally had the throttle up on start once and it revvs up very impressively… it’s just not happy at Idle.

I dropped the powerhead on the LDrive and put it in storage, been there ever since.

I’ve been selling online all year and with all this government molesting the economy with all their inflation/stimulus and PUA/jacked up unemployment I’ve been selling stuff like CRAZY. So I pulled this jetboat out and trying to get it sold.

[Author]

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