1989 70 - update carbs for better idle?

I have had a 1989 70 for about a year now, and it does not like to go slow. After you've been running at speed for a while, you can idle in gear for about 1 minute, maybe a bit less, but then it starts to stumble and cuts out. The motor is fine above about 1200 RPM. I've completely rebuilt the carbs, including removing the core plugs to make sure the idle circuit is clear, but the low speed problem remains. It will idle forever on the muffs, or when it's not in gear, but put it into gear and you're on borrowed time. Needless to say that can make docking a little interesting.

OMC switched the 70 over to carbs with an adjustable idle mixture in mid-year 1989, but my motor has the fixed carbs. Last winter, I picked up the adjustable carbs from a 1991 70 on eBay for cheap, thinking to use them as spares, but now I am wondering if they are the solution to my low speed / idle in gear problem. If I install the newer carbs, and get the idle mixture adjusted properly, will that make a difference? Or is the low speed fussiness just a characteristic of this motor? I do know that loopers don't run as well at low speed as crossflows, but the current situation is unpleasant.

Thanks for your information and experiences.

Tim

Re: 1989 70 - update carbs for better idle?

Some of the 1989 70's were jetted a little too lean at idle. If your sync-link is ok, and the engine is running at the correct temp, try installing one or two sizes smaller idle air bleeds jets. Stock they should be size 33.

Re: 1989 70 - update carbs for better idle?

Thanks - that's very helpful. It does seem like the motor is fuel-starved at idle, so I bet you're on the right track. As jets are about $8 apiece, I'd like to keep experimentation to a minimum - would you go down 1 size or 2 sizes for the first try?

Also, I notice that earlier years had the same #33 idle orifice - did they also have the problem? You seem to have specific knowledge about the 1989 - not questioning that, but just would like to know the details.

Tim

Re: 1989 70 - update carbs for better idle?

I think that I have rejetted several from 87-88 also, but it seems like the problem was more common on the 89's. If I were to bet on a jet size, I would go down two jet sizes. P/N for a 31 jet is 318823.

Todd

Re: 1989 70 - update carbs for better idle?

Thanks a lot Todd - I'll get them ordered! Really appreciate the help - I'll let you know how the story turns out.

Tim

Re: 1989 70 - update carbs for better idle?

i had that problem on my 70 after I rebuilt it with oversized pistons.Why,I dont know,but I worked in a rebuild shop and we found the problem a few times.save money,you can solder the jet closed on backside is easier,then get a small drill index with bits that have size you need,you can then experiment,some of the cheap tool houses like harbor frieght and northern hydraulics,a dremel is a good tool for drill motor

Re: 1989 70 - update carbs for better idle?

Thanks Mike! I'm not sure I have drill bits that small, so I'm trying the 31 orifices. I do appreciate the money saving idea - if the 31 don't work I may try that.

Tim

Re: 1989 70 - update carbs for better idle?

Well, it's been a long spring and I FINALLY got a chance to run with the new idle orifices. I went down 2 sizes, as advised. I was only able to do a quick test but things seem much improved. Still not smooth at idle, but it did keep running, which was a nice change.

Todd, thanks for the tip!

Tim

Re: 1989 70 - update carbs for better idle?

After some more testing, the idle has improved, but I still can't idle very long in gear. What's different with the new jets is that I can just drop it into neutral and it will keep going forever.

Is it possible that having the idle speed set too HIGH is causing the stalling? I set the motor to about 1200 on the muffs, which worked out to around 850 RPM on the water, against the spec of 700.

I would have thought that having too low an idle speed would make it stall, but maybe too high is worse?

Thanks for any additional input.

Tim