70 johnson idle mixture

I have a 1981 70 horse johnson on a 16' rinkerbuilt tri hull. I lost spark to the center cylinder last weekend. After replacing the powerpack today I found that the bottom cylinder was not working at idle. I'm sure it has been like this for a long time. It is working off idle as boat jumps on a plane quickly and runs about 30mph. any way I swapped the top and bottom carbs. It now misses on the top cylinder. I believe this one carb is a little lean as it sneezes repeatedly when running at idle on muffs. I played around with the syncronization to no avail, it either doesn't work or sneezes and works intermittently.

My question is can I put the adjustable idle mixture screws in my carbs or would i need to find the carbs that are adjustable(I think if I could adjust the idle mixture this problem would be solved, a little richer on this one carb)

Would you just ream the idle jet on the offending carb. out a little bigger(if I was sure this was the problem I would have already done this)

I have removed plugs from carbs and cleaned all passages. Compression is a little low(about 110psi) but is even.

Any input is greatly appreciated. Thank you in advance.

Re: 70 johnson idle mixture

may still be trash in carb. if not get some jets and richin up that cyl,
if your leaning out one. spray some premix and see if it idles up
check the jet sizes in all carbs and match to brp numbers on there site to see if there correct .

Re: 70 johnson idle mixture

These idle jets come in different sizes?

Re: 70 johnson idle mixture

Have you rebuilt the carbs yet. If not, I would do that before I tried anything else. Sounds to me like the low speed jet has some trash in it.

Re: 70 johnson idle mixture

if you have good spark and compression, do the carbs, make sure they are clean, do not get parts mixed up, or jets out of place.

same basic idea as this, http://forums.iboats.com/showthread.php?t=244013

Re: 70 johnson idle mixture

The cover screw for the jets are different, i always keep the jet sitting with the cover screw so they don't get mixxed up.

Probably don't go messing with jet sizes, that is best left to proffesionals and I don't think that is the problem, just alittle dirt in the ones you have and you can really mess the engine up if you get it wrong.

Use a stranded copper wire to clean the jets, pull back strands till it just fits then run it through the jet several times.

Be sure to run a wire through the pasages of the carb and use compressed air to blow everything out and work on an old T shirt to keep things clean.

It really isn't hard, take your time and do one at a time then once you have done it you know how.

If you don't have an OEM manual I would get one, they pay for themselves the first time you use it.

Re: 70 johnson idle mixture

Oh, missed the obvious.

After 74ish they took off any adjustments on the carb and went to jets so there is no way to adjust anything, all you can do is clean the jets.

There are 3 jets in each carb ,idle, mid and high speed and if you were turning a screw thinking it was an adjustment it is probabaly a cover screw for a jet and won't do anything.

Re: 70 johnson idle mixture

5150 abf, I just did the carbs on my 77 70hp and only found 2 jets. The idel jet above the intake and the high speed jet in the bottom of the bowl. Did I miss something? Also I always read about not getting the jets mixed up, I don't know how you could do that as they are nothing alike.

Re: 70 johnson idle mixture

77 only had the 2 jets, idle, and high speed, 81 had 3 idle, mid, high.

Re: 70 johnson idle mixture

Thanks TD!!!~!! Thought I missed something. Sure idels better now.