84 70hp hesitation.

[Kh0432]

Engine starts and idles great. When first started I can give it throttle and get on plane with no problem. After the motor is warm, it will start fine but as you give it gas in the mid range it will bog down. If I press the primer, it will take off and I can get on plane and run wot. I can also back down on the throttle and it will run fine. Sometimes it won't bog down at all. Carbs have been cleaned, a missing jet replaced, new fuel pump, tank and lines flushed and a filter installed. Runs like a stripped ape at wot. Timing has been set and carbs synced. Thanks for any ideas. Keith

 

[fireman57]

Re: 84 70hp hesitation.

If you can hit your primer and it takes off then it is definitely fuel. You missed something in the carbs.

 

[Kh0432]

Re: 84 70hp hesitation.

Why does it not do it all the time? There are no adjustments for these carburetors and it doesn't look like different jets are available. I would think they would sell jets to compensate for high altitude usage. This boat is used at sea level.

 

[fireman57]

Re: 84 70hp hesitation.

Because you have a piece of crud probably in your low or midrange jet and it will get sucked in and block it for a bit. Once you get going it may fall back out only to be sucked up again and cause your problems. Could be dirt or a piece of fuel line that has been broken down from ethanol. You will get very efficient at taking carbs off and cleaning them over the life of your engine. I'm a little confused by your last line. Why would you worry about high altitude jets if you are running at sea level

 

[Kh0432]

Re: 84 70hp hesitation.

The reason I ask about jet size is because the owners manual states that the carbs should be re jetted for high altitude but the parts breakdown doesn't show different sizes. When I bought this boat I spent a lot of time trying to get it to run. The seller told me the only problem was that it wouldn't idle. When I overhauled the carbs, I found 1 jet missing from only 1 carburetor. Found later that the power head was blown and found another motor that was the same year and model. I figured it would be an east swap and wound up with a lot of extra parts. Almost recouped my money selling the extras on eBay. Compression was good on this motor but I could never get it quite right so I sent it to a marine mechanic and he called and told me a jet was missing in 1 carb. I remember seeing a technical bulletin for an 83 70hp which stated to remove 1 jet from 1 carb to cure the hesitation. I find it odd that I had 2 exact motors missing a jet.

 

[bonz_d]

Re: 84 70hp hesitation.

check the floats also and see if one is hanging up.

 

[fireman57]

Re: 84 70hp hesitation.

Wow, that is strange that a service bulletin would say to remove a jet completely. I know that on the '76 70hp's you had to drill them out as I have one of them. You might do a search for all service bulletins for your engine and see what you come up with. It is definitely a fuel delivery problem though.

 

[Kh0432]

Re: 84 70hp hesitation.

One thing I forgot to mention. Once I get the boat to wot, I can back off on the throttle to mid speed and it will still run fine. I would think if the intermediate passages were clogged it would die out. I can also go from mid speed to wot with no problem. I'm leaning towards the intermediate jets being too small. The jets are relatively cheep so I may consider buying them and drilling them a little larger. I don't think I even need to take the carbs off to change them. Keith

 

[Pink Shibi]

Re: 84 70hp hesitation.

i had two similar issues on my 2001 johnson 90hp.
bogged down at 2k rpm because i forgot to put the jets back in. this problem only happened in the water under load.
second time my carb bowls were warped and letting air in the sides of the bowls reducing the amount of fuel getting to the engine also letting fuel out which was pretty dangerous in my opinion. so it would bog out. then if i got it over the hump it would run ok not great. but it made the fuel pump(VRO pump) work harder(and put more oil than it needed) and i would have to pump the fuel ball a few times now and again. my carb bowls are made of plastic and i dont know how they warped so bad but changing them helped alot.

 

[Daviet]

Re: 84 70hp hesitation.

Contact your local dealer, they should have a chart for jets at different altitudes.

 

[boobie]

Re: 84 70hp hesitation.

What tech bulletin was that that said to remove one jet from one carb ??

 

[fireman57]

Re: 84 70hp hesitation.

If you have plastic carb bowls I would inspect them first for warpage before I started rejetting.

 

[Kh0432]

Re: 84 70hp hesitation.

I have metal bowls. No gas leakage anywhere.

 

[Pink Shibi]

Re: 84 70hp hesitation.



so you have changed fuel pump, fuel filter, flushed tank, and lines, carbs have been checked. now some other things to think about. Ethanol being the first thing that came to my mind. Ethanol eats rubber and here are some examples. you said you flushed the fuel lines, but did you see any particles/plastic/crud coming out? it can eat the lining of your fuel lines especially if its older fuel lines not designed for ethanol. also can eat o-rings. if a flap was formed from the shrinking/hardening of interior lining it could fold over and restrict fuel inside fo fuel lines. this would be why your motor is running like a "stripped ape" which i believe means fast ie. your running lean because of a fuel restriction. the other way around is ethanol ate an oring which would let air in the line and reduce fuel to engine also making the engine run lean. the air in fuel line could be checked by splicing in a piece of clear tubing to check for air bubbles(dont forget to take it back off though). i would do this as close to the carbs as possible. the fuel line you would have to cut to check and see.

 

[Kh0432]

Re: 84 70hp hesitation.

All new fuel lines from tank to carbs, including primer bulb.

 

[fireman57]

Re: 84 70hp hesitation.

Still would like to see that service bulletin. It is established that you have a fuel delivery problem that shows up going to mid-range and not coming back down to it. That tells me that the low speed jets are fine because it idles properly, there is a problem in the mid-range but once it gets passed that the high speed jets are fine. Kind of narrows it down in my mind but then I have tunnel vision and can't get the thought out of my mind that you missed something in a carb or two. Are you sure that the little brown "three leaf clover" gasket that fits over the pickup tube is in each of the carbs. They fall off all the time and are easy to miss putting things back together.

 

[Kh0432]

Re: 84 70hp hesitation.

They were on there when I rebuilt the carbs but they have since been taken apart twice by a mechanic. I can live with the problem for now since the boating season here is almost over. I will probably go back through them over the winter but I'm still hoping that someone can tell me if there is a modification that I need to do to them. I saw a bulletin for 1975 that addresses the problem and would think that Johnson corrected it in later years.

 

[fireman57]

Re: 84 70hp hesitation.

The bulletin was for a '76 70hp, the problem was supposedly corrected for the '77 but I have found one '77 that needed it done. I suppose that someone swapped carbs from a '76 to it. That bulletin will have nothing at all to do with your engine. If you remember seeing a bulletin saying to remove a jet we all would like to see it. I can't find one anywhere that I have looked and am out of places to look for them. You can't just have a gaping hole in a carb and expect it to work.

 

[Kh0432]

Re: 84 70hp hesitation.

I agree with you 100%. I still can't find the bulletin that I stumbled across. But it's still odd that 2 identical motors of the same year and model were both missing 1 jet. It seems to make more sense to rejet all 3 carburetors than taking out 1 jet in 1 carb.

 

[fireman57]

Re: 84 70hp hesitation.

'zactly.