Johnson won't idle trimmed down all the way

[NekoYasha]

Greeting everybody!<br /><br />I recently bought a used Johnson 1991 90hp V4 as a replacement for a Force engine that died on me. Generally it works great, but in idle will choke up and eventually stall when trimmed down too far. <br /><br />It's fine if trimmed so it's more or less level, but, on my small boat, that can causes too much bow rise. According to the dealer, the carbs were all freshly cleaned and rebuilt, but I don't really know what kind of job they did. Also, although the same HP, it does weigh more than my old engine which, I think, may add to the problem.<br /><br />Anyhow I'm wondering if anyone knows if there's something that may help it idle better when angled down slightly, or is it something I'll just have to live with?<br /><br />Thanks!

 

[rodbolt]

Re: Johnson won't idle trimmed down all the way

hello<br /> I have had several 88,90 and 115 OMC's that did this. barring dirt dauber nests in the idle reliefs, I have found if it idles better as I trim it up then look to see if the carbs have plastic Minlon fuel bowls. I have seen the bowls warp slightly and leak at the gasket passage that seals from the bowl to the body and create a flooding problem at idle. usually it gets better as the engine is trimmed sometimes the idle speed will change 500 rpm or more.if no mechanical defects are present and the bowls are not warped you may try slightly larger idle air jets in the carb throats. good luck and have fun.<br /> Tom

 

[Solittle]

Re: Johnson won't idle trimmed down all the way

I had a simular problem - Do you know if your motor has the thermostats in it? If not they do not get up to operating temperature and one of the effects of that is poor idling. Raising the motor a tad changes the level of the fuel in the float chamber which helps the idle.

 

[ob]

Re: Johnson won't idle trimmed down all the way

NekoYasha,Also consider the height that the engine is mounted on transom.ie(X demension).Is the anti ventilation plate of lower unit even with the bottom of the boat when trimmed level?I'm thinking your engine may be mounted too low.This will also cause the problem you are experiencing.

 

[rickdb1boat]

Re: Johnson won't idle trimmed down all the way

I remember reading about that OB, but I can't remember the reason behind it. Something to do with exhaust??

 

[NekoYasha]

Re: Johnson won't idle trimmed down all the way

Thanks all!<br /><br />OB: Yes, it's about the right height AFAICT, but the back end of the boat does sit very low in the water making the boat itself not quite level while floating. Although rated for up to 100hp, somehow I think the engine may be heavier than the boat was ideally designed for. <br /><br />Still, I have the feeling this may be carburetor related too. The upper exhaust ports are clear of the water line, but you can really hear the engine bog down as the trim becomes less level. Level it out again and everything's fine. I'll have to check the bowls as rodbolt mentioned.<br /><br />Also it may be set to run a bit rich. Since our water pressure is low, out of the water I'll idle the motor using a plastic tank rather than ears (works much better) and I noticed a lot of unburnt oil from the exhaust coating the tank. <br /><br />I've ordered an owners manual for the engine so hopefully I'll have a better idea how to check the adjustments. Really I'm not even 100% sure where they are - though I'd guess the four small adjustment screws set at an angle in the barrels of each carburetor. <br /><br />For now I just raise the trim while I'm idling then lower it before giving it any power. This seems to work ok, but it would be nice if I could keep the trim in a better position for the boat.

 

[Hooty]

Re: Johnson won't idle trimmed down all the way

Yeah.<br />If the exhaust reliefs are under water, it sure would effect the idle.<br /><br />c/6<br /><br />Hooty

 

[rodbolt]

Re: Johnson won't idle trimmed down all the way

hello <br /> buy a service manual as well as the owners. the owners has very little usable technical information. if the bowls are warped you can buy new metal bowl kits from omc,but then you have to retap the main jet threads for your old jets or buy new ones. what I do is either surface the bowls if not badly warped or find a set of metal bowls from an older crossflow v-4 or v-6. the jet location for the intermediate jet may change but most bowls were interchangable.the little scres going up at about a 45 degree angle are idle air bleeds. dont play with them or they may end up being loose. loose ilde jets are a bummer.<br />if spark,compression and carbs are all fine and the cause is excessive backpressure due to the engine sitting to low in the water you may try changing the idle air jets. bigger jet means leaner idle fuel mix. I dont reccommend changing jets until a last resort.<br />and double check the heads with a temp gun or stick to insure t stats are working. most 2 strokes load up when overcooled and run slowly<br /> on the idle air jets i put a dab of gel seal on the threads making sure not to get any in the jet.<br /> good luck

 

[NekoYasha]

Re: Johnson won't idle trimmed down all the way

Thanks for your help everyone!<br /><br />So I guess the fuel/air mixture uses fixed jets for both low and high speed then? In my previous engine the high speed jets were fixed but there were still adjustment screws for low speed. <br /><br />Just doing a rough check, the temp on all four heads seems about right (once warm they'll be hot, but not too hot to touch for several seconds), but I haven't done anything more accurate than that.