78' 200 hp Carburetor conversion

[mtaggart]

I have a 78 Johnson 200 and was curious to know the effects of updating the carbs to the primer choke system with a type III carb versus the butterfly choke system and the type II Carb. Is there a notable advantage in having the intermediate orifice? I was also wondering what the effects of using a carb with 1 3/8 throats vs the current carbs at 1 5/16 throat? <br /><br />Currently when I put down the throttle on my 19ft it hits a dead spot and then takes off if it doesn't die. The engine idles fine and runs fine at full throtle. I'd like to know if the intermediate jet/orifice would help with this?<br /><br />Looking in a book it looks like they went to a type III in 1979 and kept that type until 1983. Curious to note that the 200hp was then gone for two years, and then the CU IN displacement changes to a larger number when it came back. I suppose that was due to where the horse power was rated from?<br /><br />Another question is concerning the advance. I have read discussions where it suggests advancing the timing 1 degree for every 3,000 feet in elevation. Is there any truth to this?<br /><br />Thanks in advance,<br /><br />Matt

 

[emdsapmgr]

Re: 78' 200 hp Carburetor conversion

The newer carbs with the fuel primer system work very well. A relatively easy swap, with newer fuel hoses. You will get into rejetting issues if you go to the 1 3/8 carbs. Most of these carbs were installed on the larger 160 cubic inch models. Some of the last XP/GT 150's made used these large carbs. They can be adapted to your engine-with some patience. I think the story about altitude is that you decrease jet size slightly as you go up in altitude. Do not think you ever want to advance timing.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

Thanks for the input. I found a carb set with a solenoid, that is the same size as mine 1 5/16. Using BRP site I'm unable to pull up the carb from the number stamped next to the throat size. Is there another way of determining the year and info on the Carb? If I buy them I'd like to get the right carb kit.

 

[Dhadley]

Re: 78' 200 hp Carburetor conversion

All crossflows us pretty much the same kit, 439076. Dont change the jetting, use it as produced as long as youre sure it's all stock.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

Thanks for the kit number Dhadley. I've been unable to determine the year of the carb, once I get the carbs, does anyone know a way of finding out if what I have is stock or not?

 

[emdsapmgr]

Re: 78' 200 hp Carburetor conversion

There are a few things you can do to try to determine what the year is. The top of the carb varies somewhat and you may be able to isolate the year to a series of years, based on what the top of the carb looks like. The newer crossflows have an idle circuit on top. Also, when you overhaul the carbs, take the three sets of jets out and note their sizes. You may be able to match that size combination in the OMC parts catalogs and determine the year and horsepower.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

emdsapmgr it looks like you've got some early mornings or some really late nights. I was able to do as you said. Two things that helped, the top where the primer goes and the low speed orifice. Based on those two things and some good help from the BRP diagrams, I know it's somewhere between 1983 and 1986. Was it common with OMC to keep the same size throat for the same HP through the years?

 

[emdsapmgr]

Re: 78' 200 hp Carburetor conversion

Yes. Through changes in the design of the carbs, they sometimes are able to reduce the jet size on newer models (to burn less fuel,) yet provide designed hp. General rule: the 1 3/16 carb was common on some 150's. The 1 5/16 carb on the 150 XP/GT models, 175 and 200's. The 1 3/8 on the 160 cubic inch models (235 and 2.6) There are exceptions. Your 200 is a high port engine that will take a lot of fuel. If you put a newer 1 5/16 carbs on your 200, be sure it will flow the fuel you need. If the carbs are 1 5/16, but from a 150 XP/GT, you may need to jet it up to keep your same performance and not run the engine lean. Just transferring your older high speed jets may not work perfectly with the new carb,due to carb design improvements. A little patience on re-jetting and plug-reading will get you where you want to go.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

I was given some low speed jets that are quite small. In playing with the jetting it can get pricey paying $6 bucks a pop if you don't nail it the first time. I've heard different things about drilling them out. What's the general concensus on drilling out the jets?<br /><br />Most of my lakes are still frozen, but I'm getting excited to get out and play.

 

[Dhadley]

Re: 78' 200 hp Carburetor conversion

Do you know for sure you need to rejet yet?

 

[emdsapmgr]

Re: 78' 200 hp Carburetor conversion

Drilling jets is not a good idea. They are specially machined and may even have a specific twist finish inside. A regular drill will not put the right finish on the bore, and it could even flow less fuel after drilling. Get the high speed jetting correct first before you worry about the low speed ones.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

I have no idea if I have to rejet. I only assumed I would here in the rocky mountains at about 4500 feet. One manuel I have said to reduce the high speed 2 sizes for every 3,000 feet. I've also heard due to less atmospheric pressure or altitude the low speed is actually opposite of what you might think, and you should actually increase the size. Any thoughts on this? I also received what might be an OMC bulletin that shows what you should do for different carbs. It is a little different than what some have told me. I'd be happy to scan the page if someone could tell me how to get it on?<br /><br />If drilling is a bad option it would be nice to have a bunch of demo jets I could try until I find what I like. Just dreaming.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

Well, I picked up the 1 5/16 Carb on E-bay. I actually got a bonus set that's 1 1/4. I suppose it may come in handy if I need to swap the jets and they happen to be the right size.<br /><br />I tried to put the link so you could see what they look like. Hope it works.<br /> http://cgi.ebay.com/ebaymotors/ws/e...ndexURL=0&photoDisplayType=2#ebayphotohosting

 

[emdsapmgr]

Re: 78' 200 hp Carburetor conversion

From the looks of the pics, you are all set to do some testing. Good luck. BTW, the primer solenoid really helps out when you want to do high speed jet testing.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

I'm curious to have you expound on what you said.<br /><br />"BTW, the primer solenoid really helps out when you want to do high speed jet testing."<br /><br />What should I look for?<br /><br />Also I've heard you can use engine tuner through the primer solenoid. Is that a matter of pulling the hose and shooting it in, or is there another preferred way? In the past I would take off the air silencer cover and shoot it in through the carb throats.

 

[emdsapmgr]

Re: 78' 200 hp Carburetor conversion

Depending on the vintage, the primer solenoid may have the schrader valve. It has a red cap. Very early models did not have the valve. (If yours does not, you can add it.) You can directly connect both OMC's direct injection Fogging Spray or Engine Tuner into that valve. You no logner need to remove the airbox cover. When jetting the engine, it helps to have the primer solenoid. When running flat out, if you hit the primer solenoid and the speed\rpm picks up, the engine is running lean. Time to jet up.

 

[mtaggart]

Re: 78' 200 hp Carburetor conversion

I received the carbs, they turn out to be 1983/84 stock carbs with a .033 air, .031 idle and .060 high. I totally cleaned them out. Turns out the Carb cleaner also does a good job at removing the prints from my fingers. I received the carb kits in the mail last night. On my old 78'carbs the original jetting was set up for .061 high, and .031 idle. To make up for the altitude at 4500 the dealer installed .058 for high and .32 for idle. A bulletin I believe to be correct shows what should be done for 80'-82'carbs on a 200hp at 3000-6000 it decreases air by .004 increases idle by .002 and decreases high by .003. With the extra carbs and my old carbs I can pretty much make the changes with all of the extra orifices except for the high speed I only have .055,.058 and .060. If any one has any thoughts on making this adjustment or an updated bulletin for this series of carbs I'd really appreciate it. The orifice I'd like to get right the first time is the idle fuel or low speed (the one that is inside the body) Thanks again for all the help and patience.

 

[emdsapmgr]

Re: 78' 200 hp Carburetor conversion

An engine that sits deep in the water at idle will have more backpressure than one that sits higher. This affects the idle speed setting-even can affect idle jetting. You may want to run it first and note operating characterisics at all rpm ranges. Keep notes when you run it and check the spark plug color when running high speed.