carb assembly replacement compatibility

I am in the process of rebuilding the carbs in my 1979 Johnson 100hp only to find that the upper carb is in pretty bad shape...lots of scratches and nicks in the coreplug casting, lowspeed orifice, stripped screws etc. I'm affraid this is why I'm having problems at low rpms. If I have to replace the carb it's self, will I have to replace with a '79 100hp carb only or will other year and or V4 model carbs work...if so what year and hp carbs are compatible?

Re: carb assembly replacement compatibility

According to the BRP parts manual site, the 85HP, 100HP, 115HP, and 140HP J/E all used the same carburetors in 1979: 0389989 upper; 0389988 lower. Jetting would of course be different for each HP.

There will of course be others that will work. Check the salvage yards, I am sure there are plenty of those around.

Re: carb assembly replacement compatibility

ezeke said:

According to the BRP parts manual site, the 85HP, 100HP, 115HP, and 140HP J/E all used the same carburetors in 1979: 0389989 upper; 0389988 lower. Jetting would of course be different for each HP.

There will of course be others that will work. Check the salvage yards, I am sure there are plenty of those around.

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so when you say the jetting would be different and that these other carbs would work...do you mean I could possibly use assemblies for different HP motors but use the orifice plugs I already have to make it 100hp?

would you mind posting a link for the "parts manual site" just for my future reference? Thanks

Re: carb assembly replacement compatibility

Any V4 crossflow carb from 73 on will work on your V4 crossflow. The carbs are jetted for the airflow going thru them, not for what motor they're on. A carb has no way to know what it's bolted to but it does know how much air is being pulled thru it's throat.

If I were going to change one carb on a 79 100, I'd get the latest V4 crossflow carbs (both) as I could find and change them as a set. Leave the jetting as is on whatever carb you use as long as that jetting is stock.

Re: carb assembly replacement compatibility

Dhadley said:

Any V4 crossflow carb from 73 on will work on your V4 crossflow. The carbs are jetted for the airflow going thru them, not for what motor they're on. A carb has no way to know what it's bolted to but it does know how much air is being pulled thru it's throat.

If I were going to change one carb on a 79 100, I'd get the latest V4 crossflow carbs (both) as I could find and change them as a set. Leave the jetting as is on whatever carb you use as long as that jetting is stock.

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so let's say I found a set of carbs that were for a cross flow V4 140hp and installed them on my 100hp...how would it change the way my motor ran/performed? I know you said the carb has no idea what it's bolted to but how would the motor react to carbs that were intended to run on a motor designed for a significant increae in power?
Thanks for the help!!

Re: carb assembly replacement compatibility

There are at least a few sizes of carbs/bodies that went on OMC crossflows. One of the differences is the throat diameter. This diameter was cast into the front lip of the carb. Some of the small 85 hp engines used a 1" throat. The mid hp engines used 1 3/16" and the high performance engines used a 1 5/16" throat. You may have the 1 3/16" throat carbs. The suggestion of replacing in pairs is a great idea. Look for a set of the large throat carbs and bolt them on. Ebay has them all the time-inexpensive. These larget carbs were used on the 135/140's and the 1990's 115's. The later year carbs have no choke plates, so you will need to change to a fuel primer system and a new fuel rail. (bolt on parts) You will make significantly more hp. The carbs through the years are designed internally differently, and the jets in the carb are matched to it, so start out with the jets that come with the new carbs. The downside is that the larger carbs may flow more fuel than your engine needs. You may need to jet it down very slightly for optimal performance on your block. If you are somewhat handy, the increase in performance is worth the time you will invest in making the change.

Re: carb assembly replacement compatibility

There's a general rule of thumb that you can use with carbs. The larger the venturi, the more top rpm you'll have. The tradeoff is acceleration. The smaller the venturi's, the more acceleration. The trade off is top rpm's. You can see where we use that on race boats and performance boats that have pretty constant loads. With recreational boats you have to factor in differences in loads and the motor's ability to carry a heavier load from time to time. This is why we factor in intended use.

If your boat is used for skiing, you'll be looking for acceleration rather than top end. If it's a performance hull, you'll most likely be looking for top rpm.

As for performance on your particular rig, it's hard to say what effect bigger carbs will have unless we know more about your set up. If you're set up in the low to mid 5000 range the largest carbs probably won't do much except help get rid of weight in the gas tank faster. If you're set up close to 6000, they may help some.

You also need to factor in the port timing of the 79 100. It won't be as aggressive as the same year 140.

The later the carbs you use, the closer they will be calibrated for the fuel we have today.

Re: carb assembly replacement compatibility

My boat is a 1979 16 foot tri-hull bass boat. I am topping out right at 5000 rpm's (may be a hair over) with a full load. The motor is already pretty thirsty on fuel..so, I don't want that to get any worse. So would I be better off trying to find carbs closer to the 100hp range? What about using the jets from my damaged carb in a bigger carb? Thanks!!

Re: carb assembly replacement compatibility

BRP parts manual:

"http://epc.brp.com/default.aspx?brands=ej&lang=E"

Copy the url to your browser without the quotation marks.

Re: carb assembly replacement compatibility

Putting smaller jets in a carb makes for a leaner condition. At 5000 rpm as the top rpm you're already creating some high combustion temps (lugging). Leaner is not what you want to do.

Regardless of what carbs you use, I'd work with the set up to gain rpm -- and that doesn't necessarily mean changing to a smaller prop. At least not right away.

Re: carb assembly replacement compatibility

My OEM manual says my rpm range at WOT is 4500 to 5500...if I'm at 5000, what should I do to gain the extra 500 rpms? If I do something that causes the rpms to excede 5500, am I asking for trouble as well?

Re: carb assembly replacement compatibility

I'd suggest working with the set up, mainly engine height (X dimension) to try to gain some rpm. This is all assuming the motor is healthy and is running right. There really isn't such a thing as "overreving" a needle bearing 2 stroke as long as there's a load on the prop. 5800 will not hurt your motor at all.

Re: carb assembly replacement compatibility

so why is 5000 bad if the WOT range is 4500 to 5500?

Re: carb assembly replacement compatibility

That's the range the motor was rated at. It has a lot to do with the NMMA rules of how outboards are rated for advertised horsepower. Back in the day when the gas had lead it didn't hurt to prop your motor out at 5000. Performance wasn't too good and gas was cheap. As the fuel changed we saw some motors die a horrible, premature death from coking (black, sticky carbon-like substance).

The basic motor hadn't changed but the fuel did. People started figuring out why it broke rings and how to prevent it. The main people looking into this were the outboard remanufacturers. They had to figure it out because they had to warranty the motors they sold.

It became very clear, once outboards were run on a true engine dyno, that lugging caused additional combustion heat. Without the lead in the fuel to cool it down, coking formed on the rings. As the top rpm came up, the combustion temps came down (to a point). As the temps came down, the life of the motor expanded. As the rpm's came up so did efficiency. And so did performance.

The bottom line is that a motor like yours propped out a 5000 won't live as long as one propped out at 5500. And one propped out at 5800 will outlive that one.

Re: carb assembly replacement compatibility

ahhhh good info to know. I have had leaky fuel hoses that I'm in the process of replacing...doing carbs too...Hopefully when I start giving my motor the fuel it wants, my rpms will increase at WOT

Thanks!!

Re: carb assembly replacement compatibility

Dhadley said:

Putting smaller jets in a carb makes for a leaner condition. At 5000 rpm as the top rpm you're already creating some high combustion temps (lugging). Leaner is not what you want to do.

Regardless of what carbs you use, I'd work with the set up to gain rpm -- and that doesn't necessarily mean changing to a smaller prop. At least not right away.

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Dhadley, I just took a look at my prop and it's a 13 1/4 X 19...I looked in my Johnson manual and it does not list this size/pitch combo as a replacement option...could this be the reason I'm lugging? What prop would be ideal for my set up.