"Right in the middle of it!!" Carb question??!?

[Mickyfinn]

Hi there,

I have a 1990 Johnson 120 V4 that's been running rough at an idle since I bought it this spring. I pulled the carbs right off and even though they looked spotless I cleaned them pretty thoroughly and re-installed. Given the shape of them, I wasn't surprised that it made no difference. Finally broke down and decided to pull them again and throw in new OEM kits. As I was cleaning them again, I was blowing air and through the pickup tubes that run from the bowl up to the idle & mid range jets. Air was coming through, but I decided to hold a light to one end and look through. The mid-range was completely clear the whole way, the same size as the brass tube. The low range seemed to be restricted a bit. I tried air and then come carb cleaner with no change. I ended up trying to clear it with a tip cleaner/reamer for my welder. What I discovered is that the hole is restricted to a fair amount smaller than the #41 jet. This would certainly make some sense out of the rough idle. I pulled another carb and found the exact same situation, clear mid and restricted low.

Thinking that the tube should be clear that whole way like the mid-range, I figured I'd ream it out. It does NOT clean easily and I might have to buy a drill small enough to try to drill it out. It's just bending the reamer.

Before I went and reamed this hole larger, I was hoping someone would confirm that this makes sense. I'm assuming that the fuel flows from the bowl through these tubes to the jets and not the other way around.

They should be completely clear, right???!

Thanks again,

Evan

 

[F_R]

Re: "Right in the middle of it!!" Carb question??!?

I really don't know the answer, but I suspect you are about to destroy your carbs. Especially since the second carb is the same.

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Well.... that's the hesitation and why I'm pulling my hair out. It would only make sense that if the fuel flowed from the bowl up through these tubes to and out the jet, then there shouldn't be a oriface smaller than the jet to feed it. Otherwise, wouldn't they just use a smaller jet??

Any thoughts? Looking for input, but I certainly understand that whatever I do is my decision and responsibility. The biggest item in the plus column is that it would go a long way to explaining the poor idle while the motor purrs at any other speed.

Thanks!

 

[offshore100]

Re: "Right in the middle of it!!" Carb question??!?

when you say, "restricted," after having probed a bit, do you have any better idea as to what the restriction is? If it's gummy green gunk then fire away; brass filings, not so much....

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Can't quite tell, haven't tried to ream it too much yet. It is a hard material, but if anything it probably wouldn't be brass unless they crimped the end of the tube before installing it. If it isn't some sort of built up deposit, then my best guess would be the adhesive, or whatever, that holds the tube into the plastic body. The hole in the body's channel where the tube flows into is much larger than the restriction, possibly the outside diameter of the tube itself.

I'd feel alot better about this if the carb was cruddy, or even if the mid-range tube was fouled. This motor came out of a fresh top-end rebuild from a local shop that was done and sitting around for almost three years prior to me getting it.

I'm gonna go pull the rest of the carbs and check them as well, that may give an answer to some of this.

Thanks

 

[offshore100]

Re: "Right in the middle of it!!" Carb question??!?

Sorry but i can't quite picture what you're saying. I will say though that BRP strongly advises against using tip cleaners or drill bits to clean orifices, etc. They recommend a toothpick; I've used copper wire strands of the appropriate dimension....good luck but tread carefully here.

ps, do your carbs have idle mix adjusting screws? It'd be great if that was the problem.

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Okay, all four carbs are the same. Now the question would be, are the jets for determining the flow of fuel going out through them or the flow of air coming in the other way? The channels that both tubes flow into are like "T"s. The tube comes in the bottom of the channel which has the jet on one end and another orificace on the other end that goes through into the aluminum throttle body. I always assumed the jets controlled the flow of fuel into the carb, but maybe they are controlling the amount of air, like a venturi, for the other hole.

Anyone know how these carbs work??

 

[Silvertip]

Re: "Right in the middle of it!!" Carb question??!?

Those are called "emulsion tubes" and you would be very wise to not ream them out as they are in effect a jet of sorts. You are to invert the carb, fill the idle circuit with isopropyl alcohol and if any leakage is noted around the tubes, dry the area and seal it with OMC Ultra Lock. Those tubes draw the fuel into each side of the carb where it is distributed via the high, midrange and idle circuits. The service manual has a note that warns as follows:

After a period of time, the idle chamber cover gasket can be sucked down into the L-shaped passage in the carburetor body, restricting idle fuel flow. To prevent this from occurring, carfeully cut out a L-shaped portion of the gasket corresponding to the idle passage in the body.

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Okay, broke out the camera to give you an idea of what I'm looking at. The tube on the left dumps into the lower of the two channels in the side of the body. The lower brass jet is for the idle, it screws into the lower channel that the shorter brass tube (the restricted one) dumps into. The other end of that channel ends in a fixed brass insert with a small oriface (smaller than the #41 jet but slightly larger than the restricted tube) that when the carb is attached to the aluminum body lines up with an opening that is ported to the aluminum throat with three tiny holes.

Sooooo, does fuel come up from the bowl and out both ends of the channel (jet and fixed oriface) or does fuel come up and get mixed with air coming from either end of the channel?

The "choices" are:\

1. If fuel comes up from the bowl and out both tubes, why would they bother with a jet or fixed brass oriface that combined would allow 4-5 times the capacity of the flow of what was feeding them?

2. If fuel flowed up and out only one of the orifaces, shouldn't the feed still be larger than the outflow?

3. The tube is actually a fixed "jet" feeding the idle and someone is making us buy an extra #41 jet for no apparent reason (although engineers almost always have an apparent reason for everything.....)

4. The guy who did the rebuild replaced the carbs with crappy aftermarkets from a company with little regard for quality control

or

5. There are some very hard deposits, remarkably consistently built up on only the low jets in all the otherwise clean carbs that is leaning out my idle and making it run poorly.



I think the best question of all if why does this stuff always seem to come my way????

Any thoughts??? I'm grabbing a beer to mull this over with....

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Thanks for the reply Silvertip. I'm trying to be wise by asking questions first...not normally a trait of mine. This one has me stumped though as to why the jet would be fed by something a fraction of its size..... I think I understand what the manual is saying, that the cover gasket may be restricting the idle fuel flow. The new kit contains a new gasket, so I assume that shouldn't be a problem once installed. What I should probably do is just throw the kit in and see what I get, but it just doesn't make any sense why there would be a restriction THERE, deep into the tube. If it were truly supposed to be there, wouldn't it make sense to a least put it on the other, exposed, end for easier cleaning. Or better yet, just use a smaller jet???

Anyway, thanks for the help. I'm going to go try the alcohol test.

Evan

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Thanks everyone for the input. I did the alcohol test, and everything seems to be tight. I'm just going to trust that everything is the way it's supposed to be and chaulk this one up to just one more thing that just doesn't make sense to me (it's a looong list...). I did do some digging and have come to the conclusion that the "jets" are for air flow and not fuel like I originally thought. The tube must be controlling the fuel flow being slightly smaller than the brass orifice feeding the aluminum body and the #41 controls the air mixture. Very different from anything I've ever worked on before, but then again, so is everything elso on this motor. I'm slowly learning though, and thank you all again for contributing to my education!!

I'll throw in the kits and see what I get. ;Hopefully I'll see a difference!


Thanks again,

Evan

 

[reload]

Re: "Right in the middle of it!!" Carb question??!?

There is no way the tube feeding the jet would be smaller in size then the jet itself, what sense would that make? I would not use a drill bit or tip cleaner. I usually use some very fine copper or steel wire. Also I have had some luck with brake cleaner verses carb cleaner. I had some recently that I let soak in kroil for a couple of days before I finally got them clean.

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Update:

Threw it all back together with the new kits, leveled the floats and re-synched it and it still coughs at an idle. It runs flawlessly at any speed above an idle, but it still has that ragged low end. I'm not entirely convinced that those carbs are right, but conventional wisdom leads to other issues. If I adjust the idle speed to @ 1100 in neutral, in the water, it evens right out. That's what was making me think it was running lean at the proper idle. I haven't checked the idle timing on the water but my thought was that I was changing the timing enough to affect the running by turning it out enough to lower the idle speed.

Not sure where to turn from here...I'll dig some more through the forum and see if I can figure where to turn next.

Any thoughts???

Evan

 

[Dhadley]

Re: "Right in the middle of it!!" Carb question??!?

FYI -- the idle and mid range jets meter air, not fuel.

 

[iwombat]

Re: "Right in the middle of it!!" Carb question??!?

Check the flange of the carb for warping. I can't tell you how many carbs I've had that were overtorqued and warped and leaked at the manifold. They tend to have a rough idle and run fine, but a little lean, as the rpms increase. Eventually, they blow the intake gasket and then you know for sure.

I guess some folks just figure if tight is good, then super-tight is better.

 

[Mickyfinn]

Re: "Right in the middle of it!!" Carb question??!?

Thanks,

I hate to pull those carbs one more time, but the overtighten issue might be worth pursuing. They were very tight when I first pulled them off, I suppose they might have warped. They also had some sort of grease liberally applied to the o-rings, I assumed for sealing purposes. Is that correct or at least common?

And Dhadley, thanks for the input. That was the question I was hoping to get answered when I first posted the thread. I've since figured that out but appreciate the confirmation. Unfortunately I can't easily get hold of the mechanice that did the original top-end rebuild before I bought it, but I remember him saying something about boring out the cylinders and oversized rings/pistons???. He was trying to sell it to me and commented that it was probably pushing out close to 140 hp after the rebuild. At the time, I just wrote it off as a sales push and hadn't paid much attention (that whole 20/20 hindsight is catching up.....). Is it possible to bore it out enough that it would require a different jet for the low end? I can't imagine that it could make that much of a difference, but at this point I'm open to about anything.

The only other thing I can think of is that although I'm getting strong spark with the plugs out, that maybe with them installed under compression I'm dropping spark on one of the cylinders. Any suggestions for the best way to test for this?

Thanks again, you've all been very helpful in my education here!

Evan

 

[Walker]

Re: "Right in the middle of it!!" Carb question??!?

Thanks,


The only other thing I can think of is that although I'm getting strong spark with the plugs out, that maybe with them installed under compression I'm dropping spark on one of the cylinders. Any suggestions for the best way to test for this?

Thanks again, you've all been very helpful in my education here!

Evan

An inductive timing light will do that for you. Test one cylinder at a time under load.
Also check your recirculation valves for proper operation. And check your idle timing.

 

[andy6374]

Re: "Right in the middle of it!!" Carb question??!?

-Have you made sure that the throttle shutters (or butterflies) in the carb throats are completely closed at idle? This will definitely cause a rough idle. Does hitting the choke smooth things out at all?

-Also, have you checked to make sure the idle jets are the correct size per the OEM manual. It's always possible that during the rebuild something got swapped, changed...etc...

 

[Dhadley]

Re: "Right in the middle of it!!" Carb question??!?

Boring it does not mean you have to rejet. We take the small bore motors (3.500") to big bore (3.685" -- .185" bigger) all the time and don't necessairly have to rejet. And a stock 120 is the same powerhead as a 140 so boring it didn't add any horsepower.

 

[iwombat]

Re: "Right in the middle of it!!" Carb question??!?

Let me know if you find any warping. Restoring brit bikes you see warped Amal carbs all the time. When those warp the slides bind and you've got a stuck throttle. I've developed a few tricks to take the warp out that work most of the time.