souped up johnny 40

[jamason]

Somebody put 55 comm carbs on my recently purchased 1987 40hp Johnny and boy does it move my little 14'9" trihull at higher RPM, but it takes a lot of coaxing to get it there. I'm told by a old time boat racer that I should port the exhaust as back pressure at low and intermediate speed is causing this sluggish/sputtering problem. I have already changed the entire fuel and electrical system, got great compression and adequate cranking voltages. This modification entails a lot of work and frankly it is not an option for me. So what does that leave besides the obvious reinstallation of 40hp carbs? Could I install a restrictor plate on the carbs to cut down on airflow or reduce the jet size or both? Spent alot already and need to smooth this out on the cheap so wife won't---I'll leave the rest to your imagination.

 

[JB]

Re: souped up johnny 40

Howdy JAM.<br /><br />Sounds to me like you need a "shorter" prop. What rpm are you getting?

 

[Chinewalker]

Re: souped up johnny 40

Or, it could simply be running too lean at mid and low range, so fattening the low/mid jetting might help. <br /><br />- Scott

 

[jamason]

Re: souped up johnny 40

Sorry JB, no RPM gage but it's a new 11.75" 15 pitch SS Rapture prop, not a speed prop. <br /><br />Scott, the high jet is a 56D, the midrange jet is a 35 and the low is a 46, like in the BRP parts cataloge. What upsize jets might you recommend for Florida? <br /><br />So what do y'all think about my exhaust restriction theory? Did you ever know anybody that slapped 55's on a 40 and they ran OK?<br /><br />P.S. When I said adequate cranking voltages I meant 128 peak volts for the new charge coil and .434 for the new trigger, close to minimum. I had suspected that weak flywheel magnets might somehow negatively impact low and intermediate running but not high speed. However the boat mechanics I talk to say they've never run across a weak magnet problem so I didn't chase that one!

 

[jamason]

Re: souped up johnny 40

Guess there isn't any further idea's or did everybody fall asleep at the wheel???

 

[Chinewalker]

Re: souped up johnny 40

Hi JAM,<br /> I don't know what jets would work for you - I'd ask at a dealership... They've likely had more experience with jetting for your altitude, weather, etc...<br />- Scott

 

[jamason]

Re: souped up johnny 40

Thanks anyway Scott.

 

[Dhadley]

Re: souped up johnny 40

On a 44 cid 40 the restriction isn't the carbs or the exhaust. The reed stops are closed up. You can put the biggest carbs in the world on it and it'll only flow a certian amount of fuel with stock reeds at "stock" rpms. <br /><br />We see folks put 60 hp carbs on 40's all the time and wonder why they don't get more than they did. The first step is to repalce the reeds with something like the Boyesens. Then you need a tach to see where you're at.<br /><br />Opening the exhaust is easy and it may help but only if you dry stack the exhaust too. Simply opening the exhaust without dry stacking could actually hurt the rpm's in certian cases.<br /><br />Remember, with larger throat carbs you'll get slightly more top rpms but the low amd mid range response will be less. Smaller bore carbs have better response but less top rpm.

 

[jamason]

Re: souped up johnny 40

Allright, now we're getting somewhere!<br />Thanks "D"

 

[jamason]

Re: souped up johnny 40

Installed upsize jets today and ran the boat and as expected no increase in performance was noticed, in fact there was a slight decrease, will install the Boyesens next. I was wondering why not just change the old leaf stop for the higher angle of the 55 stop and get the same results as the Boyesens? The leaf valve itself for the 40 and 55 have the same part number!

 

[Dhadley]

Re: souped up johnny 40

The stop is the same part but it's closed up. We used a #1 drill bit as a gauge to open the stop. The Boyesens will still react quicker.

 

[jamason]

Re: souped up johnny 40

Not entirely correct, the stop for the 40 is part number 0330675 and for the 55 its 0323601; wonder what the difference is? Anyway, the Boyesens have me intrigued and I must have a set!<br />Hope everything I need to change out the reeds comes with this set I'm ordering from RickRacers.com.

 

[jamason]

Re: souped up johnny 40

Installed the Boyesens----NO DIFFERENCE! Still got sluggish performance,like it's starvin. Tore down the carbs again and there clean as a whistle and they had plenty of fuel in the bowl. Sooooo back to the electrical side, I disconnected the connector coming from the NEW charge coil and the New sensor coil(trigger)and checked cranking voltages again; 106 peak vac for the charge coil and .45 for the sensor. Now I gotta believe I've got bad magnets as both NEW coils read low volts and the only other thing involved with this test is the FLYWHEEL! Can I get an agreement on this from a Johnny guru? Thanks in advance.

 

[jamason]

Re: souped up johnny 40

HELLO OUT THERE!!! Am I getting too technical for you all, or is this malfunction too far out in left field where no one can handle it, or do I need to get this topic moved to the Johnson forum where the experts peruse more frequently? By the way, pretty much everything to do with fuel or electric is new on this motor and the rest has been rebuilt except the stator (stator being the individual coil that only charges the battery) and the stator ohms checked OK. <br />Never had a problem this thorny when troubleshooting F4E fighter jets!

 

[Paul Moir]

Re: souped up johnny 40

I don't know anything about your engine or modifications but if you do have weak flywheel magnets I think you would be the first person to have that problem since about 1947.<br /><br />Are you testing with a DVA meter, or a multimeter set to AC volts? What are your cranking RPMs?

 

[Chinewalker]

Re: souped up johnny 40

Yep, flywheel magnets don't go bad. They're permanent magnets, unlike the ones used prior to WWII which may have to be recharged periodically. <br /><br />If you've got an issue that's common to both coils, then I'd look at grounds, wiring, etc. However, being that the motor is running fine otherwise, I don't think your issue is ignition related. More likely propping, carburetion or compression. We still don't know where your motor is taching at, and sluggish behavior can definitely be attributed to the wrong prop. Start with the easy stuff before you start changing out more ignition components. Borrow a tach and get your baseline set, then go from there. Then we can start eliminating what it ISN'T....<br /><br />- Scott

 

[jamason]

Re: souped up johnny 40

Thanks for the reply, now please forget everything else about this problem; there may be other things wrong but for now concentrate on the Ignition coils strength at cranking speed. I tested these coils at an adequate cranking speed with a regular digital multimeter on the AC scale then multiplied by 1.414 to arrive at the figures stated above (106 vac for the Charge coil and .45 for the sensor coil). This was by the book (service manual) for my motor. Like I stated previously, both coils are new, and they came with the wires and pins attached. As you probably know this cranking test only measures the strength of the coils as energised by the magnets, no battery or prop or carbs involved. With the five wire connector DISCONNECTED from the Power Pack the voltage should be 230 volts or greater for the Charge coil and 1.5 for the Sensor coil. It's not happening! The coils or wires or pins are not shorted, I did the test for this also and the coils have the correct ohmic value. From my way of thinking this only leaves the MAGNETS as the bad guys! Only three things involved with the cranking test----CHARGE COIL, SENSOR COIL, MAGNETS!!!

 

[jamason]

Re: souped up johnny 40

Have a safe boating weekend ya'll! I will, cause my boat is still "high'n'dry".

 

[Pony]

Re: souped up johnny 40

You would probably get more help if you lost some of the condescending attitude. These guys take their free time to help out here, and will likely pass over a thread like this.<br /><br />Two people....both highly respected and EXTREMELY knowlegeable people, told you that the magnets don't go bad. I would suggest you take their advice. <br /><br />They have pointed you to a great starting point. You are "high and dry" right now because without any compression numbers or rpm numbers no one can even hypothesize what condition your motor is in and wether its even capable of reaching the rpms you want it to.

 

[Paul Moir]

Re: souped up johnny 40

Some multimeters do some strange stuff on their AC settings below 50Hz or so, unless specifically called out in the manual. If you have a true RMS meter chances are it'll handle the lower frequencies better. Usually this is a function of their DC blocking filter.<br />When cranking at say 300 rpm, the meter is seeing a 25Hz sinusoidal wave off the stator. Take the plugs out and retest at a hopefully higher RPM, and therefore closer to 50Hz. Or with a different meter. This is part of the reason they use peak numbers rather than making assumptions about your multimeter and how you read it. There's no guarantee that the signal is sinusoidal (1.414) even.<br />Making a cheap peak reader adapter for your meter is an idea too. Just a bridge rectifier and a capacitor will do. <br /><br />I've heard of magnets cracking on larger engines, but never a 40hp. <br /><br />Perhaps you ought to start a new topic in the J/E repair section. It seems to me there's some sort of a problem if you've got sluggishness. Is it possible to retest with the 40hp carbs? If you do, put some clear information in the title so the people who know can pick up on the topic, such as "1992 modified 40hp sluggish". There's a lot of traffic this time of year and posts tend to get missed.