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1988 Johnson 48 SPL Fuel problem

K

KenU

Member
Good Morning All.
Best wishes to the Moms out there.
Ok. 1988 Johnson 48 SPL ( with AB Jet) on a 1988 Lowe 160. Got it for a good price for my 15yo son who's turned into an avid fisherman. When purchased it would start only with throttle advance all the way. wouldn't idle. Rebuilt carbs (complete), checked spark, timing, compression. All good, except compression seemed low 95 top, 90 bottom) Got it adjusted so it would run and idle without the advance. The tach would bang up over 5500 RPMs on the hose. Yeah! not bad i figured. Put it in the river, and would shoot to 4000 rpm and bog its way down to 3400. If you backed off the throttle a tad, it would find a sweet spot at 3200 where it was moving well. Timing was spot on in the water also. I twisted the choke solenoid red lever to bypass and it jumped. ran great at 4400 rpms. So i figured it wasn't getting enough fuel. Easy, Rebuild fuel pump. Also got kit for choke solenoid. Put it back in yesterday, and same result. Worse actually. only hit 4000rpm.
Any gurus out there seen this. what am i missing.

BTW, New fuel tank pickup, hose, bulb, & connector. New CDI ignition.

Thanks in advance.
Ken
 
First of all winding it up on the trailer means nothing, could damage the motor !-----------Check for a sheared flywheel key.---It affects the timing.----And no you can not use a timing light to find a sheared key-----When you open the primer valve , fuel sprays into the motor.----That fuel bypasses the jets in the carburetors.-----You might want to test compression with another gauge.
 
Thank you. Key is not sheared. We pulled flywheel. I didn't think the primer valve "bypassed" the jets, just additional fuel?
 
The primer valve replaces choke function and sprays fuel when open during cranking.----Or when key is pushed in when running.----This fuel does not go into the carburetor bowl at all !!
 
Cold or hot ----Throttle open or closed.-----Generally not much difference.----A good battery and snappy starter do make a difference.
 
Ok , here's the update. Spoke to a tech and he suggested that the issue may not be "not getting enough fuel" but getting too much air. Tried the propane torch trick to see if we could locate and air leak at the intake gasket and the gel seal joint. Nothing. So we pulled carbs and intake. When flywheel was rotated a "gush" could be heard coming back out of the lower cylinder towards the intake. Oh boy, time to pull the head. Pistons were slopping around in the cylinder. Top bore measured .010" over spec and ...... Wait for it... Bottom measured .034" over. Stock pistons too. ring gap was, i'd have to guess 1/4" LOL. Think we found the issue. I'm pretty convinced the compression coming back through the rings was restricting the fuel delivery to the cylinder, hence the increased RPMS when the choke solenoid was put on bypass. Going to bore, hone and rebuild. SHould be a tight little powerhouse when all said and done.
Thanks for the suggestions.

BTW, i now have a good comp tester, good leak down, and fuel pressure gauge. We'll get to use them on the next one.
 
New question. one of the 3 intake ports (towards intake side of the piston) in the sleeve is not ported flush with the block port. Looking through the cylinder ( from the head), it sits at about 2 oclock position (starboard ). there are 2 more, 1 at 11 oclock and 1 at 5 oclock which measure .875" on the sleeve. The 2 oclock port only measures .375" on the sleeve but the block port is the same as the other 2 (.875") Should this be ground out to match the block????
 
1 thing to note: on the smaller of the 3 ports the block port extends past the sleeve. The overall opening is 1.0625" while the other 2 are 1.1875" So i guess the actual opening is close.
 
Happy Memorial Day. Many thanks to the men and women who serve! So, I'm STUMPED. I bored and honed the cylinders, new pistons, rings, Crank bearing, & seals. Great compression now. 135 top, and 130 on bottom. Major improvement. When we put it in the water, i still had to twist choke solenoid to bypass to get it to run smooth without bogging at full throttle. I was so disappointed. We Still only got 4500-4600 RPMS at WOT. Maybe thats just where that motor will be. we may need to adjust trim angle to find a sweet spot. I did figure out the motor was about 1 1/4" to low in the water according to Outboard Jets installation manual. I played around with cutting down the air flow into the carb and it helped. Turned the choke solenoid back to normal (off) and blocked the intake ports with my fingers. Seemed to maintain rpms (small reduction 4400-4500) . I made 4 teardrop aluminum restrictor plates and tapped the intake housing so we can rotate to open or close off the air ports. We're going to put it in the water and run tests, idle, half and full throttle. Checking the plugs as we go. I really don't know what else to try. Its confusing. It is basically a new motor at this point. Why would there be a need to restrict the air???? I thought of Drilling the jets, but they are stock and the thing uses enough fuel as it is. While on bypass, about 40 minutes of running WOT we used about 5 gallons of fuel.
 
Happy Memorial Day. Many thanks to the men and women who serve! So, I'm STUMPED. I bored and honed the cylinders, new pistons, rings, Crank bearing, & seals. Great compression now. 135 top, and 130 on bottom. Major improvement. When we put it in the water, i still had to twist choke solenoid to bypass to get it to run smooth without bogging at full throttle. I was so disappointed. We Still only got 4500-4600 RPMS at WOT. Maybe thats just where that motor will be. we may need to adjust trim angle to find a sweet spot. I did figure out the motor was about 1 1/4" to low in the water according to Outboard Jets installation manual. I played around with cutting down the air flow into the carb and it helped. Turned the choke solenoid back to normal (off) and blocked the intake ports with my fingers. Seemed to maintain rpms (small reduction 4400-4500) . I made 4 teardrop aluminum restrictor plates and tapped the intake housing so we can rotate to open or close off the air ports. We're going to put it in the water and run tests, idle, half and full throttle. Checking the plugs as we go. I really don't know what else to try. Its confusing. It is basically a new motor at this point. Why would there be a need to restrict the air???? I thought of Drilling the jets, but they are stock and the thing uses enough fuel as it is. While on bypass, about 40 minutes of running WOT we used about 5 gallons of fuel.
 
Did you clean the high speed jets in the bottom of the carburetor float bowls , yes or no ?-----If there is a fuel restriction you may well be doing this repair again soon !
 
the restrictor plates worked. we got them adjusted so it would run good high, mid and low speed. I'm still puzzled by this. So I'm going back to carb. There are 2 screen covered orrifices on the carb. 1 on the left and 1 on the right. Both lead to the bowl, however 1 is blocked with a welch plug. The other is clear to the bowl. Only 1 hole had a should for a welch plug which we replaced. There was another welch plug with a diameter that would fit where the opposing screen is located. My question, should we use the additional welch plug and fill in the second screen location? This screen is allowing air into the bowl. Could that be limiting the amount of fuel getting to the cylinder? The fuel is pulled by vacuum through the jet. doesn't the air reduce the vacuum?
 

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What size high speed jet does this motor have in it? Parts book shows a 50D, but seems to me I remember a jet unit needing more fuel. The models built from the factory were alway rated lower HP due to the ineffiency of the jet unit. I can't find a unit built from a 48hp motor in the parts manuals. Not sure what the RPM at WOT is suppose to be, but I do know that many of those models had lower compression heads on them & richer jets to combat detonation/engine rattle. Since this engine was a conversion it may not have gotten those changes? The motor obviously desires more fuel. Let me see what I can find out. Take it easy on the motor until you hear something.
 
a 1985 50hp carb has a 51d jet only $13 versus $58 for a 50d ??? why such a difference?
Part Number 331567 - ID Number 51D - Orifice Size 0.051 inches - 1/4-20 thread size
 
I have been told you should contact the jet unit maker for recommendations as to jet sizing. I was thinking it was in the 10% up area? That would be a 55D. You need a jet that when you push in on the primer the motor no longer speeds up. Slight drop off would be nice! That's how you tell you are in the right area. At WOT. About 4800 RPM is what you are looking for according to my sources, but you can talk to the jet unit maker about that also. The jet unit is not efficient after a certain RPM. Let me know if I can help with jets? I have plenty. No rhyme or reason behind pricing! PM me.
 
t2stroke said:
I have been told you should contact the jet unit maker for recommendations as to jet sizing. I was thinking it was in the 10% up area? That would be a 55D. You need a jet that when you push in on the primer the motor no longer speeds up. Slight drop off would be nice! That's how you tell you are in the right area. At WOT. About 4800 RPM is what you are looking for according to my sources, but you can talk to the jet unit maker about that also. The jet unit is not efficient after a certain RPM. Let me know if I can help with jets? I have plenty. No rhyme or reason behind pricing! PM me.
Click to expand...

I did read a review recently on glastron making bascially the same boat with outboard, inboard and jet. The jets fuel consumption was higher. with my restictor plates on and adjusted we were getting 4300 consistently.
 
The jets (orifices actually) are reamed not drilled & flowed. You can ream yours and see what happens. The idea would be to sneak up on it so,you don't go too far. Hard to put the brass back! HA!
I have a supply of jets if you decide what you need. Happy to send out a couple of sizes.
Interesting the manufacter didn't think engine needed modifying? I know OMC did several things to the ones they made. They used the same unit on their jets. Everybody uses that jet unit, forever.
They make jets in just about every .001 increment. I think you will find you need to make a pretty big move the first time. At least .003/.004. Just remember I can send you new jets so nothing to lose.
Be very interesting to see how the engine responds.
 
Could the stock Jet size have been the issue all along? Initially, the cylinders were oversized from wear( and possibly a halfassed hone job) by .010" & .032".
 
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