Evinrude Big Twin: runs on choke only


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Apr 17, 2018
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Hi all - continuing the story of this original post: https://tinboats.net/forum/viewtopic.php?f=4&t=47014

I finally got a 58-year-old engine running yesterday. I have to admit, it felt pretty good. I let it run in neutral at idle for 20-30 minutes and then tried to open the choke... it died. Several more tries, with the same result. I'll post a short video from my phone.

Here's a list of what I've done (1961 Evinrude 40hp, model 35524). All replaced/rebuilt parts are either NOS OMC or brand-new BRP.
. Removed, disassembled, cleaned, and reassembled every part from the powerhead up
. Replaced broken crankshaft, installed new omc spaghetti seal and crankshaft seals
. Replaced every gasket on the engine with omc gaskets, sealing with either Permatex 3 or 3M 847 per Joe Reeves' articles (note: no seal on 2 intake gaskets or carb gasket)
. Torqued every bolt to spec in the condensed service manual (where listed); sealed bolts with either Permatex 3 or aluminum anti-seize
. Replaced thermostat and gasket
. Rebuilt fuel pump and sediment bowl
. Rebuilt vacuum cut-out switch
. Replaced all ignition components (condenser, point, coil, ignition wire, boot, plug)
. Replaced all fuel, vacuum, and hot water lines
. Rebuilt hot-water choke and carburetor (including seat/needle valve and both plugs)

Since choke-only operation usually means fuel delivery, I pulled off the carburetor again to run all the checks I know how to do. Here is what I did.
. Remove the inbound fuel line (supply from pump to carb), blow on the line, fuel blows out and then air passes through the carburetor
. Remove carburetor from intake manifold, turn carb upside down, blow on the inbound fuel line, air does not pass
. Remove the bowl, check (new oem) gasket for any tears, etc.
. Check the float (level to carb body)
. Confirm seat needle isn't stuck, and float moves freely without any noise or sticking
. Remove float/spring/seat/jet; inspect; look for dirt or debris; spray each part with carb cleaner, spray with compressed air to check all the holes are open
. Confirm idle circuit and high speed circuit are free, using compressed air, spraying both both ways
. Reassemble the carburetor
. Re-check the float/needle/seat by blowing into the fuel line (right side up, air passes; upside down, no air passes)
. Install
. Verified idle adjustment valve is backed off a soft seated position by 1-1/2 turns (tried 1 turn and 1-1/4 turns as well; no difference in the outcome)

This last time, I also removed the fuel pump, checked for dirt/debris, sprayed carb cleaner/compressed air, checked the sediment bowl and gaskets, etc. There was a little dirt in the glass bowl (very little), so I cleaned it out.

So the punch line: I don't think this is the carb. I've checked the fuel tank / vent / bulb / line. I use this same setup for my 15hp 70s Johnson, and it works great. Just to be sure, I dumped the last gallon or so of fuel on a burn pile and used brand new gas and oil.

Things I'm wondering about:
. Carb float: looks like it's cork (sort of a wheat/yellow color, not red) and not plastic. It's a brand new BRP kit that's listed for this engine (439074) - but should I research a same-size plastic float and replace it?
. Potential vacuum leak in carb: I did make a mistake when cleaning / soaking this carb. I pulled everything apart (removed all non-metal pieces) , dropped in the Berryman container, and left it in there for 2-3 months while I got sidetracked with another project. Any possibility I damaged it? The fluid was extra dirty when I removed the carb, so I used the ultrasonic cleaner as well (3 parts water, 1 part Simple Green). Carb looks nice and clean, and nothing suspect... just not sure if I could have done damage in some way without realizing or being able to see it.
. Potential vacuum leak in crankcase: I have no reason to believe this is true; just read and saw threads on this, so I sprayed the top crank seal and the joint where the crankcase halves come together with soapy water while pressurizing the cylinders and turning the shaft... no bubbles. Any other ways of testing this? Or any reason to worry about it?
. Potential vacuum leak in manifold: I didn't seal the manifold to reed plate or reed plate to block gaskets (consistent with many threads and some direct answers I got to my question on the other thread). Given the gaskets are new and there's no sign of damage on any of the metal surfaces (I went back to my photos), I don't think this is it... just wondering if anyone has suggestions to definitive tests to eliminate this as a possibility.
. Timing (now I'm starting to doubt that again): points open when flywheel mark / timing tool mark enters the 2-mark gap on the magneto cam plate. Position of the magneto: I had the magneto fully retarded when doing this adjustment. Is this correct? Alternative positions that I think I remember using on other engines: "pickup point" position (magneto cam scribe mark is even with the intake manifold sync point); or fully advanced magneto (in forward gear, "WOT" timing setting).

Appreciate any direct experience anyone may have on this. We're so close with this one! Thanks in advance.
I should add (until I get my video posted)... here's what the video shows.
. Choke is between "Manual" and "Auto"... about 1/8" toward "Manual" from the "Auto" dimple... just enough to keep the hot water and vacuum from opening the butterfly, but not 100% closed.
. At this choke setting, and the throttle set to "Start," the engine starts every time. I have the idle stop set a little high... about 1/2 way between "Start" and "Slow"
. After starting, the throttle adjusts to the idle stop, and it will run there without any feathering or rescuing (I've left it for 10-20 minutes at a time).
. When I move the choke to "Auto" the engine picks up rpm for somewhere between 2-3 seconds and dies. Every time, no exceptions, doesn't matter if the engine is cold or hot.
. When I move the choke to "Off" the engine seems to maintain rpm (maybe a slight increase but not like when I move it to "Auto") and dies within 2-3 seconds.
. When I move the choke all the way to "Manual" it just dies (no increased rpm)
. Pumping the fuel line bulb doesn't have any effect
After you soaked the carb, did you replace the idle mixture needle's packing? Maybe that dissolved & it is leaking air thru needle threads.
Good question... I did replace the packing washers with new ones. 2 plastic and 2 cork.

On the topic of needle valve: I noticed the needle valve bushing (pn 303939) came out of the carb during the soaking. I put it back in, but I'd never seen one of these drop out of the carb casting on other rebuilds.
Vacuum leak seems to be #1 theory.

Since you have already done so much work, maybe it is time to try something else. Plug intake & exhaust, then pressurize motor to 5 PSI & spray with soapy water everywhere to see where to leak is.
I can recall a similar situation. The needle valve bushing might be the cause. There was a burr on the thread of the needle that jammed in the bushing. So when I was turning the needle I was merely spinning the bushing.... That would be my first take apart and examine... completely new packing needs to be compressed pretty hard to seal well.... Make sure you crushed it with the packing nut....
Here are the videos.
Thanks CRS and Shaugh.
. Idle bushing and packing: I'll take a look at this. Any way to seal that bushing (like we would inside the block or gear case) without risking the sealant material getting into the idle circuit? Will also get serious about the packing and nut and see what that does. Right now it's about as tight as it goes without restricting movement of the adjustment needle. Needle is NOS, and the bevels nice and straight.
. Leakdown test: I've done a poor man's version of this (just enough pressure to test the top crank seal with soapy water while turning the crank), but admittedly I wasn't getting enough pressure on the whole crank case to really run a good bubble test. Would you just make an aluminum or steel plate cover for the carb/manifold fitting and the exhaust cover plate?
It's a common mistake to not compress new needle packing enough. It doesn't take much pressure to lock the needle from turning but there is still a lot of space that the packing needs to fill, seal and lock up the bushing. I usually put grease on the needle because those new packing are very grippy.... I'm still betting this is your problem.... really crush the packing then loosen the nut a bit and gradually work the needle free... Then reset your turns....
Thanks Shaugh. I usually say, "think horses, not zebras" - so hopefully it's something basic and straightforward like this. Will get back to it this weekend, do exactly the process you outlined, and see what happens.
Have been following this thread and staying out of it because others seem to be on the right track.

Ditch the pressurization of the block idea (Sorry). Let's look at more simple checks and hopefully solutions first.

Have had the bushings come out a few times. Not a big deal. Once the bushing is back in place, the old packings picked out and new ones properly installed the packing nut will hold the bushing/packings in place. Hint...if your needle valve goes in and out as you turn it the bushing is in and doing it's job.

Let me ask a simple question.... on the idle circuit you mentioned 1-1 1/2 turns. Ever try to simply richen the needle until it runs regardless of the number of turns? What is the initial setting on your High speed needle? All fuel for the low speed comes through the high speed circuit. If the high speed is almost closed that may be restricting flow to the idle or low speed circuit.

Will you take a photo of the top of your carburetor for me? From straight above.
Thanks Pappy ; appreciate the questions. I'm focused on fuel at this point, based on consistent advice. I'll have a chance to get back to this next weekend, so until then I'm planning what I'll do next.
. Idle mix bearing/packing: I'm going to make sure the 4 washers are fully compressed, then loosen the nut just enough to let the needle turn. Right now it's about as tight as it goes without restricting movement of the adjustment needle, but it may have further to go (I didn't compress them all the way, then back off, who've I'll do next). Needle is NOS, and the bevels are nice and straight.
. Idle mix needle: Fully lightly seat the idle mix needle (which it has been doing), then turn out anywhere between 1 and 4 turns is what people have suggested so far. I didn't try to back the idle mix needle out more than 1-1/2 turns since the final spot is always in from there, but I see that's a good idea for diagnosing, so I'm planning to do that.
. High speed orifice: Next, I'm thinking I'll remove and re-confirm the high speed orifice is clear. My carb doesn't have a high speed adjustment.
. Failing all that, I'll pull the carb and dis-assemble it again. Probably remove the idle circuit compression plug, so I can re-confirm all 8 holes under there are still clear (they show clear in my rebuild photos, but I could have stirred up something since then). Also want to visually inspect that the idle mix needle is seating (nothing blocking, no old debris or shavings), which will require me to remove and replace with a new plug. Will also check needle seat for main passage and verify every nozzle hole is clear.

I'll definitely post a photo from the top, as soon as I get back in front of the engine (will be next weekend before I can do that). I'm the meantime, here is a photo of it assembled (not installed ; not sure if that helps).

Can you think of anything else I should do as a next step?
Did you inspect the reed block to make sure the reeds are seating? I had a similar symptom (only running on full choke, fully cleaned rebuilt carb, good spark & compression etc.) and the reeds were the culprit.
Thanks for the idea! I remember checking the reed to plate contact (no daylight in between reed and plate once assembled).
1. Your question did make me wonder whether I had installed them the right direction, so I went back to check. Here are the process photos. Looks like they're facing the right way. Since this is my first time with such an extensive rebuild... Anything that looks off to you?
2. Unless my idle mix needle / packing or high speed orifice checks do the trick, I'm probably looking at rebuilding the carb again (so I can watch the idle mix needle seat properly under what's currently covered with the new idle circuit plug, check the float needle and valve operation, verify every hole and passage, etc.).... While I have the starter and carb off, is there anything I should do to check the reeds while I have access to the intake cover plate?
3. Speaking of intake cover plate, it looks from my photo like I used aluminum anti seize on these machine screws. I remember thinking I shouldn't use Permatex 3, but I didn't remember using anything on them. Should I have left them clean?20190224_170757.jpg
Photo of the top of the carb not good enough or close enough.
Also want to see a photo of the carb upside down with the bowl off and nothing removed or touched other than the bowl separated from the main body. you said you are going to go through it again anyway.
Here they are, from when I initially rebuilt the carb in early October. I took these thinking maybe one day I would post the project online, but I didn't get every step. For example, I think I have the wire needle retainer that people sometimes call a spring running through the hole in the brass float arm, which is not shown in the photo.

I've had the carb apart again since then, but the cork jet gasket, float, and seat are all still in place. Do you see anything that looks off? When I get back in front of the engine (next weekend) I can grab some more photos if there's something else you need to see that you think might help solve.

Did you pull the plug on the top of the carb? Can't quite tell if it is the original or not.
If not the replacement plug should have been with the kit. Underneath are around three calibration holes for the idle and off-idle circuits. Debris can get in to the calibration pocket and block one or more holes there.
When you look at the carb again take a look at the high speed nozzle well gasket and see if there is a good imprint on it from the bowl. That has to be well sealed to create the vacuum necessary to pull fuel up to the idle passages.
Edit. While apart make sure that the passageway up to the idle circuit is clear...double check it. While running on choke you are pretty much by-passing the idle circuit.
Thanks Pappy for the questions. I replaced both plugs, and the photos show my idle circuit holes were clear at the time of initial rebuild (3 medium sized holes in a triangle, and 5 tiny ones above them).

When I get the carb apart again, if a few basic checks don't solve it (idle mix needle packing washers fully compressed, trying the idle mix needle anywhere from 1 to 4 turns out, confirming the nozzle gasket is being compressed by the bowl), I'll need to rebuild the carb to check that all the things that were clear at one time don't have any debris in them now (holes behind the idle circuit plug, float valve needle and seat, watch the idle mix needle with the plug removed to confirm it seats, high speed orifice clear, nozzle holes clear, etc.).
Onthewater102, did you see anything in the reed / intake photos that looked off?

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