40EL75 Rebirth

[oldrem]

Need a little more water in the tub -- three or four inches above the pump (i.e., above the l.u. joint). Let it run at idle for a while and check the temp on the top of the head (130-140F, or so). Then shut it down and retorque the head bolts to spec.

A backfire (puff) back through the carb can mean an issue with the reed valves behind the carb -- think I would work on the air fuel mix a little first. You may be a little on the lean side, but I would do the counterintuitive thing first and lean it out just slightly. If you get a clear lean sneeze or stall, back it out 1/4 turn richer and try again (should be close for the mix).

This is a long shaft (5" extension), so I have the water well above the pump but can add more. I did a bore scope inspection of the reeds when I rebuilt the carb and they looked good. Will try the richer mixture and re torque tomorrow.

Thanks

 

[oldboat1]

Water depth sounds right -- my mistake. As long as the leafs are intact and laying flat, it should be fine.

 

[oldrem]

Here's what I did today.
1) re torqued head
2) pulled and cleaned plugs
3) opened idle screw 1/4 turn and warmed up engine. Still had a fair amount of sneezing
4) sprayed about a half can of Power Tune and let motor sit for 25 minutes
5) re torqued head again
6) warmed engine and revved it awhile per Power tune instructions then adjusted idle screw to where it was a smooth constant idle - was getting readings mostly in the 670 to 710 range. The only time I get the sneeze and choke now is when I back down the throttle after revving in gear. It's smooth and quiet through acceleration (I took it up to about 4200 RPMs). When I back down it sneezes in the 1000 to 1500 range. If I apply some light throttle at that point I can bring it down to the smooth idle again with no issues.
7) pulled and cleaned plugs again
8) ran another compression test. One cylinder came up 2 psi from my initial tests and the other was +4, so slight improvement there

Anything more I can do - or just live with it?

 

[juno pierrat]

this is a long post so forgive if you've done this before, did you remove core plug on top of carb and clean very small holes under it? that is part of the idle circuit. also disconnect linkage to carb, engine should run, this eliminates the possibility that carb is slightly open at idle causing lean condition.
third thing - have heard of a case were the rod that turns the throttle plate had very worn bushing, thus letting in more air thru the bushings giving a lean condition, just a thought, your getting close, get it in the water and enjoy.

 

[oldrem]

Thanks for chiming in juno. Yes, I did the full carb rebuild, removed and replaced the welch plugs, thoroughly cleaned all holes with a carb brush/wire set so I feel confident about the carb itself. Will take a look at the throttle plate bushing when I can get back to it. I feel I'm so close but just missing something that isn't quite right.

 

[oldrem]

Could fuel make a difference? I've been using E10 mixed 50:1 with semi-synthetic then add Marvel Mystery oil and a little Sta-bil Marine - actual mixture brings it to about 45:1. The only E0 available in my area is Avgas 100 octane (which also contains lead) out at the airport.

 

[oldboat1]

Run a couple of gallons E10 with only 2-cyle (50:1). See if it runs better.