1959 35 hp high speed jet significantly effecting idle question

[islander9]

Hello, I am working on a beautifully restored 35 hp Lark that has a strange (to me) fuel delivery issue. I suspect it has had this problem since being restored a few years ago and the owner didn't recognize it wasn't running like it should.

The high speed jet needs to be almost completely closed for the low speed jet to have any effect to the fuel delivery at idle. If I turn the high speed jet out to approx 3/4-1 turn out, the engine is idling rich on the fuel being sucked up through the main jet.
Have had the carb off and replaced the needle/seat/float and given it a good cleaning.

Suspecting the throttle plate may be slightly open thus causing air to suck fuel up the main jet, I have adjusted the plate the very small amount it can be, to be closed. Link/sync as I know it on newer engines.

Does someone with more experience on this vintage gem have any suggestions on this, is it normal for this engine?

The only observation that seems odd to me is the high speed jet has a blunt end on it, vs a sharp point, is this normal for this engine? When completely closed, the tip of the jet is about 250 thou away from the well inside the float bowl, which doesn't seem right. I would have expected the point on the tip of the high speed jet (which it doesn't have) to enter into the well/seat thus allowing fine adjustment.

Appreciate any thoughts.

 

[oldboat1]

can add a couple of points -- High speed needle is blunted, vs. the point on the l.s. needle. The setting for the h.s. needle is usually about 3/4 out from seated. The l.s. setting (initial) is about 1 1/2 turns out, and then is adjusted from there (usually ends up leaner -- clockwise). Lot's of folks don't like to remove the domed cap at the top, but that's necessary to clean the idle passages. You may be stuck with that (an easy job, but need a new cap from a kit). The fuel mix ratio would be 24:1.

[ed. might check to see if the choke plate is sticking partly closed -- rich running.]

 

[F_R]

Hello, I am working on a beautifully restored 35 hp Lark that has a strange (to me) fuel delivery issue. I suspect it has had this problem since being restored a few years ago and the owner didn't recognize it wasn't running like it should.

The high speed jet needs to be almost completely closed for the low speed jet to have any effect to the fuel delivery at idle. If I turn the high speed jet out to approx 3/4-1 turn out, the engine is idling rich on the fuel being sucked up through the main jet.
Have had the carb off and replaced the needle/seat/float and given it a good cleaning.

Suspecting the throttle plate may be slightly open thus causing air to suck fuel up the main jet, I have adjusted the plate the very small amount it can be, to be closed. Link/sync as I know it on newer engines.

Does someone with more experience on this vintage gem have any suggestions on this, is it normal for this engine?

The only observation that seems odd to me is the high speed jet has a blunt end on it, vs a sharp point, is this normal for this engine? When completely closed, the tip of the jet is about 250 thou away from the well inside the float bowl, which doesn't seem right. I would have expected the point on the tip of the high speed jet (which it doesn't have) to enter into the well/seat thus allowing fine adjustment.

Appreciate any thoughts.

Read your last paragraph. If the H.S. needle is .250" away from the seat, that is just crazy wrong. It needs to go all the way in to the seat, then back off 3/4 turn. Final position after adjusting on a boat on the lake will probably be more like 5/8 turnout.

You cannot adjust it without being on a boat on the lake. Adjust the HS at full throttle first, then slow down and do the idle.

 

[oldboat1]

too many needle washers, maybe, or jammed up? Probably a good idea to just back it all the way out and reseat the needle. Might need a new washer or two, but maybe can save the old ones. It's possible the PO cross threaded it, so might be a good idea to run the needle fully in and out a couple of times without washers, then reseat for use if you have the feel of it seating properly.

 

[HighTrim]

Always adjust the high speed needle first at WOT, then idle down and adjust the low speed needle. As Frank stated, after tuning they are leaner than the static 3/4 out from seated, mine usually end up roughly 1/2 turn out once tuned on the water.

To linc sync, you will see a pointer cast into the intake manifold. Use that to set, NOT the roller. The carb linkage should just start to open when the pointer is aligned with the hash mark on the brass cam.

 

[islander9]

Thanks gent's for the advise. We have adjusted the high speed for max rpm and it runs good at high speed, when we slow down to idle the engine is running rich and in order to lean it out we have to close the high speed jet approx 1/8th of a turn. At that setting for good idle it is then too lean for high speed. The low speed jet has no effect what so ever on the idle fuel mixture.

I know the high speed jet shouldn't be controlling the low speed fuel mixture so profoundly and I'm thinking this is what needs to figured out to fix it. Going to try and find another carb to see if we have some miss matched parts? Not sure just where to look from here without another carb.

 

[oldboat1]

It sounds like you have issues with both the high speed and low speed carb circuits -- easy to remedy. Take the carb off and take it apart, including removal of the domed cap on the top. Soak everything metal including top and bottom of the carb body (can leave exterior linkages in place). After soaking, go through every opening with thin wire or a plastic probe, then spray out with carb spray. Pay special attention to the small openings underneath the domed plug on top and the orifice where the l.s. needle seats -- make sure the orifice is not blocked with debris or even the broken tip of another needle. The vertical nozzle in the carb middle likewise has to be carefully cleaned, probing each of the small holes -- likewise openings and passages in the carb body.

An oem carb kit might contain some instructions and parts descriptions.

 

[lindy46]

If adjusting the slow speed needle has no effect on the idling of the motor, then there is a problem with the idle circuit. As Oldboat suggests, you need to remove the plug on top of the carb and check out inside to see if the passages are blocked or if the tip of the needle is broken off in the orifice.

 

[F_R]

Low speed needle has no effect? It would have effect if it were clear and adjusted anywhere near correctly. Maybe it is like the HS was, not going in all the way?? All we can do from here is guess. You are the guy that has to do an on-site inspection.

Is the throttle butterfly opening at the correct time?

 

[islander9]

Thanks again all, I know I'm getting good advise from experienced folk's here and it is much appreciated. Yes we removed the welch plug and have verified the fuel passages are completely clear. Also installed a BRP carb kit including needle/seat/float and appropriate gaskets.

FR, I have verified/adjusted the throttle plate is completely closed. I have been trying to figure out why the HS jet is supplying fuel at idle and based on my experience with fuel systems the main/hs jet only supplies fuel after the throttle plate starts to open and the venturi starts to create a higher vacuum.
Clearly on this beast the HS jet is flooding the engine and I don't think it should be supplying any/much fuel till the throttle plate starts to open. This is what is confusing to me and when the devil is found I will once again say why didn't I see that sooner.

Oldboat1 and Indy46, I hear you and completely agree, except on this vintage project the engine is flooding from fuel being supplied fuel from the HS jet. I can close the LS jet all the way and the HS jet can be leaned from the proper setting to get it to idle, still unstable, but it will stay running.

In case I wasn't able to articulate the situation, the HS jet set at approx 5/8-3/4 turns out gives a good full throttle fuel supply, coming down to idle I need to close the HS jet to approx 3/8-1/2 turns out to lean the engine enough for a reasonable idle. Enriching the LS jet just makes the mixture richer.

I have been stumped and have learned lots many times before and expect by the time I am through this PIA I will have learned something new, but I have been servicing every type of engine for 45 years with Merc, OMC, Chrysler and BRP factory training (years ago), and very confident the issue isn't related to any fuel flow restrictions through the carb.

Also verified the fuel pump diaphragm isn't leaking fuel back into the crankcase and that both crank seals are not leaking fuel. Compression is 135lb's.

I'm in the middle of nowhere in northern Ontario and unfortunately don't have another carb at hand to see if it has been dinked with to know we are not fighting a bunch of mismatched parts.

I'm stuck on the fact the HS jet has a blunt end on it (which thanks to you folk's I understand is what was original to the carb), and the fact that when the HS jet is screwed in all the way in it still leaves a 250 thou gap between the tip of the jet and the fuel supply well/port molded into the float bowl that supplies mid/HS fuel. Long time ago finally found a missing gasket that seals the HS well/port to the base of the carb on a 15HP OMC that caused a similar flooding problem, but I don't see that that gasket is used based on OMC parts diagram I have.

I'm also lost as to why the HS jet is able to get so much fuel up through the HS jet with the throttle plate completely closed when I don't think it should. I know/think it has to be something else. Maybe should have paid more attention when I was a cocky kid to know.

I also appreciate how difficult it can be to help someone via the written word on the net when you are not sure you can trust the info/experience you are dealing with.

Open and appreciate any/all suggestions to figure this out.

 

[jimmbo]

Is the float set correctly? Is it a new float or something 20 yrs old? If old it may have lost its buoyancy resulting in a high fuel level

 

[islander9]

Thanks, and yes we installed a new BRP carb kit and set the float level to the carb body.

I found this image online and would like to ask if anyone knows if this carb uses the small round gasket to seal the main jet well to the bottom of the carb (two of the carbs in this pic have the gasket and one does not). My parts diagram does not show that it uses the gasket and it didn't have one when we took it apart.

 

[oldboat1]

Fair question. I'm pretty sure the gaskets show up for the larger motors beginning in about '61, along with the fixed h.s. jets.