Yamaha F100A sneeze

[Herman Melville]

Hi I have serviced my Yamaha F100A but it has not cured the sneeze.

I have done:

New plugs
New fuel filters
New Anodes
New timing belt
4 carb kits carbs carefully serviced spray and wire and daylight through every hole. Float level checked and set on all 4.
Brand new super fuel.
Service if the prime starters - similar wire / spray through all holes etc...

But in idle it sneezes - see the video.

I have thought (maybe wrongly) that it was lean sneeze. The stock needle setting is 2 out. I have run it at 1.5 out. 2 out. 2.5 out. 3 out.... etc.. no difference.

I have also run it while pumping the bulb to keeo that hard to make 100% sure that each float bowl is fuly full. No difference.

It starts perfect. Runs smoothly. Just sneezes.

I don't know if it will do this under load - I can't test that at the moment.

Any ideas would be much appreciated.

[Herman Melville]

https://photos.app.goo.gl/9LkdEfWB9uq1g8gS8

[Herman Melville]

https://youtube.com/shorts/ZorKdaSsk7g?feature=share

[Peter_C]

Use a little propane to see if it cleans up. If so it is lean. Using a thin tube you can force it to individual carbs to try to find the culprit. Carb cleaner or brake parts cleaner can also work. I use a propane torch I modified with a hose and a small orifice to meter output.

Have the carbs been balanced using gauges? If not it is necessary, and the gauges are pretty cheap to purchase.

Were the carbs COMPLETELY torn down to every orifice? If the emulsion tubes were not removed and properly cleaned there will be a lean condition. Cleaning carbs requires a powerful ultrasonic cleaner, high pressure blow gun, and tedious cleaning.

[beamishken]

Have the valve clearances ever been checked? Assuming there not hydraulic. seems like you tried all the obvious things maybe need to start digging a little deeper.
9 out of 10 times its carb related though another look & cycle in an ultrasonic bath might not hurt. Shouldn't realy be probing carb jets or orifice's with wire

[Herman Melville]

Thanks all for the replies.

As far as I know the valve clearances have never been checked. I best take a look.

Yes I probably do need to sonic clean the carbs.

I do have a set of carb balance guages and will aim to do that after I sonic clean them.

I am looking for clues of course and the only one I have is that it wants to idle higher - 1200. I have wound the master idle screw out as far as it wil go but it still idles at 1200 rpm... so that is unusual....

[Herman Melville]

Carbs sonic cleaned and balanced.

Compression test 175PSI+ on all four pots - rules out sticky valve I think.

Primary side and secondary side coils tested to spec

HT leads tested to spec

Prime start resistance to spec. Prime start movement observed. Starting is fine though.

Spark jumps the specified 11mm gap.

Still misses.....

[A1an]

do you have a picture of the carbs fitted?

[Herman Melville]

Attached pics of the carbs cheers

[Herman Melville]

I was checking for air leaks due to the prime starters - were the orings in place etc..

I saw what I thought was a crack in the prime start body. Pretty easily a piece came off - it had been glued / bodged.

The piece missing is right from the bottom - I think it takes fuel in from the bowl and uses it to enrich the mixture at the start.

Anyway maybe air has been getting in there.... ?

I don't see how this will solve the prob because it didn't show on the vaccuum balance and also when the prime starter is warmed up it expands the plunger and blocks all this up....

In other words when the engine is warm and missing like the video then this hole is plugged anyway...

But it is so far the only component that I can find that has any defect so it's the best lead I have got at the moment.

£266 plus shipping ouch.

[A1an]

Disconnect the link arm that links all three carbs and make sure the throttle plates are shut

[Peter_C]

Originally Posted by A1an

Disconnect the link arm that links all three carbs and make sure the throttle plates are shut

I do that when balancing the carbs *shrugs*

Back to checking for a lean condition...inject propane or use a carb cleaner to see if enrichening it improves the running condition. If yes then try to isolate spray location to individual carbs.

A jet or emulsion tube that appears clean can have a teany tiny blockage causing issues as such.

[Herman Melville]

Originally Posted by A1an

Disconnect the link arm that links all three carbs and make sure the throttle plates are shut

Thanks @A1an I will try anything at moment.

I was going to say that the tick was not there at idle but the truth is that I think it is. You can barely tell - it's more like a slight knock and the slightest movement in the engine - but I think it is there at idle. I put it down to being an old engine but maybe it is the same tick as when in higher revs.

But in order to make it really show it needs to be 2500rpm or above.

[Herman Melville]

Originally Posted by Peter_C

I do that when balancing the carbs *shrugs*

Back to checking for a lean condition...inject propane or use a carb cleaner to see if enrichening it improves the running condition. If yes then try to isolate spray location to individual carbs.

A jet or emulsion tube that appears clean can have a teany tiny blockage causing issues as such.

Thanks @Peter_C

The emulsion tube is the one going up from the main jet and pokes into the throat of the carb right?

I will try carb cleaner spray I am sure I've got a can on a shelf somewhere - I presume the airbox will suck it through?

I must say I have cleaned the life out of that part of the carb but I will try.

Presumably if it is lean and there are no air leaks then the suspect is float level in the carb? The spec is 14mm for float level and I have set them all carefully using digital calipers to that. I measured from the carb housing to the top of the float as it lays flat when upside down with no gasket in the way. This also happens to be when the float level is dead flat.

Please let me know if I have set that wrongly - I totally could not understand the service manual when it said how to check it.

[Peter_C]

Carb cleaner can be sprayed into the plenum to start. If trying to figure out which carb, then you need to inject directly into each individual carb either using a long tube, into a line present at each carb (Even ruling out one carb is good), or if possible remove the air box and shoot directly into the carb which would isolate them completely.

You could also spray carb cleaner carefully on the outside of the carbs and intake area, making sure to let it dry between fairly long spurts in case it catches on fire. If there is a vacuum leak it should smooth out and increase RPM.

Float levels are most likely not the problem. Disconnect the fuel line and run it for a minute or so on what is in the carbs. If it runs that long there is plenty of fuel.

Of note zip ties are not a good practice for fuel lines as they disintegrate too easily and fail.

[Herman Melville]

OK how about ir getting in behind the fuel pumps past the mounting o-ring ?

[Herman Melville]

OK thanks to all the people who took time to try and help me.

Seems to be fixed so I thought I would post here what happened.

The prime start body (metal unit under the electrothermal valve) has a tiny crack that I had inherited and it was glue bodged I think air was getting in. I bought a new OEM part for insane money.

Carbs cleaned so much you can see them from space.

There is an air check valve in the carb air boost line into carb 4 (only carb 4) and this looked scummy. I could not blow much through it the right way and seemed like it could blow a tiny bit the wrong way... so I had that off and ran WD40 / compressed air through it back and forward...seemed to open it well the right direction and really close it tight in the reverse direction.... so I blew out any remaining WD-40 and refitted that.

So when it was finally reassembled it ran kinda OK but there was now the odd misfire at idle on carbs 1 and 2 (misfire was at speed before and on carb 4 I think). With the air box off I could see it was popping back out of the carbs so that is a lean condition right?

I set the idle needle screws to the best I could figure - further out than stock 2 turns and where it idled better, then set the idle screw on 4 (the master) then I set the idle screws best I could on the other 3.

Then I got a carb balancer and managed to set the vacuum on each carb to be about the same (they were not the same at all) - I am pretty new to that and found it hard but got them all there or thereabouts in the end.

Once I had balanced the carbs it was finally fixed.

I dont know if I have 100% fixed the problem becuase I had to adjust the idle screws on the top 2 carbs quite a lot (to their limit in fact) to try and set the vacuum to be right. I don't know if that means:
1) I am useless at carb balancing
2) There is some deep problem I am just covering up
3) It's really fine I just don't think it's fine.

Anyway doing pretty good now so above it what I did and thanks for your support and help.