Blowing out of Carburettor

[Gannet]

I have a problem on an early series Forty of poor starting and running.
Yesterday, when I fired it up in the tank, I realised that when it was running, it was also continually blowing out of the carburettor. The amount of air and I guess fuel being blown seem to be enormous. I surprised it was able to run, especially as the prop was fitted and the water was really being churned up.

This is obviously a big problem and needs to be fixed. But what is going on?

It might be something to do with port timing, and so I removed the carburettor and saw the piston moving over the inlet port in what I thought was a satisfactory way. The piston skirt was undamaged so timing was probably not being changed by that.

I obtained this engine recently and so do not know its history.

Can somebody give me some advice.

Thanks in anticipation.

Jeremy

[skyetoyman]

I would suggest new rings. A compression test might show the problem.

[Stelios_Rjk]

I am very curious about this matter. I had faced it before and I think there was also a relation with the timing. Let us know of your findings Jeremy.
Where is the timing screw located?

[Gannet]

Thanks,
I forgot to say in my inital post that the compression is good at 55psi - measure cold and with no added oil
Jeremy

[Gannet]

Stelios,
When I referred to timing, it was not ignition timing, but inlet port timing.
Jeremy

[Stelios_Rjk]

Yes I understand that. I am just saying that there may be something wrong with the ignition timing causing that condition.
So, is the timing screw where it should be?

[phil]

55 psi is ok for a Century or 102 motor,but in my experience is it too low for a forty series motor. My 40s that run well all are 80 psi or higher.
Maybe needs rings or a head gasket or both. Hopefully the cylinder is ok.

[Stelios_Rjk]

It's a little forties series. Those type of engine is OK even with 40psi. Later forties indeed work on 80-100psi.

And I have an idea. Maybe the spark happens later than it should. In such case, I imagine, the mixture could be partially burned, some of it after the explosion gets out from the carb. The engine also runs weak.
You don't loose anything trying to retard it a little. But I don't know whether you should open or close the points' gap to achieve a retarded spark.

[headdownarseup]

jeremy
double check the carb size
somebody might have inadvertently swapped it for a larger carb and not realised.
just had the same thing with my LS. it should have a 1/2" carb but was fitted with 7/16" carb. no wonder it didn't want to start easy!

i think the carb might be too big. remember that "little model 40" are much smaller displacement than "normal" 40's so dont need the bigger carb. (too much air for them and will probably "sneeze" through the inlet cowl)

jon

[headdownarseup]

just as a comparison, compression on my FV (cold engine) 60 psi.
so not much in it.
i would consider compression to be good in yours JEREMY.
try the carb?

jon

[Stelios_Rjk]

Hi again guys,

I found how to retard the spark. You have to reduce the points' gap. Could you please try that Jeremy? That would reveal whether my theory is right or wrong.
Check also please the spark plug gap.

If you find that retarding the spark isn't working, advance it by opening the points.

I know I may sound odd but there must be a relation between timing and sneezing. Worn piston rings would lead to backfires.


Thanks
S

[Gannet]

Stelios, Jon and Phil, thanks for your help.

I think 55psi as Stelios says is ok for an FV (this one is FV6353). My limited experience suggests that at 30psi they don't run properly, but have not seen this 'blowing out of the carb' problem even with 30psi.
Jon - I have checked the carb size. It is 7/16 - in fact it is a carb temporarily borrowed from FV2821, which runs very well.
Stelios - sorry for the misunderstanding ref timimg. I have checked the points gap at 0.018. Will check that the baseplate positioning screw is located correctly (the baseplate is a bit tight on its location, which might have misled me)- good point - thanks.

Jeremy

[Stelios_Rjk]

Hi Jeremy, happy to help.

I would suggest trying with a points' gap of 10thou (retarding the spark) and if that wouldn't work then advancing the spark setting the points' gap to 30thou.
You can try with a step of 5thou to see what works best. Let us know please. I am very interested in your findings.


Cheers
S

[Keith.P]

2-stroke's "fog" a bit out the carb. Generally it is due to exhaust restriction if or wear on the piston/ring, so running the motor in a tank with a high water level will make it worse if you have piston wear or are down on compression.

[headdownarseup]

good point KEITH
slightly off topic here but my race minimoto (46cc watercooled) has a 20mm carb fitted to it with a foam air filter.
after most engine runs when in the pits i have to "ring out" the filter as its so saturated with fuel.
fine at full power runs but gets a bit "boggy" lower down the rev range.
probably a bit too much carb for it but goes really well. lots of fun.

so with seagull carbs yes they do cough a bit.
it might help if ALL seagulls were fitted with reed valves on the inlet side. this might help A LOT with fueling problems particularly with "sneezing" as you put it.
might help to increase power outputs as well, not that seagulls are in any way performance motors just good old plodders!

jon