4 stroke carb - diagnose my problem

alahol2

alahol2

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www.troppo.co.uk
Trying to get an old Yam 9.9 4 stroke running again. I have taken off and cleaned the carb as best I am able. I can get the engine running reasonably well but only if I've got 6 layers (not 5 or 7!) of j-cloth over the carb air intake. This is when the engine has warmed up. To start it I have to cover the intake with my fingers, allowing only a small gap. Can anybody make any suggestion about where or what I should be looking at please?
 
You may have an air leak. Check the flange gaskets and the manifold itself for cracks. Possibly the throttle butterfly spindle bearings.

Alternatively your main jet is still blocked. Will it rev with the j cloths in situ or are we talking about just managing to idle? Did you blow them and the fuel passages out with an air line (wear goggles)

I'd still look for an air leak.
 
It would seem that the carb will only deliver enough fuel when a lot of vacuum is applied.
I Would therefore look at:
Blocked jets
very low float level
blocked airways to jets
wrong assembly of carb
air leak is possible

Is there a fuel pump built into the carb on this model? Diaphragm pulsed by the the crankcase pressure type?
This could be faulty. Maybe the vacuum is needed to get fuel into the float chamber?
Try feeding the carb with a remote header tank with about six inches of head. An inline filter pre-filled with fuel will do, with the top open to air.
Air leak in the fuel line or pump area may do the same.

good luck.
 
Not familiar with this model but if it has a remote tank you may also have either a defective fuel line/fuel line connections or, more likely, the reed valves/diapraghm in the fuel pump has failed on the engine. Covering the air intake will force the engine to suck fuel assisting the failing pump.
 
Definitely no cracks and the gaskets look fine. The butterfly seemed firm enough but I'll have another look. I think the main jet is OK because it will rev quite happily once warm and with the j-cloths in place. Just had another look and there appears to be an air adjustment on the engine side of the butterfly, I'll try adjusting that tomorrow. There's also a "bi-metal vacuum switching valve for cylinder wall temperature", I guess I'll have to look at that as well. Thanks for the pointers.
 
cliveshep and lw395
I think the fuel supply is OK because the engine will tick over fine once running and will also accelerate on the throttle.
I think it is definitely an air problem somewhere but these carbs are a lot more complicated the tha old SU's I last messed with...
 
[ QUOTE ]
cliveshep and lw395
I think the fuel supply is OK because the engine will tick over fine once running and will also accelerate on the throttle.
I think it is definitely an air problem somewhere but these carbs are a lot more complicated the tha old SU's I last messed with...

[/ QUOTE ]
Yes, far more complicated though the basic principles are the same. They now have to meet emission control regs and that means fuel vapour being drawn back into the carb rather than being vented.

My immediate thought is that you have a blocked main jet. There will probably be two jets -- one slow-running or tickover and one main jet.

You need a prick. If you wait around long enough, one is sure to turn up /forums/images/graemlins/wink.gif

Otherwise some very fine fuse wire might do.
 
To add to the above - Have you had the float assembly off, or the float needle out?

If so, check whether the float needle is supposed to sit on top of the float arm, or if there is a little fork on the float arm that's supposed to capture the end of the float needle. I don't know this particular engine, but if the latter type is assembled as if it were the former, then you will have a *very* low float level.

0.02p
 
Lemain, misterg
I'm 99% sure the main jet is OK, it's big enough to see daylight through it, I've cleaned and checked it twice now. The float needle is 'captured' and in the correct position. I've checked the quantity of fuel that I can drain out of the chamber when the engine is stopped and it's pretty much a chamber full.
 
I am not sure on the 4stroke but the 2stroke, you normally have a main jet and a diffuser tube. Have you checked this out - they are normally held in by the main jet or are screwed in behind the main jet. Check the side holes on this are clear and the the airway into this area.
 
[ QUOTE ]
Lemain, misterg
I'm 99% sure the main jet is OK, it's big enough to see daylight through it, I've cleaned and checked it twice now. The float needle is 'captured' and in the correct position. I've checked the quantity of fuel that I can drain out of the chamber when the engine is stopped and it's pretty much a chamber full.

[/ QUOTE ]

If the idle jet & 'mixture' screw are OK then it's got to be an air leak, or maybe a stuck valve, then.

You could try spraying WD40 around the carb - engine connections whilst the engine is running. If the revs change, you've got an air leak.

Dunno otherwise.

Andy
 
Well, progress is being made (I think)...
On disassembling the carb for the tenth time I found another jet (idle jet?) at the bottom of a very deep hole in the bowl, it was blocked. Having cleaned this jet I can now get rid of the j-cloths and the engine ticks over on choke like a sewing machine. Now I can't turn off the choke otherwise the engine dies. If I repeatedly 'blip' the throttle I can get it to rev up to high speed but, again, if I turn off the choke the engine dies. The main jet is definitely clear along with the diffuser holes along its length. There are so many tiny air holes and inaccessible airways that I don't know how to ensure they are clean, I don't have access to any HP airlines. Any ideas chaps?
 
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