90 model 5hp yamaha will only rev past idle with partial choke on

inflight · 07 January 2009, 20:49

Has anyone a sollution to this problem this motor idles fine but when you go to rev it up it just dies unless you turn the choke slightly on i have pulled the carby down 2 times now to clean it and looked at the fuel pump it looks ok though the rubber diaphram seems a bit stiff it has 85 psi compression i dont know what the minimum compression on pull start single cylinder 2 stoke should be but my manual tells me over 100 psi minimum for a electric start can anyone help thanks

Nos4r2 · 07 January 2009, 20:51

Quote:

Originally Posted by inflight

Has anyone a sollution to this problem this motor idles fine but when you go to rev it up it just dies unless you turn the choke slightly on i have pulled the carby down 2 times now to clean it and looked at the fuel pump it looks ok though the rubber diaphram seems a bit stiff it has 85 psi compression i dont know what the minimum compression on pull start single cylinder 2 stoke should be but my manual tells me over 100 psi minimum for a electric start can anyone help thanks

Should run at 85psi-though it's a bit low. Try holding the fuel tank above the engine-if it carries on running with the fuel supply higher than the carb then it's the pump.

inflight · 07 January 2009, 21:15

it has an over head tank fitted which i emptied cleaned and refilled it could you tell me what would be the minimum compression for this engine and can it be accuratley tested by the pull starting as i cant find much information on compression testing a pull start engine best regards

Nos4r2 · 07 January 2009, 21:22

Quote:

Originally Posted by inflight

it has an over head tank fitted which i emptied cleaned and refilled it could you tell me what would be the minimum compression for this engine and can it be accuratley tested by the pull starting as i cant find much information on compression testing a pull start engine best regards

Compression readings depend on how fast you turn it over. I'd want around 100psi really-but if it ticks over in gear in the water it's not compression that's the problem

Chris · 07 January 2009, 21:39

I had problems similar to this when the engine would starve of fuel when you tried to rev unless the choke was on and it was a blocked jet, have you stripped out all the jets etc and blown them all out and used some wire to push through them?

matt h · 07 January 2009, 21:45

second that, sounds like the main jet is blocked

Nos4r2 · 07 January 2009, 21:49

Quote:

Originally Posted by matt h

second that, sounds like the main jet is blocked

That's what I thought-but he said he'd cleaned the carb out.

inflight · 07 January 2009, 21:57

i have the carby off again and was going to try and soak it can you suggest any solutions for cleaning carbies that i can leave the carby soaking in that wont damage it thanks

inflight · 07 January 2009, 22:11

I pulled the float out checked the needle undid the main jet and pulled out the venturi under it they where all clean if it i have it in pieces again now for the 3 time i was going to leave it soaking but would like to know what the best sollution would be to leave it soaking in for a couple of days cheers

Nos4r2 · 07 January 2009, 22:22

Have you removed the jets and made sure they are clear? It's worth putting compressed air through all the passages too.

Locozodiac · 08 January 2009, 13:05

Try canned carb cleaner spray or Acrylic Thinner, don't soak for days could be dangerous to metal & plastic parts, 18 years of use it's a lot of time, replace a new gas pump kit, or at least a new pump diphragm, seems this is the issue with your 1990 Yamaha. If your gas hose has an interior double hose, check for gas obstructions along the line including primer bulb, with time, interior hose tends to break in tiny pieces blocking gas passage tank to engine. Check also, clean internal tank filter for obstructions.

Happy Boating

matt h · 08 January 2009, 14:27

it if runs with the choke on and dies when you turn it off its the main jet or main jet feed...you need to look at, get that main jet immaculate, check the needle for comparison,(eg, do the needle valve and main jet go together) have they been swapped and the incorrect one put back in, is the needle valve bent, is the needle valve too long?? you can adjust it i think...also where the fuel enters the carb, blow down it ,it it clear ??

matt h · 08 January 2009, 17:00

actually it probally has not got a needle, but its fuel feed related anyway

9D280 · 09 January 2009, 11:43

I had a Suz25 had exactly that problem. If you blip the choke I assume it runs OK at higher RPM?

If so, it's your idle mix needs slightly richened up. Either the fuel or air jet needs adjusting (don't know the carb, could be fitted with one or other or both).

inflight · 10 January 2009, 05:31

This is a great site i will try the acrylic thinners this carb doesnt have a needle i buggered up and put the wrong year down its a 2000 mod thanks to you all of you for your help