Carb Spitting at below 2200RPM ?cause

[metcalfej]

I have a 1977 Merc 700 EL (70HP).

It doesn't like to idle, indeed it doesn't like to go below 900 revs at this level it gets variable and sooner rather than later cuts out, the WOT is low aswell (but this could be prop pitch etc - dealt with elsewhere).

The spark, Carb and fuel pump issues are being dealt with in other threads. My problem with this thread is Carb spitting. I noticed whilst trying to tune the idle mixture screws that the top carb of two is spitting gas and air back out of the air intake, this is quite noticeable at low rev's and dissapears above 2200 rev's. The bottom carb sucks normally.

All three cylinders have 125PSI

What is the cause and the solution and could it be the cause of the poor idle

Thanks to all those high ranking people with advice in advance

 

[andy6374]

Re: Carb Spitting at below 2200RPM ?cause

Bad reeds?
Needle not seated or floats set too high

 

[metcalfej]

Re: Carb Spitting at below 2200RPM ?cause

Why would the needle not seated or the floats being set too high result in back pressure through the air intake. If this is the reeds how difficult are there replacement and is it a costly procedure. Are there any other things you would do at the same time?

 

[Chris1956]

Re: Carb Spitting at below 2200RPM ?cause

Replacement of the reeds requires that the powerhead be removed, the starter, carbs, crankshaft end caps and crankcase cover be removed and the crankshaft and connecting rod bearings be removed. Actually, you may be able to remove the reed cages without removing the rod bearings, by slightly lifting the crank out of the crankcase, and undoing the cage bolts and sliding the cages out. Since your compression is good, you do not need to do anything else. You will want to try to make sure your reeds are bad. How about if you remove the transfer cover for the bad reed's cylinder, put the piston at TDC, and see if the crankcase is open to the transfer port. If so, drain the carb of fuel and put a vacuum pump on the carb venturi and try to draw air thru the carb (backwards). You might be able to do it with your lungs, since you should be able to detect a small amount of leakage, and any leakage is bad. Do not suck up the gas or fumes:'(

 

[metcalfej]

Re: Carb Spitting at below 2200RPM ?cause

Thanks Chris, this is great advice, can I avoid taking off the transfer cover etc. With the piston at TDC (viewed through the spark plug orifice can I not just take the carbs off and see whether I can draw air back through the inlet, I am going to be overhauling the carbs so there coming off in anycase- I have little knowledge of 2 strokes so excuse my ignorance if I have misunderstood.

 

[gungalunge]

Re: Carb Spitting at below 2200RPM ?cause

got new reeds on mine when i rebuilt the engine hard to find and expensive . but got to say no idle or low speed troubles . and the carbs tune right at 2 1/2 to 2 3/4 all 3 600 rpm or even lower way lower . runs like a new weed wacker .good reeds new reeds. knowing those new in my engine ,, i have to say i am not experiencing low speed troubles like others i hear out here.

 

[gungalunge]

Re: Carb Spitting at below 2200RPM ?cause

the spitting can be a ignition miss too. high resistance in a plug wire . 2200 rpm hummm ..

 

[gungalunge]

Re: Carb Spitting at below 2200RPM ?cause

when you go into a fine tune . look for your plug wires doing there job .how do you know if they are good . resistance across the wire lenght .a bad spark plug wire will tell you alot . its easy but hard to rule out it being a problem . like swr on a ham radio antenna . i can go on but use an ohm meter .think about it .. what can a faulty ignition wire do ??? just one on a 6 banger make that right then see what happens." mix up all of our ham radio and mechanical experience and computer knoledge . i think we got a medium . our stuff will run right . go boating now ...captain of a ship "what a human ..beat that >>fishing is easy ...............