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Volvo model 250 RPM fluctuation

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dlib

New member
I have a 1992 volvo model 250, 140 HP I/O which when running at temperature begins to have RPM fluctuation of 100 to 1000 and eventually quits running and will not restart until I let engine sit for about 20 minutes or more. Condition comes back again when up to temperature. i installed a new Racor filter to no avail. Also tried a new coil - no use. help!!!!

Thanks ,
Dan
 
I would start by checking that both the idle mixture and idle speed are properly adjusted in the carb, that the fuel pump is not defective (this is uncommon, though), and that the anti-syphon valve in the fuel tank is not stuck. For the latter, you could try (for test purposes only) to supply the fuel to the engine from a portable fuel tank, like the ones used for outboard motor.
 
Thankyou for replying. I experienced since owning the boat ( Rosborough 246) that idle (900 rpm) did not occur until engine has run for several minutes ( when cold). Seemed to be firing on less than 4 cylinders. i didn't pay much attention to this as it came around to 9oo RPM eventually.
Because of the present problem I have been having and the engine quitting I adjusted the idle on both carbs to try and keep it running. i played around with this for awhile and tried to get the RPM as close to 900 as I could. It didn't help. i had it idling at 1200rpm but the symptom didn't go away when i slipped the mooring and test ran it up to approx 3000 rpm and it began fluctuating RPM again ( at temperature).

Is the anti spyhon valve standard on Rosborough's ?
 
The anti-syphon valve is in the fuel tank fitting where the fuel line going to the engine is attached.


Also, are the carbs properly synchronized? If you feel that the engine behaves like it is firing on less than 4 cylinders, chances are that the carbs are out of sync. One of the best ways to synchronize them is with a uni-syn tool

http://www.speedwaymotors.com/Edelbrock-Uni-Syn-Carb-Synchronizing-Tool,267.html

the Seloc manual instructs you how to use it, and otherwise the instructions that come with the tool are quite clear.

You can always try the instructions in the OEM manual, but synchronizing wiht the uni-syn tool gives better results.
 
I don't understand the purpose of the anti syphon valve and why it would cause the engine to quit at temperature.
oem manual says for this engine adjust ideling screw so that when it hits the cam add one additional turn. Hard to tell when it hits the cam but i think it was adjusted right before i meassed with it. I don't understand how that would cause engine to start flucuating RPM at 3600 since idle adjustment should be out of play at this speed?? Or not?
 
When you want to understand something, before asking it is helpful to either search the forum or to use google. If you google the terms anti-syphon valve purpose, I'm sure you will get lots of hits. But basically, the anti-syphon valve is a spring loaded check valve that prevents fuel from being spilled in the bilges should the fuel line rupture between the fuel tank and the fuel pump. If the valve becomes stuck, the amount of fuel reaching the carbs will greatly diminish and even become zero.

When it comes to erratic speed on a dual carb engine, unbalanced carbs will do that. For all the cylinders to fire with the same power, all of them should be receiving the same amount of fuel/air mixture, but if the carbs are not balanced that will not happen. The purpose of the uni-syn tool is to verify that you have the same flow of air on each carb at the time. It is basically a manometer designed to measure vacuum.

If you have the Seloc manual, it explains how to use it. We have also touched this subject in the past, so you may want to search the forum.
 
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I'm sorry but I still don't understand why this condtion only occurs at temperature! and at RPM other than idle?
 
I have stated in my previous post that unbalanced carbs could do that. At idle speed, you are feeding the engine with the gasoline that enters the carb via the idle jet (adjustable with the idle mixture screw) and the minimal amount of air entering the cylinder is adjusted via the idle speed screw, which slightly opens the butterfly valve. When you accelerate, you open the butterfly valves and the air passing through the carbs draws fuel from the main jets (Venturi effect). The amount of fuel mixing with the air depends on how much each butterfly valve is opened. This is why both carbs must be synchronized.

Of course, there can be other causes, but if you don't start a process of elimination you will never find the problem.
 
Another thing you may want to check is that there is no air ingress between the carbs and the intake manifold, or between the intake manifold and the head.
 
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