315R Carburettor - What's Going On... ?

[riffraffretro]

Any advice would be much appreciated...

I can't get things settled down with the fuelling on my 315R: starts easily from cold with choke, but shortly after the choke is closed things go pear shaped!

Throttle response gets sloppy pretty quickly - almost as if there's a disconnect between what the twist grip is doing and how the engine revs-up, with a noticeable delay and general feeling of sloppiness and lumpiness, until the point comes when the engine stalls.

The only method of keeping the engine running (and to momentarily achieve a normal throttle connection), is to open the choke, but the instant it's closed again, things go back to where they were.

Points to a carburettor/air issue to me, but I'm unsure. Carb air adjust screw is set to 1 turn out as per manual, fresh pre-mix has been added and the airbox/filter cleaned/replaced.

Anyone able to help? It's a '98 model year bike.

 

 

 

 

 

Edited August 11, 2017 by riffraffretro
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[bullylover]

That sounds as if the pilot jet is clogged. Pull the carb off and pull all the jets out and clean them all and all the fuel and air passages in the carby. Air is a good way to clean them all. Don`t use anything metal to clean the jets as this will change the hole in the jet. Graham.

[dave_t]

I agree that pilot jets are probably blocked, had a '99 bike some years ago exactly the same. Taking it that bike has original carb it has two pilot jets one behind the other in a recess, bit of dirt or water used to get between them. Needs a really good small screwdriver to remove them to clean properly.

[nigel dabster]

Make sure all jets are clear and clean, the banjo is spotless and the float height is 18 .5 then a new plug. If it still does it its some thing else other than carb. Youve got it fully turned on the tap as these are always awkward.

[riffraffretro]

Thanks guys... looks like I'll be removing and stripping the carb this weekend.

Help and pointers really appreciated.

 

 

[martin belair]

When you remove the long inner pilot jet do yourself a favor...whiz it at bin. You don't need it and all it does is clog up. Be very careful removing it. Do not bugger the threads. Use a narrow screwdriver that will fit into the hole.

[dwayne]

Buy a 26mm oko carb on ebay £29.99,  works great, get a 50mm foam filter too, £10ish.  It's an easy fix.

The problem with the dellorto's is:-  they have no overflow, if there is a problem with the floats or needle valve etc, fuel leaks into the crankcase,

that makes it hard to start with poor running etc.

[dwayne]

Quick update on the OKO carb, I felt it a bit dull to pull away so I followed this advice:-

It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.

Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing.

Before you start the jet testing, Install a fresh plug. Warm the bike completely, and shut it off.

As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling. Turn the air screw slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.

Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.

The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.

The full article is here https://thumpertalk.com/forums/topic/347305-is-your-two-stroke-running-rich-read-here/

Basically I turned the air screw out ( about half a turn ) until the revs were at their highest then turned the idle screw out about a turn to lower the revs back down to a good tickover, it now pulls like a train right off the bottom. I was surprised how much difference it made.

The above is the same advice for a Delorto carb  or any other 2 stroke. It helped me I hope some one else benefits too!