Vacuum Leak (?) Diagnosis

[rockgardener]

New member here, and this is the first post on my recently purchased 2001 315r. Sorry to start my forum participation with a novel, but I thought I'd better provide all the information I can about my mysterious problem.

The bike is hard to start, and when it does tick over with the choke on, the revs go sky high. I have to hold the choke in a half-on position while the motor warms up to keep it from either screaming or dying (is this normal?). Once warm, the idle hangs after revving and warbles (1-2 oscillations per second) when it finally drops, which it does very gradually over about 5-10 seconds. However, to keep it running, I have to set the idle rather high; otherwise it dies after about 15 seconds. Adjustment of the air screw does almost nothing. The bike seems to run okay above idle, though I have nothing to compare it to, since this is my first trials bike. (It feels quite weak compared to my two-stroke KTM 250, but I assume this is because of the 315r's heavy flywheel and big differences in tuning - I did NOT expect it to pull like a woods bike!)

I completely disassembled the carb and cleaned everything thoroughly. I also pulled out the reed valve, disassembled and cleaned it, and replaced the gasket. The reeds (carbon) looked fine after I removed some white waxy residue that was also on much of the reed cage (bad gas?). The rubber boot that connects the carb had no apparent defect. I pulled the ignition cover and there were no signs of a leak on that side, nor were there any obvious signs of a leak around the cylinder base gasket or right-side case seal.

The only visible clue that something MIGHT be leaking was a very small amount of clean transmission oil in the center of the skid plate. However, that could easily have been left from a recent oil change, as the previous owner replaced the tranny oil prior to selling it. I have thoroughly cleaned the skid plate and the bottom of the engine, and will look to see if any more oil appears there. So far, this area has remained clean, though I've only ridden the bike about 10 minutes since cleaning it. It's no fun to ride a trials bike in the yard that won't run properly at slow speeds.

I've been told that the tell-tale sign of a right-side case seal leak is that the motor will run more smoothly when leaned way over to the right. My bike does seem to idle ever-so-slightly better in that position (continues to warble, just less dramatically), but still hangs after revving. I've also been told to sprinkle baby powder around the gasket edges and then look for any sign of dampness, since a case leak on a two-stroke will involve both an inhaling and an exhaling action. This idea falls apart in practice, though, as I can't really get powder everywhere there might be a leak. I've also heard that leaks can be detected by spraying carb cleaner around the gaskets while the engine is running, but I don't know what I'm supposed to be looking for with that, and again, I can't reach/see all the way around some gaskets' edges.

One other potentially relevant piece of data is this: I've read about a small air filter that was fitted to earlier model 315r carbs. My carb doesn't have such a filter, and the hole in the right side of the carb where it's tubing would go appears to be sealed and without function - carb cleaner squirted in there just fills it up and doesn't exit somewhere else. Nevertheless, the 2002 owner's manual (the closest to my year I could find online) clearly shows the sub-air filter in the routing diagrams, and there is a tab jutting off my rear brake fluid reservoir bracket that might have once been used to mount such a filter. I've read elsewhere that the sub-air filter was eliminated by 2001, so perhaps the diagrams in the manual weren't updated. Or maybe my sub-air filter is actually missing - I can't tell. My carb does look identical to the one pictured later in the manual, in the carburetor section, and there's no sub-air filter visible in those pictures.

Assuming no more oil collects in my skid plate, how can I determine where the leak is? Could a missing sub-air filter produce the symptoms I've described? Many thanks in advance for your help!

[lineaway]

So did you buy the bike this way? Or what changed when this happend. It sounds like a carb issue. Did you check for main bearing play when you had the cover off. Air leaks other than mains and seals are uncommon and an oil leak should have nothing to do with it.

[rockgardener]

Thanks for your reply, Lineaway.

I bought the bike in a neighboring state and only rode it briefly in the parking lot of the dealer who was brokering the consignment sale of this bike. All I did when test riding it was check to make sure the transmission worked properly and the bike sounded okay while running through the gears. I don't remember it having this idling problem then, but I can't be certain. I did not check for main bearing play when I had the ignition cover off - not sure how to do this. Would I just try to move the flywheel up and down by hand? I didn't think an oil leak would have anything to do with it, but I'm lost here, so I included everything I noticed.

[lineaway]

Yes you can just `grab` the flywheel to check the mains. If nothing else it helps to eliminate problems. Best advice would be to try a `buddies` carb. We can talk for hours about what you have done to the carb, nothing like a good carb to swap. Which I know can be hard to come by. Did you remove the pilot jet and choke jet, clean and check both circuits were not blocked? The pilot actually flows out of two locations. Directly under the slide and a small hole in front of the slide. Keep trying they are a nice bike when running properly.

[dombush]

Rockgardener

you are looking in all of the right areas, and i like the suggestions so far.

I would doubt its a carb fault because of the behaviour with the choke on, this suggests a big air leak rather than a jetting failure.

The most likely places for an air leak (non crank seals) are around the intake rubber / reed valve joint, Given you have checked these and all looks good, i would move onto replacing the mains as a next step. Not sure if the mains can be replaced with the engine in situ on a 315r, but this is possible on most modern engines.

Start with the ignition / flywheel side first as its the easiest and is only a 30 minute job (at least on a Gas Gas or Sherco).

Moving onto the other side, usualy a sign of a leaking clutch side seal is air being sucked / blown out of the gearbox breather. Clearly if air is being drawn into the crankcase via the gearbox, this air has to enter the gearbox via a route. This route is usually the breather, so a simple dip of the breather pipe into a glass of water is a good identification technique.

Dom

[rockgardener]

Thanks for the replies.

When cleaning the carb I removed all jets and sprayed all passageways, although I did not notice whether the pilot circuit spray exited at the two locations Lineaway specified. I can recheck. I forgot to mention that I also checked the float height and it was spot on. As for swapping carbs with someone, I don't know anyone else with a 315R. I have read there are advantages to using a PWK28 (Keihin or copy), and I'd be wiling to eliminate the carb as a possible source of this problem by fitting one of those, if that seems appropriate/advantageous to those of you in the know.

Dom, I assume you are saying that the ignition/flywheel main could be leaking, even though there is no trace of oil or fuel residue under that cover, correct?. And I'm not clear on putting the breather pipe into a glass of water. What would I be looking for, bubbles? If it is actually sucking air in through that pipe, wouldn't I risk sucking water into the gearbox? (I suppose that wouldn't be so terrible if it provided a conclusive diagnosis.) There are two breather pipes exiting the cases. You're talking about the one toward the rear of the engine, right?

Thanks again for the help.

[brucey]

Hi,

Before you get too carried away read my post on the Gas Gas Section titled 'Slow return to idle'. It might save you a lot of time and trouble!

Brucey.

[lineaway]

p>What Brucey is saying you could have bad gas. Good thing to replace the fuel no matter how `new` it is. I have just seen the same symptoms on a Honda power washer at work. Needs half choke, idles poorly and makes lousy power! Worth a shot

[rockgardener]

Thanks for the latest replies. I'll swap out the fuel. Sometimes the easiest/simplest fixes are the ones we overlook.

[naichuff]

I do not have 2001 parts/workshop manual but I have 1999 one. The carb is a PHBH26CS

Main jet is 135 slow jet is B46 ( upper ) 46(lower) jet needle 3rd from top air screw one turn out (to start with) Float level 18.5mmThere is two slow jets one you cane see the other is above it There is two breather pipes one each side there was no secondery air filter Hope this helps

[dombush]

Dom, I assume you are saying that the ignition/flywheel main could be leaking, even though there is no trace of oil or fuel residue under that cover, correct?. And I'm not clear on putting the breather pipe into a glass of water. What would I be looking for, bubbles? If it is actually sucking air in through that pipe, wouldn't I risk sucking water into the gearbox? (I suppose that wouldn't be so terrible if it provided a conclusive diagnosis.) There are two breather pipes exiting the cases. You're talking about the one toward the rear of the engine, right

Often when the seal leaks air it doesnt pour oil out. Normaly there will be some under the flywheel when removed, all engines instances are slightly different. It can also depend on what sort of main bearing seals (shields) are fitted.

As far as the breathers are concerned. You only need to dip them for long enough to see bubbles / suck up some water. It will be very obvious very quickly. Not sure if the 315r has 2 breather hoses, someone else can pehaps comment. If it has, then blocking one while testing the other and vice versa.

The air leak you are looking for sounds quite big, so youre looking for something reasonably obvious. Remember that the crank seals are about £10 for the pair, its just the fitting that requires a bit of experience.

[naichuff]

One tube is the gearbox the other is from the generator

[martin belair]

The choke slide is dirty.

[rockgardener]

Thanks for the input from recent posters.

I checked with the mechanic at the (somewhat) local Montesa dealer today. He assured me this was almost certainly a carb problem and advised that I begin by going back through it again with not just carb cleaner but also compressed air. He said that often the passageways are partially cleared by solvent, but require air pressure to fully clear. He added that it's necessary to cover one exit hole at a time to force all the air through the second exit hole when blowing out the choke circuit and the pilot circuit, as each of those splits into two routes when blown from the float bowl side. He also told me it may be necessary to run a thin wire through the pilot and choke jets to get them completely clear, even though I can see light through them already and the holes look nice and round. I may also go ahead and soak all parts in solvent to be absolutely sure they're totally clean. I'd have done this already, but the inside of the carb actually looked quite clean - much cleaner than many I've opened up.

Three other things he told me to check: First, the tiny o-ring at the bottom of the choke jet must seal well against the float bowl - if it's hard, cracked or mishapen, it must be replaced. Second, he said the idle set screw (throttle stop) must not be turned in too far or it will interfere with starting because it will prop up the slide and produce the same result as twisting open the throttle while kicking, which ruins the mixture. Because I've been trying to keep the motor running by setting the idle high, I'm sure I've contributed to the starting difficulties on this count. Third, he advised me to set the fuel screw further out than the single turn that the manual recommends; he said to turn it out until 1.5-1.75 threads were showing, and use this as my starting point.

So, I'm going to re-clean my carb and check these details. I will also use fresh fuel and spark plug, and will report back on my efforts as soon as I get time to complete them. If unsuccessful, I'll resume detective work on the cases, but I'm really hoping the dealership mechanic is right and this is just a carb issue...

NOTE: I definitely do not have the 1999 PHBH26CS carb mentioned by naichuff, with the combined pilot jet and diffuser. There is nothing under/above my pilot jet. For what it's worth, my jets are as follows: 110 main, 35 pilot (slow jet), 60 choke ("by starter jet"), K270 needle jet. I've got a D37 needle with the clip in the third position.

[ham2]

Because of safety issues I feel the need to talk hypothetically: Suppose a person with a trials bike wished to elimate ( very quickly) the possibility of an external crankcase-carb-reed block air leak? Would he get quick results from opening a can of butane in the vicinity of the idling trials bike engine with regards to stabilising the tick-over speed, this could help rule out/ locate a range of air leak problems?

You don't smoke, do you?

If it all goes wrong, will you let me know and I'll delete this post