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Discussion Starter #1
Hi, My 82 sr250 which I bought in the spring of 2014, exploded that same year in the late autumn. What to do? I proceeded to look into how much it would cost to rebuild her and at the same time looked into finding another bike for less than a grand. I didnt find another bike, great I can do this, right? I found all the parts i needed for the rebuild at cheapcycleparts.com and took on the task. I have pics of all of the damage and parts that were affected but I dont know how to post these on this forum. I am a novice and I have never worked on a mbike before all of this happened. I have however changed brake shoes and cables and oil. That is it, not much experience, I know but i do have the factory service manual and whilst it may not be for the inexperienced and uneducated I figured, what the hell, I can do this. The parts that needed to be replaced were the piston, the camshaft, the valves and the cylinder head. Yes also the sparkplug and all of the gaskets. Oh yea the rocker arms too. I was fortunate to have been introduced to Jim Bauer from werks engineering and he rebored my cylinder and inspected the cyinder head I bought through ebay. All of the parts were installed by me with his guidance and after inspecting the clutch and trannny I was good to go. I had wanted to at this time change the airbox to an pod filter which I thought would be best for the bike. What do I know? Absolutely nothing!!! I start the bike and rode her for about five minutes and I get this backfiring along with a jerk and a serious lack of performance. I am told that this is because she is running lean so she needs more fuel. The bike still has the mikuni bs34 carburetor it was built with and the factory has it set to function proper with the airbox it came with. I replace the pod filter with the airbox and try again...still the same issue. I am riding her around the neighbourhood and I notice when i shift and accelerate she slows down then speeds up. Is this a carb issue, and why? It is the same exhaust and air filter , the sparkplug is new, the valves have the suggested clearance and the oil and fuel are new. do I need a new main jet? I am quite frustrated and yes I understand I am not an professional but do I need to bring her to a shop and spend minimum 100/hr for her to run proper? What am I not doing right? Thank you.
 

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Discussion Starter #2
I forgot to mention that I have also thoroughly cleaned the carburetor and used high pressure air to blow out whatever might be in the jet passages, There are also new hoses from fuel tank to the carburetor and I have checked for vacuum
leaks. The battery is brand new.
 

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Okay, correct me if I am mistaken, you've got the bike back to full stock set up, air filter is an OEM style, stock exhaust system, stock and new cam (so the lobes aren't worn down and not lifting valves). The cylinder has been bored out a little bit IIRC from your intro thread.

I'm of the mind you're probably okay running stock jetting. And stock jet needle clip position (the needle that lives in the carburetor slide and inserts into the main jet).

What you need to determine is where the "backfire" really is occurring.

True backfiring is the combustion of unburned fuel in the intake tract. The big bang that comes out the exhaust is actually after burn.

Backfiring could be due to an intake valve not closing fully or if the ignition or valve timing is off. Afterburn can be due to too rich a mixture, too lean a mixture, incorrect valve timing, air leaks at the head on the exhaust.

Get the bike in a dark area and see if you can see a flash coming out the pipe when it pops. Or out the back of the carb with the airbox removed. That will have the bike running lean though. Don't do that in a dusty environment either.

You need to make certain you got the valve train set up properly so the cam and crank sprockets are in the right position. Being a tooth off can foul the whole thing up. Being too far out may cause damage if the piston strikes an open valve. The correct procedure should be outlined in the manual. Usually there are some kind of index marks on the sprockets and/or timing gear that you need to line up.

You need to make certain your valve clearances are set up properly. These should be set when the engine is stone cold.

You need to be certain the ignition timing is spot on.

Make certain the intake tract is sealed. Old intake rubber boots can be dry rotted and leak fouling the mixture. A worn out carb can have air leaks around various moving components and that can mess things up. Even a poorly fit boot from the air box can let in extra air, again fouling the mixture.
 

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Discussion Starter #4
Ok I am making sure that timing is precisely set and checking valve clearances now that engine is cold. I have done this many times before with same results, however, I also feel that it might be just that, only I have done as suggested in manual. There are markings on cylinder head for cam and on crankcase too. For valve clearances I align T mark on flywheel with mark on crankcase putting piston on TDC. I have removed carb and will set idle mixture screw to factory setting. That will be first, if same result I will change main jet which is 122.5 for 125 which I received today. HERES HOPING FOR THE BEST BUT EXPECTING THE... The backfiring has been from exhaust as i have noticed. What really troubles me is that when I accelerate it doesnt respond until after it has slowed then it takes off. Never experienced anything like it.
 

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Discussion Starter #5
Right then, unbeknownst to me is the fact that there are two small holes,one smaller on the bottom of the diaphragm assembly. One of these has an A besides it. Could this be it? Have i incorrectly assembled this carb? May not be the problem but it is most certainly a problem methinks. I have not read anything in the manual about how to position these. I will proceed to have the A face the air filter and see what difference it makes. Does the A refer to air and are these two openings what i think it is fuel and air? This has been a great education but I want to ride without the worries. Ok will keep all of this documented hope it can help another poor novice.
 

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By your answers I am not entirely certain you picked up on one of my suspicions.

You replaced the cam. That means you have to install it so that the valves open and close at the appropriate time with respect to the piston's location in the stroke. If the chain is one tooth off, the valve timing is incorrect. The cam driving the valves will be a little out of sync with the crankshaft. This is not to be confused with ignition timing which is the time the spark plug fires with respect to piston location before TDC.

To assess this you will need to remove the valve cover and the engine side cover over the marks specified for positioning the crankshaft with respect to the cam.

I'm not entirely certain what can go wrong with diaphragm orientation. The diaphragm should be keyed to the slide and the top of the carburetor. If it isn't the cutaway on the slide will be positioned wrong. Some carbs have the slide notched up one side with a tab sticking out of the barrel in the carb where it lives such that it keeps the slide oriented correctly in the carb.
 

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Discussion Starter #7
Apologies for not being clear. I have aligned mark on sprocket with the cylinder head mark and crank with crankcase mark after doing so I put cam chain tension adjuster on as instructed in manual only after do I proceed to do valve clearances whilst engine is cold. In regards to diaphragm assembly markings I cant find anything in manual. Maybe it doesnt matter I'll know more today.
 

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There has to be some means of orienting the slide correctly inside the bore of the carb. The slide has a cutaway on the portion that is seen through the throat of the fully assembled carb. It should be on the engine side of the carb. That is where the tab on the inside of the carb would line up with a slot on the side of the slide. For CV carbs there will usually be some sort of tab/blob/thick spot/mark on the diaphragm's perimeter that has an appropriately located depression/slot/hole on top of the carb to engage it.
 

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Discussion Starter #9
Ok ill have another look after work. Raining here today so after work I'll be wotking on this fix. I am expecting new battery tomorrow. Will keep posting result good or bad, thanks for advise
 

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Be sure the battery is brought into service correctly. It should receive a full charge according to the manufacturer's directions.
 

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Discussion Starter #12
Still having trouble although with the new battery it starts immediately. I am stuck...I was advised by another rider to remove the cylinder head and cylinder and check the rings. The rings were installed correctly since the engineer that rebored the cylinder put them on the piston. I wil now try a bigger jet 127.5 and if that fails....
 

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If the valve clearances are set properly you don't need to take the head off to evaluate the combustion chamber. A compression check is your first look. You'll need a compression gauge - essentially a pressure gauge with a fitting on it to screw into the spark plug hole. It should be in spec, if it is then you're probably okay on the cylinder.

A leak down test is another possibility but requires a pair of pressure gauges and the right fitting between them. I'm not of the mind it is needed on a fresh top end though.

Also see if the slide is oriented properly in the carb.

I'm in a bit of a hurry to be looking at fiches now, but how is the ignition timing set? Is it all bolt up stuff that you install and cannot adjust or is there a mechanical timing advance along with the ability to adjust ignition timing? I'm thinking there isn't and you just install the components as they're all electronic with no need to adjust.
 

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CRAP!

The dang puter ate my reply.

You don't need to pop the head off to check the integrity of the combustion chamber. You need a compression gauge to check to see if the engine can build up adequate pressure for the big bang.

I'm also curious about that carb assembly with the diaphragm and the slide being in correctly.
 

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Discussion Starter #15
Ive had a look at the diaphragm and have assembled it correctly. adjusted the idle mix screw which is on engine side and on top whilst the bike idled, it sounds good. As for compression I dont own anything to test with but I have put my thumb over the plug socket and it appears to have compression. Tomorrow I'll take her to a local mechanic whom isnt going to rip me off, i've been introduced thru a local salesperson at the home depot. He works on her husband's bikes and will be glad to help me figure out this. More than likely he has the compression gauge and can ascertain if it is right. As for the ignition timing, in the manual the timing is set by aligning the mark on cam sprocket with mark on cylinder head and aligning the mark on crank with a mark on the crankcase. It is then instructed to install flywheel and align the T mark on flywheel with mark on crankcase to check valve clearances and adjust if needed. I am not familiar with any other process for timing. Thanks for your advise I will continue to post results, perhaps after tomorrow, I'll be posting positive ones.
 

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Ive had a look at the diaphragm and have assembled it correctly. adjusted the idle mix screw which is on engine side and on top whilst the bike idled, it sounds good. As for compression I dont own anything to test with but I have put my thumb over the plug socket and it appears to have compression. Tomorrow I'll take her to a local mechanic whom isnt going to rip me off, i've been introduced thru a local salesperson at the home depot. He works on her husband's bikes and will be glad to help me figure out this. More than likely he has the compression gauge and can ascertain if it is right. As for the ignition timing, in the manual the timing is set by aligning the mark on cam sprocket with mark on cylinder head and aligning the mark on crank with a mark on the crankcase. It is then instructed to install flywheel and align the T mark on flywheel with mark on crankcase to check valve clearances and adjust if needed. I am not familiar with any other process for timing. Thanks for your advise I will continue to post results, perhaps after tomorrow, I'll be posting positive ones.
:biggrin: more confusion. I was talking about ignition timing this time. Not valve timing. On older bikes, say ones with points, you'd have a points plate and a cam that was situated on a set of weights with springs holding the weights in. You'd set the timing with the bike idling with a timing light. That would be a strobe light that you attached to the bike - the strobe would light up each time the spark plug fired and that would illuminate the bits that were spinning as well as pointed fixed to the crank case. The idea was to get the timing marks to line up, if you see what I mean. The set of spring loaded weights would act to advance the timing when the engine speed reached the point where centrifugal force overcame the spring tension and that would rotate the cam that opened the points to generate the spark. It would rotate it to make the spark occur a couple degrees sooner (before top dead center).

Most newer bikes have electronic components that do all the ignition advance stuff as part of their design. They're all made to spec so you just bolt them in place and go, no need to do any ignition timing. If you bolt everything down you can't mess up the ignition timing with these. If the ignition timing is out, the bike will run like crap. As engine speed increases there is a need to advance the timing - making it so the spark occurs before top dead center is reached by the piston. That allows flame front propagation to occur such that combustion applies maximum pressure to the top of the piston at a time when it will punch the piston back down for the best effect. Too soon and the combustion is going to fight the rising piston. Too late and it is kind of like trying to shove someone who is already running away from you.


OTOH, valve timing can be set up incorrectly when the timing chain is installed if you get it off a tooth or so. Or if you just slap things together. Valve opening and closing occurs in a fashion that doesn't give best performance.

Valve timing affects the way an engine breaths, ignition timing affects the way it fires.
 

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Discussion Starter #17
yea I see what you mean there is a difference. No the manual states that ignition timing on the sr250 is not adjustable. It can however be checked with an timing light which I do not own. Tomorrow I will mention it to Mac, he will know more about this. Thanks I have to enroll in motorcyle repair and maintenance, so I can understand what the experts are trying to say. I will too. Bloody hell, I never even thought that there was an ignition timing issue since the manual only mentions it in the second chapter but not in the rebuild chapters. At the end of the chapter for rebuild it has a checklist but no mention of ignition timing. Now i understand what you are talking about. ha, I am daft. So because it is not adjustable, that may not be the issue?
 

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They have to be hosted by a photo sharing site
 

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yea I see what you mean there is a difference. No the manual states that ignition timing on the sr250 is not adjustable. It can however be checked with an timing light which I do not own. Tomorrow I will mention it to Mac, he will know more about this. Thanks I have to enroll in motorcyle repair and maintenance, so I can understand what the experts are trying to say. I will too. Bloody hell, I never even thought that there was an ignition timing issue since the manual only mentions it in the second chapter but not in the rebuild chapters. At the end of the chapter for rebuild it has a checklist but no mention of ignition timing. Now i understand what you are talking about. ha, I am daft. So because it is not adjustable, that may not be the issue?
Don't worry, when you stop learning new stuff, you're dead.

If it isn't adjustable it probably isn't an issue. When ignition components in this type of system develop faults the bike typically just won't run.


To post an image scroll further down the reply window to additional options. You'll see a button "manage attachments." That is what you want to click on. That'll bring up another window. Click on "browse" and locate the picture on your computer.

I must have shot that picture before getting my eyes worked on. :)
 

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