Motorcycle backfires inconsistently

So... I got this epic pile of shit TDM850. I traded like 3 grand worth of crap for it too. fucking whoops, it's hands down my greatest moto regret

Anyway, a year later i've got maybe 3 miles on it.

It starts and runs and seems to ride ok EXCEPT maybe once or twice a minute it'll backfire and blow back out the intake. It's not like a lean backfire on decel, it's not a timing issue, it doesnt seem to care about throttle position or load

bike is big carbed parallel twin. valves are shimmed and timed exactly. carbs cleaned meticulously. new plugs and filters, every thing else pretty factory. it's got quite a few miles, 20k or something.

but it does worry me, it's gotta be kinda hard on the engine i mean it's pretty hard when it does it. and it shouldnt.

My thought? electric? but where to address it? I dunno.

Honestly i'd rather just make this thing disappear but i've got way too much time into it to just abandon. maybe i'll leave it out one night so it can disappear without me feeling like I chose to quit it.

Re: Motorcycle backfires inconsistently

> Born to be WillD Wrote:

> -------------------------------------------------------

> So... I got this epic pile of shit TDM850. I traded like 3 grand worth

> of crap for it too. fucking whoops, it's hands down my greatest moto

> regret

>

> Anyway, a year later i've got maybe 3 miles on it.

>

> It starts and runs and seems to ride ok EXCEPT maybe once or twice a

> minute it'll backfire and blow back out the intake. It's not like a lean

> backfire on decel, it's not a timing issue, it doesnt seem to care about

> throttle position or load

>

> bike is big carbed parallel twin. valves are shimmed and timed exactly.

> carbs cleaned meticulously. new plugs and filters, every thing else

> pretty factory. it's got quite a few miles, 20k or something.

>

> but it does worry me, it's gotta be kinda hard on the engine i mean it's

> pretty hard when it does it. and it shouldnt.

>

> My thought? electric? but where to address it? I dunno.

>

> Honestly i'd rather just make this thing disappear but i've got way too

> much time into it to just abandon. maybe i'll leave it out one night so

> it can disappear without me feeling like I chose to quit it.

You say it's not a timing issue but it sure sounds like one cylinder is igniting too prematurely, when the intake valve is still open.

Re: Motorcycle backfires inconsistently

"Used TDMs–and there aren’t many around–should be checked for misfiring due to stuffed-up carbs, and for flat spots in the powerband that can often be traced to the same source. Paint and finish issues have been reported, along with faulty cooling fans."

from https://ridermagazine.com/2018/10/23/retrospective-1992-1993-yamaha-tdm850/

Re: Motorcycle backfires inconsistently

Thats a 5 valve head. From your description it could be an intake valve issue. An intake valve could be too tight and not closing all the way. That may have shim under bucket valves, meaning the adjustment shims are under the lifters. This requires measurement before taking apart then removal of the cams to pull lifters and retrieve shims. Shims will have size etched on them, do the math to achieve proper clearance and insert appropriate shim before putting it back together. The other possibility could be a burnt intake valve or seat. I heard of a guy that had one and had to have the head reworked at 10,000 miles. In your case you MUST do a compression test on both cylinders before anything else. (edited)

Re: Motorcycle backfires inconsistently

That's a cool looking bike, one of the reviews said misfire was worse in the wet, says electrical to me? New plug wires and dielectric grease may be in order, check all wiring in ignition circuit, sounds like you may be getting an extra spark, or bleed over from the cyl that should be firing? My guess anyway.

Re: Motorcycle backfires inconsistently

> live ɘvil Wrote:

> -------------------------------------------------------

> Thats a 5 valve head. From your description it could be an intake valve

> issue. An intake valve could be too tight and not closing all the way.

> That may have shim under bucket valves, meaning the adjustment shims are

> under the lifters. This requires measurement before taking apart then

> removal of the cams to pull lifters and retrieve shims. Shims will have

> size etched on them, do the math to achieve proper clearance and insert

> appropriate shim before putting it back together. The other possibility

> could be a burnt intake valve or seat. I heard of a guy that had one and

> had to have the head reworked at 10,000 miles. In your case you MUST do

> a compression test on both cylinders before anything else.

If it was a valve, would it not run for shit all the time, or not run at all? Vs. misfire 2 times a minute? Not saying your wrong, but just thinking aloud....

Re: Motorcycle backfires inconsistently

Sticking intake valve/intake tract air leak/inductive cross-spark/loose bit in exhaust/???

Re: Motorcycle backfires inconsistently

^ That "inductive cross spark thing" never heard the term, but that's where my money is. Thanks for sharing the name!

Re: Motorcycle backfires inconsistently

A compression test cost nothing if you have a compression guage. It will help rule out "the mechanics" of the motor before chasing electrical gremlins.

Re: Motorcycle backfires inconsistently

Change the spark plugs anyway, but it is probably valves. When that happens on older cars, it is usually a cracked distributor cap or rotor. (edited)

Re: Motorcycle backfires inconsistently

^ This is a motorcycle, not an older car, do try and focus.

Re: Motorcycle backfires inconsistently

> Stephen Keller Wrote:

> -------------------------------------------------------

> ^ This is a motorcycle, not an older car, do try and focus.

Musing about a problem from several angles helps solve it.

Re: Motorcycle backfires inconsistently

Not musing about a totally different vehicle, you really are slipping. A motorcycle does not have a distributor cap....you are too straight to drug test, you just need the last word even if you don't know what you are talking about. It's ok to not know everything. Please, quit this forum, join a trump group or some other hyper conservative outfit, you will only enjoy ego strokes, and we will be free of you.

It is ok to not know everything, I never heard of this bike before his post, I researched, and others who did agreed, others thought otherwise, but I still believe it's electrical, but I honestly don't know, and, sit down oldster, that's ok, we are guessing, as we can't see the bike! "Because I said so" is no longer a valid reason for anything. Sit down, and shut the fuck up, your time of absolutes is OVER. Be still, no one cares anymore, for one simple reason, your wilful ignorance embarrases you constantly. Be still old one, the darkness will soon be upon you. Embrace it, we will glory in your silence.

Re: Motorcycle backfires inconsistently

Looked it up. Needs valves adjusted every 25K miles. Make sure cam chains are in good shape.

Re: Motorcycle backfires inconsistently

Can the carbs be synced with a manometer?

Also what is cam chain tension at?

Re: Motorcycle backfires inconsistently

> Stephen Keller Wrote:

> -------------------------------------------------------

> > live ɘvil Wrote:

>

> > -------------------------------------------------------

>

> > Thats a 5 valve head. From your description it could be an intake

> valve

>

> > issue. An intake valve could be too tight and not closing all the way.

>

> > That may have shim under bucket valves, meaning the adjustment shims

> are

>

> > under the lifters. This requires measurement before taking apart then

>

> > removal of the cams to pull lifters and retrieve shims. Shims will

> have

>

> > size etched on them, do the math to achieve proper clearance and

> insert

>

> > appropriate shim before putting it back together. The other

> possibility

>

> > could be a burnt intake valve or seat. I heard of a guy that had one

> and

>

> > had to have the head reworked at 10,000 miles. In your case you MUST

> do

>

> > a compression test on both cylinders before anything else.

>

> If it was a valve, would it not run for shit all the time, or not run at

> all? Vs. misfire 2 times a minute? Not saying your wrong, but just

> thinking aloud....

It depends on how bad the valve is burnt or out of adjustment. Typically a misfire out the intake is a valve issue. Remember an intake valve should be closed when the plug fires.

Re: Motorcycle backfires inconsistently

> Stephen Keller Wrote:

> -------------------------------------------------------

> ^ That "inductive cross spark thing" never heard the term, but that's

> where my money is. Thanks for sharing the name!

Every jap bike I ever worked on had a "waste spark" ignition system. Meaning that cylinders fire in pairs twice. Once on compression stroke (when its supposed to) and again on exhaust stroke (after air/fuel charge has been ignited). I just don't think its an ignition problem because its misfiring not backfiring. (edited)

Re: Motorcycle backfires inconsistently

> D dB Wrote:

> -------------------------------------------------------

> Can the carbs be synced with a manometer?

>

> Also what is cam chain tension at?

Carbs out of sync will produce a sluggish throttle response and or a throttle that "hangs" and is slow to return to idle. If a cam chain is loose enough to cause mechanical timing issues then it will show with a compression test. Every jap bike has timing marks to check timing of both overhead cams and crank in relationship to each other. It will require the removal of the valve cover and the crank cover. (edited)

Re: Motorcycle backfires inconsistently

> Don Ohio Wrote:

> -------------------------------------------------------

> > Stephen Keller Wrote:

>

> > -------------------------------------------------------

>

> > ^ This is a motorcycle, not an older car, do try and focus.

>

> Musing about a problem from several angles helps solve it.

I'm surprised you didn't give him the fuel filter lecture.

Re: Motorcycle backfires inconsistently

Usually when something wears in the valve train on a jap bike its the valve face and valve seat. The valve then has to drop a little deeper to close. If its bad enough the valve may actually hang open partially (which can accelerate even more wear). That said, most of the time when adjusting valve's, the valve clearances are too tight and thinner shims are needed. You almost never see a valve clearance become too loose unless a cam lobe, lifter or valve stem is worn. (edited)

Re: Motorcycle backfires inconsistently

Interesting, @evil, I see where you are coming from, but I still don't see why it is not backfiring all the time or not running at all? Also, why is it reported worse in the wet, not in Will's case, but he has only put 3 miles on the machine, so.... waste spark system was explained to me in my Crown thread, as it has it, to me it seems that could be shorting or something grounding and one of those could be being delivered at the wrong time, no matter what, I'm interested to see what this turns out to be.

Re: Motorcycle backfires inconsistently

> Stephen Keller Wrote:

> -------------------------------------------------------

> Interesting, @evil, I see where you are coming from, but I still don't

> see why it is not backfiring all the time or not running at all? Also,

> why is it reported worse in the wet, not in Will's case, but he has

> only put 3 miles on the machine, so.... waste spark system was explained

> to me in my Crown thread, as it has it, to me it seems that could be

> shorting or something grounding and one of those could be being

> delivered at the wrong time, no matter what, I'm interested to see what

> this turns out to be.

If it was a spark issue it would simply fire at one of two times. Once when it should and once on exhaust stroke when it doesnt matter. A burnt or tight valve hanging open does exactly what Will described. It just hasn't got bad enough to cause other issues yet. Remember this is a 5 valve head. Thats 3 intake 2 exhaust valves per cylinder. One valve can leak some and it still run ok. A compression test will most likely show one cylinder down on compression compared to the other cylinder. At that point will can decide if he wants to throw anymore time or money into it. If indeed compression is down on one cylinder then the next step would be to check valve clearances. If valve clearances are good, then its most likely a burnt intake valve/seat. (edited)

Re: Motorcycle backfires inconsistently

Ok...., but what if the spark from the cyl that should be firing, especially the waste, but easily both, is misrouted to entirely the wrong cyl? (bad ground,wires, etc) That's what I'm thinking, but I do see what you are saying as well.

Re: Motorcycle backfires inconsistently

> Stephen Keller Wrote:

> -------------------------------------------------------

> Ok...., but what if the spark from the cyl that should be firing,

> especially the waste, but easily both, is misrouted to entirely the

> wrong cyl? (bad ground,wires, etc) That's what I'm thinking, but I do

> see what you are saying as well.

Its an in line twin motor. One coil for both cylinders. When one cylinder fires on compression the other is firing on exhaust stroke. There is nothing to burn or misfire during the exhaust stoke.

Re: Motorcycle backfires inconsistently

Only one coil? Ok, the twins I worked on had two, it could run on one half of the engine....I'm old remember, but now I'm with you on valve train, but I still wonder about the review that claimed misfire was worse in the wet.

Re: Motorcycle backfires inconsistently

Anyway, im not trying to say I'm right. I am explaining why I would do a compression test before anything else. Then i would check the valve clearances. I have the tools to do this so its only my time and its part of an investigation process. Try doing a few four cylinder motors...

(edited)

Re: Motorcycle backfires inconsistently

I don't think you are saying you are right, just putting forth different ideas, pay attention internet kids, this is how people who grew up with manners deal with different opinions/thoughts, in a civil manner. :) I gave up trying to sync carbs once there got to be four of them, two was more than enough! I hope Will gets it sorted and we can learn what's what. I strangely care now....lol!

Re: Motorcycle backfires inconsistently

> Don Ohio Wrote:

> -------------------------------------------------------

> Change the spark plugs anyway, but it is probably valves. When that

> happens on older cars, it is usually a cracked distributor cap or rotor.

One should "read" the current plugs first parts changer.

Re: Motorcycle backfires inconsistently

I did the valves, theyre spot on, but I think it was run for a while with them pretty out of spec and also the carbs pretty messy

Re: Motorcycle backfires inconsistently

> live ɘvil Wrote:

> -------------------------------------------------------

> > Don Ohio Wrote:

>

> > -------------------------------------------------------

>

> > Change the spark plugs anyway, but it is probably valves. When that

>

> > happens on older cars, it is usually a cracked distributor cap or

> rotor.

>

> One should "read" the current plugs first parts changer.

Any good mechanic reads every single used spark plug. That goes without saying.

Well, did you check the cam chains condition?

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