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Discussion Starter · #1 ·
Just stumbled across this on the Mikuni site:-

Backfires Through Carburetor

The information pertains to Harley's specifically but has some relevance to our bikes because they also use a "dual fire" or "wasted spark" system.

Of particular interest is the section on cam design. This would suggest that bikes with high overlap cams, such as early 790's or bikes fitted with aftermarket cams, would be more prone to backfiring through the carbs as the wasted spark actually occurs when the intake valve of that cylinder is partially open. I suspect that this would most likely occur at low RPM's or at cranking speeds.

I have noticed that even my standard 865 with its very low overlap cams has backfired through the carbs a couple of times when cranking, although this was probably due more to a lean condition, and the result is not very nice at all. I guess this is why some bikes have spat the starter idler gear under these circumstances.

Has anybody with high overlap cams fitted to their bike noticed an increased tendency to backfire through the carbs particularly when cranking?
 

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Compared to other motors these bikes dont have high over lap cams.4 valve heads use far less cam to get the job done then 2 valve HD motors do.A 280 DEG duration cam in a 4 valve head is about the same as a 315+ duration cam in a 2 valve head motor.Stock cams even the 01 790 cams are very mild on these bikes.Weak case design is what causes some to bust the cases at starter idler thats why they beefed up the newer cases in that area.What happens is when the starter does not have power enough to kick motor all they way over and it kicks back.Its just like the old kick start bikes if you dont kick them all the way through they will hurt your leg.
 

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Mike, How did you starter do with that big stroker motor? I can tell the new pistons put a little more load on mine.
 

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Discussion Starter · #4 ·
Compared to other motors these bikes dont have high over lap cams.4 valve heads use far less cam to get the job done then 2 valve HD motors do.A 280 DEG duration cam in a 4 valve head is about the same as a 315+ duration cam in a 2 valve head motor.Stock cams even the 01 790 cams are very mild on these bikes.
So, are you saying then that the wasted spark does not occur when the intake valve is partially open even with early 790 cams? If so I would have to question that because the early 790 cams opened the intake valve at 22*BTDC which would surely be within the range of the spark occuring at cranking speeds and aftermarket cams would probably open the intake even earlier. I'm just interested to know if this situation causes any increased tendency to backfire through the carbs that's all.

Weak case design is what causes some to bust the cases at starter idler thats why they beefed up the newer cases in that area.
Yes, I'm aware that the cases were beefed up at some stage to minimise the possibility of the starter idler gear being spat. Still makes me cringe when my bike kicks back while cranking though.

What happens is when the starter does not have power enough to kick motor all they way over and it kicks back.Its just like the old kick start bikes if you dont kick them all the way through they will hurt your leg.
Slightly disagree with that statement. :D It's not that the starter doesn't have enough grunt to turn the engine all the way over it's because combustion has occured before the piston has reached TDC and the inertia of the flywheel is not enough to carry the piston over TDC and stop the engine kicking back in the opposite direction. The starter doesn't offer sufficient torque to assist much in preventing this anyway it's primarily down to engine inertia.

I also know about kickstarting older bikes, my first real bike was a Matchless 500 single when I was fifteen years old. I was only a flyweight at that age and quickly learned the right technique for kickstarting a large capacity single cylinder motorcyle. The trick is correct positioning of the piston (just past TDC after the compression stroke) before you kicked the bike so that you had two full revolutions before the next fire then you jumped real hard on the kickstarter but released most pressure towards the end of the kickstarters stroke making sure that you had a bended knee at that point just in case. I still bear the scars. :D
 

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If so I would have to question that because the early 790 cams opened the intake valve at 22*BTDC which would surely be within the range of the spark occuring at cranking speeds and aftermarket cams would probably open the intake even earlier.
I don't think any of the aftermarket cams exceed 19 BTDC. Do you know of any?
 

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aussie the starter unlike a man kicking thebike over does not know where the motor was at when it was turned off.Oh and by the way most motors fire before tdc . You worry about to much stuff that is all ready known about these bikes.If anything these bikes with bigger cams need more advance to start good as with most motors.
 

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Discussion Starter · #11 ·
aussie the starter unlike a man kicking thebike over does not know where the motor was at when it was turned off.
Not sure of the relevance of that statement. :confused:

Oh and by the way most motors fire before tdc .
I've been very well aware of that basic fact since I was about 12 years old and fiddling with my first lawnmower engine. :D

You worry about to much stuff that is all ready known about these bikes.
You confuse worry with an inquisitive mind.
 

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maybe things turn backwards down under lol dont water rotate backwards down there lol
 

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I know nothing about Hardleys, but I thought they were 270 degree engines....wouldn't you be better off comparing them with the 270 degree twins. I seriously doubt if the 360 degree twins ever fire the wasted spark side after the intake valve is opening. I don't know how many degrees these new 360 twins fire BTDC, but my old BSA Hornet fired at 38 degrees BTDC.
 

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I know nothing about Hardleys, but I thought they were 270 degree engines....wouldn't you be better off comparing them with the 270 degree twins. I seriously doubt if the 360 degree twins ever fire the wasted spark side after the intake valve is opening. I don't know how many degrees these new 360 twins fire BTDC, but my old BSA Hornet fired at 38 degrees BTDC.
Harleys are a single crankpin 45deg v-twin, which means they're 360deg-45deg =
315 deg engine.
 

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Discussion Starter · #16 · (Edited)
I seriously doubt if the 360 degree twins ever fire the wasted spark side after the intake valve is opening. I don't know how many degrees these new 360 twins fire BTDC, but my old BSA Hornet fired at 38 degrees BTDC.
Spark advance would be at its minimum during cranking- throttle closed & revs below 1000 so the questions are:-

1. What is the ignition timing exactly during cranking?

2. How many degrees BTDC does the intake valve open (known 22* for early 790 cams)?

3. Does the wasted spark therefore occur when the intake valve is partially open?

4. If so does this increase the tendency for the bike to backfire through the carbs?

I'm considering fitting aftermarket cams to my bike and do not like it at all when my bike backfires through the carbs when cranking because there is a high potential for something breaking under those circumstances so I am interested in knowing the answer to these questions.
 

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If one of these bikes spits through the carbs on start up its not a cam or wasted spark trouble.It is a out of tune or vac leak that causes that.Mine starts better now then it did stock.Most of the troubles people have is due to the cvk carbs .Get some good carbs and them tuned and all that goes away.
 

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Discussion Starter · #18 ·
If one of these bikes spits through the carbs on start up its not a cam or wasted spark trouble.It is a out of tune or vac leak that causes that.Mine starts better now then it did stock.Most of the troubles people have is due to the cvk carbs .Get some good carbs and them tuned and all that goes away.
That's all well and good Mike, I know that your standard answer for just about any problem with these bikes is to fit Mikuni 42's, but you haven't really directly addressed any of the four questions that I posed. :SadSigh:
 

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Aussie,
I have to chime in here because I find your comments to be a bit disrespectful. I spent a lot of time reviewing EFI with you and that thread didn't end with even a thank you and you ended up misinterpreting a lot of what I wrote anyway. Your comments to Mike are disrespectful. Mike is probably the smartest motorhead on this site.
Your questions are a combination of pretty much unknowable to anybody on here but if you had the information, you couldn't make a decision anyway. The reason is you aren't smart enough to assimilate the information and then judge if it will affect the outcome. This is not a slight on you...it applies to everybody here...why you have to build configurations and then determine if it works. What development engineers do by the way and I am a card carrying member. If you want to deduce ignition timing...then contact Pieman. He knows the most about ignition on this site. After you gather all your data however including precise cam spec's you will will not be able to determine if the motor will backfire through the carbs however.
Lastly...and only a suggestion is...try to be a bit more respectful of others on this site as they assist you in your quest for knowledge.
Good Luck,
George
 

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Thank you George. You said just what many of us have been thinking. I don't like seeing people, that don't know what they are talking about, come on here and mislead new people. I might not be right all the time but I try to post what I have learned and what I think is true.
It's easy to believe the wrong info. Sometimes you have to learn the hard way.
You will hear some vendors talk bad about other vendors parts and you will hear people from that vendors camp down grade other vendors because of what their vendors have told them. With out being long winded let me tell you new guys a little something you should know.
I guess y'all see me defend Carlos at TPUSA and let me tell you why, and what kind of guy I know he is.
I used to be in a different vendor camp (there were 2 vendor camps back then) I used to give Carlos a lot of crap about his products based on what I had heard from my camp.
When I built my 989 I couldnt get it to run in the high rpms. I got to thinking my air filters where not good enough. Carlos sent me a pm and told me he had some UNI filters that would cure my problem and sent them to me. It fixed my problems. Then I had trouble with sleeves on my motor. I got angry and called Carlos. I wanted to buy one of his kits. You know what he told me? He said Mike, you will be wasting money if you dont let the other guy get your sleeves right. Wait and see if it gets fixed. How many people selling parts would do that?
That support and guidance told me how great of guy Carlos really is. I decided right there and then I would try his kit. We have had many talks on the phone. Carlos and Matt both have helped me alot.
Carlos when you read this I want to apologize to you for all the crap I gave you in the past about your products. I WAS WRONG. I didn't know what I was talking about because I had bad information to go on. I want you to know I consider you one of my best friends and not just another vendor. My 904 TPUSA motor runs smoother and more quiet than the WIESCO ever did. I want to thank you.

Mike
 
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