| Author | Message | ||
     General_ulysses |
I'm in the process of troubleshooting my Mikuni HSR42 carb on my '97 S1 (all stock other than the carb). I've got a problem with it backfiring through the carb, but I need to understand the build of the stock Buell motor first. My understanding is that Buell back in '97 simply took a mostly stock Harley Sportster motor and plugged it into the S1 frame. But I thought I remember reading that they did swap out cams for something a little more aggressive...and maybe did something with the ignition as well? I know the '98 S1W got hopped up heads, pistons, exhaust, carb jetting, cams and ignition...but I'm not sure what they did with the earlier 96-97 models (other than they were a bit tamer with about 10hp less). Can anyone clear that up for me so I can have a better idea how to tune the carb properly? Thanks in advance for any help. PS>> If they did put a more aggressive cam in it, can you explain how it differs from the stock Sportster cams and maybe include specs etc? | ||
| Phelan |
'96-up S1s had a lighter crank and Lightning heads, which are essentially standard 1200 head with extra material added to net a 10* angle cast squish band and 10:1 compression. Otherwise they are identical to 1200 motors. | ||
| 1313 |
Lightning cams also... Higher volume oil pump didn't come until '98, right? 1313 | ||
| Tombo |
The 96 S1 and up cams are more aggressive than the Sportster, and they are the same as the S1W. Heads, as mentioned are high compression and the stock exhaust and air cleaner are less restrictive than the Sportster. Lighter flywheels, as mentioned, and one other thing not previously mentioned is the ignition. The S1 has a different curve that does not shut down as early. 1313 is correct, oil pump change came in 98. Larger exhaust and T-Storm heads came in 98 for the S1W and S3, but not the standard S1. By the way, if I recall correctly, I am running 185 and 42 jets in my HSR on a 96 S1 and no issues. | ||
| Cyclonecharlie |
Those would be good numbers for the CV, but way off for the HSR 42 on a stock motor. More like 165 to170 and 17.5 to 20 on the pilot.(at sea level)  | ||
| Cyclonecharlie |
Those would be good numbers for the CV, but way off for the HSR 42 on a stock motor. More like 165 to170 and 17.5 to 20 on the pilot.(at sea level) | ||
| General_ulysses |
Thanks for the responses. So Lighting heads (higher compression), Lightning cams (with unknown profile) and tweaked ignition advance curve. The reason I'm trying to figure out the cam timing is that I want to make sure the intake valve isn't open while the exhaust valve is also open. That might allow an explosion in the exhaust system due to rich, unburned exhaust suddenly igniting when air becomes available from the intake valve opening. Also, these bikes apparently have a dual fire ignition system. This means it fires both plugs simultaneously every time, instead of one at a time only when they are needed in each respective cylinder. That dual fire system may also be contributing or outright causing the carb backfiring by triggering an explosion in one cylinder with rich exhaust gases left over and/or with some intake charge present. One way or another, a fuel/air charge of some type is managing to be ignited when the intake valve is open. Another thing I was wondering: do the valves need to be adjusted on these engines? I thought I remember reading they have hydraulic lifters/tappets and they don't need it. Is that right? I could see if they fell out of adjustment and the intake valve wasn't fully seating or was otherwise remaining open when the plug fires it could cause this problem too. As far as jetting for the HSR42, it comes stock set up for Harley 1200s with a 160 main jet and a 20 idle jet. There's also a jet for the accelerator pump, I think that was a 60, but not sure offhand. I swapped out my 20 idle jet with a 25 hoping to get rid of the backfiring. Didn't help and seemed to make it a little worse even. My experiments with adjusting the idle air screw (as per Mikuni tuning manual) indicate the 25 idle jet is too rich. The bike seemed to run rich even with the 20, with fairly sooty plugs. I reinstalled the 20 I have, but may very well get something closer to the 17.5 Cyclonecharlie recommends. I'm running a stock exhaust and ride pretty close to sea level most of the time. For now I set the needle screw to the lowest position, which will help delay the onset of the opening of the main jet. Hopefully this will help, otherwise I have to get the smaller idle jet and try that. | ||
| Buelliedan |
97 S1s and 98 S1Ws did not come with the same cams. | ||
| 1313 |
97 S1s and 98 S1Ws did not come with the same cams. News to me... 1996 S1 started the Lightning cams, which were carried through until the High Contact Ratio cams were introduced (which should have the same cam specs) in 2001. Pray tell, what was different between 1998 S1W and 1997 S1 (or 1996 S1, for that matter) cams? 1313 | ||
| Screamer |
S1, S1W, S3, X1 cams - all are essentially the same. (If I remember correctly) there was a tweak to the lobe profile when cam production went from an outside vendor to in-house production. The changes were subtle but I don't think lift or duration were affected (can't remember for sure). I may still have notes somewhere. | ||
| Sportyeric |
The specs for the S1 cams are more or less the same as Andrews N4, so those specs could be obtained from their website. And, no, the valves don't ever need to be adjusted, which is one of the beauties of these bike (from a Ducati owner who would know.) However, neither the cams nor the design of the ignition system are likely culprits to your problem (unless you have removed and re-installed the cams), as most bikes have those things without having your problem. I think you should be asking the collective intellect here, "What could be causing my engine to backfire?" Sorry, I can't help you there. | ||
| General_ulysses |
Sportyeric - I did ask specifically what might cause carb backfiring in a post shortly below this one (i.e. very recently). I've addressed almost every single issue systematically, including: Intake air leaks - All new seals and gaskets replaced less than a year ago. Tested all intake junctures using propane emitted from a small hose - no change in idle whatsoever. If that same propane is admitted directly into the air filter, the engine stalls immediately. Intake system is airtight and in good working order. Exhaust seals - Exhaust seals replaced with brand new ones. Exhaust system in excellent condition (all stock, including muffler) with no cracks or leaks. No change in carb backfiring. Ignition System - Replaced old plugs with new Autolites. Old plugs appeared original from new (8300 miles) and indicated the bike was running rich (black and sooty)). Replaced old plug wires with new. Most descriptions of what causes carb backfiring revolve around running too lean. So I increased the stock 20 idle jet (that came with the HSR42 setup for a Harley 1200) to a 25. This resulted in no improvement whatsoever, and it even seemed to get worse. Watching the carb function (with the air filter off) as it idles and then lightly blipping the throttle (i.e. what causes the backfiring most), you can see the effects of reversion. That is, you can see the intake charge actually momentarily coming out of the carb instead of going in (it pulsates in and out with the rhythm of the motor). It looks and smells rich, you can even see the atomized fuel in the charge going in and out of the carb throat. You can see the accelerator pump squirting raw fuel directly into the carb throat as you blip the throttle. Hard for me to believe the bike is running lean at all with all the gas I see and smell going into the carb. Especially considering with the smaller size 20 idle jet it had black plugs and relatively poor gas mileage. As to other people who have similar S1's that don't have this problem? I dunno, maybe so. The only other one I personally know of is my friend's 1996 S1, which is stock other than a V&H pipe and an HSR42 like mine. His does the same thing mine does. I doubt this problem is that rare with stock tune S1s, at least with Mikuni HSR42s. So that's why I'm asking about the cams on these bikes. If there is overlap on the intake and exhaust valves (which there probably is for these cams), that would mean the intake valve is open during some part of the exhaust purge. That would make it possible for the residual intake charge to ignite after contact with the exhaust and then blow up through the carb throat. Or, in a dual fire system like these bikes have, the spark plug fires on one cylinder (the one with a "wasted" spark) just 10 deg in on the intake cycle. If residual rich exhaust was in there mixed with a little air, it would explode and again be blown out of the carb inlet. Here's a quote from the Mikuni site: Ignition: Harley ignition systems have been dual fire for decades. Virtually all stock Evolution engines, Big Twin & Sportster, have dual fire ignitions. The exceptions are the EFI touring bikes and the 98 & later Sportster Sport models. All Twin Cam engines are fitted with single fire ignitions. Under normal conditions dual fire ignitions present no problems. However, when combined with high performance long duration cams the stock ignition can cause premature ignition of an air/fuel mixture entering the rear cylinder. This, in turn, results in backfiring through the open intake valve into the intake system. Dual fire ignitions fire front and rear cylinder spark plugs together. One of the sparks starts combustion while the other is wasted in other cylinder which is not on its firing stroke. When the rear cylinder is getting a useful spark, the front cylinder's spark is occurring near the middle of its exhaust stroke. There is nothing to burn in the front cylinder at this time. However, when the front cylinder is getting its useful spark, the rear cylinder is on its intake stroke and a combustible mixture may be present. If that mixture is ignited by the wasted spark, then a backfire occurs as the burning mixture forces its way past the intake valve and out through the intake manifold and carburetor. Single fire ignitions can often eliminate carburetor backfiring since they do not produce a wasted spark in the rear cylinder. In fact, single fire ignitions can generally eliminate backfiring in any Harley. For instance, EFI and Twin Cam engines very seldom backfire through their intakes; both have single fire ignition systems. Cam design: The earlier the intake valve opens the more likely the dual fire ignition will ignite air/fuel mixture in the rear cylinder. High performance long duration cams open the intake valves earlier than the stock one. This is the main reason why modified Harley engines tend to backfire through the carburetor more frequently than stock engines. Also, if you look on Buellidan's fantastic website, he has excellent write-ups about cam design and dual fire ignitions as specifically pertaining to the peculiar Harley 45 deg, single crank-pin V-twin setup (by peculiar, I mean a very old design not like modern, smoother running Japanese V-twins). Here's an excerpt: To understand the effect of the wasted spark, the first thing you need to understand is the HD engine configuration. The two connecting rods are attached to a common crankshaft pin, and the cylinders are spaced 45 crankshaft degrees apart, which means simply that the front piston will reach top dead center (TDC) just 45 degrees after the rear piston does. Now the engineers at HD could have set up the engine to fire those two cylinders 45 degrees apart, and then sent the crank around almost two full revolutions and repeated the process. Dual fire wouldn’t work with the scheduled firings so close together, and this wouldn’t be a good design from a power or reliability point of view. So they placed the sparks physically as far apart as they could within the constraints of the design. From the time the rear cylinder fires, the engine will rotate 405 degrees (one full revolution plus 45 degrees) and then fire the front cylinder. From that point, the engine will rotate 315 degrees (one full revolution less 45 degrees) and fire the rear cylinder again, thus completing it’s 720 degree four stroke cycle. This uneven arrangement is what generates the trademark (well, they tried) potato-potato sound and characteristic vibration. The common crankpin design also lends itself to a “knife and fork” style connecting rod arrangement, which enables a “uniplanar” chassis, but I digress. Okay, so what does all that mean for the wasted spark? Well, let’s assume 35 degrees of spark advance and look at where the opposite piston is positioned when the fire is lit. If we hit the rear cylinder 35 degrees before TDC, the front cylinder is 80 degrees before TDC on his exhaust stroke (35 plus 45), or just past the mid point. Is there anything in the front cylinder to burn? And if there’s anything to burn, will it generate any pressure on the piston, pushing it the wrong way? The answer to these questions is “it depends”. First off, how complete was the combustion that occurred 315 degrees ago? Spark intensity, air/fuel ratio, compression, turbulence, and combustion chamber flame propagation will all play a role. The better that burn was, the less potential exists for the wasted spark to do anything. Second, assuming there is something left to burn, how much downward pressure can you generate with your exhaust valve hanging wide open and exhaust rushing out, which is exactly what’s happening at that point in the cycle? Assuming your exhaust system works, of course. Now let’s look at the other wasted spark. Again assuming 35 degrees of advance, the rear piston will be positioned at 10 degrees after TDC on it’s intake stroke (45 minus 35), or in the very early stages. Is there anything in the rear cylinder to burn? And if it burns, does it cause a problem? Once again, the answer is “it depends”. The first 10 degrees after TDC of crankshaft rotation hardly moves the piston at all, so the piston hasn’t really had time to start yanking on the intake charge. A charge that, by the way, got yanked in the opposite direction very recently by the front cylinder and isn’t necessarily anxious to get yanked back this way again. But don’t forget the effect of overlap! Even though the piston hasn’t significantly yanked on the intake charge, the exhaust system might have done it. So, thinking about it logically, it would seem that an effective overlap event, which is driven by cam timing, the exhaust system, and the independence of the carbs, will increase the chances that there’s fuel available to burn when the wasted spark happens in the rear cylinder. Now, if there is fuel available to burn, and the spark plug manages to light it, it could certainly be disruptive to the incoming charge, resulting in less cylinder fill. It won’t push the piston in the wrong direction, however, as the piston is on it’s way down at this point anyway. | ||
| Buelliedan |
To my knowledge the 1998 S1W and 98 S3 were the first to have the thunderstorm heads and the N4 type cams. The standard S1s came with lightning heads and D cams. Someone please correct me if I am wrong. | ||
| Screamer |
You're right on the introduction on the Thunderstorm components, but the '96 S1 and the '97 S1/S3 used the SE "Bolt-In" (N4) cams. | ||
| Buelliedan |
You are correct. I should have done better research before I trusted my memory. I don't know why I was convinced that they had D cams in them. Doh!!! Sorry Brankin!! (Message edited by buelliedan on April 07, 2015) | ||
| Ralph |
Dan, you were probably thinking of the M2. They had the lower lift cams. | ||
| Buelliedan |
Yes I was thinking the older non WL S1s and M2s all had the same cams. Just a stupid assumption I made. |
