Modern Vespa

On my restored 2005 GT200 I have a little problem. My problem is I backfire when coasting or downshifting (not that I downshift on an auto). It doesn't seem to interfere with performance, but it might.

It was in an accident, thus the reason for restoration. I assumed the backfire was because I added a bottle of carb-cleaner to the gas before I had tried her out. There was 4000km on the OD, but now after adding another thousand km on the odometer last weekend it should have diluted that stuff out by now.

I'm thinking it might be a vacuum hose on the choke but it looks like an electric choke. I'll have to read up on that. I can't see a vacuum hose that's off or pinched. Might be plugged though.

So I'm guessing it's either the sparkplug or the timing is out. I'll have to find out which sparkplug it takes with the proper spacing, And I'll have to find the specs for timing the engine. I assume the timing is set on the starboard (muffler) side.

Does anyone have any other suggestions of what might be wrong? I think it would be unusual for an electric choke to be failing, but that's the last thing I want to be wrong.

Does the timing go out? Maybe as a result of an impact? Maybe 5000km has worn the sparkplug out? Somehow I doubt it's the sparkplug itself.

Mine does that once in a while when I gun it and back off immediately. It's a little pop, not a really loud backfire, and it started after I changed plugs to an NGK Iridium (CR8EIX).

Hi,

Interesting about the non-oem Plug making the Pop. Aftermarket Spark Plugs are mostly hype and rarely outperform OEM.

A couple of issues here. First, the OEM Plug was designed into the engine. So, it is properly indexed to both the thread count and pitch in the Head Tap. This assures that the electrode is positioned properly (when torqued to spec.) relative to the desired Flame Front Propogation. With an Aftermarket Plug, this is rarely the case, and if so, it's mere coincidence. Without marking the electrode orientation in both plugs and comparing, you cannot be sure you're getting the proper index. It's a relatively minor thing, but max Performance and max Range can be adversely affected, but almost never positively so. Aftermarket Plugs are essentially Universal, but each engine is different which is why the Mfgr. spec's a specific Plug, in the case of the GT 200 this is the CHAMPION RG6YC.

The second issue is the Heat Range of the Plug. There is no universal standard on Heat Ranges. One Manufacturers #7 may not correlate to the #7 from another manufacturer, and there's little published information for you, the consumer, to make any comparisons. The significance of this lies in the Plug's ability to extract Heat from the Combustion Cylinder.

A Spark Plug's secondary (but by no means less important) function is to extract the Heat from the Combustion Chamber and transfer it to the Head where it is then carried away by the Coolant or Airflow to atmosphere and out of the System (ever notice that a Spark Plug looks like a Heat Sink? This is why).

If the proper amount of Heat Transfer does not take place, the results can be Plug Fouling, because too much Heat is transferred, leaving too little to keep the Plug Clean. Or, in the case of not enough Heat being transferred, the excess Heat in the Cylinder can lead to detonation (which is what I suspect you are experiencing). Detonation is VERY BAD! It can lead to premature wear on the Crank Bearing(s), cracked Rods, and/or Pistons and Heads.

Someone is always trying to make a better Spark Plug and tap into the lucrative Aftermarket. They try to convince you, usually with little emprical evidence, only Marketing Hype, that their Plug creates more power, or produces a Hotter Spark (a case where More is not Better and Enough (OEM) is all you need), or that their Plugs last longer.

The last one is interesting in that a Spark Plug is maybe the most taxed piece of equipment in the engine (firing thousands of times a minute, every minute the engine is in operation). Yet, it is one of the cheapest and simplest forms of maintenance one can do to an engine. Super longevity is not really a desired trait, especially in an Engine with only one Cylinder and Plug - it all depends upon that thing doing it's job reliably and consistently.

Exotic Metals such as Platinum, Iridium etc. all promise longevity and lower resistance and sound incredibly sexy - that's the idea! But, minute differences in resistance mean nothing when the Plug is fed 25,000-50,000 Volts from the HT lead. Any resistance is easily overcome with plenty of remaining Spark energy to ignite the Fuel Mixture. And, there have been increasing reports about these exotic metal plugs damaging engines because their metal tips are more brittle and prone to break off and rattle around inside the cylinder. And, this potential rises as the plugs are in use longer and longer.

But, the Aftermarket continues to try and find new ways to enter, or one-up, the very stiff competition. Back in the late '70's, Firestone (a Tire and Rubber Co.) tried to enter this apparently lucrative market by introducing a Polonium (radioactive metal) tipped Spark Plug. They claimed that the radioactivity would Ionize the Air in the Gap and lower resistance, allowing the Spark to jump more easily through the Gap.

This was the same year as the China Syndrome, and people's natural aversion to anything radioactive, as well as the EPAs concern that the Landfills would glow at night from all the used Spark Plugs, pretty much killed any chance Firestone had - it was a dismal failure.

So, stick with OEM Plugs. Without extensive testing on your engine, testing which was already done by the Engine Mfgr., you're just rolling the dice and most likely not improving a thing, just paying more.

Sorry to hi-jack the thread slightly, but back to the original query, I think the listers issue may be a timing one, perhaps even a mechanical timing issue. In a crash, it's possible that a jolt caused the timing to jump. A slight jump would only result in a couple degrees Advance or Retardation. First check for leaks, incl. the Cylinder Head. Then the Fuel System, incl. any Evap equipment, and the SAS Tube. Simply replace the Spark Plug, it's cheap and easy - make sure it's properly gapped. The Change Interval for this Plug s/b every 7500 miles (less if driving conditions are severe).

If all else fails, a Timing Light and Degree Wheel may be necessary to rule out any timing issues. I am certain the answer lies somewhere in these areas. Good Luck, and report back your progress...

Happy Motoring!... Jim

I think backfiring is due to timing. Backfiring in a 2-stroke usually means you're close to blowing a hole in your piston. On 4-strokes you don't have to worry about that, but the cause is usally the same. The CDI may need to be reprogrammed or replaced.

True & true, but...
In the GT200's case, backfiring on decel is usually caused by a misbehaving SAS air valve.

I also have some backfire issues. It happens when the engine is cold and when I give a some throttle and then let it idle to let the engine do the breaking. It may give a couple of puffs and bangs, but after a couple of minutes it's gone. I use the Denso IU24 spark plug and PM-tuning exhaust.

If it only happens with a cold engine, it because the carburetor enricher is in-play. That's "normal".

Thanks! I didn't think it was a big problem, but you reassure me with what you say, and it makes perfectly sense to me.

I'm not familiar with the SAS tube/valve. What am I looking for? Off the carb?

Somehow I think it's the auto choke misbehaving. The tech manual (although I havn't had time to read it closely) seems to suggest there's either an electric or an air one. I seem to think I have an electric one. Just looking at it. But engines are something I know virtually nothing about.

I'm going to change the sparkplug to the Champion one you quoted. What's the gap supposed to be set at?

I also had a backfire on my ET2. It seems to have been solved by a new sparkplug... except when it's raining out. I'm pretty sure what that's all about but I don't want to deal with that until after I've got the GT200 right.


Thanks to everyone who has commented.

Hi,

The Spark Plug Gap s/b 0.028 - 0.031 in. (0.7 - 0.8mm).

The problem most often associated with the SAS (Secondary Air System) is a kinked or disconnected vacuum line which does not allow the Valve to close properly. This can be checked using a MityVac or similar tool. Connect MityVac to the SAS Valve and pump it up to -8.7 to -11.6 PSI. If you cannot achieve, or hold, this vac pressure, the membrane inside the Valve has failed requiring it's replacement. You can also use this tool, or a Vacuum Gauge to check that adequate Vacuum is being produced by the engine. To do this, connect the MityVac or Gauge to the 'T' fitting of the Valve Vacuum Tube. Start engine. Gauge should read vac. press. in the same -8.7 - -11.6 PSI range. If not, check for possible vacuum leaks.

If you shoot me a PM w/ your email addy, I will forward the Workshop Manual procedure to you as a Word Document. Hope this helps...

Happy Motoring!... Jim

since I have my PM, mine backfires all the time. Whenever I go off the throttle, it backfires most of the times. Didn't think too much about it since it sounds really cool and I hear it a lot on motorcycles...

Exactly what mine is doing. Not LOUD backfires but little pops. I know enough to know that's not normal.

It's kinda like livin' with the pain of a bad ankle. Looks really cool to walk with a cane and you see many of the old folks with canes. Better to try to solve it now before you need to replace a knee or a hip, eh?

Dammit I stayed on the computer too long and the shops are now closed. A lot of suggestions it might just be the spark plug. I'll try that first thing after work, then set about the timing. Those seem easiest to resolve.

I'm leaving for Amerivespa in two weeks and want to be able to make it there and back. My Modern Vespas are getting more vintage-like all the time. [/i]

Hey Kory...do you think that maybe in the accident the jet(s) got outta whack and need adjustment? The reason I thought of this is when I had that pollution crap taken out of my BV250, it causes a slight backfiring and rev adjusted the jets as best they could. I think that I would need to change the jet(s) to the ones the Euro BV's use as they don't get that carbon canister crap in their bikes.

Just a thought!

Alt

Hi,

@ Atltaman - FYI, you don't adjust Jets. You Clean 'em or Change 'em, no adjustment is available, maybe you're thinking of the Idle or Mix adjustments?...

Happy Motoring!... Jim

like I said, I only hear the backfiring since I have the PM on it, the backfiring is really loud but so is the sound of the PM. When it's cold and I'm off the throttle I get about 5 really loud ones, when it's warmer I usually get about one when I'm off the throttle after I had the throttle open for a while. I'll have greasy look at it, but honestly I like the sound, of course if it damages something I'd rather have it fixed...

Ditto on both counts - the behaviour since fitting the PM and I think it sounds cool, too.

The ignition timing is fixed. There's no adjustment. On a stock GT200, check the SAS for proper operation. The spark plug is a dead-end lead. On a GT200 with an aftermarket exhaust, the popping is due to air leaks between the header and can.

I finally got around to removing the SAS thing. All looks ok. The secondary filter is clean as Disney. The first filter had nominal amounts of dirt so I've washed it. No cracks or wearing anywhere. I very much doubt the SAS has anything to do with my backfiring.

4.6.1 of my Owner's manual states, "The system is provided with a control valve that excludes the function in deceleration, in order to prevent abnormal noise." I assume that's the valve I removed in order to remove the secondary air filter. I don't have access to a mityvac so I've given a rather amateur test of it with my mouth and here's what I found out..

It's a one way valve. The SAS can blow extra air into the catalytic, but not suck it back. But I can blow backwards through it ever so slightly.

I looked closely at the gasket that joins the exhaust to the extension. It looks like it's a lead fiber thing. It looks pretty sick so I'm wondering if that's where my 'pop ' is coming from.

I'm still looking for that champion sparkplug. Seems it's not listed in the 2006 NGK book at AutoValue. Interesting side note; the only Vespas listed are old ones. Not even any ET's.

I use NGK CR8EIX and NGK POWER CABLE to feel than original saving the fuel even more!

bump

I also have the same issue, with only 1k miles on my GT... I just assumed this was the norm.

I've disconnected my EVAP tube awhile back, and the machine generally runs smoother with no startup issues, but still the slight sputter/backfire when slow coasting on deceleration.

Any further progress with yours no cattle?

Hi. This is my first reply though I've been lurking for a while soaking up as much info as I can and this has been a great resource. I've had my LXV-150 for about a week now and 90 miles on it. (first scooter ever and absolutely love it!)

I've have the same issues described on this discussion since day 1. popping sounds on decel. Been hoping it was either normal since it doesn't bother me all that much, or due to a bad tank of gas from dealer or something that would clear up after break-in period. I removed the EVAP for a ride and it didn't seem to help.

I had planned to just leave it alone until my 600 mile checkup and ask mechanic about it.

Mine does it on deaccelaration from a cold engine, only once or twice - and I happen to think it's cool too!

Geeze, this was originally posted over a year ago. I've got the pop-pop on deceleration down to somewhere between rare and occasional. I never did pin it down to a specific cause though. Things that helped was I clean my carb semi-regularly, removed Evap, changed spark plug brand from NGK to Champion, etc. Nothing much beyond routine maintenance.