Holed piston!!

[Adam_Winstone]

I'm not sure that there is a debate to be had here? What we seem to be saying, perhaps even agreeing on , is that dynos serve as a very useful tool for setting up motors but that they are only as good as their operator. Unless a motor is being built for the track, there should always be a safety margin designed in to the final setup, otherwise an air leak, exhaust coming loose or change of fuel will cause a motor to have problems. Setting a motor to be 'on the button' leaves no safety margin and puts the motor and rider at risk.

However, this thread was started to discuss poor fuel issues and, as someone who has suffered from this, do think that the problem with fuel needs to be acknowledged. My experience with poor fuel, which some of you may have seen me post before, was coming back from Euro Netherlands. I'd ridden there with the Cambridge Lambretta boys and my bike had been without issue, was set very safe and was a pretty much standard GP200 motor. However, we al filled up just before getting on the ferry for the return voyage, with me filling up at the pump available, which I think happened to be super unleaded. I was the only one of our travelling group to use this pump. Onto the ferry at walking pace, then off the other side, with the rest of my group heading north and me heading west to get onto the M25. However, when getting onto the first fast road from the port, I realised that the bike was not running as usual and that it gave all the signs of running very hot. I pulled over immediately and changed my plug grade and went bigger on my pilot and MUCH bigger on my main jet. The bike was still not right but it felt like I might be able to nurse it further. Not long after this I pulled into a layby that had a couple of Lambrettas pulled up so that I could see if I could help, only to hear that they had filled up at the same station and were having problems with bikes that had been running fine until then. What really compounded that the fuel was the cause of the problems was that a bloke walked over to ask us what was wrong as he was waiting to have his new family/works car recovered as he had filled up at the same station and suddenly had all of his engine management warning lights come on and instruct him to stop using the car and phone for recovery! Sadly, at least one of the 2 Lambies was beyond further use, probably with a holed piston, and was going to need recovery too. With nothing that I could do to help, I limped on until the next fuel stop and quietly dumped my fuel (safe in the knowledge that petrol stations have fuel/oil interceptors to cope) and then refilled with new fuel. 50 yards after pulling out of the petrol station I had to pull over again as the bike was un-ride able, with it being so rich on the revised set-up so I put back in the original jetting and the bike was immediately back to its usual self, which I held flat out (standard 200 motor) for the remaining trip around the M25 and up the M40, with the motor not missing a beat.

Without a shadow of a doubt, these issues were 100% related to fuel.

Another pal phoned me for help with a very reliable bike that would suddenly would not start or run. First question was, “What’s changed? Fuel?”, to which he said not. He then went to see a dealer, who sold him a new crank, bearings, seals, mag housing and gaskets, only for him to rebuild it all and find that it was exactly the same! He badgered me into saying that I’d look at it so he paid to hire a van and bring the bike to mine. Before lifting a spanner I asked him to talk me through it all again, however, the story changed this time to say that he had indeed just put fresh fuel in from his local (big brand) petrol station. As such, he knew that the fuel was fresh so had discounted this as a possible problem. As the bike had a spark and the plug was wet, it should have been at least trying to fire. I drained the tank and emptied the bowl, replacing it with fuel drained from one of my bikes that had not moved for quite some time. Immediately the bike fired and after smoking like mad for a minute, whilst burning off the crap that had made its way into the exhaust, the bike ran perfectly and all was resolved, other than the fact that the owner then decided to strip the motor back down to take out the un-shimmed crank that the dealer had sold him and to put back in the original shimmed race crank. This folly cost the owner hundreds of pounds, many hours of travel and working on the bike, and the use of the bike for between 2 weeks and a month, all because of sh!t fuel from a big named outlet!

Bad fuel is out there and will wreck engines, however, it often gets blamed as an easy get out for people looking for a factor to blame. As such, it is probably a factor in only a few of the cases for which it is blamed. However, in this case I think that it is a very likely cause as it does seem unlikely that 2 bikes would suffer the same fate so soon after filling up.

Regarding my exhaust width comments, yes, the kit that has increased its exhaust width has also had to revise its piston rings from 1.5mm to 1.0mm as the rings couldn’t cope with the wider exhaust… indeed, I know of one of the early wide exhaust versions being sent out with a 1.5mm ring piston, which lasted no time before dropping a ring. Luckily for the owner, the bore plating was not damaged and the port edges just needed cleaning up a little. This sort of failure does not make me think that the porting is right for road use, just that the designer is relying on ring quality to try to cope with what I see as a design flaw! If the motor can’t cope with the quality of rings, at 1.5mm, that have been standard in high performance Jap motors for years then I think they’ve taken it too far. Also, the squaring off wear damage that shows on over-wide exhaust ports suggests that a well-shaped (but too wide) exhaust port will eventually suffer wear and ring breakage, regardless of shape and ring quality allowing all to be well to begin with. IMO motors like this are best left on the dyno or track but are not well suited to long-term road use.

Adam

[Rich_T]

Hi Darrell,

I wrote a detailed reply then it didn't post and got lost...darn. Anyway the much shorter reply would be arriving at the PHBL25 settings by lots of testing on several engines about 12 months worth. I measured loads of successful and reliable Lambretta barrels to arrive at a trapped compression ratio (between 5.1-5.33 : 1). Dynos were used to confirm AFR and BHP but final numbers were based of riding experience and quite a bit richer than dyno settings.

I'm gathering data on other carbs before seriosuly testing again but this is a long way off and it takes time. What ever gets published will always be on the safe side to compensate for variables that are not controllable such as weather and fuel.

The GT kits and manual are produced to cover the majority of applications, 2 recommended choices of crank, 2-3 recommended choices of carb and 2 recommended choices of exhaust, this is about 8 combinations. All the information in the manual is based around safe settings allowing for variables in fuel and weather. The manual information is for the majority of users to cover the majority of installations with good advice and safe settings.

Can the GT kits do more, definately, you proved that yourself. Would I want to promote that side of the kit potential, probably not as at some point it will detract from the reliability. Blueprinting and optimising performance is your arena, mine is to deliver reliability (inside the design criteria) first off and secondly offer potential for improvement.

Gotta go.

[Darrell Taylor]

thats right Adam no debate at all just good discussions of various points of view which if good conclusions can be drawn from them can be of great benefit to all
whats interesting about your bad fuel scenario is that when upjetted for it and running well then returning to good fuel it ran sick,this does show that even jetting on the safest of sides up to the point of 4 stroking it still wont protect against the possible bad fuel that may be dropped in your tank on that rare occasion.id suggest anyone who suspects bad fuel as a quick and temporary safety measure could try fully richening the mixture screw/run with the choke on/carry then fit a short reach plug to drop the compression ratio

regarding ex ports yamahas rd500 4 cylinder production bike has 70% widths as standard and quite a square shape to the top edge but has a very good chamfer on it to create a large top radius rings are 1.5mm and service limits are as per a std 2 stroke so does show quite well that they can be applied when done correctly,but your exactly right about long term road use,although the type of customer who looks for a high output tune isnt expecting long term high mileage service from it and id be the first to let them know,i recently tuned a scoot for andy (drew2) and wrote 2 paragraphs to him telling him that its moving into the realms and power output of a full race motor and the maintenance regime to go with it of which he accepted and decided to proceed,with any performance vehicle service periods are reduced considerably,not to say that a tuned set up with reliability in mind cant be as reliable as a std kit should that be the customers brief,its all about delivering whats asked for and highlighting the expected service life ,i heard the other day af are quoting 2000 miles on the rb kit?

[Darrell Taylor]

Hi Darrell,

I wrote a detailed reply then it didn't post and got lost...darn. Anyway the much shorter reply would be arriving at the PHBL25 settings by lots of testing on several engines about 12 months worth. I measured loads of successful and reliable Lambretta barrels to arrive at a trapped compression ratio (between 5.1-5.33 : 1). Dynos were used to confirm AFR and BHP but final numbers were based of riding experience and quite a bit richer than dyno settings.

I'm gathering data on other carbs before seriosuly testing again but this is a long way off and it takes time. What ever gets published will always be on the safe side to compensate for variables that are not controllable such as weather and fuel.

The GT kits and manual are produced to cover the majority of applications, 2 recommended choices of crank, 2-3 recommended choices of carb and 2 recommended choices of exhaust, this is about 8 combinations. All the information in the manual is based around safe settings allowing for variables in fuel and weather. The manual information is for the majority of users to cover the majority of installations with good advice and safe settings.

Can the GT kits do more, definately, you proved that yourself. Would I want to promote that side of the kit potential, probably not as at some point it will detract from the reliability. Blueprinting and optimising performance is your arena, mine is to deliver reliability (inside the design criteria) first off and secondly offer potential for improvement.

Gotta go.

thats around 11 to 11.5 cr the very upper end of what id use on a for race air cooled motor so im quite surprised at that ,although its nice to see the baselines were found from dyno testing
i had a really interesting mugello last week that was quite alarming to think it could have been jetted on the road as a 126 main jet produced 11-1 and a few top end splutters a 124 gave 12-1 and a 122 gave 13-1 so a rider on the road dropping from a too rich 126 to a 122 could go out and seize it ,this scenario was produced from a lean large tipped needle being in use

[Adam_Winstone]

^... yep, all good. Indeed, I have a road bike with exhaust width at 70% bore with good shape and have not had issue, however, I'd not go quite so far again for general use.

"id suggest anyone who suspects bad fuel as a quick and temporary safety measure could try fully richening the mixture screw/run with the choke on", yep good advice here too. Frustratingly, some people just don't get it and a guy local to me did listen when I said about testing his motor by flicking on the choke at marked throttle positions, however, he has since said that he's glad that he took note as now, whenever he's on the motorway he runs with the choke on and "it runs really well and gives a much better plug colour"!!!! I've tried to talk him through it and have suggested that he take it to be dyno'ed however, he has said that he'll not bother, rather, he'll ride around town with the choke off and then turn it on at higher speeds + higher throttle positions.... meaning that he's using it as a manual powerjet.

Don't you just love it!

[Darrell Taylor]

i have a 15 min pet talk with all my dyno customers about general 2 stroke riding techniques and the choke trick is part of it for flat out riding ,a lot use there heel to turn it on but a few have adapted handlebar chokes like ive used on my race bikes for the last 12 years
the big issue i find is lots of riders shut off on the overrun then downshift/engine brake all on a closed throttle generally followed by a nip up i insist on clutching to remove load from motor and a few blips of throttle at this most vulnerable time.

[Rich_T]

thats around 11 to 11.5 cr the very upper end of what id use on a for race air cooled motor so im quite surprised at that ,although its nice to see the baselines were found from dyno testing
i had a really interesting mugello last week that was quite alarming to think it could have been jetted on the road as a 126 main jet produced 11-1 and a few top end splutters a 124 gave 12-1 and a 122 gave 13-1 so a rider on the road dropping from a too rich 126 to a 122 could go out and seize it ,this scenario was produced from a lean large tipped needle being in use

I didn't get it that high. The GT186 heads have a volume of 24.92cc +/- a tad depending on the deck height tol. On a GT kit this gives a trapped volume of 5.27:1 on an un-modified barrel or 8.5:1 . I think thats pretty safe for road use on a Lambretta.

[Adam_Winstone]

^... yes, it works and is a good 'get out of jail free' option but it is no substitute for getting your jetting right in the first place. Sure, there will always be a time when this works to your benefit, I've done it at times and even recommended it to Train Driver when he ran into issues on the way back from Spain (riding all the way home with the choke on and no further problems... or so I seem to remember him saying). Indeed, if you remember, we both commented on how our respective motors were running weak/hot at certain rev ranges on the Spanish fuel and there were times that I ran with the choke on to get over the problem of running at those throttle positions with other riders. Indeed, had we not taken note of how our motors were responding to the fuel available we may have been holing pistons, especially if we took the attitude of 'Our bikes have been dyno'ed so I'll ride care-free (blind faith) and not listen to what the motor is telling me'.

Choke on is an option that is always worth considering but the local guys bike is weak at higher throttle positions and could do with getting it resolved, not just masking the issue.

Adam