southern Posted June 29, 2006 Report Share Posted June 29, 2006 I've been trying to get a 7 series Fantic 50 running properly for a freind but am failing....... The symptoms are: When engine is under high load ie attacking slope, the engine will start to bog down (even with 5st rider!) and eventualy die. Riding around on the flat the bike seems ok, when it is running!! Additionally starting is a problem. Other things noted: Air screw on carb has to be fully wound in to get the bike running ok (indicating air leak???). when reving the engine and shutting the throttle response seems normal (revs drop with no significant run on). If throttle is snapped open the engine seems to struggle and bog down, sometimes dieing and sometimes eventually clearing and picking up ok. There are no obviouse air leaks and no thick smoke. Head and base gasket have been replaced. There is a good spark however it appears more orange/red in coulor rather than blue? I would be realy greatful for any suggestions of things to try and get this bike running properly...... Thanks Quote Link to comment Share on other sites More sharing options...
mcman56 Posted June 29, 2006 Report Share Posted June 29, 2006 I'm not sure what a series 7 Fantic is but I recently had a similar experience with a Fantic 80 Clubman. It says 49.xx cc on the cases so I believe it uses the same bottom end as the 50. The bike started fine but did have an uneven idle. I found that the carb slide was worn and air was getting around the slide at idle. (The slide looked lumpy and the wear was obvious.) The bike also had the high load bog issue. You never really felt it come "on the pipe" but it would really "fall off the pipe" like you describe. (This 80 is a full size bike.) Very rich main jets had some minor benefit but did not resolve the issue. Eventually I looked inside the motor and found the squish clearance (piston to head) to be around 0.080". There was also some significant misalignment of the transfer ports on one side of the case/ cylinder interface (up to 3/16"). To retard the port timing for more low end power, I machined 0.035" off of the bottom of the cylinder and used a slightly thinner base gasket. In addition to retarding the port timing, this bosted the compression and brought the squish clearance closer to where is should be. I then used a base gasket to match the cylinder and case tranfer ports with a die grinder. At the same time, I removed any gross defects/ bumps that I saw in all of the ports. I have never done this type of mod before but the change was amazing. Peak power feels the same but the mid range and bottom end of the power curve have been filled in. The change feels like an improvement of 50% or more at some areas of the power curve. It will no longer "fall off the pipe". The last thing I did was to add a flywheel weight. It was a nice addition and made everything smoother and hard to stall. This was a rewarding project but now my daughter is not interested in riding. Quote Link to comment Share on other sites More sharing options...
southern Posted June 30, 2006 Author Report Share Posted June 30, 2006 Thanks for the reply I will have a play with the carb this weekend and see if I can improve things a little.... Do you think the carb would cause the problem of the engine bogging down under load?? Quote Link to comment Share on other sites More sharing options...
mcman56 Posted June 30, 2006 Report Share Posted June 30, 2006 I think the 80 I have ran that way from the factory. It has no reed valve and was tuned like a road racer. Peak power is pretty impressive for an 80 and may be more than my TLR200. This is the only one I have seen in the US so I can not compare it to another but have seen things written saying it was peaky. Is there another 50 in your area that you could compare it to? I hear that Bill Pye is the Fantic expert in the UK so maybe you should give him a call for some advice. What year is your 50? Going up 10 or so main jet sizes helped a slightly. I spent a lot of time unsucessfully trying to "tune" the problem away before looking inside the motor. I even replaced the needle thinking it may be worn. A plugged exhaust pipe could have a similar effect. (Look at the volume of an MX pipe verses a road race pipe.) I cleaned mine with no affect. I also checked timing and looked for air leaks. I do have a copy of the original owners manual that shows detailed carb settings and other specs. It is from 1993 and covers the 50. I think it shows the peak power of the 50 about 1000 rpm higher than the 80. I could scan and send you a copy if you send me an e-mail address. I put a lot of thought in to the mods I did but it was not a huge amount of work. If there is sufficient piston to cylinder clearance, cutting the bottom of the cylinder is a fairly straightforward task on the lathe. The mod can be reversed by using multiple base gaskets. While researching on the internet, I found that a lot of this type of work is done on scooters like Vespas and they have similar sized engines. See http://www.mbdevelopments.co.uk/lambretta/lt5.html Quote Link to comment Share on other sites More sharing options...
southern Posted August 17, 2006 Author Report Share Posted August 17, 2006 Sorry I have been having computer & email problems so have not been able to get online at home recently but: the latest on the 50 is I may have solved the problem.....not sure until the lad whos bike it is returns from holiday where he is testing it, but it did sound a lot better than before and I am quite hopeful. It would appear that someone had fitted a brass washer under the choke asembly causing the choke to always be open...... Will let you know if this little brass washer was the cause of all my problems when I find out.... Quote Link to comment Share on other sites More sharing options...
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