Ossa Tr77 Carb

[tomtom2000]

Hi,

 

Hope someone can help? Ive just purchased a TR77 250 which was sold as a poor runner with the previous owner thinking it needs new points or main seals which were both supplied with the bike.

 

On getting the bike home it starts up very easy but struggles to rev up at all. The bike is fitted with a Dellorto carb which I presume is wrong so maybe causing bike to run bad.

 

Can someone confirm what model of carb this bike needs if current one is incorrect and also where I might be able to get the correct one from?

 

Many thanks,

Tom.

[woody]

The TR77 had a MK2 Amal as standard which you can still buy new from Amal Carburetttors.

 

But before you do that - Is this the bike from the classified ads on this site which the previous owner was asking for help with poor running - see previous topic in the Ossa forum

[bondy]

hi sorry to butt in Woody mine is the TR77 350 and has a bing carb on it seems to run very well was it just the 250s that had the Amal?

Edited September 11, 2016 by bondy

[tomtom2000]

Hi Woody,

 

Yes it looks like its that bike. Ive had it running and it sounds sweet with no rumbles engine wise but as stated it won't rev out cleanly. I could start changing everything to sort but feel it needs the correct carb as a base first.

 

Tom.

[woody]

The symptoms of the poor running bescribed by the previous owner sounded more like electrical problems than carb  -  it's on the other topic which is just titled 'Ossa TR77'

 

http://www.trialscentral.com/forums/topic/61308-ossa-tr77/

 

I suggested some things to check on there which are common issues on the wiring harness of Ossa if they still follow the original routing

 

If the dellorto has been taken of a GasGas or suchlike, it should be pretty close to Ossa jetting and at least run without the symptoms described. It might need fine tuning to get it top run perfectly but it should still rev up.

 

 I've just had a similar problem on my Bultaco when it suddenly began missing, backfiring, spitting back and wouldn't rev up. It was the spade terminal onto the coil which had corroded and was hanging by a thread. It was enough to give a spark when checking but under compression the spark wasn't strong enough and the bike wouldn't run properly. New spade connector and it ran perfectly again. Personally I'd check the electrics first before spending about £150 on a carb you may not need.

 

Also, from the previous owner's description I didn't realise it was a dellorto fitted. If it is the conventional type, PHBH or PHBL, they don't have an airscrew, it's a fuel screw if it is on the inlet side of the carb in relation to the idle screw and they work the opposite way around to an airscrew - In to weaken the mixture and out to richen, so it's possible that is out of adjustment too

[woody]

hi sorry to butt in Woody mine is the TR77 350 and has a bing carb on it seems to run very well was it just the 250s that had the Amal?

 

Yes 350 Bing, 250 Amal

[woody]

Just seen the pictures of the bike on the classified ad.

 

If you don't find any electrical issues, the silencer is not standard so I'd also check that as well. There is no reason another silencer won't work but that is from some other bike so you need to be sure it isn't restricting the engine or blocked. It looks as though it is the type that has a perforated tube running through it with packing, if so the tube needs to be roughly 25mm diameter. As it looks a repackable type it may be worth pulling it apart and repacking. Try running the bike with it removed first.

 

The standard silencer is welded onto the middle box on the green bikes, they are one unit, so someone has cut the tailpipe off and replaced it with that one. Not a problem if it is a suitable type. The silencer on the green bikes is considered by some to be restrictive by some, I've never had one so can't comment but I've known them to be modified to be less restrictive

 

Also, it's been fitted with a MK2 swingarm which is the same length but the shock mounts are further forward. On the TR77 they are over the rear spindle. If the shocks are the standard length, standard is about 13.5", this would have the effect of raising the back of the bike. However, the shocks look shorter than standard but the back still looks high - it's possible the the rear mudguard loop has been bent upwards a little to give more clearance. The green models had longer forks than the MAR by about 2" and generally, they sat a bit high at the front, so usually the forks were raised through the yokes with about 1.5 to 2" protruding from the top yoke to drop the front and level the bike up, as well as reducing the chopper effect. Your forks are flush so it's going to feel like a tall bike with that set up

[tomtom2000]

Hi Woody,

 

Thanks for your help. the rear shocks are 13.5 inches in length. I had realised looking at other photos that the rear swing arm was different and was unsure whether length would be different. If not just a case of removing shock mounts and replacing over spindle. also with what you say about the front forks explains why side stand is 4 inch too short.

 

I will do as you say first and go through the electrical side of things first. The stator wires have been modded so they come out the top of the cases then run under the cylinder.

 

Cheers for now.

 

Tom.

[tomtom2000]

Just got round to finding the time to sort this poor running.

 

Turned out it had snapped the woodruff key so ignition was all out of sync. Runs like a dream now.

 

Tom