GT250A engine racing..

[daxman]

Hi all

Had another go at setting the timing last night on the 250A (just by using the alternator markings for L and R.) and it starts ok but only on choke but races straightaway. I can 'feather' the revs a bit on the choke but I get no real response from the throttle. The carbs are currently open to air and I have cleaned them thoroughly (thought I had found a problem with blocked pilots but after cleaning still poor).
I'm running from aux plastic fuel tank so contamination should be negligible - although the fuel pipe itself could be cruddy (hmm) - idle screws are backed right out for the moment and there's slack in the cable. I can see the slides look ok.

Any other thoughts? would the open carbs make s big difference (I'm thinking not so much.)

[Craig380]

Have you blocked the vacuum fuel tap take-off on the carbs? That sucks a surprising amount of air.

Also, don't run the engine under any load on the road with the timing set according to the marks on the generator - the marks never changed through all the GT models but the A/B/C models had a LOT less advance than the earlier K/L models, it really needs to be set with a dial gauge.

[daxman]

Thanks - good tips.. I did block off the vacuum tube but maybe not well enough (just an m6 bolt so I'll wrap some PTFE too). Cheers James

[jabcb]

The timing marks for the GT250A are explained in Service Bulletin GT-30. http://www.ozebook.com/compendium/techb ... 027-33.pdf
As Graig380 said, a dial indicator will help you sort this out.

[dorT500]

................maybe not well enough (just an m6 bolt so I'll wrap some PTFE too)......

Yep, smells like that might be the leak...a 'spiraling' one at that. Should be able to get some rubber vacuum blank off caps at any auto supply house.

[dollydog]

agree with the air leak will make it race, but so will lack of 2 stroke oil if you can't get hold of a blanking cap, a temp fix would be to get some good fitting rubber tube, push it on the vacuum pipe, then bend it double and hold it shut with a little jubilee type clip. bit heath robinson but it works.
another thing, can you clarify, you say you've got the idle screws backed right off. do you mean the tickover screws or the pilot air screws?
easy way to test for leaks is to spray some carb cleaner around the rubber manifolds when it's running and see if the revs get higher. those rubber manifolds are notorious for splitting, especially after 40 odd years. also the flats on them get bowed. what i'd do is run it for a few seconds, then whip one or both carbs off as fast as you can. look for petrol on the flanges. if they are wet, there's at least one leak. presume you've got a gasket on there with a smear of grease either side? and don't use vesra gaskets, blotting paper is better than those
cheers, dd.

[daxman]

Thanks guys - all very helpful.

I'm going to zone in on the air leak idea. Engine number begins 9--- and I haven't noticed any red timing marks so hopefully teh key-way is angled correctly and the marks I have are good.
BTW running on a bit of pre-mix ATM until I have it idling and I can check for flow from the pump.

Tickover screws are backed out - thems that adjust the slide height..

[dollydog]

don't forget though - premix plus working oil pump equals lean mixture more oil, less fuel, weak mixture. don't run it too long
cheers, dd.

[daxman]

don't forget though - premix plus working oil pump equals lean mixture more oil, less fuel, weak mixture. don't run it too long
cheers, dd.

Understood..

[Indy650]

Actually the change in fuel-oil ratio has a very small effect on your fuel/air ratio. Its there but its negligible.

[Vintageman]

Actually the change in fuel-oil ratio has a very small effect on your fuel/air ratio. Its there but its negligible.

+1 That has been my experience too. If you are jetted well centered you can put up with little variation. I mix oil in my tank always (80-100:1). If I feel friskey I will go more like 64:1 (I am never that exact when measuring to be honest). This has saved my arse many times for various reasons.

I have also read when you substitute oil for fuel you must subtract the amount from the fuel side and recalc Fuel to Air which would be a little leaner (assume normal amount of oil added). But simply ignoring the fact oil is not present seems wrong to me. It burns so I think some percentage should be counted with fuel side? I am not a chemist, but did sleep in a Holliday a few times.

If I am wrong I must be jetted well centered or rich (yes a little on main to keep things cool) for as said when I ride harder I add more oil premix and haven't had a problem ever. Dirty carbs, air leaks, can't shed heat, worn piston cyl high firstion. Bad cranks, now that stuff sneaks up on you when frisky.

Oh when frisky I will run a color plug so don't hole piston, but will carry spares if I plan to stop... can foul up plug on cold start. Have to warm up around the home before I take off so good chance won't foul on we a few miles down the road. Most of the time just the stock plug and psychological governs how long I'll hammer it hard before cool of period

[Indy650]

Vintageman I'm curious about your DS7 with the R5 upgrade. Is it a simple top end swap making the DS7 a 350cc? I do remember the DS7 and R5 were the only Yami twins that were piston port(from what I remember anyway) But I didn't realize the top ends could interchange if in fact thats what the mod you have is.

[Vintageman]

Vintageman I'm curious about your DS7 with the R5 upgrade. Is it a simple top end swap making the DS7 a 350cc? I do remember the DS7 and R5 were the only Yami twins that were piston port(from what I remember anyway) But I didn't realize the top ends could interchange if in fact thats what the mod you have is.

Just seeing this

You need to change oil pump and gear (else too little oil)
you need to change carbs (else 26mm vs 28mm)
you need to change clutch and primary gear (else geared too low)
Obviously cylinders. heads. and pistons

Think that is it

I run one tooth larger front sprocket, D&G chambers. Those chambers work so well clutch slipped like crazy, stiffer springs solved.... well RD 350 added one more clutch disk. I like the R5 for EX port 32mm (similar to T350) vs RD 350 29.5mm. The chambers pull it 1000 maybe even 1500 past redline pulling strong all the may. I back off for afraid to blow it up.... no rev limiter back then . If you could find and r5 or ds7, buy it. Only complaint, seat padding too thin... later T350 and Gt250 can ride a lot longer. Maybe I need new foam from HVCCycle, but my seat seams OK I have T350 cycls I am boring out 2mm. So 335cc. The R5 I have is 1.25 Os so 362 cc. More torque than t350 and can hold 70mph on hilly highways in VT better than T350. But for some reason I find myself riding the T350s more (or Gt250s with T350 engines).

Just put Daytona Cycs on my 77 rd400 ( hard to find). So much nicer and stronger midrange, they are the version where ex port 2mm lower so less peaky, but again the D&G pull it well beyond redline. Now that is a comfy (ugly) seat. The bike is so much more enjoyable to ride now... getting old I guess like torque and soon be riding a Harley