LCGB Forums

Hi,
I've just put 200 miles on my 225 Rapido Classic with new piston after its winter rebuild. I have put on it a Gammatech CHT sensor in exactly the same position on the cyl head as my cast 200 motor in my other GP. Both on 30mm dellys with expansion pipes and Augusto 6000's retarding to 17 deg @ 6000 rpm.

The problem is that the Rapido is running about 20 deg higher than the cast motor at any given speed/load. The temp rises quickly given any load and I have also detected pinking at about half throttle/5k revs and it holds its revs a little when you pull the clutch in while under hard acceleration to change gear and surges back and forth a bit when you shut the throttle suddenly to slow down. It sounds weak to me?

Apart from a rebore and new bearing its the same motor that has previously run fine for 6000 miles. I know a dyno is the best answer, after it's run in, but there's not one local to me.

There are no air leaks and the jets are the same as pre rebuild except it is now on a AV268 atomiser instead of a 266. The rest of the spec is:

55 pilot, 40 slide, X7 2nd clip, 300 float valve, 1.4mm squish, large remote Ramair.

I was considering trying different needles and or clip positions. What are other people's readings/specs, and, yes, I appreciate all motors are different. Thanks, Nick.

It does sound weak but your jetting is in the right ball park. In fact, with the AV268 you will have increased the fuel available via the needle through the range. Although your old AV266 may have worn to be bigger than the AV268, it seems unlikely.

However, that should be the first thing you substitute.

Your squish could be a tad tighter, but not worth worrying about.

What piston are you using & what was the clearance to the bore, prior to the 200 miles you’ve done?

How certain are you that there is no air leak?

Sounds like an air leak
Did you do a leak down test?
If so check the bits you might not have tested
Carb rubber exhaust.

I've seen Rapido's leak around the liner to alloy joints and also up the cylinder studs as there's not a lot of gasket face between the lower stud holes and the transfers.

Post by Wack » Wed May 18, 2016 5:37 pm
I've seen Rapido's leak around the liner to alloy joints and also up the cylinder studs as there's not a lot of gasket face between the lower stud holes and the transfers.

Wack’s correct. Two of mine needed a tubular sleeve fitted in the barrel stud hole nearest the inlet port. Stainless tube on eBay does the job.

Hi, thanks for the replies so far.

The piston is a Wossner, bored and honed by AF, so I assume the clearance is correct. I leak tested the engine after the rebuild from the exhaust stub to the carb rubber and all was OK. Because of that, and the fact that the problem started straight away, it could only have an air leak if it developed it immediately I started running it in. I might test it again to make sure.

Forgot to mention before that the X7 needle and the atomiser are new and it has a130 ain jet.

Do you think it's worth playing with the needle to try and cure it (assuming no air leak found) and what temperatures are your motors running?

Edit: Putting the choke on stops the pinking but it starts to bog down.

You had mentioned a new atomiser was fitted, an AV268 which is bigger than the AV266 previously fitted. A fresh needle will be more restrictive than the old worn one. However, it doesn’t bode too well if potentially your engine should be richer through the range yet has all the indications of running weak......

If I were you & thought playing with the needle would cure it, I would do so. Then again, I would try anything within reason to prevent the engine being potentially wrecked.

I don’t run a CHT gauge. If I were to run anything, I figure it would be an EGT but as my machine is built deliberately to replicate something I would have aspired to have owned in the early seventies, it still runs 12 volt with points. The only concessions are modern tyres & oil, reverse pull cable disc with sintered pads, the VHB (converted to two stroke) uses a filter & it is Rapido powered.

That’s how I know you are in the right ball park because prior to fitting the VHB I was running a PHBH.

Hi,

Just leak checked it and no leaks found. Also, re checked the squish which is 1.48mm at the bottom of the piston crown, so that seems ok?

I forgot to mention that it is 60mm crank.

Any suggestions as to the next step to try and cure it.

Cheers, Nick

To summarise, everything following the rebuild is identical with the exception of:

New bearing fitted.

Atomiser now a bigger, new AV268 replacing the AV266.

Needle now a new X7 replacing the old, identical needle.

Barrel has been bored & fitted with a new piston, bore clearance unknown.

Presumably the 60 mm stroke is as previously used & the slightly large squish similar @ 1.4 mm.

You have checked for leaks (presumably in the exhaust as well as inlet)

If there truly are no leaks & the ignition is functioning correctly & retarding as desired, it doesn’t leave many options other than suspect the bore clearance which you are unaware of.

As for the new bearing fitted, which has not been discussed as to the side, can you confirm it is properly sealed? With drive side bearing seals, it is far too easy to invert the seal lips.

Sorry to not be more help, but this will only get solved by a meticulous, methodical approach because there has to be something different to what you previously had.

Just a thought,
are you running sensor under plug?

Thanks for the replies so far.

Drive and mag bearings replaced. If they had been leaking it would have shown up on the leak/pressure test? The gauge never dropped at all.

The sensor is drilled and tapped into one of the fins next to the spark plug.

Is a poor piston (large/small?) clearance capable of causing a pinking and high running temp problem? And is it possible when I've run it and can ride in a normal fashion that the increased cooling from the higher fan speeds would allow it to run cooler. With the 4.8 gearing it is not easy to ride at such low speeds constantly and as a result unwanted extra heat can't be dissipated quickly enough.

Phew! I’m clutching @ straws!

Long Shot #1:

The leak test is only positive pressure so although the seals may hold up in that direction, they may allow air to be sucked in under vacuum.

Long Shot #2:

I've just put 200 miles on my 225 Rapido Classic with new piston after its winter rebuild.

If the bore was tighter than, say, 0.0035”, it could be the cause, but I doubt it TBH.

Squish?

Do you recommend a narrower squish?

If your in the 1.2 to 1.5mm area of squish and youve ruled out leaks and timing issues then it only really leaves carburation.

I had similar issues and strpiped and reassembled the top end and it went away.
I would check the fuel filter under the banjo and give the carb a good clean.
Good luck.

I've got a few more miles on it now and I think it may be carb related as it pinks at about 5k rpm/half throttle. You can ride into and out of the weak (?) area. When it starts to pink open the throttle fully stops it and as you come back down you re enter the weak area.

Is adjusting or changing the needle position the best way to eradicate this?

PSC GP200 » Fri Jun 17, 2016 10:41 pm
I've got a few more miles on it now and I think it may be carb related as it pinks at about 5k rpm/half throttle. You can ride into and out of the weak (?) area. When it starts to pink open the throttle fully stops it and as you come back down you re enter the weak area.

Is adjusting or changing the needle position the best way to eradicate this?

Yes, lifting the needle a clip or two is going in the right direction & might eliminate the problem but you might even want to try a richer needle for the costs involved.

I’d recommend you keep a note of any changes & do one alteration @ a time.

You can check the charts on-line but I believe an X13 & then X2 are the next two needles in the range to have a longer taper with the same nominal small & large diameters, so should come into play that bit sooner.

I hope that helps.

Running commentary of head temps on a new motor

Eden.

That’s an interesting insight into your set-up. To my mind, the motor sounds very strong & unstressed. It doesn’t sound as if it’s ‘too’ clean which is the correct side of the ideal stoichiometric (air fuel ratio) to be on, but you were able to analyse as you said, in your running commentary, areas of concern which centred upon your main jet & slide.

What did you conclude?

What changes did you make as the result?

My apologies in advance as I maybe should know but is this set-up with the addition of a power jet? If so, is your data analyser/logger functional in the electronic operation of the power jet or is it purely venturi, or otherwise, controlled?

I have not yet changed anything since that run yesterday. I day tripped the Big7 rally down south today. close on 400 mile round trip on my GT250.
I won't be messing with the GP again till after the Euro but next I'll try 145 main and a 40 slide and see how that feels.

My gamma unit does have the capability of powering a solenoid which could turn on a power jet at any programmed rpm or egt or cht ranges but I haven't yet got my arse in gear and fitting one lol. The carb on it which is a phbh 30mm does not have a power jet, I do have a couple of power jets I could put on but from experience I know a will be able to jet it in without one, it's just a mater of having time to fuck about. I can also adjust the programmable CDI to move the heat around, this engine was only built last Thursday so it's early days.