14 replies to this topic

[mrpj6]

Hi all. I am trying to build a trials bike using an engine from a 1978 Kawasaki KL 250. This build was inspired by some fantastic pictures posted on this site by Booma.... My problem is, i am not using the same frame he used and am having trouble getting the carb to fit in the frame. My question is... Can i lengthen the inlet manifold? If i can lengthen the tube from the carb to the head by about 30mm it will make fitting far far easier. The problem i have is, i have no idea what effect this will have on the mixture and running of the engine? If anyone has any knowledge of this subject? I would greatly appreciate the help. Many thanks.

[JSE]

mrpj6, on 02 February 2011 - 01:42 PM, said:

Hi all. I am trying to build a trials bike using an engine from a 1978 Kawasaki KL 250. This build was inspired by some fantastic pictures posted on this site by Booma.... My problem is, i am not using the same frame he used and am having trouble getting the carb to fit in the frame. My question is... Can i lengthen the inlet manifold? If i can lengthen the tube from the carb to the head by about 30mm it will make fitting far far easier. The problem i have is, i have no idea what effect this will have on the mixture and running of the engine? If anyone has any knowledge of this subject? I would greatly appreciate the help. Many thanks.

Lengthing the intake will have an effect of tract resonance but this will probably tend to improve lower RPM torque which should not be too big a problem. You will probably need to tune the carb as there will undoubtedly be other modifications in the project's intake/exhaust. This should not be a big problem and they big thing to remember is to try to keep the intake tube diameter as uniform as possible.

Jon

[andyrothers]

Hi I built a couple of trials bikes with honda engines that required lengthening the inlet manifold in one case by >45mm. Both bikes ran fine without problems. I didn't want to mofify the frame to get the carbs in so opted for the longer inlet route.
Andy

[mrpj6]

JSE, on 02 February 2011 - 03:17 PM, said:

Lengthing the intake will have an effect of tract resonance but this will probably tend to improve lower RPM torque which should not be too big a problem. You will probably need to tune the carb as there will undoubtedly be other modifications in the project's intake/exhaust. This should not be a big problem and they big thing to remember is to try to keep the intake tube diameter as uniform as possible.

Jon

Thanks Jon... Lost me i am afraid with tract resonance, But better low RPM torque can only be good. I will be making my own header pipe and air box i would have thought. So you are right about other modifications. When you say tune the carb.. Are you thinking fuel air mixture? or jetting?

[mrpj6]

andyrothers, on 02 February 2011 - 03:58 PM, said:

Hi I built a couple of trials bikes with honda engines that required lengthening the inlet manifold in one case by >45mm. Both bikes ran fine without problems. I didn't want to mofify the frame to get the carbs in so opted for the longer inlet route.
Andy

Thanks Andy...Nice to know someone has been there and done that. I have cut the frame and welded it to get the engine in, but i want to keep it to a minimum.. so a longer intake would save quite a bit of work 45mm would be luxury! Thanks again..... Pete.

[JSE]

mrpj6, on 02 February 2011 - 06:58 PM, said:

Thanks Jon... Lost me i am afraid with tract resonance, But better low RPM torque can only be good. I will be making my own header pipe and air box i would have thought. So you are right about other modifications. When you say tune the carb.. Are you thinking fuel air mixture? or jetting?

You may need to change the jetting a little but probably not much from stock. Mainly to tune the off-idle response but you can post how the engine is running and someone here will be able to give you some good advice.

Jon

[Old trials fanatic]

Dont forget to post some piccys too

Sounds an interesting project.

[Woody]

I lengthened the inlet of my BSA C15 by about the same distance in an attempt to slow throttle response off idle and soften the power coming in off idle, mainly so I could shut it down to nothing (I don't run a tickover) and bring it back in again smoothly and with no jerkiness. The theory is also that it would give a bit more torque so if it did, that would be a bonus.

It was smooth enough, not sure I could tell any difference in torque but the real surprise and bonus was that it virtually cured the spit stall when opening the throttle too quickly. I have to be pretty savage to make it cough now.

In my case no jet changes were needed, carb is MK1 Amal.

[JSE]

Woody, on 03 February 2011 - 12:54 AM, said:

I lengthened the inlet of my BSA C15 by about the same distance in an attempt to slow throttle response off idle and soften the power coming in off idle, mainly so I could shut it down to nothing (I don't run a tickover) and bring it back in again smoothly and with no jerkiness. The theory is also that it would give a bit more torque so if it did, that would be a bonus.

It was smooth enough, not sure I could tell any difference in torque but the real surprise and bonus was that it virtually cured the spit stall when opening the throttle too quickly. I have to be pretty savage to make it cough now.

In my case no jet changes were needed, carb is MK1 Amal.

My guess is that the slightly longer intake, with the added stored volume of air/fuel, acted as a sort of small "boost bottle", allowing the engine to draw just a little more fuel at the start of the intake phase/opening throttle. Interesting side effect and it would be fun to do some testing.

Jon

[mrpj6]

Thank you to all the above for the info, I feel happy to go with the 30 to 40mm extension now. I am a long way from the picture stage, but i will try and post some when there is something to show. I may well be back for more help with the jetting and probably before that!.... Good to know you are not alone when you are out in the shed knocking the skin off your knuckles... Thanks again.. Pete.

[andyrothers]

In terms of jetting, it was pretty much as standard although did take a little bit of setting up to get it running sweetly with the new airbox and exhaust. Keeping the noise down was biggest challenge as both the conversions I did had very little space for required intermediate silencer. TL125 front pipe from Sammy miller was a good starting point for the contruction of the pipe as the bend seemed to allow the right sort of shape threading between cylinder on one side and leg /boot on the other side. Kickstart /footrest was another challenge and petrol tank clearance to cylinder head...

One mistake I did make was although I (thought) had thoroughly cleaned the carb out, the donor bike had been stood for couple of years and took me ages to figure out why the bike wouldn't run on more than 1/4 throttle. Cleaned the carb out several times, still no joy then finally cleaned it 'properly' and suddenly the bike ran cleanly....

Good luck!

Andy

ps. you'll never stop tinkering either....

[belldane]

OK Bluey
I am back mate. You seem like a determined little bugger so go for it. Will do you an email soon. See ya mate

[B40RT]

mrpj6, on 02 February 2011 - 01:42 PM, said:

Hi all. I am trying to build a trials bike using an engine from a 1978 Kawasaki KL 250. This build was inspired by some fantastic pictures posted on this site by Booma.... My problem is, i am not using the same frame he used and am having trouble getting the carb to fit in the frame. My question is... Can i lengthen the inlet manifold? If i can lengthen the tube from the carb to the head by about 30mm it will make fitting far far easier. The problem i have is, i have no idea what effect this will have on the mixture and running of the engine? If anyone has any knowledge of this subject? I would greatly appreciate the help. Many thanks.

Ever seen the length of the inlet tract on a Rotax disc valve motor ?

Must be 150mm.

[mrpj6]

belldane, on 12 February 2011 - 06:53 AM, said:

OK Bluey
I am back mate. You seem like a determined little bugger so go for it. Will do you an email soon. See ya mate

Ahh Master.. You are back? still got those pebbles in your hand? Kung Fu reference.... (long story)

[mrpj6]

B40RT, on 12 February 2011 - 01:49 PM, said:

Ever seen the length of the inlet tract on a Rotax disc valve motor ?

Must be 150mm.

The answer to that is no. But whilst i was looking on the web to try and see if 30mm extension would make a difference... I saw a picture if a Royal Enfield with a reed valve fitted behind the carb and ... What looked like 300mm of old car radiator hose connecting it to the engine? I also found a thing called a trombone inlet? This was concertina tubing to make it more compact.. And the statement was that the most efficient inlet was a tube a metre long! If i am honest, I moved on.. As i was a bit out my depth. I have now found an intake from a KLF Quad bike that is the right length,, And sort of fits. So that is where i am at the moment... Off now to look up rotax disc valve.