Lack of power

[A.clarke]

Hi all
I have rebuilt my Arrow engine having had a rebore and new pistons, also centre oil seal regrind all new bearings and seals. The bike now starts first kick hot or cold but when out on the road has a lack of power, when I drop into top gear at about 35 mph it won't accelerate and tends to slow down, having rebore and new pistons I don't want to rev the engine too high. I am running with expansion boxes on and wondered if it could be these as they rely on back pressure, any thoughts

[Ivor Collins]

Hi
I doubt that the expansion boxes would make have a significant effect, have you done a compression test, ideally looking for approaching 150psi. What type of Pistons have you fitted?
Assume you have rechecked ignition timing and carb settings

Regards
Ivor

[A.clarke]

I have 110 psi at the moment obviously that will get better as the pistons bed in, they are Suzuki pistons, timing ok, carb set one notch off maximum.

[Ivor Collins]

110 is enough to give you good performance but Unless you have had the barrels shortened, the Suzi Pistons will only give a CR of around 8.3 : 1. More experienced members say that this is not detrimental but I somewhat sceptical and think that a return to 10:1 must be beneficial.

If you don't suspect carb or ignition faults, have you thought about checking the cranks are 180degrees apart, I have heard of worn keys/key ways causing problems

Also, have you thought removing one HT lead to see if one cylinder is less effective than the other to give you a clue as to where to look?

[A.clarke]

Hi Ivor
I checked for 180 degree before final tightening i too was sceptical about the pistons but as you say everybody says they are no problem but as there were no originals about I had to fit these. Don't want to skim the barrels as this would then make it so at a later stage I can't go back. I will have another tinker this afternoon.

[john.whiting]

May just be drag from the new parts.Check the ignition timing once more,and give it a chance to free up before doing anything drastic.Regards John.

[Ivor Collins]

I agree, the last thing you want to do is shorten the barrels, after all, you have paid for the rebore right to the top!
If you want to evaluate the effects of a low CR My solution would be to get a pair of heads, skim their faces and then re form a reduced 'top hat' volume to bump the CR back up. I have some '12:1' heads which were easily turned up by spinning them in a lathe from the spark plug hole.

[A.clarke]

Some great advice in the above, does anyone know what sort of speeds to run in at I'm thinking 40 for the first 100 Mile then up it to 50.

[A.clarke]

Hi Ivor
How much did you skim off can you remember?

[bh.sprint]

Hello,

Before cutting any metal take the head gasket out but check that there is still 45 thou( .045'' ) or over between the piston and the head squish band-- use old plasticene or soft solder to check the gap. This should give you an idea if this is the cause of the lack of performance.
The gasket is usually .020''

Brian H.