Tiger Cub Rebore?

[brucey]

I have recently fitted new UPB PVL type electronic ignition and UPB Delorto Carb to my cub in an attempt to make it run better. I consider these good pieces of kit and already set up to run with my engine. (Bob is very helpful). The engine currently oils plugs and appears to run rich. It dies when the throttle is opened quickly and will not rev above 1/2 throttle. The symptoms get a bit better each time a new plug is fitted but not great.

I didn't rebore the barrel when I built it as it seemed o.k. I fitted a good standard piston with new rings (all at +40"). This is my 1st engine build. It seemed to run o.k once the rings bedded in but has got proressivly worse. It smokes a bit on start up but not excessively when hot.

I have removed the enginge and head to sort an irritating oil leak from the rocker shaft near the oil feeds.

I have a brand new Hepolite +60 piston and rings sitting in my shed. I'm wondering if I should have it rebored to + 60" and fit this for good measure.

The engine is running with an Alfa roller big end and Morgo high capacity oil pump, Alan Whitton breather mods and various other goodies. It has a square barrel and head.

I'm begining to loose heart with something I have spent so much time and money trying to get right on what should be a simple engine. However, I am not going to let it beat me!

What would you do next?

Any advice appreciated,

Bruce.

Edited March 28, 2012 by brucey

[gii]

What would you do next?

Bruce.

Borrow a carb from someone else, one that is known to work well enough on an engine of the same size.

I say this because the Dellorto I borrowed from a pal that worked well on his 199cc Cub was useless on my 246cc Cub

You say the bike oils the plug and appears to run rich. These are 2 different faults, are you sure you have both?

A bike oiling the plug will be burning large quantities of oil and will produce a large amount of blue smoke at all times, especially when hot and after it's been idling for a while. This is normally caused by a worn piston / barrel, worn valves / guides, or blocked breather.

A bike running very rich will have a black sooty plug that soon fouls / misfires, it will tend to produce black smoke, especially when accelerating , or blipping the throttle quickly, it will run poorly all of the time once warmed up.

A bike burning an embarassing amount of oil can often still run well though it may be a little down on power, I'd guess you are suffering from overly rich carburation hence the suggestion to swap carbs with someone.

[losty220]

Hi Bruce, you prob don`t need a rebore the oil is most likely worn valve guides, mine dont last long, i need to bite the bullet and fit bronze ones. I run an ngk bp4hs and its a bit better, ive borrowed carbs from bikes that rev clean and hardly work at all on either of mine, i think you have to just muck about with settings and jets, cubs vary so much.i use a modified villiers but hear there is a good amal available now, i think cubs were made to be frustrating so the reward is greater when you get it right, dont give up all you need is time and cash!

[fivebunny]

Could be the valve stems and or the the valve guides that are worn letting oil past down into the bore. Dont think either the valve or the guide will be too expensive and their striaght forward to do.

[brucey]

Thanks for the ideas and comments guys,

Now the head is off, I'll get the valve guides checked out. I'd tend to agree with your thoughts that the bike is running rich but playing with the needle height has made little difference. I'm running a 100 main jet in the 20mm Delorto and i think a 35 pilot jet.

I'll also get somebody else take a quick look at the engine while it's apart to get a second opinion.

Bruce.

Edited March 29, 2012 by brucey

[totalshell]

a couple of points.. you ve fitted new ignition and carb and the bike doesnt run well.. did it foul/ run rich before the mods? if not then you ve a very obvious couple of things to look at. if you ve a clean piston and rings then putting more of the same in will not a make a difference. head would certainly be the thing i'd look at.

pvl is now though easily eclipsed by the electrix ignition and bill beveridge has a simple piston swap that transforms cubs into a smooth as cream power on demand bike when linked to this electrix ign.

[colin350]

I don't know about Tiger Cubs, but I often find that it helps to put a thick piece of insulation between the carb and manifold. Thereby keeping the carb cooler.

[brucey]

Thanks Totalshell and Alan,

My thoughts are the same about the Delorto rubber connecting hose and this isn't a 'hot only' running problem. The Eletrix ignition looks the same spec (and components) as the ignition I have just fitted from UPB.

I guess I could pop the head back on and check the compression. I currently have the engine mounted in a rather handy stand I bought at Kempton Park Autojumble.

I acquired a Kawasaki piston and made some gudgeon pin bushes for my engine but couldn't obtain rings and didn't have enough experience (or bottle) to start cutting bits off the skirt and redirect oil return channels to do it myself. This is still an option but I don't want to introduce another set of variables!

I fitted the new carb and ignition in an attempt to resolve running problems once and for all but they have only partially cured the problem.

It maybe time to take all the bits to someone who knows what they are doing. Now I just have to decide who!

Bruce.

[brucey]

I refitted the head and pushrods. I get 125 psi on the compression tester. This is kicking the engine over with it attached to an engine stand (a bit slower compared to kicking it over in the bike) and with all the oils drained.

I'm also slightly concerned that my splined kickstart shaft splines seem to be disintegrating! This might be a good opportunity to go back to a cotterpin shaft set up.

[esteve]

Terry Weedy - Avery Products - stocks the splined kickstart shafts.

[stork955]

Gday, before doing anything else, tell us what the spark plug looks like when it is fouled - does it look "wet" or "dry"?

Cheers,

Stork

[brucey]

Esteve,

Cheers, looks like I may be sending some more money Terry's way! The only thing I hate with his kick start is the way it flicks and gets stuck under my Sammy Miller Footrest Mount. The special splined shaft and alloy kick start UPB offer prevents this but at a price!

Alan,

My Dellorto Carb is a PHBL 20. The main jet is screwed in the bottom of the float bowel. Although I did have a main jet unscrew itself on a Mikunni carb and had very similar running problems (died when the throttle was opened).

Carb setting are as follows:

Main Jet: 100

Pilot Jet: 40

Needle D40 (currently set 2nd groove from top out of 4 grooves total)

Timing was set as instructions (align yellow marks with piston at TDC) which I believe is an actual static timing of 3mm btdc.

Stork995

I have tried a number of plugs with the following results:

NGK B6HS (carb needle was in middle position, ran for approx 5 mins from cold) black and wet (Oil?)

NGK B5HS (carb needle in 2nd to top position, ran for approx 15 mins) black, not as oily

NGK BPR4HS (carb needle in 2nd to top position, ran for approx 1 hour although would not rev past 1/3 throttle) very dark brown, dry.

Generaly Mikunni carbs have always resulted in black sooty plugs. I tried a 'pit bike carb' with 'standard' and 'reccomended Cub' jet sizes and a slightly larger Mikunni carb that ran pretty well for several trials. Then I started getting problems with not reving out and bad starting at which point I decided to invest in the Dellorto carb and new ingnition.

The last plug (using the dellorto carb) has reduced the 'sootyness' to some extent. The bike does not puff out bellows of black smoke when you rev it as if it were running way too rich but dies above 1/3 throttle opening which makes bigger hills interesting!

Bruce.

Edited April 3, 2012 by brucey

[stork955]

Gday Bruce, you have carb problems. Throttle openings above about 1/4 are mainly controlled by the needle jet and needle. The slide cutaway will also have a major effect on initial opening - i.e. a flat spot on itiial acceleration. In my experience, you will never get a carb out of the box to work well, even when pre-jetted, especially if you have modified your bike to your own specs. All you can do is go through a rejetting procedure. You have a pretty hot plug in the bike now, which while masking symptoms, can have far-reaching consequenses (melted pistons....) My advice would be to start with a plug close to the original recommended range and then jet/repair to suit. With the B6 you mentioned that it was wet - definately oil fouled. Careful with being too easy when running in new engines - they need to be loaded to approx 3/4 load in the initial running ASAP to seat the rings. If the bike is rich this will stop the run in as well by bore-washing so you need to get that close before going further. As far as jetting goes, it is always a hit or miss procedure and there is a heap of information available on the net. The Mikuni guide is particularly good. It takes time, money and patience...

Cheers,

Stork

[brucey]

Stork,

Thanks for the information, your advice is appreciated. I'm not sure what happened to my previous post but I have edited it to make it a bit clearer.

I'll contact the guy who sold me the carb and see what jets and needles he would reccomend trying. Now that the engine is out of the bike, I'll pop the barrel off and check the rings, small end (which I made on my lathe), piston and bore. I may also get the bore glaze busted as I didn't do this on first assembly (my first engine build). The bike has run well in the past but always fouled plugs. I can see no reason why I shouldn't be able to get the bike running well at all revs with a plug the proper 'straw' colour.

Bruce.

[brucey]

Just a bit of an update,

I have contacted the guy who sold me the ignition and carb and been advised to try retarding the ignition as this has cured similar symptoms on other Cubs.

As this costs nothing, I will probably put it back together and try it.

Bruce.