Tiger Cub Carb - Ongoing Problems

[brucey]

After stripping and checking my cub engine I was disappointed to find I had much the same running problems with not revving past 1/3 throttle and sooting up plugs on reassembly.

Convinced I had a problem with my new 20mm Dellorto carb, I tried a friends 22mm Dellorto carb (from his 250 Serco Cub). This appeared to work much better. The engine revved pretty cleanly and the plug even appeared to go from black to straw colour! I must admit we didn't do an extended test due to time constraints but the bike was vastly improved.

Bob at UPB kindly exchanged my carb for the larger one. The only difference is that my new 22mm carb has a 100 main jet and the main jet in my friend’s carb was 110. As my Cub is 200cc, I thought it would be better.

Full of anticipation, I fitted my new 22mm carb to find it doesn’t run unless the choke is on! Even though it revs past 1/3 throttle (with a bit of popping and banging), as soon as the choke is turned off, the engine just dies even after running for 10 mins. I tried raising the needle (2^nd notch to bottom notch) but it made no difference.

The plug is back to it’s usual black/sooty colour.

Would 110 to 100 main jet really make that much difference? (I guess it’s fairly easy to prove).

I can’t believe I’m having so much trouble getting this engine to run properly and seriously thinking about giving up altogether and getting a modern 4 stroke bike instead.

Bruce.

[stork955]

Gday Bruce, it can make a difference -also you need to know if the needle, slide cutaway, pilot jet and needle jet are also the same or different to the test carb you fitted. The only way is to look at the components in the test carb and get the numbers. Carb tuning can be a long and frustrating thing, they never work perfectly out of the box but the satisfaction of getting it sorted is worth it in the end. Check those numbers and let us know how you go.

Cheers,

Stork

[stanmet]

hi before u go any futher try some freshly bought petrol i had a simler broblem stan

[brucey]

Thanks for the comments guys, I just ordered a 110 main jet from e bay.

I'm tempted to give my £20 Micuni pit bike carb another go. At least it ran on that one, just not very well!

[naichuff]

My Moto Guzzi V50 uses twin Dell,Orto carbs VHB24 The mixture screw is out 1 1/2 turns this sets the tick over mixture If you go to the Amal web site there is a fault and tuning down load chart showing throttle postions and problem cures www.amalcarb.co.uk the main jet should on restrict fuel at full throttle slide/needle mid range and pilot mixture jet/screw setting at idle Hope this is a help

[totalshell]

well if you have had another carb on and it ran okay with that order an exact duplicate of that and you should be sorted , though as someone has pointed out above setting up carbs can be long and drawn out but once done immensly satisfying and once done DONT mess with it.

[brucey]

looking on the internet, it may be possible I have an air leak. I will fit the 110 main jet, start the bike and give it a squirt of carb cleaner to see if it speeds up.

I have noticed a small oil leak which I think is comming from my base gasket. Can I lift the head & barrel slightly, remove the thin gasket and apply some RTV sealer around the cylinder base instead of using a gasket? (I would need to watch the oilways!)

Would a slight leak around the cylinder base cause a fuelling problem on a 4 stroke?

It must be something fairly simple as the effect is so dramatic.

I am now using a Dellorto PHBL 22 Carb.

100 (soon to be 110) Main Jet

35 Pilot Jet

60 Choke Jet D40 needle and 260BZ Needle Jet

[totalshell]

some where on thses pages many moons ago i posted the settings that martin adams reccomended for his serco cubs a quick search might/ ought to find them

[big_red_bike]

Hi the main jet size will make no effect to your engine cutting out .The problem is in the pilot system check jet size and if fuel ways are clear even in a new carb there could be a blockage.The black plug could be because you are using choke to get the bike to run.

[goudrons]

Brucey, you need to make sure everything else is right before attempting to tune in a carb.

Timing out, valves incorrectly set (I had a similar problem when my inlet valve guide wore and leaked, evident by black crude in the inlet tract) an air leak and you'll be chasing your own a*** around forever.

From the carb and jets etc you listed, you shouldn't be that far off, 35 to 40 pilot, 100 to 110 main, though I would add to that list a 40 slide.

By what you write, running on choke and dying without it with that carb and settings, I'd hazzard it's sucking in air from somewhere as whatever is effecting it isn't only effecting pilot (slow running) but main circuit as well.

[goudrons]

I've just noticed your club and area, feel free to contact me as I'm fairly local and ride in your area.

[brucey]

Just a quick update.

I fitted the 110 mainjet (god bless ebay!), checked all the other jets and blew everything though with carb cleaner (it's all new anyway). All appeared to be o.k.

It's just possible I didn't have the carb seated in the rubber manifold groove properly the 1st time I tried it as I think it went a bit further on this time.

I'll put yet another new plug in and some fresh Shell Vmax tomorrow and see what happens. If that fails, I will have to start with the basics all over again.....................or just give up and buy a Honda!

Goudrons, where abouts are you?

[goudrons]

Make sure you support the carb, especially if you're running a bolt on filter and there is nothing holding that end of the carb up or you'll split that connector in no time.

So it idles, picks up and revs out ok now?

I'll be at the evening series at the heights next month, if you're going I'll probably see you there.

[brucey]

Hello to anyone still interested!

Just tried it again with fresh fuel with absolutely no difference. However I did notice it blowing a lot though the crankcase breather pipe (Alan Whitton item).

There was very little black smoke (even when revved) and compression test results were o.k. This combined with the heavy breathing convinces me there is an internal air leak within the engine. Maybe valve guides of a seal somewhere?

I don't mind stripping the engine to rectify the fault, or even giving it to an expert to sort out, but I would like to confirm the actual fault before I remove the engine from the bike.

I think I will just put it back in the shed for a while untill I feel enthusiastic enough to start all over again.

Bruce.

[big_red_bike]

Hi if the compression test was good there is not a leak which would effect running,if the valve guides are worn you will get alot of smoke but the engine will still run ok .If it ran ok with the other carb then the problem has to be the carb,check fuel flow to carb, float valve fuel level in float chamber .If still no luck try the other carb again and if that works get another carb.