'67 Sebring carb

[Frank528]

I picked up a '67 Sebring a few years back that seems to have been set up for dirt track racing. Engine, frame and 6 volt electricals with mechanical points seem to be original. Mikuni carb fitted on a 3/4" spacer which I cleaned and replaced. Fully charged 6 volt battery. Got it started a few times-held it at quarter throttle-loud and strong. Never idled. Now it won't start and I'm too old to keep kicking. I know that there are many things to check and I don't expert my first post in this forum to be a complete tuning guide. There is one thing I wonder if someone out there can explain: after one unsuccessful kicking session I pulled the spark plug and there was a cloud of smoke or haze in the chamber. Plug was a little wet with fuel. Is it possible to partially ignite? Did it fire once and put it self out on the next cycle?
Got spark got compression got fuel. Maybe a short list of things to check next.
If you've read this far I thank you for your time.
Frank

[Ventodue]

Hi Frank and welcome to the Forum,

I picked up a '67 Sebring a few years back that seems to have been set up for dirt track racing. Engine, frame and 6 volt electricals with mechanical points seem to be original. Mikuni carb fitted on a 3/4" spacer which I cleaned and replaced. Fully charged 6 volt battery. Got it started a few times-held it at quarter throttle-loud and strong. Never idled. Now it won't start and I'm too old to keep kicking.

My advice? Don't keep kicking - either they go or they don't. If the latter, best to stop exhausting yourself and instead find out what's not right

There is one thing I wonder if someone out there can explain: after one unsuccessful kicking session I pulled the spark plug and there was a cloud of smoke or haze in the chamber. Plug was a little wet with fuel. Is it possible to partially ignite? Did it fire once and put it self out on the next cycle?

Yes - the engine has fired, but died. Happens - nothing especially untoward (someone more knowledgeable than me may come along to say exactly what occurs). But, as you say, it at least confirms you've got the basics of compression + spark + fuel. However, the evidence suggests that you haven't got the right amounts of each - and/or possibly at the right time. But the good news is that you can't be that far off.
P.s. The wet plug was possibly the consequence of you kicking it over too often (excess fuel in the chamber may even have contributed to it dieing ...)

Maybe a short list of things to check next.

Here's what I would do :
1. New plug. Even if your bike isn't entirely stock, I'd go for the standard NGKB7HS which seems to suit most of the w/cs.
1a) Make sure the plug cap and the HT cable are well connected, electrically-speaking, to the coil. Also check the coil has a good ground.
1c) Also check the battery has a good ground. In fact, just check all the wiring
2. Reset the points: 0.3- 0.4mm. Even better, change them (and ideally the condenser/capacitor).
3. Check the valve clearances. Again, I don't have the specs for the Sebring to hand, but 0.05 - 0.10mm (inlet and exhauust) was the standard w/c setting. Once you're confident that the clearances are ok, and if you have a compression tester, take a reading and let us know what you get.
4. (I'm sure you haven't, but ... ) Don't use old gas.

For the carb, it not being stock, it' more difficult. I know other people have mounted Mikunis succesfully, but I don't know what settings they used. Hopefully someone can advise.

After that, if it's stil 'No Go', you need to check the ignition timing. But that's a different story ...

HTH and Good Luck!

Craig

[Frank528]

Thanks Craig
A perfect place to stop. The short list will keep me busy for a week or two. I'll go over the wiring and replace old plug, points and condenser. I'll get the proper gaps for all including valve clearances and set them to the best of my ability. We can talk timing procedures after that I hope.

Frank

[Nick]

Get rid of the stock carb. Wrap it up in a plastic bag and put it away. Get a Mikuni 28mm or any other modern/semi-modern carb with a 26~28mm bore. If it came off a bike with approximately 350cc of displacement, the jetting should be pretty close. An Amal concentric or even an Amal Monobloc would also be fine. Or, just go to a junkyard and find something.

Disassemble whatever carb you use, soak all metal parts in lacquer thinning, blast clear with compressed air and make sure ALL the jets are clear. If you don't do this, just park the bike and forget about the whole thing. Ruining the kickstart mechanism by jumping up and down on it won't accomplish anything. Push start the bike instead.

I run a 28 Mikuni on my Sebring and it starts 1st or 2nd kick every time, idles well and runs way stronger than it did with the stock carb. Rode it to work yesterday, including a blast on the freeway, and was smiling all the way! The Sebring makes a great daily rider.

[Ventodue]

Get rid of the stock carb. Wrap it up in a plastic bag and put it away. Get a Mikuni 28mm <snip>

? ... Frank's already got a Mikuni fitted, Nick . (And he's even had a go at cleaning it ).

Mikuni carb fitted on a 3/4" spacer which I cleaned and replaced.

I run a 28 Mikuni on my Sebring

Have you got the details of the internals? Might help Frank with his ...

[machten]

Hmmm....I think the VHB works fine on my bike. I can also recall a 450 scrambler with a VHB that I never saw take more than 1 kick to start....



I think that would be your bike, Craig...

Kev

[amartina75]

I agree, I have a 250 and a 450 with a vhb and have no problems with them. When I build my 350 it will have a vhb on it as well. I don't see any point in modifying things to fit a mikuni. It doesn't matter what carb you have the tuning procedure is basically the same. Since the poster already has a mikuni fitted he may as well keep it for now at least. Maybe someone who is using one can post their setup. I believe their is a post on here called "mikuni on a monza" that may help as well.
Good luck

[amartina75]

http://www.motoscrubs.com/forum/viewtopic.php?f=3&t=1239&hilit=Mikuni+on+monza

[Nick]

Quick and dirty timing check:

First check points gap, don't have a manual here, but it should be around .016~.018" (of an inch) I believe. If it is smaller than that, that means the rubbing block has worn and the timing is retarded. Gap points properly as per manual, then put a small dab of grease on the points cam.

File the points mating surface smooth, then rotate engine until points are closed, put piece of cellophane from CD wrapper or cigarette pack between points. Remove spark plug and put engine in 3rd gear, bike on centerstand. Now, sitting on the left side of bike reach around with left hand and tug very lightly on cellophane, look down plug hole or place small straw or thin piece of wood down plug and slowly bring piston up towards TDC. (Use leg to nudge rear wheel to turn engine). Points should release cellophane just a few degrees before TDC (just before TDC). This will get you close enough for starting. From there on just advance the timing incrementally until you notice detonation or get kickback on starting, then retard accordingly.

Make sure plug is good (not just clean looking, but good.) Should produce nice spark when grounded on head.

Re Mikuni: think I've got a 15 pilot and about a 150 main. In any case, ensure that pilot jet is clear (make sure you can see though it.) Sudco can supply good base settings, but you can use the Mikuni settings for a twin-carb Triumph 650 Bonneville or even a 500 as a starting point. (My 28mm came off a Triumph 650 and worked fine as is.

Check high tension lead at both ends, coil and plug cap; also check wires to coil for corrosions, etc. Consider replacing the condenser. (It might be 40 years old.)

Close enough for government work.