The 390 won't start

Hi Gents. Well, got the 390 to a point where I could try to start the engine, but no joy. Not even a sniff of life.

I have had the Motoplat tested by Steve Hardaker out in Spain and he said that it is fine.

I have checked that I have sparks and they are healthy looking to me. I have checked the timing and it seems to be set correctly. I have tried NGK B8ES and B8EG plugs, both have good sparks, but still no life.

The big difference from the stock set up is that I am running a Mikuni flat slide 38 mm carb originally from a 1984 Husky CR250. I used to have this model and it ran fine with a 400 main and from memory a 45 or 50 pilot jet. I have fitted a 430 main and a 45 pilot to the carb for the 390 engine.

The carb is nice and clean and I set the float height to 19 mm.

If I take the plug out after trying to start it, fuel is present, but it isn't soaking wet. Is it possible that I am not getting enough fuel through to the plug to fire it up?

On stripping the reed block I found it needed a good clean but the reeds themselves looked ok. There was no sign of any damage that I could see, so I just gave the whole thing a good clean and put it back together. Is it important which way the reeds go in the block?

Does anyone have any suggestions as to what else I can check?

Probably to basic, but a dirty pilot jet will make starting difficult.
Do you get any kind of pop at all?
You might recheck the timing again to, some times it can move a bit when you go back to tighten the backing plate.
Are you using a dial indicator to locate the piston?
Ron

Thanks for your suggestions. Yes, a clean pilot jet was one of the first things I made sure of, and also that the carb is clean as well. I have just rechecked the timing and retarded it a tiny bit, will try again tomorrow morning.

I am not using anything more scientific than a screwdriver down the plug hole. It is quite easy to feel TDC. The timing was set to 21.8 mm BTDC but I have taken it back a bit, to about 21.3 mm BTDC now.

I not following how your finding 21.3mm BTDC with a screwdriver in the plug hole?
Ron

Sorry, I did not explain that very clearly. I followed the 390 manual and it describes turning the external rotor back by 21.8 mm from TDC to set the timing. This obviously doesn't mean the piston is 21.8 mm BTDC, I got my terminology a bit mixed up.

Anyway, I have been having a bit more of a think and I took the carb off again to check. The slide is completely down as far as it can go and there is no possible way air can be going though the carb to fuel the motor. It seems to me that the slide should not be completely down even when the throttle is fully closed, so I have been looking at the carb to see how to adjust the slide. On a round slide Mikuni I remember there is a small screw on the side of the carb for adjusting the tickover which lifts the slide up and down as you screw it in and out, but on this flat slide carb there is no such screw, so it seems that the slide is adjusted simply using the cable. I have adjusted the cable so that now there is a small gap between the bottom of the slide and the carb throat. It seems to me that if the slide is completely down and closed then no air will be drawn into the engine, hence no fuel. Does that sound reasonable or am I missing something about this type of carb? When I was trying to start it today I did not use any throttle at all, so surely no air/fuel mix would have been drawn into the engine.

Has anyone had any experience of adjusting this type of carb? It is a flat slide from a 1984 CR250. I used to have one of these but cannot remember how it was done.

It should be able to start with the throttle closed and the choke on, but I agree the throttle has to be open to draw air and mixture from the pilot jet.
I had a flat slide carb on my Honda CR500 and the idle was adjusted on top of the carb.
Yours may be similar.
Does your flywheel have TDC and the 21.8mm timing marks on it?
I think the manual is a little confusing if you don't have the marks on the flywheel to line it up with already.
I don't have these marks on mine. I think the manual is saying you should make the marks for future timing reference.
Otherwise, you'll need a dial indicator to locate the piston. I always have to take the head off mine because my dial indicator won't reach that deep.
The factory spec for timing the 390 is 21.8.
That "does" mean the piston should be held at 21.8mm BTDC when you stick the 2mm pin through the flywheel and align the stator plate with the flywheel.
That's what I was refering to when I said the stator some times moves, when you remove the flywheel to tighten the stator down.
Best to recheck the timing after everything is tightened back up.
Hope I'm not making this more confusing for you.
Ron

You could use a screwdriver if you can make a mark on the screwdiver at TDC, then make another mark .8574" (55/64") lower and use that to locate the piston.

Thanks for your help Ron. I will let you know how I get on tomorrow.

If anyone does have any info on how to set up the flat slide carb off a 1984 CR250, then please let me know.

Thanks

Well, I have had to admit defeat.

I have checked and double checked everything. Timing, jets, float height, everything I can think of, still no joy, so I am going to have to give it to someone who knows what they are really doing.

Will keep you informed.

have to ask, did you manualy check for spark??
if so....id drag it down the street with a tow rope....

If its got spark and the timing is close put a few drops of gas in the cyl. and put the plug in and it should at least fire.

Thanks for the suggestions gents. I have had another look this evening and I am sure the ignition is working well enough to get her to fire up. I will also try the starting technique you suggest Ruwfo, thanks.

So I am going to try looking at the carb again on Wednesday. Struggling to understand why this carb might be causing problems as it is off a Husky, albeit a later 250, but I am hoping to borrow a round slide carb from Charlie Preston (Husky Sales UK) in case this is the problem. I am determined to crack this. I have never been beaten by a motocross bike before and I don't plan to start now. Grrr!

Well, I have finally admitted defeat, and the bike is going to Husky UK on Saturday. I went through the ignition again and even tried a borrowed round slide carb, still no life. Nearly wore out the back tyre dragging it up and down the road behind the car (so compression is healthy! ) Starting to think it's the ignition again as the sparks are still a bit intermittent, although they look reasonably heathly when it does spark.

Anyway, Charlie Preston knows his Huskies so should find out what the problem is quite quickly then I will know what needs to be done to get it sorted.

So gave the YZ125 a good service and then at least I will have a bike for the season opener on March 20th (this one started third kick despite not being run since October).

Yes, I have checked and rechecked all ground points and made sure all the power coat is removed from the engine cases and the paint from the frame. I have also run an extra earth wire from the coil mounting plate down to the stator plate as well, to be certain they are all grounded. I will run through it again once more tomorrow evening just to be sure.

With all the kicking and towing I would look at how much fuel is in the crankcase. You might want to clean the crankcase out, fresh plug and try again. Problem with fuel getting in the crankcase is the gas evaporates and the oil stays. When getting new fuel charge into it, it then is oil rich and fouls the plug. Had this problem with both of my 500's. I drilled and tapped a 1/16th pipe plug in both of those crankcases to be able to clean them from fuel. A plug that will spark outside of the combustion chamber is not a sure thing that it will fire in the chamber. Just my 2c

Check the workshop manual for the timing specs once again. Having the engine fire when the piston is at 21.3mm BTDC is definitely incorrect. It should be 2.13 mm or there abouts. If you are at 21.3mm you are waaaay too advanced. The timing for the 250 is at 2.43mm BTDC.
Peter

Wow! Went back and read the timing posts. I just timed my 84 XC250 at 2.3mm BTDC.

I pulled this off the Yahoo Groups Husky website:

1983-4 500cc: 18 Deg btdc, or 2.8mm btdc on piston.
1972-5 450cc: 18 Deg btdc, or 2.54mm btdc on piston.
1981-3 430cc: 17 Deg btdc, or 2.2mm btdc on piston.
1970-5 400cc: 22 Deg btdc, or 3.55mm btdc on piston.
1977-80 390cc: 18 Deg btdc, or 2.19mm btdc on piston.
Note: the 390 AUTO is 16 Deg btdc.
1975-7 360cc: 22 Deg btdc, or 3.02mm on piston.
1972-3 360cc: 16 Deg btdc.
1970-84 250cc: 20 Deg btdc, or 2.45mm on piston.
1974 250cc MAG: 18 Deg btdc, or 1.98mm on piston.