I have a 1989 (I think it's an 89') Moto 4 250 that I have just put back together after being disassembled for 2 years. Frame has been painted and the top has been rebuilt. It had blown up from the timing chain stretching and breaking causing the valves to bend. I had the cylinder bored to the 3rd size over and valve head machined. I also rebuilt the carb, however I am still missing the plastic washer/dish that goes around the main jet. It was not in the rebuild kit and was missing in the carb. I'm not sure if that would cause my backfire but I doubt it. I did notice it popping air back threw the air box on backfire, and it just sound horrible. The backfire only happens while coasting @ speed, or while revving down. Next thing I was considering is the cam possibly gone soft.


I'm hoping there's some gurus out there who can help me get this machine running right. any ideas what it could be?

 

I have measurements of the cam that I took 2 years ago when I first took the engine apart. I just found them now.


Lobe A:
intake- 1.435 (1.439 limit)
exhaust- 1.437 (1.440 limit)


Lobe B:
intake- 1.183 (1.186 limit)
exhaust - 1.187 (1.190 limit)


every measurement is slightly below tolerance, would that be enough to cause these backfire issues?

 

I would maybe check the float height on your carb to make sure you aren't running too rich and slightly flooding your engine with fuel. Otherwise, I would find that plastic washer or use a metal one of similar size/shape temporarily to see if it changes anything. Not sure about the cam question, but the first two options are cheapest to check

 

thanks hoowahfun, that would certainly be the reasonable route. But I just decided to spend the money and fix everything. I'm going to install a new cam just because it is under tolerance and only going to get worse. I had the engine rebuilt and it still doesn't have the power I think it is capable of. Also addressing the carb issue at the same time so I can cross that off.




other things on the list:
air box parts; missing original filter. only have a k&n filter that doesn't fit 100% snug. Sand can still get in through the carb and I don't want to damage the work and $ I've put into this machine.


front brake cables


new front and rear tires

 

update:
still having backfire issues, and it backfires through the carb. I have replaced the cam, re-set timing and valves, added the plastic washer over the main jet in the carb that was missing. Only thing left would be electrical????


I should add the bike has always backfired, even before it blew up and ever since I've owned it. It's also a bitch to start now, not sure if that's just because it's cold out or something else.

 

Hmm..I dunno. How does the plug look? Maybe it's running too lean. Considering that you've bored it to the 3rd size over, it may not have enough fuel and could require a bigger jet. Maybe fiddle with the carb settings and see if it helps. Does it have a needle? A lot of options there. Also you mentioned that it backfired ever since you owned it....was it already bored over before all the engine work?

 

yes it was bored out before. I will try some carb tuning. At this time main jet is set 1 3/4 turns out as per manual.

 

Yeah if it was bored out before it may not be getting enough fuel. A look at the plug can also help you determine if it's too rich or too lean.